Oceanic Wave Energy Conversion: Advancement of Electrical Generators 9789819998135, 9789819998142

Table of contents :
Preface
Contents
About the Editors
Symbols
Acronyms
List of Figures
List of Tables
1 Introduction to the Principles of Wave Energy Conversion
1.1 Introduction to the Oceanic Wave Energy
1.2 Motion of a Particle in Oceanic Wave
1.3 Estimation of Wave Power
1.4 Wave Energy
1.5 Recent Wave Energy Projects
1.6 Basic Principles of Wave Energy Devices
1.6.1 Point Absorber: Power Buoy
1.6.2 Overtopping WEC
1.6.3 Oscillating Water Column
1.6.4 Attenuators
1.6.5 Oscillating Wave Surge Converters
1.6.6 Submerged Pressure Differential
1.6.7 Rotating Mass
1.6.8 Bulge Wave Converter Device
1.7 State of the Art
1.8 Summary
References
2 Oceanic Wave Energy Devices
2.1 Introduction
2.2 Wave Resources and Energy Centers
2.3 Wave Energy Converters
2.3.1 WET-NZ
2.3.2 Sea Oyster
2.3.3 Onshore Device Limpet
2.3.4 Oscillating Water Column
2.3.5 Penguin as a Rotational Energy Capturing Device
2.3.6 Pelamis as a Floating Offshore Device
2.3.7 SEAREV Wave Energy Converter
2.3.8 Columbian Wave Energy Converter SeaRAY
2.3.9 Delos-Reyes Morrow Pressure Device
2.3.10 Tapchan
2.3.11 SeaRaser
2.3.12 Power Buoy
2.4 Design Objective of a Wave Energy Device
2.5 Modeling and Simulation of Point Absorber
2.5.1 Time Domain Simulation
2.5.2 Method of Extracting Maximum Energy
2.5.3 Latching Method for Extracting Maximum Energy
2.6 Research and Education Approach
2.7 Recent Development
2.8 Summary
References
3 Pelamis Wave Energy Converter
3.1 Introduction to Pelamis
3.2 Background
3.2.1 Energy Policy Drivers—UK Perspective
3.2.2 Wave Energy
3.2.3 US Wave Energy Resource
3.2.4 Grid Integration
3.3 Pelamis Technology
3.3.1 Power Train
3.3.2 Survivability of Pelamis
3.4 Power Capture by Pelamis
3.5 Example of Resonant Response
3.6 The Tuned Response of Pelamis
3.7 Features of Pelamis
3.8 Strength, Weakness, Opportunity, and Threat of Pelamis
3.8.1 Strength
3.8.2 Weakness
3.8.3 Opportunity
3.8.4 Threat
3.9 Renewable Ocean Energy
3.9.1 Wave Energy Technology and Devices
3.9.2 Renewable Oceanic Energy Challenges
3.9.3 Investment and Operational Cost
3.10 Installation Challenges
3.10.1 Power Take-Off
3.10.2 Direct Drive Device
3.11 Uninterrupted Power Supply
3.12 Challenges and Solutions
3.12.1 Environment and Ecology
3.12.2 Economic Aspect
3.12.3 Installation-Related Factors
3.13 Summary
References
4 Resonant Wave Energy Converter
4.1 Introduction
4.2 State of the Art of Wave Energy Device
4.3 Resonant Effect in Oceanic Wave
4.3.1 Resonance as a Swing
4.3.2 Tank Testing of Wave Energy Device
4.3.3 Stage 3 Pilot Project
4.3.4 Full System Test Rig
4.3.5 Half Scale Device
4.4 Experimental Setup of Wave Energy Converter
4.4.1 The Shallow Water Wave Basin
4.4.2 Experimental Arrangement of the WECWake
4.5 Instrumentation and Acquired Data
4.5.1 Measurements of the Heave Displacement and Surge Force
4.5.2 Video Acquisition
4.6 Challenges
4.7 Conclusion
References
5 Mathematical Model, Design, and Cost Analysis of a Linear Electrical Generator
5.1 Introduction
5.2 Wave Power Formula
5.3 Wave Energy and Wave Energy Flux
5.4 Oceanic Wave and Linear Generator
5.5 Electricity Generation of the FSLEG
5.6 Description of the Magnetic Material
5.7 Simulation Results
5.8 Cost Analysis of the FSLEG
5.8.1 Active Material Used in the FSLEG
5.8.2 Cost Analysis of Active Material
5.8.3 Cost Calculation of Active Material
5.9 Future Scope
5.10 Summary
References
6 Dual-Port Linear Electrical Generator: Solution of the Existing Limitation of Power Generation
6.1 Introduction
6.2 The Existing Linear Generators
6.3 Limitation of the Existing Linear Generator
6.4 Dual-Port Linear Generator
6.5 Construction of the DPLG
6.6 Optimization of the DPLG
6.7 Results and Discussion
6.8 Summary
References
7 Flux Switching Linear Generator: Design, Analysis, and Optimization
7.1 Introduction
7.2 Manufacturer of Oceanic Wave Energy Devices
7.3 Prospects and Challenges
7.3.1 Prospects of Wave Energy
7.3.2 Challenges of Oceanic Wave Energy
7.4 Oceanic Wave Model
7.5 Limitations of the Existing FSLG
7.6 Optimization of the Existing FSLG
7.6.1 Working Principle
7.6.2 Architecture and Vector Diagram
7.6.3 Phasor Diagram
7.6.4 Equivalent Circuit Diagram
7.6.5 Electricity Generation
7.7 Simulation Results
7.8 Summary
References
8 Linear Electrical Generator for Hydraulic Free Piston Engine
8.1 Introduction
8.2 Parameter Analysis: Stroke Length
8.3 Output Power of the Generator
8.4 Frequency of the Generated Voltage
8.5 Voltage Rating
8.6 Simulation Results of an FPLG
8.7 Improvement
8.8 Future Scope
8.9 Summary
References

Citation preview

Omar Farrok Md Rabiul Islam   Editors

Oceanic Wave Energy Conversion Advancement of Electrical Generators

Oceanic Wave Energy Conversion

Omar Farrok · Md Rabiul Islam Editors

Oceanic Wave Energy Conversion Advancement of Electrical Generators

Editors Omar Farrok Department of Electrical and Electronic Engineering Ahsanullah University of Science and Technology Dhaka, Bangladesh

Md Rabiul Islam Faculty of Engineering and Information Sciences School of Electrical, Computer, and Telecommunications Engineering (SECTE) University of Wollongong Wollongong, NSW, Australia

ISBN 978-981-99-9813-5 ISBN 978-981-99-9814-2 (eBook) https://doi.org/10.1007/978-981-99-9814-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore Paper in this product is recyclable.

Preface

Nowadays, harvesting renewable energy is considered as one of the most significant fields of research. As conventional fossil fuels are rapidly burning out and electrical power demand is increasing exponentially, there is no alternative other than using renewable energy. Researchers have been working on utilizing different renewable energy sources for producing electricity since many years. Most of the renewable energy has a limited capacity of power generation. Although, oceanic wave energy has tremendous power potential for electricity generation, its harvesting is still in the developing stage. It is thought to be a new member of the renewable energy compared to solar and wind energy. Recently engineers are working hard to produce electrical power from the oceanic wave in an effective way. It is expected that if the untapped energy from the oceanic wave can be captured, it can contribute a significant amount of global electrical energy consumption. This book describes the fundamental concept of generating electricity from the oceanic wave. In the Chap. 1, “Introduction”, principles of wave energy conversion are explained. Firstly, fundamentals of wave energy and motion of a particle in the ocean are depicted. It is supportive to understand the working principle of wave energy devices. Estimation of wave power and energy is illustrated mathematically which is useful for design and size selection of the wave energy converter. Recent wave energy projects around the globe are described in a separate section. From the study of various types of wave energy converters, it is found that point absorber type wave energy device with direct drive linear electrical generator is widely used for harvesting oceanic wave energy. Recent developments of the linear generator are summarized in a table at the end of this chapter. The Chap. 2 named “Oceanic Wave Energy Devices” presents various wave energy converters (WECs) and devices with their operation. The resources along with the main renewable energy centers, the technical, environmental, and social aspects of the procedures are discussed. Several methods of wave energy extraction and wave termination are also explained. Construction of the WEC or devices such as WET-NZ, Oyster, Limpet, and Penguin along with their visual structures and working principles

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Preface

are demonstrated. Additionally, many other devices such as Paramus, Power buoy, SeaRev, SeaRay, etc., are studied that can harvest the power from the wave. Finally, the construction and operational formula of point absorber WEC is discussed along with its related factors. The chapter concludes with the discussion of current research and educational approach to wave energy. The Chap. 3, “Pelamis Wave Energy Converter” describes different aspects of Pelamis technology along with its features. The current energy policy and estimation of wave energy are presented. Power capture by Pelamis and its survivability attributes are depicted. The power train of Pelamis, its resonant and benign response along with the tuned response are described. The strength, weakness, opportunity, and threat of this device are mentioned in detail with their challenges and possible solutions. An integration of energy storage and its importance are illustrated to obtain intermittent power. Environmental, ecological, and economic factors are discussed as well. The Chap. 4, “Resonant Wave Energy Converter” describes the concept and explanation of the resonance effect of wave energy converter. Placement of resonant and other WECs is discussed at the beginning of this chapter. Resonant WEC is usually a floating type of device which can be placed near shore. It is found that resonant WEC can enhance the amplitude of the swinging buoy with comparatively less effort than the conventional one. An experimental setup of a wave basin is presented, which is used for testing resonant WECs. Its various components and setup for parametric measurements are illustrated. Phase control issue is one of the key factors for this WEC. At the end of this chapter, necessity of implementing resonant WEC and other effective renewable source-based power plants is explained. The simplified mathematical model of a flux switching linear electrical generators for wave power extraction is presented in the Chap. 5. Then design and simulation of a double-sided flat flux switching linear electrical generator (FSLEG) is presented. Characteristics of the FSLEG for harvesting oceanic wave energy are analyzed. To enhance the performance of FSLEG, a special Kool Mμ powder core with N46SH permanent magnet is applied. It is found from the simulation that because of using Kool Mμ powder core, core loss is minimized. On the other hand, power generation is reduced. To increase power generation, high graded N46SH material is applied again. Thus, with proper combination of Kool Mμ and N46SH, both parameters are improved, i.e., increase in electrical power generation and decrease in core loss. Cost analysis is provided for the active material, i.e., copper, permanent magnet, and magnetic core. Then the tentative material cost of the FSLEG is calculated. As application of Kool Mμ powder core to the linear electrical generator is relatively new, future recommendation is listed at the end of this chapter. In the Chap. 6, “Dual port linear electrical generator: solution of the existing limitation of power generation”, a new design of dual port linear generator (DPLG) is presented. It can produce enough electrical power from the oceanic wave with adequate amount of voltage while the translator reaches the top and bottom ends. Stator tooth design greatly affects the efficiency of the DPLG. Genetic algorithm is

Preface

vii

suitable to determine the optimized stator tooth. The shape optimization method of the stator teeth is presented to justify the performance of DPLG. The force ripples of the DPLG are reduced up to 40.89% by improving its stator tooth shape. It improves the power conversion efficiency of the DPLG as shown in the results and discussion section. The analysis is illustrated with multiphysics simulation and the finite element method to determine the electromagnetic performance. Simulation results along with laboratory prototype are also presented for validation of the DPLG topology. Experimental and simulation results from the prototype show the special interest of applying DPLG as it generates adequate voltage even at zero vertical velocity of the translator obtained from the oceanic wave. It is not possible to achieve this by using the conversional SPLG which is mathematically shown. Thus, the production of more electrical power from the DPLG is ensured even at the moment of no vertical velocity of its translator. In Chap. 7, “Flux Switching Linear Generator: Design, Analysis, and Optimization” a new design of flux switching linear generator (FSLG) is presented along with a model where the translator weight is reduced and magnetic flux linkage of the main stator is improved by applying static steel core in the secondary stator. The produced voltage, current, power, efficiency, core loss, force ripples, and cogging force reduction for the FLSG are presented. The new translator is lightweight and can generate enough electrical energy from the oceanic wave as shown in the dynamic model. Genetic algorithm is used to find out the optimal design of the translator and stator before it is finally selected. To observe the improvement and possibility to utilize this design of FSLG, finite element analysis is conducted by utilizing ANSYS/Ansoft. In the Chap. 8, the application of linear electrical generator and its advancement are discussed. It is explained that the construction and working principle of the linear generator is similar for both oceanic wave energy conversion and free piston generator applications. Therefore, advancement of linear electrical generator for wave energy conversion can be applicable to a free piston linear generator (FPLG). In an FPLG, a free piston engine drives a linear electrical generator to produce electrical power. In this chapter, different parameters of an FPLG are analyzed. The parameters include stroke length, power, voltage frequency, and voltage. Firstly, the construction and working principle of an FPLG is explained. Among the parameters of FPLG, stroke length is discussed in the beginning. It is found that the minimum and maximum stroke lengths are 20 mm and 152.4 mm, respectively. Stroke lengths of a free piston engine and a linear electrical generator must match each other. Then output powers of different FPLGs are described and tabulated. The power range is found from 22.23 W to 95 kW. Voltage frequency of the FPLG ranges from 2 Hz to 67 Hz which is listed in a separate table. The maximum output voltage is found to be 400 V whereas most of the FPLG produce less than 300 V. Performance of the FPLG depends on its design, different parameters, and construction material. Then simulation results of an FPLG are presented for using a conventional and the proposed XFlux materials. Voltage, current, power, flux linkage, and core loss of the FPLG are plotted using different magnetic cores to observe their relative performance. It is found that because of

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Preface

applying XFlux to the FPLG, minimum core loss occurs. Recent progress of FPLGs and their specialty are summarized. The advancement and future scope of the FPLG has been proposed at the end of this chapter. Dhaka, Bangladesh Wollongong, Australia

Omar Farrok Md Rabiul Islam

Contents

1 Introduction to the Principles of Wave Energy Conversion . . . . . . . . . Omar Farrok, Mohamud Mohamed Farah, and Md Rabiul Islam

1

2 Oceanic Wave Energy Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tanwi Howlader, Omar Farrok, Md Ahsan Kabir, Md. Abdullah-Al-Mamun, and Md Sawkat Ali

17

3 Pelamis Wave Energy Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mushfiqur Rahman Shipon, Md Sawkat Ali, Md Ahsan Kabir, Md. Abdullah-Al-Mamun, and Omar Farrok

45

4 Resonant Wave Energy Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Md. Mahedi Hasan Sujon, Md Ahsan Kabir, Md Sawkat Ali, Md. Abdullah-Al-Mamun, and Omar Farrok

67

5 Mathematical Model, Design, and Cost Analysis of a Linear Electrical Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohamud Mohamed Farah, Omar Farrok, and Mahamudul Hasan Uzzal 6 Dual-Port Linear Electrical Generator: Solution of the Existing Limitation of Power Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohamud Mohamed Farah, Md. Abdullah-Al-Mamun, Md Rabiul Islam, and Omar Farrok

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7 Flux Switching Linear Generator: Design, Analysis, and Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Abdirazak Dahir Tahlil, Md. Abdullah-Al-Mamun, Md Rabiul Islam, and Omar Farrok 8 Linear Electrical Generator for Hydraulic Free Piston Engine . . . . . . 145 Abdirazak Dahir Tahlil, Omar Farrok, Md. Abdullah-Al-Mamun, and Mahamudul Hasan Uzzal

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About the Editors

Omar Farrok received a Ph.D. degree from the Department of Electrical and Electronic Engineering (EEE), Rajshahi University of Engineering and Technology (RUET), Rajshahi, Bangladesh, in 2016. Since 2020, he has been a Distinct Professor with the Department of EEE, Ahsanullah University of Science and Technology (AUST), Dhaka, Bangladesh. He has authored or co-authored around 100 technical papers in international journals and conference proceedings, including a few chapters and 15 IEEE Transactions/Journal articles. His research interests include design of electrical machine, linear electrical generator, magnetic material, renewable energy system, oceanic wave energy converter, electromagnetics, power electronics, electrical machine, and drive. Currently, he is developing an oceanic wave energy converter test setup at his University (AUST), which is going to be the first test setup in Bangladesh. He has received a Research Grant for this experimental setup. He received Six (6) Best Paper Awards from the IEEE ICEMS, Sydney, NSW, Australia, in 2017; ASEMD, Tianjin, China, in 2018; ICEMS, Harbin, China, in 2019; PEEIACON, Dhaka, in 2019; and ASEMD in 2020, Tianjin, China. He was invited to different international conferences as Session Chair/Keynote Speaker in a few countries. He was elected as the Chair, Co-Chair, and nominated as a member of several technical committees formed by the Bangladesh Government under the Ministry of Industries and others. He is a Life Fellow of the Institution of Engineers, Bangladesh (IEB). He was registered xi

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About the Editors

with the Board of Bangladesh Professional Engineers Registration Board (BPERB) as a Professional Engineer (P.Eng.) in 2017. He is a member of the Editorial Board of a few international journal publishers. Md Rabiul Islam received a Ph.D. degree from University of Technology Sydney (UTS), Sydney, Australia, in 2014 in electrical engineering. He is currently a Senior Lecturer with the School of Electrical, Computer, and Telecommunications Engineering (SECTE), University of Wollongong (UOW), New South Wales, Australia. He is a Senior Member of IEEE. His research interests are in the fields of power electronic converters, renewable energy technologies, power quality, electrical machines, electric vehicles, and smart grid. He has authored or co-authored more than 400 papers including more than 100 IEEE Transactions/ IEEE Journal papers. He has written or edited 8 technical books published by Springer and Taylor & Francis. He is serving as an Associate Editor for the IEEE Transactions on Industrial Electronics and IEEE Transactions on Energy Conversion. As a Lead Guest Editor, he has organized the first joint IEEE Industrial Electronics Society and IEEE Power & Energy Society Special Section entitled “Advances in High-frequency Isolated Power Converters”. He is an Editor of the Book Series entitled “Advanced in Power Electronic Converters” for CRC Press, Taylor & Francis Group. He is the Vice-Chair of IEEE New South Wales (Australia) Joint Chapter IAS/ IES/PELS. He is the Australia/Oceania Liaison Officer for the IEEE IAS Transportation Systems Committee. He has organized many international conferences in different countries and was the Keynote Speaker for many international conferences. He has received several funding from Government and Industries including in total $5.48 million from Australian Government through Australian Research Council (ARC) Discovery Project (DP) 2020 entitled “A Next Generation Smart SolidState Transformer for Power Grid Applications” and ARC Industrial Transformation Training Centre Project 2021 entitled “ARC Training Centre in Energy Technologies for Future Grids”. He is one of the Chief Investigators of the ARC Training Centre in Energy Technologies for Future Grids and Theme Leader of Australian Power and Energy Research Institute.

Symbols

Introduction to the Principles of Wave Energy Conversion P Hm Te ρ g E cg

Wave energy flux per unit length of the wave crest Significant wave height Time period of wave Mass density of water Gravitational acceleration Average energy density of the wave per unit horizontal area Group velocity

Oceanic Wave Energy Devices F e (s) F r (s) F pto (s) m V (s) ρ g A mb madd Tw T0 ω

Excitation force on a float by the wave Radiation force by the wave Force on the float from power take-off device Mass of the float Vertical velocity Mass density of the fluid (water) Gravitational acceleration Area of the buoy Mass of the buoy Added mass due to wave motion Wave period Natural period of the buoy Natural frequency

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Symbols

Mathematical Model, Design, and Cost Analysis of a Linear Electrical Generator a atr Aw Bair cg E av Ei Ew Eϕ fE fw g h H m0 Id Iq KE kw Mb Mw M tr N WT Pw Rr Rv T vmean Vq Vt vtr vw Ws Xd X d Xq X q E g E q ztr zw θj

Buoy radius Translator acceleration Wave amplitude Air gap flux density Group velocity Excitation voltage Generated no load rms voltage Overall wave power density per unit horizontal area Induced voltage per winding per phase Frequency of generated voltage Wave frequency Gravitational acceleration Ocean depth Wave height Direct axis current Quadrature axis current Voltage constant Winding factor Buoy mass Added mass due to sea water Translator mass Number of turns per phase Wave power Radiation resistance Viscous resistance of ocean water Wave period Average velocity of translator Quadrature axis voltage Terminal voltage Velocity of translator Wave velocity Stator width Synchronous reactance of direct axes Transient reactance of direct axes Synchronous reactance of quadrature axes Transient reactance of quadrature axes Electromotive force behind transient reactance Transient electromotive force along q-axis Displacement of translator Vertical position of wave Initial phase angle

Symbols

λ ρ τ tr Φ PM

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Wavelength Density of ocean water Pole or tooth pitch Permanent magnet excitation flux

Dual Port Linear Electrical Generator: Solution of the Existing Limitation of Power Generation z v Amax T αv Pm Pe vt F F drvr F drvn m z1 z2 Ke km ω ωH ωL AL AH ϕ F float zfloat vfloat afloat g r float Rvis Rrad ρ vwave zwave Mt

Elevation of translator Perpendicular velocity of translator Peak level of the translator Time period Phase angle Input mechanical power of the generator Electrical power generated from linear generator Velocity of the translator Applied force Applied force to the driver translator Force transferred to the driven translator Translator mass Vertical displacement of driver translator Vertical displacement of driven translator Upper end and lower end spring constant Spring constant of the spring connecting driver and driven translator Natural frequency Maximum value of natural frequency Minimum value of natural frequency Magnitudes of the driver translator Magnitudes of the driven translator Initial phase angle Buoy force Vertical displacement of buoy Vertical velocity of buoy Vertical acceleration of buoy Gravitational acceleration Radius of buoy Viscous resistance Radiation resistance Mass density of ocean water Vertical velocity of ocean water Vertical displacement of ocean water Translator mass

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M float Mw E driver E driven L winding Rwinding Lf Cf Rload E gen K lg zt τp Ft Fe Fl λ at W co B H F obj s F vir J Bθ θ h d Pdc V dc I dc F tr η

Symbols

Buoy mass Additional mass due to oceanic water Generated voltage of driver generator Generated voltage of driven generator Equivalent series inductance of winding Winding resistance Filtering inductance Filtering capacitance Load resistance Generated voltage of dual port linear generator Construction constant of dual port linear generator Vertical position of translator Pole pitch of translator Total force of the translator Electromagnetic force of translator External load force Damping factor Acceleration of translator Magnetic co-energy of the system Magnetic flux density Magnetic field intensity Force on any part or object of translator in the moving direction Distance Instantaneous virtual force on translator Conduction current density Magnetic flux density at a specific time Angular movement Height of stator tooth Width of stator tooth Generated electrical power measured at dc bus Voltage of dc bus Line current Applied mechanical force to the translator Efficiency of dual port linear generator

Flux Switching Linear Generator: Design, Analysis, and Optimization zw vw Aw fw

Vertical position of wave Wave velocity Wave amplitude Frequency of the wave

Symbols

θj z tr vtr atr ρ g a Rr Rv Mb Mw M tr atr Hc Brem μr τ tr KE Eϕ N WT Ws Bair vmean Ei fE Φ PM Kw Φg net l Fd Fq p Ia Kd Kq Kf K fd K fq Kw ωc ps Xd Xq X d

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Initial phase angle Displacement of translator Velocity of translator Translator acceleration Mass density of sea water Gravitational acceleration Cylinder or buoy radius Radiation resistance Viscous resistance of the sea water Mass of the buoy Added mass due to sea water Translator mass Acceleration of the translator Magnetic coercivity Remanence Relative permeability Pole pitch Voltage constant Induced voltage per winding per phase Number of turns per phase Effective stator width Flux density of air gap length Average speed of translator No load rms voltage Frequency of induced voltage The excitation flux of permanent magnet Winding factor Air gap flux Net flux Leakage flux Magnetomotive forces for d-axis Magnetomotive forces for q-axis Number of pole pairs Armature current Reaction factor for d-axis Reaction factor for q-axis Excitation field form factor Excitation field form factor for d-axis Excitation field form factor for q-axis Winding factor Coil pitch Permeance of the introduced FSLG Synchronous reactance for direct axis Synchronous reactance for quadrature axis Transient reactance for direct axis

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X q E av P F ar Id Iq Vq Vt E g E q Fga

Symbols

Transient reactance for quadrature axis Excitation voltage Generated power of the FSLG Force of the armature reaction Current for d-axis Current for q-axis Voltage for q-axis Terminal voltage Electromotive force behind transient reactance Transient electromotive force along q-axis Gravitational acceleration force

Acronyms

Introduction to the Principles of Wave Energy Conversion WEC OWE OWSC

Wave energy converter Oceanic wave energy Oscillating wave surge converter

Oceanic Wave Energy Devices RES WEC PCF PCU OWC

Renewable energy source Wave energy converter Power connector frame Power capture unit Oscillating water column

Pelamis Wave Energy Converter OTEC WEC

Ocean thermal energy conversion Wave energy converter

Resonant Wave Energy Converter WEC WG

Wave energy converter Wave gauge

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Acronyms

Mathematical Model, Design, and Cost Analysis of a Linear Electrical Generator FSLEG NdFeB SmCo PM

Flux switching linear electrical generator Neodymium iron boron Samarium cobalt Permanent magnet

Dual Port Linear Electrical Generator: Solution of the Existing Limitation of Power Generation SPLG DPLG RER OWE

Single port linear generators Dual port linear generator Renewable energy resource Oceanic wave energy

Flux Switching Linear Generator: Design, Analysis, and Optimization RES OWE FSLG WEC

Renewable energy source Oceanic wave energy Flux switching linear generator Wave energy converter

Linear Electrical Generator for Hydraulic Free Piston Engine FPLG CRGO DPI TSI

Free piston linear generator Cold rolled grain oriented Diesel pilot ignition Traditional spark ignition

List of Figures

Fig. 1.1 Fig. 1.2 Fig. 1.3 Fig. 1.4 Fig. 1.5 Fig. 1.6 Fig. 1.7 Fig. 1.8 Fig. 1.9 Fig. 1.10 Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6

Fig. 2.7 Fig. 2.8 Fig. 2.9

Movement of a floating object in a ripple on the surface of oceanic wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oceanic wave propagation and motion of a particle in sea water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic diagram of a point absorber type wave energy converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of the overtopping device . . . . . . . . . . . . . . . . . . . . . . Structure of an oscillating water column . . . . . . . . . . . . . . . . . . . . An attenuator wave energy harnessing device . . . . . . . . . . . . . . . . An oscillating wave surge converter device . . . . . . . . . . . . . . . . . . A submerged pressure differential with a moving float . . . . . . . . Rotating mass wave energy technology . . . . . . . . . . . . . . . . . . . . . Bulge wave energy converter system . . . . . . . . . . . . . . . . . . . . . . . The wave resources in the world . . . . . . . . . . . . . . . . . . . . . . . . . . Global energy consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some marine renewable energy centers established by the US Department of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . Wave energy floating device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wave energy device using oscillating water column . . . . . . . . . . Different types of WEC devices according to their working principles: a oscillating wave surge converter b oscillating water column c point absorber: heaving buoy d rotating mass e attenuators f submerged pressure differential . . . . . . . . . . A WEC device called WET-NZ with a float . . . . . . . . . . . . . . . . . Oscillating wave surge converter . . . . . . . . . . . . . . . . . . . . . . . . . . Construction of a sea Oyster . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 4 7 8 9 10 10 11 12 13 19 20 21 21 22

23 23 24 25

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Fig. 2.10

Fig. 2.11 Fig. 2.12 Fig. 2.13 Fig. 2.14 Fig. 2.15 Fig. 2.16 Fig. 2.17 Fig. 2.18 Fig. 2.19 Fig. 2.20 Fig. 2.21 Fig. 2.22 Fig. 2.23 Fig. 2.24 Fig. 2.25 Fig. 2.26 Fig. 2.27 Fig. 3.1 Fig. 3.2 Fig. 3.3 Fig. 3.4

Fig. 3.5

List of Figures

Sequential operation of a sea Oyster. a Hydraulic pistons are in the initial position. b Movement of the flap when the waves come. c Maximum expansion of the flaps of the Oyster when the whole cylinder is filled with water and high pressurized water is fed to the onshore hydroelectric turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An onshore wave energy device called Limpet . . . . . . . . . . . . . . . An oscillating water column mounted at seashore . . . . . . . . . . . . An efficient and affordable wave energy device called Penguin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Pelamis that uses the motion of oceanic wave for producing electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEAREV wave energy converter: a floating state and b internal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An array of floating wave energy device named SeaRAY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Delos-Reyes Morrow pressure device situated on the ocean floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A tapered channel WEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Construction of a typical SeaRaser WEC . . . . . . . . . . . . . . . . . . . Block diagram representation for a model of power buoy . . . . . . Visual structure of a point absorber . . . . . . . . . . . . . . . . . . . . . . . . Wave termination of wave energy device . . . . . . . . . . . . . . . . . . . Maximum power extraction by using a a hydrofoil and b two hydrofoils into the water . . . . . . . . . . . . . . . . . . . . . . . . Latching method for extracting maximum energy: a latching and b unlatching of the buoy . . . . . . . . . . . . . . . . . . . . Suboptimal latching process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental implementation of a latching method . . . . . . . . . . . Some aspects of the research and education approach . . . . . . . . . A typical Pelamis wave energy converter . . . . . . . . . . . . . . . . . . . Components of the oceanic wave . . . . . . . . . . . . . . . . . . . . . . . . . . Resonant response and benign response . . . . . . . . . . . . . . . . . . . . The device position for the a first, b second, and c third stages due to resonant response in the water (front view). Position for the d upward and e downward directions due to its movement for resonant response (bottom view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fixed and slow tune responses of Pelamis for a low frequency at fixed tune response, b low frequency at slow tune response, c high frequency at fixed tune response, d high frequency at slow tune response, e medium frequency at fixed tune response, and f medium frequency at slow tune response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26 27 27 28 29 29 31 31 32 33 34 34 35 37 38 39 40 40 47 49 51

53

54

List of Figures

Fig. 3.6 Fig. 3.7 Fig. 3.8 Fig. 3.9 Fig. 3.10 Fig. 4.1 Fig. 4.2 Fig. 4.3 Fig. 4.4 Fig. 4.5 Fig. 4.6 Fig. 5.1 Fig. 5.2 Fig. 5.3 Fig. 5.4 Fig. 5.5

Fig. 5.6 Fig. 6.1

Fig. 6.2 Fig. 6.3 Fig. 6.4 Fig. 6.5 Fig. 6.6 Fig. 6.7 Fig. 6.8 Fig. 6.9

Fig. 6.10 Fig. 6.11

The range of non-renewable electricity cost . . . . . . . . . . . . . . . . . Multistage power take-off system . . . . . . . . . . . . . . . . . . . . . . . . . Magnet gearing system by ferrite material . . . . . . . . . . . . . . . . . . Consistent power by using storage to meet baseload . . . . . . . . . . Ocean compressed air energy storage . . . . . . . . . . . . . . . . . . . . . . Different locations of wave energy devices from the shoreline to nearshore . . . . . . . . . . . . . . . . . . . . . . . . . . . Swinging of a child as an explanation of resonance effect . . . . . . Plan view of the experimental arrangement in the wave basin and standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Side view of the wave basin at DHI (Hørsholm, Denmark) . . . . . Rear view (from the wave generator). a 5 × 5 wave energy converter rectilinear array and b its various components . . . . . . . Test arrangement of the experimental setup . . . . . . . . . . . . . . . . . Phasor diagram of the FSLEG (under consideration) for cos ϕ = 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetizing curve of Kool Mμ with other magnetic cores . . . . . Demagnetization curve of the permanent magnet used in the linear generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparisons of a voltage and b current waveforms of the FSLEG with the proposed material . . . . . . . . . . . . . . . . . . . Comparisons of a core loss and b instantaneous power of the FSLEG for the conventional core and Kool Mμ materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of instantaneous power of the FSLEG with the conventional and the proposed materials . . . . . . . . . . . . Forecast of annual electrical energy generation from 2010–2050 (CCS stands for carbon capture and storage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Illustration of the conventional linear generator . . . . . . . . . . . . . . Positions of the driver-translator and driven-translator for a specific value of ω . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Positions of Translator-U and Translator-D for ωL = 2 and ωH = 2.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vertical velocity of the translators for ωL = 2 and ωH = 2.2 . . . . Vertical speed of the translators for ωL = 2 and ωH = 2.2 . . . . . . a Architecture of the DPLG (one of the identical segments) and its b top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equivalent circuit diagram of the DPLG . . . . . . . . . . . . . . . . . . . . Determination of stator pole geometry. a Selection of the slope by and b cross section of the stator pole shoe with labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Selection of stator pole curvature and b selected pole face with slope and curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flowchart of the optimization process of the DPLG . . . . . . . . . .

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58 60 61 61 62 70 72 74 75 76 78 85 87 87 89

89 89

101 103 105 105 106 106 108 109

109 109 111

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Fig. 6.12

Fig. 6.13 Fig. 6.14 Fig. 6.15 Fig. 6.16 Fig. 6.17 Fig. 6.18 Fig. 7.1 Fig. 7.2 Fig. 7.3

Fig. 7.4 Fig. 7.5 Fig. 7.6 Fig. 7.7 Fig. 7.8 Fig. 7.9 Fig. 7.10 Fig. 7.11

Fig. 7.12 Fig. 7.13 Fig. 7.14 Fig. 7.15 Fig. 7.16 Fig. 7.17 Fig. 7.18 Fig. 7.19

List of Figures

Rainbow spectrum of a magnetic flux, b B, and c H. d Magnetic flux lines, e B, and f H of the DPLG without shape (slope and arc) optimization. g Magnetic flux lines, h B, and i H of the DPLG with optimization . . . . . . . . Rectified voltage of the DPLG that shows filtering effect . . . . . . Electrical parameters of the DPLG with proper filtering . . . . . . . Mechanical force of the translator with and without optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . Core losses of the DPLG before and after applying optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selected curvature of the stator pole teeth . . . . . . . . . . . . . . . . . . . Experimental and simulation results of the generated voltages of the DPLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Power generation in various regions by renewable energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Front view of a linear generator coupled to a floater on sea surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a The conventional FSLG. b Alignment of the stator and translator teeth at a particular time. c Teeth alignment is changed for movement of the translator . . . . . . . . . . . . . . . . . . . Magnetic flux lines that travel in the existing FSLG . . . . . . . . . . Translator and secondary stator cores of the proposed FSLG . . . Flux switching of the proposed FSLG for two different positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Construction of the proposed FSLG . . . . . . . . . . . . . . . . . . . . . . . Vector diagram of the coil voltages of the proposed FSLG . . . . . Phasor diagram of the proposed FSLG . . . . . . . . . . . . . . . . . . . . . Design of the a traditional flux switching permanent magnet linear generator and b the proposed FSLG . . . . . . . . . . . A few possible secondary stator shapes: a rectangular, b triangular, c trapezoidal, d circular, e elliptical, and f special curve for initialize population for shape optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equivalent circuit of the proposed FSLG . . . . . . . . . . . . . . . . . . . Generated powers for different resistive loads of the FSLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Load currents for different inductive loads of the FSLG . . . . . . . The current, induced voltage, and power for a capacitive load of the FSLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The resultant cogging force of the proposed FSLG . . . . . . . . . . . Minimization of the force ripples of the proposed FSLG . . . . . . . Comparison of the generated electrical powers for different translator widths of the proposed and the existing FSLGs . . . . . . Equivalent translator width for the a existing and b the proposed FSLG translator design . . . . . . . . . . . . . . . . .

113 114 114 114 115 115 117 121 126

127 128 129 129 130 131 132 132

133 134 136 136 137 137 138 138 139

List of Figures

Fig. 7.20 Fig. 7.21 Fig. 7.22 Fig. 7.23 Fig. 8.1 Fig. 8.2 Fig. 8.3 Fig. 8.4 Fig. 8.5

Plot of five significant parameters for different load conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plot of seven significant parameters versus speed variation . . . . . Core losses measured in the stator and the split translator core of the proposed FSLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generated electrical powers for different length of the air gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A typical FPLG with its different accessories . . . . . . . . . . . . . . . . Comparison of core loss curves of three different magnetic cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Output voltage and current waveforms of the proposed FPLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Power and flux linkage of the FPLG with respect to time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Core loss of the FPLG for using three different magnetic cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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139 140 141 141 146 159 159 159 160

List of Tables

Table 1.1 Table 3.1 Table 3.2 Table 4.1 Table 5.1 Table 5.2 Table 5.3 Table 5.4 Table 5.5 Table 6.1 Table 6.2 Table 6.3 Table 7.1 Table 7.2 Table 8.1 Table 8.2 Table 8.3 Table 8.4 Table 8.5

Recent development of the linear electrical generator . . . . . . . . . Total wave power densities at different locations in the US . . . . Recoverable wave power densities at different locations in the US . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of structural efficiency of different wave energy devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters of FSLEG with the proposed material . . . . . . . . . . . . Cost analysis of copper used in the winding . . . . . . . . . . . . . . . . . Cost analysis of different permanent magnets . . . . . . . . . . . . . . . Cost analysis of different magnetic cores . . . . . . . . . . . . . . . . . . . Cost calculation summary of the active material applied to the FSLEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geometry of the DPLG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of parameters a and b (for stator teeth) of the DPLG . . . . Dimensions of the DPLG prototype . . . . . . . . . . . . . . . . . . . . . . . Common parameters of the FSLG . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of the traditional and the proposed FSLG . . . . . . . . Stroke length of various FPLGs . . . . . . . . . . . . . . . . . . . . . . . . . . . Output power of different FPLGs . . . . . . . . . . . . . . . . . . . . . . . . . Frequency of the generated voltage of different FPLGs . . . . . . . Output voltage of the FPLGs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A few recent developments of the FPLG . . . . . . . . . . . . . . . . . . .

14 49 49 70 88 91 92 93 94 116 116 117 140 141 151 154 156 158 161

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Chapter 1

Introduction to the Principles of Wave Energy Conversion Omar Farrok, Mohamud Mohamed Farah, and Md Rabiul Islam

Abstract Oceanic wave energy is one of the most significant renewable energy resources because of its availability and high power density compared to the other sources. It covers around 70% of the total earth’s surface. In this chapter, principles of wave energy conversion are explained. Firstly, fundamentals of wave energy and motion of a particle in the ocean are depicted. It is supportive to understand the working principle of wave energy devices. Estimation of wave power and energy is illustrated mathematically which is useful for design and size selection of the wave energy converter. Recent wave energy projects around the globe are described in a separate section. From the study of various types of wave energy converters, it is found that point absorber type wave energy device with direct drive linear electrical generator is widely used for harvesting oceanic wave energy. Recent developments of the linear generator are summarized in a table at the end of this chapter. Keywords Oceanic wave · Wave energy converter · Wave energy devices · Wave energy harvesting · Wave energy projects · Wave power

1.1 Introduction to the Oceanic Wave Energy Due to wind flow, ocean wave transports the energy through the surface of the ocean. The wave energy can be utilized for various types of necessary usage like electrical power generation, water desalination, pumping of fluids, etc. Greenhouse gas emission can be mostly avoided by energy harvesting from oceanic wave [1]. Wave power O. Farrok · M. M. Farah Department of Electrical and Electronic Engineering (EEE), Ahsanullah University of Science and Technology (AUST), 141-142 Love Road, Tejgaon, Dhaka 1208, Bangladesh e-mail: [email protected] M. R. Islam (B) School of Electrical, Computer and Telecommunications Engineering (SECTE), Faculty of Engineering and Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_1

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and tidal power are different. Tidal power is produced due to gravitational attraction among the earth, the sun, and the moon. Ocean currents are different from both tides and ocean waves. Ocean current is generated due to different forces including wind, breaking waves, the Coriolis effect, diversity in salinity, temperature difference, and cabbeling. When two different water parcels get mixed to form a new one that sinks below both parents, it is known as cabbeling. Breaking of the surface water can occur anywhere including in mid-ocean. When the amplitude reaches a critical level that the crest of the wave overturns, wave breaking occurs. When a reference frame rotates with respect to any inertial frame, Coriolis force acts on an object which is in motion within the reference frame. The object is deflected because of the Coriolis force. This deflection phenomenon is known as Coriolis effect. This is the cause for the creation of ocean current. Ocean wave can be used as a potential source of renewable energy to meet the energy crisis of the existing world. Because of high power density, the attempt for utilizing this source has been taken since 1890. Power density of the solar panel (photovoltaic) is 1 kW/m2 on average depending on location at peak solar insolation. Power density of wind is similar to solar for adequate wind speed. On the other hand, the mean power density (yearly) of the waves at the coast of San Francisco is found 25 kW/m which is quite high [2]. Islay LIMPET was the first well known wave power device used for commercial purposes. It was installed on the coastal area of Islay in Scotland and connected to the national grid [3]. World’s first multi generator-based wave farm was opened experimentally in Portugal in 2008. Wave motion varies naturally specially its amplitude, frequency, and direction. Because of this reason, a device is required to convert the irregular motion into a regular pattern so that it can be exploited properly. The device that utilizes wave power is known as wave energy converter (WEC). Often wave energy devices and WEC are interchangeably used. But basically, it is found that wave energy devices convert irregular wave motion into regular mechanical motion. Then, an electrical generator converts mechanical power into electricity. Therefore, electrical generator is often called power/energy converter. On the other hand, wave energy device means a physical or mechanical structure in which electrical generators are mounted for harvesting oceanic wave energy. Principles of wave energy conversion by different WECs are described in [4]. Various kinds of electrical generators are widely applied for this purpose. Energy conversion occurs by electromagnetic induction or through piezoelectric effect [5]. A design methodology of linear electrical generator is extensively presented with a flowchart in [6]. Generally, permanent magnet synchronous generator is applied for WEC. Neodymium iron boron permanent magnet is mostly used in this type of generator. As neodymium is a rare earth element, an alternative permanent magnet is applied to the generator as reported in [7]. In a wave farm, a lot of generators are connected to the electrical power grid. The necessity of using isolation in such applications is illustrated in [8].

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In this chapter, the working principle of wave energy converters with fundamentals are described. Various types of wave energy converters, point absorber type power take of devices, and direct drive linear electrical generators are explained for harvesting oceanic wave energy. Recent developments of the linear generator are summarized at the end.

1.2 Motion of a Particle in Oceanic Wave When wind passes on the sea surface, wind energy is transferred, and oceanic waves are generated. The difference in atmospheric pressure between the backward and forward side of a wave peak mainly builds the oceanic wave. Frictional force in between the water surface and wind also creates shearing force that contributes to increase in the wave size [9]. Wave height depends on several facts including the wind speed, duration of wind blowing time, the distance over which the waves are instigated by wind known as fetch, depth, and area of the sea surface. Although, growth of oceanic waves greatly depends on wind speed over distance or time. But practically, there is a maximum limit of wind speed beyond which it cannot produce larger waves. When the limit is achieved, the wave is considered as fully developed. Wave power depends on water density, length, and propagation speed of the wave. But generally larger waves are considered more powerful. While moving up and down with a ripple in ocean water, an object follows mostly elliptical or circular pathway (Fig. 1.1). Motion of the oscillation is maximum at the water surface. It is reduced exponentially with depth as illustrated in Fig. 1.2. For the standing wave in a reflecting coastal area, wave energy available in the form of pressure oscillations in deep water can produce micro-seisms [9]. Pressure

Fig. 1.1 Movement of a floating object in a ripple on the surface of oceanic wave

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Fig. 1.2 Oceanic wave propagation and motion of a particle in sea water

fluctuation is quite small in this region. Wave energy flows horizontally with a group velocity while moving forward to the ocean surface.

1.3 Estimation of Wave Power If the water depth is larger compared to the half of wavelength of the oceanic wave, it is considered as deep water. It is one of the most general situations in the sea. The wave energy flux can be calculated as   ρg 2 2 kW 2 H Te ≈ 0.5 Hmo P= Te 64π m m3 s

(1.1)

where P is the wave energy flux per unit length of the wave crest, H m is the average wave height (crest to trough) of the highest third of the waves known as significant wave height, T e is the time period of wave, ρ is the density of water, and g is the gravitational acceleration. Wave power has a proportional relation with wave energy, time period, and square of the wave height. For significant wave height given in meter and time period in second, the resultant wave power is in kilowatt per meter (kW/m) of wavefront length [10].

1.4 Wave Energy Wave energy flux is defined as the average transport rate of wave energy through a vertical plane of a unit width which is considered in parallel to a wave crest. From the concept of linear wave, the mean energy density per unit area of gravity waves is

1 Introduction to the Principles of Wave Energy Conversion

5

directly proportional to the square of the wave height [9, 11] which can be expressed as E=

1 ρg Hm2 16

(1.2)

where E is the average energy density of the wave per unit horizontal area (J/m2 ). It is the sum of potential and kinetic energy density. Both types of energy densities per unit horizontal area contribute the same and are equal to the half of E [9]. While propagating, wave energy is transported from one place to another with a velocity which is known as group velocity. Thus, the wave energy flux through a vertical plane which is perpendicular to the direction of wave propagation and has unit width can be given as P = Ecg

(1.3)

where cg is the group velocity (m/s) that depends on the time period of the wave, T e . Group velocity is different for different water depths [9, 11]. Propagation and transportation of energy both are faster in deep water level. The phase velocity is twice than the group velocity in deep water. On the other hand, group velocity and phase velocity both are same near the coast [12].

1.5 Recent Wave Energy Projects Electricity generation from oceanic wave energy is increasing rapidly day by day worldwide. Recently, the USA, UK, Sweden, Canada, and Germany have made great contributions to develop the projects of oceanic wave energy (OWE) to produce large amounts of electricity. In 2015, the CCell wave energy converter was mounted in the UK. It is basically a directional wave surge converter comprising of curved flap. There are two main advantages of the curved device over other flat oscillating wave surge converters. Firstly, the energy is dissipated over a long curve minimizing the wave height. Secondly, the curved device cuts through the waves which decreases turbulence on the borders. Furthermore, disparate from other oscillating wave surge converters, the modern version of CCell is considered to float just under the sea surface, expanding the obtainable wave energy. The designers of the CCell declare that it has the highest efficiency compared to other wave energy devices [13]. The multiple point absorber type wave energy convertors are built in the Vancouver coast of Canada from 2010 to 2019. The wave energy is harnessed with several float pistons inhibited from moving vertically up and down masses. The reciprocation motion of float pistons is converted into single way rotational motion which allows to generate electricity from up and down directions (bobbing motion) [14]. An oceanic wavebased industry collaborated with the Swedish energy agency in 2015. The first wave power energy park is situated in the Northwest of Smögen on the Swedish West coast.

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Initially the ocean wave energy park was installed in March 2015 which includes 36 oceanic wave energy converters (WEC) and a single substation [15]. In 2014, a nearshore WEC named “SINN Power” was deployed in Germany which contains several buoys connected to a steel frame. The linear generator produces electricity when the translator of the generator creates bobbing motion with respect to the stator [16]. Due to the wave motion, the translator moves along with the buoy because of the physical connection between them. By 2015, the single module of SINN Power wave energy converter is also tested in the Greek Island Crete [17]. The surface dynamic vibration absorber is situated on the shore of Cornwall, UK. The AMOG wave energy converter is progressing offshore technology which is developed by the Cornwall Development Company and the European Union Regional Development Grant. A 1/3rd scale of this device was installed in Europe in the summer of 2019. The device was constructed by Mainstay Marine in Wales. It contains an arc shaped frame with an in-air pendulum adjusted to absorb the wave energy instead of the hull. A power take-off device is placed on the top part of the pendulum which contributes to electricity generation. The maximum rating of this device is about 75 kW.

1.6 Basic Principles of Wave Energy Devices Since the last 30 years, wave energy devices have been considered as significant members of electrical energy conversion. There are various types of wave energy devices depending on different points of views. Depending on location, it could be situated onshore, nearshore, or offshore. Based on vertical orientation, they can be floating, partially submerged, or fully submerged. Depending on the sea water depth, they can be classified as shallow water, intermediate depth water, and deep water. Depending on the working principle, wave power devices can be classified into 8 (eight) major categories. They are point absorber (heaving buoy), overtopping WEC, oscillating water column, attenuators, oscillating wave surge converters, submerged pressure differential, rotating mass, and bulge wave converter device. The following subsections describe each type of wave energy converter or device based on working principle.

1.6.1 Point Absorber: Power Buoy Many scientists consider point absorber as a significant economical device for electricity generation that can produce bulk amount of power. It has moving parts where the radius is less than its length. It is vertically placed in the ocean as shown in Fig. 1.3. It has two major portions: the upper portion where the float or buoy is mounted and the lower portion where a heavy base is anchored on the seabed with a rope.

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Fig. 1.3 Schematic diagram of a point absorber type wave energy converter

The bottom part (base) is almost motionless and is connected deep into the sea and the other part (float) exists on the sea surface. It moves vertically to extract wave power to drive the translator of the linear electrical generator. Power buoy is one of the common examples of point absorber type device which is developed by Ocean Power Technology [18]. It is shown in Fig. 1.3 that the generated energy from each buoy is connected to an undersea substation.

1.6.2 Overtopping WEC Researchers have been focusing on the device since 1990 to improve this technology. In 2011, major development of the device was made. The device can be installed both onshore and offshore. Overtopping wave energy converter is known as terminator device according to its principle of operation. Its structure is quite large as compared to other wave energy devices. It has a giant basin that can contain large amount of sea water as shown in Fig. 1.4. When oceanic wave passes across the ramp, the basin is filled with water making a reservoir. Due to the gravitational force, the water passes through the funnel toward the bottom of the water reservoir. A water turbine is placed inside the funnel and the turbine makes the generator rotate to produce electrical power. An overtopping offshore device is installed in Denmark which generates a good amount of electricity.

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Fig. 1.4 Illustration of the overtopping device

The width of this device is nearly 390 m, which can accommodate large volume of sea water (1,400–1,500 m3 ). Overtopping devices such as Wave Dragon [19] have good efficiency and can be used for a long time.

1.6.3 Oscillating Water Column Oscillating water column is another type of wave energy converter which generates electricity by the differential wave pressure available to the onshore as shown in Fig. 1.5. Generally, it is situated at the seashore where the chamber wall is nearly situated at the seabed. When the water wave enters the chamber, the air inside is compressed. While increasing the water level, excessive air pressure is created which drives a turbine. When the water returns to the ocean, the pressure is minimized which drives the turbine again for which it can produce electricity. Power take-off system has bidirectional turbine that rotates identical direction nevertheless the direction of airflow. It is widely known as Well’s turbine that helps to produce electrical energy continuously. The device is mostly installed at nearshore up to 10 m of water and offshore where the sea water depth is around 40–80 m which is basically shallow water depth [20]. WaveGen Limpet is a popular oscillating water column device.

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Fig. 1.5 Structure of an oscillating water column

1.6.4 Attenuators Attenuator wave energy converter is a floating device that aligns with the direction of wave propagation. It is made of a few floating cylinders (segments) connected to each other by special flexible joints. The joint or bend moves with the segment when oceanic wave falls on the consecutive segments of this device sequentially. A popular example of this type of WEC is Pelamis which is developed by Pelamis Wave Power. The structure of Pelamis is shown in Fig. 1.6, which is commonly found in different wave energy projects. Hydraulic cylinder and piston arrangements are there in between two consecutive segments. When oceanic wave passes across the device, the piston pumps oil to the hydraulic motor that functions as a prime mover for an electrical generator. Engineering research was conducted for many years prior to design Pelamis. It is the first wave energy device which is used commercially to produce electricity from the ocean. The device was developed in Scotland. In the beginning, there was a plan to generate 2.5 MW of electricity. But the first device installed in Portugal produced 750 kW [21]. Still, it is considered one of the benchmarks in the area of harvesting oceanic wave.

1.6.5 Oscillating Wave Surge Converters Oscillating wave surge converter (OWSC) uses horizontal wave motion. The system is mounted to the seabed at a nearshore location. It is placed at such a region where the water depth reaches up to 10–20 m. The widely used example of OWSC is pendulum

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Fig. 1.6 An attenuator wave energy harnessing device

arm flap type device which is mounted at the seabed. When the wave propagates toward the flap, it moves back and forth. The movement drives a water pump which further make rotation to the water turbine connected to a rotating electrical generator [4]. Wave Roller and Sea Oyster are two well known OWSCs, as shown in Fig. 1.7.

Fig. 1.7 An oscillating wave surge converter device

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Fig. 1.8 A submerged pressure differential with a moving float

1.6.6 Submerged Pressure Differential Submerged pressure differential type OWE converter is generally vertically oriented and anchored to the seabed. While oceanic wave peak passes through the converter, it causes the sea water overhead the float as shown in Fig. 1.8. Thus, it pushes down the float. A mechanical spring is connected in between the float and stationery center spar. For this reason, spring is compressed and stored mechanical energy. On the other hand, while oceanic wave trough passes through the converter, it causes the sea water to fall from its head. At this moment, the spring pushes the float in an upward direction and releases its mechanical energy. The process continues periodically, and this vertical motion is fed to a linear generator to produce electricity directly. Thus, it allows direct drive power take-off system, which is also a point absorber type device as it receives wave power at a single point on sea surface. Archimedes wave swing is a well known submerged pressure differential wave energy converter [4].

1.6.7 Rotating Mass Rotating mass wave energy converter rolls a physical body or mass in which center of gravity is unbalanced in its axis of rotation. For this reason, when oceanic wave passes through the device, it inclines depending on the direction of wave propagation and the angular displacement of rotating mass. This device is of floating type and the rotating mass is mounted to a gear box so that its angular speed can be increased. The rotating mass has high mechanical inertia which is suitable to drive a rotating electrical generator to produce electricity. The concept is illustrated in Fig. 1.9 where the oceanic wave transfers mechanical energy to the electrical generator through the rotating mass. Some examples of this

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Fig. 1.9 Rotating mass wave energy technology

type of device are Wello by Penguin, Witt Energy, Enorasy Labs by Robotic Juggler, etc. [4].

1.6.8 Bulge Wave Converter Device Bulge head wave energy converter as shown in Fig. 1.10 is called Anaconda wave power device [4]. It can produce electrical power by using instant water thrust obtained from the sea wave. It is made of flexible hollow cylindrical structure which is occupied with water and then it is embedded into the ocean. The flexible structure has two ends. One is kept stationary and the other is free to move and connected to an electrical generator through a turbine. The device lies in the sea water at low pressure. For the incident of oceanic wave, it generates a bulge wave that propagates very quickly toward the turbine. As a result, the turbine makes the generator rotate for which electrical power is generated. The device was invented in 2005 by Checkmate Sea Energy Limited. The advantage of Bulge wave converter is that it is cheaper than Pelamis. It does not have any intersections to crack. The company forecasts the cost of Bulge wave device can reach approximately 2–3 million GBP. It can produce 1 MW of electrical power [22].

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Fig. 1.10 Bulge wave energy converter system

1.7 State of the Art As the obtained power from oceanic wave is mechanical, it requires to convert into electrical power by using an electrical generator. The generator converts mechanical power into electrical power, therefore, it can be called wave energy converter in this respect. Although there are several linear electrical generators for wave energy conversion, permanent magnet linear generator is widely used. Prevention of demagnetization for this kind of generator is essential because of the limitation of permanent magnet. Piezoelectric material-based generator can also be used for the same purpose. A cooling system can be utilized to minimize the temperature rise of the linear generator. Protection of the generator under natural disaster is required as well where an automatic control using programmable wireless device can be applied. Table 1.1 presents recent progress of the linear generator describing their purposes.

1.8 Summary It is found that oceanic wave energy can be converted into electrical energy by using different types of wave energy converters and devices. Among them, point absorber type converters are very popular and widely used. Some of them are placed nearshore, on shore, or offshore based on their horizontal position in the ocean. Based on their altitude, they can be floating, partially or fully submerged, or even could be placed on the seabed. Each of the devices has its own pros and cons based on the location. On shore and floating devices face the challenges of natural calamity whereas the submerged one is mostly free from natural disaster. It can be concluded that proper

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Table 1.1 Recent development of the linear electrical generator Attribute

Description

References

Design

As the performance of a linear electrical generator much depends on its design, a guideline is provided for its outstanding operation

[6]

Dynamics

Instead of using the conventional massive magnetic core, J [23] ferrite material is applied to a linear generator to improve its dynamics

Optimization

Translator optimization improves the generator performance

[24]

Permanent magnet

Advanced permanent magnet is applied to the linear generator to generate more electrical power

[25]

Demagnetization

A technique is applied to avoid degradation of permanent magnet because of demagnetization

[26]

Electromagnet

Instead of using permanent magnet, electromagnet is applied for magnetic flux production

[27]

selection of the wave energy converter depends on its principles of operation and geographical location of installation.

References 1. Molla S, Farrok O, Alam MJ (2023) Zero Carbon emission based electrical power plant by harvesting oceanic wave energy: minimization of environmental impact in Bangladesh. In: Climate change and ocean renewable energy. CCORE 2022. Springer proceedings in earth and environmental sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-26967-7_1 2. Czech B, Bauer P (2012) Wave energy converter concepts: design challenges and classification. IEEE Ind Electron Mag 6(2):4–16. https://doi.org/10.1109/MIE.2012.2193290 3. Wave power. https://en.wikipedia.org/wiki/Wave_power. Visited 21 Jan 2020 4. Farrok O, Ahmed K, Tahlil AD, Farah MM, Kiran MR, Islam MR (2020) Electrical power generation from the oceanic wave for sustainable advancement in renewable energy technologies. Sustainability 12(6):1–23. https://doi.org/10.3390/su12062178 5. Kiran MR, Farrok O, Mamun MAA, Islam MR, Xu W (2020) Progress in piezoelectric material based oceanic wave energy conversion technology. IEEE Access 8:146428–146449. https:// doi.org/10.1109/ACCESS.2020.3015821 6. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 7. Molla S, Farrok O, Islam MR, Muttaqi KM (2020) Application of iron nitride compound as alternative permanent magnet for designing linear generators to harvest oceanic wave energy. IET Electr Power Appl 14(5):762–770. https://doi.org/10.1049/iet-epa.2019.0372 8. Islam MR et al (2020) Design and characterisation of advanced magnetic material-based core for isolated power converters used in wave energy generation systems. IET Electr Power Appl 14(5):733–741. https://doi.org/10.1049/iet-epa.2019.0299 9. Phillips OM (1977) The dynamics of the upper ocean, 2nd edn. Cambridge University Press. ISBN: 978-0-521-29801-8

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10. Matching Renewable Electricity Generation with Demand (2006) University of Edinburgh. https://www2.gov.scot/resource/doc/112589/0027358.pdf. Visited 25 Jan 2020. 11. Goda Y (2000) Random seas and design of maritime structures. World Scientific. ISBN 978981-02-3256-6 12. Dean RG, Dalrymple RA (1991) Water wave mechanics for engineers and scientists. Advanced series on ocean engineering, 2. World Scientific, Singapore, pp 64–65. ISBN: 978-981-02-04204 13. List of wave power projects. https://en.wikipedia.org/wiki/List_of_wave_power_projects# cite_note-15. Visited 2 Nov 2019 14. Nepturewave.ca. https://www.neptunewave.ca/history. Visited 29 Oct 2019 15. The European marine energy Centre ltd (EMEC). http://www.emec.org.uk/marine-energy/ wave-developers/. Visited 5 Nov 2019 16. Sinn power. https://www.sinnpower.com/. Visited 1 Dec 2019 17. Sinn power. https://www.sinnpower.com/waveenergy. Visited 14 Dec 2019 18. Energy and the environment—a coastal perspective. http://coastalenergyandenvironment.web. unc.edu/. Visited 28 Nov 2019 19. Energy and the environment—a coastal perspective. http://coastalenergyandenvironment.web. unc.edu/ocean-energy-generating%20technologies/wave-energy/overtopping-terminator/. Visited 27 Sept 2019 20. Energy and the environment—a coastal perspective. http://coastalenergyandenvironment.web. unc.edu/ocean-energy-generating%20technologies/wave-energy/oscillating-water-column/. Visited 18 Oct 2019 21. Energy and the environment—a coastal perspective. http://coastalenergyandenvironment.web. unc.edu/ocean-energy-generating-technologies/wave-energy/the-pelamis-wave-energy-con verter/. Visited 12 Dec 2019 22. The renewable energy website. http://www.reuk.co.uk/wordpress/wave/anaconda-bulge-wavepower-generator/. Visited 7 Jan 2020 23. Molla S, Farrok O, Alam MJ (2022) J material based lightweight linear electrical generator with improved dynamics for harvesting oceanic wave energy. In: 12th International conference on electrical and computer engineering (ICECE), Dhaka, Bangladesh, pp 92–95. https://doi. org/10.1109/ICECE57408.2022.10089129 24. Molla S, Farrok O, Alam MJ (2023) Translator optimization of a linear electrical generator for harvesting oceanic wave energy. In: IEEE IAS global conference on emerging technologies (GlobConET), London, United Kingdom, pp 1–6. https://doi.org/10.1109/GlobConET56651. 2023.10150079 25. Bashir MS, Farrok O (2019) Generation of electrical power by using high graded permanent magnet linear generator in wave energy conversion. In: 1st International conference on advances in science, engineering and robotics technology (ICASERT), pp 1–5. https://doi.org/10.1109/ ICASERT.2019.8934591 26. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Lei G (2017) A novel method to avoid degradation due to demagnetization of PM linear generators for oceanic wave energy extraction. In: 2017 20th International conference on electrical machines and systems (ICEMS), pp 1–6. https://doi.org/10.1109/ICEMS.2017.8056471 27. Farrok O, Islam MR, Sheikh MRI, Guo YG, Zhu JG, Xu W (2015) Analysis and design of a novel linear generator for harvesting oceanic wave energy. In: 2015 IEEE international conference on applied superconductivity and electromagnetic devices (ASEMD), pp 272–273. https://doi.org/10.1109/ASEMD.2015.7453569

Chapter 2

Oceanic Wave Energy Devices Tanwi Howlader, Omar Farrok, Md Ahsan Kabir, Md. Abdullah-Al-Mamun, and Md Sawkat Ali

Abstract The global renewable energy sources (RESs) are comprised of solar photovoltaic, wind energy, marine energy, geothermal energy, and so on. Among the RESs, wave energy is considered as the prominent energy resource as the oceans cover around three-fourths of total area of the earth’s surface and it contains a huge amount of energy that can be extracted from the ocean. However, to extract the energy from this source, it is essential to know about its exact location. This chapter presents various wave energy converters (WECs) and devices with their operation. The resources along with the main renewable energy centers, the technical, environmental, and social aspects of the procedures are discussed. Several methods of wave energy extraction and wave termination are also explained. Construction of the WEC or devices such as WET-NZ, Oyster, Limpet, and Penguin along with their visual structures and working principles are demonstrated. Additionally, many other devices such as Paramus, Power buoy, SEAREV, SeaRay, etc., are studied that can harvest the power from the wave. Finally, the construction and operational formula of point absorber WEC is discussed along with its related factors. The chapter concludes with the discussion of current research and educational approach to wave energy. Keywords Maximum wave energy extraction · Oceanic wave · Point absorber · Wave energy converters · Wave energy resources

T. Howlader · O. Farrok (B) · Md. Abdullah-Al-Mamun Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected]; [email protected] M. A. Kabir Department of Electrical, Electronic and Communication Engineering, Military Institute of Science and Technology, Dhaka 1216, Bangladesh e-mail: [email protected] M. S. Ali Department of Computer Science and Engineering, East West University, Dhaka 1212, Bangladesh © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_2

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2.1 Introduction Renewable energy sources (RESs) have a lower rate of greenhouse gas emission and consequently less impact on the environment as compared to non-renewable ones [1]. According to the REN21’S 2017 report, RESs such as geothermal, solar photovoltaic, tide, wave, and wind energy contribute 19.3% of the total energy demand around the world and out of that, 3.9% of energy is produced from the wave resources. The use of wave energy is more beneficial than other RESs as it is a consistent energy source whereas solar energy is harnessed during a particular time. Wave energy converter uses float-based technology which makes the system cost effective as it is free of heavy foundations. Some technologies such as Deep Green by Minesto, Sweden, MeyGen project, the WEC device Penguin by Wello company, Finland, the CETO technology by Carnegie Clean Energy, Australia are generating energy from waves and creating a huge impact on the enhancement of oceanic energy harvesting. Though wave energy has some great benefits, the harnessing efficiency is quite low in some cases. Therefore, over the last two decades, the development has been progressing in this field to improve its efficiency [2]. To harness an adequate amount of wave energy, power electronic converters are usually placed in rows and interact with each other. The incident or incoming oceanic waves are transmitted, absorbed, reflected, and create a positive or negative impact on the nearby wave energy converters (WECs). The WECs are categorized into a few types. One of them is “oscillating water column” which is installed onshore. In this converter, the water column continues to oscillate for the incident oceanic waves. It creates air pressure deviation inside the column because of extracting wave energy. The other one is an overtopping device where the incident waves are overtopping in a basin at a higher level than the sea surface. In this type of WEC, the slack moored and fixed type base devices are used [3]. Point absorber type direct drive wave energy converter is quite advantageous because additional mechanical gear and intermediate devices are not required. A linear electrical generator can be mounted to it to produce electricity directly [4]. Permanent magnet linear generators are mostly used for this purpose. A systematic design approach for this kind of generator is extensively presented in [5]. An isolation between a linear generator and an electrical grid is essential for safe operation [6]. This chapter describes the wave resources, various types of wave energy devices, and the present approach in wave research and educational development. A wave energy device named point absorber is also discussed along with its formula and time domain results. It summarizes the wave energy test facility and state of the art of wave resources.

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2.2 Wave Resources and Energy Centers Oceanic waves are created by the wind flow on the ocean surface. The world wave energy resources are shown in Fig. 2.1, where the red and orange parts indicate better hub of energy. It is also observed that better waves are found from the South to all the way up into Alaska, USA [7]. It represents the average annual wave power in kW/m. Harvesting of oceanic waves has several challenges. Such as the high initial establishment cost, failures of the previously installed devices, inadequate operating stations, etc. [8]. The marine research centers are working to overcome the shortcomings and try to find out better solutions for harvesting wave power. The global wave energy resource is approximately 2 TW and only 5–25% of energy can be extracted using the wave energy converters [9]. The extraction percentage depends on the converter’s controllability and the wave vulnerability. Moreover, the wave energy is harnessed only by 0.001% (Fig. 2.2) as compared to the total energy that comes from renewable sources. The Water Power Technologies Office, Department of Energy, US founded four National Marine Energy Centers. One of them is very recently established. They spread over different places in the US. The purpose is to deal with different marine energy resources, regional expertise, testing, and advance marine energy technologies. The centers are as follows: • Pacific Marine Energy Center (previously known as Northwest National Marine Renewable Energy Center), formed in 2008 by the University of Washington, Oregon State University, and the University of Alaska Fairbanks. • Hawai’i National Marine Renewable Energy Center, founded in 2008 by Hawai’i Natural Energy Institute.

Fig. 2.1 The wave resources in the world [7]

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Fig. 2.2 Global energy consumption [8]

• Southeast National Marine Renewable Energy Center, founded in 2010 by Florida Atlantic University. Oregon State University has focused on wave research due to the opportunity of having a great resource on the coast. University of Washington deals with the tidal force for its availability. The centers conduct basic research and facilitate the commercialization of the wave energy industry. The centers also collaborate with the industries to demonstrate the progress of wave energy systems. Besides, a close relationship has been established with the government organization that deals with energy and regulatory agencies to make new policies and decisions. Their locations are shown in Fig. 2.3. The center works on wave forecasting technologies for integrating wave power with the existing power grid. However, the integration of wave power into the utility grid is a challenging task because of variable wave energy and electrical demand for the time being. According to the wave forecasting technologies, the wave energy converter has been tuned to optimize the supplied power to the grid. In this way, the power network has been protected from the intermittent nature of wave energy.

2.3 Wave Energy Converters This section presents an overview of wave energy conversion principles and various types of converters used for wave energy conversion. The wave energy device floats on or near the sea surface and moves in response to the form of wave incident. For

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Fig. 2.3 Some marine renewable energy centers established by the US Department of Energy [7]

submersible maneuvers, it fluctuates up and down with the differences in underwater pressure. Almost all kinds of wave contour devices hover on the sea surface completely engrossing the wave energy in all directions with the traveling waves on or near the surface. The only wave energy device used for wave profiling has been utilized for a while and applied for driving triangulation buoys on a small scale [3]. If the relative practical size of the wave contour device to the episodic length of the wave is quite small, then the device is usually known as a point absorber. In opposite, if the device size is larger than the typical episodic wavelength, it is well known as linear absorber or wave attenuator. This moving device in seawater is moored to the ocean floor [3]. Figure 2.4 shows a set of floating devices [9].

Fig. 2.4 Wave energy floating device [9]

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Fig. 2.5 Wave energy device using oscillating water column [10]

Figure 2.5 shows a wave energy device using the principle of the oscillating water column [10]. The device is situated on the coastline where a turbine and generator is mounted inside it. The oscillation of the water column is produced from the up and down oscillating motion of water. Then the air is moved toward the turbine to make it rotate. Thus, electrical energy is produced. There are different ways of receiving energy from waves which are demonstrated in [11]. A swimming pool wave machine is an example of such an application. The pool water wave is affected when the air is gusted in and out of a cavity near the pool, which marks the water bob up and down. At a wave power station, the water in the cavity rises and falls due to incoming waves, which causes the air to enforce in and out of the hole in the top of the cavity. A turbine exists in this hole and is operated by the air hastening in and out. Then the generator is run by this turbine. The noisy hissing air is considered a constraint of this system unless a silencer is incorporated into the turbine. Sometimes the wave noise minimizes the effect of hissing air noise. Figure 2.6 presents different WEC devices according to their principles of operation. In the previous chapter, a total of 8 (eight) principles of operation are explained. The following sub-sections describe the operation of various wave energy devices that fall under the basic principles of operation as already described.

2.3.1 WET-NZ The Wave Energy Technology New Zealand (WET-NZ) is a point absorber WEC in which the float can rotate constantly 360° and thus the device can extract wave energy at higher efficiency. The external structure of WET-NZ is shown in Fig. 2.7. The device is run by the float that moves up, down, back, and forth respectively. In

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Fig. 2.6 Different types of WEC devices according to their working principles: a oscillating wave surge converter b oscillating water column c point absorber: heaving buoy d rotating mass e attenuators f submerged pressure differential

this way, the wave energy is harvested. Besides, the “ocean sentinel” is situated in the corner [7] and provides dynamic information to the researcher and authorities [12]. Electrical power can be generated by this kind of point absorber type WEC by using a permanent magnet linear generator [13, 14]. A linear generator excited by electromagnet can be alternatively applied to the similar system [15].

Fig. 2.7 A WEC device called WET-NZ with a float [7]

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2.3.2 Sea Oyster Sea Oyster is an oscillating wave surge converter device that uses wave motions to capture energy as shown in Fig. 2.8. It has two main components—a power connector frame (PCF) and a power capture unit (PCU). The PCF is bolted with the seabed where PCU moves back and forth with the incident wave. This movement helps the hydraulic pistons in an operation mode where it feeds the pressurized water to the onshore turbine. The turbine is coupled to a generator that generates electricity [16]. Currently, Oyster is widely used as it has less risk than using fossil fuels. According to the Carbon Trust, each Oyster can avoid over 500 t of carbon dioxide from being released into the atmosphere annually [7]. A detailed view of a Sea Oyster is shown in Fig. 2.9 [7]. Its simple construction makes it more sustainable for harvesting wave energy. Oyster hinged flaps can easily sustain under the large waves during critical weather. Also, this whole operation is easy to maintain because its components are situated onshore. However, the challenges to use an Oyster are (i) installation process is not easy and (ii) the production cost is high. Besides, the total weight of this device is above 200 t and its installation is done in several stages. The sequence of its operation is sketched in Fig. 2.10. Firstly, it must be carried to the location in a huge flat top barge. The PCF must be accurately placed and bolted in the seabed to cope with uneven seabed. Around 120 t of water needs to be pumped into the ballast tank of PCU. It provides negative buoyancy to support its existence in water that would be hinged to PCF. These prolonged steps require a huge cost and manpower.

Fig. 2.8 Oscillating wave surge converter [17]

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Fig. 2.9 Construction of a sea Oyster

The disadvantages include the generator and turbine situated offshore produce noise creating a disturbance in wildlife. Besides, it can harm mammals as well as fish lives. The underwater noise and vibration produced by the movement of this device affect the natural sound. It also poses threats like producing stress or hearing loss to aquatic creatures. Installing many oysters can disturb the ecosystem of the species living in that area [16]. It is made by a company in Scotland called “Aquamarine” and is located near the shore. Among various types of offshore and nearshore wave energy devices such as Archimedis wave swing and Power buoy, linear electrical generators are widely used. When a wave peak or trough reaches the device the vertical velocity of the mover becomes zero. For this reason, the conventional linear generator cannot produce electrical power. This limitation and its solution are described in [18]. A way to improve dynamics by translator weight minimization is described for a linear generator [19]. Generally, the linear generator consists of permanent magnets which may often demagnetized. In this regard, a solution is pointed out and illustrated in [20]. The magnitude and frequency of oceanic waves are not constant. Therefore, proper control is required for effective energy conversion.

2.3.3 Onshore Device Limpet A WEC device Limpet is installed on the seashore. Wavegen Ireland along with Queen’s University Belfast built this device on the Isle of Islay’s solid rock coastline. For generating 500 kW of power, it uses a single turbine having 2.6 m diameter with a collection chamber consisting of three 6 by 6 m tubes. A Limpet built on a coastline is shown in Fig. 2.11 [21].

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Fig. 2.10 Sequential operation of a sea Oyster. a Hydraulic pistons are in the initial position. b Movement of the flap when the waves come. c Maximum expansion of the flaps of the Oyster when the whole cylinder is filled with water and high pressurized water is fed to the onshore hydroelectric turbine

2.3.4 Oscillating Water Column An oscillating water column (OWC) extracts energy by oscillation of a water column. Due to wave motion, oscillations are generated in a hollow chamber. As it has a low impact on the environment, many companies come forward to make this device more efficient. An OWC is discussed in [22] where the hollow structure contains the turbine and air tunnel as shown in Fig. 2.12. The wave enters and exits due to its motion by creating a force that makes the water column act like a piston going back and forth. This movement creates a bidirectional flow with a huge velocity of air. For further conversion or to convert the airflow into

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Fig. 2.11 An onshore wave energy device called Limpet [21]

Fig. 2.12 An oscillating water column mounted at seashore [22]

electricity, a bidirectional turbine has been introduced. The turbine rotates in the same direction and it is coupled to a generator that produces electricity.

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Fig. 2.13 An efficient and affordable wave energy device called Penguin [23]

2.3.5 Penguin as a Rotational Energy Capturing Device Penguin is an efficient and affordable wave energy device. It floats on the sea surface similar to a boat. It is a rotational device. This device has a height of 9 m, a weight of 1600 t, a length of 30 m, and a draft of approximately 7 m. Only upper 2 m can be seen above the water, other parts are below the surface of seawater. The revolution of flywheel is obtained by the motion of the hull. The spinning flywheel drives an electrical generator to generate electricity. Finally, this electricity is transferred through a subsea cable. This device looks like a water vessel as shown in Fig. 2.13. It is from the Finnish company Wello Oy on the European Marine Energy Center wave energy test field in Bilia Croo Bay. It is situated near Stromness in OrkneyMainland, Scotland. Its power rating is 500 kW [23].

2.3.6 Pelamis as a Floating Offshore Device A floating offshore device, Pelamis, is called a sea snake. It is also known as Paramus. It lies on the top of the water surface. As ocean waves go under Paramus, it constantly squeezes and expands to produce energy. To harvest more power, the latest device incarnation is about 600 ft long [7]. Paramus or Pelamis is basically an attenuator type of wave energy device. It is elaborately discussed in the next chapter. Three

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Pelamis machines are installed on the Aguçadoura Wave Park situated in Portugal. One of them is presented in Fig. 2.14 [24].

Fig. 2.14 A Pelamis that uses the motion of oceanic wave for producing electricity [24]

Fig. 2.15 SEAREV wave energy converter: a floating state and b internal structure [25, 26]

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2.3.7 SEAREV Wave Energy Converter SEAREV is called the second generation offshore WEC. The floating SEAREV [25] and its internal structure [26] are shown in Fig. 2.15a, b, respectively. It contains a watertight floater. Inside the floater, there is a wheel having a 9 m horizontal axis. Its upper half remains empty where the lower half is filled with concrete. The weight in the lower half creates a pendulum effect. The wheel swings back and forth when the floater moves with the sea wave. The relative movement between the floater and wheel results in initiated hydraulic conversion and transforms the mechanical energy into electric power. Hydraulic pumps are cascaded with the wheels where accumulator changes due to the developed high pressure. In this way, the hydraulic pump eventually operates as a power generator. After that, through underwater power transmitting cables, the generated power is transmitted. The floaters can be operated offshore because it results in low impact on the environment. For checkup and maintenance purposes, the individual parts can be dismantled and sent to the nearest offshore port.

2.3.8 Columbian Wave Energy Converter SeaRAY The Columbian WEC SeaRAY consists of two wings and a cell that is placed in between the wings where each wing and the cell can move independently. A linear electrical generator can be installed on this device. The generator is placed inside the nacelle above the vertical spars. When the wings rotate with the wave, the generator creates electric energy through mechanical energy conversion (Fig. 2.16) [27].

2.3.9 Delos-Reyes Morrow Pressure Device Delos-Reyes Morrow pressure device is placed between 50 and 100 ft below sea level. As the device is placed on the seabed, it does not hamper coastal view or does not create an impact on the sea surface. This device is shown in Fig. 2.17 [28]. The water pressure is used to inflate and deflate the airbags. It turns on a central turbine to run an electrical generator.

2.3.10 Tapchan Tapchan or tapered channel WEC consists of a tapered channel where the channel is connected to an overhead reservoir. When the wave gets narrowed, the channel captures the wave and gradually the wave water flows through the reservoir. Finally,

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Fig. 2.16 An array of floating wave energy device named SeaRAY [27] Fig. 2.17 Delos-Reyes Morrow pressure device situated on the ocean floor [28]

Ocean surface

Pressure device

Seabed

it makes the water elevated, drives a turbine, and produces electrical power from the oceanic wave. After completing the power generation process, the used water is dispersed in the sea [22]. The schematic diagram of a tapered channel WEC is presented in Fig. 2.18. This Tapchan can meet the extra demand of electricity during the wave surge. Besides, it has the advantage of lifting sea water into a reservoir. It creates less environmental impact due to its small size (tidal range). Technologically, it is easy to maintain due to its resources having low production costs and availability in the

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Fig. 2.18 A tapered channel WEC

market. One of the disadvantages is that its usability is limited to nearshore area only. It also requires sufficient depth at nearshore and higher cliffs [22].

2.3.11 SeaRaser The SeaRaser is a distinctive WEC that can produce energy from sea waves above the surface. It contains a top float, reactor float, double acting piston, and single point mooring. The corrosion due to constant contact of the device with salt water creates problem. Thus, saltwater is used for lubrication in the device instead of petroleumbased lubricants because it is problematic for the marine ecosystem. Figure 2.19 presents a schematic diagram of a SeaRaser [29].

2.3.12 Power Buoy Power buoy harnesses power from ocean waves which is made by a company called Ocean Power Technologies. Length and width of the buoy are 18 and 10 ft, respectively. This device works with the up and downward motions of the incoming wave to harness electricity. The system uses a weighted metal plate to create a movement that is situated approximately 100 ft down from the buoy. Besides, there is a large hydraulic cylinder between the buoy and the undersea metallic plate. The cylinder has a piston that is pushed and pulled as the buoy rises and falls with the surface wave.

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Fig. 2.19 Construction of a typical SeaRaser WEC [29]

Therefore, the hydraulic fluid then goes to the hydraulic motor due to the piston’s force and turns the generator on [30]. Another type of direct drive point absorber type converter is available, which utilizes a linear electrical generator to produce electricity directly. It also has a heaving buoy. It displaces the water and generates interference between the incident wave and radiated wave with the aid of an oscillating material. The model consists of a heaving buoy, a damping plate, and a spar. The buoy is a device that is generally used to convert wave energy. A block diagram representation of this device is presented in Fig. 2.20 where L denotes the wavelength, H is the wave height, m is mass (both m1 and m2), and h is the water depth. A detailed view of this device has been shown in Fig. 2.21. It contains a float that goes up and down into the water surface where the damper plate at the bottom holds the structure in place. There is a coil in the spar at the center and there are magnets in the buoy. It produces electrical energy by Faraday’s law of electromagnetic induction because of their relative vertical motion. Another type of point absorber type WEC is constructed with submerged and circular shaped buoys. The buoys relate to the seabed pump which is situated at 25–50 m depth from the ocean surface. The buoys use the incoming water motion which helps to run the pump. This operation makes the high pressurized water go toward a pipeline on the ocean floor. Then from there, the water is transferred to the hydraulic power station which is situated onshore. Finally, the pressurized water runs the turbine and harvests the wave energy. The working principle of different WECs with the functionality of each component has been discussed in this section. The design and simulation of a point absorber type WEC is presented in the following sections to understand its technology in detail.

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Fig. 2.20 Block diagram representation for a model of power buoy [10]

Fig. 2.21 Visual structure of a point absorber [10]

2.4 Design Objective of a Wave Energy Device During energy extraction, a secondary wave is produced by the point absorber type WEC with a heaving buoy such as the Power buoy. The wave possesses the same wave height and length as the main harmonic wave. However, the secondary wave is exactly out of phase concerning the main wave. It happens because when a wave generates a crest, the other one creates a trough and vice versa. The secondary and resultant waves are shown in Fig. 2.22.

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Fig. 2.22 Wave termination of wave energy device [11]

Figure 2.22 summarizes the fact that the wave energy converter creates another wave. If the newly created wave is synchronized with the ocean wave, they oppose each other because of having opposite phases. Thus, the newly created wave has the tendency to cancel out the incoming waves. On the other hand, because of power extraction from the oceanic wave, its magnitude decreases practically. It is possible to extract maximum energy by exploitation of this phenomena.

2.5 Modeling and Simulation of Point Absorber To extract the energy from heaves, a simple mathematical expression of force (force = mass × acceleration) is considered and three types of force such as excitation force, radiation force from wave, and force on the float are used to calculate the energy extraction value. The following formula [7] is applied here. Fe (s) + Fr (s) + F pto (s) = s V (s)m

(2.1)

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where F e (s) is the excitation force on a float by the wave, F r (s) is the radiation force by the wave, F pto (s) is the force on the float from the power take off device, m is the float mass, sV (s) is the vertical acceleration where s is the differential operator and V (s) is the vertical velocity. For extraction force, LS-Dyna modeling fluid–structure interactions are used on the devices. All the different sources of force on this device are possible. Besides that, they are trying to come up with reduced order models that are tractable for real time estimation and can control these devices.

2.5.1 Time Domain Simulation In this section, related factors for time domain simulation [7] are discussed. It is well known that some of the devices can produce more energy when they use a back control. Life extending control is one of the processes that is conducted in simulation. However, sometimes the devices can face obstacles such as storm and as a result they may lose their structural integrity. Therefore, it is essential to control dynamics, reliability, sustainability, and the cost of energy. The lifetime of these devices can be extended by controlling these devices. To meet the above objective, a controlling analysis has been done by putting the real wave data into computational models and corresponding simulation results are shown in [7] that mainly represent the displacement concerning the result with a spar plate and buoy. Displacement works vertically or in other words toward its height. It indicates the dynamics of the system. It possesses a constant suitable value. Besides, it signifies efficiency and the buoy position with respect to the wave height.

2.5.2 Method of Extracting Maximum Energy One of the widely used modern technologies for energy conversion is Hydrofoil [7]. It provides benefits like hydrodynamic loss reduction and more energy conversion from the wave. To get the wave peak, a hydrofoil is placed into the water which is shown in Fig. 2.23a. To control the wing pressure, the aerodynamics principle is used. Also, Fig. 2.23b demonstrates the maximum power extraction process using two hydrofoils. Two bidirectional hydrofoils are pitched to create two opposite directional traveling waves. The generated peaks canceled each other as the waves are apart 180° phase shifted. As a result, the maximum energy is being harvested from the oceanic wave. Besides, a cycloidal whack has also been used for extracting the wave power by considering two hydrofoils. The hydrofoils are pitched in such a way that when the cycloidal whack is spined it creates peaks and troughs. They can perfectly cancel away if it is timed with the way it comes in. With this process, it is possible to extract 95% of the wave energy [7]. However, a method of extracting maximum power is discussed in the following.

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Fig. 2.23 Maximum power extraction by using a a hydrofoil and b two hydrofoils into the water [7]

To extract maximum power from the oceanic wave, another method called “Latching” is presented in [8]. In this method, the velocity of the buoy is in phase with the excitation force. When the natural frequency of the buoy is equal to the wave frequency, the optimal oscillations would happen. The natural frequency, ω of a buoy can be expressed as follows. / ω=

ρg A m add + m b

(2.2)

where ρ is the fluid density, g is the acceleration due to gravity, A is the plane area of the buoy, mb is the mass of the buoy, and madd is the added mass due to the wave motion.

2.5.3 Latching Method for Extracting Maximum Energy To discuss the latching method, two cases have been described here. For the first case, as shown in Fig. 2.24a, the buoy velocity becomes zero due to its locked position. For this reason, with the wave motion, the buoy fails to reach its previous location. Therefore, it remains partially or fully submerged. Besides, in Fig. 2.24b, an unlatching state is illustrated where the buoy oscillates with the wave. Therefore, latching and unlatching are alternatively going on throughout the process. Latching is executed by locking the buoy at certain moment. Unlatching is deployed after a certain interval from the latching period. This process continues repeatedly. The latching time is given as Latching time = Tw − T0

(2.3)

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Fig. 2.24 Latching method for extracting maximum energy: a latching and b unlatching of the buoy [8]

where T w is the wave period and T 0 is the natural period of the buoy. However, this formula is only applicable for regular waves while ocean waves are mostly irregular. Therefore, the process of latching becomes complex while working with practical oceanic waves. To overcome the nonlinear wave complexity, suboptimal latching is used which is shown in Fig. 2.25. Suboptimal latching works well when the velocity and excitation force are in the same direction and the buoy is free to move [8]. In this case, the buoy has only one degree of freedom, namely heave. However, the buoy is influenced by multiple forces such as hydrostatic and hydrodynamic acting upon it where moving force is neglected. Here, the power take off systems movement extracts the energy from each motion. Besides, an improved latching model has been introduced in [8], where a mathematical model has been developed and implemented physically. The implementation of a latching control is done by building a wave tank (1 m by 0.4–0.45 m) where the paddle pushes the aluminum sheets tank water to move up and down and creates the reciprocating wave motion. The generated wave depends on the vertical displacement of the paddle range. The frequency follows the rate at which the paddle moves. When the buoy moves up, only the right side of the system is engaged to drive the flywheel in an anti-clockwise direction. While moving downward, the left side of the system is engaged again to run the flywheel in the same direction. For bidirectional motion of buoy, there is only one direction in the flywheel. This is how the experimentation of latching method works. Its working principle can be easily understood by observing Fig. 2.26.

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Fig. 2.25 Suboptimal latching process [8]

2.6 Research and Education Approach Characteristics of the WEC make wave energy harvesting beneficial compared to other renewable energy sources. The common drawback of solar photovoltaic and wind energy is that the supply continuity is not possible to maintain from these sources due to the nature of intermittencies. On the other hand, wave energy is much more predictable and persistent compared to other renewable energy sources. Some research and educational approaches of the test devices are presented in Fig. 2.27. From Fig. 2.27, it is found that the approaches mainly focus on the oceanography, engineering, and mooring system as well as the ships which are used to deploy. Generally, the technical fields are led by the US or UK. Meanwhile, the US focuses on environmental issues where much concern is found in the water resource-based energy sectors. They are also concerned about social aspects such as people living in the coastal area and the fishing grounds. According to this issue, they have established three to four years of territorial sea planning. This sea planning would be helpful to make the decision about site selection. For example, identify a suitable sea area to put the energy converter devices for harvesting wave power. Besides, researchers can predict what is going on with sea waves over time. It balances out various attributes that are available in different regions in an integrated way.

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Fig. 2.26 Experimental implementation of a latching method [8]

Fig. 2.27 Some aspects of the research and education approach [7]

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2.7 Recent Development Although a lot of new oceanic wave energy devices are invented, without an efficient electrical generator, the total effectiveness of the system would be degraded. Because an electrical generator mainly converts mechanical power into electrical power. On the other hand, wave energy devices convert irregular wave motion into a regular mechanical motion. Therefore, utilizing proper wave energy devices is not sufficient only for producing electricity effectively. Rather, improved design of the electrical generator [5], application of high graded magnetic cores, [19] permanent magnets [13], and their optimization [18] are required as well to improve the overall performance of the total wave energy conversion system. The power loss due to armature resistance can be minimized by applying superconductor [31]. The size of a superconducting electrical machine for high power application is smaller than the conventional one for the same power rating. Mostly, conventional linear electrical generators are of electromagnetic type, but it is found that piezoelectric generators can also be used for the same purpose [32].

2.8 Summary Wave energy has been treated as a potential alternative source of energy. The growth and the level of technology readiness are worth praising. Though harnessing all the possible energy from this resource may seem far-fetched it should be considered that most of the companies working in this field are comparatively new (only one or two decades old). Therefore, there is a lot of scope to improve wave energy converter technology. Other extraction devices of renewable sources like the wind or solar structure are enough mature. To harness wind power, tall and white blades are used and for extracting solar power, a solar panel is used. But for extracting the wave energy, deciding the shape of the harnessing device is quite difficult. Many companies are trying to figure out proper shape for harnessing maximum energy. Harnessed wave energy is still challenging to supply power to the grid, but the researchers are trying their best to do so. Another challenge for extracting the wave energy is that waves neither flow normally like wind nor radiate like sun rays. The wave rolls, pitches, and heaves making the structures of harnessing devices so unpredictable. Hence, chances are most of the projected designs fail to meet the requirement while few provide the benefits. Some wave energy converters may help to provide coastal protection through erosion reduction. However some of these devices can affect wildlife interaction. Sea animals may get detoured by the lengthy cables of floating energy converters. Before deploying the devices, researchers should study the ecological situation of that place. One more problem can be the noise generated by these devices. Many sea animals can be disoriented by the sonar. But now researchers ensure that they become comparatively noise free after the installation. But it can have a positive impact if the

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corals or algae tend to grow on these structures. Such devices can protect fish or other sea life. To build a successful wave energy converter, an integrated system should be introduced. Sustainable measures should be taken to preserve environmental balance. Further exploitation of the renewable sources for meeting energy needs in a balanced way can introduce an efficient and sustainable energy sector.

References 1. Molla S, Farrok O, Alam MJ (2023) Zero Carbon emission based electrical power plant by harvesting oceanic wave energy: minimization of environmental impact in Bangladesh. In: Climate change and ocean renewable energy. CCORE 2022. Springer proceedings in earth and environmental sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-26967-7_1 2. MIT News, Race Develop Renewable Energy Technologies-1218 [online]: http://news.mit. edu/2019/race-develop-renewable-energy-technologies-1218. Accessed 12 Jan 2020 3. Beels C, Torch P, Backer GD, Vantorre M, Rouck JD (2010) Numerical implementation and sensitivity analysis of a wave energy converter in a time-dependent mild-slop equation model. Coast Eng 57(5):471–492. https://doi.org/10.1016/j.coastaleng.2009.11.003 4. Molla S, Farrok O, Islam MR, Muttaqi KM (2020) Application of iron nitride compound as alternative permanent magnet for designing linear generators to harvest oceanic wave energy. IET Electr Power Appl 14(5):762–770. https://doi.org/10.1049/iet-epa.2019.0372 5. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 6. Islam MR et al (2020) Design and characterisation of advanced magnetic material-based core for isolated power converters used in wave energy generation systems. IET Electr Power Appl 14(5):733–741. https://doi.org/10.1049/iet-epa.2019.0299 7. AAAS MemberCentral (Online) Advancements in wave energy with Belinda Batten [Video]. You Tube. https://www.youtube.com/watch?v=3vz8xma45yM. Accessed on 23 Dec 2020 8. Yousouf (Online) Modeling and control of a Wave Energy Converter for Maximum Power Extraction [Video]. You Tube. https://www.youtube.com/watch?v=7FpDx54x-jo. Accessed 23 Dec 2020 9. Farrok O, Ahmed K, Tahlil AD, Farah MM, Kiran MR, Islam MR (2020) Electrical power generation from the oceanic wave for sustainable advancement in renewable energy technologies. Sustainability 12(6):1–23. https://doi.org/10.3390/su12062178 10. Wave Energy (Online) In Wikipedia. https://openei.org/wiki/Wave_Energy. Accessed 10 Jan 2020 11. Atargis Energy Corporation, CycWEC design features (Online): https://atargis.com/CycWEC. html#Torque. Accessed 10 Jan 2020 12. Early Life, Ocean Sentinel [Online]: https://earlylife-erc.com/ocean-sentinel. Accessed 9 Jan 2020 13. Bashir MS, Farrok O (2019) Generation of electrical power by using high graded permanent magnet linear generator in wave energy conversion. In: 1st International conference on advances in science, engineering and robotics technology (ICASERT), pp 1–5. https://doi.org/10.1109/ ICASERT.2019.8934591 14. Molla S, Farrok O, Alam MJ (2023) Translator optimization of a linear electrical generator for harvesting oceanic wave energy. In: IEEE IAS Global Conference on Emerging Technologies (GlobConET), London, United Kingdom, pp 1–6. https://doi.org/10.1109/GlobConET56651. 2023.10150079

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15. Farrok O, Islam MR, Sheikh MRI, Guo YG, Zhu JG, Xu W (2015) Analysis and design of a novel linear generator for harvesting oceanic wave energy. In: 2015 IEEE international conference on applied superconductivity and electromagnetic devices (ASEMD), pp 272–273. https://doi.org/10.1109/ASEMD.2015.7453569 16. Oyster Wave Energy Converter (online) In Wikipedia: https://en.wikipedia.org/wiki/Oyster_ wave_energy_converter. Accessed 19 Oct 2019 17. Costal Energy and Environment, Oscillating surge converter (online): http://coastalenergyan denvironment.web.unc.edu/ocean-energy-generating-technologies/wave-energy/surge-conver ters-2/environmental-effects. Accessed 9 Jan 2020 18. Farrok O, Islam MR, Muttaqi KM, Sutanto D, Zhu J (2020) Design and optimization of a novel dual-port linear generator for oceanic wave energy conversion. IEEE Trans Ind Electron 67(5):3409–3418. https://doi.org/10.1109/TIE.2019.2921293 19. Molla S, Farrok O, Alam MJ (2022) J material based lightweight linear electrical generator with improved dynamics for harvesting oceanic wave energy. In: 12th International conference on electrical and computer engineering (ICECE), Dhaka, Bangladesh, pp 92–95. https://doi. org/10.1109/ICECE57408.2022.10089129 20. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Lei G (2017) A novel method to avoid degradation due to demagnetization of PM linear generators for oceanic wave energy extraction. In: 2017 20th International conference on electrical machines and systems (ICEMS), pp 1–6. https://doi.org/10.1109/ICEMS.2017.8056471 21. Decommissioned Islay Limpet wave power plant. Anhn, CC BY-SA 4.0 [Online]: https://com mons.wikimedia.org/w/index.php?curid=72766157. Accessed 30 Sep 2023 22. Oscillating Water Column (online) In Wikipedia: https://en.wikipedia.org/wiki/Oscillating_ water_column. Accessed 30 Sep 2023 23. EMEC, Wello Oy [Online]: http://www.emec.org.uk/about-us/wave-clients/wello-oy/, Wave power plant “Penguin” (500 kW) Wello Oy, CC BY-SA 4.0 (September 2023) Photo: Jan Oelker, https://commons.wikimedia.org/wiki/File:JOE140514_128_Orkney.jpg [Online]: https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons. Accessed 30 Sep 2023 24. Pelamis bursts out of a wave (September 2023) [Online]: https://commons.wikimedia.org/ wiki/File:Pelamis_bursts_out_of_a_wave.JPG#/media/File:Pelamis_bursts_out_of_a_wave. JPG. Accessed 30 Sep 2023 25. Faizal M, Ahmed MR, Lee Y (2015) A design outline for floating point absorber wave energy converters. Adv Mech Eng 6:1–18, Article ID 846097. https://doi.org/10.1155/2014/846097 26. de António F, Falcão O (2010) Wave energy utilization: a review of the technologies. Renew Sustain Energy Rev 14(3):899–918. https://doi.org/10.1016/j.rser.2009.11.003 27. Columbia POWER, Power from the next wave. [Online]: https://renews.biz/38612/us-wavegets-european-patent/. Accessed 9 Jan 2020 28. The Corvallis Advocate-Ocean Power: A Perfect Fit for Oregon (September 2023) [Online]: https://www.corvallisadvocate.com/2012/ocean-power-a-perfect-fit-for-oregon/. Image: http://www.corvallisadvocate.com/wp-content/uploads/2012/06/wave-energy_600. jpg. Accessed 30 Sep 2023 29. The Searaser [Online]: http://www.alt-energy.info/wave-power/the-searaser-a-new-wave-ene rgy-device-that-simplifies-electricity-generation. Accessed 12 Jan 2020 30. Scitechdaily, Floating Power Buoy creates Electricity from ocean waves. [Online]: https:// scitechdaily.com/floating-power-buoy-creates-electricity-from-ocean-waves. Accessed 12 Jan 2020 31. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Xu W (2016) A novel superconducting magnet excited linear generator for wave energy conversion system. IEEE Trans Appl Supercond 26(7):1–5, Art no. 5207105. https://doi.org/10.1109/TASC.2016.2574351 32. Kiran MR, Farrok O, Mamun MAA, Islam MR, Xu W (2020) Progress in piezoelectric material based oceanic wave energy conversion technology. IEEE Access 8:146428–146449. https:// doi.org/10.1109/ACCESS.2020.3015821

Chapter 3

Pelamis Wave Energy Converter Mushfiqur Rahman Shipon, Md Sawkat Ali, Md Ahsan Kabir, Md. Abdullah-Al-Mamun, and Omar Farrok

Abstract To meet the growing renewable energy need, many utilities have been built to extract energy from natural resources. The Pelamis wave energy converter is one of them. It is basically an offshore semi-submerged float type device operating in such a location where the depth of sea water is 50 m or more. This chapter describes different aspects of Pelamis technology along with its features. The current energy policy and estimation of wave energy are presented. Power capture by Pelamis and its survivability attributes are depicted. The power train of Pelamis, its resonant and benign response along with the tuned response are described. The strength, weakness, opportunity, and threat of this device are mentioned in detail with their challenges and possible solutions. An integration of energy storage and its importance are illustrated to obtain intermittent power. Environmental, ecological, and economic factors are discussed as well. Keywords Grid integration · Oceanic wave energy · Offshore energy · Pelamis · Resonant response · Wave energy device

M. R. Shipon · Md. Abdullah-Al-Mamun · O. Farrok Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected] M. S. Ali (B) Department of Computer Science and Engineering, East West University, Dhaka 1212, Bangladesh e-mail: [email protected] M. A. Kabir Department of Electrical, Electronic and Communication Engineering, Military Institute of Science and Technology, Dhaka 1216, Bangladesh e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_3

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3.1 Introduction to Pelamis To meet the massive power demand all over the world, both renewable and nonrenewable resources are used for electricity generation. Till now the maximum electricity is produced by fossil fuels and the world must look forward for alternative resources to mitigate the environmental pollution as well as the large energy demand issues. Therefore, renewable energy resources have an economic production opportunity for its unlimited natural energy source and environmental sustainability. As a result, potentiality of different renewable energy sources is investigated with a continuous development of modern devices in recent times. Many power scientists, corporations, and electrical power technological firms have developed different types of renewable energy appliances and their prototypes based on the geographical location, accessibility of the resources, and favorable surroundings. To utilize the wave energy, Pelamis is a popular commercial wave energy converter (WEC) device which is used in the wave energy conversion. The main advantages of this converter are low investment cost and less CO2 emission to the environment in comparison to other renewable energy devices. The CO2 emission is low for both wave turbine and the modern wind turbine for the same power output capability. Therefore, the environmental impact is quite low for both due to their less emission. Electricity generation from the oceanic wave can be supportive for minimizing carbon emission target for many countries [1, 2]. Due to the acceptability of Pelamis, a commercial off coast wave farm, Agucadoura in north Portugal has been started by a Scottish company in 2008. The project follows four tabular sections where around 750 kW of wave power is harvested. However, generation of the world’s first commercial Pelamis is stopped due to some technical issues which have been covered by the upgraded Pelamis (P2 machine) installed in Orkney in the Northern Scotland [3]. This chapter describes background of the pelamis wave energy converter in a separate section. Difference between the conventional WEC and Pelamis is depicted as well. Wave energy estimation in the US is tabulated separately based on total and recoverable wave power densities. Environmental and economic analyses are also presented.

3.2 Background The world’s first full-scale pre-production prototype of Pelamis was tested at the European Marine Energy Centre, Orkney. The farm started with three Pelamis generators in September 2008 with a power generation capacity of 2.25 MW. A typical Pelamis wave energy device is sketched in Fig. 3.1. It is a wave energy device which converts offshore wave energy working with the water depth of 50 m or more. The device is formed of a series of multiple semi-submerged cylindrical sections attached by flexible joints. When waves push the submerged portion, all the sections

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Fig. 3.1 A typical Pelamis wave energy converter

are moved relative to one another [4]. The motion of the sections is produced by wave through a hydraulic cylinder which pumps pressurized oil to the hydraulic motors through smoothing hydraulic accumulators. The motors run the generators to produce electricity. The electricity produced from all the section joints is supplied through a single umbilical cable. To get more power, multiple machines have been attached and linked by an individual seabed cable [4, 5]. Pelamis and Power buoy wave energy converter float on the sea surface whereas Archimedes wave swing is submerged. Power buoy and Archimedes wave swing are point absorber type devices. Point absorber type devices are popular and widely used as they have the flexibility to mount a direct drive linear electrical generator. Point absorber requires less area for producing adequate power because it is placed vertically. Electrical power generation does not depend on the wave energy devices only. Rather it depends on the design of electrical generator [6]. A method of dynamics improvement of a linear generator has been described in [7]. The translator weight is reduced for this purpose by improvement of its design. The weight can also be minimized by applying ferrite magnetic core to the translator to improve its dynamics [8]. Application of an advanced permanent magnet results in increase in electrical power generation [9]. Optimization is necessary for the electrical generator to improve its performance [10]. Voltage, current, and power control of the generator are also required for practical implementation. There are some electrical losses in the generator which can be avoided by using superconductor as proposed in [11]. To improve further performance and safety of the electrical generators, prevention of demagnetization [12], cooling system, and protection systems are required as well. For direct drive application, permanent magnet linear generator is widely used [13]. Piezoelectric material-based electrical generators can also be used for wave energy conversion [14]. A power converter is essential for transferring electrical power to the grid for

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renewable energy-based power generation such as solar photovoltaic, wind, etc. and wave energy converter is not an exception [15]. On the contrary, Pelamis wave energy device is a line absorber type device. It is placed horizontally on the sea surface. Therefore, it requires relatively large area for the same power rating.

3.2.1 Energy Policy Drivers—UK Perspective The current energy policy has been set by the Department of Energy and Climate Change, UK. It was developed in 2003 and low carbon transition plan was reported in 2009. The recommendations were suggested for the energy market restructuring, smart meter integration, and improvement of green energy efficiency [16]. The driven factors of the policy are mentioned as follows: a. Environmental factors in EU/UK for climate change due to greenhouse gas emission and Kyoto commitments. b. Socio-economic facts arethe invisible expense of current generation, renewable increasingly competitive, and dynamic industrial-economic scenario. c. Energy supply security facts increase due to enormous demand of electricity and operating power plants have reached the end of life.

3.2.2 Wave Energy The wave energy is produced from the dynamic waves where their energy is gained from the wind blowing through the surface of the sea. It can transmit energy over further distances with very few wasting of energy and thus, it is considered as one of the most useful renewable energy resources [17]. The wave components are presented in Fig. 3.2.

3.2.3 US Wave Energy Resource Wave power density is calculated as 15 MW/km on the coastline. The entire renewable wave energy resource can be determined by multiplying the total length with wave power density. The entire possible energy resource along the US global shelf edge is calculated to be 2,640 TWh/year as confined in Table 3.1. During the estimation, both the thresholds and the maximum operating conditions in terms of input wave power density are considered. Thus, total recoverable resource along the US continental shelf edge is 1,170 TWh/year as shown in Table 3.2 [7].

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Fig. 3.2 Components of the oceanic wave

Table 3.1 Total wave power densities at different locations in the US [18]

Location

Wave power densities (TWh/year)

West Coast

590

East Coast

240

Gulf of Mexico

80

Alaska

1,570

Hawaii

130

Puerto Rico

Table 3.2 Recoverable wave power densities at different locations in the US [7]

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Location

Wave power densities (TWh/year)

West Coast

250

East Coast

160

Gulf of Mexico

60

Alaska

620

Hawaii

80

Puerto Rico

20

3.2.4 Grid Integration Ocean wave energy can be found in offshore, onshore, and nearshore. Offshore wave energy has some features which are presented as follows. a. b. c. d.

Optimum resource in deep water Enough space and high reaching power density Merest environment impact Smallest visual incursion

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To extract oceanic wave energy, wave energy device is to be installed at first. Before device installation, various parameters must be forecasted properly as follows. a. b. c. d.

Sufficient calibrated wind-wave designs Remaining offshore forecasting ministration Local weather effects Short hourly and stable variation

Oceanic waves are irregular in nature. To extract this energy wave energy devices are used. The wave energy device is mounted to drive a linear generator. The produced electricity from linear generator is converted into dc power. Then the dc power is converted to ac power such that it can be synchronized to the grid. Many countries have established renewable energy technologies to fulfill the target of mitigating excessive power demand, promoting sustainability, and maintaining low carbon emission. To obtain the target, power system planning and operations must be reinforced. Consequently, grid integration refers to a productive way of development to supply renewable energy-based power to the grid. While maintaining system stability, different methods are applied so that grid integration can be cost effective. The future grid integration comprises the following issues [19, 20]. a. b. c. d.

New renewable energy generation New transmission Increased system flexibility Planning for a high renewable energy

3.3 Pelamis Technology Pelamis can be considered as a line (or series of point) absorber. It uses the change in height of the wave crest to operate this wave energy converter. It has a few horizontal cylindrical sections connected to each other. When it is placed on the sea wave, its power capturing unit or floating platform movement is vertical. From the top view, it is seen that the Pelamis wave device also moves along the surface orthogonal to the wave direction. This movement is converted into electricity [21].

3.3.1 Power Train The internal construction of Pelamis consists of a hinged joint sway, a hydraulic ram, high pressure accumulator, motor generator set, manifold, reservoir, and a hinged joint heave. A Pelamis contains a few similar segments. Different segments are cascaded to each other. The energy is resisted by the hydraulic pistons which come from the wave motion through device joints. The smoothing accumulators like hydraulic motors are used for generating green electric power. A similar approach

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has been done for the other Pelamis converters. Finally, generated power has been collected through a single feeder line [22].

3.3.2 Survivability of Pelamis For survivability consideration, Pelamis wave energy converter withstands a very wide range of input power than that of the other existing renewable energy-based technology. During oceanic abnormal situations such as storms, the levels of input peak energy can be 100 times higher than the normal operational conditions. However, the wave energy converter inherently restricts its absorption during the vulnerable wave nature. Pelamis proved better energy capture efficiency with the inherent survivability with the vulnerable wave nature in the sea. As wavelength rises, little frontal transverse section and short drag portion of the pelamis permit to dive under the wave crests. Therefore, controlled power absorption occurs by limiting wave motion and loads. In extreme wave conditions, the most excessive loads are faced due to high wave velocity. Pelamis has the generic protecting capability against such hydrodynamic loading. This process is matched with a surfer jumping under the wave peaks when surfing from the seashore [23].

3.4 Power Capture by Pelamis The power capture technique by pelamis wave energy converter technology is discussed in the following. Figure 3.3 shows the resonant response and benign response for pelamis converter.

Fig. 3.3 Resonant response and benign response [23]

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a. Benign articulated raft in large waves limits loads and motions. b. Selectable and tunable resonant response in small sea optimizes absorption. c. Most of the WEC uses the resonance of waves to capture more power. Pelamis in non-resonant condition restraints that depend on the pitch and yaw axes. The response is vertical and non-resonant. On the other hand, Pelamis in resonant condition restraints much greater about pitch axis than yaw axis. The response is inclined and resonant. Pelamis operates on the idea of tunable resonance response. In resonant situations, the system requires to cope with huge wave during the stormy condition. A cross-coupled resonant wave energy converter has been developed to control the joints. However, the normal condition (non-resonant) has the inherent capability to deal with extreme conditions. In theory, resonance occurred properly through an advanced power take-off system by tuning its internal impedance. The execution of cross-coupled resonance restricts transfer of highly reactive energy. As a substitute, the common ratio between perfect damping barriers was implemented at each of the paired axes. By developing an inclined response, the external impedance of the device is changed. The inclined angle response calculated the actual period as pointed by the fraction of vertical hydrostatic stiffness, which maintains the inertia. The effect of internal gravity wave is obvious, in which the frequency of excitation determines the angle of inclination of freely oscillating fluid particles. This capability to match machine response to the waves in each sea-state is similar to the concept of wind turbine, i.e., industry-standard variable speed rotors and variable pitch blades. The turbine speed and blade angle are continuously adjusted to optimize energy capture and avoid stress [23]. Some other features of Pelamis WEC technology for power capture are mentioned as follows: a. Pelamis is basically a line absorber. b. Ultimate power capture limit < wavelength/6. c. Ultimate power capture limit > wavelength/2.

3.5 Example of Resonant Response Figure 3.4 shows a Pelamis model tested in a laboratory, where the water waves are created artificially. The experimental results claimed the movement happened to the Pelamis test device due to the resonance of water [24]. Figure 3.4b shows movement of the device due to resonance in the water where the movement of the joint is examined (bottom view). Movement of the joint further produces electricity with other internal devices [24].

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Fig. 3.4 The device position for the a first, b second, and c third stages due to resonant response in the water (front view). Position for the d upward and e downward directions due to its movement for resonant response (bottom view) [17, 24]

3.6 The Tuned Response of Pelamis The power spectrum curve due to tuned response is shown in Fig. 3.5. The yellow shaded area represents the wave spectrum of the sea with its variance with time [17]. The curve shows the changes in frequency with the area of the power spectrum due to change in frequency.

3.7 Features of Pelamis Unique characteristics of Pelamis make it popular, which includes but are not limited to the following [17]: • • • • • • • • • • •

Critical inherent survivability Sufficiently high power take-off Hydraulic power take-off Power smoothing The patented core technology Independently verifiable Forecastable output Negligible visual intrusion Less impact on environmental Minimal construction work in on-site Maintenance work in off-side

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Fig. 3.5 Fixed and slow tune responses of Pelamis for a low frequency at fixed tune response, b low frequency at slow tune response, c high frequency at fixed tune response, d high frequency at slow tune response, e medium frequency at fixed tune response, and f medium frequency at slow tune response [17]

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Some perspectives about its cost effective development are: • • • • • • •

Concept development Primary research, development, and modeling 7th scale prototype Full-scale research, development, and design Large-scale joint test Manufacturer friendly prototype Cost reduction and performance enhancement

3.8 Strength, Weakness, Opportunity, and Threat of Pelamis This section presents the strength, weakness, opportunity, and threat analysis of pelamis considering the strength, present weakness, and future development scopes with the potential threats.

3.8.1 Strength The strength of Pelamis is its robustness, technological maturity, and flexible modular design [25]. • Robustness: Its construction is robust. Because of its operating principle, it can withstand harsh environments. Most of the accessories are placed inside the cylindrical structure. It makes the system reliable. • Technological maturity: Maintenance cost is comparatively less than other renewable energy source-based devices. It provides a better output power generation with minimal cost. • The flexible modular design: For the electrical system of this wave energy converter, easy power scalability can be maintained by adding new modules for progressively increasing power rating for the existing one. This process is only possible due to its flexible modular design.

3.8.2 Weakness Three main weaknesses of Pelamis are its electrical system, generation system, and controller system [25, 26].

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• Electrical system: The major shortcoming of the pelamis WEC is the capability of the system’s power processing. In continuation, there is no power electronicbased power processor. As a result, the controller operating point is not precisely defined that impacted the system operation. • System controller: Pelamis lags in its control structure where the hierarchical control approach is not properly synchronized. The control platform needs to take measurements on different levels and communicate between them.

3.8.3 Opportunity Pelamis has several opportunities that make the system suitable for harnessing wave energy. The following opportunities are included [25]. • • • • •

At the joints, there is more than one degree of freedom. Adjustable geometries and the materials have been developed to use. Modified the mooring system for higher water depth. Enhanced multi-input multi-output control to increase the system efficiency. Gather electrical engineering knowledge to address the problem efficiently and to connect the grid. • Developed monitoring and methodology. • Grid cable cost could be reduced by checking the performance on depths (intermediate and shallow) • Scope for investments in mass production.

3.8.4 Threat There are many non-perceptible threats in the wave energy sector itself which are pointed out in the following [25]. • • • • •

Developed many other ideas which are not so flexible. Dynamic energy market which impacted small wave power generation company. To continue entrepreneurship, huge investment is needed. Dependence on a few suppliers. High competition is developed nowadays due to offshore wind farm progress.

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3.9 Renewable Ocean Energy Renewable energy is available in the water in a few different forms, which are listed in the following. • Marine Hydrokinetic Energy: Marine hydrokinetic energy is a kind of energy which is generated by oceanic water movements. This includes the wave tidal current and ocean current. • Ocean Thermal Energy Conversion: Ocean thermal energy conversion (OTEC) is a way of energy harness process produced by the temperature differences between the warm surface water of the ocean (heated by the sun) and cold deep water. The power cycle has been continued to pump sufficiently warm and cold water with the help of OTEC plants to generate electricity. There is also another energy conversation process which is developed by the salinity concentration differences between saline ocean water and freshwater draining. The process is fulfilled by land and fresh salinity gradient technologies. It harnesses osmotic forces because the salinity concentration difference between the two water masses tries to reach its equilibrium.

3.9.1 Wave Energy Technology and Devices In the marine oceanic energy extraction process, different types of technologies are involved that have been divided into a few classes as listed in the following. • • • • • • • •

Attenuator Point absorber Surge converter Oscillating water column Overtopping terminator Submerged pressure differential Bulge wave energy converter Rotating mass

The above mentioned principles are considered as basic wave energy harvesting technology. There are some accessories and devices that support the main wave energy device. They are as follows. • • • • •

Horizontal axis turbine Vertical axis turbine Oscillating hydrofoil Enclosed venture tips Tidal kite

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• Archimedes screw A horizontal axis turbine can be set up by a series of turbines along with a gate or there is another setup called an eco-pod where the horizontal axis turbine is on a single buoy.

3.9.2 Renewable Oceanic Energy Challenges The technological, economic, and societal challenges are key factors for the further development of oceanic energy. The development of wave energy prototype hasn’t faced any major technical obstacles. But many issues have been raised under construction, design, and deployment operation which can be eliminated by transferring technology from other industries especially offshore industries. This section describes the challenges in detail.

3.9.3 Investment and Operational Cost There is a high capital expense involved in the device construction as well as the installation. The high expense is also involved in maintaining the devices, especially in the harsh ocean environment. These costs mean that the price for marine hydrokinetic energy is not cost competitive compared to the non-renewable energy resources. A typical range of costs has been shown in Fig. 3.6. From Fig. 3.6, the renewable energy costs are presented that range from 3 to 96 cents per kilowatt-hour of energy. Another challenge is the intermittent nature of

Fig. 3.6 The range of non-renewable electricity cost

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renewable energy sources especially wind and wave energy. There is a lot of variation in height and power density of the wave in a location over time.

3.10 Installation Challenges There are a few challenges regarding the installation of Pelamis wave energy device. It includes collection of historical background, feasibility, adequate statistical information, estimation of wave energy, etc. Besides, maintaining synergies between marine hydrokinetic energy and other renewable sources is another challenge because they often conflict with each other. The wave energy researchers addressed all the challenges and proposed possible ways of solution that have been discussed in the next section.

3.10.1 Power Take-Off The common platform of power capturing process for both the wave and wind is to generate kinetic energy after converting the energy into work. It is a well-known phenomenon that wave power needs to operate the generator with a specific range of speed from the wave or the wind. To fulfill that job, the generator operates with the gearboxes. Almost all the traditional wave generators have expired due to the reliability issue of the power extraction process. Besides, maintenance is a vital issue which has impact on the marine environment and the power take-off cost. Figure 3.7 shows a typical power take-off process from translational to rotational motion conversion.

3.10.2 Direct Drive Device Another power take-off process from the ocean wave is the direct drive generator. The generator mostly falls under the category of permanent magnet synchronous generator. Although there are different types of permanent magnets, rare earth magnets produce the highest magnetic flux density. On the other hand, to produce sufficient electromotive force, a high rate of change of flux is required that can be obtained from rare earth magnets. However, it is alarming that the demand of rare earth magnets is continuously growing since it has been widely used in different machineries. China and the US are in the leading position for its mining. However, due to environmental concerns, the US stops rare earth magnets mining. On this vulnerable situation of the earth magnets, oceanic wave power harvester scientist of the North Carolina State University offered an alternative solution.

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Fig. 3.7 Multistage power take-off system

Often translational motion obtained from a wave energy device is required to convert into rotation motion. Generally, it is executed with rack and pinion arrangement or using other mechanical gearing system. Traditional mechanical gear has some technical problems such as friction, noise, etc. due to its physical limitations. Application of magnetic gearing can be one of the alternative solutions. Instead of using rare earth magnets, ferrite magnets can be used in a magnetic gearing system. It is known that the cost of ferrite magnet is chipper than rare earth magnet. However, the deficiency of ferrite material is its relatively weak magnetic field compared to that of rare earth magnets. To overcome this issue, a magnetic gear using ferrite magnets is presented in [27] that can produce enough torque. Construction of this magnetic gear is shown in Fig. 3.8.

3.11 Uninterrupted Power Supply Uninterrupted power supply is another challenge for oceanic wave energy. It is required to meet electrical power demand of the customers by using intermittent wave energy sources. Figure 3.9 shows a typical power generation curve in a day where the blue shadow represents daily wave power generation and the red line indicates the daily base demand. Both curves demonstrate the vulnerability that will be impacted by the demand and supply management. The storage-based solution that might overcome the intermittent wave power generation. When demand is less than generation, power will be stored. On the other hand, at the period, where the wave

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Fig. 3.8 Magnet gearing system by ferrite material [27]

energy harnessed is less than the demand, the storage power fulfilled the local load demands. This storing is called compressed air energy process. However, power generation is a vital issue to manage power demand. When power is required, the proposed storage releases that power through a turbine. However, the excess wave power generation requires very large area of storage, besides, there is a risk of blowouts. Therefore, ocean compressed air energy storage system has come up to overcome the challenge. This energy storage system works to air being stored under the water that uses available hydrostatic pressure. Therefore, the proposed ocean compressed air storage system has fewer blowouts risk compared to the traditional one and can overcome the vulnerable supply from the wave energy system.

Fig. 3.9 Consistent power by using storage to meet baseload

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Fig. 3.10 Ocean compressed air energy storage [28]

When miners finished mining and clearing salt domes, then they left behind the caverns. It can be used to store energy that can be applied by power utility when required. The pressurized air is sent to the salt caverns when there is an excess of generated power. When there is power shortage, it can be released through a turbine. But it requires a large area to store and there is also a risk of blowout. Therefore, it can be an effective solution to use ocean compressed air energy storage if it is properly investigated and monitored. It is a little bit different from the conventional system because the stored air underwater uses hydrostatic pressure. For that reason, a large storage volume minimizes the risk of blowout. Various sections of an ocean compressed air energy storage system are illustrated in Fig. 3.10 [28]. Ocean compressed air energy storage has the potential to solve this challenge. It utilizes hydrostatic underwater pressure that can solve the intermittent problems associated with many forms of renewable energy. Thereby it provides base load power. Water movement in the ocean is complex and dynamic. It operates at a specific range. A computational design tool can be helpful for the solution associated with water particles as well as for marine hydrokinetic energy that is developed by Billy Edge Robert and Kevin Gamal’s oceanic research group. This research effort has been led by Billy Edge Robert and Kevin Gamal. It is interesting to watch this research progress because it is a big group of students coming from all over the world. They are allowed two years to solve this problem. Two different surge converters are used there to optimize the design of the device so that optimal amount of energy can be captured.

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3.12 Challenges and Solutions There are a few open ocean test facilities in the Jenette’s Pier Wave Energy Test Center in the US. Therefore, to ensure the open ocean testing device optimization, issues presented by harsh ocean environment need to be addressed. The assessment of interaction with environment, ecology, and human uses requires to be considered. To solve the problem, a shallow water platform is offered for testing wave devices. The following progressing steps are found to mitigate the challenges. • • • •

Testing of resolute Marine’s surge WEC in 2011. Permit application for 2 test berths near Jenette’s Pier submitted Demonstration of ocean compressed air energy storage to be installed Interest expressed by additional companies

3.12.1 Environment and Ecology Any wave power plant might be interacting with the environment, ecology, and existing uses of the coastal ocean so that any conflicts can be minimized and any synergies can be maximized. Suitable site selection is needed to avoid ecologically sensitive area. Culturally and recreationally important places should also be taken into consideration. The next step can be environmental and stakeholder assessment. Identifying the potential conflicts and synergies in the coastal ocean should be the next step. Its area mining for spatial extent should be considered. A research team worked on a similar assessment for wind energy offshore in the North Carolina. It is expected that this kind of assessment can be supportive for a guideline for installation of wave power plant. Therefore, interactions are a little bit different from marine hydrokinetic energy versus wind energy. In marine life, there are a lot of areas that are important for different organisms such as marine mammals, fishes, coastal birds, sea birds, sea turtles, etc. The wave power plant can endanger species such as sea turtles, Olenek sturgeon, and important habitat areas like hard bottom sargassum. Macroalgae is carried in the gulf stream current. Sometimes it washes up on the beaches, but it is important for productivity and nutrient cycling in the gulf stream. To harness energy from the sensitive areas of the ocean, devices are placed in such a way where ecological risk is low. It does not mean that devices will not be installed and explored harnessing energy from the areas where sea animals exist. It means that we need to be careful and make sure that all the risks can be avoided by searching for other forms of energy. Marine mammals, especially whales are frequently found near the North Carolina coast. Also, cultural resources like shipwrecks are placed on the sea floors. Energy harvesting devices may have an impact on marine research. Therefore, shipwrecks are another important thing that needs to be considered and avoided during site selection of devices. Wave power plant can be standalone or a part of some other project. Interaction of wave energy devices with the environment and different environmental

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processes need to be pointed out. The effect of any project should be carefully measured for the safety of marine environment.

3.12.2 Economic Aspect Economic analysis is usually constructed for a flexible cost model that allows anyone to investigate how key factors affect cost. From the study it is found that cost minimization depends on identifying high quality ocean energy resources with geographical precision (resource assessments), the scale, scope of offshore installing, national energy policies, project longevity, capital operation, and maintenance cost. Improvements in several areas would be necessary to make it competitive with wind and solar energy technology.

3.12.3 Installation-Related Factors Before installation of any power plant, there are several factors to be considered. Wave power plants are not an exception in this regard. The following analysis should be conducted before any wave power plant installation. • • • •

Complexity, expense, project time duration, risk factors, etc. Scale of the wave power plant Lack of information due to novelty Mitigation of environmental and ecological impact

After installation of wave power plant, the stakeholders need to deal with the following factors. • Engaging communities whose livelihoods and reaction could potentially be affected • To learn about concerns • To allow researchers to utilize information from specialized knowledge

3.13 Summary The climate is changing drastically around the world. Everyone is concerned about climate change. Burning fossil fuel is one of the major reasons in that issue. Therefore, reduction of fossil fuel burning is a must to reduce carbon emission rate. To meet the increasing power demand renewable energy resources can play a vital role. Soon, renewable energy resources such as solar, wind, and ocean waves are going to be the main victuals to produce electrical power. Therefore, the use of renewable energy resources must be increased throughout the world. In the present day,

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many researchers and researching organizations have invented a new oceanic wave energy converter device by using technical knowledge and theory. Pelamis is one of them and it is one of the oldest (version) devices among all oceanic wave energy converter devices. Electrical generators installed in Pelamis or any other device must be efficient to produce adequate electricity. Although Pelamis is a good converter device, it has some disadvantages too. It blocks more area compared to its generation capacity and is harmful to marine life. As a result, it is now an open challenge to think about the possible solution. Therefore, researchers need to improve the size, shape, and design of this device to make it productive and efficient. The ocean environment must be clean, unpolluted, and protected from harmful things ensuring the safety for all the living beings near the oceans. Finally, the renewable energy converter device creates a new dimension in recent electrical power generation if it is implemented successfully with proper research work.

References 1. Rahman A, Farrok O, Haque MM (2022) Environmental impact of renewable energy source based electrical power plants: solar, wind, hydroelectric, biomass, geothermal, tidal, ocean, and osmotic. Renew Sustain Energy Rev 161(112279):1–29. https://doi.org/10.1016/j.rser.2022. 112279 2. Molla S, Farrok O, Alam MJ (2023) Zero Carbon emission based electrical power plant by harvesting oceanic wave energy: minimization of environmental impact in Bangladesh. In: Climate change and ocean renewable energy. CCORE 2022. Springer proceedings in earth and environmental sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-26967-7_1 3. Inhabitat, Portugal builds world’s first commercial-wave-farm. [Online]: https://inhabitat.com/ portugal-builds-worlds-first-commerical-wave-farm. Accessed 15 Dec 2020 4. Pelamis wave energy converter (Online) In Wikipedia: https://en.wikipedia.org/wiki/Pelamis_ Wave_Energy_Converter. Accessed on 3 May 2020 5. Pelamis Wave Power (2011) Pelamis at Agucadoura [Online]: YouTube. https://www.youtube. com/watch?v=slawyq4PXxE. Accessed 3 May 2020 6. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 7. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J (2018) A split translator secondary stator permanent magnet linear generator for oceanic wave energy conversion. IEEE Trans Ind Electron 65(9):7600–7609. https://doi.org/10.1109/TIE.2017.2767521 8. Molla S, Farrok O, Alam MJ (2022) J material based lightweight linear electrical generator with improved dynamics for harvesting oceanic wave energy. In: 12th International conference on electrical and computer engineering (ICECE), Dhaka, Bangladesh, pp 92–95. https://doi. org/10.1109/ICECE57408.2022.10089129 9. Bashir MS, Farrok O (2019) Generation of electrical power by using high graded permanent magnet linear generator in wave energy conversion. In: 1st International conference on advances in science, engineering and robotics technology (ICASERT), pp 1–5. https://doi.org/10.1109/ ICASERT.2019.8934591 10. Molla S, Farrok O, Alam MJ (2023) Translator optimization of a linear electrical generator for harvesting oceanic wave energy. In: IEEE IAS global conference on emerging technologies (GlobConET), London, United Kingdom, pp 1–6. https://doi.org/10.1109/GlobConET56651. 2023.10150079

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11. Kiran MR, Farrok O, Guo Y (2019) Superconducting linear machines for electrical power generation from the oceanic wave. In: Advanced linear machines and drive systems, pp 281– 302. https://doi.org/10.1007/978-981-13-9616-8_8 12. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Lei G (2017) A novel method to avoid degradation due to demagnetization of PM linear generators for oceanic wave energy extraction. In: 2017 20th International conference on electrical machines and systems (ICEMS), pp 1–6. https://doi.org/10.1109/ICEMS.2017.8056471 13. Molla S, Farrok O, Islam MR, Muttaqi KM (2020) Application of iron nitride compound as alternative permanent magnet for designing linear generators to harvest oceanic wave energy. IET Electr Power Appl 14(5):762–770. https://doi.org/10.1049/iet-epa.2019.0372 14. Kiran MR, Farrok O, Mamun MAA, Islam MR, Xu W (2020) Progress in piezoelectric material based oceanic wave energy conversion technology. IEEE Access 8:146428–146449. https:// doi.org/10.1109/ACCESS.2020.3015821 15. Haque MM et al (2023) Three-port converters for energy conversion of PV-BES integrated systems—a review. IEEE Access 11:6551–6573. https://doi.org/10.1109/ACCESS.2023.323 5924 16. Energy policy of the United Kingdom (Online) In Wikipedia: https://en.wikipedia.org/wiki/ Energy_policy_of_the_United_Kingdom. Accessed 25 Dec 2020 17. Ocean Wave Energy [Online]: https://youtu.be/ovw-pHqyP7E. Accessed 30 Sep 2023 18. Mapping and Assessment of the United States Ocean Wave Energy Resource. [Online]: https:// www.osti.gov/biblio/1060943. Accessed 30 July 2023 19. Greening the grid, Overview of grid integration issues. [Online]: https://greeningthegrid.org/ quick-reads. Accessed 26 July 2019 20. Coastal Studies Institute (2013) Science on the sound series: renewable ocean energy for North Carolina [Video] YouTube. https://www.youtube.com/watch?v=_VNjVCFumjQ. Accessed 26 July 2019 21. Renewable energy, Ocean wave technologies. [Online]: https://baonguyen1994.wordpress. com/introduction-to-wave-energy/ocean-wave-technologies. Accessed 26 July 2019 22. Forcedgreen, A green energy red sea serpent [Online]: http://www.forcedgreen.com/2010/06/ a-green-energy-red-sea-serpent. Accessed 26 July 2019 23. Yamm R, Pizer D, Retzler C, Henderson R (2012) Pelamis: experience from concept to connection. Philosophical transactions. Ser A Math Phys Eng Sci 370(1959):365–380. https://doi.org/ 10.1098/rsta.2011.0312 24. Mindseyecreative (2007) Pelamis Wave—35th scale model testing [Video] You Tube. https:// www.youtube.com/watch?v=s_52_aZqB1Y. Accessed 25 Nov 2007 25. ABENGOA RESEARCH, Technical assessment of the Pelamis Wave Energy Converter concept (Online). https://www.researchgate.net/profile/Jannette_Frandsen/publication/317 014924_Technical_assessment_of_the_Pelamis_wave_energy_converter_concept/links/596 09e9a458515a357df002c/Technical-assessment-of-the-Pelamis-wave-energy-converter-con cept.pdf. Accessed 20 Aug 2012 26. Atargis, Cyc. WEC Design Features. [Online]: https://atargis.com/CycWEC.html#Torque. Accessed 25 July 2019 27. Uppalapati K, Bird J (2012) A flux focusing ferrite magnetic gear. In: 6th IET international conference on power electronics, machines and drives (PEMD 2012), Bristol, pp 1–6. https:// doi.org/10.1049/cp.2012.0303 28. Lim SD, Mazzoleni AP, Park J-k, Ro PI, Quinlan B (2012) Conceptual design of ocean compressed air energy storage system. In: 2012 Oceans, Hampton Roads, VA, USA, pp 1–8. https://doi.org/10.1109/OCEANS.2012.6404909

Chapter 4

Resonant Wave Energy Converter Md. Mahedi Hasan Sujon, Md Ahsan Kabir, Md Sawkat Ali, Md. Abdullah-Al-Mamun, and Omar Farrok

Abstract To harvest wave energy, different processes are being investigated throughout the world. Among them, the resonant wave energy converter (WEC) has been developed where power generation efficacy depends on the resonant effect. This chapter explains concept and explanation of the resonance effect of wave energy converter. Placement of resonant and other WECs is discussed in the beginning of this chapter. Resonant WEC is usually a floating type of device which can be placed near shore. It is found that resonant WEC can enhance the amplitude of the swinging buoy with comparatively less effort than the conventional one. An experimental setup of a wave basin is presented, which is used for testing resonant WECs. Its various components and setup for parametric measurements are illustrated. Phase control issue is one of the key factors for this WEC. At the end of this chapter, necessity of implementing resonant WEC and other effective renewable source-based power plant are explained. Keywords Linear absorber · Point absorber · Resonant effect · Resonant wave energy · Wave energy converters

Md. Mahedi Hasan Sujon · Md. Abdullah-Al-Mamun (B) · O. Farrok Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected] O. Farrok e-mail: [email protected] M. Ahsan Kabir Department of Electrical, Electronic and Communication Engineering, Military Institute of Science and Technology, Dhaka 1216, Bangladesh e-mail: [email protected] M. Sawkat Ali Department of Computer Science and Engineering, East West University, Dhaka 1212, Bangladesh © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_4

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4.1 Introduction The electricity generation from renewable energy sources is increasing day by day for its cleanliness and environment-friendly nature compared to fossil fuel-based power plants. Oceanic wave energy is one of the remarkable renewable energy sources. A high yield of energy can be obtained by using oceanic waves. Present world is concerned about electricity generation from traditional fossil fuel due to environmental issue. At present, the non-renewable energy sources like coal, oil, and natural gas are used to meet almost 70% of global energy demand. Therefore, the environment is affected as the burning of fossil fuels produces CO2 and other greenhouse gases that are responsible for global warming [1]. For an example, it is found that, around 42% of NOx and 38% of SO2 have been emitted from coal-based power plants in China in 2015. As a result, average global temperature would increase rapidly for the enormous emission of greenhouse gases. Unfortunately, significant amount of electricity is still generated by using coal even in this modern age. On the other hand, renewable energy resources (RES) such as solar, wind, hydro, biomass, and wave energy are replenished in nature to meet zero carbon emission target for the environment [2]. Additionally, the UN sustainable development goals such as poverty reduction, improvement for good health, excellence in education, sanitation, and clean water goals can be achieved by using sustainable energy that mainly depends on the progression of renewable energy technologies. Oceanic wave energy can produce around 1–10 TW of electrical power worldwide. Wave energy has the benefit of having high power density that is predictable and available throughout the year. However, during energy conversion from renewable energy sources, one of the main challenges is to maintain continuous electricity supply. For example, during a cloudy day, solar energy-based devices have limited power generation capacity. This type of resource has no crank up capability when more loads are connected suddenly. To solve this issue, battery storage system is used as an instantaneous power backup for the solar-based power systems. The solar system with battery storage makes the system cost higher. Thus, the economic issues come in front while compared to other conventional systems. Besides, lifetime of the battery energy storage is also a major concern for this type of renewable energy source. On the other hand, nuclear energy is an alternative clean energy source that uses nuclear fission. The generated power from the nuclear plant has no link up with air pollution by emitting carbon dioxide to the environment. Thus, large-scale nuclear power plants have installed continually to fulfill the energy demand all over the world in parallel to solar and wind energy sources. In 2018, researchers from the Massachusetts Institute of Technology, Massachusetts, USA, raised an issue that without incorporating nuclear energy to renewable energy sources, the decarbonate cost will be increased, and the environment will suffer in the long run. However, nuclear energy resources have some impacts on the environment because of using the radioactive uranium as a catalyst. Naturally limited availability of uranium, its mining, extraction, and

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waste management of the by-products of the nuclear reactor are some of the major limitations of nuclear power plant. To overcome the constraints, researchers are now looking for wave energy as an alternative source of renewable energy. Almost 71% of the earth contains water and it is possible to produce more electrical power by its utilization with minimal environmental impact. In the twenty-first century, several investigations on wave energy harvesting have been conducted. Now many concepts are being developed and implemented for full-scale sea trials. Besides, to harvest more wave power, some of the methods have already been tested. Some other methods are under improvement and upgradation stage. From the tested wave energy harvesting process, one of the leading concepts is resonant wave energy converter (WEC). Like other RESs it has no running cost for electricity generation. Only installation cost is required to harvest wave energy. Generated energy from resonance converter is highly predictable, and eco-friendly. In this chapter, state of art of electricity generation from the oceanic wave energy including the resonant wave energy converter and its benign response are discussed. It presents physical construction and operating principle of resonant WEC. Power conversion, its possible outcome, challenges, and solutions are also depicted.

4.2 State of the Art of Wave Energy Device Waves are generated by the wind flow across the ocean and from the regular tidal forces formed by the gravitation with the moon due to earth rotation on its axis. In comparison to other renewable resources such as solar and wind, waves are more certain, steadier, and available. The density of wave energy is quite higher than solar and wind energies. In comparison to nuclear power, the wave energy potential is comparable to the global nuclear-installed capacity. The electricity demand and wave power both vary over the year. In addition, the electrical power generated from wave energy can be supplied to around 60% of the world’s population who are living across 50 miles of coastline along with freshwater. The wave energy is capable of powering up the process of desalination as many plants are needed to install on coastal areas to access freshwater [3]. To extract wave power from the ocean, different prototypes such as offshore, onshore, and near shore devices are designed and mounted or in operations. Shoreline devices are placed on the water surface. On the other hand, some nearshore devices are floating, and some of them are placed at the bottom. The offshore devices are mostly submerged. Resonant wave energy devices are mostly floating type and they can be placed nearshore. Figure 4.1 illustrates the schematic presentation of the position of shoreline, nearshore, and offshore devices. It is seen that oscillating water column device is situated onshore whereas Archimedes wave swing, pelamis, and the resonant wave energy device are located not too far from the shore. In this Fig. 4.1, a Power buoy which is a point absorber type device exhibits resonance effect.

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Fig. 4.1 Different locations of wave energy devices from the shoreline to nearshore

To design wave energy devices, the structural efficiency (measured in MWh/ t) is used in the power industry to eradicate different inconsistent initial design conventions. Up to now, around 1–1.5 MWh/t of wave devices are tested in the ocean whereas the wind turbine structural efficiency is found at 10 MWh/t. To make the wave power structure efficient and cost effective, several technical initiatives are needed to study. A compact highly efficient wave energy converter has been developed that was initially inspired by the pumping principles. Table 4.1 presents a comparison of efficiency for multiple wave energy devices [4, 5]. Various electrical generators are used for converting wave energy into electrical energy. Linear electrical generator can be mounted to a direct drive device. Magnetic excitation of the linear generator used for wave energy conversion is generally obtained by permanent magnet [6]. It is found that dynamics of a lightweight translator are better than the conventional massive translator for utilizing oceanic wave [7]. Table 4.1 Comparison of structural efficiency of different wave energy devices [4, 5] Wave energy device

Low

Medium

High

Approximated value of absorbed energy/mass (MWh/t) Small bottom reference heaving buoy

0.75

0.9

1.15

Bottom reference submerged heave buoy

0.6

0.95

1.2

Floating two body heaving converters

0.2

0.35

0.5

Bottom fixed heave buoy array

1.2

1.55

1.9

Floating heave buoy array

0.6

0.7

0.9

Bottom fixed oscillating flap

0.9

1

1.4

Floating three bodies oscillating flap

0.45

0.7

0.9

Floating OWC

1.05

1.55

2.1

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Overall performance of the wave energy conversion by a linear generator depends on several factors such as its design [8], armature winding, suitable ferromagnetic core, high graded permanent magnet, optimization [9], cooling system, control, protection system, isolation, prevention of demagnetization [10], etc. The linear electrical generator is suitable for a point absorber type power take off system. Therefore, resonance behavior is attainable for this type of system.

4.3 Resonant Effect in Oceanic Wave The phenomenon of tide exasperation in a vibrant mode of the ocean is usually known as tidal resonance in the study of oceanography. A tidal wave is reinforced between the coast and shelf edge by producing a span of tidal range in the sea beach. The consequence of tidal wave is found significant when a continental sand bed is about a quarter wavelength wide. In the Bay of Fundy in the Northwest part of Canada, the world’s extreme tides are found. The other categorical areas with extreme tides are found in the Patagonian Shelf and the inland shelf of Northwest Australia [11]. Therefore, to get the optimum power from the resonance effect, the recent developments in wave energy production have been pointed out in the following sections.

4.3.1 Resonance as a Swing The hydrodynamics of a wave power device describes the natural period of oscillations of the oceanic wave. Theoretically, the amplitude of vertical swing of a float/ buoy on the ocean can be less than or equal to the amplitude of the oceanic wave. But it is possible to obtain more vertical swing by the same float using resonance effect. For example, the swinging of a child creates a positive feedback loop which makes the child reach a higher level at each swing. It is illustrated in Fig. 4.2. In other words, the resonance effect can be explained by this swinging. Because, a wider amplitude can be achieved with comparatively less effort by this technique. Similarly, when a device is on its upswing, the lifting force from the incoming wave is found at the same time and the device wants to restore itself to a lower position naturally. Therefore, a positive feedback loop is formed and the device can jump in and out of the water giving linear motion. Then, the wave energy is produced and the spring synchronizes and maintains this resonant effect for different sea states. It can be achieved by latching and unlatching of a buoy followed by the incident wave. The wave spring is considered as a significant tool for future wave power that achieves and maintains the resonance effect with the incoming waves. It amplifies the motion of the device considering the wave time duration or the direction which produces greater power output. A highly efficient gearbox is now commercially available to cope up with the resonant WEC.

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Fig. 4.2 Swinging of a child as an explanation of resonance effect

4.3.2 Tank Testing of Wave Energy Device The phase control system is a key part of a wave energy device which is composed of phase control on and off. The system is turned on and off as required. This phenomenon provides the idea of storm survivability. This device can synchronize naturally, and the waves get beyond a comfortable level for the device in a similar way that wind turbines pitch back into the wind. It can be detuned, confiscate the resonant effect, and greatly lower the efficiency. Therefore, it lowers the loading on wave energy device and makes it much more survivable.

4.3.3 Stage 3 Pilot Project The test site was selected for the current pilot project at the Northern Scotland for a prodigious wave condition and the duration of test sites functionality was more than 10 years. The pilot project was conducted with a partnership with the ridge Rolla Spanish utility company. There is stage 3 pilot of high efficiency wave power and following sections describe the test system.

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4.3.4 Full System Test Rig A full system test rig is considered as the entire wave power device. It has turned it sideways and implemented in full storm conditions. The process is composed of verification and debugging of all defined functions and de-rising the west test program by rigorous dry testing including full storm loading on land.

4.3.5 Half Scale Device The half scale device was designed at industrial level. The composite buoy is under the process of modulating testing and unit assembly. The full scale device is expected to be tested in a short period of time.

4.4 Experimental Setup of Wave Energy Converter Oceanic wave is considered as a global resource which draws more attention around the world for its enormous energy. Europe is in the leading position because of existing funding and partnership where other continents are increasing their attention on oceanic wave. In this section, different experimental setups have been introduced to get well optimized wave power which will be discussed one after another. An individual wave energy converter is known as a point absorber device. It is mainly constructed with three main parts (a) a curved ended buoy of diameter 31.5 cm and draft of 31.5 cm with a total height of 60 cm (b) a vertical steel shaft of 40 mm square section enclosed in a gravity metal base, (c) a friction brakes based on power take off (PTO) system comprising a few blocks and 4 linear springs. The dry mass of the buoy is 20.49 kg and the response time of regular wave is 1.176 s as calculated by decay test during natural period. The PTO system is set in the top of the buoy which is a horizontal cover made of polyvinyl chloride. In addition, a measuring instrument is connected to calculate the heave displacement of the device. The initial prototype testing of a few WEC units was completed in the workshop of Ghent University and later, twenty-five identical WEC units were built in the same lab. The initial results of prototype testing show a judicious value of two parameters such as response amplitude operator and power output, in comparison to the predicted measured linear time domain model. Complete description of the unit development, assessment, and experimental study for the WECwakes project can be found in Stratigaki et al. [12]. Its plan view is illustrated in Fig. 4.3. The dotted region along the x-axis at the bottom indicates the extent of wave paddles. At the upper side, the wave absorbing slope is indicated with a hatched area. Vertical guide walls are used at both of its sides. Water depth is 0.7 m. 5 × 5 WEC rectilinear array [12]

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Fig. 4.3 Plan view of the experimental arrangement in the wave basin and standard

Guide walls were installed on both sides of the wave basin as indicated in Fig. 4.3 to minimize spreading of the produced waves. It causes a relatively greater effective domain for this experiment. They are stretched 2 m beyond the absorbing slope so that the generated waves are not returned back to the wave basin. Thus, it ensures directional wave propagation.

4.4.1 The Shallow Water Wave Basin An experiment of DHI in the shallow water wave basin is conducted in Horsholm, Denmark. The empty wave basin is constructed with 25 m long, 35 m width, and 0.8 m water depth test facility. There are forty-four piston type wave paddles with 1.2 m height and 0.5 m width of each paddle to produce waves at one end of the basin which is presented in Fig. 4.4. Then, total width of the wave generator is measured at 22 m. The paddles are set in two segments where each segment dimension is 18 and 4 m with a 20 cm step between them [12]. It is designed to operate at the water depth between 0.2 and 0.8 m.

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Generator array

Wave basin

Wave padel

Fig. 4.4 Side view of the wave basin at DHI (Hørsholm, Denmark) [12]

The active wave absorption control system is installed in the wave generator to control the unexpected re-reflection of waves and deal only with incident waves. On the opposite side of the wave basin, a gravel beach with a slope of 1/5.59 is built for wave energy absorption. An array of the wave energy converter is shown in Fig. 4.5a. The following components are required to install WEC units in a wave basin: (i) WEC steel vertical shafts (ii) connecting steel frame and (iii) the WEC metal gravity bases. The components are labeled in Fig. 4.5b.

4.4.2 Experimental Arrangement of the WECWake The WEC units are tested separately at different locations inside the basin after intricacy of tested array layout increases steadily. Therefore, multiple WECs have been tested with several geometric arrangements. Schematic view of the general test configuration including 5 by 5 WEC rectilinear array is presented in Fig. 4.3. The inter center spacing between WECs is lateral (w) = longitudinal (l) = 5D = 1.575 m where WEC diameter D = 0.315 m. In the wave basin, total length of the wave generator is 22 m and cannot spread across the entire basin width of 35 m. Therefore, the vertical guide walls are built in both sides of the basin to evade diffraction of the generated waves. This installation of vertical guide walls inside the basin produces a specific operative area and simplifies

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Fig. 4.5 Rear view (from the wave generator). a 5 × 5 wave energy converter rectilinear array and b its various components [12]

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the numerical analysis of the setup like the uses of complete reflective boundaries for simulation. The internal space between the outermost WECs of the array and the guide walls is approximately 5 × 5 D = 25 D = 7.875 m. As a result, less effect is observed as the wave reflections are scattered and radiated by the array.

4.5 Instrumentation and Acquired Data To measure wave elevation, the resistive wave gauges (WGs) are used at different positions inside the wave basin. According to the tested WEC array geometric arrangement, forty-one WGs are used at different specific areas of the WEC units. To measure the wave directionality and reflection, a CERC 5 wave gauge array is installed in front of the WEC arrays. To conduct the wave field measurement, two WG plans have been used. The first WG plan is used for the recording of wave elevations near the WEC units whereas the second one is used in all locations where WEC units have been installed and tested within all WEC (array) arrangement. Test arrangement for this setup is depicted in Fig. 4.6. In the experimental setup of second WG plan, all WEC units and additional structures have been replaced by the wave gauges of first WG plan at the center points. Then, a unique identification number is allocated to forty-one WGs. The undistributed wave field has been recorded in an empty wave basin (without any WEC or support structure) using both WG plans [12].

4.5.1 Measurements of the Heave Displacement and Surge Force To estimate the time varying heave displacement, a total of 25 potentiometers have been used by installing the device at each WEC unit as shown in Fig. 4.5. This analysis helps to get the WEC response information and data for scheming power absorption of the WEC units [12]. To measure the wave induced surge force on each WEC unit, two load cells are attached at the upper and lower part of WEC shaft. They are connected to an auxiliary parallel axis and in the longitudinal direction of the wave basin. The sum of the recorded signals is extracted from the load cells to calculate the surge force on a WEC unit. Then, the surge force is measured on five WECs positioned in the central column of the WEC array. This surge force measurement experiment is conducted with ten load cells and constructed at the workshop of Ghent University [12].

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Fig. 4.6 Test arrangement of the experimental setup [12]

4.5.2 Video Acquisition The video recording (with 40 frames per second) has been established at all WEC array arrangements. The first place is located behind the wave generator and the second place is at the opposite end of the basin, behind the wave absorbing beach [12].

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4.6 Challenges Energy is the backbone of modern civilization. It has made a huge contribution to the global economy. According to the US chamber of commerce, the US energy production stimulated a manufacturing renaissance by adding yearly $20 billion to the economy and a huge number of new jobs. For example, the new projects on the US Gulf Coast by ExxonMobil are expected to create more than 45,000 jobs. It is predicted that the global energy demand is going to increase by a huge amount. One of the main reasons for this expansion is that the global population growth is expected to be close to 2 billion. The second one is repaid expansion of middle class in emerging economies which will make double the world economic output. Therefore, this uprising demand creates two significant challenges: ensuring uninterrupted energy supply to the consumers when needed and effectively resisting environmental degradation. This challenge also arises on the renewable energy sources to make it economic and reliable. Switching to more sustainable sources in wave power manifests itself in a unique challenge of designing devices. The devices should be robust enough to survive under natural disaster while also being efficient enough to provide a significant amount of energy conversion. Therefore, the climate impacts need to be addressed seriously by all related stakeholders such as the consumer, company, and government. Several global organizations, for example, ExxonMobil have taken initiatives to reduce carbon emission, carbon capture, and storage according to the Paris Accord and consider alternative energy efficiency measures [13]. These challenges can be mitigated by implementing resonant WEC and other effective renewable source-based power plants. At present, the existing WECs are mostly found in small-scale applications. With the advent of research and development it is expected that it would be possible to generate large-scale electricity in the near future.

4.7 Conclusion Resonant wave energy converter is relatively a newer concept of harvesting wave energy. There is a total of six degrees of freedom, but mostly the heave motion is considered for this kind of energy converter. Although the oceanic wave is approximated as sinusoidal, practically two or more linear sinusoidal waves form an irregular wave motion. Sometimes, resonant wave energy converter may perform better even for the irregular waves because of its resonance behavior. The main category of the wave energy converter as presented in this chapter is point absorber type. Therefore, a direct drive power take off system could be utilized.

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References 1. Kabir MA et al (2023) Net-metering and Feed-in-Tariff policies for the optimum billing scheme for future industrial PV systems in Bangladesh. Alex Eng J 63:157–174. https://doi.org/10. 1016/j.aej.2022.08.004 2. Molla S, Farrok O and Alam, MJ (2023) Zero Carbon emission based electrical power plant by harvesting oceanic wave energy: minimization of environmental impact in Bangladesh. In: Climate change and ocean renewable energy. CCORE 2022. Springer Proceedings in Earth and Environmental Sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-26967-7_1 3. Rahman A, Farrok O, Haque MM (2022) Environmental impact of renewable energy source based electrical power plants: solar, wind, hydroelectric, biomass, geothermal, tidal, ocean, and osmotic. Renew Sustain Energy Rev 161(112279):1–29. https://doi.org/10.1016/j.rser.2022. 112279 4. Resonant Wave Energy Converter by CorPower Ocean: Sweden—2016 Ocean Exchange Finalist. https://www.youtube.com/watch?v=NVwye-ZwZNA&t=772s. Accessed 5 Apr 2019 5. AAAS MemberCentral, Advancements in wave energy with Belinda Batten. https://www.you tube.com/watch?v=3vz8xma45yM. Accessed 23 Dec 2020 6. Molla S, Farrok O, Islam MR, Muttaqi KM (2020) Application of iron nitride compound as alternative permanent magnet for designing linear generators to harvest oceanic wave energy. IET Electr Power Appl 14(5):762–770. https://doi.org/10.1049/iet-epa.2019.0372 7. Molla S, Farrok O, Alam MJ (2022) J material based lightweight linear electrical generator with improved dynamics for harvesting oceanic wave energy. In: 12th international conference on electrical and computer engineering (ICECE), Dhaka, Bangladesh, pp 92–95. https://doi. org/10.1109/ICECE57408.2022.10089129 8. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 9. Farrok O, Islam MR, Sheikh MRI, Xu W (2016) A new optimization methodology of the linear generator for wave energy conversion systems. In: 2016 IEEE international conference on industrial technology (ICIT), pp 1412–1417. https://doi.org/10.1109/ICIT.2016.7474965 10. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Lei G (2017) A novel method to avoid degradation due to demagnetization of PM linear generators for oceanic wave energy extraction. In: 2017 20th international conference on electrical machines and systems (ICEMS), pp 1–6. https://doi.org/10.1109/ICEMS.2017.8056471 11. Tidal resonance (Online) in Wikipedia. https://en.wikipedia.org/wiki/Tidal_resonance. Accessed 13 Apr 2020 12. Stratigaki V et al (2014) Physical modelling of wave energy converter arrays in a large-scale wave basin: the WECwakes project. In: Proceedings of the HYDRALAB IV joint user meeting, Lisbon, pp 1–11 13. The future of energy-opportunities and challenges. https://energyfactor.exxonmobil.com/per spectives/the-future-of-energy-opportunities-and-challenges. Accessed 25 July 2019

Chapter 5

Mathematical Model, Design, and Cost Analysis of a Linear Electrical Generator Mohamud Mohamed Farah, Omar Farrok, and Mahamudul Hasan Uzzal

Abstract The simplified mathematical model of a flux switching linear electrical generator for wave power extraction is presented in this chapter. Then design and simulation of a double-sided flat flux switching linear electrical generator (FSLEG) is presented. Characteristics of the FSLEG for harvesting oceanic wave energy are analyzed. To enhance the performance of FSLEG, a special Kool Mμ powder core with N46SH permanent magnet is applied. It is found from the simulation that because of using Kool Mμ powder core, core loss is minimized. On the other hand, power generation is reduced. To increase power generation, high graded N46SH material is applied again. Thus, with proper combination of Kool Mμ and N46SH, both parameters are improved, i.e., increase in electrical power generation and decrease in core loss. Cost analysis is provided for the active material, i.e., copper, permanent magnet, and magnetic core. Then the tentative material cost of the FSLEG is calculated. As application of Kool Mμ powder core to the linear electrical generator is relatively new, future recommendation is listed at the end of this chapter. Keywords Cost analysis · Design · Flux switching linear generator · Linear electrical generator · Mathematical model

5.1 Introduction In this chapter, a double-sided flat flux switching linear electrical generator (FSLEG) with advanced magnetic materials is analyzed. The core size is reduced and power generation is increased in the proposed FSLEG by applying advanced magnetic materials. However, if it is needed to produce more power (kW range), a larger size of generator can be applied. It is known that generator size and its power output are proportional to each other. Power rating can also be increased by cascading multiple small generators. FSLEG is a specific type of linear electrical generator M. M. Farah · O. Farrok (B) · M. H. Uzzal Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_5

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in which magnetic field is usually created with a permanent magnet and electrical power is produced by magnetic flux switching during its operation [1]. It has several advantages over the other linear electrical generators. A few of the benefits are its simple constructional architecture and necessity of using only a permanent magnet for each individual unit. From the literature review of linear generators, it is found that uses of existing permanent magnet (PM) and magnetic core in FSLEG cause high core loss and less power generation [2]. Based on this research gap, there is a lot of scope for its upgradation. Mathematical model and design procedure of a linear electrical generator is depicted for different oceanic wave conditions in [3]. A few parameters of the linear generator from different research works are analyzed. Common parameters of a linear generator are stroke length, frequency, output power, voltage, current, etc. They often studied to select suitable materials for the proposed FSLEG. Moreover, various types of hard magnetic materials, such as ferrite PM, neodymium iron boron (NdFeB), and samarium cobalt (SmCo), have been studied. It is found that NdFeB permanent magnet is mostly used for designing linear electrical generator because of its high magnetic energy product [4]. The magnetic properties, applications, strength, and weakness of each of these materials were investigated. Characteristics of different types of magnetic materials used in electrical machines are analyzed in Rahman et al. [2]. A novel design optimization methodology with a characteristic curve of the magnetic material is presented in Farrok et al. [5]. Then by applying this method to a test FSLEG, its performance is increased. In this context, analysis of an FSLEG is illustrated in this chapter, which is based on the design as presented in Farah et al. [6]. Proper magnetic materials and parameters are selected for the FSLEG design. Then the FSLEG is analyzed using finite element method by ANSYS/Ansoft software. It splits the whole linear generator into smaller parts that are known as finite elements. The finite elements of the linear generator are determined by fine mesh setup. The physical forms of finite elements are triangular and tetrahedral for two-dimensional and three-dimensional analysis, respectively. Maxwell’s equations are essentially a set of four differential equations that form the theoretical basis for describing classical electromagnetism. Results of the finite elements are gathered into a larger system of equations by this software that models the entire design. Contents of this chapter are listed in the following. • The major drawback of the existing FSLEG used for electricity generation from oceanic wave energy is the application of conventional magnetic materials that cause more core loss and low power output. • With the application of ordinary core and PM, Kool Mμ powder core with ordinary PM, and Kool Mμ powder core with advanced PM, the performance of FSLEG for wave energy conversion is analyzed and further enhanced. • By using suitable magnetic materials, Kool Mμ powder core with N46SH in the FSLEG, the power generation is increased while core loss is minimized. • The tentative material cost of the FSLEG is calculated for copper, permanent magnet, and magnetic core.

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5.2 Wave Power Formula Under the ocean where depth of the water exceeds half the wavelength, the wave power is   ρg 2 2 kW 2 Hm0 Pw = Te H Te ≈ 0.5 3 64π m0 m .s

(5.1)

where Pw is the wave power per unit of wave crest length, H m0 is the wave height, T e is the period of wave energy, ρ is the density of water, and g is the gravitational acceleration. The Oceanic wave power is proportional to the period of wave energy and the square of wave height, according to the previous formula. The wave power in kilowatts per meter of wavefront length is calculated when the significant wave height is measured in meters and the wave period is measured in seconds. For instance, let us consider a medium ocean wave with a wave height of 3 m and a wave energy period of 8 s in the deep ocean which is a few kilometers from the coast. By applying the formula, the power can be calculated as Pw ≈ 0.5

kW kW (3m)2 (8s) ≈ 36 3 m •s m

(5.2)

which means there are 36 kW of possible output power per meter of oceanic wave front. The highest offshore waves are over 15 m high in big storms and have a time cycle of around 15 s. The waves deliver around 1.7 MW of power through each meter of wavefront for such conditions. It is determined by applying the same formula. Mathematical description of the oceanic wave is expressed in Molla et al. [3].

5.3 Wave Energy and Wave Energy Flux The overall power density per unit area of the waves on the ocean water surface can be expressed by Ew =

1 2 ρg Hm0 16

(5.3)

where E w is the overall wave power density per unit horizontal area (J/m2 ), the sum of potential and kinetic energy density per unit horizontal area. The potential power density is equivalent to the kinetic energy, they contribute half of the wave energy density of E, as can be assumed from the equipartition theorem. Surface roughness impacts on oceanic waves are very small for wavelengths larger than just a few decimeters. The wave energy is transmitted as the wave propagates. The wave power of this propagation is as

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Pw = Ecg

(5.4)

where cg is the group velocity. Because of the dispersion relation of oceanic waves under the effect of gravity, the group velocity is dependent on the wave period or the wavelength λ. Additionally, the dispersion relationship is also a function of the ocean depth. Consequently, at the limits of deep, shallow water, and at moderate depth, the group velocity works differently.

5.4 Oceanic Wave and Linear Generator Usually, the dynamics of a wave energy are considered sinusoidal for numerical study. If the vertical position of wave is zw (t), the wave velocity is vw (t), the initial phase angle is θ j , the wave amplitude is Aw , and the frequency is f w , the motion of wave in the ocean can be described by the following formula. Fnet = m

d2x = −kx dt 2

vw (t) = 2 Aw π f w cos(2π f w t ± θ j )

(5.5) (5.6)

In a simplified example, a translator attached to a buoy closely follows the surface of the wave. The buoy location, however, varies from the location of the wave surface due to the armature reaction. Because the location and velocity of the buoy vary from those of the wave, the movement of the translator or buoy is represented by two state variables: displacement (ztr ) and velocity (vtr ). Velocity is considered to represent the motion of the translator or buoy. As a result, the motion can be described as follows. d d (z tr ) = vtr ; (vtr ) = atr dt dt

(5.7)

d2 ρgπa 2 [z tr − z w (t)] + (Rr + Rv )[vtr − vw (t)] = (z ) tr Mb + Mw + Mtr dt 2

(5.8)

where the translator acceleration is atr , the mass density of ocean water is ρ, the buoy radius is a, the radiation resistance is Rr , the viscous resistance of sea water is Rv , the buoy mass is M b , the added mass due to the sea water is M w , and the translator mass is M tr . The values of Rr and Rv are calculated using hydrodynamics. It is clear from (5.2.8) that atr reduces as the translator’s mass increases, resulting in a reduced translator speed. vtr and atr are minimized due to the reduced applied force. There is a relationship between vtr and the production of electrical power, that is described in (5.2.9). If vtr is increased, the generated electromotive force will also be increased and vice versa. In other words, atr is inversely proportional to weight for a certain elevation force generated by the buoy. Thus, a heavy translator would result

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in a smaller peak velocity or less electrical power generation for a given length of movement. As a result, the translator with a larger mass is the cause of low electricity output. The power generation of linear electrical generator and its relationship to the oceanic wave are depicted in Farrok et al. [1] and Mollaet al. [3].

5.5 Electricity Generation of the FSLEG A flux switching linear electrical generator along with the phasor diagram is sketched in Fig. 5.1. If the tooth pitch is τ tr , and the voltage constant is K E , the induced voltage per winding per phase, E ϕ of the linear electrical generator can be written as 

 z tr E ϕ = K E cos π vtr τtr where K E = N W T Bair Ws vmean

(5.9)

and the number of turns per phase is N WT , the flux density of air gap is Bair , the stator width is W s , and the translator’s average velocity is vmean . The generated no load rms voltage is calculated without considering the effect of armature reaction. The induced voltage E i per phase is calculated as √ E i = π 2 f E N W T kw  P M

Fig. 5.1 Phasor diagram of the FSLEG (under consideration) for cos ϕ = 1 [6]

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where fE =

vtr 2τtr

(5.10)

and the generated voltage frequency is f E , the PM excitation flux is Φ PM , and the winding factor is k w . If X d , X q , X d , and X q are the direct and quadrature axes of synchronous and transient reactance respectively, the excitation voltage, E av and power, Pw can be written as   E av = Id X d − X d + Vq + X d Id 



Pw = Vt Id sin(δ) + Iq cos(δ) =

E q Vt X d

(5.11)

 VT2 1 1 sin(δ) + −  sin(2δ) (5.12) 2 Xq Xd

where d-axis current is I d , q-axis current is I q , q-axis voltage is V q , and the terminal voltage is V t . E q is the transient electromotive force along q-axis. The second order harmonic can be ignored for the sake of simplicity. The magnitude difference between E g and E q is negligible where E g can be called the electromotive force behind transient reactance. As a result, Eq. (5.12) can be summarized as follows. Pg =

E g Vt X d

  sin δg .

(5.13)

5.6 Description of the Magnetic Material In most of the linear electrical generators, permanent magnets are used for creating magnetic field. The analyzed FSLEG consists of the stator and mover which is also known as translator. Magnetic core is required for constructing both. Instead of using conventional steel core, Kool Mμ powder core is proposed in Farah et al. [6]. Magnetic properties of three different magnetic cores are shown in Fig. 5.2. A permanent magnet exists in between two “U” shaped stator cores (Fig. 5.1). Demagnetization curves of this magnet are shown in Fig. 5.3 for different temperatures.

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Fig. 5.2 Magnetizing curve of Kool Mμ with other magnetic cores [6]

Fig. 5.3 Demagnetization curve of the permanent magnet used in the linear generator [6]

5.7 Simulation Results In the simulation setup, 60 °C is considered for the working environment. The mover/ translator is allowed to move in between two sets of stators (Fig. 5.1). With the movement, the conductor cuts the magnetic flux which creates electricity. The main parameters used in the simulation setup of the FSLEG are presented in Table 5.1. Figure 5.4 shows terminal voltages and load currents where the peak voltage and current of the FSLEG with proposed Kool Mμ powder core are 230 V and 2.83 A, respectively. Figure 5.5 shows core loss and power output of the FSLEG where the

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Table 5.1 Parameters of FSLEG with the proposed material [6] Name of the parameter

Value

Unit

Length of stator

100

mm

Length of translator

200

mm

Stroke length of the translator

100

mm

Tooth width of translator

4.5

mm

Tooth pitch of translator

10

mm

Slot width of translator

5.5

mm

Translator tooth/slot depth

5

mm

Stator tooth pitch

15.5

mm

Stator tooth width

4.5

mm

Magnetic remanence of the permanent magnet

1.1

T

Thickness of the permanent magnet

5.5

mm

Coercive force of the permanent magnet

−838

kAm−1

Relative permeability of the PM

1.044573

Depth of the LG

0.25

Winding factor

0.6

The conductivity of copper

58

MS

Winding resistance

3



Turn number of copper coil

200

turns

Load resistance

5–30



Air gap

1

mm

Mass density of the steel core

7.9

g/cm3

Velocity of translator

2

m/s

m

average core loss and output power for using the proposed Kool Mμ powder core are 5.75 and 494.89 W, respectively. If an ordinary core and NdFeB PM are used in the simulation setup of the FSLEG, the core loss is found high, and the power output is 504.44 W. However, by applying Kool Mμ powder core and NdFeB PM to the same generator, the core loss is minimized up to 74.5%. But less amount of output power is obtained, which is 494.89 W. Again, by using Kool Mμ powder core and N46SH in the same FSLEG, the output power is increased to 608.7 W with the minimized core loss because of using Kool Mμ powder core as shown in Fig. 5.6.

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Fig. 5.4 Comparisons of a voltage and b current waveforms of the FSLEG with the proposed material [6]

Fig. 5.5 Comparisons of a core loss and b instantaneous power of the FSLEG for the conventional core and Kool Mμ materials [6]

Fig. 5.6 Comparison of instantaneous power of the FSLEG with the conventional and the proposed materials [6]

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5.8 Cost Analysis of the FSLEG In this section, the cost calculation of FSLEG is described. The price of active materials viz. PM, copper conductor, and magnetic core are analyzed in this study. Finally, the building cost of FSLEG is mentioned.

5.8.1 Active Material Used in the FSLEG The volume and mass of the active material that are utilized in the construction of FSLEG are calculated. The calculation of translator is shown as follows. Translator height = 12.45 cm Translator width = 3 cm Cross sectional area of the translator = height × width = (12.45 × 3) cm2 = 37.35 cm2 Model depth of translator core is 10 cm. Volume of the translator = area × model depth = (37.35 × 10) cm3 = 373.5 cm3 Mass density of the translator core varies from 7.7 to 7.8 g/cm3 Therefore, mass density of 7.75 g/cm3 is conceded for the translator core in this calculation. Mass of the translator core = (7.75 × 373.5) g = 2894.625 g = 2.894625 kg Calculation of the stator is shown as follows. Stator height = 2.39 cm Stator width = 4.26 cm Cross sectional area of the stator = height × width = (2.39 × 4.26) cm2 = 10.18 cm2 Model depth of the stator is 10 cm. Therefore, the volume of each stator = area × model depth = (10.18 × 10) cm3 = 101.8 cm3 The volume of four stators = (4 × 101.8) cm3 = 407.2 cm3 Mass density of the translator core varies from 7.7 to 0.8 g/cm3 . Therefore, the mass density of 7.75 g/cm3 is conceded in this calculation. Mass of four stators = (4 × 7.75 × 101.8) g = 3155.8 g = 3.1558 kg Mass of the magnetic core of translator and stator cores = 2.894625 kg + 3.1558 kg = 6.05043 kg The calculation of copper conductor is shown in the following. The calculated volume of each copper conductor = 26.65 cm3 Therefore, volume of four copper conductors = (4 × 26.65) cm3 = 106.6 cm3 Mass density of copper is 8.96 g/cm3 .

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Table 5.2 Cost analysis of copper used in the winding Reference

Material name

Price per unit

Price per cm3

Price per kg

Volume (cm3 )

Mass density (g/ cm3 )

Mass (g)

[7]

Copper

$17.45 or BDT 1478.69 per kg

$ 0.156 or $17.45 or BDT 13.25 BDT 1478.69

111.607

8.96

1000

Mass of all (four) copper conductors = (4 × 8.96 × 26.65) g = 955.136 g = 0.955136 kg The calculation of NdFeB PM is shown as follows. Height of the PM = 0.56 cm Width of PM = 3.15 cm Cross sectional area of the PM = height × width = (0.56 × 3.15) cm2 = 1.764 cm2 Model depth of the PM is 10 cm. The volume of each PM = (1.764 × 10) cm3 = 17.64 cm3 The volume of two PMs = (2 × 17.64) cm3 = 35.28 cm3 As the mass density of PM (NdFeB) varies from 7.1– 7.6 g/cm3 . Therefore, conceding the mass density of 7.6 g/cm3 in this calculation Mass of two PMs = (2 × 7.6 × 17.64) g = 268.128 g = 0.268128 kg.

5.8.2 Cost Analysis of Active Material The cost analysis of different types of active materials which are used in electrical machines is analyzed. The costs of various active materials from different suppliers are presented in Tables 5.2, 5.3 and 5.4. In the tables, the currency $ means US dollar, £ means British pound sterling, | means Indian rupee, and Bangladeshi currency is written as BDT.

5.8.3 Cost Calculation of Active Material In this section, the cost of active material for the FSLEG is calculated. The price of active materials applied to the FSLEG is summarized in Table 5.5. The cost calculation of copper used in the FSLEG is calculated as follows. The cost of 1 kg of copper is $17.45. Mass of copper used in FSLEG is equal to 0.95514 kg. The cost of 0.95514 kg copper is $16.667. Or

SmCo

NdFeB N52

NdFeB N52

NdFeB N35

NdFeB N45

[11]

[12]

[13]

[14]

[15]

£ 17.59 or BDT $24.47 or BDT 2068.62 per piece

£ 9.99 or $13.9 or BDT 1175per piece £ 2.86 or $ 3.98 or BDT 336.63

£ 2.33 or $ 3.24 or BDT 273.86

$ 19.9 or BDT 1488 $ 0.199 or BDT per piece 14.88

£ 403.16 or $ 560.85 BDT 47,412.79

£ 327.98 or $ 456.35 or BDT 38,571.52

$ 28.03 or BDT 2096.23

6.145

7.62cm × 1.27cm × 0.635cm

7.1 − 7.6

7.1 − 7.6 4.29

6.6 cm × 1.3 cm × 0.5 cm

7.1 − 7.6 7.1 − 7.6

8

8.3 − 8.5

10 cm × 5 cm 100 × 2 cm

| 0.625 or $ 0.0084 | 88.03 or $ 1.18 or BDT 4 cm × 2 cm or BDT 0.71 93.421 × 1 cm

1.0242

1.27 cm × 1.27 cm × 0.635 cm

| 5 or $ 0.067 or BDT 5.68 per Piece

$11.76−1176.47 or BDT995.3−99.517.65

0.3

0.5 cm × 1.2 cm × 0.5 cm

6.9 − 7.3

6.9 − 7.3

4.95

1.1 cm × 0.6 cm × 7.5 cm

$ 0.098−9.76 or BDT 7.436−825.95

| 47,342.99 or $ 637.68 or BDT 53,777.78

£ 180.65 or $ 250.91 or BDT 21,244.62

43.630 − 4.67

30.459 − 32.604

710 − 760

56.8 − 60.8

8.5 − 8.71

2.07 − 2.19

34.155 − 36.135

48

Mass density Mass (g) (g/cm3 ) 4.8

Volume (cm3 )

4 cm × 2.5 cm 10 × 1 cm

Dimension

$ 0.1−10 or BDT 8.46−845.94 per piece

| 326.67 or $ 4.4 or BDT 371.1

| 98 or $1.32 or BDT 111.32 per piece

AlNiCo

[10]

£ 1.25 or $ 1.73 or BDT 146.59

AlNiCo

[9]

Price per kg

$ 0.009−0.015 or $ 1.875 − 3.125 or BDT BDT 0.762−1.269 158.75 − 264.375

Price per cm3

£ 6.17 or $ 8.57 or BDT 725.61 per piece

Ferrite PM $ 0.09−0.15 or BDT 7.62−12.69 per piece

[8]

Price per unit

Material name

References

Table 5.3 Cost analysis of different permanent magnets

92 M. M. Farah et al.

Material name

Permalloy 80 core

Ferrite core

Silicon steel core

Kool Mμ power core

CRGO

References

[16]

[17]

[18]

[19]

[20]

$ 0.31 or BDT 26.07 | 1.45 or $ 0.019 or BDT 1.65

| 190 or $ 2.5 or BDT 216 per kg

$ 0.019−0.046 or BDT 1.62−3.88

$ 0.00049−0.0017 or BDT 0.0415−0.145

$ 0.258−0.301 or BDT 21.83−25.471

Price per cm3

$ 2.83 or BDT 239.48 per piece

$ 2.5−6 or BDT 212−508 per kg

$ 0.1−0.35 or BDT 8.46−29.62 per kg

$ 30–35 or BDT 2539−2962 per kg

Price per unit

Table 5.4 Cost analysis of different magnetic cores 116.279

Volume (cm3 )

| 190 or $ 2.5 or BDT 215.82

$ 44.01 or BDT 3724.42

$ 2.5−6 or BDT 211.55−507.73

130.729

9.186

130.729

$ 0.1−35 or BDT 204.082−200 8.46−29.62

$ 30−3 or BDT 2538.64−2962

Price per kg

7.65

7

7.65

4.9−5

8.6

Mass density (g/cm3 )

1000

64.3

1000

1000

1000

Mass (g)

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Table 5.5 Cost calculation summary of the active material applied to the FSLEG Material

Mass (kg)

Price ($/kg)

Magnetic core

6.05043

44.01

Price ($) 266.279

Price (BDT) 22,506.38

Copper

0.95514

17.45

16.667

1,408.72

PMs

0.26813

28.03

7.516

635.23

Total = $290.462

Total = 24,550.33

Mass of copper used in the FSLEG = 0.95514 kg The price of 1 kg of copper = $17.45 Therefore, price of 0.95514 kg of copper = 0.95514 × $17.45 = $16.667

The cost calculation of the PM used in this generator is described here. The cost of 1 kg of NdFeB PM is $28.03. Mass of NdFeB PM used in FSLEG is equal to 0.26813 kg. Therefore, the cost of NdFeB PM of 0.26813 kg is $7.516. Or Mass of NdFeB PM used in the FSLEG = 0.26813 kg The price of 1kg of NdFeB PM = $28.03 Therefore, price of 0.26813 kg of NdFeB PM = 0.26813 × $ 28.03= $7.516

The cost calculation of magnetic core used in this generator is presented in the following. The cost of 1 kg of Kool Mμ power core is $44.01. Mass of Kool Mμ power core is 6.05043 kg. Therefore, the cost of 6.05043 kg Kool Mμ power core is $266.279. Or Mass of Kool Mμ power core used in the FSLEG = 6.05043 kg The price of 1 kg of Kool Mμ power core = $44.01 Therefore, price of 6.05043 kg of Kool Mμ power core = 6.05043 × $44.01= $266.279

In ESMAP Technical Paper 122/09 [21], it is mentioned that compared to the material cost of the generator, the manufacturing cost is double. Therefore, the tentative manufacturing cost of the FSLEG can be considered approximately $581 (which is two times of its material cost). The material cost can be reduced if a bulk amount of material is purchased. According to Yazawa and Shakouri [22], manufacturing cost can be minimized for the large-scale production. From the cost analysis, it is found that the price of an ordinary magnetic material is low, but its required size is large. That is why, higher amount of material would require extra cost and added copper conductor for a larger size material again increases the

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cost. An example of using an ordinary magnetic core for magnetic levitation is remarkable that uses a large ferromagnetic core for lifting a tiny lightweight flap [23]. On the other hand, the physical size of a high graded magnetic core would be smaller for similar strength of magnetic field. In the conventional linear electrical generator, copper loss occurs [24] due to having internal winding resistance which can be avoided by using superconductor [25]. It is reported in Molla et al. [26] that site selection for oceanic wave energy conversion is significant to maximize electrical power generation. Therefore, besides increasing power generation and core loss minimization, it is required to install the generator to the proper location.

5.9 Future Scope In this chapter, various works are accomplished such as analyzing the parameters of magnetic materials used in a flux switching linear electrical generator and their simulations. However, there are several opportunities for further development in this topic. The future scope can be summarized as follows. • Further research can be done by using Kool Mμ powder core with other designs including rotational and linear electrical machines. • By using the FSLEG design, the researchers can investigate other advanced magnetic core and PM materials. • Using the combination of different magnetic materials, the FSLEG can be designed economically for commercial applications.

5.10 Summary The oceanic wave dynamics and mathematical model of an FSLEG are presented in the beginning of this chapter. Then, a simulation work of a double-sided FSLEG is presented. Simulation results show that the FSLEG produces more electrical power while reducing core loss than the conventional one. It produces 4.8% high amount of electrical power (approximately) while the core loss is reduced to around 74.5% than the conventional one. Because of using the proposed Kool Mμ powder core with advanced PM. The temperature rating is considered 60 °C for both cases. From the cost analysis it is found that the material cost for the FSLEG is quite low for using the selected materials as tabulated in Table 5.5. Therefore, it can be considered as an economically viable generator.

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References 1. Farrok O, Islam MR, Sheikh MRI, Guo Y and Zhu J (2017) Design and analysis of a novel lightweight translator permanent magnet linear generator for oceanic wave energy conversion. IEEE Trans Magnet 53(11):1–4, Art no. 8207304. https://doi.org/10.1109/TMAG.2017.271 3770 2. Rahman A, Farrok O, Islam MR, Xu W (2020) Recent progress in electrical generators for oceanic wave energy conversion. IEEE Access 8:138595–138615. https://doi.org/10.1109/ ACCESS.2020.3012662 3. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 4. Molla S, Farrok O, Islam MR, Muttaqi KM (2020) Application of iron nitride compound as alternative permanent magnet for designing linear generators to harvest oceanic wave energy. IET Electr Power Appl 14(5):762–770. https://doi.org/10.1049/iet-epa.2019.0372 5. Farrok O, Islam MR, Guo Y, Zhu J, Xu W (2018) A novel design procedure for designing linear generators. IEEE Trans Industr Electron 65(2):1846–1854. https://doi.org/10.1109/TIE. 2017.2739694 6. Farah MM, Farrok O, Ahmed K (2019) Kool Mμ powder core used in a flux switching linear electrical machine for electricity generation from the oceanic wave. In: IEEE international conference on power, electrical, and electronics and industrial applications (PEEIACON), pp 127–130. https://doi.org/10.1109/PEEIACON48840.2019.9071936 7. SINDABAD.COM, Coper Wire. https://sindabad.com/gazi-super-copper-enameled-wire-perk-g-20.html. Accessed 27 June 2021 8. Alibaba.com, Ferrite Magnet Bar. https://www.alibaba.com/product-detail/Ferrite-MagnetBar-Ferrite-Block-Magnet_1600091866694.html?spm=a2700.galleryofferlist.normal_offer. d_title.28567163OrgqQL. Accessed 28 June 2021 9. Amazon.co.uk, Alnico Bar Magnet. https://www.amazon.co.uk/Magnet-Expert-EducationalAlnico-Bar/dp/B001RX0KFO/ref=sr_1_5?dchild=1. Accessed 26 June 2021 10. Indiamart, Alnico Bar Magnet. https://www.indiamart.com/proddetail/alnico-bar-magnet14861026830.html?pos=12&kwd=alnico%20magnet&tags=CC||||8256.8125|Price|Product. Accessed 29 June 2021 11. Alibaba.com, SmCo Magnet. https://www.alibaba.com/product-detail/SmCo-Magnet-Block1-2-X1_60336051408.html?spm=a2700.galleryofferlist.normal_offer.d_title.589a1da7t zDxgj. Accessed 06 July 2021 12. Indiamart, Ndfeb Magnet. https://www.indiamart.com/proddetail/neodymium-magnet-smpbrand-22292101762.html. Accessed 3 July 2021 13. Alibaba.com, Rare Earth Neodymium N52 Block Magnet. https://www.alibaba.com/productdetail/Large-Rare-Earth-Neodymium-N52-html?spm=a2700.details.0.0.7b714871V5m2m3. Accessed 8 July 2021 14. Amazon.co.uk, Rare Earth Neodymium Magnet. https://www.amazon.co.uk/efeel-Super-Str ong-Neodymium-Magnets/dp/B01H7K2Y/ref=sr_1_40?dchild=1. Accessed 2 July 2021 15. Amazon.co.uk, Super Strong Neodymium Magnet. https://www.amazon.co.uk/NeodymiumPermanent-Strongest-Magnets-Applied/dp/B00KFLBI8W/ref=sr_1_28?dchild=1. Accessed on 05 July 2021 16. Alibaba.com, Permalloy 80 Core. https://www.alibaba.com/product-detail/Permalloy-80-Per malloy-strip-Mumetal-Permalloy_1600075151433.html?spm=a2700.7724857.normal_offer. d_title.690c42faxmNMlv. Accessed 4 July 2021 17. Alibaba.com, Ferrite cores. https://www.alibaba.com/product-detail/toroidal-core-magnetFerrite-core%20toroidal_60802130586.html?spm=a2700.%207724857.normal_offer.%20d_ image.77ba49b6jsYkVT. Accessed 7 July 2021 18. Alibaba.com, Silicon Steel Cores. https://www.alibaba.com/product-detail/Silicon-SteelTape-Current-Transformer-Rectangular_60511146257.html?spm=a2700.galleryofferlist.nor mal_offer.d_title.23754cf7R5QfGf&s=p. Accessed 28 June 2021

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19. DXT Magnetics, Kool Mu Cores. https://dxtmagnetics.com/product/t39-9-x-24-1-x-14-5kool-mu-060-perm/. Accessed 25 June 2021 20. Indiamart, CRGO Transformer Lamination Core. https://www.indiamart.com/proddetail/crgotransformer-lamination-core-22172136755.html?pos=3&pla=n. Accessed 13 July 2021 21. ESMAP Technical Paper 122/09, Pauschert D (2009) Study of equipment prices in the power sector. Energy sector management assistance program (ESMAP), Washington, DC 22. Yazawa K, Shakouri A (2011) Cost-efficiency trade-off and the design of thermoelectric power generators. Environ Sci Technol 45(17):7548–7553. https://doi.org/10.1021/es2005418 23. Ahmed M, Hossen MF, Hoque ME, Farrok O, Mynuddin M (2016) Design and construction of a magnetic levitation system using programmable logic controller. Am J Mech Eng 4(3):99–107. https://doi.org/10.12691/ajme-4-3-3 24. Farrok O, Islam MR, Sheikh MRI, Guo YG, Zhu JG, Xu W (2015) Analysis and design of a novel linear generator for harvesting oceanic wave energy. In: 2015 IEEE International Conference on Applied Superconductivity and Electromagnetic Devices (ASEMD), pp 272– 273. https://doi.org/10.1109/ASEMD.2015.7453569 25. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Xu W (2016) A novel superconducting magnet excited linear generator for wave energy conversion system. IEEE Trans Appl Superconduct 26(7):1–5, Art no. 5207105. https://doi.org/10.1109/TASC.2016.2574351 26. Molla S, Farrok O, Alam, MJ (2023) Zero Carbon emission based electrical power plant by harvesting oceanic wave energy: Minimization of environmental impact in Bangladesh. In: Climate change and ocean renewable energy. CCORE 2022. Springer Proceedings in Earth and Environmental Sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-26967-7_1

Chapter 6

Dual-Port Linear Electrical Generator: Solution of the Existing Limitation of Power Generation Mohamud Mohamed Farah, Md. Abdullah-Al-Mamun, Md Rabiul Islam, and Omar Farrok

Abstract Almost all linear generators used for harvesting wave energy are singleport linear generators (SPLG). The transferred oceanic wave energy to the traditional SPLG is discontinuous in nature due to the variation of waves and its principle of operation. In this chapter, a new design of dual-port linear generator (DPLG) is presented. It can produce enough electrical power from the oceanic wave with adequate amount of voltage while the translator reaches to the top and bottom ends. Stator tooth design greatly affects the efficiency of the DPLG. Genetic algorithm is suitable to determine the optimized stator tooth. The shape optimization method of the stator teeth is presented to justify the performance of DPLG. The force ripples of the DPLG are reduced up to 40.89% by improving its stator tooth shape. It improves the power conversion efficiency of the DPLG as shown in the results and discussion section. The analysis is illustrated with multiphysics simulation and the finite element method to determine the electromagnetic performance. Simulation results along with laboratory prototype are also presented for validation of the DPLG topology. Experimental and simulation results from the prototype show the special interest of applying DPLG as it generates adequate voltage even at zero vertical velocity of the translator obtained from the oceanic wave. It is not possible to achieve by using the conversional SPLG which is mathematically shown. Thus, production of more electrical power from the DPLG is ensured even at the moment of no vertical velocity of its translator. Keywords Direct drive generator · Dual-port linear generator · Linear electrical generator · Oceanic wave · Optimization · Permanent magnet machine · Single-port linear generator · Wave energy converter M. M. Farah · Md. Abdullah-Al-Mamun · O. Farrok (B) Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected] M. R. Islam School of Electrical, Computer and Telecommunications Engineering (SECTE), Faculty of Engineering and Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_6

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6.1 Introduction Due to the huge population growth and technological development, the demand of electrical energy is increasing all over the world. Therefore, large amount of power generation is required to meet the demand. Electrical power generation systems are mostly based on traditional fossil fuels such as petroleum, coal, and natural gas. The storage of fossil fuels is diminishing rapidly. It is harmful for the environment and responsible for global warming [1] that can result in rise in the sea level. However, to ensure pollution free and green source of energy, renewable energy resources (RERs) are required to be utilized rather than using fossil fuels. Consciousness of the reduction of conventional energy resources and the environmental effects of the use of fossil fuels are increased nowadays. Because of the limitations in traditional energy resources, the importance of power generation from renewable energy resources is boosted in most countries. The EU and the US have formed their policies about increase in power plants based on RERs [2]. Several inspiration schemes with devices have been introducing since the last few years specifically to help renewable energy source development, such as tax credits, carbon taxes, cap-and-trade systems, and feed-in tariffs. Therefore, the researchers pay attention to generate electricity from the RERs rather than traditional fuels. RERs are considered as an operative element to meet increasing energy demand viable. The existing well known RERs are geothermal energy, wind, solar, biomass, hydropower, and oceanic wave energy (OWE). Among the conventional renewable energy sources, the energy density of OWE is the highest. The convertible power potential from the OWE is approximately 1000– 10,000 GW, which can contribute a lot of electrical power to the total electrical load demand in the world. As the nominal capacity factor increases, the power generation from oceanic wave energy gets great attention in Victoria. It is expected that by 2050, the amount of electrical energy generation from wave energy would increase notably. The wave energy was initially 1.6 TWh in 2019 but according to the forecast, it would increase to 25.7 TWh by 2050 as shown in Fig. 6.1. It is found that annual electrical energy generation by RERs including OWE is growing fast compared to the others that illustrates their importance [3]. The values of worldwide OWE potential are defined and presented in [4] for various frequency and amplitude of oceanic wave. The major sources of RERs are wind, biomass, solar, tidal, hydro, geothermal, etc., that are used either separately or their combination in the hybrid form [5]. For using any of the power sources, a storage system would be required to supply power during unavailability of the RERs. On the other hand, OWE is referred as vital source of renewable energy. Comparing to the other existing RERs, OWE is more predictable and highly available. Electrical power production from the oceanic wave was initiated many years ago but its commercial viability is recently considered. Since the ocean covers more than 70% of the earth surface, wave energy is well-thought-out one of the best RERs comparing all other sources for electrical power plant. Various wave energy devices can be used for energy conversion [3]. Among them point absorber type devices are very popular and used frequently whereas an example

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Fig. 6.1 Forecast of annual electrical energy generation from 2010–2050 (CCS stands for carbon capture and storage) [3]

of less frequent wave energy device is Bulge wave converter. The device was invented in 2005 by a company named Checkmate Sea Energy Ltd. The advantage of Bulge wave converter is that it is cheap compared to the Pelamis. It does not have any intersections to crack. The company forecasts that the cost of Bulge wave device can reach approximately 2–3 million GBP. It can produce 1 MW of electrical power [6]. On the contrary, point absorber type devices have other attractive features over the other devices. Such as ease of installation, requirement of less space, ability to drive the linear generator directly, etc. In general, linear generators are mostly used to convert OWE to electrical energy because it has several advantages over the conventional rotating electrical generator. The existing linear generator consists of a stator and a translator. The translator mounted to the buoy moves vertically with respect to the stator for the wave propagation causing electrical power generator. But it is not possible for the traditional single-port linear generators (SPLGs) to produce electricity at zero vertical speed of the translator as it is the main reason to produce electricity. In this chapter, an effective method of wave energy extraction scheme is extensively described. A dual-port linear electrical generator (DPLG) is presented in comparison to the conventional linear electrical generators. The proposed DPLG can generate enough amount of electricity even at zero vertical speed of the translator. The architecture of DPLG is strong and compact in construction. The overall power generation is increased, and the force ripples are reduced by optimizing the tooth shape of the stator of the DPLG. Force ripples occur due to cogging force and end effect force. The additional optimization in the stator tooth shape of the DPLG

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produces more power and decreases the force ripples of the translator to avoid the possible damage in its structure. Section 6.2 of this chapter describes the existing linear generator. In Sect. 6.3, limitation of the conventional single-port linear generator is explained. Model of the DPLG with mathematical expression is illustrated in Sect. 6.4. In Sect. 6.5, its construction is illustrated. The stator tooth shape optimization of the DPLG is explained in Sect. 6.6. In Sect. 6.7, the simulation setup and its results are presented. Finally, the advantage of the DPLG and its specification are briefly concluded in the next section.

6.2 The Existing Linear Generators There are two types of linear generators, viz. tubular and flat type. To improve the performance of the linear generator, various approaches have been proposed by scientists and researchers recently. Construction of a flat type of linear generator is simple. It is found that proper design and optimization of linear a generator is required for its satisfactory performance [7]. From the oceanic wave dynamics, it is understood that lightweight translator is suitable for producing electrical power from the oceanic wave [8]. Because of using an ordinary steel as magnetic material, size or volume of the magnetic core becomes quite large for producing adequate magnetic field. It is found that for magnetic levitation of a tiny lightweight flap, a large electromagnet is implemented because of using the conventional magnetic core [9]. Therefore, application of high graded magnetic core in the linear generator is needful for creating strong magnetic field. Besides, design improvement and application of high graded materials, incorporating a cooling system, control methodology, and protection system are required for sustainable development of the linear generator. Similar to the other renewable energy source based power generation, a suitable converter is required to utilize the generated electricity by harvesting wave energy. Various power converters are described in [10] which can be supportive to select a suitable one. An isolation system is needful for safe operation of a wave farm connected to the grid. Because a lot of linear electrical generators are connected to this system. If any of them get damaged, the isolator can separate the faulty one [11].

6.3 Limitation of the Existing Linear Generator From the literature review it is found that most of the linear electrical generator for harvesting oceanic wave uses single-port topology. A generator with the SPLG topology has a single mechanical input terminal and an output electrical terminal. For a conventional SPLG, let us assume that the translator moves vertically with the incident of sea wave. As the oceanic wave is generally approximated to a sine wave, the translator motion is periodic. Considering the initial position of each quarter

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cycle, a total of four translator positions are achieved where position 1 represents 1st quarter, position 2, position 3, and position 4 represent 2nd, 3rd, and 4th quarters, respectively as shown in Fig. 6.2. Positions 1 and 3 indicate the translator location for the peak and trough of the oceanic wave, respectively. As the slope for positions 1 and 3 is zero, there is no vertical velocity of the translator. On the other hand, magnitude of slop for positions 2 and 4 is maximum for which translator velocity is also the highest. Usually, translator motion in the linear generator due to wave propagation can be mathematical expressed as: ) ( 2π t ± αz (6.1) z(t) = Amax sin T ) ( 2π 2 Amax π cos t ± αv vt (t) = T T where the elevation is denoted by z(t), the perpendicular velocity of the translator is vt (t), the peak level of the translator is Amax , the time duration is T, and the phase angle is α v . The mechanical power, Pm = Fv, gained from the oceanic wave energy is converted to the electrical power, Pe by means of a linear generator where v the velocity of the translator and F the applied force. From Fig. 6.2 it is illustrated that vertical velocity of the translator is zero at positions 1 and 3, and non-zero at positions 2 and 4. For zero velocity of the translator, the existing SPLG does not produce power. The relation of the oceanic wave nature, translator position, velocity, and electrical

Fig. 6.2 Illustration of the conventional linear generator [12]

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parameters are explained in the following sections. Similarly, it is shown that a high amount of electrical energy is produced when the velocity of translator is increased and vice-versa.

6.4 Dual-Port Linear Generator Limitation of the existing SPLG can be avoided by utilizing DPLG topology which is introduced and proposed in [12]. The DPLG is constructed with two stators and translator sets. The translator frame of the DPLG contains two main (different) translators. One is the driver-translator situated in the upper side of the DPLG. Therefore, it is denoted as translator-up (Translator-U). The other is the driven-translator which is situated at the lower side of Translator-U. It is denoted by translator-down (Translator-D). The buoy is directly connected to Translator-U only. A mechanical spring is physically connected in between the driver- and driven-translator. The vertical motion of Translator-U and Translator-D of the DPLG can be formulated in the following as Fdr vr = −(ke + km )z 1 + km z 2 = −{ke z 1 − km (z 2 − z 1 )}

(6.2)

Fdr vn = −km z 1 − (ke + km )z 2 = −{ke z 2 + km (z 2 − z 1 )} where the applied force to Translator-U is denoted by F drvr , the force shifted to Translator-D through the spring is F drvn , the translator mass is m, the vertical displacement of Translator-U is z1 , the vertical displacement of the Translator-D is z2 , spring constant of the spring positioned at the both ends is K e , and the spring constant of the spring is k m connecting Translator-U and Translator-D. The natural frequency, ω can be found by solving (6.2) and can be written as follows. ] [ ] z z 1 (t) = 1 e− jωt z2 z 2 (t) [ ] d2 2 z1 e− jωt (t) = 2 z(t) = −ω z2 dt ][ ] [ km z 1 − jωt −(ke + km ) e F(t) = km −(ke + km ) z 2 [

z(t) =

(6.3)

(6.4)

(6.5)

By solving (6.4) and (6.5), natural frequency can be expressed as / ω=

ke ke + 2k m = ωL , = ωH m m

(6.6)

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where ωH and ωL are the maximum and minimum values of the natural frequencies of the driven- and driver-translators, respectively. They mostly depend on the translators mass and the spring constants. General equations of motion are given as: z 1 (t) =

1 {A L cos(ω L t + ϕ L ) + A H cos(ω H t + ϕ H )} 2

(6.7)

z 2 (t) =

1 {A L cos(ω L t + ϕ L ) − A H cos(ω H t + ϕ H )} 2

(6.8)

where the magnitudes of the driver- and driven-translators are AL and AH , respectively. The initial phase angle is ϕ. √ The mass and spring constants are needed to be tuned as, ωL = 2 and ωH = 2 2. The vertical positions of the translators are plotted in Fig. 6.3. Different values of ωL and ωH determine the variation of vertical displacement of the driver- and driven-translator. Figures 6.4, 6.5, 6.6 plot the vertical positions, velocities, and speeds of TranslatorU and Translator-D for ωL = 2 and ωH = 2.2, respectively.

Fig. 6.3 Positions of the driver-translator and driven-translator for a specific value of ω

Fig. 6.4 Positions of Translator-U and Translator-D for ωL = 2 and ωH = 2.2

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Fig. 6.5 Vertical velocity of the translators for ωL = 2 and ωH = 2.2

Fig. 6.6 Vertical speed of the translators for ωL = 2 and ωH = 2.2

The focus of Fig. 6.6 is that at least one of the translators’ speeds is not zero for any time interval. Therefore, electrical power generation is possible. It is important to consider the mathematical model of floating buoy, as the translator is associated with it. The force of the buoy or float, F float can be written as d 2 z f loat dv f loat (6.9) = a f loat = 2 dt dt [ ] ] [ F f loat (t) = ρgπr 2f loat z f loat − z wave (t) + (Rr + Rv ) v f loat − vwave (t) (6.10) where the vertical displacement, velocity, and acceleration of the float are zfloat , vfloat , and afloat , respectively. The gravitational acceleration is g, the buoys radius is r float , the viscous resistance is Rvis , the radiation resistance is Rrad , and the water mass density is ρ. The vertical velocity and displacement of the oceanic water is vwave and zwave , respectively. Therefore, the buoy acceleration transferred to the translator of the DPLG can be presented as a f loat =

F f loat (t) Mt + M f loat + Mw

(6.11)

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where the translator mass is M t, the buoy mass is M float , and the additional mass due to the oceanic water is M w .

6.5 Construction of the DPLG There are two sets of stator and translator in the DPLG. It consists of two equal segments which are the driver- and driven-part of the linear generators. Both stators are the same and they are connected to the cage. Similarly, both Translator-U and Translator-D are the same. They are linked with one another by a mechanical spring. The length of spring must be similar to the stoke length of the translator. Then the spring constant depends on the stroke length, mass of Translator-U or Translator-D, and the vertical force obtained from the oceanic wave. Figure 6.7 depicts one of the identical segments of the DPLG which is comprised of 10 windings. Every winding contains two coils in series which can produce 50 W electrical power. Hence, rated power of the identical segment is 0.5 kW. The pole/slots/phase number is four and the winding factor is 0.65. The aim of the stator core connector is to link specific single segment to each other. This arrangement can increase the stability and physical strength of the DPLG. In this design, standard iron core is applied. The translator contains neodymium iron boron permanent magnet with a remanence magnetic flux density of 1.1 T and coercive force of −837 kA/m. Equivalent circuit diagram of the DPLG is shown in Fig. 6.8 where the generated voltage of the driver-generator is E drvr , the generated voltage of the driven-generator is E drvn , the equivalent series inductance of the winding is L wind , the resistance of the winding is Rwind , the filtering inductance is L f , the filtering capacitance is C f , and the load resistance is RL . The dashed lines around RL and LC filter show the dc bus that can be connected to other DPLGs.

6.6 Optimization of the DPLG The shape of the stator tooth determines the power production of a linear generator as shown in the simulation results. It is also found that, by applying this technique it is possible to increase the performance of the linear generator. To realize this, it may be summarized in the following steps: (a) selecting permanent magnet geometry and height of stator tooth, (b) determining the parameters a and b, , (c) finding the proper arc. Steps (a) and (b) are illustrated in Fig. 6.9 and step (c) is depicted in Fig. 6.10. The term, emd or h(1 − e−bx ) is simplified representation of the arc, where m and h determine the curvature. The equation of generated voltage from the DPLG can be written as

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Fig. 6.7 a Architecture of the DPLG (one of the identical segments) and its b top view [12]

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Fig. 6.8 Equivalent circuit diagram of the DPLG [12]

Fig. 6.9 Determination of stator pole geometry. a Selection of the slope by and b cross section of the stator pole shoe with labels

Fig. 6.10 a Selection of stator pole curvature and b selected pole face with slope and curvature

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( E gen (t) = K lg cos

) π 2π vt (t) zt + k τp p

(6.12)

where the generated voltage is E gen , k = −2, −1, 0, 1, 2; p = 2, the construction constant of the machine is K lg , the vertical position is zt , the pole pitch is τ p , and the translator velocity is vt . As the electromagnetic field and estimation of applied force are determined by using finite element method, a suitable mesh setup is significant to achieve exact outcomes. The simulation of both forms of stator tooth shapes is there in this design. One has flat plane surfaces only, and the other has curved surfaces. The finite element mesh is essentially a triangular pyramid that has sliced or meshed surface. Surfaces of the plane tooth shapes match exactly the meshed surfaces, while the actual surfaces of the curved tooth shapes closely match the meshed surfaces maintaining a distance. For this reason, a surface deviation occurs for approximating curved surface by a plane surface. The increase of mesh density can reduce the surface deviation which leads to the certainty of the simulated results. Again, additional computational time would be required for selecting extremely high mesh density. It would require much computational time because the finite element method is iterative. Therefore, there is a high chance of divergence in the result. A new technique is used (Fig. 6.11), to make the suitable mesh setup to minimize the simulation time, and to realize the improved results. The stator tooth shape is optimized by using genetic algorithm as shown in Fig. 6.11. By using this optimization method not only additional power is generated but also the force ripples are reduced. From the force applied to the translator, the force ripples are calculated, and the force can be written as Ft = Fe + Fl = m t at + λvt

(6.13)

where the total force of the translator is F t , the electromagnetic force is F e , the external load force is F l , the damping is λ, the acceleration is at , the translator mass is mt , and the velocity of the translator is vt . If the magnetic co-energy of the system is W co , the magnetic flux density is B, the magnetic field intensity is H, and the suffix l and u denote the lower and upper limits of the moving object, respectively. Then the force on any part/object, F obj of the translator in the moving direction at a distance, s for constant i a can be expressed by

Fobj

⎤ ⎞ ⎡ yu x z ⎛ H { {u {u { d ∂⎣ ⎝ B.d H ⎠d xd ydz ⎦ = {Wco (s, i a )} = dl ∂l y=xl x=yl z=zl

(6.14)

0

where x, y, and z axes represent a 3D Cartesian coordinate system. The oscillation of the translator force is related to the current and magnetic field. The instantaneous virtual force, F vir on the translator can be expressed as

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Fig. 6.11 Flowchart of the optimization process of the DPLG

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Fvir =

1 2

[{

] { | |− − → | |→ Re| J × B |dv + Re| J × B θ |dv

(6.15)

− → where the conduction current density is denoted by J , magnetic flux density at a specific time is B θ , and θ is the angular movement. The chosen arc of the stator teeth can be presented by exponential or polynomial as h = f (d) = a1 d 4 + a2 d 3 + a3 d 2 + a4 d + a5

(6.16)

where the polynomial coefficients of a single indeterminate are a1 , a2 , …, a5 and the arc is expressed in terms of d and h. Height of the stator tooth is h which is the function of width, d.

6.7 Results and Discussion In the simulation setup, the translator moves vertically at the velocity of 1 m/s with respect to its stator maintaining a 2 mm air gap. Figure 6.12 shows magnetic properties of the DPLG before and after the optimization is conducted. Figure 6.13 explains the effects of applying various LC filters after being rectified with a bridge rectifier. To obtain a suitable filtered output and to reduce the overshoot, appropriate selection of capacitance and inductance is required. They were selected as 2 mF and 25 mH, respectively. Ripple minimization shows the effect of filtering as plotted in Figs. 6.13, 6.14 shows the voltage, current, and power of the chosen LC filter. The effect of optimization is shown in Fig. 6.15 where the mechanical force on driver-translator of DPLG is plotted. The effectiveness of optimization is also observed for core loss of the DPLG which is illustrated in Fig. 6.16. In the optimized DPLG, core loss and force ripples are significantly minimized. The approximation and the chosen curved face (arc) of the stator tooth are depicted in Fig. 6.17. Table 6.1 presents geometry of the DPLG. The comparative results for various stator tooth shapes are shown in Table 6.2. The tooth shapes are specified in terms of a, b, and stator tooth faces. A total of four cases are analyzed where the cases are Condition—1, 2, 3, and 4. The conditions are different by the terms of a and b. By optimizing the stator tooth shape, force ripples are mostly reduced. A small-scale model and simulator of sea wave motion are incorporated in the laboratory for experimental results. The obtained results are found to be more suitable for DPLG in comparison to the existing SPLG topology. The time period, stroke length, and maximum power rating of the DPLG prototype are about 1500 ms, 360 mm, and 0.1 kW respectively. Air gap length is 2 mm for the prototype.

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Fig. 6.12 Rainbow spectrum of a magnetic flux, b B, and c H. d Magnetic flux lines, e B, and f H of the DPLG without shape (slope and arc) optimization. g Magnetic flux lines, h B, and i H of the DPLG with optimization [12]

Figure 6.18 illustrates the unique benefit of the DPLG that is shown by experimental results of the prototype. The driven-translator can produce acceptable voltage even at zero vertical velocity of the sea wave. Comparison of voltages which is found from the experimental and simulation results of the DPLG are similar. The volt- and time-per division are 25 V/div and 50 ms/div, respectively for both the simulation and experimental results. The generation of electrical power, Pdc is measured at the dc bus (Fig. 6.14). After applying Pdc = V dc × I dc where voltage of the dc bus is denoted as V dc and the line current is I dc . The mechanical power, Pm is obtained by

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Fig. 6.13 Rectified voltage of the DPLG that shows filtering effect

Fig. 6.14 Electrical parameters of the DPLG with proper filtering

Fig. 6.15 Mechanical force of the translator with and without optimization

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Fig. 6.16 Core losses of the DPLG before and after applying optimization

Fig. 6.17 Selected curvature of the stator pole teeth [12]

applying Pm = F tr × vt where the applied mechanical force on the translator is F tr and the translator velocity is vt . Then, the efficiency is calculated by η = Pdc /Pm . The feature of producing electrical power with the DPLG for the translator position at both ends cannot be realized by applying the existing SPLG. Because the SPLG cannot produce electrical power under this condition for its operation and the periodic nature of marine waves. The ripples of the voltage found from simulation and experimental results for various filtering are similar. Due to the sampling rate of the oscilloscope (experimental) and step size in the simulation setup, a little variation is observed in the simulation and experimental results. Table 6.3 shows physical size of the proposed DPLG.

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Table 6.1 Geometry of the DPLG [12] Attributes Thickness

Width

Distance

Length

Dimension

Value

Permanent magnet (cm)

1.1

Stator pole (cm)

0.6

Translator core (cm)

1

Stator pole shoe (cm)

0.3

Cross section of the copper coil (cm)

1.8

Stator poles (cm)

1.9

Total stator poles (cm)

2.2

Stator core connector (cm)

0.8

Stator poles of the same winding (cm)

2.1

Stator poles of the same side (cm)

2.8

Translator pole pitch (cm)

4.2

Two consecutive stator poles (vertically) (cm)

2.5

Stator pole shoe (vertically) (cm)

2

Stator pole shoe (cm)

0.7

Cross section of the copper coil (cm)

0.6

Air gap (cm)

0.1

Table 6.2 Effect of parameters a and b (for stator teeth) of the DPLG [12] Parameters

Condition 1 a = 0 mm, b = 0 mm

Condition 2 (straight face) a = 2 mm, b = 4 mm

Condition 3 (straight face) a = 3 mm, b = 7 mm

Condition 4 (curvature) a = 3 mm, b = mm 7

Generated power (W)

445.4

443.1

441.6

463.4

Ripple of the dc power

13.56

Force ripples

−10.96

Maximum value of B(T) Standard deviation of force

2.824 57.01

10.14 −12.15 2.672 63.87

10.12

12.53

−12.54

−6.4

2.76 64.64

2.865 33.7

6.8 Summary For simulation of the DPLG, finite element method is applied to it by using ANSYS/Ansoft software. The results found from simulation and experiment of the DPLG show its unique property. Enough electrical power is generated even at no vertical velocity of the translator. The optimization technique used for the

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Fig. 6.18 Experimental and simulation results of the generated voltages of the DPLG [12]

Table 6.3 Dimensions of the DPLG prototype [12] Attributes

Dimension

Value

Height

Prototype generator (cm)

43.2

Stator pole shoe (cm)

2

Prototype generator (cm)

10.6

The stator pole (cm)

2.4

Inside the bobbin for the copper winding (cm)

1.8

Width

DPLG by improving the design, size, and slope of the stator tooth successfully minimizes the force ripples. The optimized DPLG produces 17.98W additional electricity compared to the non-optimized DPLG. Although optimization procedure is applied to the DPLG, the same method is applicable to the other linear generators. The full load efficiency of the DPLG is found to be around 83.92%. Optimization of the stator tooth shape applied to the DPLG as a part of the design process is also suitable for other linear generators. By applying the lower and upper mechanical springs, both ends of lower and upper translators are safe from collision to its cage of the DPLG. Therefore, end stoppers are no longer required for this design. It can be treated as one of the benefits of the DPLG as presented in this chapter. The waveforms found by the simulation and experimental results seem close to one another.

References 1. Kabir MA et al (2023) Net-metering and Feed-in-Tariff policies for the optimum billing scheme for future industrial PV systems in Bangladesh. Alex Eng J 63:157–174. https://doi.org/10. 1016/j.aej.2022.08.004 2. Kiran MR, Farrok O, Mamun MAA, Islam MR, Xu W (2020) Progress in piezoelectric material based oceanic wave energy conversion technology. IEEE Access 8:146428–146449. https:// doi.org/10.1109/ACCESS.2020.3015821 3. Ocean renewable energy (2015–2050) An analysis of ocean energy in Australia. https://public ations.csiro.au/rpr/download?pid=csiro:EP113441&dsid=DS2. Visited 14 Jan 2020

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4. Robertson B, Hiles C, Luczko E, Buckham B (2016) Quantifying wave power and wave energy converter array production potential. Int J Marine Energy 14:143–160. https://doi.org/10.1016/ j.ijome.2015.10.001 5. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Lei G (2018) Oceanic wave energy conversion by a novel permanent magnet linear generator capable of preventing demagnetization. IEEE Trans Ind Appl 54(6):6005–6014. https://doi.org/10.1109/TIA.2018.2863661 6. The renewable energy. http://www.reuk.co.uk/wordpress/wave/anaconda-bulge-wave-powergenerator. Visited 7 Jan 2020 7. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 8. Farrok O, Islam MR, Sheikh MRI, Guo Y and Zhu J (2017) Design and analysis of a novel lightweight translator permanent magnet linear generator for oceanic wave energy conversion. IEEE Transactions on Magnetics 53(11): 1–4, Art no. 8207304. https://doi.org/10.1109/TMAG. 2017.2713770 9. Ahmed M, Hossen MF, Hoque ME, Farrok O, Mynuddin M (2016) Design and construction of a magnetic levitation system using programmable logic controller. Am J Mech Eng 4(3):99–107. https://doi.org/10.12691/ajme-4-3-3 10. Haque MM et al. (2023) Three-port converters for energy conversion of PV-BES integrated systems—a review IEEE Access 11:6551–6573. https://doi.org/10.1109/ACCESS.2023.323 5924 11. Islam MR et al (2020) Design and characterisation of advanced magnetic material-based core for isolated power converters used in wave energy generation systems. IET Electr Power Appl 14(5):733–741. https://doi.org/10.1049/iet-epa.2019.0299 12. Farrok O, Islam MR, Muttaqi KM, Sutanto D, Zhu J (2020) Design and optimization of a novel dual-port linear generator for oceanic wave energy conversion. IEEE Trans Industr Electron 67(5):3409–3418. https://doi.org/10.1109/TIE.2019.2921293

Chapter 7

Flux Switching Linear Generator: Design, Analysis, and Optimization Abdirazak Dahir Tahlil, Md. Abdullah-Al-Mamun, Md Rabiul Islam, and Omar Farrok

Abstract Flux switching linear generator and Vernier hybrid machine comprise of solid translator with huge weight. As there are slots and teeth at both ends of the translator it should be thick enough because of its design limitation. Therefore, while generating power from the sea wave, it results in poor dynamics because of its bulk weight. In this chapter, a new design of the FSLG is presented along with the model where the translator weight is reduced and magnetic flux linkage of the main stator is improved by applying static steel core in the secondary stator. The produced voltage, current, power, efficiency, core loss, force ripples, and cogging force reduction for the FLSG is presented. The new translator is lightweight and can generate enough electrical energy from the oceanic wave as shown in the dynamic model. Genetic algorithm is used to find out the optimal design of the translator and stator before it is finally selected. To observe the improvement and possibility to utilize this design of FSLG, finite element analysis is conducted by utilizing ANSYS/Ansoft. Keywords Direct drive technology · Linear machines · Oceanic wave · Permanent magnet machines · Wave energy converters

7.1 Introduction In the last half century, because of the enormous development of human civilization, electrical energy demand increases. By using fossil fuel based traditional energy sources (such as natural gas, coal, diesel, etc.) the reserve is largely decreasing. A. D. Tahlil · Md. Abdullah-Al-Mamun · O. Farrok Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected] M. R. Islam (B) School of Electrical, Computer and Telecommunications Engineering (SECTE), Faculty of Engineering and Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_7

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Even though the world still depends on traditional energy sources which are not environment friendly due to emission of large amount of carbon dioxide gas that causes air pollution and climate change to the environment [1, 2]. To minimize the environmental issues, the scientist tried to get new ways for electricity generation. One of the suggested ways is to use renewable energy sources such as solar, wind, biomass, biogas, hydro, oceanic wave, geothermal, and fuel cell to produce electricity. Renewable energy is green and clean energy. It has numerous environmental benefits and the renewable energy sources (RESs) will not run out after electricity generation. RESs are globally accepted to cope with increased power demand with sustainability. Nowadays, RESs have the capability to produce sufficient energy to meet global demand [3]. According to a recommendation, it is expected that the US is going to increase renewable energy based power production from 30% up to 90% by 2050. Besides using other RESs, almost 50% of the energy will be produced by utilizing wind and solar photovoltaic energy [4]. In another statement about US it is found that 80% of the overall electricity will be generated by renewable energy [4]. Among various RESs, a negligible amount of oceanic wave energy has been utilized for power generation till now [5]. Since the last two decades, some companies and researchers have been encouraging to use oceanic wave energy (OWE) for electricity generation [6]. In the current situation, oceanic wave energy is harvested only 0.02% of EU energy requirement and it is mainly utilized for the generation of electrical power [7]. Oceanic wave energy can also be a solution to high carbon emission to the environment. Due to geographical location, UK has the highest potential of wave energy. In the last decade, the UK had a target to generate 20% of its electrical power demand by 2020 by utilizing oceanic wave [8]. A few years ago, China had a target to meet 15% of their total electricity demand by incorporating RES especially by the oceanic wave energy [9]. Ireland had a target to produce 500 MW electricity for OWE by the year of 2020 [10]. If it is possible to utilize only a few of the unharnessed OWE available to the ocean, it could be remarkable for the entire world [11]. Oceanic wave energy has various benefits compared to other renewable energy. It is available most of the time and it can be forecasted better than other renewable energy sources [12]. Oceanic wave energy has high power density that can produce adequate amount of energy with less cost without hampering visual impact [13]. Compared to other renewable energy like solar and wind, oceanic wave energy has the highest energy density and availability [14]. The predicted quantity of wave energy resource is projected to be 10 TW in the open sea area. It is a notable amount compared to the total global energy consumption. In Fig. 7.1, it illustrates the amount of harnessed renewable energy in different years. It also shows the data of Organization for Economic Cooperation and Development, which is greater (roughly 18% in the year of 2010 and 33% by the year 2035) than the other areas [15]. The amount of electricity generation utilizing renewable energy is increasing day by day. Total power production reached up to 20% in 2010 and it will be increased up to 31% by 2035. The first wave energy technology was built in 1799 [16]. The expansion of wave energy technology has been increased since the last 35 years [17] and EU has 16% of the total available wave energy in the world [18].

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Fig. 7.1 Power generation in various regions by renewable energy [15]

Different international companies are trying to develop various kinds of wave energy devices to convert irregular wave motion into a regular mechanical motion. Direct drive devices are widely used because linear electrical generators can be mounted to it for producing electricity. Linear generators are quite advantageous for energy conversion because of their simple and rugged design. Permanent magnet linear generators are highly utilized in different point absorber type power take off system. Flux switching linear generator falls under the category of linear electrical generator. In this chapter, a flux switching linear generator (FSLG) with optimized translator weight is presented. A brief discussion is provided about the manufacturer of oceanic wave energy devices in Sect. 7.2. Section 7.3 illustrates the prospect and utilization challenges of oceanic wave energy. A mathematical representation (model) of oceanic wave energy is presented in Sect. 7.4. Limitations of existing flux switching linear generator are mentioned in Sect. 7.5. Weight optimization of existing FSLG is discussed in Sect. 7.6. With brief discussion, simulation results are shown in Sect. 7.7. Finally, the chapter is concluded in Sect. 7.8, which describes the summary.

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7.2 Manufacturer of Oceanic Wave Energy Devices As we all know that the 70% of the earth is covered by water, so there is huge opportunity to harness energy from the ocean. It is possible to utilize oceanic wave energy throughout the year. Electricity generation from oceanic wave requires a sophisticated technology. It needs time and a huge amount of money as well. In September 2008, Aguçadoura Wave Farm developed a wave energy converter called Pelamis. It is one of the well known wave power devices. It is used to extract energy from the ocean surface to generate electricity. Two months later, the project was postponed due to technical problems. After the collapse of that project, five companies are trying to improve the device. Aquamarine Power in Scotland re-designed the shape of that device and they claim it is effective. It is an onshore type of device and it has the ability to provide energy to thousands of homes every day. Pelamis is known as surface attenuator and line absorber because of its working principle. The attenuator consists of some connected sections that move when the wave hits its connected segments sequentially. The joined units bend when the wave passes each section. Its size is comparable to a typical wavelength of the oceanic wave. It is aligned along or normal to the direction of wave propagation. As a wave energy converter, Pelamis is also developed by the company “Pelamis Wave Power.” The company claims that the design can improve the effectiveness of converting wave power into electrical power. Each section of Pelamis is around 600 ft long and it has the capability of providing energy to almost 500 residents in a year [19]. The Green Wave comprises an underwater tunnel and a cabin full of air that is situated into the sea surface. There is a small hole which has a turbine. When the waves come one after another, the device is moved with water. The movement squeezes air inside the cabin and the air is forced through the outlet. A vacuum chamber is created due to water falling. The Green Wave is relatively cost effective compared to other wave energy devices and it can overcome storm and bad weather condition. Oyster is another type of device that utilizes hydraulic to transmit high pressure water to shore. The water is stored in an overhead reservoir. While falling into sea, the water rotates a turbine coupled to a generator. The generator thus converts kinetic energy of the sea water into electrical power. Maintenance works can be done easily for that kind of devices [19]. The Ocean Power Technology introduced “PowerBuoy” which has a moving part having 5 ft diameter, 5 ft high, and 25 ft tall pole attached to it. In Victoria state of Australia, Lockheed Martin signed a contract to build 62.5 MW wave energy harnessing project commercially. This competence of Autonomous PowerBuoy allows to be the biggest energy generator of this type in the globe [19]. The AWS III was installed in Scotland in 2010. This is a floating wave energy device which has rubber skin on the outside face. When ocean wave passes, the moving air inside the chamber drives an air turbine to produce electricity. The AWS III is checked in Loch Ness in 2010. The height of this device is 60 m and its electricity production ability is approximately up to 2.5 MW. This is placed offshore that is 100 m

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deep into the water [20]. Islay LIMPET produces high energy from oceanic wave. This is a nearshore oscillating water column device which is used to harness energy. It was installed in Scotland in 1991 which can produce up to a few hundred MW of power [21]. R38/50 kW and R115/150 kW are two projects installed in UK in 2010 using an offshore attenuator wave energy device. The electricity generation ability of R38/50 kW is 50 kW while R115/150 kW can produce up to 150 kW. It works by pulling out energy from the relative motion in the middle of higher member and lower member. They use an advanced method in a device/machine that was awarded as first one from UK Trade and Investment (UKTI) Research and Development Award in 2011 [22]. The machine extracts energy from the relative motion between one upper member and one lower member. The initial (1st) generation full scale prototype of this type of device was tested offshore in 2010. The second generation of full scale prototype was tested offshore during 2011. In 2012 the first one is traded to several customers in different regions of the world. AMOG is developing through a technological approach in wave energy sector. One third of the devices are installed in Europe in 2019 summer at FaBTest. During installation, the financial support was given under the European Union Regional Development Grant and Cornwall Development Company. The device was built in Wales. It has a raft shape with air pendulum which is made to captivate wave energy. A power take off device is located on the top of the pendulum. The power is produced inside the immersion heaters placed in sea water. The highest energy can be produced up to 75 kW. In 2008, an onshore project was designed in Finland called Penguin. At EMEC test in summer 2012, the initial power was found to be up to 0.5 MW [22]. This project was improved in 2017 and then reinstalled again at Billia Croo. It was funded under Clean Energy from Ocean Waves research project. This is a 5 year project that has a goal to produce 3 MW of power [22]. The project is organized by a service company named Fortum.

7.3 Prospects and Challenges The prospects and challenges of oceanic wave energy are discussed in the following.

7.3.1 Prospects of Wave Energy Largely untapped energy is available to ocean in the form of wave, tidal, marine current, and ocean thermal. Wave energy has become an important source of electricity since the fossil fuel and other sources of energy are declining. OWE has the highest energy production capability compared to other renewable energy sources. It is available almost all the time, which is an exceptional feature compared to the solar and wind energy [14]. Approximately 8,000 to 80,000 TWh wave energy is

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available globally, whereas almost 1170 TWh/year energy can be produced from these types of sources. If 10% of wave energy is utilized, it can remarkably reduce the energy crises around the world. The West Coast, East Coast, Gulf of Mexico, Alaska, Hawaii, and Puerto Rico contribute to energy generation with a production of 250, 160, 60, 620, 80, and 20 TWh/year respectively [23]. In 2010, UK government produced 10% of total power from renewable energy [24]. Previously, there was a goal to produce up to 20% of its electricity from oceanic wave by the year of 2020. Like UK, Ireland had similar goal of producing up to 500 MW energy by the same year (2020) [25]. China has developed electricity generation technology from wave energy around 21.79 GW [26]. European countries have estimated wave power in between 20–60 kW/m of wave front. It is quite higher than other geographical locations. Therefore, there is a good opportunity to produce enough energy [27].

7.3.2 Challenges of Oceanic Wave Energy The major challenges for the development of oceanic wave energy are high project cost and environmental issue. Wave energy device faces several technical difficulties because of variable amplitude and frequency of ocean waves. So, the frequency of output voltage can be as low as 0.1 Hz which must be increased up to 50/60 Hz before its supply to the load or grid [28]. Wave energy converters face harsh marine environmental situations. These significant difficulties discourage further improvement of wave energy devices that essentially require proper operational planning. Cyclone affects wave power level to increase severely which brings uncertain condition for electricity generation. Although various models are mentioned in different literatures, no convergence of wave conversion technology is guaranteed. Hence, it is hard to anticipate the challenges and address them at the beginning stage of a wave energy converter (WEC). Recently, one of the most used wave energy devices is Pelamis. It is not cost effective in comparison to solar and wind energy [29]. Another challenge is the distance between electrical grid and location of wave energy devices. The project cost increases for long transmission. In offshore, wave motion is completely different than nearshore. Thus, it is not so easy to forecast the amount of energy harvesting. To produce electricity, unbalanced ocean wave is an additional obstacle [30]. Nearshore equipment can be positioned in ocean floor. But the output electricity is still very low compared to offshore devices. The construction and operation are inexpensive at nearshore devices. But they must sustain throughout the extreme weather conditions. Some engineers claim that offshore devices can offer better performance compared to others. Most energy is obtained to the surface at one quarter of wavelength [17]. The challenges of efficiently capturing irregular wave motion need to be considered while designing any device. The device along with whole system should be operated at the most common wave power level to gain maximum efficiency. Around 30–70 kW/m of offshore wave energy is common to British Isles and Western coasts of Europe [17]. WEC has environmental and social effect for building and operating wave energy technology where the impacts

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may be negative or positive. Integrating wave energy devices is the main cause of environmental impact compared to other renewable energy. However, as wave energy does not use any land area, operation of WEC has a little effect on marine life and fish. Careful cite selection is necessary for electricity generation from ocean wave energy. It can improve economy of a country [31].

7.4 Oceanic Wave Model Generally, oceanic wave is considered as sinusoidal in numerical analysis [32, 33]. If the vertical position of wave is z w (t), the wave velocity is vw (t), the wave amplitude is Aw , the frequency of the wave is f w , and the initial phase angle is θ j , the equation of oceanic wave motion is given below. z w (t) = Aw sin(2π f w t ± θ j )

(7.1)

vw (t) = 2 Aw π f w cos(2π f w t ± θ j )

(7.2)

Simply, a translator connected to a buoy follows the surface of oceanic wave. However, practical location of the buoy differs from that of the wave surface. Since the buoy displacement and velocity vary from those in the ocean, two state variables namely displacement (z tr ) and velocity (vtr ) are assumed to indicate the movement of the buoy or translator. So, the motion can be represented as d d (z tr ) = vtr ; (vtr ) = atr dt dt

(7.3)

d2 ρgπa 2 [z tr − z w (t)] + (Rr + Rv )[vtr − vw (t)] (z ) = 2 tr Mb + Mw + Mtr dt

(7.4)

where the translator acceleration is denoted as atr , the mass density of sea water is ρ, the acceleration due to gravity is g, the cylinder radius is a, the radiation resistance is Rr , the viscous resistance of the water of the ocean is Rv , the buoy mass is M b , the added mass due to sea water is M w , and the translator mass is M tr . From (7.4) it is found that for larger translator mass, the acceleration, atr reduces which would be the cause for low translator velocity. The ocean wave length is much more higher than the diameter of a cylindrical buoy. The surface water surrounding the buoy can be approximated by a plain surface. It can be seen from the front view of the buoy with a generator as drawn in Fig. 7.2. The power produced by a floater is restrained due to inertia of translator and buoy. For the larger mover mass of a linear generator, the net force minimizes due to elevation. For the linear generator, it works as the mechanical input. The minimized net elevation force reduces vtr and atr . There is a relation between vtr and generation

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Fig. 7.2 Front view of a linear generator coupled to a floater on sea surface [34]

of electric power. Higher electromotive force is produced for higher value of vtr and vice-versa. According to the law of Newton, atr is inversely proportional to its mass for a specified force due to elevation generated by the buoy. Therefore, for a linear generator with a specified stroke length, a massive translator would lead to a low peak velocity. Thus, translator having larger mass causes small amount of electrical power generation. Mathematical model of the oceanic wave and linear generator as presented in this chapter are partly similar to that of [35].

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7.5 Limitations of the Existing FSLG The isometric perspective of a traditional FSLG is presented in Fig. 7.3a. It comprises a permanent magnet, steel cores, stator windings, and a movable translator that moves vertically in the oceans as seen in Fig. 7.3b and c. The arrow indicates direction of the magnetic flux lines of the permanent magnet. As the translator moves during operation, its vertical position changes. As a result, translator teeth and slots appear one after another. The magnetic flux line is shown in Fig. 7.3b and c for two separate positions of the translator. The flux line travel in reverse route in stator core for two distinctive locations. However, the vertical location of translator varies because of sea waves. Flux lines shift their orientation over the time. So, the flux line orientation varies along the stator coils and that is the cause for electrical power production. The velocity of translator is generally assumed to be 0.5–2 m/s in various research papers such as reported in [33]. The magnetic flux of the traditional FSLG for WEC is investigated and presented in Fig. 7.4 as a rainbow spectrum. Even though the vertical movement of translator, the number of

Fig. 7.3 a The conventional FSLG. b Alignment of the stator and translator teeth at a particular time. c Teeth alignment is changed for movement of the translator [34]

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Fig. 7.4 Magnetic flux lines that travel in the existing FSLG [34]

magnetic flux lines is huge in certain sections at a particular time, and small in other sections of the translator as shown in Fig. 7.4. After some time interval, both the area having higher and lower flux density shifts according to its movement. Consequently, all portion of the translator is not required to provide pathway of magnetic flux to generate electricity. It is marked as one of the limitations of FSLGs. The following section describes the method of optimization to overcome this limitation. Another problem of this generator is the bulk weight of its translator. Poor dynamics occur for a massive translator, which can be visualized by (7.4).

7.6 Optimization of the Existing FSLG The permanent magnet of NdFeB is applied to this suggested FSLG. Here the magnetic coercivity, H c = −837.5 kA/m; remanence, Brem = 1.129 T; and relative permeability, μr = 1.045 are considered to produce magnetic flux. Copper conductors are used for winding. For their appropriate specifications, silicon steel sheets are used to build the magnetic cores of the FSLG. Figure 7.5 indicates the corresponding model of secondary stators with the introduced translator.

7.6.1 Working Principle Unlike the other FSLGs, total magnetic flux, Φ PM from primary stator of the introduced FSLG will not return to the stator through the translator. It divides into two components. Among them Φ SS travels through the secondary stator and rest of the magnetic flux, Φ ST passes through the translator. It is shown in Fig. 7.6 for two different translator positions. For magnetic segregation, nonmagnetic material is used around the secondary stator. It is indicated by blue color. The key difference

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Fig. 7.5 Translator and secondary stator cores of the proposed FSLG [34]

Fig. 7.6 Flux switching of the proposed FSLG for two different positions [34]

between the traditional FSLG and the proposed one is that magnetic flux switching only takes place in the primary stator core, not in the secondary stator core.

7.6.2 Architecture and Vector Diagram Figure 7.7 represents the whole structure of the FSLG. The translator has a hollow segment with secondary stators that is enclosed by nonmagnetic material.

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Fig. 7.7 Construction of the proposed FSLG [34]

Every winding of the proposed generator has two coils in each phase. In Fig. 7.7 W 1a (winding -1) has two coils which are indicated by C 1 and C 2 . Thus, there are a total of 12 coils. The vector diagram of winding voltage of the introduced FSLG is shown in Fig. 7.8 where θ = 60º. The difference between phase-n and phase-n' is 60º while n = a, b, and c. The proposed FSLG decreases force ripples and cogging

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Fig. 7.8 Vector diagram of the coil voltages of the proposed FSLG [34]

forces by shifting the stator position. In Fig. 7.9, the phasor diagram of the introduced generator is shown.

7.6.3 Phasor Diagram The static portion or stator of a traditional FSLG is shown in Fig. 10a. However, the translator size and mass were not reduced. Optimization is required to reduce the weight of the translator. The magnetic core of translator conducts essential flux. To reduce the weight of translator, number of cores should be reduced. Less cores reduce the magnetic flux linkage between stator and translator. This problem was solved by optimizing the shape and size of both split translator and secondary stator to obtain an effective design. The weight is decreased in the introduced FSLG model that is presented in Fig. 7.10b. The difference between the introduced and previous designs of FSLG are shown in Fig. 7.10 using dotted lines. The stator core model of FSLG was derived from a genetic algorithm. The initial population for selection of

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Fig. 7.9 Phasor diagram of the proposed FSLG [34]

secondary stator shape has been sketched in Fig. 7.11. Two out of six basic shapes, Fig. 7.11c and f exhibit better performance. In Figs. 7.5 and 7.6, it is explained that two comparable secondary stator shapes have been developed. Finite element method is used to calculate the produced electricity. For better efficiency, size optimization is required for compact structure. Unnecessary large core size increases core loss. On the other hand, if the secondary

Fig. 7.10 Design of the a traditional flux switching permanent magnet linear generator and b the proposed FSLG [34]

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Fig. 7.11 A few possible secondary stator shapes: a rectangular, b triangular, c trapezoidal, d circular, e elliptical, and f special curve for initialize population for shape optimization [34]

stator has less amount of core, it is not possible to produce the required magnetic flux. The introduced design balances both the weight and efficiency of the FSLG.

7.6.4 Equivalent Circuit Diagram The equivalent circuit diagram of introduced FSLG is drawn in Fig. 7.12. E 1a , E 2a , E 3a , E' 1a , E' 2a , and E' 3a are generated voltages in the windings. The dotted lines indicate identical sections of voltages from E 1a to E' 3a and bridge diodes. Here Ra and L a are armature resistance and winding inductance, respectively. The produced ac power from windings is rectified by bridge diodes and provide it to the dc link.

7.6.5 Electricity Generation Electricity generation depends on pole pitch and voltage constant along with other common parameters. Considering pole pitch τ tr and the voltage constant K E , the induced voltage per winding per phase, E ϕ of the linear generator can be written as (

) Z tr π vtr τtr

(7.5)

K E = N W T Bair Ws vmean

(7.6)

E ϕ = K E cos where

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Fig. 7.12 Equivalent circuit of the proposed FSLG [34]

where N WT is the number of turns per phase, effective width of the stator is W s , the flux density of air gap is Bair , and the average speed of translator is vmean . No load rms voltage, E i of each phase can be represented as √ Ei = π 2 f E NW T K w ϕ P M

(7.7)

Where fE =

vtr 2τ tr

(7.8)

where the frequency of induced voltage is denoted by f E , the excitation flux of permanent magnet is represented by ϕ P M , and the winding factor is considered as K w . The air gap flux, Φ g resists Φ PM for armature reaction. The net flux ϕnet = ϕg − ϕl is the magnetic flux which flows through the core, and the leakage flux is ϕl . Thus, altering Φ PM by Φ net in (7.7) defines the amount of electromotive force at loaded condition. Considering armature reaction, d-axis magnetomotive forces, F d and q-axis magnetomotive forces, F q can be calculated by ( √ ) 3 2 NW T Fd = K d K w Ia sina π p ( √ ) 3 2 NW T Fq = K q K w Ia sina π p

(7.9)

(7.10)

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where the number of pole pair is p and the armature current is Ia . The d-axis reaction factor, K d and q-axis reaction factor, K q can be calculated by Kd =

kfd K fq ; Kq = Kf Kf

(7.11)

where excitation field form factors are K f . K f d and K f q represent the factor for daxis and q-axis. The winding factor, K w established for fundamental space harmonic is represented as K W 1 = K d1 K p1 ωc ) 2 τ (π ) sin 6 ) ( = π 5 × ps sin 30× ps

K p1 = sin K d1

(π

×

(7.12) (7.13)

(7.14)

where ωc is the coil pitch and the permeance of the introduced FSLG is denoted by ' ps . If X d , X q , X d, and X q' are the synchronous and transient reactances for direct and quadrature axes, respectively, the excitation voltage, E av and power, P can be determined by ( ) E av = Id X d − X d' + Vq + X d' Id

(7.15)

{ } P = Vt Id sin(δ) + Iq cos(δ) or, P =

E q' Vt '

Xd

sin(δ) +

] [ Vt2 1 1 − ' sin(2δ) 2 Xq Xd

(7.16)

where d-axis current is denoted by Id , q-axis current is indicated by Iq , q-axis voltage is denoted by Vq, and the terminal voltage is represented by Vt . E q ' is the transient electromotive force along q-axis. The second order harmonic component can be ignored for simplification. The angle between E g ' and E q ' is very low as shown in the phasor diagram. Hence, (7.16) can be reduced as P=

E g' Vt X d'

sin(δg )

where E g ' can be called the electromotive force behind transient reactance.

(7.17)

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7.7 Simulation Results The translator velocity is set to 1 m/s in the simulation platform. For proper surface approximation, the angle is considered 3° in the mesh design for both in the primary and secondary stators of the suggested FSLG. Fine mesh is considered for air gap of the FSLG design. During investigation, it is found that adequate magnetic flux travels inside the core of the secondary stator. A few flux lines flow inside the translator and the remaining flux lines pass through secondary stator. To determine the efficacy of the secondary stator whether it works properly or not, the magnetic flux lines and flux density are required to be analyzed. The highest magnetic flux density in the split translator and the secondary stator of the FSLG are found 1.6 T and 0.85 T, respectively, from simulation results. The produced electricity for 7 Ω, 15 Ω, and 25 Ω resistive loads of the FSLG are plotted in Fig. 7.13. The load currents for the resistive inductive loads (15 Ω with 100 mH, 250 mH, and 500 mH) are shown in Fig. 7.14. The induced voltage, current, and the power for resistive capacitive loads (15 Ω with 2 mF) are shown in Fig. 7.15.

Fig. 7.13 Generated powers for different resistive loads of the FSLG [34]

Fig. 7.14 Load currents for different inductive loads of the FSLG [34]

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Fig. 7.15 The current, induced voltage, and power for a capacitive load of the FSLG [34]

The cogging force minimization is illustrated in Fig. 7.16, where the cogging force for 1a, 2a, and 3a result for the stators on the right hand side of the z-axis as labeled in Fig. 7.7. However, cogging force for 1a' , 2a' , and 3a' means the same for the stators located on the left hand side of the z-axis. Force ripples reduction is shown in Fig. 7.17. They are horizontal force components. Minimization of the force ripples for the proposed FSLG is plotted in Fig. 7.18 for phase a, b, and c that act on the right hand side of the z-axis (Fig. 7.7). On the other hand, the resultant force a' , b' , and c' act on the left hand side of the z-axis (Fig. 7.7) are plotted in Fig. 7.18 as well. It is seen that they mostly cancel each other. Therefore, the total resultant force ripple is minimized. The produced electricity for various translator width of the traditional FSLG along with introduced (proposed) FSLG translator are presented in Fig. 7.18. The objective is to get equivalent translator size for the same power generation. The translator width of preferred FSLG is 10 mm and it is equivalent to 24 mm with the traditional one.

Fig. 7.16 The resultant cogging force of the proposed FSLG [34]

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Fig. 7.17 Minimization of the force ripples of the proposed FSLG [34]

Fig. 7.18 Comparison of the generated electrical powers for different translator widths of the proposed and the existing FSLGs [34]

The front view of the traditional FSLG translator is presented in Fig. 7.19a and that of the split translator of the introduced one is presented in Fig. 7.19b. The applied force, voltage, current, power, and efficiency at different load conditions are shown in Fig. 7.20. In Fig. 7.21, efficiency, voltage, force, power, current, and core loss at various speeds are plotted. Efficiency of the traditional and proposed FSLGs are determined based on the force for the armature reaction, Far as follows η=

Vdc Idc × 100% vtr Far

(7.18)

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Fig. 7.19 Equivalent translator width for the a existing and b the proposed FSLG translator design [34]

Fig. 7.20 Plot of five significant parameters for different load conditions [34]

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Fig. 7.21 Plot of seven significant parameters versus speed variation [34]

where the rectified voltage is indicated by V dc and the load current is I dc . Table 7.1 demonstrates typical parameters that are used both for the traditional and the proposed FSLGs. The eddy current loss, hysteresis loss, and the core loss of the split translator and stator of the proposed FSLG are shown in Fig. 7.22. The air gap in between split translators and secondary stator is called secondary air gap as shown in Fig. 7.6b. The produced electricity for different air gaps is presented in Fig. 7.23. A comparison for the traditional and the proposed FSLG is presented in Table 7.2. Considering the gravitational acceleration force Fga , specific gravity of 7.87 and gravitational acceleration of g = 9.8 m/s2 , the efficiency is calculated as Table 7.1 Common parameters of the FSLG [34]

Parameters name

Value

Unit

Translator tooth width

4.5

mm

Translator slot width

5.5

mm

Translator tooth/slot depth

5

mm

Translator length

1.6

m

Translator tooth pitch

10

mm

Turn number of copper coil

130

turns

Depth of LG

0.25

m

Winding factor

0.6

–

Velocity of translator

0.5–2

m/s

Thickness of pm

6

mm

Stroke length

0.8

m

Load resistance

5–30

Ω

Winding resistance

3

Ω

Air gap

1

mm

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Fig. 7.22 Core losses measured in the stator and the split translator core of the proposed FSLG [34]

Fig. 7.23 Generated electrical powers for different length of the air gaps [34] Table 7.2 Comparison of the traditional and the proposed FSLG [34] Name of the item

Unit

Traditional FSLG

Proposed FSLG

Power generation of a unit for default condition

W

500

500

Width of the translator

cm

2.4

1×2

Mass of the translator

kg

40.6

31.74

Required upward force

kN

0.945

0.859

Efficiency

%

57.93

63.79

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η=

Vtr

(

Vdc Idc ) × 100% Far + Fga

(7.19)

There are a few limitations of the conventional linear electrical generator including FSLG. One of them is the zero velocity of the translator at both upper and lower ends of the generator during operation. This problem can be avoided by applying the method as described in [36]. The other problem is variable voltage and frequency of the generated voltage due to the nature of oceanic waves. This is a common problem of the power generation by other renewable energy sources. A converter is required to apply for obtaining regulated power and other electrical parameters. Different converters are discussed in [37] that can also be applicable to the oceanic wave energy based power plants. As a wave farm consists of many electrical generators, their protection is required for safe operation. An isolation concept is illustrated in [38] for safety purpose of a wave farm.

7.8 Summary The limitation of the conventional FPLG and its solution is mainly discussed in this chapter. Bulky and massive translator is identified as the problem and minimization of its weight is considered as the solution. The translator weight of the FSLG as described is less than the traditional one with slightly different working principle. Thus, it can produce higher velocity and more power because of its improved dynamics. The proposed split translator has 21.82% less weight than that of the traditional one. The efficiency is improved of the suggested FSLG by 5.86% than the traditional one with the same incident wave as the buoyancy force must eliminate some gravitational force because of the reduced mass of the translator.

References 1. Molla S, Farrok O and Alam, MJ (2023) Zero carbon emission based electrical power plant by harvesting oceanic wave energy: Minimization of environmental impact in Bangladesh. In: Climate change and ocean renewable energy. CCORE 2022. Springer Proceedings in Earth and Environmental Sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-26967-7_1 2. Kabir MA et al (2023) Net-metering and Feed-in-Tariff policies for the optimum billing scheme for future industrial PV systems in Bangladesh. Alex Eng J 63:157–174. https://doi.org/10. 1016/j.aej.2022.08.004 3. Robin P, Fujita RM (2002) Renewable energy from the ocean. Marine Policy, Elsevier 26(6):471–479 4. Mai T et al (2014) Renewable electricity futures for the United States. IEEE trans sustain Energy 5(2):372–378. https://doi.org/10.1109/TSTE.2013.2290472

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5. Leijon M et al. (2008) Wave Energy from the North Sea. Experiences from the Lysekil Research Site. Surveys in Geophysics, 29(3): 221–240. https://doi.org/10.1007/s10712-008-9047-x 6. Zhang H et al. (2013) Design and simulation of SMES system using YBCO tapes for direct drive wave energy converters. IEEE Trans Appl Supercond., 23(3), Article no. 5700704. https:// doi.org/10.1109/TASC.2012.2232954 7. European Ocean Energy Association (2010) Oceans of energy: European ocean energy roadmap 2010–2050 8. Farrok O, Ahmed K, Tahlil AD, Farah MM, Kiran MR, Islam MR (2020) Electrical power generation from the oceanic wave for sustainable advancement in renewable energy technologies. Sustainability 12(6):1–23. https://doi.org/10.3390/su12062178 9. World Offshore Renewables Report (2004–2005) Department of Trade and Industry 10. Ocean Renewable Energy Coalition (2011) U. S. marine and hydrokinetic renewable energy roadmap. Power 1–30 11. Liu C, Yu H, Hu M, Liu Q, Zhou S, Huang L (2014) Research on a permanent magnet tubular linear generator for direct drive wave energy conversion. IET Renew Power Gener 8:281–288. https://doi.org/10.1049/iet-rpg.2012.0364 12. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J, Lei G (2018) Oceanic wave energy conversion by a novel permanent magnet linear generator capable of preventing demagnetization. IEEE Trans Ind Appl 54(6):6005–6014. https://doi.org/10.1109/TIA.2018.2863661 13. Leijon M et al (2009) Catch the wave to electricity. IEEE Power Energy Mag 7(1):50–54. https://doi.org/10.1109/MPE.2008.930658 14. Drew B, Plummer AR, Sahinkaya MN (2009) A review of wave energy converter technology. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 223(8):887–902. https://doi.org/10.1243/09576509JPE782 15. Islam MA, Hasanuzzaman M, Rahim NR, Nahar A, Hosenuzzaman M (2014) Global renewable Energy-Based electricity generation and smart grid system for energy security, Hindawi Publishing Corporation the Scientific World Journal, Article no.197136. https://doi.org/10. 1155/2014/197136 16. Binh PC, Tri NM, Dung DT, Ahn KK, Kim SJ, Koo W (2016) Analysis, design and experiment investigation of a novel wave energy converter. IET Gener Transm Distrib 10(2):460–469. https://doi.org/10.1049/iet-gtd.2015.0821 17. Truong DQ, Ahn KK (2014) Development of a novel point absorber in heave for wave energy conversion. Renew Energy 65:183–191. https://doi.org/10.1016/j.renene.2013.08.028 18. Rodrigues L (2008) Wave power conversion systems for electrical energy production. Renew Energy Power Qual J. 1:601–607. https://doi.org/10.24084/repqj06.380 19. Wave Power: 5 Bright Ideas to Capture the Ocean’s Energy: The Pelamis Wave Energy Converter https://www.popularmechanics.com/science/energy/g497/5-bright-ideas-tocapture-the-oceans-energy/?slide=3. Visited 30 September 23 20. AWS Technology. AWS Ocean. Retrieved https://awsocean.com Visited 17 November 2019 21. International Water Power and Dam Construction https://www.waterpowermagazine.com/pro jectprofiles/industry_projectprofiles_archive.html. Visited 17 November 2019 22. List of wave power project: https://en.wikipedia.org/wiki/List_of_wave_power_projects. Visited 15 December 2019 23. Hagerman G, Scott G (2011) Mapping and assessment of the United States Ocean wave energy resource, https://www.osti.gov/servlets/purl/1060943#:~:text=The%20average% 20annual%20and%2012,the%20limit%20out%20to%20which. Visited 15 December 2019 24. Josseph P, Martin S, Ean M, Ted B, Annette VJ (2010) A permanent-magnet tubular linear generator for ocean wave energy conversion. IEEE Trans Ind Appl 46(6):2392–2400. https:// doi.org/10.1109/TIA.2010.2073433 25. Zhang D, Wei L, Yonggang L (2009) Wave energy in China: Current status and perspectives. Renew Energy 34(10):2089–2092. https://doi.org/10.1016/j.renene.2009.03.014 26. Yang X, Song Y, Wang G, Wang W (2010) A comprehensive review on the development of sustainable energy strategy and implementation in China. IEEE Trans Sustain Energy 1(2):57– 65. https://doi.org/10.1109/TSTE.2010.2051464

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27. Wu F et al (2013) Modelling control strategy and power conditioning for direct-drive wave energy conversion to operate with power grid. Proc IEEE 101(4):925–941. https://doi.org/10. 1109/JPROC.2012.2235811 28. Chenari B, Saadatian SS, Ferreira A (2014) Wave energy systems: An overview of different wave energy converters and recommendation for future improvements. In: 8th International Technology, Education and Development Conference, pp 6266–6272 29. Mueller M, Jeffrey H (2008) UKERC marine (wave and tidal current) renewable energy technology roadmap: summary report. University of Edinburgh, Edinburgh, UK, UK Energy Research Centre 30. Mustapa MA, Yaakob OB, Ahmed YM et al (2017) Wave energy device and breakwater integration: A review. Renew Sustain Energy Rev 77:43–58. https://doi.org/10.1016/j.rser.2017. 03.110 31. Tom N, Yeung RW (2016) Experimental confirmation of nonlinear-model- predictive control applied offline to a permanent magnet linear generator for ocean-wave energy conversion. IEEE J Oceanic Eng 41(2):281–295. https://doi.org/10.1109/JOE.2015.2439871 32. Farrok O, Islam MR and Sheikh MRI (2016) Analysis of the oceanic wave dynamics for generation of electrical energy using a linear generator. J Energy, 1–14, Art. no. 3437027. https://doi.org/10.1155/2016/3437027 33. Molla S, Farrok O, Islam MR, Xu W (2023) A systematic approach for designing a highly efficient linear electrical generator for harvesting oceanic wave energy. Renew Energy 204:152– 165. https://doi.org/10.1016/j.renene.2023.01.020 34. Farrok O, Islam MR, Sheikh MRI, Guo Y, Zhu J (2018) A split translator secondary stator permanent magnet linear generator for oceanic wave energy conversion. IEEE Trans Industr Electron 65(9):7600–7609. https://doi.org/10.1109/TIE.2017.2767521 35. Molla S, Farrok O, Islam MR, Muttaqi KM (2020) Application of iron nitride compound as alternative permanent magnet for designing linear generators to harvest oceanic wave energy. IET Electr Power Appl 14(5):762–770. https://doi.org/10.1049/iet-epa.2019.0372 36. Farrok O, Islam MR, Muttaqi KM, Sutanto D, Zhu J (2020) Design and optimization of a novel dual-port linear generator for oceanic wave energy conversion. IEEE Trans Industr Electron 67(5):3409–3418. https://doi.org/10.1109/TIE.2019.2921293 37. Haque MM et al. (2023) Three-port converters for energy conversion of PV-BES integrated systems—a review IEEE Access 11: 6551–6573. https://doi.org/10.1109/ACCESS.2023.323 5924 38. Islam MR et al (2020) Design and characterisation of advanced magnetic material-based core for isolated power converters used in wave energy generation systems. IET Electr Power Appl 14(5):733–741. https://doi.org/10.1049/iet-epa.2019.0299

Chapter 8

Linear Electrical Generator for Hydraulic Free Piston Engine Abdirazak Dahir Tahlil, Omar Farrok, Md. Abdullah-Al-Mamun, and Mahamudul Hasan Uzzal

Abstract In a free piston linear generator (FPLG), a free piston engine drives a linear electrical generator to produce electrical power. In this chapter, different parameters of an FPLG are analyzed. The parameters include stroke length, power, voltage frequency, and voltage. Firstly, the construction and working principle of an FPLG are explained. Among the parameters of FPLG, stroke length is discussed in the beginning. It is found that the minimum and maximum stroke lengths are 20 mm and 152.4 mm, respectively. Stroke lengths of a free piston engine and a linear electrical generator must match each other. Then output powers of different FPLGs are described and tabulated. The power range is found from 22.23 W to 95 kW. Voltage frequency of the FPLG ranges from 2 to 67 Hz which is listed in a separate table. The maximum output voltage is found to be 400 V whereas most of the FPLG produce less than 300 V. Performance of the FPLG depends on its design, different parameters, and construction material. Then simulation results of an FPLG are presented for using a conventional and the proposed XFlux materials. Voltage, current, power, flux linkage, and core loss of the FPLG are plotted with using different magnetic cores to observe their relative performance. It is found that because of applying XFlux to the FPLG, minimum core loss occurs. Recent progress of FPLGs and their specialty are summarized. The advancement and future scope of the FPLG has been proposed at the end of this chapter. Keywords FPLG · Linear electrical machine · Parameter analysis · Power · Stroke length · Voltage rating · Voltage frequency

A. D. Tahlil · O. Farrok (B) · Md. Abdullah-Al-Mamun · M. H. Uzzal Department of Electrical and Electronic Engineering, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 O. Farrok and Md R. Islam (eds.), Oceanic Wave Energy Conversion, https://doi.org/10.1007/978-981-99-9814-2_8

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8.1 Introduction Free piston linear generator (FPLG) is getting popular because of its attractive features. The feature includes lightweight and slender construction compared to the conventional internal combustion engine driven generator. In an FPLG, a linear electrical generator is coupled to a free piston engine as shown in Fig. 8.1. The generator receives mechanical power from the motion of a piston. Performance of an FPLG depends on several parameters such as its stroke length, generated voltage, frequency, power, etc. In general, an engine controller unit is connected to the free piston engine, which receives the corresponding signals of power and distance due to the piston movement and provides necessary feedback. FPLG can have energy storage as shown in Fig. 8.1 that consists of a few supercapacitors. In that case, a power controller is needed to

Fig. 8.1 A typical FPLG with its different accessories [1]

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control the current flow. Performance of a linear electrical generator depends on its design and construction material. In this chapter, different parameters of various FPLGs are presented. From the study, typical stroke length, design of the generator, and materials are found. Different types of materials which are used to design the generator are studied. The magnetic materials include cold rolled grain oriented (CRGO), nano crystalline core, CRGO ferrite core, and Permalloy 80. The performance of some magnetic materials used in an FPLG is observed. It is also analyzed that core loss occurs in the FPLG because of its high speed operation and application of conventional magnetic cores. There are various alternative ways to improve output power of an FPLG. When the generator design is improved, it is possible to produce more power. By using special materials to the FPLG, its overall performance can be improved. At the end of this chapter, characteristic curves of three magnetic materials are presented. One of them is conventional and the other two are advanced magnetic cores namely DI-Max-HF-10X and XFlux. Analysis of core loss of a free piston linear generator is illustrated for using them. The maximum core loss is found for using ordinary steel core. Because of using special magnetic material, core loss is minimized. The FPLG with DI-Max-HF-10X results in more core loss compared to that of using XFlux. Therefore, XFlux magnetic core is selected for designing the FPLG. Contributions of this chapter are summarized in the following. • The stroke length, power, voltage frequency, and voltage of various FPLGs are investigated. • The FPLG is analyzed with different magnetic materials. • Minimization of core loss is performed by applying XFlux magnetic core.

8.2 Parameter Analysis: Stroke Length Performance and characterization analysis of a linear electrical generator depends on different parameters such as the stroke length, power rating, voltage frequency, voltage, etc. An engine piston travels from one end to the other end in the cylinder. The linear distance traveled by a piston is called stroke length. When the piston reaches the lower position, it is often called bottom dead center. Similarly, when the piston reaches the upper position of the cylinder, it is called top dead center. When a piston travels between top dead center to bottom dead center in four different cycles, it is called a four stroke engine. The four strokes are suction, compression, power or expansion, and exhaust. Different linear electrical generators mounted to a free piston engine are described in the following. A plate moving magnet linear generator is coupled to a two stroke free piston engine [1]. The magnetic circuit of this linear generator is extensively examined and designed. Its maximum stroke length is 40 mm. Both flat and tubular structure of this FPLG is discussed as well. The core loss of a linear generator used in a free piston engine system is modeled and simulated in [2]. Simplified diagram of an Alpha generator with a figure is presented there. The design of a linear generator is

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described as well. Stroke length of this machine is 22 mm. The specifications and factors that affect a linear generator design are depicted in [3]. An optimal linear generator is designed to drive it with a free piston engine. An FPLG is considered that has a combustion chamber, energy storage, and a load. The free piston engine and linear generator are mounted together. Design concept of this FPLG is described. The piston begins to accelerate with combustion. The gas spring is compressed during the process where the stroke length of the linear generator is 62 mm. The experimental findings from a hydrogen fueled spark ignited dual piston FPLG prototype working in two stroke and four stroke modes are discussed in [4]. The FPLG testing is completed successfully with a compression ratio of 3.7. The engine speed varies from 5 to 11 Hz at various equivalence ratios. Its stroke length is 40 mm. Design of the FPLG is presented along with a prototype schematic configuration. A tubular permanent magnet moving coil linear generator is built for a free piston engine as described in [5]. The linear generator’s electromagnetic properties are investigated using a two dimensional finite element model. The basic structure of this energy converter system is shown. It is powered by a gasoline engine which has a stroke length of 102 mm. A linear generator connected to a free piston engine is figuratively illustrated. Pilot ignition technology is numerically evaluated for a hydrogen fuel free piston engine generator [6]. The effect of pilot ignition technology is simulated and compared to that for spark ignition technology. It discusses a two stroke engine which has two cylinders and two pistons. It has 58 mm nominal stroke length and 94 mm standard stroke length [6]. Schematic diagram of an experimental test bench of the hydrogen FPLG is explained with a figure. An air driven free piston expander linear generator test rig and a GT-SUITE/Simulink based FPLG simulation model are developed in [7] for investigation and research purpose. The FPLG test bench includes a free piston engine, servo motor, compressed air tank, linear generator, rectifier bridge, resistance box, compressor, data acquisition module, and numerous sensors. A two stroke two cylinder tubular permanent magnet linear generator is described there. Stroke length of the FPLG is 100 mm. The performance of an opposite side FPLG with a combustion chamber from both ends of the mover is discussed in [8]. It describes a two stroke movable permanent magnet linear generator. Instead of an air spring chamber, combustion chamber is contrasted to that of a one sided FPLG. The opposite side FPLG with the combustion chambers on both sides of the piston is presented as well. The one sided stroke length is 136 mm and that for the opposite sided is 107.9 mm. A free piston expander coupled to a linear generator prototype is proposed to efficiently recover emissions waste heat from a vehicle engine [9]. In a small scale organic Rankine cycle device, the FPLG can effectively convert the thermodynamic energy of the working fluid into electricity. A two stroke, two cylinder, dual opposite piston engine is depicted. The system consists of a computer, air storage tank, right servo motor, the left cylinder, right cylinder, piston, external load resistance, rectifier circuit, linear generator, the left servo motor, and a data unit of acquisition. The stroke length of this FPLG is 100 mm. It also presents a pictorial view of the FPLG.

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The effects of hydrogen addition on the performance of an FPLG with glow plug ignition is presented in [10]. It is a two stroke and two cylinder engine. It shows an FPLG experimental model as well as a schematic diagram. The stroke length of this FPLG is 32 mm. Electrical energy generated by the linear electrical generator is measured with the output voltage and current consumed by an external resistance. The induced voltage is not exactly sinusoidal, rather it is quasi sinusoidal. According to the experimental results, its stable FPLG operating state is achieved. The FPLG can be started manually. A Prototype and a diagram of the FPLG are illustrated as well. The dc bus voltage regulation strategy of the FPLG in a stable operating state is described in [11]. The linear electrical machine and three phase rectifier models are constructed. It is a single piston two stroke FPLG with a spark plug. The stroke length of this FPLG is 120 mm. A plate type generator is used and it is mentioned superior to the conventional tubular generator. Mechanical structure of the FPLG is graphically explained. The concept of various FPLG packages for electric vehicles with a generating unit is depicted in [12]. A two cylinder engine is described. Two separate FPLG modules for future production cars have been built for this purpose and the kit for various combinations has been demonstrated in a vehicle model. This comparison gives an idea of a future potential vehicle model. It shows a basic structure of the FPLG module with graphical representation. The parametric study of a dual piston type free piston engine generator and the model validation results are described in [13]. It illustrates the forces acting on the pistons of the FPLG during the generating process. The peak in-cylinder pressure decreases linearly as the stroke lengthens, while the achieved piston top dead center increases gradually. When the stroke falls below 70 mm, the top dead center does not meet the ignition position requirements. It results in engine misfire. The forces acting on the mover of the FPLG and performance of the engine with two stroke and four stroke are presented. A linear synchronous machine is compared to a linear transverse flux machine in [14]. A cylindrical permanent magnet machine is discussed. Both machines are designed with an intent of serving as the power take off for a free piston engine. Since both features are cylindrical, they cannot be built with flat laminations alone. Therefore, alternate methods are defined, illustrated, and a model is developed. The stroke length of the linear synchronous machine is 120 mm and that for the transverse flux machine is 152.4 mm. An experiment with a three phase six slot/seven pole tubular axially magnetized PMLG and its controller is presented in [15]. Research findings show that it is a good candidate to operate with FPLG for space applications. The structure of the system is described and the stroke length of the machine is 42 mm. A free piston Stirling engine is mechanically coupled to a translator. Fuel spray and mixture formation characteristics of a direct injection gasoline FPLG is conducted in [16]. The FPLG prototype used in this study is based on a traditional direct injection gasoline engine, whose structure diagram and key parameters are presented. Although the nominal stroke of this engine is 88 mm but its maximum effective stroke length is 63 mm. Configuration of this free piston engine generator is presented. A hemispherical

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shaped permanent magnet linear generator and two stroke two cylinder engine are depicted. Article [17] concentrates on a Vernier machine (Vernier 5/6), which has five armature pole pairs that coincide with six stator teeth. Several geometric parameters such as magnet height, slot pitch ratio, and magnet length, are varied to find the best configuration for high thrust force and low force ripple. It concludes with the determination of losses and electrical efficiency. It illustrates the generator’s mechanical construction. Both one cylinder and two cylinder operate in contrast to one another, which can be compared. The traditional topology and the Vernier machine both are described. Five armature pole pairs are mounted on six stator teeth in the design. Mechanical structure of the generator and comparison between the conventional and Vernier machine are depicted. A tubular permanent magnet linear generator is discussed. The first section of [18] describes a 100 W electrical free piston Stirling engine. Secondly, a description of a one dimensional computational model that solves both the engine thermodynamics and kinematic behaviors as an initial value problem is presented. The stroke length is 20 mm. Table 8.1 summarizes the stroke length of various FPLGs.

8.3 Output Power of the Generator Output power of various electrical generators is analyzed in this section. Maintaining other parameters, the FPLG can be designed with a goal to produce desirable output power. When the linear generator is powered by a conventional reciprocating motion by free piston engine, the coil current, flux linkage, copper loss, terminal voltage, and generated power are plotted in [1]. The coil current is regulated as a normal sinusoidal current with 40 A amplitude and 50 Hz frequency. The typical output power is around 2 kW. To avoid unnecessary disturbances to the generator activity, an acceptable range of equivalence ratio is suggested in [4] for each set of defined FPLG operation parameters. At 11 Hz, the FPLG produces a maximum power output of 650 W. At a working frequency of 10 Hz, the linear generator has a peak power of about 16.77 kW and an averaged generating power of 3.37 kW [5]. Efficiency at peak power is around 95.2% and its average generating efficiency is around 87.5%. It shows the tested and simulated electric power. The diesel pilot ignition (DPI) peak electricity generation is 10.7 kW whereas that for the traditional spark ignition (TSI) is 8.4 kW [6]. Although the mean power of a single cycle exceeds 5.1 kW under traditional spark ignition conditions, it is increased up to 6.5 kW when DPI technology is used. As a result, the DPI technology has a remarkable benefit in terms of producing electricity because it transforms more fuel chemical energy into electrical energy. With the external load resistance, the peak power output shows a changing (increasing and then decreasing) pattern [17]. The highest peak power output is 22.23 W for a certain external load resistance at 373 K temperature. Fluctuation of output power for various external load resistance

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Table 8.1 Stroke length of various FPLGs Description of the FPLG

Stroke length (mm)

References

A plate moving magnet linear generator is driven by a free piston engine

40

[1]

Tubular permanent magnet linear generator used in a free piston engine system

22

[2]

Tubular linear generator is designed for high force density

62

[3]

Hydrogen fueled spark ignited dual piston FPLG prototype is working on two stroke and four stroke modes

40

[4]

Tubular permanent magnet moving coil linear generator is considered for free piston engine

120

[5]

Pilot ignition technology is numerically evaluated for the hydrogen fuel free piston engine generator

58, 94

[6]

Air driven free piston expander linear 100 generator test rig and a GT-Suite/ Simulink based FPLG simulation model is described

[7]

An opposite side FPLG with a combustion chamber from both ends of the mover is applied rather than an air spring chamber of a one sided FPLG

136 (One sided) 107.9 (Opposite sided) [8]

Free piston expander is coupled to a linear generator prototype

100

[9]

It shows the FPLG experimental model where a two stroke two cylinder engine is used

32

[10]

A dc bus voltage regulation strategy of the FPLG in a stable operating state is described

120

[11]

Various package concepts of the FPLG Variable are depicted for electric vehicles with a generating unit. Two separate FPLG modules for future production cars have been built for this purpose

[12]

A dual piston type free piston engine generator is acting on the pistons of the FPLG during the generating process

70

[13]

A linear synchronous machine is compared to a linear transverse flux machine

120 (Linear synchronous machine) 152.4 (Linear transverse flux machine)

[14]

(continued)

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Table 8.1 (continued) Description of the FPLG

Stroke length (mm)

References

Experiment is conducted with a three 42 phase six slot /seven pole tubular axially magnetized PM linear alternator and its controller

[15]

An examination into the fuel spray and mixture formation characteristics of a direct injection gasoline FPLG is depicted

[16]

63 (effective), 88 (nominal)

A Vernier machine (Vernier 5/6) with 90 five armature pole pairs coincides within six stator teeth

[17]

A 100 W electrical free piston Stirling engine is considered that has 20 mm stroke length

[18]

20

is observed. Variation of peak power output with the temperature for external load resistance are figuratively explained. The compression ratio control of an FPLG by artificial neural network is shown in [19]. A two stroke single cylinder spark plug ignition engine is considered there. The peak producing power at compression stroke is around 3 kW whereas it is 7 kW for expansion stroke. The peak generating power for compression stroke is 5 kW and that for the expansion stroke is 8 kW. Because of the variation in efficiency between the two different generators, their produced powers also vary. When the frequency of the piston for its reciprocating motion is 60 Hz, The FPLG device produces 6.3 kW of output power. It has a thermal efficiency of 42.8% and a power generation efficiency of 37.4%. Generating power of linear electrical machine in one cycle is discussed. Tubular permanent magnet generator and plate type generator are depicted. The simulation results of a one sided FPLG are described in [8]. It illustrates the FPLG effects on the opposite side. The efficiency of power generation is increased by using opposite side FPLG, because copper and iron losses are reduced. When the frequency is set to 1 Hz and the external load resistance is 20 Ω, the total peak velocity is around 0.69 m/s and the highest power output is 96 W [9]. The output power is measured by using various external load resistances. Rankine cycle is a thermodynamic cycle that converts heat into mechanical energy. In the conventional Rankine cycle inorganic substances are used as working fluid. The organic Rankine cycle is the Rankine cycle in which an organic substance is used instead of water vapor. As it does not emit harmful substances like CO, CO2 , NOx , SOx , etc. Therefore, it is an environmentally friendly system. Organic Rankine cycle is simple to implement as it is applicable to low temperature differences. An organic Rankine cycle is presented that produces electricity where R245fa is used as the working fluid [20]. It is experimentally studied. The factors that affect the developed organic Rankine cycle performance are evaluated and addressed. The highest average cycle, turbine efficiency, and electric power are found as 5.22%, 78.7%, and

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32.7 kW, respectively. For the organic Rankine cycle method, an experimental system is designed and a few tests are carried out with single screw type expander and diesel engine [21]. The effects on performance indexes of a single screw expander and heat work conversion efficiency are investigated. According to the findings, the maximum power output is 10.38 kW, and the highest organic Rankine cycle performance and overall system efficiency are 6.48% and 43.8%, respectively. The power output and effectiveness of a tubular permanent magnet linear generator (PMLG) for various loads are illustrated in [22]. A two piston two chamber engine is used in the FPLG. As load resistance increases, output power and efficiency increase, then decrease after a certain value. At a constant speed of 10 m/s and a load resistance of 2 Ω, the output power is 8.7 kW, and the performance is increased a lot where the efficiency is 93.96%. The main output in generating mode is projected with a figure in [23]. A flat PMLG with a two stroke two cylinder engine is depicted. The output power shows a pattern of first rising and then decreasing as the load changes from 10 Ω to 20 Ω at the same motion where the frequency of the generator is 25 Hz and the maximum power is 18.69 kW for 6 Ω load. Output power and efficiency of the generator is plotted with various load resistance at 25 Hz frequency. It is observed that the power output throughout the expansion stroke is greater than that of the compression stroke [24]. The maximum velocity of the expansion and compression stroke is around 12 m/s, and 10 m/s, respectively. Simulation results show that the maximum power output is 52.7 kW, with a mean power of 26.36 kW. The total estimated efficiency is 36.32%. Electrical power output of a two stroke FPLG is figuratively described. The produced instantaneous power fluctuation results in output voltage ripple [15]. It occurs during each stroke period due to the alternator’s damping force being proportional to piston velocity. The dc output power is 98.8 W, and the output voltage is 96.9 V with around 0.7% ripple. It is suitable for low power applications mostly. An FPLG with a flat PMLG is described in [25]. Both the double and single sided linear generator are discussed. It includes inner and outer pistons as well as unique inner and outer movers. Outcome of the experimental results demonstrate that this FPLG has outstanding efficiency, particularly in terms of output power capacity. It can reach up to 95 kW and a mean value of 46.35 kW when the load resistance is 6Ω. The structure of a two stroke two cylinder piston shaped FPLG is depicted to allow stable continuous operation [26]. The cycle simulation results show that for both spark ignition and premixed charge compression ignition combustion cases, the required output generating power of 10 kW is achieved. Experimental results are presented as well. Both single cylinder and double cylinder auto cycle linear Joule engine generator are presented in [27]. For the evaluation of the system dynamics and thermodynamic characteristics, a comprehensive numerical model is presented. The linear alternator’s output power is around 4.4 kW, and the engine’s thermal efficiency can exceed 34% with a generating efficiency of 30%. The output power of different FPLGs with some of their descriptions are summarized in Table 8.2. The output power of the studied electrical machines ranges from a few watts to tens of kilowatts. The lowest power output is found 22.23 W and highest power

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Table 8.2 Output power of different FPLGs Description of the system

Output power (W)

References

A plate moving magnet linear generator is driven by a 2,000 free piston engine that produces the conventional reciprocating motion

[1]

An acceptable range of equivalence ratio is suggested 650 for each set of defined FPLG operation parameters

[4]

It shows the tested and simulated electric power for a 16,770 (peak), 3,370 linear generator (average)

[5]

The diesel pilot ignition peak electricity generation under traditional spark ignition conditions are simulated

10,700 (DPI), 8,400 (TSI)

[6]

A two sided FPLG is described in the simulation result Simulation results of a one sided FPLG are described and the efficiency of power generation (output) is increased by using opposite side FPLG

10,516 (output power for one sided FPLG) and 10,688 (for the opposite side FPLG)

[8]

The highest power is measured for the frequency of 1 Hz, load resistance of 20 Ω, and peak velocity of 0.69 m/s

96 (the highest power)

[9]

Output voltage ripple is observed for the alternator’s damping force that produced power fluctuation

98.8

[15]

It focuses on a Vernier machine (Vernier 5/6), which has five armature pole pairs that coincide with six stator teeth. Electrical efficiency and losses are determined

22.23

[17]

The compression ratio control of an FPLG is 6,300 simulated. Generating power of FPLG in one cycle is considered

[19]

Electricity is produced where R245fa is used as the 32,700 working fluid of a system with organic Rankine cycle

[20]

An organic Rankine cycle method is experimented where tests are carried out with various expander torques and diesel engine loads

10,380

[21]

Permanent magnet linear generator is considered for various loads

8,700

[22]

Output power and efficiency with various load resistance at 25 Hz of a flat PMLG is plotted

18,690 (maximum)

[23]

Electrical power output throughout the expansion and 52,570 generating stroke of a two stroke FPLG is figuratively described

[24]

An FPLG with both double and single sided PMLG are described. It includes inner and outer pistons as well as unique inner and outer movers

[25]

95,000

(continued)

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Table 8.2 (continued) Description of the system

Output power (W)

References

The structure of a two stroke, two cylinder, piston shaped FPLG is depicted to allow stable continuous operation

10,000

[26]

Design of a linear Joule engine generator for renewable energy sources is presented

4,400

[27]

output is 95 kW. Power rating of the generator vitally depends on its design and construction material.

8.4 Frequency of the Generated Voltage Frequency of the generated voltage of the FPLG depends on a few factors. Generally, PMLG is mounted to the FPLG. In that case, frequency of the generator depends on speed of the free piston. At the rated operating condition, the plate moving linear generator generates sinusoidal voltage waveform. Magnitude and frequency of the current are 40 A and 50 Hz, respectively as reported in [1]. The in-cylinder pressure for a two stroke thermodynamic cycle is presented in [4]. It also presents the in-cylinder pressure for a four stroke thermodynamic cycle during compression and power strokes. For both thermodynamic cycles, the equivalency ratio and frequency are 0.4365 and 5 Hz, respectively. A tubular moving coil linear generator is designed in [5] that is applicable to a free piston engine. This machine works at a frequency of 10 Hz. The engine operation frequency is enhanced from 31.6 Hz to 35.3 Hz by applying diesel pilot ignition [6]. As a result, the corresponding generator speed is increased from 1896 to 2118 rpm. When the piston’s reciprocating frequency is 60 Hz, the FPLG produces 6.3 kW of output power [19]. It has a thermal efficiency of 42.8% and an output power generation efficiency of 37.4%. With an intake pressure of 2.6 bar and a frequency of 2.0 Hz, the FPLG energy conversion efficiency is found to be 45.82% [9]. The FPLG with 20 Hz operating frequency corresponding to different angular speeds of a traditional two stroke and four stroke internal combustion engine are described in [28]. As a result, higher operating frequency should be required. A two sided moving permanent magnet linear generator is depicted in [24]. It operates in a stable manner where the average stable cycle duration is 29.64 ms. Therefore, frequency of the generated voltage is approximately 33.74 Hz. A study of the scavenging process in a two stroke FPLG using computational fluid dynamics is illustrated in [29]. The simulation results demonstrate that at the optimum operating frequency of 33 Hz, the efficiency reaches a maximum of 83%. Furthermore, trapping performance is improved by increasing exhaust port distance and lowering inlet pressure. For a tubular PMLG, the experimental core loss for the Alpha generator is 1 W for 67 Hz frequency [30]. Its power rating is 3.34 W

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Table 8.3 Frequency of the generated voltage of different FPLGs Description

Frequency (Hz)

References

Sinusoidal voltage and current waveforms are obtained by a plate moving magnet linear generator driven by a free piston engine

50

[1]

The in-cylinder pressure for a two stroke thermodynamic cycle is presented

5

[4]

A tubular moving coil linear generator is designed for a free piston engine

10

[5]

The effect of pilot ignition technology is simulated and compared with that for spark ignition technology

31.6 to 35.3

[6]

An FPLG system with an intake pressure of 2.6 bar is considered in which the energy conversion efficiency is reached up to 45.82%

2

[9]

Generating power of FPLG in one cycle is considered. FPLG produces 6.3 kW of output power

60

[19]

A two sided moving permanent magnet linear generator is described where the average stable cycle duration of the machine is 29.64 ms

33.74

[24]

The FPLG corresponds to different angular speeds of a traditional internal combustion engine

20

[28]

The scavenging process in a two stroke FPLG using computational fluid dynamics is illustrated

33

[29]

Simulation and experimental results of equivalent circuit parameters and core loss in a tubular PMLG for FPLG application are presented

67

[30]

for air core and 4.3 W for iron core. The stator of this generator has windings, laminations, and back iron. The moving portion of this generator has permanent magnets, translator, and aluminum drum. In Table 8.3, voltage frequency of different FPLGs is summarized. It is found from the literature review that construction of a free piston linear generator is sometimes very similar to the linear generator which is proposed for oceanic wave energy conversion. For example, construction of the FPLG found in [6, 8], and [11] is similar to the linear generator reported in [31–33].

8.5 Voltage Rating This section describes the generated voltage of various FPLGs. It is found that some of the generators produce up to 300–400 V and some of them produce only less than 20 V. A tubular moving coil linear generator is built for free piston engine [5]. A supercapacitor is used in this generator where the dc voltage rating is 200 V. The supercapacitor is charged or discharged according to the working state of this generator. As the intake temperature and pressure are increased, the peak voltage output of the FPLG increases linearly [7]. Intake pressure has a greater effect on

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peak voltage performance than temperature. When 33 Ω resistive load is applied, the peak output voltage is found 23.6 V. An organic Rankine cycle method is experimented in [21] where exhaust heat is recovered from the diesel engine. Overall efficiency versus output power, overall efficiency versus torque, volume efficiency versus torque, inlet pressure versus torque, volume versus torque, heat exchange versus torque, and gas exhaust versus torque are plotted as well. The peak voltage is 18.08 V as found from the simulation. It is 0.19 V higher than the experimental peak voltage of 17.89 V. At a displacement of 25– 30 mm, the maximum voltage difference between the simulation and experimental result is 0.4 V, and the maximum relative error is 2.5%. A 1 kW axially magnetized permanent magnet linear alternator and its control approach for free piston Stirling engines are investigated in [15]. The output voltage is 96.9 V with roughly 0.7% ripple. It is adequate for most applications where large power efficiency is not a major concern. An analytical model of a free piston expander linear generator is experimented for a small scale organic Rankine cycle [34]. The free piston engine has dual opposite piston and it is operated at two stroke and four stroke. Output voltage of the FPLG is found to be around 78 V. A dc bus voltage control technique for the FPLG in a stable operating condition is proposed in [11]. It effectively addresses the fluctuating ac voltage into stable dc bus voltage. It maintains a steady dc bus voltage of around 400 V and ensures the FPLG system’s stability. A free piston expander linear generator test setup for an organic Rankine cycle waste heat recovery sensor is presented in [35]. The highest peak voltage output is 44.4 V when the intake pressure is 3 bar. The frequency is 1.5 Hz, and the generator is connected to the external load resistance. A two cylinder engine with two stroke and four stroke is depicted as well. The finite element method is used to evaluate and describe the characteristics of a tubular type linear generator for a free piston engine [36]. The highest output voltage is 300 V. Design of a free piston expander linear generator model for a small scale organic Rankine cycle waste heat recovery system is illustrated in [37]. Single piston and dual opposed piston engine with permanent magnet generator are described. Experimental setup is carried on as well. The peak voltage output (highest) with cam plate A is reported 13 V where there are a total of three cam plates namely A, B, and C. The peak voltage is found for the intake pressure of 1.9 bar, operating frequency of 2 Hz, and external load resistance of 9 Ω. A free piston expander linear generator prototype for a small scale organic Rankine cycle system has been presented in [38]. With the intake pressure ranging from 0.14–0.2 MPa, the maximum voltage is between 2.18–10.41 V. It uses a flat PMLG and a two stroke engine with two opposed cylinders and pistons. Various generated voltages of the FPLG are tabulated in Table 8.4.

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Table 8.4 Output voltage of the FPLGs Description

Voltage (V)

References

A tubular moving coil linear generator is coupled to a free piston engine

120 to 240

[5]

As the intake temperature and pressure are increased, the peak voltage 23.6 output of the FPLG increases linearly

[7]

A dc bus voltage control technique for the FPLG in a stable operating 400 condition is proposed that addresses the fluctuating ac voltage into stable dc bus voltage

[11]

A 1 kW axially magnetized permanent magnet linear alternator and its control approach for free piston Stirling engines are investigated

[15]

96.9

Experiments were conducted for different expander torque and diesel 18.08 engine loads

[21]

An analytical model of a free piston expander linear generator is carried on for a small scale organic Rankine cycle

78

[34]

A free piston expander linear generator test setup is arranged for an organic Rankine cycle waste heat recovery system

44.4

[35]

The finite element method is used to evaluate and describe the characteristics of a tubular type linear generator for a free piston engine

300

[36]

The peak voltage (highest) is found for the intake pressure of 1.9 bar, operating frequency of 2 Hz, and external load resistance of 9 Ω

13 (highest)

[37]

A free piston expander linear generator prototype for a small scale organic Rankine cycle system has been presented

2.18–10.41 [38]

8.6 Simulation Results of an FPLG Before conducting simulation of an FPLG, its design and all the active material properties are required to be analyzed first. Then initial machine parameters, such as air gap length between the stator and the translator, the translator’s stroke length, and the major parameters are identified. Then they are inputted into the simulation set up. Thus, the results have been obtained. It is found from [39] that output parameters of a free piston linear generator depend on its design and active materials. Special cold rolled grain oriented magnetic core (XFlux) is proposed in [39] for the FPLG for decreasing core loss compared to that of using the ordinary one. Generated voltage, frequency, and output power of an FPLG vary with the translator velocity. Output power and core loss greatly depends on the magnetic materials. Magnetizing curves of DI-Max-HF-10X, ordinary steel core, and special magnetic core, XFlux are plotted in Fig. 8.2. Output voltage and current of the proposed FPLG are plotted in Fig. 8.3. Peak value of the generated voltage reaches nearly 59 V. Magnetic flux linkage and output power are plotted in Fig. 8.4.

8 Linear Electrical Generator for Hydraulic Free Piston Engine

Fig. 8.2 Comparison of core loss curves of three different magnetic cores [39]

Fig. 8.3 Output voltage and current waveforms of the proposed FPLG [39]

Fig. 8.4 Power and flux linkage of the FPLG with respect to time [39]

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Fig. 8.5 Core loss of the FPLG for using three different magnetic cores [39]

Magnetic core losses of the FPLG for applying three different magnetic cores are observed. They are presented in Fig. 8.5 where it is found that by applying DIMax-HF-10X to the FPLG, the core loss is minimized compared to that of using conventional iron core. Again, the FPLG with XFlux material results in less core loss compared to that of using DI-Max-HF-10X. Therefore, XFlux magnetic core is recommended for designing the FPLG.

8.7 Improvement The conventional FPLG utilizes fossil fuels that produce greenhouse gas which is harmful for the environment. For large scale implementation, it would be a threat to the target of net zero carbon emission of many countries [40]. Hydrogen fuel based FPLG can solve this limitation to a great extent. It is reported that it has sufficient velocity and acceleration [4]. Some recent improvements of the FPLG are listed in Table 8.5. The linear electrical generator used in a free piston engine and oceanic wave energy conversion system is similar. For this reason, several methods or techniques for advancement of a linear electrical generator for wave energy conversion are mostly applicable to the FPLG because similar permanent magnet electrical generator is generally used. Their working principles are almost the same for both. But the generator size used in an FPLG is smaller than that for the wave energy conversion. Proposals for the further advancement of FPLGs should be carefully considered if the concept of improvement is adopted from the linear generator for wave energy conversion.

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Table 8.5 A few recent developments of the FPLG Improvement

Specialty

References

A higher weight ratio is gained for Influencing factors of the generator such as the double sided planar machine winding factor and limiting factor are used

[3]

There are no side forces between the piston and the side cylinder

This is because of elimination of crank shaft mechanism

[4]

It results in faster cycle, larger compression ratio, and high generating power

The conventional single point spark plug is replaced by the area ignition of pilot fuel

[6]

The peak voltage output of FPLG rises linearly by increasing the intake temperature and intake pressure

The higher the intake pressure is, the more the [7] intake temperature affects the peak voltage output

The power generation efficiency is It is achieved by decreasing maximum value improved of the combustion thrust

[8]

The system efficiency is increased It is achieved by using a dual piston air driven [12] FPLG The axially magnetized PM linear Energy storage devices are employed to alternator exhibits higher power provide transient power supply and reduce density and better voltage ripple manufacturability

[15]

Higher force density and efficiency are achieved

[17]

The armature was equipped with different magnet circuit structures

A little amount of electrical power The geometric layout of the engine prototype is generated than that of self was based on sun power engine. A unique sustained operation addition of a linear motor is directly mounted to the displacer shaft

[18]

The FPLG system has high output power. The system thermal efficiency and the power generation efficiency is increased

The compression ratio control strategy takes [19] the starting position of the compression stroke and uses the target generating current amplitude of the compression stroke as the output value

The FPLG has higher efficiency, power density, and ultra low emission

It is a new structure with inner and outer pistons and special inner and outer movers

[25]

The output generating power (10 With the prototype FPLG with the “W kW) and thermal efficiency (42%) shaped” piston and two stroke spark ignition is achieved in a premixed charged combustion system are used compression ignition combustion case

[26]

Efficient energy conversion FPLG benefits were shown due to the crank reduces the energy transfer chain, connecting rod mechanism was removed multi combustion mode, and multi fuel adaptability

[28]

(continued)

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Table 8.5 (continued) Improvement

Specialty

It enhances the trapping efficiency The increase in exhaust port distance as well as minimization of inlet pressure can help to increase the trapping efficiency It has high efficiency and reliability

References [29]

A ring shaped iron with an axially magnetized [36] magnet rather than radial one is used

8.8 Future Scope Analysis of electrical properties of FPLG and application of the magnetic materials are presented in this chapter. There are different opportunities for further improvement of the FPLG which are listed in the following. • There is a scope to apply other advanced magnetic materials to the FPLG. • Different types of generators can be designed by using the proposed magnetic material. • Using different magnetic materials such as amorphous can be applied to the FPLG design to minimize core loss. • The concept of advancement of the FPLG can be adopted from the linear generator for wave energy conversion because of their similar principles of operation.

8.9 Summary The construction, working principle, and parameters of the FPLG are investigated in this chapter. A short description of each of the FPLGs along with their parameter is depicted. Different tables are presented that summarize stroke length, power, voltage frequency, and voltage of the FPLG. Parameters of the FPLG are quite significant for their proper design. For example, stroke length of the free piston engine must match with the linear electrical generator mounted to it. The generated electrical power depends on the input power of the prime mover. The parameters found in each table can be compared among the FPLGs. The way of increasing performance of the FPLG is presented with different perspective. Linear electrical machines are utilized in various applications such as FPLG and wave energy conversion as reported in this chapter. Identification of suitable model structures for them, building materials, as well as estimation of model parameters are significant. Finally, advancement of the linear electrical generator and their future scopes for further improvement are discussed.

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