Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation [1st ed.] 978-3-030-17282-4;978-3-030-17283-1

Table of contents :
Front Matter ....Pages i-xi
Solar Energy Resource and Its Global Distribution (Zhongzhu Qiu, Peng Li)....Pages 1-30
Solar Heating, Cooling and Power Generation—Current Profiles and Future Potentials (Wei He, Xinghui Zhang, Xingxing Zhang)....Pages 31-78
Heat Pipe and Loop Heat Pipe Technologies and Their Applications in Solar Systems (Zhangyuan Wang, Haopeng Zhang, Fucheng Chen, Siming Zheng, Zicong Huang, Xudong Zhao)....Pages 79-100
PCM and PCM Slurries and Their Application in Solar Systems (Zhongzhu Qiu, Peng Li, Zhangyuan Wang, Han Zhao, Xudong Zhao)....Pages 101-141
Modular Solar System for Building Integration (Gang Ren, Zishang Zhu, Yanyi Sun, Xudong Zhao)....Pages 143-163
Micro (Mini)-Channels and Their Applications in Solar Systems (Thierno Diallo, Min Yu, Jinzhi Zhou, Yi Fan, Xudong Zhao)....Pages 165-209
Solar Desiccant (Absorption/Adsorption) Cooling/Dehumidification Technologies (Wansheng Yang, Shuli Liu, Xiaoqiang Zhai, Yin Bi, Zhangyuan Wang, Xudong Zhao)....Pages 211-286
Solar Ejector Cooling Technologies (Xiaoli Ma, Wei Zhang, Fenglei Li, S. B. Riffat)....Pages 287-309
Heat Pump Technologies and Their Applications in Solar Systems (Xingxing Zhang, Manxuan Xiao, Wei He, Zhongzhu Qiu, Xudong Zhao)....Pages 311-339
Solar Thermoelectric Technologies for Power Generation (Guiqiang Li, Xiaoli Ma, Samson Shittu, Xudong Zhao)....Pages 341-371
Solar Systems for Urban Building Applications—Heating, Cooling, Hot Water, and Power Supply (Bin Li, Xuemei Chen, Xiwen Cheng, Xiaoqiang Zhai, Xudong Zhao)....Pages 373-416
Solar System Design and Energy Performance Assessment Approaches (Xingxing Zhang, Xinru Wang, Xudong Zhao)....Pages 417-451
Solar Systems’ Economic and Environmental Performance Assessment (Xingxing Zhang, Yixuan Wei, Wei He, Zhongzhu Qiu, Xudong Zhao)....Pages 453-486
Solar Heating, Cooling, and Power Generation Projects—Case Studies (Yi Fan, Jinzhi Zhou, Xiaoqiang Zhai, Han Zhao, Xudong Zhao)....Pages 487-539

Citation preview

Green Energy and Technology

Xudong Zhao Xiaoli Ma Editors

Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation

Green Energy and Technology

Climate change, environmental impact and the limited natural resources urge scientific research and novel technical solutions. The monograph series Green Energy and Technology serves as a publishing platform for scientific and technological approaches to “green”—i.e. environmentally friendly and sustainable—technologies. While a focus lies on energy and power supply, it also covers “green” solutions in industrial engineering and engineering design. Green Energy and Technology addresses researchers, advanced students, technical consultants as well as decision makers in industries and politics. Hence, the level of presentation spans from instructional to highly technical. **Indexed in Scopus**.

More information about this series at http://www.springer.com/series/8059

Xudong Zhao Xiaoli Ma •

Editors

Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation

123

Editors Xudong Zhao School of Engineering and Computer Science University of Hull Hull, UK

Xiaoli Ma School of Engineering and Computer Science University of Hull Hull, UK

ISSN 1865-3529 ISSN 1865-3537 (electronic) Green Energy and Technology ISBN 978-3-030-17282-4 ISBN 978-3-030-17283-1 (eBook) https://doi.org/10.1007/978-3-030-17283-1 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Overview Based on the research experience and outcomes established by the authors, the proposed book will address a range of advanced energy efficiency technologies and their applications in solar heating, cooling and power generation, thus delivering the solutions to tackling the low efficiency problems remaining with the current systems. In this book, the global solar resource will be briefly presented and the currently available solar systems will be illustrated. A number of advanced energy efficiency technologies, including heat pipes and loop heat pipes, PCM and PCM slurries, micro (mini)-channel panels, building integrate-able modular technologies, desiccant (adsorption and absorption) cycling, ejector cooling, heat pumps, as well as solar concentration and thermoelectric technologies will then be studied and characterised. Meanwhile, the applications of these technologies in various solar systems will be investigated. With the successful implementation of these advanced technologies, a few innovative solar systems applicable to rural and urban buildings are characterised. Based on the above analyses, the design principle and associated energy performance assessment method for these advanced solar systems will be delivered, while the associated economic and environmental performance analytic measures will also be discussed. In the end, a range of selected solar heating, cooling and power generation projects will be studied. The proposed book will provide the readers with the latest technologies and methods that can significantly improving the performance of solar systems, thus enabling them to design, construct and apply high-performing solar systems in buildings and elsewhere. The publication of the book will promote wide deployment of advanced renewable solar technologies on the global scale. The book will deliver a systematic introduction of the latest energy efficiency technologies and their applications in solar heating, cooling and power generation. By going through the dedicated illustration of the technologies, including heat pipes and loop heat pipes, PCM and PCM slurries, micro (mini)-channel panels, building

v

vi

Preface

integrate-able modular technologies, desiccant/adsorption cycling, ejector cooling, heat pumps, as well as solar concentration and thermoelectric units, readers will be able to gain the knowledge of advanced solar energy technologies on a fast-track route. Through the study of the dedicated design method, energy and environmental performance, as well as the practical engineering cases, readers will be able to quickly grasp the sense on how to implement these innovative solar systems into practicality. The publication of the book will therefore promote wide deployment of advanced solar heating, cooling and power generation technologies in buildings and elsewhere at global extent. Hull, UK

Xudong Zhao Xiaoli Ma

Contents

Solar Energy Resource and Its Global Distribution . . . . . . . . . . . . . . . . Zhongzhu Qiu and Peng Li Solar Heating, Cooling and Power Generation—Current Profiles and Future Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wei He, Xinghui Zhang and Xingxing Zhang Heat Pipe and Loop Heat Pipe Technologies and Their Applications in Solar Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zhangyuan Wang, Haopeng Zhang, Fucheng Chen, Siming Zheng, Zicong Huang and Xudong Zhao

1

31

79

PCM and PCM Slurries and Their Application in Solar Systems . . . . . 101 Zhongzhu Qiu, Peng Li, Zhangyuan Wang, Han Zhao and Xudong Zhao Modular Solar System for Building Integration . . . . . . . . . . . . . . . . . . . 143 Gang Ren, Zishang Zhu, Yanyi Sun and Xudong Zhao Micro (Mini)-Channels and Their Applications in Solar Systems . . . . . . 165 Thierno Diallo, Min Yu, Jinzhi Zhou, Yi Fan and Xudong Zhao Solar Desiccant (Absorption/Adsorption) Cooling/Dehumidification Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Wansheng Yang, Shuli Liu, Xiaoqiang Zhai, Yin Bi, Zhangyuan Wang and Xudong Zhao Solar Ejector Cooling Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Xiaoli Ma, Wei Zhang, Fenglei Li and S. B. Riffat Heat Pump Technologies and Their Applications in Solar Systems . . . . 311 Xingxing Zhang, Manxuan Xiao, Wei He, Zhongzhu Qiu and Xudong Zhao Solar Thermoelectric Technologies for Power Generation . . . . . . . . . . . 341 Guiqiang Li, Xiaoli Ma, Samson Shittu and Xudong Zhao

vii

viii

Contents

Solar Systems for Urban Building Applications—Heating, Cooling, Hot Water, and Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Bin Li, Xuemei Chen, Xiwen Cheng, Xiaoqiang Zhai and Xudong Zhao Solar System Design and Energy Performance Assessment Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417 Xingxing Zhang, Xinru Wang and Xudong Zhao Solar Systems’ Economic and Environmental Performance Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 Xingxing Zhang, Yixuan Wei, Wei He, Zhongzhu Qiu and Xudong Zhao Solar Heating, Cooling, and Power Generation Projects—Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 Yi Fan, Jinzhi Zhou, Xiaoqiang Zhai, Han Zhao and Xudong Zhao

Contributors

Yin Bi School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Fucheng Chen School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Xuemei Chen Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China Xiwen Cheng Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China Thierno Diallo School of Engineering and Computer Science, University of Hull, Hull, UK Yi Fan School of Engineering and Computer Science, University of Hull, Hull, UK Wei He Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, China; Department of Building Environment and Equipment, Hefei University of Technology, Hefei, China Zicong Huang School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Bin Li Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China Fenglei Li College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan, Shangxi, China Guiqiang Li School of Engineering and Computer Science, University of Hull, Hull, UK

ix

x

Contributors

Peng Li College of Mechanical and Energy Engineering, Tongji University, Shanghai, China Shuli Liu Department of Civil Engineering, Architecture and Building, Faculty of Engineering and Computing, Coventry University, Coventry, UK Xiaoli Ma School of Engineering and Computer Science, University of Hull, Hull, UK Zhongzhu Qiu College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, China Gang Ren Harbin Institute of Technology, Heilongjiang, People’s Republic of China

Nangang

District,

Harbin,

S. B. Riffat Department of Architecture and Built Environment, University of Nottingham, Nottingham, UK Samson Shittu School of Engineering and Computer Science, University of Hull, Hull, UK Yanyi Sun Department of Architecture and Built Environment, Faculty of Engineering, The University of Nottingham, University Park, Nottingham, UK Xinru Wang Department of Architecture and Built Environment, University of Nottingham Ningbo China, Ningbo, China Zhangyuan Wang School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Yixuan Wei Department of Architecture and Built Environment, University of Nottingham Ningbo China, Ningbo, China Manxuan Xiao Department of Architecture and Built Environment, University of Nottingham Ningbo China, Ningbo, China Wansheng Yang School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Min Yu School of Engineering and Computer Science, University of Hull, Hull, UK Xiaoqiang Zhai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China; School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China Haopeng Zhang School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Wei Zhang College of Architecture and Environment, Sichuan University, Chengdu, China

Contributors

xi

Xinghui Zhang College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, People’s Republic of China Xingxing Zhang Department of Energy, Forest and Built Environments, Dalarna University, Falun, Sweden Han Zhao Green Building & Low-Carbon Technology Development Center, Reading, UK Xudong Zhao School of Engineering and Computer Science, University of Hull, Hull, UK Siming Zheng School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong, China Jinzhi Zhou School of Engineering and Computer Science, University of Hull, Hull, UK Zishang Zhu School of Engineering and Computer Science, University of Hull, Hull, UK

Solar Energy Resource and Its Global Distribution Zhongzhu Qiu and Peng Li

Abstract The sun is an extremely powerful energy resource, and the solar energy is an important renewable energy. Solar energy can be used for producing heat and generating electricity. The amount of solar energy incident on earth is enormous, and it is larger than current and predicted energy requirements in the future. In this chapter, the basic concepts and parametrical performance of the sun and its radiation across the space and earth surface including solar irradiance on earth(i.e., diffuse irradiance and direct normal irradiance, the solar constant, extraterrestrial solar spectrum, extraterrestrial solar irradiance, and extraterrestrial solar radiation on a surface), ground-level solar radiation characteristics(atmosphere effects and solar spectrum), solar angles(i.e., the earth–sun angles, hour angle, declination angle, latitude angle, solar altitude, zenith, and azimuth angles), are described. Solar energy global distribution by belt and nation at variable geometrical regions on the globe is also presented. Keywords Solar · Energy resource · Basic concepts · Parametrical performance · Solar radiation distribution

1 Solar Irradiance and Solar Angles [1] 1.1 Solar Irradiance on the Earth The rate at which solar energy reaches a unit area on the earth is defined as the “solar irradiance” or “insolation,” which are measured in the units of watts per square meter (W/m2 ). Solar irradiance is an instantaneous measure of solar rate and can vary Z. Qiu (B) College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China e-mail: [email protected] P. Li College of Mechanical and Energy Engineering, Tongji University, Shanghai 200092, China © Springer Nature Switzerland AG 2019 X. Zhao and X. Ma (eds.), Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation, Green Energy and Technology, https://doi.org/10.1007/978-3-030-17283-1_1

1

2

Z. Qiu and P. Li

time by time. The solar irradiance integrated with time is called solar irradiation, or insolation, or solar exposure. In reality, insolation is often used interchangeably with irradiance. There are several types of measurement approaches for solar irradiance. Direct normal irradiance (DNI), or beam radiation, is measured on the surface of the earth at a given location with a surface element perpendicular to the sun. Direct irradiance is equal to the extraterrestrial irradiance above the atmosphere minus the atmospheric losses due to absorption and scattering. The losses are dependent upon the time of day (i.e., the length of light’s path through the atmosphere depending upon the solar elevation angle), cloud cover, moisture content, and other contents. The irradiance above the atmosphere also varies timely throughout a year owing to the variation of the distance between the earth and the sun, although this effect is generally less significant compared to the effect of losses on DNI. Diffuse horizontal irradiance (DHI), or diffuse sky radiation, is the radiation at the earth’s surface from light scattered by the atmosphere. It is measured on a horizontal surface with radiation coming from all points in the sky excluding circumsolar radiation, i.e., radiation coming from the sun disk. There would be almost no DHI in the absence of atmosphere. Global horizontal irradiance (GHI) is the total irradiance from the sun on a horizontal surface of the earth. It is the sum of the direct irradiance by counting the solar zenith angle of the sun z, as well as the diffuse horizontal irradiance: GHI = DHI + DNI × cos(Z )

1.1.1

(1)

The Solar Constant

The radiation intensity on the surface of the sun is approximately 6.33 × 107 W/m2 . Since radiation spreads out as the distance squared, by the time it travels to the earth (1.496 × 1011 m or 1 AU is the average earth–sun distance), the radiant energy falling onto the 1 m2 of surface area is reduced to 1367 W, as shown in Fig. 1.

Fig. 1 Divergence of energy from the sun to the earth

Solar Energy Resource and Its Global Distribution

3

The intensity of the radiation leaving the sun is relatively constant. Therefore, the intensity of solar radiation at a distance of 1 AU is called the solar constant (represented by Isc) which is currently recognized as 1367 [2, 3]. Other values for the solar constant are found in the historical literature with the value 1353 W/m2 appearing in many publications. It is now generally believed that most of the historical discrepancies have been caused by the instrument calibration error [4]. Satellite and rocket data [5] have confirmed that the 1353 W/m2 value was low. Also, these data confirm that there are daily and monthly variations, which is less than ± 0.25% and changes over the 11-year sunspot cycle of by about 1%. Although none of these variations are of prime importance to the design of a solar energy system, studies are continuing to help explain the potential impact of these variations on our climate. It has been estimated that a drop of only 1% in the sun’s output of radiation would decrease the earth’s mean global temperature by more than 1 °C. The entire earth would be covered with ice if the sun’s radiation decreased by only 6%. The apparent angular size of the solar disk may be calculated from the diameter of the photosphere and the earth–sun distance. At 1 AU, this is 9.3 mrad (0.553°). As the earth–sun distance varies throughout a year, the apparent size of the sun varies by ± 1.7%.

1.1.2

Extraterrestrial Solar Spectrum

The spectrum of the sun’s radiation above the earth’s atmosphere and the spectral irradiance from a blackbody at 6050 K (10,430 °F) are shown in Fig. 2. Meanwhile, the integrated value of the spectral solar flux density, which shows the cumulative amount of energy radiated at wavelengths between the shortest wavelength and the abscissa value, is also shown in this figure. It should be stressed that the data shown in the diagram are derived from the presumed solar constant value of 1353 (W/m2 ) [6]. It is interesting to notice that about 45% of the sun’s energy reaching the earth is at wavelengths of the visible spectrum, which are in the range 0.3–0.7 μm. It is also seen that only a little more than 1% of the sun’s energy is at shorter wavelengths (UV and X-solar radiation) and the remaining (around 54%) is in the infrared (IR) region. The spectrum of the sun’s radiation changes slightly as it passes through the earth’s atmosphere. This will be discussed in the following section. Knowledge of the relative amount of energy contained in sunlight of different wavelengths enables the engineers to evaluate the impact of wavelengths on the total collected energy. To give an example, a solar collector making use of a glass cover on its receiver aperture can transmit 90% of the sun’s energy at wavelengths of less than 1 μm and zero energy at longer wavelengths. By using Fig. 5, it is found that 69.5% of the solar energy falls into the wavelengths of less than 1 μm. By taking a simple calculation, it is found that the glass receiver cover allows 62.5% of the sun’s energy to pass through into the receiver.

4

Z. Qiu and P. Li

Fig. 2 Standard extraterrestrial solar spectral irradiance curve

Owing to the wavelength-dependent nature, i.e., glass transmittance, surface absorptance, or photocell response, these parameters are often plotted against the ratio of shorter wavelengths related energy to the total sun’s energy. The area under this curve represents the percentage of the collected energy across the full wavelengths relative to the full sun’s energy striking on the collector’s surface. As such, a secondary nonlinear scale that can represent wavelength may be included, on the basis of the relationship between sun’s energy and wavelength as shown in Fig. 3. This enables the wavelength-dependent parameters to be presented clearly on the diagraph. The relative spectral response of a silicon photovoltaic cell is shown in Fig. 3, indicating that the photovoltaic cells can make use of 58% of the sun’s energy, with shorter-wavelength energy loss of 11% and longer-wavelength energy loss of 31%.

1.1.3

Extraterrestrial Solar Irradiance

Owing to the elliptical shape of the earth’s orbit, the intensity of the solar radiation above the earth’s atmosphere is proportion to the square of the earth–sun distance. Solar irradiance varies by ± 3.4% with the maximum irradiance occurring at the perihelion, i.e., January 3–5 in which the earth is closest to the sun, and the minimum occurring at July 5 in which the earth is at the aphelion. This variation may be approximately expressed as:      360N W/m2 (2) I0 = Isc 1 + 0.034 cos 365.25

Solar Energy Resource and Its Global Distribution

5

Fig. 3 Example of a distorted wavelength scale plot

where I0 is the extraterrestrial solar irradiance above the earth’s atmosphere and N is the number of the day (counted from 1st January). An instructional concept, which is often used in solar irradiance models, is the extraterrestrial solar irradiance striking on a horizontal surface. Considering a flat surface that is above the earth’s atmosphere with a parallel layout to the earth’s surface, when this surface is vertically facing to the sun (i.e., normal to a central ray), the solar irradiance striking onto it will be I0 , which is the maximum solar irradiance; when the surface is not vertically facing to the sun, the solar irradiance striking onto it will be lower than the maximum, owing to the angle between the normal line of the surface and a centralized ray from the sun. This concept is diagrammatically illustrated in Fig. 4. It is seen that the rates of sun’s energy striking onto the both surfaces are the same each other. However, the area of the surface A is greater than its projection, while the hypothetical surface B, which receives the indicated solar irradiance, is more coincident with the surface A, rather than surface B. The extraterrestrial solar irradiance striking on a surface parallel to the ground is expressed as:

6

Z. Qiu and P. Li

Fig. 4 Cosine effect as it relates to the concept of extraterrestrial horizontal irradiance

  Io,h = I0 cos θz W/m2

(3)

where I0 is the extraterrestrial solar irradiance, and θz is the solar zenith angle which is defined as the angle between the two surfaces. Reduction in radiation by the cosine of the angle between the solar radiation and a normal surface is defined as the cosine effect. The cosine effect is an extremely important concept in optimizing the orientation of solar collectors. Owing to the cosine effect, the extraterrestrial solar irradiance striking on a horizontal surface varies cyclically with the earth spins on its axis. The amount of solar radiation received by a horizontal surface above the atmosphere forms an upper limit to the amount of radiation striking onto a horizontal surface beneath the earth’s atmosphere, in case that the factors of air mass and cloud cover are not taken into consideration. Values of Io,h above a specified location on a given day at a particular time may be calculated by using Eq. (3), and the values of the zenith angle can be determined by using Eqs. (23) and (14), while the extraterrestrial radiation can be determined by using Eq. (2).

1.1.4

Extraterrestrial Solar Radiation on a Surface [1]

The total amount of energy stored on a surface over a period of time is determined by integrating (or summing) solar irradiance across this time duration. This sum (H), defined as the solar radiation with the units of energy per unit area (J/m2 ), is expressed as: sunset   Ho,h = ∫ Io,h dt J/m2 . sunrise

(4)

Solar Energy Resource and Its Global Distribution

7

Fig. 5 Seasonal variation of the daily extraterrestrial solar radiation (irradiation) incident on a horizontal surface outside the earth’s atmosphere in the northern hemisphere

It is always of interest of people to determine the total amount of energy striking onto a surface over a full day duration above the earth’s atmosphere. The daily extraterrestrial solar radiation on a horizontal surface Ho,h may be calculated by using the instantaneous values of extraterrestrial solar irradiance, while the time t (in seconds) is counted from sunrise to sunset. By using the angle between a surface parallel to the earth and to the sun, the daily extraterrestrial solar radiation striking onto a surface parallel to the earth can be expressed as: Ho,h =

  86, 400Io ω sin φ sin δ + cos δ cos φ cos ω J/m2 . π

(5)

The definitions of the angles φ and δ (latitude and declination) are indicated in Sect. 1.1.4. The hour angle of sunset, ωS , can be calculated by using Eq. (25). It should be noted that above-indicated angles, expressed as a function of other directional angles, has the units of ‘radians’ in this equation. Since the constant 86,400 has the implied units of “seconds”, the units of the extraterrestrial solar irradiance, I o , must be in watts per square meter (W/m2 ). Values of the daily extraterrestrial solar radiation on a horizontal surface, which is calculated over 1-year duration at three latitudes using Eq. (5), are presented in Fig. 5. This figure shows a number of interesting points relating to the solar energy input. It should be noted that the greatest amount of incident energy during a single day occurs at the northernmost latitude, while the highest solar radiation occurs at summer during which the sun never fall.

8

Z. Qiu and P. Li

Fig. 6 Seasonal variation of the daily extraterrestrial solar radiation incident on a surface always pointed normal to the sun’s rays outside the earth’s atmosphere in the northern hemisphere

It should also be noted that at the equator, the highest solar radiation occurs during the spring and fall (at the equinoxes), rather than summer. A maximum value of the solar irradiance at summer occurs only at latitudes of above 21.11°. The total amount of energy gathered during a year is defined as the average solar radiation which is shown in Fig. 5. As one might expect, the sum value during a single year of the daily solar radiation on a horizontal surface, occurs at the equator. The surfaces at higher (polar) latitudes lose more available energy as a result of the cosine effect indicated above. If a surface is always pointed toward the sun (but still outside of the earth’s atmosphere), the daily extraterrestrial solar radiation on the surface will vary as shown in Fig. 6. As latitude increases to the north, there is more energy available in the summer and less in the winter. It is interesting to see from Fig. 6 that the annual overall solar radiation striking on a surface normal to the sun’s rays is essentially the same regardless of the latitude. This is because at any place of the earth, the daytime hour number is always 4380 h during a single a year, while the average length of day is 12 h. Therefore, apart from the slight difference caused by the winter extraterrestrial solar irradiance which is around 6% higher than the summer solar irradiance, the total annum extraterrestrial normal solar irradiance is essentially the same at any place of the earth. It should be noted that the yearly average of the daily normal solar radiation is very close to the product of 12 (the average length of daylight) times the solar constant, giving a value of 59.1 MJ/m2 . It should also be noted that during a single-year period, the cosine effect leads to a reduction in the solar radiation striking onto a horizontal

Solar Energy Resource and Its Global Distribution

9

surface by 39% at the equator, and the reductions in the solar radiation by 52% at 40° latitude and by 74% at 80° latitude. It is clear that Sect. (1.1) provides some understanding of extraterrestrial solar radiation on hypothetical surfaces above the earth’s atmosphere that gives the readers some idea of the solar energy resource and the effects of the mechanics of the earth–sun system. The next section will address the effects of the earth’s atmosphere (i.e., water vapor, carbon dioxide, clouds, smog, particulates) on the solar radiation traveling across the atmosphere.

1.2 Ground-Level Solar Radiation Characteristics As solar radiation passes through the earth’s atmosphere, it is absorbed (the reason for some atmospheric heating), reflected (the reason astronauts can see the earth from outer space), scattered (the reason one can read this book in the shade under a tree), and transmitted directly (the reason there are shadows). At the surface of the earth, the sun has a lower intensity, a different color, and a different shape from that observed above the atmosphere.

1.2.1

Atmosphere Effects

The atmosphere causes a reduction in the extraterrestrial solar input by about 30% on a very clear day to nearly 90% on a very cloudy day. Figure 7 gives an indication of the range of the absorption and scattering (forward and backward) caused by different components of the atmosphere. On the surface of the earth, we perceive a beam or direct solar irradiance that comes directly from the disk of the sun and a diffuse or scattered solar irradiance that appear to come from all directions over the entire sky. In this text, we will use the term direct to signify solar irradiance coming directly from the sun’s disc, and the term diffuse to indicate solar irradiance coming from all other directions. We use the traditional subscript b to represent the direct component of solar irradiance and the subscript d to indicate the diffuse component. The sum of direct and diffuse solar irradiance is called the global or total solar irradiance and is identified by the traditional subscript t. In this book, we will use the term global to indicate this sum. On a clear day, direct solar irradiance represents about 80 or 90% of the total amount of solar energy reaching the surface of the earth. Local blockage of the direct component of solar irradiance produces shadows. On a cloudy or foggy day when “you can’t see the sun,” the direct component of solar irradiance is essentially zero and there are no shadows. The direct component of solar irradiance is of the greatest interest to designers of high-temperature solar energy systems because it can be concentrated on small areas using mirrors or lenses, whereas the diffuse component cannot.

10

Z. Qiu and P. Li

Fig. 7 Nominal range of clear sky absorption and scattering of incident solar energy. Values are typically for one air mass [6]

Solar Energy Resource and Its Global Distribution

11

The diffuse or scattered component of solar irradiance is what permits us to see in the shade. If there was no diffuse component of solar irradiance, the sky would appear black as at night and stars would be visible throughout the day. The first astronauts vividly described this phenomenon to us from the moon where there is no atmosphere to scatter the solar radiation. As depicted on Fig. 7, diffuse radiation is the result of downward scattering of solar irradiance by nitrogen, oxygen, and water molecules, water droplets, and dust particles in the atmosphere. The amount of this scattering depends on the amount of water and dust in the atmosphere and the altitude of the observer above sea level. Since diffuse solar irradiance cannot be concentrated, only flat-plate (nonconcentrating) solar collectors and some low-temperature types of concentrators (having wide acceptance angles) can collect diffuse solar irradiance. Few of the collectors used in industrial applications can utilize the diffuse component of solar radiation. The variation in these factors, especially that of water droplets (i.e. clouds) as they attenuate the direct component and change the diffuse component, is the major unknown parameter in the design of systems to collect solar energy. Consequently, a considerable amount of effort has been and is being spent in measuring, cataloging, and developing analytical models to predict these effects.

1.2.2

Solar Spectrum

The spectrum of solar radiation has been discussed before. In addition to a reduction in intensity, the spectrum of solar radiation reaching the surface of the earth is also modified as it passes through the atmosphere. The processes taking place include Rayleigh and particulate (dust and water) scattering and absorption by ozone, water vapor, and carbon dioxide. All of these processes depend not only on the temporal condition of the atmosphere, but also on how much of the atmosphere the sunlight passes through. This latter factor is measured in terms of the air mass, which is simply the ratio of the distance that solar radiation travels through the earth’s atmosphere (path length), to the distance (path length) it would travel if the sun were directly overhead. Radiation coming from directly overhead, therefore, is said to pass through an air mass of 1.0 at sea level. Solar irradiance coming from a zenith angle of 60° would pass through approximately twice the perpendicular path length and hence an air mass of 2.0. The following expression to approximate air mass at any zenith angle θz has been developed by Kasten and Young (1989) air mass =

1 cos θz + 0.50572(96.07995 − θz )−1.6364

(6)

where the zenith angle θz is given in degrees. At sunset (θz = 90°), this expression has a value of 37.92 and that is why there is very little solar radiation reaching the earth’s surface at sunset.

12

Z. Qiu and P. Li

Fig. 8 Solar spectral irradiance for different air mass values assuming the U.S. Standard atmosphere, 20 mm of perceptible water vapor, 3.4 mm of ozone, and very clear air [6]

For altitudes other than sea level, the air mass calculated above is reduced by the ratio of the local atmospheric pressure to standard sea-level atmospheric pressure. The effect on the earth’s atmosphere on the solar radiation spectrum is shown in Fig. 8 for different air masses of very clear, sea level air. Terrestrial solar spectrum data sets are available on the NREL internet site cited at the end of this chapter. Note in Fig. 8 the effects of the strong water vapor and carbon dioxide absorption bands in the IR region (wavelength >0.7 μm). Also note the reduction in blue and violet light (wavelength 0.3–0.4 μm) due to particulate and Rayleigh scattering and the reductions in the UV light (wavelength 0 then A = 360 − A ⎪ ⎪ ⎭ ⎩ ielse : sin ω ≤ 0 and A = A

(25)

In summary, we now have equations for both the sun’s altitude angle and azimuth angle written in terms of the latitude, declination, and hour angles. This now permits us to calculate the sun’s position in the sky, as a function of date, time and location (N, ω, φ). Example: For a site in Miami, Florida (25°, 48 min north latitude/80°, 16 min west longitude) at 10:00 AM solar time on August 1 (not a leap year), find the sun’s altitude, zenith and azimuth angles … For these conditions, the declination angle is calculated to be 17.90°, the hour angle −30° and the sun’s altitude angle is then 61.13°, the zenith angle 28.87° and the azimuth angle 99.77°.

2 Solar Energy Global Distribution 2.1 Solar Energy Global Distribution by Belt It is common knowledge that solar radiation is unevenly distributed, and that it varies in intensity from one geographic location to another depending upon the latitude, season, and time of day. Until recently, valid records for solar radiation have been very scanty in the vast majority of the developing countries. In the absence of such useful information as a guide for the proper exploitation of solar energy, only general hints can be offered regarding the geographic areas with favorable conditions for solar energy applications. For convenience and simplicity, the geographic distribution of total solar radiation on a global scale is divided in terms of intensity into four broad belts around the earth. These are illustrated in Fig. 17 and also described briefly hereunder with respect to the northern hemisphere, with the understanding that the same conditions apply to the corresponding belts in the southern hemisphere:

26

Z. Qiu and P. Li

• The most favorable belt. This belt, lying between latitudes 15°N, and 35°N, embraces the regions that are naturally endowed with the most favorable conditions for solar energy applications. These semiarid regions are characterized by having the greatest amount of solar radiation, more than 90% of which comes as direct radiation because of the limited cloud coverage and rainfall (less than 250 mm per year). Moreover, there is usually over 3000 h of sunshine per year. • Moderately favorable belt. This belt lies between the equator and latitude 15°N and is the next most favorable region for the purpose previously mentioned. Because the humidity is high, and cloud cover is frequent, the proportion of scattered radiation is quite high. There is a total of about 2500 h of sunshine per year. The solar intensity is almost uniform throughout the year as the seasonal variations are only slight. • Less favorable belt. This belt lies between latitude 35°N and 45°N. Although the average solar intensity is roughly about the same as for the other two belts, there are marked seasonal variations in both radiation intensity and daylight hours. During the winter months solar radiation is relatively lower than in the rest of the year. • Least favorable belt. The regions in this belt lie beyond latitude 45°N. They include the USSR, and the greater parts of northern Europe and North America. Here, about half of the total radiation is diffuse radiation, with a higher proportion in winter than in summer primarily because of the rather frequent and extensive cloud coverage [10].

2.2 Solar Energy Global Distribution by Nation NREL researchers have calculated the solar irradiance of many countries all over the world based on their weather data. These estimates are derived from the best available solar resource data provided by NREL, detailed in Figs. 18, 19, 20 and 21. Resolution varies spatially from 1 km to 1° (approximately 100 km) depending on the data source. High-spatial-resolution datasets (1–40 km cells) were modeled to support country or regional projects. Here high-resolution datasets were not available, data from NASA’s Surface Meteorology and Solar Energy (SSE) version 6 database were used [11]. The data represent total potential solar energy per year as a function of land area per solar class (KWh/m2 /day). Each solar class correlates to a specific 0.5 kWh/m2 /day range. Energy is calculated by multiplying the productive land by the class, conversion efficiency and number of days per year. In this case, a standard calendar year of 365 days was used. The conversion efficiency rate applied was 10%. E = Productive Land ∗

kWh m2

day

∗ 365 days ∗ 10% efficiency

(26)

Solar Energy Resource and Its Global Distribution

Fig. 17 Best solar collective area distributes between 35°N and 35°S

Fig. 18 Solar DNI distribution map

27

28

Z. Qiu and P. Li

Fig. 19 Solar GHI distribution map

Fig. 20 Top 30 nation of solar resource distribution trillion Watthour per year

The solar data have been derived from solar data measured or modeled between 1961 and 2008, depending on the dataset.

Solar Energy Resource and Its Global Distribution

29

Fig. 21 Percent of G20 nation solar resource

3 Summary A series of basic concepts in relation to solar energy application were presented, involving in classifications of solar irradiance on earth, ground-level solar radiation characteristics, solar angles. The diffuse irradiance, direct normal irradiance, solar constant, extraterrestrial solar spectrum, extraterrestrial solar irradiance, and extraterrestrial solar radiation on a surface were proposed to describe the solar irradiance on earth, sometime named as solar radiation on earth or solar insolation or earth. The atmosphere effects and solar spectrum were presented in order to define ground-level solar radiation characteristics. Then a variety of angle relevant to earth, sun, and time, such as the earth–sun angles, hour angle, declination angle, latitude angle, solar altitude, zenith, and azimuth angles, were defined and illustrated. Finally, the solar energy global distribution was summed up by belt and nation, providing a 20-nation list which accounts for more solar irradiance than others. Such list enables to evaluate the applicability of a solar utilization engineering project.

References 1. Johnson L, Matloff GL, Bangs C (2010) Power from the Sun. Paradise regained. Springer, New York 2. Fröhlich C, Brusa RW (1981) Solar radiation and its variation in time. Solar Phys 74:209

30

Z. Qiu and P. Li

3. Iqbal M (1983) An introduction to solar radiation. Academic Press, New York 4. White OR (ed) (1977) The solar output and its variation. Colorado Associated University Press Boulder, CO 5. Duncan CH, Willson RC, Kendall JM, Harrison RG, Hickey JR (1982) Latest Rocket measurements of the solar constant. Sol Energy 28(5):385 6. Watt AD (1978) On the nature and distribution of solar radiation. U.S. Department of Energy Report HCP/T2552-01, March 7. Woolf HM (1968) On the computation of solar evaluation angles and the determination of sunrise and sunset times. National Aeronautics and Space Administration Report NASA TMX -164, September 8. Anonymous (1981) The Astronomical Almanac for the Year 1981, issued by the Nautical Almanac Office of the United States Naval Observatory 9. Lamm LO (1981) A new analytic expression for the equation of time. Solar Energy 26(5):465 10. Stanhill G (1983) The distribution of global solar radiation over the land surfaces of the earth. Solar Energy 31(1):95–104 11. https://openei.org/datasets/dataset/solar-resources-by-class-and-country. Solar Resources by Class and Country, National Renewable Energy Laboratory

Solar Heating, Cooling and Power Generation—Current Profiles and Future Potentials Wei He, Xinghui Zhang and Xingxing Zhang

Abstract Due to the large amount of consumption of the fossil fuels, the ecological environment has suffered serious pollution and damage. Solar power technologies provide the best solution to the current energy and environment issues. In past decades, global solar thermal capacity increased rapidly, and now it has been used worldwide to provide heating, cooling and power generation. However, after years of development, solar energy utilization technology still faces problems such as low efficiency, high cost, difficulty in energy storage and unstable energy supply, which have been seriously restricting its applications. This chapter briefly summarizes the concept and classification of solar heating, cooling and power generation. Furthermore, some technology development and potential applications relating to solar heating, cooling and power generation are discussed. Keywords Solar heating · Trombe wall · Solar cooling · Solar power generation

1 Solar Heating Solar thermal systems are the most mature and market available technologies in the field of renewable energy. Solar thermal applications can be classified into low-, medium- and high-temperature types. The low-temperature systems have a collecW. He (B) Department of Building Environment and Equipment, Hefei University of Technology, No. 193 Tunxi Road, Hefei 230009, China e-mail: [email protected] X. Zhang College of Environmental Science and Engineering, Taiyuan University of Technology, No. 79 Yingzexi Street, Taiyuan 030024, Shanxi, People’s Republic of China e-mail: [email protected] X. Zhang Department of Energy, Forest and Built Environments, Dalarna University, 79188 Falun, Sweden e-mail: [email protected] © Springer Nature Switzerland AG 2019 X. Zhao and X. Ma (eds.), Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation, Green Energy and Technology, https://doi.org/10.1007/978-3-030-17283-1_2

31

32

W. He et al.

tor temperature of less than 200 °C; medium-temperature systems have a collector temperature of 200–800 °C; and high-temperature systems have a collector temperature of above 800 °C. Depending on the operational temperature, solar systems can find different applications. The higher the temperature, the better the energy quality. Low-temperature solar energy applications are mainly used in solar water heaters, solar agriculture drying, seawater desalination, solar energy rooms and solar refrigeration systems. Medium-temperature systems are mainly applied for solar cookers, solar thermal power generation and industrial warm-up. High-temperature systems can be used in high-temperature solar furnaces, solar thermal chemistry and other applications. At present, the solar thermal systems have been widely applied in various areas, from people’s daily life to industrial and agricultural sectors, concerning heating, air-conditioning, refrigeration, baking, drying, seawater desalination and high-temperature power generation. Solar collectors are the key devices in solar thermal systems. In terms of heat transfer fluid, solar collectors can be classified into liquid and air types. In terms of solar radiation gathering, the collectors can be classified into concentrating and non-concentrating ones. Furthermore, the non-concentrating collectors can further be classified into the flat-plate collectors and evacuated tube collectors.

1.1 Solar Water Heating Solar water heating (SWH) is the conversion of sunlight into heat for water heating using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. Solar water heaters are widely used in residential and industrial applications [1]. A sun-facing collector heats a working fluid that passes into a storage system for later use. SWH is active (pumped) and passive (convection-driven). They use water only, or both water and a working fluid. They are heated directly or via lightconcentrating mirrors. They operate independently or as hybrids with electric or gas heaters [2]. In large-scale installations, mirrors may concentrate sunlight onto a smaller collector. The global solar thermal market is dominated by China, Europe, Japan and India, although Israel was one of the first countries to mandate installation of SWH in 1980, leading to a flourishing industry [3]. In recent years, researches on solar water heaters are focused on improving the energy efficiency of hot water systems. These include collectors, heat storage systems and heat exchangers. Dagdougui, H. et al. established a thermodynamic model for a solar flat-plate collector, studied the effect of the number of collectors and the type of cover plate on the performance of the collector, and hoped to provide the most cost-effective design for collector designers [4]. Michaelides, I. et al. conducted an experimental study on the thermal loss coefficient of the hot siphon solar water heating system at night, and there was a linear relationship between the heat loss and the ambient temperature at night [5]. Zheng, H.F. et al. studied a solar collector that

Solar Heating, Cooling and Power Generation—Current Profiles …

33

combines a heat pipe and a vacuum tube and studied the effect of the radiation characteristics of the back surface on the performance of the collector. The experimental results show that by increasing the roughness of the back surface, the overheating of the solar collectors in the high-latitude summer can be prevented [6]. Fiaschi, D. et al. used a double-glazed heat pipe vacuum tube for numerical simulation to study the performance of solar collectors absorbing solar radiation under meteorological conditions at different regions [7].

1.2 Solar Space Heating Solar heating is the application of solar thermal energy collected by solar thermal collectors to heating needs. According to the different methods of collecting solar energy, it is classified into the active and passive types. The main judgment is based on whether external driving force is needed. Two heating systems are introduced below.

1.2.1

Active Solar Heating

An active solar heater uses electricity to circulate air or a liquid through a solar collector and then distributes the heat throughout the house. There are two types of active solar heating systems: an air circulation system and a liquid circulation system. (1) Air Circulation Systems Air circulation systems are mounted on a roof or south-facing wall of a room. A large flat panel that is darkened and non-reflective is used as a solar collector. A protective, transparent cover is placed over the top darkened surface to create a narrow air chamber. As the panel absorbs the solar radiation, it heats the air in the chamber. A simple fan then circulates the heated air from the top of the unit while bringing in cold air at the bottom. The fan helps to speed the natural convection flow of the air and helps keep the room at a comfortable temperature. These systems are effective for heating rooms or open floor plans. Only indoor air is circulated in the system as the outside air may be too cold to be sufficiently warmed by the collector. In the studies of solar air active heating systems, Robert L. Reid introduced 22 years of operation of an active solar air heating system, indicating that the annual average maintenance cost of the system is 1.6% of the total system investment [8]. Waqas, A et al. studied the use of solar air heating systems with phase-change materials for residential heating [9]. (2) Liquid Circulation Systems Liquid circulation systems use a large, flat solar collector. Tubes are filled with an ethylene glycol (anti-freeze) solution which snakes back and forth behind the panel

34

W. He et al.

to absorb the solar heat. The heated liquid is then stored in an insulated tank which can then be used to heat the home. Water should not be used as a heat transfer medium as it may freeze in winter. These solar power systems work well with in-floor radiant heating systems which have already been installed. Heat exchangers can also be installed to transfer the heat from the ethylene glycol to a household hot water system. Both types of active solar heating systems are excellent supplementary energy systems for homes and can help save residential heating costs. In the research of solar hot water active heating systems, Argiriou, A. et al. introduced the study of a solar hot water active heating system in the northern part of Hellas in Greece. They combined the solar hot water system with the heating system. The system running status was analyzed by TRNSYS software. The results showed that the average solar energy guarantee rate reached 28%, and the system cost and return rate was 0.18 ECU/kWh [10]. Wang, F. et al. introduced a hot water system with a solar collector wall. The system is used for domestic hot water and heating. Through theoretical analysis of suitable experimental data, the system can meet the needs of room hot water heating. The investment return period is about 16 years [11].

1.2.2

Passive Solar Heating System

Active solar heating can provide thermal comfort and high-quality indoor air. However, the high investment and complicated control system cannot be avoided. The passive solar heating system has simple structure and convenient operation, also favorable by researchers and designers. The passive solar system absorbs solar energy as much as possible through a rational layout of windows, walls and roofs in a building. Four types passive solar heating would be introduced in the section: direct solar gains, Trombe walls, double-skin façade and sunspace. (1) Direct heat gains Passing through sunny windows, the solar radiation heats up interior wall and furniture and then heats up indoor air by thermal radiation and convection. Generally, the use of large windows glazing on south (north in the southern hemisphere) and French windows is common ways to obtain more solar radiation. In the 1980s, Lu et al. proposed a model for simulating the thermal process of the direct gain system and studied the influences of some factors on indoor environment via this model, such as the inner surface color of envelops, the structure of southern window, the climate condition and the insulation curtain [12]. Wei et al. introduced a retrofit scheme for Tibetan buildings based on exterior envelope and south-facing glazing to decrease the heat loss and increase the direct solar gain and proved that the direct gain house can meet the minimum requirement of space heating without an auxiliary heat source in this district [13].

Solar Heating, Cooling and Power Generation—Current Profiles …

35

Fig. 1 Operation modes of double-skin façade in different weathers; one of the advantages of this system is the promotion of ventilation in buildings

(2) Double-Skin Façade As one of typical envelops for direct heat gains, the skin façade is popular in buildings. However, the large amounts of energy consumption and light pollution cannot be ignored. The double-skin façade (DSF) was developed to resolve the problems. The system is a heated point in passive solar buildings and widely applied in buildings in Europe and Japan. A double-skin façade is a hybrid system made of a building façade and an external skin; thus, an air space is formed between the two layers. The air space functions as an insulating barrier against the unwanted impacts of microclimatic conditions [14]. It is not uncommon that shading devices are added in the middle of cavity. Furthermore, a small wind turbine may be installed on the bottom of the cavity in case the mechanical ventilation is required. The DSF works thanks to the phenomenon of thermal chimney. In other words, the air moves by density difference. The cavity of DSF is used to collect or evacuate the solar radiation absorbed by the façades. In cold days, the upper and lower openings on the inside skin are open for the thermal circulation between air channel and inside space, while in hot days, the vents on the outside glazing are open for the circulation between outside and air channel. As a result, the heat is drawn to the room for heating in winter and to the environment for cooling in summer. Furthermore, the lower vent on the exterior façade and the upper vent on the interior façade are open to introduce the outdoor air when the fresh air is needed. The three operation modes of DSF are shown in Fig. 1. Hashemi et al. [15] provided simulation results that the temperature differences between the external façade, the inner façade and the air channel play an important role in heating in winter. After theoretical and experimental results, Darkwa [16] indicated that the double-skin façade can provide adequate ventilation; thus, no additional source of heat needs to be supplied. From the results obtained by Khalifa [17], the high thermal mass of the double-skin façade system is beneficial for heating in winter. Furthermore, the impacts of many other aspects on the thermal

36

W. He et al.

Fig. 2 Various configurations of a solar wall: a without ventilation; b winter mode with air thermocirculation; c summer mode with cross-ventilation [19]

performance of the double-skin façade have been studied for years, such as the glass type, the shading devices and the cavity width [18]. (3) Trombe walls Trombe wall is a typical case of solar application in buildings. It was first proposed by E.S. Morse in 1881 but populated by Félix Trombe and his colleagues in 1957. Basically, the Trombe wall consists of a south-facing wall, a glazing and the air channel between them. The wall has good ability of thermal storage as well as black coating on the surface to absorb more solar radiation. The classical Trombe wall is simple with only a glazing and an air channel and thermal storage wall made of bricks, concrete, stone, adobe, etc. The Trombe wall functions by heating using greenhouse effect and thermal siphon principle. The three configurations of Trombe wall are shown in Fig. 2. Absorbing heat, the air in the channel is heated up and delivered into the inside room through an upper opening on the storage mass wall. Meanwhile, cold indoor air is drawn to the air channel through the lower opening on the wall and waits to be heated later. As a result, the indoor air is heated due to the thermal circulation in the daytime in heating seasons. During winter night, all vents are closed. The thermal mass releases solar heat absorbed and stored in the daytime. In addition, two openings are installed on the glazing for cooling in summer. With the upper vent of the thermal wall opening and lower vent on the glazing closed, hot indoor air goes outside through the opening on the floor level of the thermal wall and the upper opening on the glazing. When it comes to its drawbacks, the overheating problem in summer could not be avoided as excessive heat is brought inside by greenhouse effect of the air channel. Another problem is the large heat losses in winter night or in cold area. Furthermore, the wall is opaque, that is, the Trombe wall admits no daylight into the room. Based on these flaws, some improved and modified Trombe walls were proposed. In this paper, five different kinds of Trombe walls are discussed: zigzag Trombe wall, water Trombe wall, solar transwall, Trombe–Michel wall and Trombe wall with phasechange materials.

Solar Heating, Cooling and Power Generation—Current Profiles …

37

Fig. 3 A zigzag solar wall [9]

(a) Zigzag Trombe Wall Zigzag Trombe wall is designed to overcome the overheating problem in buildings and opaque problem. The structure of zigzag wall is shown in Fig. 3. The wall composed of a south-facing glazing as well as a southeast-facing section and a southwest-facing section which form into V shape. The south-facing section allows sunlight and heat to enter the room directly. Other two sections are traditional Trombe walls, which provide heat indirectly. The zigzag Trombe wall is helpful to reduce overheating-related problems. (b) Water Trombe Wall Compared to the classical storage mass made of concrete, masonry, etc., water Trombe wall was proposed. In this kind of wall, water acts as a substitute for traditional building materials in thermal storage wall. Generally, exterior surface of water wall is painted black to improve heat absorption rate. Absorbed by water, the heat distributes fast in water due to convection and transferred to the indoor environment due to thermal radiation. The high heat capacity of water leads to more heat storage as well as less heat loss in water than in other materials. Water also has aesthetic benefits, being translucent and thus allowing sunlight to penetrate the space [20]. However, there are no much researches on water Trombe wall, mainly owing to the less developed water seal technologies. Other than water container directly used as water wall, an array of water tubes or water cans embedded in the brick are also applicable devices [21]. (c) Solar Transwall Transwall is a kind of transparent Trombe wall. The typical transwall is composed of two glazing windows as well as capsuled water between the two windows. A semi-transparent absorbing plate is in the middle of glazing panels. Furthermore, transparent baffles are also necessary in the wall to hold the water container. Reached on the wall, part of the solar radiation is absorbed by water and semi-transparent panels, with the remaining radiation entering the room. As a result, there are both direct and indirect heat gains in the room. In the meantime, the translucent property

38

W. He et al.

Fig. 4 A cross-sectional view of transwall system [20]

of transwall meets the requirement of illumination in buildings. The transwall system is suitable for areas with high daytime temperature [22]. Ting et al. developed a transient heat balance model (THBM) to predict the thermal behavior of a semi-transparent water wall system in Sydney and suggested how to reduce overheating problem which is to reduce the transmissivity of the external panel [23]. The sketch of water walls is shown in Fig 4. (d) Trombe–Michel wall (Composite Trombe Wall) Based on the issue of heat loss, the Trombe–Michel wall (also called composite Trombe wall) was proposed. As shown in Fig. 5, the Trombe–Michel wall is like the classical Trombe wall [24]. The difference is an additional insulation wall behind the original massive wall; thus, there is a new air channel in the wall, which is between the insulation wall and the massive wall. The vents are installed on the glazing and the insulation wall. Drawn into the interior air layer by conduction in the massive wall, the heat is then transferred by convection while using the thermo-circulation phenomenon of air between the massive wall and the insulating wall. Compared to the traditional Trombe wall, the composite wall induces less heat loss in cold period and less undesired heat inputs in hot days. An anti-reverse thermal circulation system is installed on the lower vents of the insulation wall, which is appropriate to cold winter night. Shen et al. developed a model to analyze with precision the thermal performance of the Trombe wall and the composite Trombe wall and concluded that the latter behaves better in cold and/or cloudy days by the model they proposed [25]. As shown in Fig. 6, Sandra Corasaniti et al. studied the energy performances of the classical Trombe–Michel wall (with “sharp edges”) and two types of modified ones (with “rounded edges” and “guided flow”), among which the one with guided flow has the best thermal efficiency [26].

Solar Heating, Cooling and Power Generation—Current Profiles …

Fig. 5 A composite Trombe wall [24]

Fig. 6 Three configurations of a modified Trombe–Michel wall [26]

39

40

W. He et al.

(e) Trombe wall with phase-change materials A popular research topic is to integrate phase-change materials (PCMs) into the storage wall. PCMs are good heat storage materials, with considerable heat absorbed or released during melting or solidifying. For example, the heat capacity of wallboard with 30% PCM is five times more than that of ordinary wallboard of the same size [27]. Furthermore, PCM absorbs and releases heat at a nearly constant temperature, which is beneficial for mixing PCMs into concrete materials. Therefore, PCMs (paraffin, salt hydrate and so forth) are being studied to replace masonry or rocks in thick storage mass. The major problems for the development of PCM walls are the selection and production of PCMs. Currently, the thermal conductivity and transition heat of most PCMs are not high enough for cooling or heating purposes of a building. Furthermore, it is hard to find a PCM with suitable phase-change temperature. Apart from that, for some PCMs, supercooling, phase separation and corrosion would happen during the phase-change period and the durability of PCMs should be considered. Heim et al. [28] simulated a room with the PCM–gypsum walls by ESP-r. The results show that the minimum heating energy consumption could be acquired when the phase-change temperature is 2 K higher than the heating set point for the room. Furthermore, the results prove the effectiveness of the wall on reducing heating energy demand in cold days. Fiorito [29] assessed the thermal behaviors of Trombe walls with PCMs in lightweight constructions in five climate zones in order to get the optimum thickness and phase-change temperature of PCMs in different zones. Dan et al. [30] proposed a Trombe wall with PCMs on both side surfaces of the concrete block to improve the heat capacity of walls. The experimental studies indicate the good energy-saving characteristics of the system. Enghok et al. [31] carried out simulations on a novel Trombe wall where the storage wall is made of mortar mixed with microencapsulated PCMs. Furthermore, the time lag between the solar gains and the heat restitution to the room was analyzed by the dynamic simulation. As one of the most promising applications of solar energy, Trombe wall has been studied for many years. It is believed that more novel Trombe walls, such as Trombe walls with new materials, will emerge in market soon. (4) Sunspace Sunspace can be regarded as a combination of direct gain technology and Trombe wall. The schematic diagram of a sunspace is shown in Fig. 7. Generally, sunspace is a glass house integrated with a thermal storage wall that is south facing (north in the southern hemisphere); thus, a greenhouse is formed for heat absorption. The storage wall is usually painted black to increase the absorption rate. The sunspace reduces the direct heat exchange between the indoor space and outside, reducing heat loss in winter. One major drawback of the system is the overheating problem in summer, which is remedied by a shading device in the greenhouse. Sunspace is of use in some sunny climates, such as the Tibet and Gansu provinces in China, places with intense sunlight and underdeveloped economy.

Solar Heating, Cooling and Power Generation—Current Profiles …

41

Fig. 7 A sunspace [32]

1.2.3

Possible Future Development on Solar Heating System

(1) Large-scale district solar heating system Compared to individual solar heating systems which perform water heating and thermal storage for individual use, the district solar heating system has large quantity of solar collectors integrated together for multi-family use. Namely, hot water is heated and delivered to different families by a network. The system falls into two types: central system and central–individual system [33]. In the central system, the user has their own water tanks to store the hot water heated by large-scale collector field, while in the latter, the water is stored in a shared tank and supplied to every family by the network. The district heating is becoming increasingly popular in European countries and China owing to its lower cost compared to individual solar heating systems. Thirtyone new district heating plants were operated, and five existing plants were expanded in 2016 in Denmark, compared to 15 new and 3 expanded plants in 2015. In Shandong province of China, an incentive policy was announced to build central space heating systems in public buildings [34]. District solar heating system is expected to be a trend of development in the future. (2) Solar photovoltaic/thermal (PV/T) system The PV system has been a hot area of research for many years. However, the low power efficiency and high investment are the barriers for the development of PV systems. Solar photovoltaic/thermal (PV/T) system is an alternative technology that may be able to overcome this problem, which recycles and utilizes the heat generated, thus cooling down the PV cells. Among the PV/T systems, BIPV/T systems enjoyed increased popularity over the past years. PV/T systems make use of building envelope to collect heat and electricity. A BIPV/T system comes in many forms. The classification of the BIPV/T system is shown in Fig. 8.

42

W. He et al.

Fig. 8 Classification of BIPV/T system [35]

Fig. 9 A BIPV/T system with three configurations

Pantic et al. [36] proposed and examined a BIPV/T system with three configurations which is shown in Fig. 9. The first configuration was a base case of unglazed BIPV with airflow passing beneath it. The second one was the base case connected with a 1.5-m vertical-glazed solar air collector. In the third configuration, a glazing was added over the PV. The results show that the first system was suitable for the HAVC system and preheating of domestic hot water. The thermal efficiency in the second and third systems was higher than the first one. Furthermore, the electricity generated from the third system was reduced significantly, which may result in excessively high PV panel temperatures.

Solar Heating, Cooling and Power Generation—Current Profiles …

43

As an effective way to achieving the energy-efficient building and the net zeroenergy building, the BIPV/T system has always been a hot research field in HAVC subject. With the popularization of solar system, the BIPV/T system has established significant market potential.

2 Solar Cooling 2.1 Introduction Solar cooling means converting the solar energy into useful cooling for various applications such as air-conditioning. During this process, solar heat is collected to generate chilled water or conditioned air for the cooling systems. Studies on solar cooling systems started in the 1970s owing to the energy crisis and have picked up again due to greater awareness of environmental protection over the past decade [37]. Solar cooling technologies have proven to be a promising alternative for conventional cooling systems because a typical building’s cooling load peaks within 2 or 3 h of the time of maximum solar irradiation [38].

2.2 Solar Thermal-Driven Refrigeration Systems Thermally powered cooling technologies were classified into three categories: closed cycles, open cycles and thermo-mechanical cycles. Adsorption and absorption cycles represent the closed cycle. Solid and liquid desiccant cycles represent the open cycle [39].

2.2.1

Sorption Cooling

In solar sorption systems, thermal power is the driven power to the compressor. The sorbent and the sorbate are used to produce the cooling effect. Low-temperature sorption system would be a better option for Saudi Arabia because the cost of a solar collector system tends to increase with working temperature more rapidly than the COP of a sorption machine does. (1) Absorption cooling systems In absorption cooling the liquids or gases are dissolved in the bulk of the sorbent in one stage of the process and released in another stage. Absorption cooling machines generally use a closed loop comprising four steps: evaporation, absorption, regeneration and condensation.

44

W. He et al.

Single-effect absorption systems are limited in COP to about 0.7 for LiBr–water and to 0.6 for ammonia–water. Hence, they require a large solar collector area to supply the heat needed for their operation [40]. Generally, the capacity of solar absorption cooling systems has become increasingly larger. They are more suitable for large-building air-conditioning applications [41]. Previous studies show that solar absorption refrigeration will provide a promising application potential of solar cooling for buildings in the subtropical region. This kind of solar cooling system had the highest energy-saving potential in the subtropical Hong Kong and would have even better performances with advanced solar collectors [42]. (2) Adsorption cooling systems Adsorption cooling is a thermally driven cooling process powered by solar energy. Adsorption cooling is based on the evaporation and condensation of a refrigerant combined with adsorption. Adsorption cooling machines are based on solid or liquid solvents, the so-called adsorbents, which can bind gases or liquids to their surface. Adsorbed particles can be removed from the surface by heating the adsorbent. A supplementary step to regenerate or exchange exhausted adsorbent is required since the process in adsorption cooling machines is discontinuous. With small-capacity chillers, solar adsorption cooling systems are thought to be more promising in small-building air-conditioning systems [43]. (3) Comparisons between absorption and adsorption systems The main difference compared to the absorption systems is that two or more absorbers are necessary to provide continuous operation. Adsorption systems allow for lower driving temperatures, but have a lower COP compared to absorption systems under the same conditions. The use of adsorption cooling technology is preferable for small solar-powered cooling systems [41]. Based on COP, the absorption systems are preferred because higher-temperature issues can be easily handled with solar adsorption systems [39]. (4) Desiccant cooling systems Desiccant cooling involves a combination of dehumidification and evaporative cooling processes. Different desiccants can be found either in liquid or in solid phases [43]. (1) Solid desiccation Solid desiccation generally uses rotating adsorption wheels made of silica gel, zeolite or lithium chloride as sorption materials. (2) Liquid desiccation

Solar Heating, Cooling and Power Generation—Current Profiles …

45

The dehydration process in liquid desiccation is accomplished by absorption. The desiccant wheel is replaced by a dehumidifier and a regenerator. These elements provide the air’s cooing blown through an absorbent solution. The main difference compared to the absorption process lies in the varying equilibrium temperature. In a liquid desiccant, the temperature is not found by the total pressure but by the partial pressure of the liquid [44]. The liquid desiccant system has a higher thermal COP than the solid desiccant system [39].

2.2.2

Thermo-Mechanical Refrigeration

In a solar thermo-mechanical cooling system, heat gained from the solar collector is converted into mechanical work to compress the working fluid in a vapor compression cycle directly or indirectly [37]. Solar thermo-mechanical cooling systems have received a renewed attention in recent years owing to the advantages such as ability to produce low refrigeration temperatures ( 400 ρl g

(11)

where Re is the Reynolds number, μl (Pa s) the liquid dynamic viscosity, g (m/s2 ). The Reynolds number is given by: Re =

ρl Ul δlf μl

(12)

U l is the superficial velocity of the liquid film. Since the liquid flow through the holes of the micro-channel depends of the driving force of the loop which is the pressure head, it is suitable to link it with the superficial flow. To consider this effect, the

Micro (Mini)-Channels and Their Applications in Solar Systems

185

superficial velocity ul is linked to the pressure head by the following equation (it is simply expressed as the liquid flow throughout a hole caused by the pressure head):  1 2g Hhp-he 2   u l = Ah Al 1 − Cd 2 (dh /Dlh )

(13)

where Ah is the hole’s section, Hhp-he is the pressure head (height difference between the top of the evaporator and the heat exchanger), d h is the hole’s diameter, and Dlh is the diameter of liquid header. The mass velocity ul is assumed the same for the four holes; then, the film thickness is assumed the same along the adjacent wall. Al is the liquid film section expressed as follows: Sl = δlf a

(14)

a is the mini-channel port width. C d (−), the discharge coefficient of the flow from the liquid head to the hole, is expressed as follows [32]: ⎡



μl  Cd = 0.611⎣87 ρl dh g Hhp,he

1.43



4.5 μl  + 1+ ρl dh g Hhp,he

−1.26 ⎤−0.7 ⎦

(15)

The liquid thickness δlf is calculated iteratively by assuming that the liquid mass flow from the liquid header is equal to the mass flow of the liquid film. Thermal resistance of axial vapour flow, Rv,a . The vapour flow process from the evaporation section to the condensing heat exchanger experiences a certain pressure loss and consequently a temperature drop. This creates a resistance in heat transfer which could be written as: Rv,a =

Tv2 R0 Pv Nhp Nch Q u h fg Pv

(16)

The total pressure drop of the system has been expressed as given in [25]. Thermal resistance of the condensation, Rcond . Heat transfer through the heat pipe wall is a typical steady-state conduction process, and its thermal resistance can be written as: Rcond =

1 2 ↑ π d1 L hx h cond

(17)

The condensation heat transfer coefficient hcond is given in Annex 3. b. Mathematical model of the heat transfers in the heat triple heat exchanger At the heat pipe’s evaporation section, the part of the solar energy converted into heat Qu leads to the evaporation of the heat pipe working fluid. This vapour fluid, via the

186

T. Diallo et al.

Fig. 18 Sketch map of phase change process

vapour transportation line, moves forward to the condensing triple heat exchanger, where the evaporated fluid is condensed and transfers the condensation heat into the adjacent water flow and the PCM material. The condensed fluid in the central tube of the heat exchanger returns to the evaporation section, via the liquid transportation line, thus forming a complete heat transportation cycle. Figure 18 presents the heat exchanger geometry. There are two heat transfer processes in the water middle tube according to the charge of discharge of the PCM. The number of transfer unit (NTU) method is used to quantify the heat transfer rate from the water to the PCM material. Charging the PCM The heat transferred from water in the middle tube to the PCM is expressed as follows:   Q c = εc Cmin Tc,out − Tc,in

(18)

With C min (W/K), the minimum heat transfer capacity, εc , is the average effectiveness of the melting process and is expressed as follows [33]:  1  1 − exp − εc = 0

1 RT Cmin

 df

(19)

where f is the fraction of melted PCM and varies from 0 (only solid) to 1 (only liquid); RT is the total resistance from the water to the PCM material and is expressed as follows: RT = Rw + Rmt + RPCM

(20)

where Rw is the water resistance, Rmt is the resistance of the middle tube, and RPCM is the resistance of the PCM material. The different resistances are expressed as follows: Rw =

1 h w π D2 L he

(21)

The convective heat transfer coefficient h w of the water on the annulus side can be calculated by

Micro (Mini)-Channels and Their Applications in Solar Systems

hw =

Nu kw D45

187

(22)

The Nusselt number Nu of the annulus water flow is given in Annex 3. Thermal resistance of the middle tube, Rmt . The conductive thermal resistance of the middle tube is expressed as follows:   1 D4 (23) Rmt = log 2 π kmt L he D3 Thermal resistance of the PCM material, RPCM . The thermal resistance of the PCM considering the fraction of the melted PCM is expressed as the sum of the resistance of the PCM solid layer and the PCM liquid layer and is expressed as follows:

RPCM

   0.5  f R52 − R42 + R42 1 = log R4 2π L he kPCM,L   R5 1 + log     2 2 2 0.5 2π L he kPCM,S f R5 − R4 + R4

(24)

Total heat transfer rate, Qw . The total heat transferred to the water is assumed equal to the total useful heat Q u minus the heat transferred to the PCM material Q PCM,c : Q w = Q u − Q PCM,c

(25)

Discharging the PCM. The heat transferred from water in the middle tube to the PCM is expressed as follows:   Q PCM,d = εd Cmin TPCM − Tc,in

(26)

With εd , the average effectiveness of the solidification process is expressed as follows. Total heat transfer rate, Qw . The total heat transferred to the cold water is assumed equal to the sum of the heat released from the PCM material Q PCM,d and the useful heat Q u : Q w = Q PCM,d + Q u

(27)

c. Definition of the system performance The energy efficiency of the system is characterized by two main parameters: • The overall thermal efficiency ηo of the PVT module that is the sum of the electrical efficiency ηe and the thermal efficiency ηth of the system.

188

T. Diallo et al.

ηo = ηe + ηth

(28)

The coefficient of performance (COP). The coefficient of performance (COP) of the system could be defined as the ratio of system’s overall heat output (including the power output converted to heat) and power consumption of the water pump as follows (29): COP =

Q w + Q e /0.38 Ppump

(29)

The electricity consumption of the pump is expresses as follows: Ppump =

Q v ρw g H ηpump

(30)

where Qv is the water volume flow rate, ηpump the efficiency of the pump and H the total height H expressed as follows: H = f an

 2   2 uw u L he + K w D3 − D2 2g 2g

(31)

where f an is the friction factor for an annular tube given in Annex 3 and K is the sum of loss coefficient for the four 180 returned bend, inlet and outlet fittings [34, 35]. d. Presentation of the algorithm of the integrated model of the PVT system The global model is resolved by ensuring the heat balance at the micro-channel evaporator and at the PCM heat exchanger. The algorithm is summarized in Fig. 19. The algorithm is illustrated as follows: (i) (ii) (iii) (iv) (v)

Input external weather variables, system design and operating parameters. Calculate the absorbed heat equation (32) (Annex 2). Set the cold water mass flow mw rate and the water inlet temperature T c,int . Assume the outlet temperature of the water T c,out . Calculate the total heat transfer rate Qw , Eq. (25) (charge process) or Eq. (27) (Discharge process). (vi) Assume the cell temperature T p , and commence the following analysis: A. Heat balance of the glazing cover could be analysed using Eqs. (33) to (40), which gives the heat loss, QL . B. Heat balance of the PV cells gives of the converted solar electricity, Qe and heat, Qth ; Eqs. (42) and (44). C. Heat transfer from the PV cells to the PCM heat exchanger could be analysed by Eqs. (1)–(17), which gives the useful heat gain, Qu . D. If (Qth − Qu )/Qth > 0.1% (error allowance), then increase t p by 0.1 °C and return to step (vi) for re-calculation.

Micro (Mini)-Channels and Their Applications in Solar Systems

189

Fig. 19 PV/LHP thermal performance model

E. If (Qth − Qu )/Qth < −0.1% (error allowance), then decrease t p by 0.1 °C and return to step (vi) for re-calculation. F. If −0.1% ≤ (Qth − Qu )/Qth ≤ 0.1%, the system achieves heat balance. (vii) If (Qu − Qw )/Qu > 0.1% (error allowance), then increase T c,out by 0.1 °C and return to step (iii) for re-calculation. (viii) If (Qu − Qw )/Qu < −0.1% (error allowance), then decrease 0.1 °C and return to step (iii) for re-calculation. (ix) If −0.1% ≤ (Qu − Qw )/Qu ≤ 0.1%, the system achieves heat balance. (x) Calculate the module’s energetic efficiencies and the overall performance coefficient of COPPV/T using Eqs. (28)–(31) and the program stops. e. Parametric study on the energy efficiency of the novel PVT system In this part, through the established computer model, the influence of the weather conditions (external parameters) and system configuration (internal parameters) on the energy performance of the system has been investigated. Influence of the Solar Radiation By varying the solar radiation from 200 to 800 W/m2 while keeping the other external variables constant, i.e. air temperature at 25 °C and air velocity at 1 m/s, the

190

T. Diallo et al.

Fig. 20 Influence of solar radiation on: a the thermal, electrical and overall efficiencies. b PV cell temperature

energy performance of the system is assessed. The PCM is assumed at the melting temperature of 44 °C. Figure 20a shows the influence of the solar radiation I on the thermal, electrical and overall efficiencies. The increase in the solar radiation would lead to an increase in the thermal and overall efficiency and a decrease in the electrical efficiency. The thermal efficiency outweighs the electrical efficiency, and then, the overall efficiency follows the increasing trend. The increase is most significant for relatively lower solar radiations (I < 400 W/m2 ) and is nearly linear. It can be found that the trend becomes rapid in 200–400 W/m2 ; however, it slows down above the solar radiation of 400 W/m2 . For each 100 W/m2 increase, the overall efficiency increases by nearly by 7% and by 1.6% for the slow trend. The slow trend is due to the increase in the water outlet temperature, therefore the heat loss from the panel to the ambient air while increasing the solar radiation. For a solar radiation of 800 W/m2 , the PVT module reaches an overall efficiency of 67.8%. Otherwise, the increase in the solar radiation from 200 to 800 W/m2 leads to a small decrease (0.28%) of the electrical efficiency from 12.48 to 12.2%. The higher solar radiation brings more instant heat to the PV layer, resulting in an increase in the PV cell temperature as shown in Fig. 20b and the decrease in the electrical efficiency. For a solar radiation of 800 W/m2 , the PVT module reaches an overall efficiency of 67.8%. Figure 21a shows the influence of the solar radiation on the useful heat Qu transferred to the circulating water temperature, the total heat transferred Qw to the network/heat storage tank and Qpcm . Increasing the solar radiation would increase the useful Qu heat and therefore the total heat transferred to the circulating water the Qw . The heat released (discharged) by the PCM Qpcm and transferred to the water is constant as the calculation has been performed an assumed constant PCM temperature of 44 °C (Eq. 25). The amount of heat released by the PCM is 110 W. The contribution of the PCM release in the total heat transfer rate decreases with the solar radiation increases. For the solar radiation of 200 W/m2 , the heat from the PCM Qpcm represents 43% of the total heat transfer rate (Qw = 256 W), and this percentage is 11% (Qw = 987 W) for higher solar radiation of I = 800 W/m2 . It is seen that the presence of the PCM permits to increase the total heat transfer rate wick is more important for lower solar radiations. Increasing the solar radiation by

Micro (Mini)-Channels and Their Applications in Solar Systems

191

Fig. 21 Influence of solar radiation on: a the heat outputs and b the water outlet temperature Fig. 22 Influence of solar radiation on the COP

100 W/m2 would increase the water output temperature by 1.33 °C (Fig. 21b). For a solar radiation of 800 W/m2 , the system delivers a water temperature of 45.8 °C. Figure 22 presents the influence of the solar radiation the COP of the PVT system which considers both electrical and thermal outputs (Eq. 28). The COP increases linearly with the solar radiation. The high COP value (2–3 order) reflects the small power consumption of the pump. The increase in solar radiation significantly influences the COP which is multiplied by 4.3 when the solar radiation increases from 200 to 800 W/m2 . The solar radiation increase is very beneficial to the PVT system. Influence of the Ambient Temperature Varying the air temperature from 5 to 35 °C while the other external variables remained constant, i.e. solar radiation at 800 W/m2 , air velocity at 1 m/s, PCM melting temperature of 44 °C, the energy performance of the system has been assessed. Figure 23a shows that increasing the ambient temperature would increase the thermal and the overall efficiency but decrease the electrical efficiency. As the thermal efficiency outweighs the electrical efficiency, the overall efficiency follows the thermal efficiency which increases linearly. For each 5 °C ambient temperature increase, the overall efficiency increases by 4%. From 5 to 35 °C ambient temperature, the electricity efficiency decreases slightly because of the PV cell temperature rose of 2.58 °C, from 39.3 to 41.9 °C, as shown in Fig. 23b. This little increase in the PV cell tell temperature can be explained by the high capacity of the micro-channel evaporator

192

T. Diallo et al.

Fig. 23 Influence of the ambient air temperature on: a the thermal, electrical and overall efficiencies. b PV cell temperature

Fig. 24 Influence of the ambient temperature on: a the heat outputs and b the water outlet temperature

to absorb heat flux, and the two-phase heat transfer coefficient in the micro-channel evaporator is in the order of 4 × 103 W/m2 /K and by the heat loss decrease when increasing the ambient temperature. Figure 24a presents the evolution the heat output rates and the circulation water outlet temperature at the PCM heat exchanger. Higher ambient temperature would lead to a higher total heat transfer rate Qw . At 35 °C, the system achieves a total heat transfer rate of 1096 W. Figure 24b shows that from 5 to 35 °C ambient temperature, the water outlet temperature increases by 4 °C; this means for each 7.5 °C ambient temperature increase, the water outlet temperature increases by 1 °C. Figure 25 presents the influence of the ambient temperature on the COP. The COP of the system increases linearly with the increase in the ambient temperature due to the linear increase in the total heat transferred to the water Qw . The temperature increase is a favourable factor on the COP that is multiplied by 1.34 when the ambient temperature increases from 5 to 35 °C. It can be seen that the influence of the solar radiation on the COP (Fig. 22) is stronger than the influence of the ambient temperature (Fig. 25). Influence of the Wind Velocity Varying the wind velocity from 0.5 to 5 m/s and holding the other external variables constant, i.e. solar radiation at 800 W/m2 and air temperature at 25 °C, the energy performance of the system has been assessed. It was found that increasing wind

Micro (Mini)-Channels and Their Applications in Solar Systems

193

Fig. 25 Influence of ambient temperature on the COP

Fig. 26 Influence of the wind velocity on: a the thermal, electrical and overall efficiencies. b PV cell temperature

velocity would decrease slightly the overall efficiency and increase the electricity efficiency very slightly (Fig. 26a). Increasing the wind velocity by 1 m/s decreases the overall efficiency by 0.3% (from 67.9 to 66.5%). This decrease is due to the little increase in the heat loss due to the increase in the convective heat transfer coefficient from the PV to the surroundings. However, this heat transfer coefficient is favourable for the electricity efficiency that increases very slightly (almost constant) from 12.2 to 12.21%. As shown in Fig. 26b, the wind velocity increase led to a very slight decrease in the temperature of the PV cells (40.58–40.39 °C). This PV cell temperature decrease is favourable to the system but weak. Figure 27 presents the influence of the wind velocity on the total heat transfer rate and the water outlet temperature. Increasing the wind velocity by 1 m/s slightly decreases the total heat transfer rate Qw by 0.8% (Fig. 27a) and the water outlet temperature T c,out by 0.08 °C (Fig. 27b). Figure 28 shows that COP decreases slightly with an increasing the velocity. Increasing the velocity from 0.5 to 5 m/s decreases the COP by 4%. It can be seen that the influence of the wind velocity on the COP of the system is very weak compared to the influences of the solar radiation (Fig. 22) and the ambient temperature (Fig. 25).

194

T. Diallo et al.

Fig. 27 Influence of wind velocity on: a the heat outputs and b the water outlet temperature

Fig. 28 Influence of wind velocity on the COP

Influence of Number of Cover By varying the number of top glazing covers from 0 to 2 while the other parameters remained constant, solar radiation at 800 W/m2 , air temperature at 25 °C, velocity 1 m/s, it was found that increasing the number of glazing covers would increase the thermal and slightly decrease the electrical efficiency (Fig. 29a). The thermal efficiency overweighs the electrical efficiency, and the overall efficiency follows the thermal efficiency increase. The increases in the overall efficiency can be explained by the fact adding more glazing covers helps to reduce the overall heat losses and the amount of absorbed solar energy due to its reflection and reduced transmittance; therefore, the thermal efficiency and the PV cell temperature rise slightly from 39.2 to 40.9 °C (Fig. 29b), and then the electrical efficiency falls slightly (12.19–12.17%). The increase in the cover number is favourable for thermal efficiency and unfavourable for electrical efficiency. Figure 30 shows that increasing the number of cover from 1 to 2 increases the heat transfer rate by 7.3% (Fig. 30a) and water outlet temperature by 0.8 °C (Fig. 30b). Figure 31 shows that the cover number is a favourable factor for the performance of the system. From 1 to 2 number of cover, the COP increases only by 5.4% (Fig. 31);

Micro (Mini)-Channels and Their Applications in Solar Systems

195

Fig. 29 Influence of the cover number on the efficiency

Fig. 30 Influence of the cover number on heat transfer rates and the outlet water temperature Fig. 31 Influence of the cover on the COP

this means using 2 covers does not significantly increase the COP. To minimize heat loss and maximize solar energy input, the single-glazing cover was considered to be the most appropriate option. Influence of the Packing Factor The influence of the packing factor on the energy performance of the system has been investigated for a constant solar radiation at 800 W/m2 , air temperature at 25 °C and velocity 1 m/s. The packing factor has been varied by changing the number of PV cells. It was found that increasing the packing factor from 0.1 to 0.9 increases the overall efficiency from 60.4 to 67.7% (Fig. 32a). This increase is in majority due

196

T. Diallo et al.

Fig. 32 Influence of the packing on the efficiency

Fig. 33 Influence of the packing factor on heat transfer rates and the outlet water

to the significant increase in the electricity efficiency (1.5–12.2%), and the thermal efficiency decreases less (58.9–55.5%). Figure 32b shows that the increase in the packing factor slightly influences the PV cell temperature. The packing is significantly favourable to the electricity efficiency and unfavourable to the thermal efficiency. Figure 33 shows that total heat transferred to the water Qw and the outlet water temperature decreases slightly with the packing factor. As a result, from a packing factor of 0.1 to 0.9, the COP decreases slightly by 36.5% (Fig. 34). In fact, this increase in the COP is due to the increase in the electrical output that compensates the small decrease in the total heat output. Finally, it was found that higher packing factor is beneficial for the overall system performance. Influence of Number of Heat Pipe By varying the number of heat pipes from 5 to 10 while the other parameters remained constant, solar radiation at 800 W/m2 , air temperature at 25 °C, velocity 1 m/s, it was found that increasing the number of micro-channel heat pipes increases the overall efficiency of the system from 64.9 to 67.85% (Fig. 35a) and the effect is most evident for of heat pipe quantities less than 10. It is found that for the micro-channel heat pipe number (N hp ) superior to 20, the overall heat transfer coefficient is approximately constant (Fig. 35a). The total heat transfer rate tends also to be constant. Figure 36 shows also that for N hp > 20, the PV cell and the outlet water temperatures, 40.48 and 45.8 °C, respectively, are constant. The COP of system follows also this behaviour

Micro (Mini)-Channels and Their Applications in Solar Systems

197

Fig. 34 Influence of the packing factor on the COP

Fig. 35 Influence of the micro-channel heat pipe number on the efficiency

Fig. 36 Influence of micro-channel the heat pipe number on the PV cell temperature and the outlet water temperature

(Fig. 37) and is constant for a Nhp superior to 20. This means that 20 micro-channel heat pipes represent an optimal micro-channel number. Influence of Water Inlet Temperature By varying the cold water inlet temperature of the heat exchanger from 20 to 40 °C, while the other parameters remained constant, solar radiation at 800 W/m2 , air temperature at 25 °C, velocity 1 m/s, it was seen that increasing the water inlet temperature would significantly decrease the overall efficiency of the system (Fig. 38a).

198

T. Diallo et al.

Fig. 37 Influence of micro-channel heat pipe number

Fig. 38 Influence of the water inlet temperature on the efficiency

This decrease is due to the increase of the water outlet temperature (Fig. 39b) that increases the heat loss in the loop system. Figure 38a shows that the increase in the water inlet temperature significantly decreases the total heat transfer rate. This significant decrease is due to the decrease in the useful heat Qu because of the heat loss increase and the decrease in the total heat released the PCM Qcm . It can be seen that for water inlet temperature near 48 °C, the useful heat Qu is superior to the total heat transfer rate Qw (Fig. 34b). This means there is excess heat charged in the PCM material. Figure 40 shows that the COP evolution presents a maximum value where the heat excess is beginning to be charged in the PCM material and the COP falls. Influence of the Water Mass Flow By varying the water mass flow rate from 0.01 to 0.087 kg/s, while the other parameters remained constant, the influence of this variation on the system performance has been assessed. For lower mass flow rates mw < 0.04 m/s, the overall efficiency of the module increases with the flow rate; after this point, the efficiency tends to be constant (Fig. 41a). This can be explained by the fact that increasing the mass flow rate decreases the mean temperature of the water in the annular tube and then decreases

Micro (Mini)-Channels and Their Applications in Solar Systems

199

Fig. 39 Influence of the water inlet temperature on heat transfer rates and the outlet water

Fig. 40 Influence of the water inlet temperature on the COP

the heat losses in the system. Consequently, the useful heat gain increases (Fig. 42a) and the thermal efficiency increases. The electrical efficiency also increases because of the PV cell temperature decreases (Fig. 41a). Otherwise, the increase in the mass flow rate decreases the heat released by the PCM for flow rate inferior to 0.02 kg/s, above which this tends to be constant. Figure 43 shows that the increase in the mass flow rate significantly influences the COP of the system, because of the supplementary electricity consumption of the system. It is seen that the flow rate of 0.0125 kg/s would be an optimal value for high COP and high heat output simultaneously. This section presented a novel solar PVT loop heat pipe (PVT-LHP) system using a micro-channel evaporator and a PCM triple heat exchanger. A computer model was developed to assess the performance of the PVT-LHP system on the basis of a heat balance mechanism, which gave the predicted PV modules’ solar thermal, electrical and overall efficiencies and the system’s overall performance coefficient (COPPV/T ) at the specified operational conditions. The influence of the environmental parameters (i.e. solar radiation, air temperature, wind velocity), structural parameters (i.e. glazing covers, number of the absorbing heat pipes, PC cell packing factor) and the variable inputs (i.e. water inlet temperature, mass flow rate) on the energy performance of the

200

T. Diallo et al.

Fig. 41 Influence of the water mass flow rate on heat transfer rates and the outlet water

Fig. 42 Influence of the water mass flow rate on the efficiency Fig. 43 Influence of the water mass flow rate on the COP

system was investigated individually. The novel PVT-LHP has been compared with a conventional solar PVT-LHP system. It was found that: (1) Increasing the solar radiation led to an increase in thermal efficiency and a decrease in the electrical efficiency, resulting in an increase in the system’s overall performance coefficient (COPPV/T ).

Micro (Mini)-Channels and Their Applications in Solar Systems

201

(2) Increasing the ambient air temperature led to an increase in the thermal efficiency, a decrease in the electrical efficiency and an increase in the system’s overall performance coefficient (COPPV/T ). (3) Increasing the wind speed led to a slight decrease in the thermal efficiency, slight increase decrease in the electrical efficiency and slight decrease in the system’s overall performance coefficient (COPPV/T ). (4) Increasing number of the glazing covers led to increase in the module’s thermal efficiency but a decrease in the module’s electrical efficiency and in the system’s overall performance coefficient (COPPV/T ). (5) Increasing the packing factor led to decrease in the module’s thermal efficiency but an increase in the module’s electrical efficiency and in the system’s overall performance coefficient (COPPV/T ). (6) Increasing number of the heat absorbing pipes led to increase in the fin’s efficiency and in the system’s overall performance coefficient. (7) Increasing the cold water inlet temperature led to decrease in the module’s thermal efficiency, the module’s electrical efficiency and the system’s overall performance coefficient (COPPV/T ). (8) Increasing the water mass flow rate led to an increase in the module’s thermal efficiency and the module’s electrical efficiency and a decrease in the system’s overall performance coefficient (COPPV/T ) because of the increase in the pump electricity consumption. The results show that a flow rate of 0.0125 kg/s would be an optimal flow for a high heat output and high COP simultaneously. Furthermore, on the whole, the increase in solar radiation, ambient temperature, cover number, heat pipe number and packing factor has been seen as the favourable factors for the COPPV/T (coefficient of performance) of the system, whereas higher wind velocity and cold water mass flow rate have been seen to be unfavourable. Under the given design conditions, a number of micro-channel heat pipes of 20 and one glazing cover were found optimal. The electrical, thermal and overall efficiency of the PVT-LHP module was found 12.2, 55.6 and 67.8%, respectively.

202

T. Diallo et al.

Annex 1: Characteristics of Different PVT Layers

Parameters

Nomenclature

Value

Unit

Emissivity of glazing cover

εc

0.84

(−)

Absorptance of glazing cover

Ac

0.05

(−)

Transmittance of glazing cover

τc

0.90

(−)

Emissivity of PV cell

εabs

0.96

(−)

Absorptance of PV cell

α abs

0.90

(−)

Thickness of PV cell

δ abs

0.0003

m

Thermal conductivity of PV cell

λabs

148

W/m/K

Reference efficiency of PV cell

ηrc

0.18

(−)

Temperature coefficient of PV cell

β abs

0.0045

1/°C

Reference temperature of PV cell

t rc

25

°C

Number of PV cell

N pv

72

(−)

Area of single PV cell

Apv

0.156 × 0.156

m2

Thickness of EVA grease

δ eva

0.0005

m

Thermal conductivity of EVA grease

λeva

0.35

W/m/K

Thickness of electrical insulation

δ ei

0.002

m

Thermal conductivity of electrical insulation

λei

144

W/m K

Absorptance of blacken electrical insulation

Aei

0.8

(−)

Thickness of fin sheet

δf

0.005

(−)

Thermal conductivity of fin sheet

λf

203

(−)

Annex 2: Heat Transfer from Outside to the PV Surface [18] The solar energy received by the PV module is expressed as follows:    Q abs = τcNC τg,pv αp βp + αb 1 − βp Am I

(32)

Micro (Mini)-Channels and Their Applications in Solar Systems

203

Fig. 44 Thermal network of heat loss for a typical double-cover module

where τ c and τ g,pv are the visual transmittances of cover plate and the glazing layer of PV lamination, respectively; N c is the number of cover plates; α abs and α b are the absorption ratios of the PV layer and its baseboard; β p is the packing factor of PV layer; Am is the collector area of the module (m2 ). The loss heat from a double-glazed module will experience (1) heat transfer from the PV absorber surface to the inner glazing cover; (2) heat transfer from the inner cover to the outer cover; and (3) heat transfer from the outer glazing cover to the ambient air. As shown in the following Fig. 44, the three forms of heat transfer are laid in a series and achieve a balance. Therefore, the total heat loss is written as:   Q L = UL Am Tp − Ta

(33)

where QL and U L are, respectively, the total heat loss (W ) and the heat loss coefficient (W/m2 K); T p and T a are the average temperatures of PV layer and the ambient air (K), where the U L is the overall heat transfer coefficient and could be written as:  UL =

1 1 1 + + h c,p-c2 + h R,p-c2 h c,c2-c1 + h R,c2-c1 h c,c1-a + h R,c1-a

−1 (34)

204

T. Diallo et al.

where hc,p-c2 , hc,c2-c1 and hc,c1-a are, respectively, the convective heat transfer coefficients (W/m2 K) of PV layer (p) to inner cover surface (c2), inner cover surface (c2) to external cover surface (c1) and external cover surface (c1) to ambient air (a); hR,p-c2 , hR,c2-c1 and hR,c1-a are the radiative heat transfer coefficients (W/m2 K) of PV layer (p) to inner cover surface (c2), inner cover surface (c2) to external cover surface (c1) and external cover surface (c1) to ambient air (a), respectively.  +    ka,p 1708 sin(1.8θ )1.6 1708 1− h c,p-c2 = 1 + 1.446 1 − δa,p Raa,p cos θ Raa,p cos θ +  0.333 Raa,p cos θ −1 + (35) 5830 where k a,p is thermal conductivity of air gap at the average temperature of PV layer and inner cover surface (W/m k); δ a,p is the PV layer to glazing cover distance (m); θ is the collector slop (degree); the bracket with plus means zero and positive values only; Raa,p is the Rayleigh number of the air gap at PV layer and inner cover surface, given by: Raa,p =

3 g(TP − TC2 )δa,p 2 T va,p a,m

Pr

a,p

(36)

where g is the gravitational acceleration (m/s2 ) and νa is kinematic viscosity of air at the PV and inner cover surface (m2 /s); Pra,p is the Prandtl number of the air gap at PV layer and inner cover surface, which is assumed to be independent of temperature and is taken equal to 0.7; T a,m is the average air temperature of PV layer and inner cover surface which is Ta,m =

(TP + Tc2 ) 2

(37)

where T p and T c2 are, respectively, the average temperatures (K) of PV layer and inner cover surface. Converting the radiation transfer into the equivalent convective one, a radiation-relevant factor, hR, p-c2 , is expressed by:

  σ Tp + Tc2 Tp2 + Tc22

(38) h R,p-c2 = 1 1 + − 1 εp εc2 where T c1 is the average temperature of external cover surface (K); εp and εc2 are emissivity of the PV layer and inner cover surface; σ is the Stefan–Boltzmann constant (5.6679 × 10−8 W/m2 K4 ). Heat Transfer from the Inner Glazing Cover to the Outer Cover: Similarly, heat transfer from the inner glass to the outer glass can be calculated using equations (A.5)

Micro (Mini)-Channels and Their Applications in Solar Systems

205

to (A.7) to substitute corresponding parameters, including temperature, air properties and emissivity. Heat Transfer from the Cover’s Outer Surface to the Surrounding Air: For a surface exposed to the outside wind, the convective coefficient could be calculated using the Klein equation expressed as follows: h c,c1-a =

8.6 V 0.6 L 0.6

(39)

where V is the wind speed (m/s); L is the characteristic length of the collector (m). The minimum convective coefficient for a wind-exposed surface is considered to be 5 W/m2 K [7]; if the above calculation gives a lower value, this should be replaced by the minimum value since the temperature of the sky has little influence on the calculation result, it is usually represented by the air temperature, and thus   h r,c1-a = εc1 σ (Tc1 + Ta ) Tc12 + Ta2

(40)

The PV cells’ electrical efficiency is adversely proportional to their surface temperature, and this dependency can be written as:    ηe = ηrc 1 − βPV Tp − Trc

(41)

The overall electricity output is, therefore, given as: Q e = ηe βp αp τcNc τg,pv I Am

(42)

The module’s solar electrical efficiency could be calculated through ηe =

Qe I Am

(43)

Under the steady-state condition, the rate of useful heat delivered by the module equals the rate of the absorbed energy minus the overall heat loss and converted electricity, expressed as Q th = Q abs − Q L − Q e

(44)

This part of the heat will eventually be converted into the heat received by the water and stored, which is denoted by Qu . In this case, the module’s thermal efficiency can be defined by: ηth =

Q th Am I

(45)

206

T. Diallo et al.

Annex 3 A. The two-phase heat transfer coefficient The two-phase heat transfer Kandlikar correlation [34] has been used and is expressed as follows:  For ReLO > 100 h TP = larger of

h TP,NBD h TP,CBD

h TP,NBD = 0.6683 Co−0.2 (1 − x)0.8 h LO + 1058.0 Bo0.7 (1 − x)0.8 FF1 h LO ⎧ ⎫ ⎪ hTP,NBD = 1.136 Co−0.9 (1 − x)0.8 hLO + 667.2 Bo0.7 (1 − x)0.8 FF1 hLO ⎪ ⎪ ⎪ ⎨ ⎬ 0.5 0.8 ρv Co = [(1 − x)/x] ρL ⎪ ⎪ ⎪ ⎪ ⎩ ⎭ Bo = G qhfg   ReLO Pr L 2f kDL 4 6

For 10 ≤ ReLO ≤ 5 × 10 h LO = 2/3 1 + 12.7 Pr L −1 ( f /2)0.5   (ReLO − 1000) Pr L 2f kDL

For 3000 ≤ ReLO ≤ 104 h LO = 2/3 1 + 12.7 Pr L −1 ( f /2)0.5 For 100 ≤ ReLO ≤ 1600 h LO =

N u LO k Dh

(46) (47)

(48)

(49)

(50) (51)

In the transition region between Reynolds numbers of 1600 and 3000, a linear interpolation is suggested for h LO . For Reynolds numbers below and equal to 100 (Re ≤ 100), the nucleate boiling mechanism governs, and the following Kandlikar correlation is proposed: For ReLO ≤ 100

(52)

h TP = h TP,NBD = 0.6683 Co−0.2 (1 − x)0.8 h LO + 1058.0 Bo0.7 (1 − x)0.8 FFl h LO (53) For R-134a, the recommended value of FFl is 1.63. The vapour quality in the channel port is estimated as: x=

π Dh Q th z mh ˙ fg (Nch Nhp)

(54)

Micro (Mini)-Channels and Their Applications in Solar Systems

207

B. The condensation heat transfer coefficient [36] For Rev < 35,000: h cond

1/4  ρl g(ρl − ρv )kl3 h fg = 0.555 μl (Tsat − Tw )D

(55)

For Rev > 35,000:  ⎫ ⎧ kl 2.22 ⎪ 0.8 0.4 ⎪ h 1 + = 0.23 Re Pr ⎪ D l D X tt0.89 ⎪ ⎬ ⎨ cond ˙ − x)/(π ˙ Dμl ) ReD = 4 m(1 ⎪ ⎪  0.9 ρv 0.5 μl 0.1 ⎪ ⎪ ⎭ ⎩ X = 1−x tt

ρl

x

(56)

μv

C. The fins efficiency in the micro-channel [37] εof = 1 − Nf Af (1 − εf )/Atf ; εf = tanh(m f L cf )/m f L cf

(57)

0.5   m f = 2 × h tp × (L e + tf )/ K hp L e tf

(58)

L cf = b +

δf ; 2

Atf = Nf Af + Ab Ab = L hp L e Af = L e δf

D. Heat transfer in the annulus [38] The flow in the annular is characterized by the following Nusselt number and friction factors: For laminar flow: Re < 2200 Re > 4000

f = 24 Re−1 Nu = 9.33

(59) 1/3

f = 0.0885 Re−0.263 Nu = 0.02 Re0.733 Pr

(60)

For 2200 ≤ Re ≤ 4000 an interpolation has been performed. hw =

Nu kw D45

(61)

where D45 is the hydraulic diameter based on wetted perimeter, m D45 =

π(D52 − D42 ) π D5 + π D4

(62)

208

T. Diallo et al.

Reynolds number Rew can be calculated by: Rew =

ρcf u 1 D45 μcf

(63)

The Prandtl number Pr w is calculated using: Pr = w

μcf cpcf kw

(64)

where μcf , cpcf , ρcf , kw are the relevant mean dynamic viscosity (kg/m s), specific heat, density, thermal conductivity of the water.

References 1. Kandlikar SG, Grande WJ (2003) Evolution of micro-channel flow passages-thermohydraulic performance and fabrication technology. Heat Trans Eng 24(1):3–17 2. Faghri A (1995) Heat pipe science and technology, 1st edn. Taylor & Francis, Washington, DC 3. Cao Y, Faghri A (1994) Micro/miniature heat pipes and operating limitations. J Enhanc Heat Transf 1(3):265–274 4. Hopkins R, Faghri A, Khrustalev D (1999) Flat miniature heat pipes with micro capillary grooves. J Heat Transf 121(1):102–109 5. Faghri A (2014) Heat pipes: review, opportunities and challenges. Front Heat Pipes (FHP) 5:1. https://doi.org/10.5098/fhp.5.1 6. Reay D, Kew P (2006) Heat pipe, 5th edn. Elsevier, Amsterdam, pp 48–52, 93–96, 122 and 234–236.10 7. Vasiliev LL et al (2005) Copper sintered powder wick structures of miniature heat pipes. In: VI Minsk international seminar ‘heat pipes, heat pumps, refrigerators. Minsk, Belarus, 12–15 Sept 2005 8. Babin BR, Peterson GP, Wu D (1990) Steady state modeling and testing of a micro heat pipe. Heat Transf ASME 8:112 9. Diaz G (2008) Performance analysis and design optimization of a mini-channel evacuated-tube solar collector. In: Proceedings of ASME IMECE 2008, Paper IMECE2008-67858. Boston, MA, pp. 1–7 10. Sharma N, Diaz G (2011) Minichannel tube solar collector. US patent US 2011/0186043 A1, Aug 2011 11. Sharma N, Diaz G (2011) Performance model of a novel evacuated tube solar collector based on minichannels. Solar Energy 85:881–890. https://doi.org/10.1016/j.solener.2011.02.001 12. Robles A, Duong V, Martin AJ, Guadarrama JL, Diaz G (2014) Aluminum minichannel solar water heater performance under year-round weather conditions. Sol Energy 110:356–364 13. Mansour MK (2013) Thermal analysis of novel minichannel-based solar flat-plate collector. Energy 60:333–343 14. Moss RW, Shire GSF, Henshall P, Eames PC, Arya F, Hyde T (2017) Optimal passage size for solar collector microchannel and tube-on-plate absorbers. Sol Energy 153:718–731 15. Jinzhi Z, Zhao X, Ma X, Qiu Z, Ji J, Du Z, Yu M (2016) Experimental investigation of a solar driven direct-expansion heat pump system employing the novel PV/micro-channels-evaporator modules. Appl Energy 178:484–495 16. Rullof J, Lambers K, Dick C, Blieske U, Hadji-Minaglou J-R, Scholzen F (2016) Experimental studies on the development of a solar hybrid module with an aluminium microchannel evaporator. In: International energy and sustainability conference (IESC). Cologne, Germany, June 2016

Micro (Mini)-Channels and Their Applications in Solar Systems

209

17. Agrawal S, Tiwari A (2011) Experimental validation of glazed hybrid micro-channel solar cell thermal tile. Sol Energy 85:3046–3056 18. Valeh-e-Sheyda P, Rahimi M, Karimi E, Asadi M (2013) Application of two-phase flow for cooling of hybrid microchannel PV cells: a comparative study. Energy Convers Manag 69:122–130 19. Zhu T, Diao Y, Zhao Y, Li F (2016) Thermal performance of a new CPC solar air collector with flat micro-heat pipe arrays. Appl Therm Eng 98:1201–1213 20. Deng YC, Quan ZH, Zhao YH et al (2013) Experimental investigations on the heat transfer characteristics of micro heat pipe array applied to flat plate solar collector. Sci China Tech Sci 56:1177, 1185. https://doi.org/10.1007/s11431-013-5204-7 21. Chen H, Zhang H, Li M, Liu H, Huang J (2018) Experimental investigation of a novel LCPV/T system with microchannel heat pipe array. Renew Energy 115:773–782 22. Modjinou M, Ji J, Li J, Yuan W, Zhou F (2017) A numerical and experimental study of microchannel heat pipe solar photovoltaics thermal system. Appl Energy 206(15):708–722 23. Wang Z, Zhao Z (2011) Analytical study of the heat transfer limits of a novel loop heat pipe system. Int J Energy Res 35:404–414 24. Zhang X, Zhao X, Xu J, Yu X (2013) Study of the heat transport capacity of a novel gravitational loop heat pipe. Int J Low Carbon Technol 8(3):210–223 25. Diallo T, Yu M, Zhou J, Zhao X (2018) Analytical investigation of the heat transfer limits of a novel solar loop-heat-pipe employing the mini-channel evaporator. Energies 11:148. https:// doi.org/10.3390/en11010148 26. Diallo TMO, Yu M, Zhou J, Zhao X, Shittu S, Li G, Ji J, Hardy D (2019) Energy performance analysis of a novel solar PVT loop heat pipe employing a microchannel heat pipe evaporator and a PCM triple heat exchanger. Energy 167:866–888 27. Yu M, Diallo TMO, Zhao X, Zhou J, Du Z, Ji J, Cheng Y (2018) Analytical study of impact of the wick’s fractal parameters on the heat transfer capacity of a novel micro-channel loop heat pipe. Energy 158:746–759 28. Kuroda M, Chang J, Gwin P, Mongia R (2013) Development of aluminium-water heat pipes. In: 17th international heat pipe conference (17th IHPC). Kanpur, India, 13–17 Oct 2013 29. He W, Hong X, Zhao X, Zhang X, Shen J, Ji J (2014) Theoretical investigation of the thermal performance of a novel solar loop-heat-pipe facade-based heat pump water heating system. Energy Build 77:180–191 30. Kalogirou SA (2009) Solar energy engineering: process and system. Elsevier Inc 31. Imura H, Kusuda H, Funatsu S (1977) Flooding velocity in a counter-current annular two-phase flow. Chem Eng Sci 32:79–87 32. Swamee PK, Swamee N (2010) Discharge equation of a circular sharp-crested orifice. J Hydraul Res 48(1):106–107 33. Tay NHS, Belusko M, Bruno F (2012) An effectiveness-NTU technique for characterising tube-in-tank phase change thermal energy storage systems. Appl Energy 91:309–319 34. White FM (2011) Fluid mechanics, 7th edn. McGraw-Hill, New York 35. Zhang X, Zhao X, Xu J, Yu X (2013) Study of the heat transport capacity of a novel gravitational loop heat pipe. Int J Low Carbon Technol 8(3):210–223 36. Kandlikar SG, Balasubramanian P (2004) An extension of the flow boiling correlation to transition, laminar and deep laminar flows in minichannels and microchannels. Heat Transfer Eng 25(3):86–93 37. Incropera FP, DeWitt DP, Bergman TL, Lavine AS (2011) Fundamentals of heat and mass transfer, 7th edn. Wiley, Hoboken, NJ 38. Tiruselvam R, Chin WM, Raghavan VR (2012) Double tube heat exchanger with novel enhancement: part II—single phase convective heat transfer. Heat Mass Transfer 2012(48):1451–1462. https://doi.org/10.1007/s00231-012-0986-x

Solar Desiccant (Absorption/Adsorption) Cooling/Dehumidification Technologies Wansheng Yang, Shuli Liu, Xiaoqiang Zhai, Yin Bi, Zhangyuan Wang and Xudong Zhao

Abstract Air dehumidification in humid climates can improve the people’s living environment to promote the life quality and improve working environment significantly to increase production rate and product quality. Desiccants are key materials used in the dehumidification technologies. In this chapter, the conventional solid desiccant materials and different types of desiccant systems are introduced. Furthermore, the performance of solid dehumidification materials is emphatically analysed. In addition, desiccant regeneration methods are summarized, and two examples of their applications are presented in the last part of the chapter, namely the novel solar solid. dehumidification/regeneration bed and solar-powered dehumidification window. This chapter would be helpful for researchers and engineers in this area to exploit the potential applications of solar desiccant technologies in building sector.

W. Yang (B) · Y. Bi · Z. Wang School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, Guangdong, China e-mail: [email protected] Y. Bi e-mail: [email protected] Z. Wang e-mail: [email protected] S. Liu Department of Civil Engineering, Architecture and Building, Faculty of Engineering and Computing, Coventry University, Coventry CV1 2HF, UK e-mail: [email protected] X. Zhai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China e-mail: [email protected] X. Zhao School of Engineering and Computer Science, University of Hull, Hull HU6 7RX, UK e-mail: [email protected] © Springer Nature Switzerland AG 2019 X. Zhao and X. Ma (eds.), Advanced Energy Efficiency Technologies for Solar Heating, Cooling and Power Generation, Green Energy and Technology, https://doi.org/10.1007/978-3-030-17283-1_7

211

212

W. Yang et al.

Keywords Air dehumidification · Solid desiccant · Regeneration methods · Solar technologies

1 Introduction Hot and humid weather affects people’s comfort, bringing inconveniences to people’s life and work, as well as influences industrial productivity, thus reducing the quality of process products. Taking an example of South China, most time in a year in South China that is humid, its annual average relative humidity is above 70% [1] and the daily average moisture content in the air-conditioning season is 20 g/kg [2]; air dehumidification becomes more and more important in modern life in such an area with the rapid development of economic and increased life quality demand. Currently, the commonly used air dehumidification methods are cooling dehumidification, compressed air dehumidification, liquid absorption dehumidification and solid adsorption dehumidification, or combination of the above dehumidification methods. Meanwhile, some scholars put forward some new dehumidification technologies, such as membrane dehumidification, electrochemical dehumidification, heat pipe dehumidification and heat pump dehumidification [3]. The selection of air dehumidification methods is mainly based on the processed air parameters and environment, and the comparison of common air dehumidification methods is shown in Table 1 [4]. Among different methods, solid adsorption dehumidification has the advantages of large air volume, strong dehumidification capacity, energy-saving, simple structure and no pollution. Solid adsorption dehumidification usually realizes air dehumidification by loading desiccant material in the air flow channel. According to the structure, solid dehumidification includes rotary dehumidification and packed-bed dehumidification. A rotary dehumidifier has a complex structure, high cost, easy running wet, high regeneration temperature (90–150 °C); thus, the use of low-grade heat source is inhibited [5]. Furthermore, its rotating parts make difficult to achieve the process of internal cooling dehumidification and thermal regeneration, which reduce the dehumidification and regeneration performance. Packed-bed dehumidification has the advantages of large air volume, low-grade energy regeneration, easy of achieving internal cooling dehumidification and thermal regeneration, simple structure and maintenance, low noise and reliable operation, etc. [6–8]. When the packed-bed is saturated, it needs to be regenerated to achieve the operation cycle. The regeneration performance is one of the most important factors which affect the dehumidification performance of the packed-bed [9]. The traditional regeneration method is the electrical heating regeneration, which regenerates the solid desiccant material of the packed-bed by the air that is directly heated by electrical energy. This regeneration method has the following major disadvantages: (1) Low regeneration efficiency Two reasons lead to low regeneration efficiency. One is the two-step heating process, i.e. regeneration air is heated firstly by electricity and then solid desiccant material heated by the regeneration air, lead-

Solar Desiccant (Absorption/Adsorption) …

213

Table 1 Comparison of the dehumidification methods Dehumidification method

Cooling dehumidification

Liquid dehumidification

Rotary dehumidification

Membrane dehumidification

Solid dehumidification

Separation mode

Condensation Absorption

Adsorption

Infiltration

Adsorption

Dew point temperature after dehumidification/°C

0–20

0–30

−30 to 50

−20 to 40

−30 to 50

Handling air volume/m3 min−1

0–30

100–2000

0–200

0–100

0–2000

Equipment area

Middle

Large

Small

Small

Large

Operation and maintenance

Middle

Difficult

Difficult

Middle

Middle

Existing problems

Difficult to achieve low dew point, high energy consumption

Adsorbent corrosion

High energy consumption

High membrane requirements

Repeated regeneration

ing to the decrease in the regeneration efficiency. Another is that in the heating and regeneration process the moisture movement direction in the solid desiccant material is opposite to the heat transfer and therefore further reduces the regeneration efficiency. (2) Large regeneration energy consumption. The thermal resistance of the solid desiccant material is large, so the heat in regeneration air is difficult to transfer to interior of the dehumidification material, causing most of the heat to be discharged with the air, and the high temperature (greater than 80 °C) of the regeneration process leads to a large loss of energy consumption [10]. After regeneration, the temperature of solid desiccant material is high, so it is necessary to cool the material before performing the further dehumidification. In the system cycle, heat and cold cancel each other out, further increasing the regeneration energy consumption of the packed-bed. (3) Long regeneration time. Due to the increase in regeneration temperature, the energy consumption of regeneration will increase, and the high temperature will destroy the structure of solid desiccant material; therefore, the regeneration air temperature of electric heating cannot be too high, resulting in its long regeneration time, which is difficult to meet the engineering application. In order to solve the problems of low efficiency, large energy consumption and long regeneration time, many scholars have proposed new regeneration methods, including solar regeneration, waste heat regeneration, ultrasonic regeneration, electroosmotic regeneration and microwave regeneration.

214

W. Yang et al.

Among these new regeneration methods, solar regeneration has a good energysaving effect, which can effectively alleviate the pollution caused by the burning of fossil fuels, and it will not cause harm to human body. Taking an example of South China which is a hot and humid area, the annual total sunshine hours in South China ranges from 1200 to 2200 h and solar radiation range is about 4086.6–5225.1 MJ/m2 [10]. Rich solar energy source provides a good environment for the solar regeneration in this area.

2 Desiccant Materials The solid desiccant material in a solid dehumidification device is a key factor which affects the dehumidification and the regeneration performance. The solid desiccant materials used commonly include the activated alumina, the molecular sieve, the activated carbon and the silica gel, and the basic properties of each solid desiccant material are given as follows:

2.1 Activated Alumina Activated alumina (Al2 O3 ) is a kind of high microporous particle, which is mainly made of aluminium hydroxide by hydroxyl reaction, and the capillary structure of activated alumina makes the specific surface area of the internal channel is large and has high activity. The specific surface area of activated alumina is about 100–200 m3 /g and the pore diameter is 1.5–6 nm, and the adsorption heat is about 3000 kJ/kg. Activated alumina has higher mechanical strength than silica gel [11]. Moreover, it also has stronger adsorption capacity for water molecules, and its capacity on water can reach about 60% of its own weight [12]. Activated alumina can be used for the deep environmental dehumidification. Under experimental conditions, the dehumidification of the activated alumina to the air can reach the air dew point below −70 °C [13].

2.2 Molecular Sieve Molecular sieve is mainly composed of crystalline silicate or aluminosilicate, which can be divided into micropore (50 nm) according to the size of the pore. Molecular sieves can also be divided into 3A, 4A, 5A, 10X and 13X according to the chemical composition. The skeleton structure of molecular sieve is stable, and it has strong corrosion resistance. In the case of low relative humidity, molecular sieve has a strong dehumidification capacity. Research has shown that at an ambient temperature of 25 °C, a relative humidity of

Solar Desiccant (Absorption/Adsorption) …

215

20%, the maximum adsorption capacity of 5A, molecular sieve is approximately 20% and the maximum adsorption capacity of microporous silica gel is approximately 5% [14, 15]. At the same time, there are some disadvantages of the molecular sieve. In general, the adsorption capacity of molecular sieve is commonly smaller than that of silica gel [16]. Due to the strong adsorption capacity to molecular sieve for water molecules, it is necessary to consume a lot of thermal energy in the regeneration stage to achieve desorption of moisture, resulting in higher heat loss and detrimental to the use of low-grade energy such as solar energy and waste heat [17]. Molecular sieve has strong adsorption to water molecules and has excellent dehumidification performance under low humidity condition, so it is suitable for low-dew-point dehumidification and special goods storage room, precision instrument storage room and other environment with high humidity requirement [18–21].

2.3 Activated Carbon Activated carbon contains carbon, oxygen and hydrogen, and carbon accounted for more than 80–90%. The adsorption performance of activated carbon is mainly determined by its micropore. The pore volume of activated carbon is usually 0.25–0.9 mL/g, and the surface area of microporous surface is about 500–1500 m2 /g. Measured by BET method, the micropore surface area can reach 3500–5000 m2 /g. Activated carbon is non-polar molecule, which is easy to adsorb non-polar adsorbed mass, while water molecule belongs to polar molecule, so the adsorption ability of the activated carbon to water molecule is poor. Adsorption of activated carbon for water molecules is V-type adsorption, it means under low water vapour pressure, the interaction between molecules is weaker, and the adsorption capacity of activated carbon is smaller. When activated carbon adsorbs part of water, the interaction between adsorbate and adsorbate is formed inside the activated carbon and the adsorption of water molecules increases [22]. Activated carbon is generally used in water adsorption process; it can also be used as a solid desiccant material, but it is poor in water absorption when used as a solid dehumidification material.

2.4 Silica Gel Silica gel is a kind of semi-transparent, non-toxic, non-corrosive solid desiccant material, and its chemical composition is mSiO2 ·nH2 O, which contains a lot of capillary and crystal block structure. The specific surface area of silica gel is 600–700 m2 /g, and the average pore size is 3.2–3.5 nm [23, 24]. The adsorption of silica to water is mostly physical adsorption, and the mass of water that it can absorb can reach 40% of its own mass. The moisture of physical adsorption can basically be removed when the regeneration temperature of silica gel is 100 °C; therefore, low-grade heat source can be used to achieve the regeneration of silica gel. In addition, silica gel adsorbs

216

W. Yang et al.

a small amount of water (about 7% of silica gel mass) by chemical adsorption. This part of water needs a higher regeneration temperature to make it desorbed. Silica gel has the advantages of high moisture adsorption capacity, low regeneration temperature, good mechanical properties and stable chemical properties [25]. At the same time, the silica gel has also some disadvantages; for example, many adsorption heats will be released during the process of dehumidification, causing a sharp decrease in its dehumidification capacity in low humidity conditions and cracks when meeting water droplet and so on.

3 Types of Desiccant Systems Initially in desiccant bed dehumidification system, solid desiccant material was placed in a closed container to dehumidify the air in the container, and then it developed into two types of desiccant bed [26]: (1) Packed-bed. The solid desiccant material is filled in the tower (cylinder) to dehumidify air. This dehumidification process is intermittent, and regeneration of the solid desiccant material in the tower is periodically; neither its operation nor control is convenient. In order to realize continuous air dehumidification, a two-tower dehumidification method has emerged: a tower for air dehumidification and another tower for the solid desiccant material regeneration. After a certain period two towers are converted, interchanging dehumidification process and regeneration process, so it can achieve continuous air dehumidification. However, in the process of air dehumidification, the adsorption heat produced by the solid desiccant material is difficult to be dissipated, which leads to the increase in temperature and the reduction of the dehumidification performance of the packedbed, so some scholars put forward a new desiccant device. (2) Desiccant-coated dehumidification packed-bed: the solid desiccant material is fixed on the air channel to dehumidify air. Due to the low thickness of the solid dehumidification material, the adsorption heat is easy to dissipate, which effectively reduces the influence of adsorption heat on the bed dehumidification performance. In order to further reduce the influence of adsorption heat, some scholars have proposed a packed-bed with cooling gas on the other side of the dehumidification channel, which can reduce the temperature and improve the dehumidification performance of the bed [27, 28], and then the fin tube with cooling water is added in packed-bed, which strengthens the heat transfer inside the packed-bed, to further improve the performance of the desiccant bed [29–31].

3.1 Packed-Bed The domestic and foreign researchers have studied the dehumidification performance and dehumidification model of packed-bed. In the aspect of dehumidification performance of the packed-bed, Kabeel [32] studied and analysed the effects of air

Solar Desiccant (Absorption/Adsorption) …

217

temperature, humidity, airflow velocity and bed thickness on the dehumidification performance of the packed-bed in the dynamic operation, which is with eight layers, and the results showed that the dehumidification quantity of the packed-bed mainly depended on the humidity of inlet air and air velocity. Song et al. [33] studied the dehumidification performance of packed-bed with different desiccant material filling modes, and they found that the average dehumidification capacity of two pieces of 50-mm-thick, 100-mm-filled desiccant modules was 16.8% greater than that of one piece of 100-mm-thick desiccant module. It is indicated that the dehumidification performance of packed-bed could be improved effectively by the sectional dehumidification of the thickness direction. In order to reduce the effect of adsorption heat on the dehumidification performance of the desiccant material in the thickness direction, the gas–liquid heat exchanger was set in the middle of the packed-bed to reduce air temperature. Ramzy et al. [34] produced a packed-bed with intercooling and compared traditional packed-bed with the experimental one. The results showed that the dehumidification capacity of the packed-bed via intercooling is 22% larger than that of the traditional packed-bed. In the dehumidification model of the packed-bed, Pesaran and Mills [35] established a solid-side resistance model (SSR) and a pseudo-gas-side-controlled model (PGC) to study the law of water transfer in the dehumidification process of the packed-bed. The results showed that the model calculated with the solid-side resistance was closer to the experimental data. In order to study the effect of heat transfer along the thickness direction of the packed-bed in the process of non-isothermal dehumidification, Ramzy et al. [36] established a solid-side resistance with axial heat conduction model (SSR-AC) that considers the direction heat transfer of the thickness of the bed based on the solid-side resistance model. And by comparing the calculation results of SSR model and SSR-AC model, the effect of heat transfer along the thickness direction of the packed-bed in the dehumidification process was studied. Ramzy et al. [37] established the pseudo-gas-side-controlled (PGC) mathematical model and compared with the experimental results. The results showed that the root mean square of errors ranges from 1.15 to 9.03% for the exit air humidity ratio and from 1.08 to 9.68% for the exit air temperature. By using the scale principle to analyse and calculate the heat and mass transfer process of the desiccant in the packed-bed and comparing with the numerical simulation results of the packed-bed dehumidification process, Mitra et al. [38] found that the scale principle can accurately describe the two-dimensional heat and mass transfer process of the packed-bed dehumidification, which provides the theoretical basis for the establishment of the packed-bed dehumidification model.

3.2 Desiccant-Coated Dehumidification Packed-Bed The research on the desiccant-coated dehumidification packed-bed at home and abroad mainly includes the dehumidification performance and the dehumidification model. In the aspect of the dehumidification performance of desiccant-coated

218

W. Yang et al.

dehumidification packed-bed, Ge [39] carried on the experimental research on the dehumidification performance of the cross-flow packed-bed and downstream-flow packed-bed. The results showed that the dehumidification capacity of the cross-flow packed-bed is greater than that of downstream-flow packed-bed, but there is a large heat resistance between the dehumidification material in the cross-flow packed-bed, resulting in the difference between the temperature of inlet and outlet air of the crossflow packed-bed is greater than that of the downstream-flow packed-bed. In order to study the effect of air channel structure on the dehumidification performance of the packed-bed, based on the turbulent boundary layer theory, Feng et al. [40] designed three types of packed-beds, including the straight channel type packedbed, the curved channel type packed-bed and the spiral channel type packed-bed, and carried on the experimental research. The results showed that the spiral channel structure had the most obvious effect on improving the motion of water molecules on the surface of the desiccant material, so the spiral channel structure had the best dehumidification effect. In order to reduce the effect of adsorption heat on the dehumidification performance of the packed-bed in dehumidification process, Worek and Lavan [41] glued silica gel on the dehumidification channel and passed the cooling gas on the other side of the channel, setting up a cross-cooled desiccant dehumidifier with cooling gas channel. The results showed the dehumidification capacity of the cross-cooled desiccant dehumidifier was 30–60 g/kg. Fathallah and Aly [42] improved the dehumidification performance of cross-cooled desiccant packed-bed. Dehumidification channel was filled with silica gel, which improved dehumidification capacity of cross-cooled desiccant packed-bed, but this method increased the heat resistance of the dehumidification material in dehumidification channel, going against to the adsorption heat dissipation. Yuan et al. [28] stick the solid desiccant material on the air channel of the plate-fin heat exchanger to produce a cross-cooled compact solid desiccant dehumidifier and compared the cross-cooled desiccant dehumidifier with it, and the results showed that the dehumidification performance of the cross-cooled compact solid desiccant dehumidifier is better than that of cross-cooled desiccant dehumidifier. Under the high humidity condition, the dehumidification rate of cross-cooled compact solid desiccant dehumidifier can reach 12.4%. The dehumidifier can use the gas–solid heat exchange to eliminate the adsorption heat generated by the solid desiccant material; however, with this heat exchange method it is difficult to improve the thermal efficiency. Peng et al. [29] proposed the method of liquid–solid heat exchange to eliminate the adsorption heat that produced by solid desiccant material. They stick the solid desiccant material on outer surface of the finned tube and pipe of heat exchanger and passed cooling water in the pipes. The results showed that when the inlet air temperature was 24.7 °C and the moisture content was 12.41 g/kg, the dehumidification rate of finned tube packed-bed can reach 43.8%. In the dehumidification model of the desiccant-coated dehumidifier, Yuan et al. [28] established the dynamic dehumidification model of the cross-cooled compact solid desiccant dehumidifier by the finite difference method, and the results showed that the error between simulation results and experimental results was less than 7%. Zhao et al. [43] studied the heat and mass transfer law of the gas side in the finned

Solar Desiccant (Absorption/Adsorption) …

219

tube packed-bed dehumidification process through experiments under conditions of a different air temperature, humidity, velocity and cold water temperature, the NU number and SH number of the finned tube packed-bed dehumidification process under various operating conditions are obtained, which provided a theoretical basis for the establishment of the finned tube packed-bed dehumidification model. Ge et al. [31] established the mathematic model of the finned tube packed-bed in dehumidification process, and the operation of the packed-bed was simulated by C++ language program and compared with the experiment, which showed that the error between the simulation results and the experimental results was less than 15%. The current research status of packed-bed dehumidification is mainly focused on the improvement of dehumidification performance and establishment of dehumidification model. The desiccant-coated dehumidifier provides an effective cooled method for dehumidification process, most of the adsorption heat created in the dehumidification process can be taken, which effectively improves the dehumidification performance of the packed-bed. But the effective dehumidification time of most desiccant-coated dehumidifier is short, which cannot meet the engineering application, and the dehumidification/regeneration switching time is too short leading to energy waste. Nowadays, packed-bed dehumidification is more common, which can deal with large amount of air and have a long effective dehumidification time, but because the solid desiccant material is loaded in the form of accumulation, the adsorption heat produced in the dehumidification process is difficult to disperse, which leads to the decrease in the dehumidification performance of packed-bed.

4 Performance of Solid Dehumidification Materials The physical properties of solid desiccant materials have important effects on their internal heat and moisture transfer. Solid desiccant material silica gel with two kinds of phase change materials, GR50 and PK52, were investigated; their basic performance parameters such as bulk density, porosity, thermal conductivity and radiation transmittance of solid dehumidification materials and phase change materials were tested, which provide a reference for solid dehumidification bed simulation and structural optimization.

4.1 Density Measurement In this section, the density of the solid desiccant materials used in the experiments was measured by a graduated cylinder method and verified by a mass-volume method. The equipment used in the test includes electronic balance, electric blast oven and cylinder. The performance parameters of each test instrument are shown in Tables 2 and 3.

220

W. Yang et al.

Table 2 Performance parameters of electronic balance Model

Range (kg)

Actual scale value (g)

Voltage (V)

Power (W)

Manufacturer

TCS-01

0–75

2

220

14

Bai Lens Electronic Weighing Apparatus Co., Ltd.

Table 3 Performance parameters of the electric drum wind drying oven Model

Temperature range (°C)

Voltage (V)

Rated power (W)

Manufacture number

Manufacturer

DHG9145A

10–300

220

2050

0,610,016

Shanghai Heng Technology Instrument Co., Ltd.

Table 4 Experimental test results of silica gel density No.

Measuring cylinder mass (g)

Measuring cylinder volume (cm3 )

Total weight of cylinder and dry material (g)

Bulk density (g/cm3 )

1

312

500

824

1.024

2

312

500

828

1.032

3

310

500

832

1.044

4

312

500

830

1.036

Average

311.5

500

828.5

1.034

Table 5 Experimental test results of PK52 phase change materials No.

Measuring cylinder mass (g)

Measuring cylinder volume (cm3 )

Total weight of cylinder and dry material (g)

Bulk density (g/cm3 )

1

310

500

638

0.656

2

310

500

640

0.660

3

310

500

634

0.648

4

312

500

640

0.656

Average

310.5

500

638

0.655

(1) Test results from cylinder method The material bulk density test results are listed in Tables 4, 5, 6, 7 and 8. It can be seen from the above test that the density of the two kinds of phase change materials is basically in line with the nominal value. GR50 had an error of 2.01% and PK52 had an error of 8.2%, which reflects the accuracy of the test

Solar Desiccant (Absorption/Adsorption) …

221

Table 6 Experimental test results of GR50 phase change materials No.

Measuring cylinder mass (g)

Measuring cylinder volume (cm3 )

Total weight of cylinder and dry material (g)

Bulk density (g/cm3 )

1

312

500

740

0.856

2

310

500

742

0.864

3

314

500

748

0.868

4

314

500

752

0.876

Average

312.5

500

745.5

0.866

Table 7 Test results of PK52 phase change materials and silica gel compound No.

Measuring cylinder mass (g)

Measuring cylinder volume (cm3 )

Total weight of cylinder and dry material (g)

Bulk density (g/cm3 )

1

310

500

682

0.744

2

310

500

690

0.760

3

310

500

684

0.748

4

312

500

684

0.744

Average

310.5

500

685

0.749

Table 8 Test results of GR50 phase change materials and silica gel compound No.

Measuring cylinder mass (g)

Measuring cylinder volume (cm3 )

Total weight of cylinder and dry material (g)

Bulk density (g/cm3 )

1

312

500

756

0.888

2

310

500

760

0.900

3

314

500

766

0.904

4

312

500

760

0.896

Average

312.0

500

760.5

0.897

results. In order to further verify the density accuracy of the dehumidification material, the mass-volume method is used for further calculation. (2) Test results from mass-volume method To correct the volume of the dehumidification and phase change mixture material, the mass-volume method was applied according to Eq. (1). The materials were homogeneously hybrid and placed in a rigid three-dimensional module with a length × width of 400 mm × 340 mm. The surface of the desiccant material was gently flattened by plate, and the thickness was measured at nine random points. The thickness of each point is listed in Table 9, and the basic test parameters for silica gel + PK52 hybrid material and silica gel + GR50 hybrid material are shown in Table 10.

222

W. Yang et al.

Table 9 Thickness parameters of hybrid materials Test no.

1

2

3

4

5

6

7

8

9

Average

Thickness of silica gel + PK52 material (cm)

14.8

14.7

14.7

15.1

14.8

15.2

15.0

15.2

15.2

15.0

Thickness of silica gel + GR50 material (cm)

12.3

12.2

12.4

12.4

12.2

12.3

12.1

12.2

12.0

12.2

Table 10 Bulk density test parameters of hybrid materials Material

Initial weight (kg)

Thickness (cm)

Volume (m3 )

Density (kg/m3 )

Thickness of silica gel + PK52 material

15.20

14.97

0.0203592

746.6

Thickness of silica gel + GR50 material

14.92

12.23

0.0166328

896.9

ρ=

m v

(1)

It can be calculated from the known data and Eq. (1) that the average density of silica gel + PK52 hybrid material is 746.6 kg/m3 . Similarly, the average density of silica gel + GR50 hybrid material is 896.9 kg/m3 , which is in accord with the results obtained by measuring cylinder method.

4.2 Porosity Calculation The porosity of material is the percentage of the pore volume of the material in the unit of the original material [44], defined by:     VP VP × 100% = × 100% (2) = V0 Vs + VP where —the porosity of the material, %; V p —the pore volume of the material, cm3 ; V 0 —the total volume of the material, cm3 ; V s —the material dense solid volume, cm3 . Like porosity, the relative density ρr is the ratio of the apparent density of the porous material to the density of the corresponding dense material. The relationship between relative density ρr and porosity  is as follows [45]:

Solar Desiccant (Absorption/Adsorption) …

223

  ρ∗ × 100%  = (1 − ρr ) × 100% = 1 − ρs

(3)

where ρr —the relative density of the material, g/cm3 ; ρ ∗ —the apparent density of the material, g/cm3 ; ρs —the dense density of the material, g/cm3 . Methods for measuring the porosity of common materials include [45]: (1) microscopic analysis; (2) mass-volume direct calculation method; (3) soaking medium method; (4) vacuum impregnation method; (5) floating method. In this paper, the adsorption medium is loose material, and formula (2) was applied to calculate the porosity of the material. The results showed that the porosity of silica gel is 0.34–0.4 L/kg and the bulk density of silica gel is 1034 kg/m3 , so the porosity of silica gel is 35.12–41.36%.

4.3 Thermal Conductivity Measurement (1) Measurement method The heat transfer in the solid desiccant material is a combination process of thermal and moisture migration effects. In order to explain the mechanism of the material’s thermal conductivity and mass transfer, the thermal conductivity of the solid desiccant material with different moisture contents is tested with the DRM-II thermal conductivity meter, as shown in Fig. 1, and the specific instrument performance parameters are shown in Table 11.

Fig. 1 The DRM-II coefficient of thermal conductivity tester

Table 11 Performance parameters of DRM-II of thermal conductivity tester Model

Voltage

Dimensions (LWH)

Measuring range W/(m K)

DRM

220 V/50 Hz

600 × 440 × 720 (mm)

0.035–107.0

224

W. Yang et al.

There are two different test methods for thermal conductivities of solid desiccant materials: steady-state method and unsteady method. The unsteady plane heat source method can be applied to measure homogeneous solid materials, heterogeneous materials and porous materials. The material thermal conductivity, specific heat capacity and other thermal properties can be obtained at the same time by only measuring the temperature changes in a sample. The following thermal conductivity tests were carried out to measure the thermal conductivities of silica gel, silica gel + PK52 hybrid material and silica gel + GR50 mixture, respectively. For the above materials, the interspace between the particle skeletons is mostly interconnected, and the fluid could pass through; therefore, it belongs to a typical porous medium [46]. The main steps of the test are as follows: (1) The dehumidification material samples with the same ratio were divided into two groups for two different tests; each group includes three pieces: one is a thin specimen (200 mm × 200 mm × 20 mm) and the other two are thick specimens (200 mm × 200 mm × 100 mm). The thickness of the specimen is uniform, and the unevenness of the thin specimen shall be less than 1% of its thickness. (2) Two different tests were carried out for the two groups of the samples, respectively. One test was the natural wet performance test, and another was performance test in the artificial humidification state. For the natural wet performance test, the pure silica gel was placed in a drying oven with a temperature of 140 °C, and the time for drying was at least 4 h so that the physical adsorption water can be taken out, and then the silica gel was cooled to the room temperature to be tested for its performance. For the performance test in the artificial humidification state, the test was carried out using the method of artificial humidification to dry the specimen to the required humidity and the moisture content change within the material was measured by a humidity meter. For each group of test pieces, the humidity difference should be less than ± 1%, and the humidity in the same specimen should be evenly distributed to study the thermal performance parameters of the material under different humidification conditions. (3) Put the samples in the test device. When the test dehumidification temperature changes within 5 min, temperature is less than 0.05 °C, and the temperature difference between the upper and lower surface of the thin specimen is less than 0.1 °C, that is, the beginning of the measurement. To verify the test results, Eq. (4) [47] is used to calculate the theoretical thermal conductivity of the materials in different humidity conditions. λwet = λdry + ϕλw

(4)

where λwet —the thermal conductivity of the material in the wet state, W/(m K); λdry —the thermal conductivity of the material in the dry state, W/(m K); ϕ—the moisture content of the material, %; λw —the thermal conductivity of water, W/(m K).

Solar Desiccant (Absorption/Adsorption) …

225

(2) Thermal conductivity test results 1. Silica gel The test results of the thermal conductivity of pure silica gel were obtained. The moisture content varies from 0 to 21.2%. The test results and theoretical calculation results are shown in Table 12. It can be seen from Table 12 and Fig. 2 that the thermal conductivity test results of pure silica gel material are in accord with the calculated values, and the thermal conductivity increases linearly with the moisture content. The correlation between the thermal conductivity and the moisture content is shown in Table 13. As shown in Table 12, the measured value of the thermal conductivity and the theoretical value are consistent, the relative error is between 0.29 and 6.84%, and the absolute error is about 0.001–0.018 W/(m K). 2. Silica gel + PK52 hybrid materials The thermal conductivity of silica gel + PK52 hybrid materials was tested. The moisture content varies from the natural dry state to 20.0% (moisture content 0–20.0%). The test results and theoretical calculation are shown in Table 14. The correlation between the thermal conductivity and the moisture content is shown in Fig. 3, and the thermal conductivity changed with moisture content is shown in Fig. 4. In the state of moisture content that is below 20%, the thermal conductivity of silica gel + PK52 hybrid material is linearly increasing with moisture content (see Fig. 4 and Table 14). This is because when the moisture content is about below 12%, the pores have a larger flow area, and the diffusion of steam by the wall dehumidification contains less. The smaller the internal air content of the material, the greater the heat and mass exchange coefficient, so the thermal conductivity increases with the increase in the moisture content [48]. When the dehumidification material is in the near saturation state, the material shows non-dehumidification and gradually produces water, the heat and mass exchange coefficient becomes smaller, and the thermal conductivity decreases. There should be a critical moisture content, at which the dehumidification of the dehumidified material has the strongest influence, the heat and mass exchange

Fig. 2 Theoretical and measured value of thermal conductivity of silica gel

0

0.210

0.210

0

0

Moisture content (%)

Measured value [W/(m K)]

Theoretical value [W/(m K)]

Absolute error [W/(m K)]

Relative error (%)

3.32

0.007

0.230

0.223

3.2

0.97

0.002

0.234

0.232

4

4.03

0.009

0.240

0.231

4.85

5.18

0.013

0.243

0.257

5.4

Table 12 Testing and theoretical equivalent coefficient of thermal conductivity of silica gel 5.5

6.84

0.018

0.243

0.261

10.35

2.61

0.007

0.273

0.281

10.60

3.49

0.009

0.276

0.266

15.2

2.86

0.008

0.303

0.294

21.2

0.29

0.001

0.343

0.344

226 W. Yang et al.

Solar Desiccant (Absorption/Adsorption) …

227

Table 13 Correlation of silicone between thermal conductivity and moisture rate Relationship

R2

Moisture content (%)

Measured value

λ = 0.0061ϕ + 0.2110

0.9411

≤21.2

Theoretical value

λ = 0.0062ϕ + 0.2094

0.9995

Table 14 Correlation of thermal conductivity and moisture content of the silica gel + PK52 hybrid materials Relationship

R2

Moisture content (%)

Measured value

λ = 0.0064ϕ + 0.2258

0.9904

≤12

Theoretical value

λ = 0.0031ϕ + 0.2347

0.9167

Fig. 3 Theoretical and measured value of thermal conductivity of the silica gel + PK52 hybrid materials

Fig. 4 Theoretical and measured value of thermal conductivity of the silica gel + PK52 hybrid materials in low moisture rate condition

228

W. Yang et al.

Table 15 Theoretical and measured thermal conductivity of hybrid materials Moisture content (%)

0

4

5.5

6.5

7.5

9

9.8

Measured value [W/(M K)]

0.237

0.238

0.256

0.268

0.279

0.280

0.289

Theoretical value [W/(M K)]

0.237

0.246

0.251

0.254

0.258

0.262

0.265

Absolute error [W/(M K)]

0

0.008

0.005

0.014

0.021

0.018

0.024

Relative error (%)

0

3.25

1.99

5.51

8.14

6.87

9.05

Moisture content (%)

10

11

12

14

15

20

–

Measured value [W/(M K)]

0.289

0.295

0.298

0.340

0.341

0.326

–

Theoretical value [W/(M K)]

0.266

0.269

0.273

0.295

0.303

0.302

–

Absolute error [W/(M K)]

0.023

0.026

0.025

0.045

0.038

0.024

–

Relative error (%)

8.65

9.66

8.39

15.2

12.5

7.9

–

Table 16 Theoretical and measured thermal conductivity of silica gel + GR50 hybrid materials Moisture content (%)

0

2

4.5

10.5

14

15.8

Measured value [W/(M K)]

0.1683

0.1813

0.1974

0.2363

0.2590

0.2706

Theoretical value [W/(M K)]

0.1683

0.1811

0.2037

0.2512

0.2787

0.2938

Absolute error [W/(M K)]

0

0.0002

0.0063

0.0149

0.0197

0.0232

Relative error (%)

0

0.08

3.19

6.31

7.62

8.57

coefficient is the largest, and the thermal conductivity is the largest. Analysis of the test data shows that the critical moisture content is 15%. It could be inferred from Table 15 and Fig. 4 that the relative error between the theoretical thermal conductivity and the actual test results was between 1.99 and 15.2%. In the case of moisture content ≤12%, the theoretical thermal conductivity was consistent with the experimental data, the minimum deviation was 1.99%, and the maximum deviation was 9.66%. In the case of moisture content >12%, the calculation results of the theoretical thermal conductivity differed much from experimental data, with the maximum deviation of 15.2%. 3. Test for the thermal conductivity of silica gel + GR50 hybrid material The test results of thermal conductivity of silica gel + GR50 hybrid material were obtained. The moisture content varies from the natural dry state to 15.8%. The test and theoretical calculation results for the thermal conductivity at different moisture contents are listed in Table 16. The thermal conductivity changes with the moisture content are shown in Fig. 5. It can be seen from Fig. 5 and Table 17 that the thermal conductivity test results of the silica gel + GR50 mixture tend to be consistent with the theoretical calculated values (relative error 760 kg/m3 , the pore volume is 0.34–0.4 L/kg, the thermal conductivity is about 0.63 kJ/(M K), the specific heat is about 0.92 kJ/(kg K), water dehumidification capacity is 30–40%, and effective dehumidification rate is 80%

Phase change material

PK52

The bulk density is 0.55 kg/L, the melting point is 49–53 °C, the average particle size is 3–5 mm, the heat storage density is 131 kJ/kg, the volume expansion rate is 8%, the specific heat capacity is 2 kJ/(kg K), the effective heat exchange area is 1 m2 /L, and operating temperature is >100 °C

GR50

The bulk density is 0.849 kg/L, the melting point is 45–51 °C, the average particle size is 1–3 mm, and the heat storage density is 55–99 kJ/kg

latent heat released. The experiment was carried out in Guangzhou at room temperature 34 °C and relative humidity 75%. It was known that the moisture content d = 25.2 g/kg, the average density ρ = 1.132 kg/m3 , the size of the duct is the length × width = 0.3 m × 0.25 m, the vaporized latent heat of the water vapour at 34 °C is 2415 kJ/kg, the adsorption rate of silica gel was 30%, effective dehumidification rate was 80%, and wind speed v = 1.5 m/s. It can be obtained that the air mass flow was 458.35 kg/h, the theoretical unit of silica gel adsorption capacity was 0.216 kg/kg, latent heat release per unit time was 1506.6 kJ/h, and the weight of dehumidification material was 6 kg. Considering the system of air leakage loss and heat loss [18], the correction factor 0.7 was applied, and the heat needed to be absorbed by that phase change material was 1054.62 kJ/h. It could be inferred from Table 21 that the storage density of phase change material was 131 kJ/kg, so 8 kg phase change material needed to be added, and mass ratio of silica gel/phase change material was 3:4.

5 Desiccant Regeneration Methods The traditional regeneration method of the solid dehumidification packed-bed is electric heating regeneration; the principle is to use the air directly heated by electric energy to regenerate the packed-bed. However, it has the disadvantages of low efficiency, high energy consumption and long regeneration time [49], so this kind of regeneration method cannot meet the engineering applications and the increasingly urgent energy-saving requirements. To solve the above problems effectively, it is vital to develop the solid dehumidification packed-bed. New regeneration methods of the solid dehumidification packed-bed are needed to control and reduce the regenerative energy consumption, improve the regenerative efficiency, save the operating cost and meet the inevitable requirements of energy-saving and emission reduction.

Solar Desiccant (Absorption/Adsorption) …

241

Aiming at the problems of low regeneration efficiency, large energy consumption and long regeneration time of electric heating regeneration, many researchers have proposed the new regeneration methods that include waste heat regeneration, ultrasonic regeneration, electro-osmotic regeneration, microwave regeneration and solar regeneration.

5.1 Waste Heat Regeneration The waste heat regeneration system of solid dehumidification packed-bed is reformed based on original production system, combining the heat transfer equipment with the packed-bed, and the basic principle is to use the waste heat generated from the production process to heat and regenerate the packed-bed. At present, the waste heat which is commonly used mainly includes the waste heat of air-conditioning system and industrial waste heat. Under the normal circumstances, the temperature of the equipment is relatively low when the air-conditioning system is in the operation, and the hot air generated by the heat exchange has a low temperature. Therefore, it is commonly used for the preheating of the packed-bed regeneration. Zhao et al. [50] designed a system, which could recover the exhaust heat of air-conditioning system for preheating the regeneration of the packed-bed, effectively improving the COPh of dehumidification cooling system. Compared to the compressed air-conditioning system, high temperature hot water can be produced by the operating process of the absorption air-conditioning system. Fathallah and Aly [42] designed a kind of waste heat regeneration system, which used the waste heat from condenser of the absorption refrigeration unit to regenerate the packed-bed, so the temperature of regeneration air was increased to 73 °C, which can be used directly for regeneration. In the use of the packed-bed regenerated by industrial waste heat, the US Department of Energy has developed an integrated energy system (IES) which uses the waste heat from the generator to regenerate the packed-bed. The system was reported by Zaltash et al. [51]. Myat et al. [52, 53] used the waste heat from factory to regenerate the multi-layer packed-bed, and in the process, they used the 55–80 °C hot water, which is heated by the factory waste heat, to achieve the regeneration of the multi-layer packed-bed. The waste heat regeneration method has the advantages of energy-saving, stable effect and no need of auxiliary heating equipment, which can effectively improve the energy efficiency and stability of the packed-bed regeneration. However, the research and application of the packed-bed waste heat regeneration are limited by the heat source place, it is difficult to popularize, and the heat exchange equipment, which is suitable for utilizing the waste heat in various industries, is still in the research stage, resulting in waste heat regeneration mode only applicable for the production of 60–140 °C waste heat site [7].

242

W. Yang et al.

5.2 Ultrasonic Regeneration Ultrasonic regeneration is to use the mechanical effect and thermal effect produced by super acoustic wave to strengthen the regeneration of the packed-bed. On the one hand, the mechanical vibration effect produced by ultrasonic wave propagates in the solid desiccant material, causing severe air disturbances in the pores of solid desiccant materials, destroying the surface water vapour film of the solid desiccant material, thus reducing the gas-side mass transfer resistance of solid desiccant materials. On the other hand, the heat effect caused by ultrasonic wave increases the internal temperature of the dehumidification material, speeding up the migration of internal moisture to the outer surface, thus increasing the gas-side mass transfer power of the dehumidification material [54, 55]. Many scholars have studied the regeneration characteristics, regeneration mechanism and regeneration model of ultrasonic regeneration packed-bed. In terms of the regeneration characteristics of ultrasonic regeneration packedbed, Yao et al. [56, 57] studied about the influence factors of ultrasonic regeneration packed-bed, including the regeneration air temperature, the moisture content of solid dehumidification materials, the ultrasonic power and frequency; then the results showed that the efficiency of the ultrasonic regenerative packed-bed increased with the decrease in regenerative air temperature and increased with the increase in the water ratio of silica gel. The results also showed the regeneration rate of the ultrasonic regenerative packed-bed increased with the increase in ultrasonic power and decreased with the increase in ultrasonic frequency, and the influence of ultrasonic power and frequency change increased with the decrease in regenerative air temperature. Yao et al. [57] found that the regenerative energy consumption of the ultrasonic regenerative packed-bed decreased with the increase in ultrasonic power and increased with the increase in ultrasonic frequency; in further research and analysis [58], they found that the regenerative energy consumption of the ultrasonic regenerative packed-bed depended mainly on the regeneration condition of the packed-bed, and the SEC index was proposed to evaluate the energy-saving characteristics of ultrasonic regenerative packed-bed under different regeneration conditions, by calculating the different regenerative air temperatures, and ultrasonic power of the SEC can find the best energy-saving conditions, which provided the theoretical basis for the selection of the working condition of the packed-bed with ultrasonic regeneration. In the regeneration mechanism of ultrasonic regenerative packed-bed, through theoretical analysis, Yao et al. [55] found that the mechanical effect and thermal effect of ultrasonic wave can not only improve the moisture diffusion rate of silica gel regeneration, but also reduce the activation energy that required for the internal moisture removal of silica gel, consequently reducing the regeneration temperature and improving the availability of low-temperature heat source in the process of packed-bed regeneration. Yang et al. [59] used two kinds of silica gel (M and SS type) as dehumidification material, to study the mechanical effect and thermal effect on promoting regeneration process, and the research results showed that the thermal effect on promoting the regeneration process was less than 14% (m type) and 20% (SS

Solar Desiccant (Absorption/Adsorption) …

243

type), which was shown that the ultrasonic regenerative process of the packed-bed was mainly promoted by mechanical effect. Yao et al. [60] studied the mechanism of ultrasonic mechanical effect and heat effect on the regeneration process; they found that the mechanical effect of ultrasound enlarged the synergy between the surrounding velocity field and the temperature field of silica gel particles, effectively promoted the convection heat and mass transfer effect of the air side and improved the regeneration rate of silica gel; the thermal effect of ultrasound promoted the diffusion of moisture and temperature in silica gel and increased the rate of silica gel regeneration. In the regeneration model of the packed-bed with ultrasonic regeneration, Yao et al. [57, 61] used six models (Page model, Lewis model, Henderson model, Logarithmic model, Gaussian model and Weibull model) to simulate and analyse that water ratio variation with time in the process of ultrasonic regeneration. The results showed that the regeneration rate constants of the Weibull model did not vary with the regeneration condition, while the regeneration rate constants of the other models varied with the regeneration condition, so the Weibull model was more suitable for the change of silica gel moisture ratio with time during the simulated ultrasonic regenerative packed-bed process. On the basis of ultrasonic mechanical effect and thermal effect mechanism, Yao et al. [62, 63] proposed one dimensional transient heat and mass transfer model of ultrasonic combined with hot air regenerative packed-bed, and the theoretical value of the model calculation was compared with the experimental value, the results showed that the average relative error between the theoretical and experimental values was less than 2%, and it showed that the model could simulate the heat and mass transfer process of ultrasonic combined with hot air regenerative packed-bed well. Ultrasonic regeneration has the advantages of high regeneration rate, small regenerative energy consumption, low regenerative temperature, and so on; at the same time, ultrasonic also has bactericidal function and can effectively reduce the solid desiccant materials and airborne bacteria concentration. However, ultrasonic regeneration has not been popularized in practical application, and most of the research remains in the laboratory stage. On the one hand, it is because the cost of the equipment is 2–3 times higher than that of the electric heating regeneration; on the other hand, it is difficult to meet the application requirement because of the production process of sonic generator; in the mechanism of ultrasonic regeneration, mechanical effect and thermal effect have been studied, while the effect of ultrasonic cavitation on the packed-bed ultrasonic regeneration is rarely reported. The results show that the cavitation effect of ultrasonic wave is the main power of ultrasonic chemistry, and the shock wave, microjet and microdisturbance are the main mechanism of strengthening ultrasonic drying in the fields of food and medicine. Therefore, it is necessary to study the effect of ultrasonic cavitation effect on the ultrasonic regenerative packed-bed [56, 64, 65].

244

W. Yang et al.

5.3 Electro-osmotic Regeneration The electro-osmotic regeneration is to regenerate the dehumidification material by using the electro-seepage effect of moisture in the solid desiccant material under electric field, when the moisture in the air is absorbed by the solid desiccant material to a certain water content, forming a double electric layer on the wall surface of the desiccant material [66]. In the electric field, the ions in the double layer migrate from the positive electrode to the negative electrode, forming the ion flow. Under the action of viscous force, the moisture in the dehumidification material transfers from the positive electrode to the negative electrode to form the electro-seepage flow and finally separates from the solid desiccant material [67]. The electro-osmosis regeneration is affected by the Joule heat and the corrosion of the electrode, the regeneration rate is low, and the duration is short [68]. Qi et al. have improved these issues, increasing the regeneration rate of zeolite to 0.0021 g/s and improving the duration time to 120 h. In addition, the moisture content of the solid desiccant material has great influence on the effect of the electro-osmotic regeneration, Zhang et al. [67] found that when the voltage at both ends of the packed-bed was 60 V, there is no electro-osmosis effect on macro-porous silica gel with initial water content of 95%, while the macro-porous silica with initial moisture content of 105 and 110% has electro-osmotic effect. The regeneration rate of electro-osmotic regeneration is lower, but compared to the traditional electric heating regeneration, this regeneration has the advantages of the lower regeneration temperature, uniform regeneration effect, dehumidification and regeneration at the same time, and no damage to the solid dehumidification material structure. Meanwhile, the electro-osmotic regeneration does not need to consume heat energy, so it can save a great deal of energy.

5.4 Microwave Regeneration Microwave regeneration places the packed-bed in a high-frequency alternating electromagnetic field with a frequency of up to hundreds of millions of times per second, and the dipole in the solid desiccant material is rearranged and oscillates with the alternating electromagnetic field. At the same time, due to the direction of electric field constantly changes, the molecules in the dehumidification material will also be constantly rearranged. In this process, the thermal motion of molecules and the friction between molecules produces a large amount of heat, which causes the internal and external temperature of the dehumidification material to rise simultaneously, and the moisture in the desiccant material is heated and vaporized, realizing the regeneration of the packed-bed [69]. The study on microwave regeneration of packed-bed mainly concentrates on the regeneration of Zeolite, the results show that the regeneration rate of microwave regeneration zeolite packed-bed is about 5 times that of hot air regeneration [70], at the same time, microwave regeneration can reduce the heat

Solar Desiccant (Absorption/Adsorption) …

245

source temperature by 16 °C [71], and microwave combined with hot air regeneration can improve the energy efficiency of initial regeneration stage of zeolite packed-bed [72]. But it is also found that the microwave-regenerated zeolite has the problems of large heat loss and easy structure damage [73]. Ohgushi et al. [74, 75] proposed that the heat loss during microwave regeneration could be reduced significantly by adding Ca-X zeolite to the zeolite. At the same time, they [76] found that the percentage of the zeolite’s dehumidification capacity decreased 1.3%/times due to structural damage. Microwave regeneration has the advantages of high regeneration rate, low temperature of heat source, high energy utilization, ease of realizing heating uniformity and sterilization in uniform microwave field, but in experiments, it is found that the combination of packed-bed and microwave device is difficult to form uniform microwave field, which results in uneven heating of solid desiccant material and even causes the local overheating of the solid desiccant material to rupture; at the same time, the combination of packed-bed and microwave device can easily produce microwave leakage and endanger the health of people; on the other hand, the microwave heating process is a complicated unsteady process, the researches on transient heat and mass transfer theory of microwave regeneration are insufficient, and it is difficult to provide an effective theoretical basis for the researches of microwave regenerative packed-bed and the development of microwave regeneration equipment and instruments [77].

5.5 Solar Regeneration Solar regeneration is an application of solar thermal effect. The basic principle is to use the collector to convert solar energy into heat to regenerate the packed-bed. Solar regeneration can be divided into direct and indirect types. Direct solar regeneration uses solar radiation to heat and regenerate the packedbed directly. Under the action of solar radiation, the temperature of solid dehumidification material in packed-bed elevates, and then the moisture vapour adsorbed in the solid desiccant material vaporizes and discharges out of the packed-bed under the action of natural convection or fan, realizing the regeneration of solid desiccant material. The packed-bed, which is directly regenerated by solar energy, was initially metal structure [78–81]. However, it is found that the metal structure packed-bed has high reflectivity of solar radiation and large heat loss, which is unfavourable to solar regeneration. Lu et al. [82] replaced the metal structure with the glass structure, and it effectively reduced the reflectivity and heat loss of the packed-bed to the solar radiation, improving the thermal efficiency of the packed-bed and the regeneration efficiency of the desiccant material. Saito [83] and Techajunta et al. [84] developed a direct solar regenerative packed-bed device suitable for tropical hot and humid climatic conditions, which further improved the applicability of direct solar regenerative packed-bed. Kumar et al. [85] developed a parabolic disc structure of the collector for the regeneration of the packed-bed, and the regeneration rate of direct solar regenerative packed-bed was improved. The results showed that the maximum

246

W. Yang et al.

regeneration rate of silica gel per unit quality is up to 0.216 kg/h, and the minimum time for regeneration of silica gel per unit quality is 110 min. Indirect solar regeneration is to set the collector and the packed-bed separate, and the collector absorbs the solar energy to heat the air (water) and uses the blower (pump) to pass the heated air (water) into the packed-bed to realize the regeneration of the packed-bed. The indirect solar regeneration system can be divided into traditional type and internal heat type according to the structure of packed-bed. The research on the traditional packed-bed regeneration mainly concentrates on the optimization of the collector. Surajitr and Exell [86] designed a composite parabolic solar air collector to regenerate the traditional packed-bed, and the results showed that the heat collector could increase the air temperature in the tropical hot and humid climate conditions by 10–50 °C, and the maximum regeneration rate of the traditional packed-bed could achieve 0.51 kg/h. Yadav and Bajpai [9] used vacuum tubular collector to regenerate traditional packed-bed. In the sunny day conditions, they got the regeneration air which temperature of is 14–27 °C higher than that of the ambient air. The results showed that under the conditions of 5 kg of silica gel and 88 and 138 kg/h of air flow, the regeneration rate of silica gel was 0.063–0.207 and 0.006–0.506 kg/h. The research on the internal heat packed-bed regeneration mainly concentrates on the optimization of packed-bed. Zhen et al. [27, 87] adopt plate-fin heat exchanger as bed body, the inner surface of the heat exchanger channel adhered to silica gel, and a cross-heated compact silica packed-bed with indirect solar regeneration is developed, as shown in Fig. 27. The packed-bed is mainly composed of the main flow channel and the secondary flow channel, the inner wall of the main channel adhered with silica gel in order to dehumidify the flowing air, and the secondary flow channel is used to regenerate the silica gel in the main channel by using regeneration air heated by the collector. The cross-heated compact silica packed-bed adopts the method of gas–solid heat transfer to regenerate the solid desiccant material, so heat exchange efficiency is difficult to improve. Ge et al. [88–90] put forward the use of liquid–solid heat-type packed-bed, as shown in Fig. 28. They stick silica gel on the fins and the outer surfaces of the finned tube heat exchangers and used solar hot water in the pipeline to regenerate silica gel, developing a heat-type packed-bed with finned tubes, and a series of studies were carried out. They studied the solar hot water temperature required for the regeneration of the silica gel-coated fin-tube packed-bed in finned tubes under various operating conditions, and the results showed that hot water with 50–80 °C temperature could meet the regeneration of packed-bed under various working conditions; they also studied the effect of hot water temperature on the COPh of the silica gel-coated fin-tube packed-bed in the solar regenerative fin tube, the results showed that when the air temperature was 30 °C, the air moisture content was 14.3 g/kg, the air velocity was 1 m/s, and the COPh of the silica gelcoated fin-tube packed-bed was up to the maximum when the hot water temperature in the tube was 70 °C; and then a mathematical model of the silica gel-coated fintube packed-bed in solar regenerative fin tube was established, and the operation of the bed body was simulated by C++ language program and compared with the

Solar Desiccant (Absorption/Adsorption) …

247

Fig. 27 Cross-heated compact silica packed-bed [27]

Fig. 28 Silica gel-coated fin-tube packed-bed [90]

experiment. The results showed that the error between the simulation results and the experimental results was less than 15%. Solar energy has the advantages of large reserves, wide distribution and no pollution, solar regenerative packed-bed has good energy-saving effect, and it can effectively alleviate the environmental pollution that caused by the burning of fossil fuels. Solar regeneration includes direct and indirect type; the efficiency of direct-type regeneration is better than that of indirect type [91], but it is also found that the regeneration efficiency of direct solar regeneration is not high and the regeneration effect is unstable in application; the main factors affecting the efficiency and stability of direct solar regeneration include solar radiation intensity, air flow rate and inlet air temperature humidity [92, 93]; it is an effective method to improve the efficiency and stability of solar energy regeneration by increasing the temperature of inlet air and reducing air humidity in the case of solar radiation intensity and air velocity constant.

248

W. Yang et al.

5.6 Existing Problems Regarding the current research on the regeneration of packed-bed worldwide, ultrasonic regeneration, electro-osmotic regeneration and microwave regeneration have effectively improved the regeneration efficiency and energy utilization of packedbed regeneration, but the above methods are all dependent on energy supply, which is a problem of high-grade energy consumption [94]. At the same time, it is difficult to popularize the regeneration device by using these methods. The waste heat regeneration can regenerate packed-bed by the low-grade energy, but it is difficult to be popularized in the engineering application because of the site restriction. Solar regeneration does not consume electricity, and the distribution of solar energy is widely and without the limitation of the site, and the production of solar regeneration device is relatively simple. In addition, there are still some technical problems about filling-type packed-bed dehumidification and solar regeneration, as follows [95]: (1) Most of the existing research results are that setting up the packed-bed dehumidification system in buildings as an additional device and seeing packed-bed dehumidification system as a single building surface attachment rather than its own structure or components, and there are less researches on the integrated design of packed-bed dehumidification system and building. (2) The adsorption heat affects the dehumidification efficiency of the packed-bed dehumidification process, and the air temperature is increased by the adsorption heat which is absorbed by the air flowing through the solid desiccant material, which causes the air to be sent indoors to increase the heat load of the air conditioner. (3) The dehumidification and regeneration effect of the packed-bed is influenced by the inlet air temperature and humidity, and the dehumidification and regeneration efficiency of the packed-bed is not high and unstable under the condition of the inlet air temperature and humidity is not good. (4) The dehumidification model of packed-bed dehumidification process is mainly a mathematical model deduced from the theory, which is complex in form, not intuitive in calculation and inconvenient for engineering application.

6 The Novel Solar Solid Dehumidification/Regeneration Bed 6.1 Introduction The independent temperature- and humidity-controlled air-conditioning systems are more and more widely used in buildings. The complete air-conditioning cycle of the system consists of the adsorption process, regeneration process and cooling pro-

Solar Desiccant (Absorption/Adsorption) …

249

cess, while the regeneration process is the core of the entire cycle. This is because the regeneration process not only affects the dehumidification performance in the adsorption process, but also affects the energy efficiency of the entire system [96]. Traditionally, one of the mostly used regeneration methods for the dehumidification materials in building’s air-conditioning systems is by means of the high temperature from the simulated solar energy [97–99]. Techajunta et al. [100] established the integrated desiccant/collector system which was regenerated by solar radiation directly, and the results proved that the silica gel can be regenerated in tropical humid climates by using the solar-heated air. Surajitr et al. [101] investigated the regeneration of silica gel desiccant by the solar air heater, and it was found that the average regeneration rate under the various weather conditions was at 0.19 kg/h per m2 of the aperture area, and the highest regeneration rate was at 0.51 kg/h in one silica gel bed with the air flow rate of 0.007 kg/s. Ram et al. [102] studied the feasibility of the regeneration of solid desiccants by using the solar parabolic dish collector, and the results showed that the maximum regeneration rate of activated charcoal was 0.24 kg/h. Dong et al. [103] designed the solar heating system which combined the technologies of evacuated tube solar air collector and rotary desiccant humidification together, and the experimental and simulation results showed that the solar heating with desiccant humidification was worthwhile being applied to improving the indoor thermal comfort in heating season. The solar regeneration method can improve the COP of the air-conditioning system and suitable for all dehumidification materials, but with low regeneration efficiency and long working time. So far, another commonly used regeneration method is called the microwave regeneration, which has been introduced due to its improved regeneration efficiency, shortened regeneration time and little damage to the dehumidification materials [104–106]. Ania et al. [107] compared the regeneration of activated carbon under electric heating and microwave irradiation. It proved that the time of the microwave regeneration was less than that of the electric heating, and the adsorption capacity of the activated carbon after the microwave irradiation was greater than that after the electric heating. Polaert et al. [108] found that the porous and molecular structure of the adsorbent had little effect on the absorption of microwave energy, while the dielectric properties of the adsorbents played a dominant role in this process. Chao et al. [109] studied the regeneration of the granular activated carbon by microwave thermal treatment, and the results also showed that in comparison with the conventional thermal regeneration, microwave regeneration reduced the processing temperature and time. Even though the microwave technology appears to be very successful in the regeneration of solid desiccant, most research cases are demonstrated on the closed microwave oven and have the drawback of non-uniformity in the regeneration process [105, 110–112]. To resolve the existing problems in the solar regeneration technologies (e.g. low energy efficiency and time-consuming) and microwave regeneration technologies (e.g. non-uniformity during the process), the combined solar/microwave regeneration method has been proposed. In this section, the novel solar solid dehumidification/regeneration bed will be investigated that the generation method which combines the microwave with the solar radiation to regenerate the dehumidification materials

250

W. Yang et al.

will be explored. The combined solar/microwave regeneration method is compared to the solar radiation regeneration and microwave regeneration regarding their regeneration performance and regenerative energy consumption. The dehumidification performance of the proposed bed is investigated. Finally, a mathematical model is proposed to predict the regeneration characteristics of the proposed system under the combined method of the microwave and solar regeneration. The research can be expected to improve the regeneration performance of the dehumidification materials and reduce the energy consumption of the regeneration process for the building’s air-conditioning systems.

6.2 System Description The proposed solar solid dehumidification/regeneration bed is made of plexiglass and wooden rectangular container filled with silica gel. The container was divided into five sub-containers using four columns; each column has two holes on it: one is on the upper part of the sub-container, and another is on the lower part of the sub-container. Figure 29 shows the schematic drawing of the proposed system.

Fig. 29 Schematic of the solar solid dehumidification/regeneration bed: a dehumidification mode; b regeneration mode

Solar Desiccant (Absorption/Adsorption) …

251

The proposed bed can operate under two modes, i.e. dehumidification mode and regeneration mode. In the dehumidification process (Fig. 29a), the outdoor air driven by the fans passes through the silica gel layer in the container, and the water vapour in the outdoor air will be absorbed by the silica gel. Then the dehumidified air is supplied to indoor of the building. In the regeneration process (Fig. 29b), the saturated silica gel will be heated by the simulated solar and microwave radiation, and the water vapour inside the silica gel will be vaporized. Then the outdoor air driven by the fans passes through the bed to bring away water vapour, and it finally dissipates to the environment. The regenerated silica gel is ready for use. The independent temperature- and humidity-controlled air-conditioning systems have been demonstrated that the novel solar solid dehumidification/regeneration bed designed based on the concept of temperature and humidity independent control is more energy efficient compared to the traditional condensate dehumidification air-conditioning systems [20, 96, 113]. Compared to the conventional dehumidification/regeneration bed, the advantages of the proposed solar solid dehumidification/regeneration bed can be presented as follows: (1) the proposed bed is composed of five dehumidification sub-containers with thin silica gel, which will enhance the dehumidification capacity and reduce the total thickness of the dehumidification layer in the conventional air-conditioning systems; (2) the structure of the proposed bed is simple so that the dehumidification and regeneration modes could be easily adjusted according to the requirements of the residents in the buildings; and (3) the saturated silica gel in the proposed bed will be regenerated using the combined method of solar radiation and microwave, reducing the energy consumption and cost of operating conventional air-conditioning systems in the buildings. The proposed solar solid desiccant/regeneration bed, as the major component of the independent temperature- and humidity-controlled air-conditioning systems for buildings, is simple in structure and easy for building integration and has excellent dehumidification performance and regeneration efficiency with the reduced energy consumption and carbon emission.

6.3 Construction of the Testing Rig In order to investigate the performance of the proposed bed, a testing rig was constructed at a laboratory in the Guangdong University of Technology, China, in Figs. 30 and 31. The testing rig was mainly consisted of the DC air conditioner, solar solid dehumidification/regeneration bed, microwave generator, xenon lamps, electronic scale, multi-channel temperature and humidity recorder and Pitot tube anemometer. The DC air conditioner was used to provide the air flow across the solar solid dehumidification/regeneration bed with controlled temperature and humidity. The size of the solar solid dehumidification/regeneration bed (shown in Fig. 32) was 700 mm × 500 mm × 250 mm. The thickness of the silica gel layer was chosen at 50 mm. This was because the authors in the previous study [114] had measured the temperature of silica gel at different thicknesses under the simulated solar radiation condition.

252

W. Yang et al.

Fig. 30 Structure of the testing rig 1—DC air conditioner; 2—solar solid dehumidification/regeneration bed; 3—silica gel; 4—microwave generator; 5—electronic scale; 6—xenon lamps; 7—temperature/humidity sensor; 8—Pitot tube anemometer

The results had shown that the temperature of the silica gel at 50 mm thickness was 60% of the surface temperature. When the thickness was more than 50 mm, the temperature of the silica gel would be significantly decreased, while when the thickness was less than 50 mm, the water contained in the air cannot be absorbed by the silica gel. The microwave generator for providing microwave radiation was mounted on the sides of the bed, and the xenon lamps were fixed above the bed for the simulation of the sunlight. The electronic scale can measure the weight change of the silica gel during the humidification and regeneration processes of the bed. The multi-channel temperature and humidity recorder and Pitot tube anemometer can measure the temperature/humidity of the air and the export wind speed, respectively. The performance parameters of each component are shown in Table 22, and the experimental test data are within the error range of the test instrument. The testing rig was operated in two processes: Regeneration process: Before testing, the silica gel was placed in the blast oven at the temperature of 120 °C for 8 h to be completely dried, which was measured for its mass reach 11.312 kg. Then the dried silica gel was put into the bed, and the moisture air-controlled by the air conditioner passed through the silica gel until the mass of the silica gel reached 13.312 kg, which was considered as the saturation point of the silica gel. In the regeneration process, the microwave generator was turned on for 10 min and turned off for 5 min in order to prevent the damage of the container, while the xenon lamps were turned on all the time. The weight change of the silica gel will be recorded every 15 min through the electronic scale. When the weight of the silica gel was no longer change, the xenon lamps and microwave generator were turned off to finish the testing. For comparison, the regeneration of the bed under the pure simulated radiation and pure microwave radiation (turn on 10 min and turn off 5 min) will be tested as well. In these processes, the power consumption for the bed operated under different regeneration methods was also monitored for analyses.

Solar Desiccant (Absorption/Adsorption) …

253

Fig. 31 Image of the testing rig

Fig. 32 Image of the solar solid dehumidification/regeneration bed

Dehumidification process: Before the testing, the silica gel was put in the blast oven at the temperature of 120 °C for drying for 8 h, and then the silica gel was weighted to ensure it was fully dried and then put into the bed. Then the air flow channels were closed as shown in Fig. 29a, and the air in different conditions (shown in Table 23) provided by the DC air conditioner passed through the silica gel layer. The weight change of the silica gel will be recorded every 10 min by the electronic scale. When

254

W. Yang et al.

Table 22 Performance parameters of each component of the testing rig Equipment

Specification

Parameter

DC air conditioner

DW11-50No2 OCS-I

Power: 105 W; voltage: 220 V/50 Hz; current: 0.48 A; air flow volume: 370 m3 /h; static pressure: 400 Pa

Microwave generator

Custom-made

Frequency: 2450 MHz; power: 0–1000 W

Electronic scale

TCS-01

Power supply: 220 V; power: 14 W; maximum weight: 75 kg

Xenon lamps

AHD1000

Wavelength range: 0.2–2 μm; power: 1000 W

Multi-channel temperature monitor

AT4340

Sensor: K-type thermocouple; temperature range: −200 to 1300 °C; measurement accuracy: ± (0.5% × value + 2); power supply: 220 V ± 10%, 50 Hz ± 2%

Multi-channel temperature and humidity recorder

BYCT-TH150B

Temperature range: −40 to 150 °C; relative humidity range: 20–90%; temperature accuracy: ±0.5 °C

Pitot tube anemometer

VF110

Range: 0–10 m/s; accuracy: 0.01 m/s

Table 23 Performance parameters of the testing rig during the dehumidification process Condition

Inlet air temperature (°C)

Inlet air relative humidity (%)

Inlet air humidity ratio (g/kg)

Mass flow (kg/s)

1

27.69

81.84

19.24

0.33992

2

26.09

89.23

19.09

0.33992

3

27.66

85.24

20.03

0.33992

4

29.17

81.68

20.98

0.33992

the weight of the silica gel was no longer changed, the DC air conditioner was turned off to complete the testing.

6.4 Test Results and Analysis The regeneration performance of the proposed solar solid dehumidification/regeneration bed was evaluated for its dry basis moisture content, speed of regeneration, regeneration degree and energy efficiency. The dehumidification per-

Solar Desiccant (Absorption/Adsorption) … Simulated solar

18

Microwave Combined simulated solar/microwave

16

Dry basis moisture content(%)

255

14 12 10 8 6 4 2 0

100

200

300

400

500

600

700

Regeneration time(min)

Fig. 33 Variation of the dry basis moisture content of the solar solid dehumidification/regeneration bed under the three regeneration methods

formance of the bed will be studied by analysing the moisture removal, dehumidification efficiency and speed of dehumidification. (1) Dry Basis Moisture Content Dry basis moisture content refers to the ratio of the mass difference between the silica gel at a time during the regeneration process and at the completely dried state to the mass of the silica gel at the completely dried state [115], which is expressed in Eq. (5). Figure 33 presents the variation of the dry basis moisture content of the solar solid dehumidification/regeneration bed under the three regeneration methods. Mτ =

mτ − mg mg

(5)

From Fig. 33, it can be found that the dry basis moisture content for the three regeneration methods presented similarly that it decreased until reached relatively stable. This is because with the increase in the regeneration degree, water vapour in the silica gel was reduced and the energy required for the regeneration was improved, leading to the reduced dry basis moisture content. The dry basis moisture content was maintained at 14.02% for the simulated solar regeneration, 4.67% for the microwave regeneration and 4.01% for the combined simulated solar/microwave regeneration, indicating that most water vapour was regenerated from the solar solid dehumidification/regeneration bed by using the combined method among the three regeneration methods.

256

W. Yang et al. Microwave Simulated solar Combined simulated solar/microwave

Speed of Regeneration(g/(kg·s))

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

Regeneration time(min)

Fig. 34 Variation of the speed of regeneration of the solar solid dehumidification/regeneration bed for the three regeneration methods

(2) Speed of Regeneration Speed of regeneration is defined as the ratio of the change of the dry basis moisture content at the unit time to the unit time [116], which is described in Eq. (6). The variation of the speed of regeneration with time is shown in Fig. 34. υ=

Mτ − Mτ +τ τ

(6)

In Fig. 34, the regeneration speeds for all the three different methods rapidly increased at the first time and then decreased slowly. This could be explained in the aspect of the molecular structure of the water vapour inside the silica gel. At the beginning of the regeneration process, the regeneration speed increased with the increase in the regeneration energy required for free water molecules, while later the regeneration speed decreased with the increase in the regeneration energy required for bound water molecules. For the simulated solar radiation condition, the speed of regeneration reached its maximum at 0.491 g/(kg s) in the first 90 min. The maximum speed of regeneration under the microwave regeneration method appeared at the 60 min with the value of 0.569 g/(kg s) The maximum speed of regeneration for the combined simulated solar/microwave regeneration method was at 0.668 g/(kg s) corresponding to the first 135 min during the testing. Compared with the pure simulated solar and microwave radiation regeneration methods, the combined method can improve the regeneration effect of the silica gel. (3) Regeneration Degree

Solar Desiccant (Absorption/Adsorption) … Simulated solar Microwave Combined simulated solar/microwave

90 80

Regeneration Degree(%)

257

70 60 50 40 30 20 10 0 0

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

Regeneration time(min)

Fig. 35 Variation of the regeneration degree for the solar solid dehumidification/regeneration bed under the three regeneration methods

The regeneration degree is the ratio of the mass of the water vapour regenerated at a time during the regeneration process to the mass of the water vapour contained in the saturated silica gel when the process starts [117], which is expressed in Eq. (7). The variations of the regeneration degree with time are shown in Fig. 35, and the regeneration degree for the simulated solar radiation, microwave and combined regeneration methods increased and kept stable at 20.7, 73.5 and 77.7%, respectively. These results showed that the combined method could regenerate more water vapour from the silica gel and therefore improve the dehumidification capacity of the silica gel. R=

Wτ W0

(7)

(4) Energy Efficiency The energy efficiency of the testing rig is defined in Eq. (8). The energy consumed by the simulated solar radiation was negligible in the practical conditions, and therefore, the variation of the energy efficiency with the regeneration degree for the microwave and combined regeneration methods is compared in Fig. 36. η=

Qr Wr · H = Qτ Qτ

(8)

In Fig. 36, the energy efficiencies firstly increased with the increase in the regeneration degree and then reached stable and finally reduced. At the point of 21% of the regeneration degree, the energy efficiency for the two regeneration

258

W. Yang et al. Microwave Combined simulated solar/microwave

Energy efficiency (%)

22

21

20

19

18

17 15

20

25

30

35

40

45

50

55

60

65

70

75

Regeneration degree (%)

Fig. 36 Variation of the energy efficiency with the regeneration degree for the solar solid dehumidification/regeneration bed under the microwave and combined regeneration methods

methods obtained the same value of 18.6%. When the regeneration degree was less than 21%, the energy efficiency of the combined method was lower than that of the microwave method, while when the regeneration degree was higher than 21%, the energy efficiency of the combined method was higher than that of the microwave method. This was because the regeneration process of the silica gel was not only related to the temperature but also to the external pressure. The maximum energy efficiency for the two methods, i.e. microwave and simulated solar/microwave, was at 19.4 and 21.7%, respectively, which indicated that the combined method improved the energy utilization efficiency during the regeneration process. (5) Moisture removal performance The moisture removal was defined as the difference of the humidity ratio between the outlet air and inlet air Eq. (9), and the results are shown in Fig. 37 [50]. d = din − dout

(9)

Figure 37 shows the variation of the moisture removal at different inlet air temperatures and relative humidity with time. It was found that the trend of the moisture removal in the four conditions was similar, which is because with the increase in the water in silica gel, the water vapour pressure in silica gel increased, and therefore, the dehumidification capacity was also reduced. Until the end of the dehumidification process, the water vapour pressure in silica gel was the same as that in the air. The maximum moisture removal of 14.1 g/kg

Solar Desiccant (Absorption/Adsorption) …

259

15

Condition 1 Condition 2 Condition 3 Condition 4

14 13 12

Moisture removal (g/kg)

11 10 9 8 7 6 5 4 3 2 1 0 50

100

150

200

250

300

350

400

450

500

550

Time (min)

Fig. 37 Variation of the moisture removal of the solar solid dehumidification/regeneration bed with time

was found when the inlet air temperature was at 26.09 °C and the air relative humidity was at 89.23%, and its average moisture removal was also at maximum value of 4.2 g/kg. This indicated that the inlet air with lower temperature and higher humidity can improve the dehumidification performance. (6) Dehumidification efficiency The dehumidification efficiency of the system, defined as the ratio of the moisture removal performance to the humidity ratio of inlet air, is expressed in Eq. (10) and shown in Fig. 38 [117]. From Fig. 38, it can be found that the maximum dehumidification efficiency, which was significantly influenced by the temperature of the inlet air, decreased with time. The maximum dehumidification efficiency was 68.0% when the inlet air temperature was 26.09 °C and the air relative humidity was 89.23%. εd =

d din

(10)

(7) Speed of dehumidification Speed of dehumidification is defined as the ratio of the moisture removal at the unit time to the mass, which is expressed in Eq. (11). Figure 39 shows the variation of the speed of dehumidification with time [118], and the maximum speed of dehumidification s appeared at the beginning of the dehumidification process since the water vapour pressure difference between the silica gel and the inlet air was biggest at this stage, resulted in a maximum speed of dehumidification. After that, the speed of dehumidification decreased slowly. The

260

W. Yang et al. 70

Condition 1 Condition 2 Condition 3 Condition 4

65

Dhumidification efficiency (%)

60 55 50 45 40 35 30 25 20 15 10 5 0 50

100

150

200

250

300

350

400

450

500

550

Time (min)

Fig. 38 Variation of the dehumidification efficiency of the solar solid dehumidification/regeneration bed with time

maximum average speed of dehumidification was 0.126 g/(kg s) at the inlet air temperature of 26.09 °C and the air relative humidity of 89.23%. υd =

m˙ · d mg

(11)

6.5 Comparison with the Previous Models The moisture ratio is the major parameter used to study the regenerative mathematical model of the dehumidification materials [119], and the calculated moisture ratio (Eq. 12) using the tested data is shown in Fig. 40. MR =

Mτ − M1 M0 − M1

(12)

For comparison, four commonly used previously studied regeneration models were selected to simulate the moisture ratio of the silica gel (shown in Table 24) [120–123], and the semi-empirical equations from the testing results are summarized in Table 25. By comparing the testing results with the modelling results using the semi-empirical models, the Page model was validated, and it can be used to predict the moisture ratio of the silica gel.

Solar Desiccant (Absorption/Adsorption) …

261

Speed of dehumidification (g/(kg·s))

0.5 Condition 1 Condition 2 Condition 3 Condition 4

0.4

0.3

0.2

0.1

0.0 50

100

150

200

250

300

350

400

450

500

550

Time (min)

Fig. 39 Variation of the speed of dehumidification of the solar solid dehumidification/regeneration bed with time

Fig. 40 Comparison of the moisture ratio from the testing and semi-empirical Page model results [120] Table 24 Previous regenerative mathematical models for thin layer silica gel Model

Expression

Lewis model [120]

MR = exp(−k · τ )

Page model [121]

MR = exp(−k · τ n )

Wang and Singh model [122]

MR = 1 + a · τ + b · τ 2

Two-term model [123]

MR = a · exp(−k0 · τ ) + b · exp(−k1 · τ )

262

W. Yang et al.

Table 25 Summary of the semi-empirical equations Regeneration method

Model

Semi-empirical equations

R2

Microwave regeneration

Lewis model [120]

MR = exp(−0.0033 · τ )

0.9414

Page model [121]

MR = exp(−1.57 · τ 1.52 )

0.9926

Wang and Singh model [122]

MR = 1 − 0.0023 · τ + 1.12 × 10−6 · τ 2

0.9956

Two-term model [123]

MR = 0.57·exp(−0.0038· τ )+0.57·exp(−0.0038·τ )

0.9577

Lewis model [120]

MR = exp(−0.0035 · τ )

0.9289

Page model [121]

MR = exp(−0.78 · τ 1.66 )

0.9973

Wang and Singh model [122]

MR = 1 − 0.0024 · τ + 1.275 × 10−6 · τ 2

0.9886

Two-term model [123]

MR = 0.584 · exp(−0.0041 · τ ) + 0.584 · exp(−0.0041 · τ )

0.9534

Combined simulated solar/microwave regeneration

6.6 Conclusion This section presents a novel solar solid dehumidification/regeneration bed using three different regeneration methods, respectively, i.e. simulated solar radiation, microwave radiation, combined simulated solar/microwave radiation. Experimental testing was carried out to investigate dehumidification performance of the system for the three different regeneration methods. The testing rig was constructed at a laboratory in Guangdong University of Technology, China. The parameters determining the characteristics of the proposed bed including dry basis moisture content, speed of regeneration, regeneration degree, energy efficiency, moisture removal, dehumidification efficiency and speed of dehumidification were evaluated. The regenerative testing results were compared to the modelling results using the previous developed models to validate the models. Both modelling and testing results were analysed to find the most appropriate regeneration method for silica gel. It was found that the speeds of regeneration rose linearly and then decreased slowly, and their maximum values were 0.491, 0.569 and 0.668 g/(kg s) for the simulated solar radiation, microwave radiation and combined regeneration methods, respectively. The maximum regeneration degree for the combined regeneration method, i.e. microwave combined with simulated solar, was 77.7%, which was 3.77 times higher than that for the simulated solar radiation and 1.05 times higher than that for the microwave radiation. The maximum energy efficiency for the testing rig under the combined regeneration method was 21.7%, and that of the microwave regeneration method was 19.4%. The testing results indicated that the performance of the combined simulated solar/microwave regeneration method was the best among the three regeneration methods. When the inlet air temperature was 26.09 °C and the air relative humidity was 89.23%, the maximum transient moisture removed was 14.1 g/kg, the maximum dehumidification efficiency was 68.0%, and the maximum

Solar Desiccant (Absorption/Adsorption) …

263

speed of dehumidification was 0.294 g/(kg s). By comparing with the previous studies, the semi-empirical Page model equation was established for the moisture ratio of the silica gel, and the results from the semi-empirical Page model were in good agreement with the testing results in the regeneration process. This study concluded that the combination of microwave and simulated solar can improve the regeneration effect and energy efficiency of the regeneration process and reduce the regeneration energy consumption for the proposed solar solid dehumidification/regeneration bed. The further work will need to be carried out to investigate the dehumidification performance of the entire system and the optimization of the experimental devices, providing an effective theoretical basis for the system use in practical applications.

7 Solar-Powered Dehumidification Window 7.1 Introduction Nowadays, energy crisis is one of the major problems faced by the whole world, and building sector is one of the largest energy end-use sectors that consume more than 40% of the global energy [124, 125]. Over half of the energy consumed by the building is contributed by the conventional vapour compression cooling system because of its air process of excess cooling and reheating [126]. A suitable alternative to this energyintensive system is called the solid desiccant cooling system [127], which integrates solid desiccant device with evaporative cooling system to realize air humidity and temperature independent control for resolving the problems of excess cooling and reheating. According to the integrated methods, the solid desiccant devices of the solid desiccant cooling system can be divided into the fluidized bed, rotary desiccant wheel and solid desiccant packed-bed. The fluidized bed has lower pressure drop than the packed-bed, but it might create dust pollution by the collision between particles [128]. The rotary desiccant wheel is widely adopted in the solid desiccant cooling system, but it was found that the adsorption heat from the desiccant dehumidification greatly lowered its performance because it was difficult to remove the heat by innercooling process due to its structure of rotary wheel [7, 129]. Since the solid desiccant packed-bed performs without the dust pollution and is relatively easy to realize innercooling dehumidification process to remove the adsorption heat, it has received much attention for application in solid desiccant cooling system [6, 8]. Over the past few years, several investigations have been conducted on solid desiccant packed-bed. Kabeel [32] studied the effect of the design parameters on the performance of a multi-layer desiccant packed-bed. Awad et al. [130] designed a radial flow hollow cylindrical packed-bed for reducing the distance travelled by the air through the vertical bed, and the maximum value of the mass transfer coefficient was at 2.2 kg/m3 . An intercooled desiccant packed-bed was proposed by Ramzy et al. [34], and 22% increase in the total adsorbed mass was achieved in comparison with

264

W. Yang et al.

the conventional packed-beds. A cross-cooled desiccant packed-bed was proposed by Yuan et al. [28], and the simulation results indicated that the dehumidification efficiency reached 12.4%. Ge et al. [30, 89] concluded that the air–liquid heat exchanger had higher heat transfer coefficient than the air-to-air heat exchanger and consequently removed more adsorption heat. Peng et al. [29] proposed a desiccant-coated packed-bed by coating desiccant material to the outside surface of the conventional fin-tube heat exchangers, and the moisture removal could reach 43.8%. Wang [90] improved the desiccant-coated packed-bed by combining the regenerative evaporative cooler, and they found that the average moisture removal of the packed-bed with regenerative evaporative cooler was 17% higher than the packed-bed without regenerative evaporative cooler. Most of the investigations have been focused on optimizing the structure and improving the adsorption capacity of the solid desiccant packed-bed. However, as the amount of the air to be treated increases, the more space and energy will be required to use the packed-bed in solid desiccant cooling system. In order to reduce the space and energy of the packed-bed required, a novel solarpowered dehumidification window (SPDW) which integrated the desiccant packedbed and photovoltaic (PV) panel into the double-glazed window of the building is proposed and presented in this section. The SPDW is used to dehumidify the outdoor fresh air that enters the residential building, and the part or the whole electric power used to drive the SPDW can be produced by PV panel. Moreover, the saturated solid desiccant of the SPDW can be regenerated by the solar radiation. In order to investigate the dehumidification and regeneration performance of the proposed system, the testing rig of the SPDW will be constructed at the laboratory of the Guangdong University of Technology (China) and tested for different conditions of inlet air and simulated solar radiation. The experimental results will be used to verify a semiempirical model for the prediction of the water content ratio of the solid desiccant modules applying the isothermal adsorption assumption during dehumidification process. This research would provide a novel energy-saving and building-integrated dehumidification technology, which would be helpful for realizing the global target of the building energy reduction and produce a new research area for researchers.

7.2 System Description The proposed SPDW was composed of a PV panel, a flat-plate glass cover, fans, orifice plates, a double shutter and desiccant modules filled with solid desiccant (i.e. silica gel) as shown in Fig. 41. The proposed window will operate under two modes, i.e. dehumidification mode and regeneration mode, as shown in Fig. 41a, b. In the dehumidification mode, the fans inside the window will be switched on, while the fans on both sides of the window will be switched off. The outdoor air will firstly enter the window from the air inlet and is dehumidified when it passes the solid desiccant modules, and then it will be driven into the building by the fans inside the window. In the regeneration mode,

Solar Desiccant (Absorption/Adsorption) …

(a)

265

(b) dehumidified air

moist air

moist air solar radiation

outdoor air outdoor air

Fig. 41 Schematic of the SPDW: a dehumidification mode; b regeneration mode 1—photovoltaic panel; 2—fans; 3—desiccant modules; 4—flat-plate glass; 5—orifice plates; 6—air inlet

the fans on both sides of the window will be switched on, while the fans inside the window will be switched off. Hence, the outdoor air will enter the window from the air inlet, bring away the vapour evaporated from the solid desiccant modules during the regeneration process by solar radiation, and is discharged to the surroundings driven the fans on both sides of the window. It can be predicted that the interference of the moist air exhaust with the outdoor air inlet will be little because the temperature of the exhaust air will be higher than that of the ambient air, and the exhaust air will move upward. Moreover, the air flow flux of the window will be small to allow the ambient air flow carrying away the exhaust air. Also, there will be no fogging on the upper level fenestration near the moist air exhaust because the temperature of fenestration will be raised due to the adsorption of solar radiation. During these processes, the electric energy consumed by the fans can be compensated from the photovoltaic panel. The novelty of the SPDW could be summarized as follows: (1) the SPDW can be potentially used to replace the existing residential building window, which can not only save the construction materials, but also reduce the space the solid desiccant packed-bed and PV panel need. This could be helpful to realize the building

266

(a)

W. Yang et al.

(b)

Fig. 42 Testing rig of the proposed SPDW: a dehumidification mode; b regeneration mode 1—air inlet; 2—electric heater; 3—ultrasonic humidifier; 4—mixing chamber; 5—double shutter; 6—temperature/humidity sensor; 7—solid desiccant module (silica gel); 8—thermocouple; 9—inner layer of the flat-plate glass; 10—outer layer of the flat-plate glass; 11—fans; 12—PV panel; 13—testing space; 14—air outlet; 15—bracket; 16—xenon lamps

integration of the proposed system component. (2) Solar radiation can be used to regenerate the solid desiccant and drive the fans in certain extent, which can significantly increase the energy efficiency of the window and eventually reduce the energy consumption of the air conditioning.

7.3 Construction of the Testing Rig In order to investigate the dehumidification and regeneration performance of the SPDW, the SPDW was designed with the total flat-plate glass area of 1 m2 . The testing rig of the proposed SPDW and the images of the testing rig are shown in Figs. 42, 43, 44 and 45, respectively. The parameters of the experimental material, apparatuses and instruments are presented in Table 26. The testing rig of the SPDW was composed of the mixing chamber, solid desiccant modules, ordinary window glass, fans, testing space, PV panel and xenon lamps. The outdoor air was pretreated in the mixing chamber where the temperature and humidity of the inlet air were controlled by using the electric heater and ultrasonic humidifier. The six desiccant modules filled with 3 kg silica gel each, having the same size of 550 mm in length, 190 mm in width and 50 mm in thickness, were used to dehumidify the air. The distance between each two desiccant modules was at 290 mm. The desiccant modules were fit into the space between the two layers of the ordinary window glass of 3 mm thickness. The inside layer of the window glass was fixed, while the outside layer was movable for conveniently replacing

Solar Desiccant (Absorption/Adsorption) …

267

Fig. 43 Image of the testing rig

the saturated desiccant modules. Two fans were installed above the solid desiccant modules to drive the outdoor air into the testing space. The testing space of 1250 mm × 1200 mm × 2000 mm was made of the colour-coated steel insulation panels of 50 mm thickness representing the building indoor room, where the temperatures and relative humidity of the air were measured every one hour for analysis. The PV panel having the size of 1150 mm in length and 500 mm in width was made of polycrystalline silicon cells and installed on the sunblind. For the evenness of the simulated solar energy, four xenon lamps were used to simulate the sunlight for different radiations during regeneration mode through regulating the input voltage of the lamps. Also, two xenon lamps were used to simulate the sunlight for the PV panel, and the simulated radiations were recorded by the pyranometer.

268

W. Yang et al.

Fig. 44 Image of the desiccant modules embedded in the window

7.4 Methods In order to investigate the dehumidification and regeneration performance of the SPDW, the transient moisture removal, dehumidification efficiency, heat transfer characteristics and regeneration rate will be analysed for the system operated under different inlet air conditions and simulated solar radiation. The transient moisture removal of the solid desiccant modules d τ is defined in Eq. (13) [50]. dτ = din − dout .

(13)

Dehumidification efficiency of the solid desiccant modules η was determined as the ratio of the transient moisture removal to the humidity ratio of the inlet air flow as shown in Eq. (14) [118]. η=

dout dτ =1− , din din

(14)

The heat transfer characteristics referred to the distribution of released adsorption heat Qi during the dehumidification process. The Qi could be considered as the

Solar Desiccant (Absorption/Adsorption) …

269

Fig. 45 Image of the xenon lamps

condensation heat of the water vapour in the SPDW and be divided into three parts: (1) the heat absorbed by the dehumidified air Qa ; (2) the heat absorbed by the solid desiccant modules Qs ; and (3) the heat losses from the window to the surroundings Qd . The relationship of the four parameters during the heat transfer processes of the SPDW is shown in Eqs. (15) to (18) [37, 131]. Ql = Qa + Qs + Qd,

(15)

Q l = dτ Gτr,

(16)

Q a = Gca ta ,

(17)

Q s = Mcs ts ,

(18)

270

W. Yang et al.

Regeneration rate of the solid desiccant modules Rc was determined as the product of the mass flow rate and the humidity ratio difference between the inlet and outlet air of the SPDW as in Eq. (19) [84]. Rc = G(dout − din )

(19)

7.5 Analysis and Discussion of the Testing Results of Dehumidification Process The experiments of the dehumidification and regeneration processes of the SPDW will be conducted, and the testing results will be analysed and discussed, respectively, for the investigation of the dehumidification and regeneration performance of the SPDW. It should be mentioned that the basis for choosing the air conditions was the typical hot and humid weather (from March through May) in South China (Guangzhou) and that the air temperature and relative humidity ranged 18–26 °C and 70–85%, respectively. In order to investigate the dehumidification performance of the SPDW, five inlet air conditions will be conducted during the tests as shown in Table 27. Four evaluation indicators, i.e. the transient moisture removal, dehumidification efficiency, temperature difference between the inlet and outlet air of the window and heat transfer characteristics, will be analysed for the system operated under different inlet air conditions. (1) Transient moisture removal Figure 46 indicates the time variation of the transient moisture removal of the solid desiccant modules during the dehumidification process at a different inlet air temperature and relative humidity. It was found that the solid desiccant modules had the maximum transient moisture removal at 7.1 g/kg when the inlet air temperature was at 19.2 °C and relative humidity was at 86.1%. It was also found that the transient moisture removal reached its maximum at the beginning of the dehumidification process due to the least accumulation of water content and adsorption heat in the modules. Moreover, it decreased sharply at the first 3 h and then slowly due to the increase in the water content in the modules, indicating that the window could maintain a relatively high humidity adsorption capacity for a long time of more than 3 h. For the window operated under the same inlet air temperature, the transient moisture removal increased with the increase in the inlet air relative humidity due to the increase in the vapour pressure difference between the air and the solid desiccant. As to the window operated under the same relative humidity of the inlet air, the transient moisture removal decreased with the increase in the inlet air temperature because of the decrease in heat transfer from the solid desiccant to the dehumidified air, resulting in the heat accumulation in solid desiccant and consequently reducing the adsorption capacity of the window.

Solar Desiccant (Absorption/Adsorption) …

271

Table 26 Parameters of the main components of the SPDW Name

Type

Manufacturer

Main parameters

–

Dongguan Tasike Material Company (China)

Average diameter: 2–4 mm; pore volume: 0.34–0.40 L/kg; particle density: >600 kg/m3 ; specific heat: 0.92 kJ/(kg K)

Electric heater

DJR

HQDRG Company (China)

Power range: 250–500 W

Ultrasonic humidifier

H-010

Guangzhou Shide Electric Company (China)

Humidification capacity: 0–800 mL/h; power: 60 W

Multi-channel temperature & humidity monitor

PC-2WS

Jinzhou Solar Science & Technology Company (China)

Accuracy: ±2% in humidity, ±0.2 °C in temperature

64-channel temperature logger

JK-XU

Changzhou Jinailian Electronic Technology Company (China)

Sensor: K-type; accuracy: ± (value × 0.5% + 1) °C

Fan

ASB20-4-1 M

Changzhou Jinling Electric Company (China)

Mass flow rate: 486 m3 /h; power: 28 W

Anemometer

405-V1

Testo Company

Range: 0–10 m/s; accuracy: 0.01 m/s

Xenon lamp

AHD2000 W

Shenzhen Anhongda Opto Technology Co. Ltd. (China)

Power: 2000 W

Pyranometer

JTTF

JT Technology Company (China)

Spectrum range: 0.3–3.2 μm; sensitivity: 7–14 mV/(kW m2 ); response time: