Photovoltaic thermal (PV t) system effect of active cooling

Photovoltaic Thermal (PV/T) System: Effect of Active Cooling TEO HAN GUAN (B.Sc Eng. (NCKU)) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements I am deeply grateful to my supervisors, Assistant Professor Lee Poh Seng and Associate Professor Hawlader M.N.A, for giving me the guidance, insight, encouragement, and independence to pursue my research. Their advice, contributions and invaluable comments inspired me. I would also like to thank Mr. Yeo Khee Ho, Mr. Chew Yew Lin, Mr. Anwar Sadat and Mrs. Roslina Bte Abdullah who helped me with the setting up of my experimental rig. Also, I would like to express my gratitude to my colleagues, Yan Lin, Wai Soong, Kim Seng, Jayaprakash, Hwang Sheng, Yong Jiun, Sivanand, Karthik B, Karthik S, Aung Myat, and Satyanarayana for their kind help and valuable advice. I would like to extend my deepest gratitude to my parents, sisters and brother for their encouragement and support for the entire duration of this project. Besides, I am also grateful to my friends, especially Miss Lim Sze Huey, who supported and encouraged me to come to Singapore for further study. Lastly, I offer my regards and blessing to all of those who love, care and supported me during the completion of the project. i Table of Contents Acknowledgements i Table of Contents ii Summary v List of Tables vii List of Figures vii Nomenclature xii Chapter 1 Introduction 1 1.1 Energy today 1 1.2 Solar Energy 3 1.2.1 Fundamental 3 1.2.2 Solar Thermal Collector 3 1.2.3 Solar Photovoltaic 4 1.2.4 Photovoltaic Thermal (PV/T) System 5 1.3 Objectives 5 1.4 Scope 6 Literature Review 7 2.1 Water cooled PV/T 7 2.2 Air cooled PV/T 15 Design of Manifold 26 3.1 Simulation of Different Configurations 26 3.2 Manifold Design of Experiment 29 Chapter 2 Chapter 3 ii Experimental Set-up 32 4.1 Description of the PV/T system 32 4.2 Experimental Components 35 4.2.1 Solar Cells 35 4.2.2 Maximum Power Point Tracker 37 4.2.3 Battery Bank 39 4.2.4 Active Cooling Device-Dc Blower and AC Blower 40 4.2.5 Solar Lamp 42 Chapter 4 4.3 4.4 Experimental Measurements 42 4.3.1 Data Logger and 20 Channels Multiplexer 42 4.3.2 Pyranometer 43 4.3.3 T-type Thermocouple 44 4.3.3.1 Ambient Temperature 44 4.3.3.2 Temperature Difference Across the PV Panel 46 4.3.3.3 Inlet and Outlet Air Temperature 47 4.3.4 Anemometer 47 4.3.5 Shunt Resistor 48 Experimental Procedures 49 Mathematical Formulation 51 5.1 Description of the numerical simulation model 52 5.2 Assumptions of the numerical simulation model 52 5.3 The analysis of heat transfer on Photovoltaic cell 52 5.4 Meteorological data of Singapore 63 Chapter 5 iii Results and Discussion 66 6.1 Thermal performance 67 6.2 Electrical performance 78 6.3 Comparison of experimental and simulated results 99 Chapter 6 Chapter 7 Conclusion 104 Chapter 8 Recommendation 106 References 109 Appendices 117 Appendix A Manufacturer’s Specifications 117 Appendix B Calibration of T-type thermocouple 121 Appendix C Derivation of the result 126 Appendix D Process log of Simulation 127 iv Summary This thesis discusses aspects of a photovoltaic/thermal (PV/T) system which has been designed to produce both electricity and hot air concurrently. Experiments were conducted under outdoor conditions to determine the influence of the temperature of the PV cell on the PV conversion efficiency. At higher operating temperatures of the PV module, the conversion efficiency of the module can be drastically reduced due to the significant reduction in the open circuit voltage of the photovoltaic cell. For this reason, the payback period of the PV system is extended and the lifespan of the PV module may also be shortened. In order to resolve this problem, several different cooling techniques can be utilised to more effectively dissipate the heat from PV module. In this work, forced convective air cooling is utilised to reduce the operating temperature of the PV module. It was found that without active air cooling, the temperature of the PV module was high and solar cells could only achieve a conversion efficiency of only 8 to 9%. However, when the PV module was operated under active air cooling condition, the temperature dropped significantly, leading to an increase in the efficiency of solar cells to between 12 and 14%. The heat which was extracted from the PV module by the cooling air can contribute to the overall energy output of the system. Hence, the overall v system efficiency is no longer only limited by PV conversion efficiency but also include the thermal efficiency which ranges between 45 to 55 %. A variable speed blower is studying the effect of flow rate on the electrical efficiency of PV module. The results showed that the optimum flow rate of this system is around 0.055 kg/s. The flow field analysis of a parallel array of ducts with inlet/outlet manifold was simulated using the commercial computational fluid dynamic (CFD) package – Fluent. The simulation results showed that with a properly designed manifold, a uniform flow distribution can be obtained. Uniform flow field can evenly dissipate the heat from the PV module and reduce the occurrence of hotspots. A mathematical model has been developed to investigate the heat transfer performance of the PV module under actual meteorological conditions. The absorptivities and transmittivities of the cover glass, Ethylene-vinyl acetate (EVA), silicon cell are also considered in the numerical simulation. The simulation results showed good agreement with the experimental results. vi List of Tables Table 5.1 Thermal Properties of the Material 62 Table 6.1 Thermal efficiency for different flow rate 76 List of Figures Figure 1.1 World energy demand 1 Figure 2.1 Water PV/T collector 7 Figure 2.2 Water and air mixed-type PV/T collectors 8 Figure 2.3 Hybrid PV/T system schematic 11 Figure 2.4 Monthly changes of available energy gain by exergetic evaluation on Electrical 12 Figure 2.5 Monthly changes of available energy gain by exergetic evaluation on Thermal 12 Figure 2.6 (a) Cross-sectional view of unglazed PV/thermal air (i) with tedlar (Model I) (ii) without tedlar (Model II). (b) Cross-sectional view of glazed PV/T air (i) with tedlar (Model III), (ii) without tedlar (Model IV) 17 Figure 2.7 (a) Hourly variation of electrical efficiency for a, b, c, d type weather conditions considering glass to glass PV module with duct 18 Figure 2.8 Daily average of electrical efficiency for a, b, c, d type weather conditions considering glass to glass PV module with duct. 18 Figure 2.9 Schematics of the double pass photovoltaic thermal solar collector 19 Figure 2.10 Schematic representation of the reflector assembly for a collector unit 20 Figure 2.11 PV module electrical efficiency as function of its operating temperature for the typical and the combined with diffuse reflector mode 25 vii Figure 3.1 FLUENT results 27 Figure 3.2 FLUENT results (cont’d) 28 Figure 3.3 3D model of parallel array air duct. Red arrows show the direction of air flow 29 Figure 3.4 Engineering sketch drawing 30 Figure 4.1 Photograph of the outdoor transient testing set up 32 Figure 4.2 Schematic diagram of the experimental set-up 34 Figure 4.3 Polycrystalline Silicon Photovoltaic Cell 35 Figure 4.4 Structure of Photovoltaic Panel 35 Figure 4.5 The working wavelength of different type of solar cells 37 Figure 4.6 IV Curve and the maximum power point 38 Figure 4.7 MPPT Solar Charger Controller 39 Figure 4.8 Deep Cycle Gel Battery 40 Figure 4.9 DC Blower 41 Figure 4.10 AC Blower 41 Figure 4.11 Solar lamp 42 Figure 4.12 Hewlett-Packard data logger 43 Figure 4.13 20-channel relay multiplexer 43 Figure 4.14 Eppley pyranometer 44 Figure 4.15 T-type Thermocouple 44 viii Figure 4.16 T type thermocouple miniature connector 45 Figure 4.17 Location which put the thermo probe 45 Figure 4.18 The arrangement of T-type thermocouple 46 Figure 4.19 Inlet thermal Probe 47 Figure 4.20 Outlet Thermal Probe 47 Figure 4.21 Anemometer 48 Figure 4.22 Voltage of the PV panel can be measured directly by connecting to datalogger. 49 Figure 5.1 Diagram of principal reflections, absorptions and transmissions for a silicon PV cell imbedded in EVA 53 Figure 5.2 Friction Factor under different Air Flow velocity 59 Figure 5.3 Heat Transfer Coefficient under different Air flow velocity 60 Figure 5.4 Solar Irradiation of Simulation 64 Figure 5.5 Ambient temperature of Simulation 65 Figure 6.1 Irradiation and Average Panel Temperature for the whole day under cooling condition (23 September 2009) 67 Figure 6.2 Irradiation and Average Panel Temperature for the whole day without cooling condition (28 September 2009) 68 Figure 6.3 Module Temperature as a function of solar irradiation 69 Figure 6.4 Temperature profile at centre the duct PV module. 69 Figure 6.5 Temperature profile at side of duct. 70 Figure 6.6 Top view of velocity contour of manifold design 71 ix Figure 6.7 Cross section view of velocity contour of manifold design 71 Figure 6.8 Top view of the pressure contour of manifold design 72 Figure 6.9 Temperature profile of the front glass of module 73 Figure 6.10 Temperature profile of inlet and outlet flow 74 Figure 6.11 Variation of temperature difference (To-Ti) with incident radiation for flow rate 0.0389 kg/s and 0.0932 kg/s 75 Figure 6.12 Thermal efficiency as a function of (Ti-Ta)/G 76 Figure 6.13 Influence of flow rate on thermal efficiency 77 Figure 6.14 Electrical efficiency as a function of PV temperature at irradiation at 1000W/m2 78 Figure 6.15 Electrical efficiency as a function of PV temperature at irradiation at 250W/m2 79 Figure 6.16 A comparison between theoretical and experimental results 81 Figure 6.17 Influence of flow rate on electrical efficiency 82 Figure 6.18 Influence of temperature difference (To-Ti) on electrical efficiency for different flow rate 84 Figure 6.19 PV electrical power output under different solar radiation 89 Figure 6.20 Solar radiation of the entire day and the corresponded PV current due to the solar radiation (23 September 2009) 89 Figure 6.21 Solar irradiation and the PV Voltage for the entire day (23 September 2009) 90 Figure 6.22 Solar radiation and the PV Voltage for the entire day (8 June 2009) x 91 Figure 6.23 Solar radiation of the entire day and the corresponded PV current due to the solar radiation (8 June 2009) 91 Figure 6.24 Battery and blower voltage of partially discharged battery bank (23 September 2009) 92 Figure 6.25 Battery and blower voltage of fully charged battery bank (8 June 2009) 93 Figure 6.26 PV current generated by module in case: (a) partially discharged battery and (b) fully charged battery 94 Figure 6.27 Electrical Efficiency of fully charged and partially discharged at the similar meteorological condition 95 Figure 6.28 Input solar radiation and thermal and electrical energy production over five days 96 Figure 6.29 Electrical and thermal energy and the total energy gain over the five days 96 Figure 6.30 A comparison of thermal and electrical efficiency over 5 days. 97 Figure 6.31 A comparison of simulation and experiment in the temperature profile of the back of PV module 100 Figure 6.32 A comparison of simulation and experiment in the temperature profile of the front of PV module 100 Figure 6.33 Temperature contour of the PV cell at 1:30 pm (highest solar radiation on that day) 101 Figure 6.34 Temperature gradient of the PV module at 1:30 pm 102 Figure 8.1 Spectral absorption coefficient of pure water (solid line) and of pure sea water (dotted line) as a function of wavelength. 107 Figure 8.2: Transparent water passage in front of the PV panel to pre-filter the solar irradiation before it strikes the solar cell. 108 xi Nomenclature A area of the PV module m2 Axs flow cross-sectional area m2 c light speed m/s cp specific heat capacity under constant pressure J/kg·k D distance from sun to the earth m Ec net energy absorbed by the cell W/m2 Ece electrical energy produced by photovoltaic cell W/m2 Ect thermal energy released by photovoltaic cell W/m2 ET rate of solar energy absorbed by Tedlar W/m2 F Radiant Flux Density W/m2 FF Fill factor -- G solar irradiation W/m2 Ho Solar Constant W/m2 Hsun radiation intensity W/m2 h Planck’s constant J·s hc convection coefficient of air W/m2·℃ hg convective heat transfer coefficient of the glass W/m2·℃ I current A IL light generated current A I0 dark saturation current of diode A IMP maximum current A Isc short circuit current A K Boltzmann’s constant J/K k thermal conductivity W/K·m xii m mass flow rate kg/s Nu Nusselt number -- n ideality factor -- P wetted perimeter m p cell packing factor -- Pel electrical power ouput W Pr Prandtl number -- q electron charge C qc the heat which convected away by the air flow W/m2·℃ Re Reynolds number -- Rsun radius of Sun m T temperature ℃ Ta ambient temperature ℃ (Ta-6℃) sky temperature ℃ Tb temperature of backsheet ℃ Tc cell temperature ℃ Tg glass temperature ℃ Ti inlet temperature of the air flow ℃ To outlet temperature of air flow ℃ V voltage V VMP maximum voltage V Voc open circuit voltage V v wind speed m/s um mean fluid velocity m/s λ wavelength of incident ray μm xiii Greek Letters σ Stefan-Boltzman constant Wm-2K-4 ψB radiated energy Wm-2 αc cell absorptivity -- τg fraction transmitted through the front glass -- ηo nominal electrical efficiency under standard condition -- ηe cell electrical efficiency -- β temperature coefficient of silicon cell C-1 αT absorptivity of the Tedlar -- εg emittance of the glass -- αg absorptivity of glass -- θ module inclination to the horizontal -- v kinematic viscosity m2s-1 μ dynamic viscosity kgm-1s-1 α thermal diffusivity Jm-3k-1 ηtotal total efficiency -- ηth thermal efficiency -- Subscripts and superscripts a ambient b backsheet c cell e electrical g glass h hydraulic xiv loss losses m mean MP maximum power oc open circuit pv Photovoltaic s sun sc short circuit SH shunt T Tedlar th thermal xv CHAPTER 1 INTRODUCTION 1.1 Energy Today Energy is currently an important issue all over the world. The demand for fossil fuel has grown steadily due to increased industrial activities in developing and developed countries. It is estimated that the world energy demand will increase by 45% between 2006 and 2030, and the rate of increase will be 1.6% per year [1]. Fig 1.1 shows the estimated world primary energy demand from 1980 to 2030. Figure 1.1 World energy demands [1] In general, fossil fuels such as oil, natural gas and coal can be considered as primary sources of energy, especially, oil is the dominant fuel of the world. The 1 increase of the energy demand may be met by utilizing fossil fuel resources but the amount of greenhouse gas emissions in the atmosphere will reach a dangerous level. The WG 3 – Mitigation of Climate Change [2] indicated that over the last three decades, greenhouse gases emissions have increased by an average 1.6% per year with carbon dioxide (CO2) emissions from the use of fossil fuels growing at a rate of 1.9% per year. According to the fourth assessment report from 2007 Inter-governmental Panel on Climate Change [3], the increases of sea level are consistent with global warming. Furthermore, global average sea level rose at an average of 1.8 mm per year over 1961 to 2003 and at an average of about 3.1 mm per year 1993 to 2003. The rise of the sea level is attributed to the melting of snow and ice in the Arctic Sea due to the global warming effect. Renewable energies including solar energy, wind power, hydropower, biofuel, geothermal energy are suggested to provide a solution to resolve the global warming problem and alleviate the potential of energy crisis. The demand of fossil fuels will be reduced when the renewable energies become popular in the energy market. Furthermore, potential climate change will be mitigated when the renewable energies replace fossil fuels in the future. Solar energy is one of the most promising energy sources with solar radiation reaching the earth’s surface at a rate approximately 80,000 TW and this figure is more than 10,000 times the present consumption of energy in the 2 world. 1.2 Solar Energy 1.2.1 Fundamental The surface temperature of the sun is 5778 K. The core temperatures of sun reach over 15 million K and the energy of the sun comes from nuclear fusion reaction from H to He that take place deep inside the sun’s core [4]. The sun can be considered as a blackbody radiator at the surface temperature. According to the Planck’s radiation law for blackbody, the solar constant is approximately to 1368W/m2. The solar constant is defined as the incoming power that Sun would deposits per unit area that is directly exposed to sunlight. To harness the great amount of solar energy, solar thermal collectors and solar photovoltaic cells have been used to convert the solar energy into heat and electricity for various applications. 1.2.2 Solar Thermal Collector Solar thermal energy can be interpreted as direct conversion of the energy from solar radiation to useful thermal energy. The heat is generated by the absorption of sun’s ray through a dark coated material, called absorber. The absorber actually is a system of pipes filled up with a heat transfer medium, and the medium flows to the collector to collect the heat from sun’s ray and goes back to the hot water store. In some systems, the heat exchanger is used to extract heat from the water-glycol mixture 3 that is circulated in a closed circuit; is called an indirect system. Other systems, in which pure water is used as the heat transfer medium, are called drainback systems. Flat plat collectors are utilised chiefly for domestic hot water heating for showering, washing and some industrial applications. The efficiency of a solar collector drops drastically at high temperatures due to heat losses from the large surface area of collector. Evacuated tube collectors are always utilized for applications at high temperature and they are very efficient since they contain several rows of glass tube and the air in the glass also removed from it to reduce the heat loss through the convection effect. Therefore, the collector can operate at high efficiency and high temperature. 1.2.3 Solar Photovoltaic cell The Photovoltaic cell is made of semiconductor materials and used to convert sunlight into direct-current electricity. When light with wavelengths less than 1100nm strikes a PV crystalline cell, electron hole pairs are created in the cell, the electric field sends the electrons from p-type to n-type silicon and the holes from n-type to p-type. Disruption of electrical neutrality occurs during the photovoltaic effect. An external load is needed to restore the equilibrium. The external load will provide a current path which allows electrons to flow from n-type to p-type silicon; electron recombines with the hole when it reaches the p-type silicon. The photocurrent is generated when the 4 electron passes through the external load. The electrical efficiency of the PV cell is significantly affected by the operating temperature. The electrical efficiency of PV cell linearly decreases when the operating temperature increases, which is an advantage of the PVT system. 1.2.4 Photovoltaic Thermal (PV/T) System A photovoltaic/thermal hybrid system (or PVT system) is a combination of photovoltaic and solar thermal system. The PVT system can produce both electricity and heat simultaneously. The PVT system refers to a system that extracts heat from the panel with using heat transfer fluid, usually water or air and sometimes both. There are several reasons which motivate the development of the PV/T system. One of the main reasons is that PV/T system can provide higher efficiency than individual PV and thermal collector system. With increased the efficiency, the payback period of the system can also be shortened. 1.3 Objectives The objectives of this study are 1. To design a manifold to ensure uniform flow distribution. 2. To investigate how active air cooling affects photovoltaic module performance. 5 3. To find the optimum flow rate for the PV/T system under operation. 1.4 Scope Chapter 1 gives an introduction and brief discussion of the importance and potential of solar energy and some solar technologies. Chapter 2 is a literature review of water and air cooled PV/T and includes a summary of the state of the art of the water and air cooled PV/T. The characteristics of the PV/T will also be investigated in detail. Since the PV/T-air cooled system is discussed in this project, the detailed description of the manifold design, which used in current experiment is provided in Chapter 3. In Chapter 4, the components and functions of the system are described clearly. Chapter 5 presents the mathematical formulation of the heat transfer on Photovoltaic cell. The results of experiment and simulation are discussed in Chapter 6. The experimental data is categorized into 3 parts, thermal performance, electrical performance and the comparison of experimental and simulation result. Chapter 7 provides conclusion to the entire study and discusses the overall performance of the experiment. Chapter 8 provides some ideas, which may significantly improve the overall performance of the PV/T system. 6 CHAPTER 2 LITERATURE REVIEW Nowadays, for the PV/T system applications, production of the electricity becomes more important. Therefore, it is necessary to keep the operating temperature of the PV module as low as possible to ensure that its conversion efficiency is maintained within an acceptable range. In these couple of years, PV/T-air and PV/T-water systems have been widely investigated and different kinds of configurations developed to test the overall performance of the combined system. Numerical simulation of PV/T systems has also provided more detailed information on the performance of the system. 2.1 Water cooled PV/T Figure 2.1 Water PV/T collector [15]. 7 Figure 2.2 Water and air mixed-type PV/T collectors [5]. Figure 2.1 shows the common configurations of current PV/T systems in use today nowadays. Water and air are the most common media utilised rather than refrigerants since the overall cost of the entire system increases due to the capital cost and maintenance cost for the refrigerant loop. Generally, water is the most effective fluid to collect the heat from the PV panel and absorber due to its high heat capacity and thermal conductivity. Basically, water type PV/T can be categorized according to the water flow pattern as shown in Fig 3.2. The parameters involved in the design are the sheet, tube, free flow, channel and absorber types [6]. Numerical analysis is more preferable in investigating the preliminary studies since it can provide an optimum model before fabricating the prototype. The first mathematical model of PV/T collector was 8 published by Florschuetz [7]. He modified the Hottel-Willier [8] analytical model for flat plat thermal collector in order to apply the equations to PV/T collectors. Some parameters (such as heat removal factor and collector efficiency factor) of the Hottel-Willier model are still available to be utilised in the PV/T collectors. A dynamical model and three steady state models have been investigated by Zondag [9]. He also carried out a prototype experiment to validate the simulated result generated by his model. All models show good agreement with the experiment within 5% accuracy. Sandnes and Reskstad [10] have developed a polymer solar collector which combines with crystalline silicon PV cell in a hybrid generating unit. This model was developed by modifying the Hottel and Willier model for flat plate thermal collector. Their experiments show that attaching PV cells onto an absorbing surface reduces the solar energy absorbed by about 10%. This is because that the absorptivity of PV cell is lower compared to the black absorber. Zakherchenko [11] showed the importance of having good thermal contact between the solar cells and thermal absorber. Their study indicates that some commercial PVT modules should not be used directly in PVT system. Huang et al. [12] investigated the performance between the integrated photovoltaic and thermal solar system IPVTS and conventional solar water heater. A corrugated polycarbonate panel was used to make the solar PV/T collector and the characteristic daily thermal 9 efficiency and primary-energy saving of the collector is 38% and 60%. A hybrid photovoltaic/thermal water-heating system with natural circulation was constructed by Jie Ji et al. [13]. Their experiment results showed that the characteristic daily primary-energy saving could reach up to 65% for this system. The simulated result also showed that the higher the packing factor and glazing transmissivity, the better is the overall system performance. Wei He et al [14] indicated that a good thermal-contact between the absorber and the PV module can significantly increase both thermal and electrical efficiency of the system. Fin performance of the heat exchanger is also a crucial factor to boost the overall efficiency. Chow [15] presented an explicit dynamic model for operation of PV/T collector since it is not suitable to use a steady state model to predict the working temperatures of the PV module and the heat removal fluid was also under fluctuating irradiance or intermittent fluid flow. For that reason, the transient case can more accurately predict the outcome of experiments. That model was developed based on the control-volume finite difference approach. The proposed model can provide a detailed analysis of the transient energy flow through different types of collector components and the instantaneous energy output can also be monitored. A simulation of PV/T system was carried out by using the well known TRNSYS program by Kalogirou [16]. They used the typical meteorological data of Cyprus and 10 the optimized water flow rate via simulation. The system consists of a series of PV panels, a battery bank, a hot water storage cylinder, a pump, a differential thermostat and an inverter (Fig 2.3). Figure 2.3 Hybrid PV/T system schematic diagram [19] Fujiwa and Tani [17] used exergy analysis to evaluate the experimental performance of a designed PV/T system since exergy can be used to qualitatively compare the thermal and electrical energy based on the same standard. 11 Figure 2.4 Monthly changes of available energy gain by exergetic evaluation on electrical [17] Figure 2.4 shows that the coverless PV/T collector produces the highest electrical exergy and Figure 2.5 shows that thermal exergy of the coverless PV/T was the lowest amongst the system considered. The latter may be due to heat losses from the top of the device. Figure 2.5 Monthly changes of available energy gain by exergetic evaluation on thermal [17] 12 The thermal exergy with monthly changes is presented in Fig 2.5.Flow rate affects the performance of PV/T system since the increase of water velocity in the tube will result in increasing the heat transfer coefficient. This will help enhance the cooling on the PV panel or collector. Bergene and Lovvik [18] investigated the relation between the geometric parameter W/D and the performance of the PV/T system. They reported thermal efficiency increases by a factor of 0.1 and the flow fate increases from 0.001 to 0.0075kg/s. Chow [15] also found that when flow rate in the tube increases from 0.002 to 0.016 kg/s, the electrical and thermal efficiencies also increase. Garg and Agarwal [19] utilised the finite difference method to investigate PV/T system with different solar cell areas and flow rate. The system comprised of a storage tank, pump, differential controller and PV modules. The optimum flow rate of this experiment was 0.03kg/s, for maximum thermal efficiency. It was shown that the electrical efficiency decreased at this flow rate and was minimum when the insolation was maximum (as the temperature of absorber is maximum). Nishikawa et al. [20] utilised the refrigerant R22 as the liquid of PV/T collector which function as the evaporator of a heat pump. A high COP was observed when the system is efficiently cooled. Ito et al [21] showed that the COP of heat pump is very low when it is under low irradiance. This is because the flat plat collector is not 13 optimised to extract the energy from the surrounding. In order to solve this problem, a 3.24 m2 multiple-fin evaporator was placed in parallel with a 2.45m2 PV/T absorber and this resulted in an increase of COP from 2 to 3 under low irradiance conditions. However, the COP of the system can attain a value of 6 when it is under high irradiance. Zondag et al [22] and Jong [23] have conducted a series of comparison between different types of PV/T design and different types of thermal systems. Those experiments generally investigated the covered and uncovered PV/T and thermal system with and without heat pump. The studies indicated that an uncovered PV/T shows improved efficiency for the case which the PV/T is utilised for low-temperature ground storage integrated with a heat pump. The high thermal efficiency of the system is because that the inflow air is always kept in low temperature. However, the net electrical efficiency of the system turns into negative because of the energy consumption of the heat pumps. Currently, PV/T systems are always installed for residential use. In order to investigate the actual condition of the residential building, the PV/T systems were installed on the roof top of a residential building. Ji et al [24] installed a 40m2 PV/T collector on a facade of the residential building in Hong Kong in order to investigate the difference between the thin film and crystalline silicon PV cell. Under the same 14 meteorological condition, it was found that the thermal efficiency of the thin film is 48% which that of the crystalline silicon is 43%. They also proposed that the system can also be utilized for pre-heating of hot water for residents in that building. Furthermore, the systems can also provide cooling for the building with absorption of heat by the wall of building reduced during the operation of PVT system. It was concluded that the hybrid system has potential to be widely advocated in a sub-tropical city such as Hong Kong. Elswijk et al [25] also claimed that PVT collector arrays installed on multi-family buildings could save about 38% in area. This is very vital due to the availability of the roof top space per house. The disadvantage of this system is that the shading angle of PVT collector must be smaller than the conventional solar thermal collector because of the shading effect. 2.2 Air cooled PV/T The first PV/T air facility was built in 1973 at the University of Delaware. This PV/T air facility was called as ‘Solar House’ and the air collectors were integrated in the roof top and façade of the house. Besides, one fourth of the collectors were embedded with CdS/Cu2S cell to generate electrical energy. After the pioneering work of University of Delaware, some laboratories such as the MIT Lincoln laboratory, 15 Sandia laboratory and Brown University also started developing the PVT air collectors. The performance of PVT air collectors fabricated by ARCO and Spectrolab [26] was insufficient but this first generation technology has become a motivation to boost the development of second generation technology. The effect of thermal gradient on electrical efficiency of PV panel was investigated by the Sandia [27]. In 1994, the French Company Cythelia [28] developed a PV-air collector, called the Capthel collector. An unglazed PVT collector with air and liquid heat extraction was developed and commercialised in Israel [29]. A new type PVT-air collector was developed by the German Company Grammer Solar and the Danish company Aidt Miljo [30,31]. This type of PVT air collector is only covered with a small PV cell which used to drive the fan. The function of this system was utilised for dehumidification purposes in vacation cottages. Both experimental and numerical simulation were implemented by Tiwari [32] to evaluate the overall performance of PV-T air collector. In this study, different kind of configurations of PVT air collector (like unglazed, glazed, with and without tedlar) which shown in Fig 2.6 were used to investigate the electrical and thermal performance. It was shown that the glazed PVT air collector without tedlar provides the best performance. 16 Figure 2.6. (a) Cross-sectional view of unglazed PV/thermal air (i) with tedlar (Model I), (ii) without tedlar (Model II). (b) Cross-sectional view of glazed PV/thermal air (i) with tedlar (Model III), (ii) without tedlar (Model IV). A PVT-air collector was investigated by Garg and Adhikari [33] using a computer simulation model. It was concluded that the thermal efficiency of the absorber without solar cell is higher than that when the absorber is covered with the solar cell. This is because that some of the incidence irradiance is converted into electrical energy. Dubey et al [34] reported the efficiency of different configurations of PVT-air collector (Case A-Glass to glass PV module with duct, Case B-Glass to glass PV module without duct, Case C-Glass to tedlar PV module with duct, Case D-Glass to tedlar PV module without duct). It was indicated that case A can give the highest efficiency among the all four cases. The annual average efficiency of case A and B is 10.41% and 9.75%, respectively. The daily average electrical efficiency of the four cases are 17 presented in Figs 2.7 and 2.8 Figure. 2.7. (a) Hourly variation of electrical efficiency for a, b, c, d type weather conditions considering glass to glass PV module with duct. Figure 2.8 Daily average of electrical efficiency for a, b, c, d type weather conditions considering glass to glass PV module with duct. Hegazy [35] did a comparative study of the performance on four types of PVT solar air collectors. Their results showed that the air flow on both sides of the absorber in a single pass demands the least fan power. 18 Figure 2.9 Schematic diagram of a double pass photovoltaic thermal solar collector Sopian [36] developed a double pass PVT air collector (Fig 2.9) for solar drying application. Solar cells were put between the glass cover and absorber plate. The air first enters the channel created by the glass cover and photovoltaic panel and next it enters the channel created between the photovoltaic panel and absorber. This configuration can greatly reduce the heat loss and increase its thermal efficiency. The thermal efficiency of this system can reach to 60%. Some simpler modifications were utilised to enhance the thermal performance of the air duct and this will help enhance heat extraction from the PVT air collector. Prasad and Saini [37] reported that the heat transfer mechanism of the solar collector can be enhanced by artificially increasing the roughness of absorber plate and wall of the channel, leading to higher thermal efficiency. However, high roughness of wall and absorber will induce a higher friction factor and therefore a higher pumping 19 power is needed. Han et al [38] and Gupta et al [39] showed that several types of ribs in the air channel can provide better performance in heat extraction but it is also accompanied by a significant increase in friction losses. Some modifications like using the pins, matrices, porous materials and perforated plates were suggested to improve the heat extraction in the air channel. However, most of them are not practical to significantly enhance the overall system performance. Garg and Datta [40] suggested several practical modifications to enhance the heat transfer in air duct. In the study, both experiment and numerical simulation were undertaken and the agreement between the theoretical predictions and experimental results has been satisfactorily. Figure 2.10 Schematic representation of the reflector assembly for a collector unit. Garg et al. [41] presented a study of a PVT air hybrid system, this system 20 comprised a plane booster and a flat plat collector mounted with photovoltaic cells (Fig 2.10). It was concluded that the electrical efficiency of photovoltaic cell will linearly decrease with increase of the absorber temperature. The results also indicated that the minimum area of photovoltaic cell needed to operate a pump at a given flow rate is a function of time. The plane boosters were utilised to reflect the extra incident rays to the photovoltaic cell in order to increase the intensity of sunlight on the photovoltaic module. Optimization the absorber geometry for solar air heating collector has been investigated by Pottler [42]. It was reported that the optimized distance between the fins is about 5 to 10 mm. The thermal efficiency of the collector can attain to 77% with optimized geometry. As the pressure drop increases drastically with decreasing fin spacing, this factor should also be considered in the design. Naphon [43] carried out a study on the performance and entropy generation of the double pass solar air heater with longitudinal fins. The study showed that the thermal efficiency increases with increasing flow rate as the heat transfer is proportional to the mass flow rate. The number and height of fins will also increase the heat transfer rate due to the increase of heat transfer area. Hence, the thermal efficiency is proportional to the number and height of fins. However, the entropy generation was found to decrease with increasing height of fins. This is because the outlet temperature increases 21 with increase of height of fins. Tonui and Tripanagnostopoulos [44] also reported an improvement of heat extraction achieved by modifying the channels of PV/T air system in low cost. Three different configurations of air ducts (simple air channel, thin aluminum sheet and rectangular fin) were investigated by experiment and numerical simulation. Some parameters (channel length, channel depth and mass flow rate) were used to study the effect on electrical and thermal efficiency. From the result of experiment and simulation, a good agreement has been presented and air duct with fins were shown more effective in enhancing the heat transfer from the wall of the channels to air flow. Sopian et al [45] presented a steady state simulation of the single and double pass combined photovoltaic thermal air collector. The simulations indicated that the double pass photovoltaic thermal collector has superior performance during the operation. The difference of thermal efficiency for single and double pass combined photovoltaic thermal collector is about 10%. The air flow in the double pass combined thermal collector can absorb more thermal energy than that in the single pass. Therefore, the thermal efficiency of the double pass is higher than that of the single pass. Due to the large amount of heat absorbed by the air flow, the temperature of the photovoltaic module decreased significantly and this causes the electrical efficiency of the double pass was higher than single pass as well. 22 Garg and Adhikari [46] developed a simulation model to investigate the performance of single glass and double glass hybrid photovoltaic thermal air heating collector. The thermal performance of the double-glass configuration was found to be better than single glass for a normal black paint absorber. This is because the extra layer of glass can reduce the radiative losses from absorber to glass cover. For a selective absorber, the thermal efficiency of single-glass is higher than that of double-glass as the effective transmittance-absorptance product also decreases when the sun ray passes through the double-glass. The parametric studies showed that the overall system efficiency increases with the increase in cell density, collector length and mass flow rate. However, the increase of duct depth will incur the decrease in system efficiency. Joshi et al [47] carried out an evaluation of a hybrid photovoltaic thermal system. Two types of PV module (glass to tedlar and glass to glass) were utilized to investigate the performance under the climate of New Delhi. The results showed that the overall performance of hybrid thermal collector with PV module glass-to-glass is better than glass-to-tedlar. Parametric studies also indicated that thermal efficiency decreases with the increase of length of the duct. It is because the thermal energy which can be extracted at the back of PV module decreases. The highest thermal efficiency obtained from the experiment was 46.28%. Thermal efficiency also increases with air velocity. 23 However, as the air velocity exceeds a certain level, thermal efficiency remains at a constant level. This could be explained as the time of contact of air with module reduces and therefore decreases the heat removal from the back of PV module. Tripanagnostopoulos et al [48] presented a hybrid PV/T experimental model to investigate the temperature effect on PV electrical efficiency. A booster diffuse reflector was also utilized to enhance the electrical and thermal performance of the system. It was found that PV electrical efficiency decreases at the rate of 0.1%/℃. However, with the diffuse reflector, the electrical efficiency decreased at the rate of 0.0957%/℃ and 0.0814%/℃ for concentration factor at 1.3 and 1.5 respectively. In this study, a comparison between water cooled and air cooled PVT were presented. The PV module with thermal insulation leads to high temperature and incurs an electrical efficiency drop (ηel/insul=0.113), and water cooled PV and air cooled PV with ηel/water=0.128, ηel/air=0.126, respectively. 24 Figure 2.11 The PV module electrical efficiency as function of its operating temperature for the typical PV and combined PV with diffuse reflector mode Tripanagnostopoulos [49] also showed that the electrical efficiency of PV module increases by 2% with using the diffuse reflector and without incurring significant penalty in temperature rise. The decreasing rate of temperature effect in electrical efficiency was also found to be 0.1%/℃. 25 CHAPTER 3 DESIGN OF MANIFOLD A parallel arrangement of air ducts underneath the PV panels is used to create the passage for the air to pass through. Fins are incorporated in the duct to increase the heat transfer rate from the PV panel to the moving fluid. Non-uniform air flow usually causes in recirculation and therefore the hot air will be trapped in the channels. This causes the panel temperature to be unevenly distributed. A hot spot will result in the panel and the electrical efficiency decreased due to the uneven temperature distribution of panel. A series of simulations with different configurations of air duct is done using the computational fluid dynamics (CFD) software, FLUENT. The results of the simulation are presented in the Figs 3.1 and 3.2. 3.1 Simulation of Different Configurations From the results, it can be seen that the re-circulated flows are significant in the C shaped and S shaped ducts. These flows will result in the fluid flowing back towards the inlet instead of the outlet, and this reverse flow will be heated up. The flow will cause the PV module which attached atop of manifold having an uneven temperature profile. 26 C-shaped duct with vanes at inlet C-shaped duct Duct with 1 inlet and 2 outlets V-shaped Figure 3.1. FLUENT results 27 S-shaped S-shaped duct with a protruding vane S-shaped duct with vanes at inlet C-shaped duct with a L-shaped Figure 3.2 FLUENT results (cont’d) 28 In the V shaped duct, the simulations indicate no reverse flow and most of the fluid flows through the centre channels resulting in the uneven heating of the PV modules. The latter will also seriously affect the overall performance of the PV module. 3.2 Manifold Design of Experiment Following the simulation results of the V shaped design, a 90° change in flow direction has been applied in current design. This will alleviate the focusing of fluid in the centre channels and recirculation flow can also be avoided in this design. Figure 3.3 3D model of parallel array air duct. Red arrows show the direction of air flow 29 Figure 3.4. Engineering sketch drawing 30 The manifold design shown in Fig 4.6 has a uniform flow field in the simulation. The simulation results are presented in Chapter 6. The engineering sketch drawings of the manifold design are presented in the Fig 3.4. All the needed dimensions and sizes are clearly shown in the sketches. The entire air duct was made of galvanized steel. Galvanized steel is the steel coated with zinc which protects the steel from corrosion. As the experimental set up was always locked on the roof top, it was important that this material could prevent the air duct from being corroded by daily exposure to the elements. The modules are incorporated in the air duct and fixed with screws. The design of air duct is also allowed for inserting the thermocouple in the centre and both sides of the ducts. During the experiment, gaskets were used to seal the gap between panel and panel in order to reduce the leakage of air. Isometric views of this configuration are presented in the Appendix. The entire air duct is put on a stainless steel rack. The height of this rack is around 1.5 meter, to avoid shading of the PV module during operation by other experimental set-ups on the roof top. Summarising, this design permits the flow to enter the ducts uniformly, hence obviating the potential hotspot problem. 31 CHAPTER 4 EXPERIMENTAL SET-UP 4.1 Description of the PV/T system A test set up was designed to investigate the thermal and electrical performances of the Photovoltaic thermal system. This system was built on the roof top of EA building at the National University of Singapore. The photograph of the set-up is shown in Fig 4.1. A schematic diagram of the complete experimental set-up is shown in Fig 4.2 Figure 4.1 Photograph of the outdoor transient testing set up 32 The current experiment is designed to investigate how the temperature affects the electrical efficiency and power output during the operation. Four polycrystalline solar panels were used in the experiment to generate the electricity. The electricity which generated by the solar panels will be stored in four deep cycle gel batteries. A direct current blower connected to the batteries, is used to extract surrounding air to cool the panels. During the operation, a maximum power point tracker (MPPT) was used to modulate the power output from solar panel to be the maximum value. Another alternating current (AC) blower is also used in this experiment because it can function as variable speed blower, so that the flow rate can be controlled by adjusting the knob of the controller. Solar irradiation was measured by the pyranometer, which was put at the same level as the solar panels. In this experiment, the air speed was measured by the anemometer and the temperature of air and PV module was obtained by using the T-type thermocouple directly connected to the datalogger. The voltage and current of the solar panels were directly recorded by the datalogger. The experiments normally operated from 8:30 am to 5:00 pm. In the experiment, PV current, PV voltage, temperature of modules, temperature of inlet and outlet, wind speed and irradiation of sunlight were collected during the operation of system. 33 Figure 4.2 Schematic diagram of the experimental set-up 34 4.2 Experimental Components 4.2.1 Solar cells Figure 4.3 Polycrystalline Silicon Photovoltaic Cell Neste polycrystalline solar cells are used in this experiment. The photovoltaic module consists of 36 cells, the open circuit voltage and short circuit current are 22.5 V and 3.47 A. At 1000 Watt/m2 and 25℃, the maximum power output of single module can reach 56.7 Watt/m2. Figure 4.4 Structure of Photovoltaic Panel 35 The structure of crystalline silicon solar cells is presented in Fig 4.4. EVA is a kind of copolymer of ethylene and vinyl acetate. The polymer encapsulant which used in PV modules serves to provide the functions like structural support, electrical insulation, physical isolation/protection and thermal conduction for the solar cell circuit [50]. The backsheet of photovoltaic module normally is a kind of material, called Tedlar. The function of Tedlar is to prevent the ingress of water of water vapour. It is a kind of polymer material, called Polyvinyl fluoride. Tedlar will also provide the functions like UV resistance, mechanical properties, strength and durability, resistance of weathering and electrical insulation. All of these functions will help PV panel to sustain at least 20 years and above. Part of the backsheet is normally made as a laminated film composite and the most common structure is the trilayer structure of Tedlar/Polyester/Tedlar, also called TPT. This kind of structure can enhance the functions of abovementioned. Fig 4.5 shows that the wavelength of polycrystalline solar cell working range is between 350 nm to 1200 nm [51]. Besides the transmission issues, the reflection of the front surface of PV panel should be low as well. A low iron glass is most usually used in the PV industry because it is of low cost, strong, stable, highly transparent, impervious to water and gases and the front contact glass also has 36 Figure 4.5 The working wavelength of different type of solar cells self-cleaning properties after raining. The specification of the Neste polycrystalline silicon solar panel used in this experiment is shown in the Appendix A. 4.2.2 Maximum Power Point Tracker (MPPT) A maximum power point tracker is utilised to maximize the power output of PV module and it is also a high efficiency DC to DC converter. Using a conventional charge controller to charge a discharged battery, it connects the PV modules to the battery directly, forcing the PV module to work at the battery voltage. Generally speaking, this voltage is always not the ideal voltage for the maximum power output of PV module. However, the Maximum Power Point tracker (MPPT) is not simply the bridge between the module and battery. The MPPT controller is able to calculate the voltage at which modules can 37 produce the maximum power output and the working voltage of PV module is at maximum power output voltage rather than battery voltage. If the whole system wiring is assumed to be 100% efficient, the battery charge current would be VMODULE /VBATTERY x IMODULE. The current which stored to the battery with using the MPPT controller rather than conventional converter will increase significantly. Fig 4.6, shows that there is always a single operating point which will produce the maximum power output of the PV cell. This point is called the maximum power point. Figure 4.6 IV Curve and the maximum power point The maximum power point tracker is used to seek this point in order to maximize the power output of the panels under the different irradiation. The power from the solar panel passes through the Maximum Power Tracker (MPPT), which modulates to the best level that the module can produce and converts it to get maximum current from the deep cycle battery. Fig 4.7 indicates the MPPT being used in this experiment. 38 Figure 4.7 MPPT Solar Charger Controller 4.2.3 Battery Bank Deep cycle gel batteries are very common in PV systems. It is designed to produce a consistent voltage when the battery discharges. Fig 4.8 shows the battery that was used in the experiment. During the discharging, the deep cycle battery can discharge down to around 20% of its charge capacity without deteriorating its performance. However, this kind of discharging cannot be applied to other types of batteries which are not designed for “deep cycle discharge”. As it will deteriorate the performance of the battery and also reduce the lifespan of the battery. In the experiment, 4 deep cycle gel batteries are connected in both series and parallel arrangements. Two 36 W solar lamp and a 24 W blower are utilised to discharge the battery to ensure that the battery is not fully charged. This is because once the batteries are fully charged, PV modules cannot properly produce the current from irradiation. 39 Therefore, battery voltage must always be monitored in the experiment. Solar Lamps are used to discharge the deep cycle gel batteries during the night. The specification of deep cycle gel battery is shown in Appendix A. Figure 4.8 Deep Cycle Gel Battery 4.2.4 Active Cooling Device-DC Blower and AC Blower In this experiment, a direct current (DC) blower and an alternating current (AC) blower are used to cool the PV modules. The power of the DC blower is supplied by the battery bank so that the rating of blower may be matched with the battery and PV modules. Besides, the size, weight and pressure drop must also be taken into account in choosing type of blower. A Sanyo Denki fan (Fig 4.9) which met the criteria is utilized in this experiment. The flow rate of this blower is 8.2m3/min, the relevant information is presented in Appendix A. 40 Figure 4.9 DC Blower Figure 4.10 AC Blower In order to investigate the effect of different flow rates of air passing through the duct, a variable speed AC fan (Fig 4.10) is needed in this experiment. The flow rate of this fan is between 2.09m3/min and 7.11 m3/min. 41 4.2.5 Solar Lamp A solar lamp is used to discharge the battery during the night. Basically, the lamps are used in street and pedestrian lighting and this type of lamp is known as Energy Saving Compact Fluorescent Lamps (CFL), 4 pin, Single U with lamp wattage of 36 W and rated light output of 2900 ± 5% lumens from Trilux-Lenze. A water tight canopy made of injection molded plastic is used to enclose the lamp to ensure that water will not permeate into the light. The ballast is only customized with 24 V lamp bulbs. The relevant information of solar lamp is attached in the Appendix A. Figure 4.11 Solar lamp 4.3 Experimental Measurements 4.3.1 Data Logger and 20 channels multiplexer A Hewlett-Packard data logger was used to record the readings at 1 minute interval. There are 20 channels for each multiplexer. In this experiment, first 15 42 channels are used to measure the temperature of PV modules, inlet temperature, outlet temperature and ambient temperature. The other 5 channels are used to capture the data in DC voltage mode. There are PV voltage, shunt resistor voltage, Battery voltage, Blower voltage and the Pyranometer voltage. Fig 4.12 and 4.13 are the data logger and multiplexer which being used in the experiment. Figure 4.12 Hewlett-Packard data logger Figure 4.13 20-channel relay multiplexer 4.3.2 Pyranometer Global radiation was measured with an Eppley pyranometer (Fig 4.14) (measures global radiation). This pyranometer is a World Meteorological Organization First Class Radiometer and it is designed for the measurement of sun and sky radiation. The 43 hemispheres of the pyranometer are made of clear WG295 glass. Hence, this instrument can measure the radiation in the spectral range 285 to 2800 nm. The response time of this instrument is 1s, therefore, the 1 minute interval which is used in our experiment is perfectly fine to get a stable value from the pyranometer. The instrument measures solar intensity in the range 0 to 2800 Watt/m2. Figure 4.14 Eppley pyranometer 4.3.3 T-type Thermocouple 4.3.3.1 Ambient Temperature Figure 4.15 T-type Thermocouple 44 Ambient temperature was measured using a T-type thermocouple. T-type thermocouples are suitable for measurements in the range at -200 to 350 °C. T-type thermocouples (Fig 4.16) comprise 2 wires, copper and costantan (copper-nickel alloy). A plastic connector (Fig 4.16) was needed in measuring the ambient temperature. Figure 4.16 T type thermocouple miniature connector Figure 4.17 Location which put the thermo probe The temperature probe was mounted beneath the duct (as shown on Fig 4.17), which is shaded from direct sunlight and rain but allows the circulation of air. A master thermometer was used to calibrate the thermometers in this experiment and the temperature range is 20 to 80 °C. The tolerance of T-type thermocouple is 0.5°C. The calibration curves are attached in Appendix C. 45 4.3.3.2 Temperature difference across the PV Panel To measure the temperature of the PV module, T type thermocouples were attached on the front and back of modules. There are 12 pieces of T-type thermocouples attached to the panels. The locations of the thermocouples are shown in Fig 4.18. Figure 4.18 The arrangement of T-type thermocouple Before doing the calibration of thermocouples, the constantan and copper wire of the T-type thermocouples need to be spot welded. The two wires then have a common joint, called the bead. During measurements, the bead must be well attached to the surface for more accurate temperature readings. 46 4.3.3.3 Inlet and Outlet air temperature Inlet and outlet temperature are used to calculate the thermal efficiency of the system. The inlet and outlet temperature were measured with the T-type thermocouple mounted in a probe inserted to the flexible hose. Figure 4.19 Inlet thermo probe Figure 4.20 Outlet thermo probe 4.3.4 Anemometer The wind speed was measured using an anemometer manufacture by Geneq Inc. This equipment was used to measure the wind speed for hourly during the experiment. A picture of the anemometer is given in Fig 4.21 47 Figure 4.21 Anemometer The volumetric flow of the system was also measured using an anemometer. As the cross section area of the flexible hose is fixed, in measuring the air flow speed, the flow rate can be obtained through the equation-Flow rate=air speed × cross section area. 4.3.5 Shunt Resistor To measure the current of the PV modules, a shunt resistor (Fig 4.22) with 0.006 ohm was used in the experiment. As the current produced by the PV modules already exceeds the working range of data logger, a shunt resistor was placed in series with the modules. The voltage across the shunt resistor was recorded by the datalogger and the current output of panels can be calculated by using the Ohm’s law shown in Eq (4.1): IPV=VSH ÷ RSH (4.1) 48 Figure 4.22 Voltage of the PV panel can be measured directly by connecting to datalogger. The electric circuit diagram of this experiment is presented in the Appendix A. 4.4 Experimental Procedures The performance of PVT system was monitored from Jun 2009 to November 2009. The following is the entire experiment procedure: (1) Switch on the mains in Thermal Process Lab 2 and to supply the power to Data logger and AC Blower. (2) Place the data logger inside the steel case and put the Pyranometer on the same level with the PV modules. (3) Set the channels of data logger into temperature and DC voltage and also set the interval of time to capture the data. (4) Set the speed of the blower to the flow rate which wants to be investigated. (5) Check that the gaskets of the system are well pasted to ensure that the leakage is very minor during the experiment. 49 (6) Check that all the thermocouples and the thermal probes are well attached. (7) Switch on the mains of the MPPT controller (8) Press the scan button on the data logger to start logging the data. 50 CHAPTER 5 MATHEMATICAL FORMULATIONS 5.1 Description of the numerical simulation model The analytical model of hybrid PV/T panel is based on the work of Raghuraman and Hendrie [52]. This simulation can be simply described as the heat transfer of photovoltaic modules under the solar irradiation. The cooling mechanism is utilised to reduce the temperature of the PV modules. The front glass surfaces of the photovoltaic modules are exposed to the surroundings and therefore radiation and convection need to be considered in the heat transfer analysis of the module. There are several layers of material in the PV panel (Fig 4.4). The Fourier conduction law can be implemented in analyzing the conduction heat transfer in between these layers. The back of photovoltaic panels is attached to the cooling duct. For that reason, forced convection is the main mechanism of heat transfer at the back of modules. This is a transient simulation and the solar irradiation and ambient temperature will be varied from time to time. The solar irradiation and ambient temperature is based on the experimental data obtained on 23 September 2009. 51 5.2 Assumptions In order to simplify the simulation model, the following assumptions are made: (1) Edge and back heat losses of the collector are neglected in the simulation studies. (2) The heat transfer in the collector is envisioned as two dimensional heat transfer process. (3) Only the single cell is simulated during the process. (4) Inter-reflections of insolation between the various surfaces are neglected. (5) The leakage of air from the collector is negligible. (6) The capacity effect of glass cover and enclosed air is also neglected. (7) An average wind speed was used to estimate the convection coefficient of collector. (8) The data of solar irradiation and ambient temperature from the experiment were used as the input of numerical simulation. (9) The ohmic losses in the solar cell are negligible. 5.3 The analysis of heat transfer on Photovoltaic Cell The net energy absorbed by the cell is: Ec = pα cτ g G (t ) (5.1) 52 where G (t) is the solar irradiation incident on the glass cover, p is the cell packing factor which defined as the ratio of area of solar cell to the area of blank absorber, αc is cell absorptivity to sunlight, τg is the fraction transmitted through the front glass and low iron glass was used in the experiment, τg =.0.95. For the wavelength less than 1.1μm the absorption length is less than the thickness of typical cell (ie: 260μm), hence the absorption process is completed before the radiation reaches the rear surface. Absorptivity of the silicon solar cell can be computed through Fig 5.1 αc=0.926×0.8+0.073×0.2 =0.7554 Figure 5.1 Diagram of principal reflections, absorptions and transmissions for a silicon PV cell imbedded in EVA [53] 53 The PV module used in this experiment is of gridded metal type; EVA absorptivity, 0.073 Polycrystalline silicon PV modules are used in the experiment. According to the Cox and Raghuraman [53] report, the insolation of wavelength above 1.1μm is transmitted through the silicon cell without any absorption and this is absorbed by the backsheet of PV module. The insolation absorbed by the solar cell can be converted into electrical and thermal energy and the equations are shown respectively below, Ece = η e pτ g I (t ) (5.2) Ect = (1 − ηe / α c ) pα cτ g G (t ) (5.3) where Ece is electrical energy produced by photovoltaic cell, Ect is thermal energy released by photovoltaic cell, ηe is the cell electrical efficiency and this parameter is functioned of the cell temperature. ηe = ηo [1 − β (Tc − To )] ηo = (5.4) Vm p I m p (5.5) GA 54 where ηo is the nominal electrical efficiency under standard condition, A is the area of the PV module, G is the irradiation and it is defined as 1000W/m2 for standard condition, Vmp is the PV voltage at maximum power point and Imp is the PV current at maximum power point. All the relevant data can be obtained from the specification of PV module which put in the Appendix., To is the temperature of standard condition, 25℃, Tc is the cell temperature, β is the temperature coefficient of silicon cell, β=0.0045℃-1. ET = τ g (1 − P )α T I (t ) (5.6) where ET is the rate of solar energy absorbed by Tedlar (Backsheet) after transmission from EVA, αT is the absorptivity of the Tedlar. Energy conservation laws are applied into the components of the collector and the equations are shown below: (1 − η e / α c ) pα cτ g G (t ) + τ gα T G (t )(1 − p ) = Eloss + qc (5.7) Eloss is the energy losses from the front glass to environment through the forced and free convection and radiation. 55 E = hg [Tg − Ta (t )] + ε gσ Tg 4 − α gσ [Ta (t ) − 6]4 (5.8) where Tg is the glass temperature, Ta is the ambient temperature, εg is the emittance of the glass, αg is the absorptivity of glass and (Ta-6℃) is assumed to be the sky temperature [54]. The solar collector was exposed to the ambient so that the heat loss is transferred by the top glass cover to the surrounding due to the combination of free and forced convection. Free convection is due to the air near the collector surface, which gets heated up producing the natural buoyancy force on the air. Forced convection is caused by the wind. Therefore, hg is the convective heat transfer coefficient of the glass to the environment and an empirical correlation from Stultz and Wen [55] report is used, hg = 1.247([Tg − Ta (t )]cos θ )1/ 3 + 2.658V (5.9) where hg is the convection coefficient of surface of collector, θ is the module inclination to the horizontal, V is the wind speed and assumed to be a constant speed, V=0.4m/s. 56 qc = hc (Tb − Tave ) (5.10) The energy balance of the air flow in the duct: • m cp dTair dx = qc dx (5.11) Where qc is the heat which convected away by the air flow in the channel, Tb is the temperature of backsheet and hc is the convection coefficient of air in the channel. Tave is the average temperature of inlet and outlet flow. To investigate the heat transfer of an internal flow within a duct, the flow condition (laminar or turbulent) is important to know and this information can be obtained through the equation below: Re D = um D (5.12) ν where um is the mean fluid velocity over the duct cross section, v is the kinematics viscosity of fluid and D is the hydraulic diameter. In a fully developed flow, to achieve a turbulence, the Reynolds number must somewhere between ReD=2300 and 10000. The smoothness of wall surface is also a factor to the affect the Reynolds number. For the hydraulic diameter, it is defined as 57 Dh = 4 Axs P (5.13) where Axs and P are the flow cross-sectional area and wetted perimeter. The Nusselt number Nu is a dimensionless measure to determine the convective heat transfer coefficient from the inside surface of a duct. It can be physically interpreted as the dimensionless temperature gradient at surface. NuD = hc D k (5.14) where hc is the heat transfer coefficient for convection, k is the thermal conductivity of the fluid, and D is the hydraulic diameter of duct. For fully developed turbulent flow, the Nusselt number is much more complicated to determine. Therefore, empirical correlation is always utilised to calculate the Nusselt number. A correlation, which is widely utilised and is attributed to Petukhov [56], is valid for 0.5＜Pr＜2000 and 104＜ReD＜5×106 NuD = ( f / 8) Re D Pr 1.07 + 12.7( f / 8)1/ 2 (Pr 2 / 3 − 1) (5.15) where Pr is the Prandtl number, which can be physically described as the ratio of the 58 momentum and thermal diffusivities, and shown below: Pr = Cpμ (5.16) k Where cp is specific heat capacity under constant pressure, μ is dynamic viscosity. f is the friction factor, which can be obtained through checking the Moody diagram or equation below: f = (0.790 ln Re D − 1.64)−2 (5.17) This correlation is valid for 3000≦ReD≦5×106 Figure 5.2 Friction Factor under different Air Flow velocity 59 Friction factor is a function of Reynolds number, as expressed in Eq (5.17) and the variation of friction factor with air flow velocity is shown in Fig 5.2. It shows that the friction factor is observed to decrease with increasing air flow rate. There is a significant drop in the range of 0 to 2 m/s. From Equations 5.10 and 5.11, hc, convection coefficient of air flow in the duct can be written as follows: hc = k ( f / 8)U m Pr ν [1.07 + 12.7( f / 8)1/ 2 (Pr 2 / 3 − 1)] (5.18) Figure 5.3 Heat Transfer Coefficient under different Air flow velocity The heat transfer coefficient in the cooling duct which is a function of the air flow velocity is plotted in Fig 5.3. With increasing air flow velocity, the heat transfer in the cooling duct will be enhanced as well as can be observed in Fig 5.3. 60 The internal heat transfer mechanism of solar cell is dominated by Fourier conduction law. The equation can be written as following equation: ρC p ∂T = ∇(k ∇T ) ∂t (5.19) A two dimensional simulation is discussed in this study and therefore the above equation can be transformed to the following equation: ∂ 2T ∂ 2T 1 ∂T + = ∂ 2 x ∂ 2 y α ∂t (5.20) Where α is the thermal diffusivity, α=k/ρcp, T is the temperature. The thermal properties (thermal conductivity, density and specific heat capacity) of the layers inside the photovoltaic module are tabulated in table 5.1 61 Table 5.1 Thermal Properties of the Material Material Thermal Conductivity (W/m-ºK) Specific Heat Capacity (KJ/kg K) Density (Kg/m3) Thermal Diffusivity Tedlar (Polyvinyl Fluoride) 0.14 1010 1450 9.56E-08 EVA (Ethylene-vinyl acetate) 0.3836 2220 1080 1.6E-07 PET (Polyethylene terephthalate) 0.24 1000 1455 1.65E-07 Silicon (Polycrystalline) 148 712 2330 8.92E-05 PV Glass 1 858 2500 4.66E-07 The thermal efficiency can be computed with the following equation: • ηth = m c p ∫ (To − Ti )dt (5.21) Ac ∫ G (t )dt where m is the mass flow rate, cp is the specific heat capacity, To is the outlet temperature of air flow, Ti is the inlet temperature of the air flow, Ac is the area of collector. The electrical efficiency of the PV module can be described as following equation: 62 ηe = ∫ VIdt A∫ G (t ) dt (5.22) The total efficiency of the hybrid PV/T system is: • ηtotal = ηth + ηe == m c p ∫ (To − Ti )dt + ∫ VIdt Ac ∫ G (t )dt (5.23) The thermal and electrical efficiencies are presented in Eqs (5.22) and (5.23). It can be seen that the solar irradiation is a function of time and those parameters which are affected by the solar irradiation, such as inlet and outlet temperatures, PV voltage and PV current, are also functions of time. That is the reason to integrate the equation with time. 5.4 Meteorological data of Simulation In order to compare the temperature profile of simulation and experiment, the same meteorological condition must be applied to the simulation programming. A second order polynomial equation was used to curve the real meteorological data on 23 September 2009. The solar irradiation of the simulation is a function of time and the equation is shown below: 63 G (t ) = −0.003t 2 + 0.6746t + 580.47 (5.24) where t is the time and the unit of t is minute. 1100 1000 Irradiation (W/m2) 900 800 700 600 500 400 300 2 200 y = -0.0003x + 0.6746x + 580.47 2 7 7 :1 16 :5 15 :2 7 2 7 2 15 :0 15 :3 14 :1 14 :4 2 13 :2 13 2 7 2 7 2 2 7 :5 12 :3 12 :0 12 :4 11 :1 11 :4 10 :1 10 9: 47 100 Time Figure 5.4 Solar irradiation of the simulation The curve seems to perfectly fit the real data in the morning, and the deviation of the curve started increasing after 1 pm. Another parameter which is used to approach the real condition for the simulation is ambient temperature. Fig 5.5 is the curve of the ambient temperature at that day. A six order polynomial equation was used to represent the meteorological data on 23 September 2009 and the equation is: 64 y = -8E-19x6 + 7E-15x5 - 2E-11x4 + 4E-08x3 - 3E-05x2 + 0.0109x + 31.271 Ambient Temperature (C) 35 34 33 32 31 :1 7 2 :5 15 16 7 :2 7 2 2 15 :0 15 :3 14 7 :4 :1 14 2 13 :2 2 7 2 7 2 2 7 13 :5 12 :3 12 :0 12 :4 11 :1 11 :4 10 :1 10 9: 47 30 Time Figure 5.5 Ambient temperature of simulation Ta(t) =-1×10-14t6 +2×10-11t5 - 1×10-8t4 +5×10-06t3 - 0.0009t2 +0.0561t +31.297 (5.25) The ambient temperature of the simulation can only averagely curve the meteorological condition on 23 September 2009. The fluctuation of the ambient temperature could be attributed to the wind blowing unsteadily. However, the polynomial equation is still acceptable to represent the ambient temperature in the numerical simulation. 65 CHAPTER 6 RESULTS AND DISCUSSION The results of the experiments and simulation are discussed in this chapter. Good agreement between the experiment and simulation results is found in this study. The electrical and thermal performances of the system are also clearly presented in this chapter and the comparison of different meteorological conditions will be provided in order to investigate the function of the PV/T system. In this study, thermal and electrical performance will be discussed respectively. In thermal aspect, those parameters like temperature of Photovoltaic module at different location, inlet and outlet temperature of air flow, ambient temperature, solar intensity, thermal efficiency, thermal gain and flow rate are investigated thoroughly in order to figure out the potential of combining the Photovoltaic module and thermal collector. Apart from these, battery voltage, Photovoltaic voltage, Photovoltaic current, external load and electrical efficiency are also needed to be elaborated. This is because that these parameters can adequately indicate whether the cooling mechanism could improve the electrical performance of system. 66 50 49 48 47 46 45 44 43 42 41 40 1100 2 Irradiation (W/m ) 1000 900 800 700 Irradiation 600 Average Temperature 500 400 9:47 10:30 11:30 12:30 13:30 14:30 15:31 TEmperature (C) 6.1 Thermal Performance 16:21 Time Figure 6.1 Irradiation and Average Panel Temperature for the whole day under cooling condition (23 September 2009) The cases with and without cooling are presented in Figs 6.1 and 6.2. The maximum solar intensity of both days occurs at around 1:30 pm. The maximum solar intensities on 23rd September and 28th September were about 1050W/m2 and 1200W/m2, respectively. For the case with cooling, the module temperature was maintained at 48℃ at maximum solar intensity. However, from Fig 6.2, it can be seen that the maximum temperature attained by module is 63℃. The temperature profile of the PV module almost corresponds to the solar intensity and this may be observed from the Figs 6.1 and 6.2 These two figures provide vital information in illustrating that how effective of using the cooling mechanism to reduce the temperature of PV module. These two figures also show that the maximum solar intensity always occurs 67 at solar noon. The local solar noon of Singapore is around 1:00 to 1:30 pm. 1200 68 Average Temperature 63 2 Irradiation (W/m ) 1000 800 58 600 53 400 48 200 43 0 AverageTemperature (C) Irradiation 38 9:46 10:14 10:41 11:50 12:02 12:49 12:55 13:18 13:43 14:25 15:15 15:48 16:42 16:57 Time Figure 6.2 Irradiation and Average Panel Temperature for the whole day without cooling condition (28 September 2009) The Photovoltaic module temperature is linearly proportionate to the irradiation and it is displayed in the Fig 6.3. Under the cooling mechanism, for every 100W/m2 increment of solar irradiation, the temperature of module increases 1.39℃. However, if the PV module is not associated with the cooling mechanism, the increase of temperature will be 1.8℃ for every 100W/m2. 68 Without Cooling Module Temperature (C) 65 With Cooling 60 y = 0.018x + 41.752 55 50 45 y = 0.0139x + 34.424 40 200 300 400 500 600 700 800 900 1000 1100 1200 Irradiation (W/m2) Figure 6.3 Module Temperature as a function of solar irradiation This also shows that the increase of temperature of PV module without the cooling mechanism is higher than with the cooling mechanism under the same solar irradiation. The variation of temperature between the cooling and non-cooling cases can can be as high as 10℃. The high temperature will seriously affect the electrical performance of the system and also degrade the Photovoltaic module. 54 T1 T2 Temperature (C) 52 T3 50 T4 48 46 44 42 40 9:47 10:30 11:30 12:30 Time 13:30 14:30 15:30 16:30 Figure 6.4 Temperature profile at centre the duct PV module. 69 Temperature (C) T7 48 47 46 45 44 43 42 41 40 39 38 T11 T12 T13 9:47 10:00 10:30 10:59 11:30 12:00 12:30 13:00 13:30 13:52 14:30 15:00 15:30 16:00 16:30 Time Figure 6.5 Temperature profile at side of duct. Figs 6.4 and 6.5 provided the temperature profile of back of PV module. The shapes of the curves are shown consistently to each other. It was observed that the temperature of the module has shown a variation in the sequence of the location of thermocouple. The temperature of PV module which near to the inlet of air flow will always be lower than the temperature of panel which near to the outlet. This could be attributed to the air flow keep absorbing the heat from the panel and the temperature difference (Tbacksheet-Tair) between the surface and air flow will become smaller. Therefore, the sequence of temperature of PV module will be T4>T3>T2>T1. From Figs 6.4 Fig 6.5, it is may be observed that the temperature of the panel in the centre channel is higher than that of the side channel. This phenomenon can be described as non-uniform flow distribution. The flow distribution simulation results are displayed in Figs 6.6 and 6.7. These figures indicate that the non-uniform flow caused 70 the temperature of PV module in the centre channel increases due to the low flow rate passing through. Figure 6.6 Top view of velocity contour of manifold design Figure 6.7 Cross section view of velocity contour of manifold design 71 The simulation results also showed that the pressure drop in this design is not very significant and therefore the energy consumption which utilised to drive the blower can be reduced. Figure 6.8 presents the pressure contour of the manifold design Figure 6.8 Top view of the pressure contour of manifold design The scale of Fig 6.8 shows that the pressure drop of this manifold design is less than 55.3 Pascal. This information is useful for the proper selection of the blower. Although the flow rate at centre channel is slightly different from the side channel, the manifold duct design can still provide the Photovoltaic module with a well distributed flow field. 72 108(C) Temperature (C) 54 109(C) 52 110(C) 50 114(C) 48 46 44 42 40 38 9:47 10:00 10:30 10:59 11:30 12:00 12:30 13:00 13:30 13:52 14:30 15:00 15:30 16:00 16:30 Time Figure 6.9 Temperature profile of the front glass of module Temperature profiles of the front glass of PV module are presented in Fig 6.9. From the comparison of Figs 6.5 and 6.9, it is clearly observed that the temperature of front glass is higher than at the back. This indicates that the convection heat transfer of front glass is not constant at the back of the PV module. The convection heat transfer of the front glass is dominated by the forced and free convection. Free convection in this experiment is insignificant as the buoyancy force involved is very small; therefore, the heat transfer is also insignificant. However, forced convection at the front glass is dominated by the wind blowing over the PV module. Hence, it is not comparable to the forced convection at the back of PV module. In addition, the wind blow at the front glass of module is very unstable. Thus, the temperature of the front glass of PV module is higher than the back of PV module. However, the trend of the temperature profile can still be observed and it still 73 corresponds to the solar intensity even though under the disturbance of unstable heat transfer mechanism. 44 Inlet (C) Temperature (C) 42 Outlet (C) 40 38 36 34 32 30 9:47 10:30 11:30 12:30 13:30 14:30 15:30 16:30 Time Figure 6.10 Temperature profile of inlet and outlet flow Inlet and outlet temperature of the air flow are also investigated in this study, as the heat gain and thermal efficiency can be computed by using the difference of inlet and outlet flow temperature. Fig 6.10 indicates the temperature profile of inlet and outlet flow over the entire day. The maximum temperature difference between the inlet and outlet flow can be 8℃ and it happened at 1:30 pm. The minimum temperature difference between the inlet and outlet flow occurred at 3:30 pm and just has 4℃ in difference. This might be the reason that the solar irradiation is low at that moment and the ambient temperature of the surrounding is still high and therefore the temperature difference of inlet and outlet is limited by those reasons. 74 Temperature Difference (C) Mass Flow= 0.0932 kg/s 10 9 8 7 6 5 4 3 2 1 0 Mass Flow= 0.0389kg/s 200 300 400 500 600 700 2 800 900 1000 Irradiation (W/m ) Figure 6.11 Variation of temperature difference (To-Ti) with incident radiation for flow rate 0.0389 kg/s and 0.0932 kg/s Temperature differences of the inlet and outlet flow are presented in Fig 6.11. The temperature difference of inlet and outlet flow will increase 0.55 ℃ for every increment of 100W/m2 of solar irradiation when the flow rate is 0.0932kg/s. However, when the flow rate at 0.0389 kg/s, the temperature difference of inlet and outlet flow will increase 0.89℃ for every increment of 100W/m2 of solar irradiation. This can be explained that the increase of flow rate will cause the temperature difference of inlet and outlet flow decreases. As the flow rate is inversely proportional to the temperature difference at a given heat gain. 75 0.600 Mass Flow=0.0932kg/s Mass Flow=0.0389kg/s 0.550 Thermal Efficiency Mass Flow=0.1379kg/s 0.500 0.450 0.400 0.350 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 (Ti-Ta)/G(K．m2W-1) Figure 6.12 Thermal efficiency as a function of (Ti-Ta)/G The thermal efficiencies of the experiments are presented in Fig 6.12. The thermal efficiency of the PV/T air collector at different mass flow rate are tabulated in table 6.1 Table 6.1 Thermal efficiency for different flow rate Mass Flow Rate (kg/s) Equation 0.0389 ηth = 0.4095-3.7491ΔT/G 0.0932 ηth = 0.5104-4.7574ΔT/G 0.1379 ηth = 0.5342-5.4084ΔT/G The thermal efficiencies of the experiments are between 40 to 55%. However, Tonui and Tripanagnostopoulos [57] reported, a PVT air collector with fins can provide the thermal efficiency at 0.30-6.14ΔT/G. The flow rate which used at that experiment was very low compared to current study. Therefore, the thermal efficiency of that study can only attain to 30%. From the result of theoretical models, high values of air flow rate, 76 long PV/T system and small air duct depth, thermal efficiency can up to 55% [48]. For the model of a finned double-pass photovoltaic-thermal solar air heater, Othman et al [58] reported, the thermal efficiency of the system can attain to around 45% and 70% at flow rate 0.027kg/s and 0.181kg/s, respectively. This may be attributed to the air having sufficient time for good a heat transfer with the PV module when the air moves from the top of the cell to the bottom. Othman et al [59] performed an experimental analysis on PVT collector of double pass with flat plat, and achieved the thermal efficiency of about 58% at 0.1kg/s of air flow rate. Thermal Efficiency (%) 60 50 40 30 Irradiation=1000W/m2 Irradiation=900W/m2 20 10 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Flow rate (kg/s) Figure 6.13 Influence of flow rate on thermal efficiency Fig 6.13 showed that, by varying the flow rate, it can be seen that the thermal efficiency will come to be a constant after the flow rate at around 0.05 kg/s. Sopian et al [45] showed that the thermal efficiency will come to a constant after the flow rate at 77 0.042 kg/s. Hegazy [35] reported that the daily thermal efficiency of the system will reach an asymptotic value when the flow rate reaches to 0.045 kg/s., Sopian et al [36] found that for a solar drying system the thermal efficiency will be maintained at 58% with the flow rate at 0.05 kg/s. The thermal efficiency of the PVT air system seems to be maintained at fixed level when the flow rate at 0.04 to 0.05 kg/s, regardless of the configuration of collector. Electrical Efficiency (%) 6.2 Electrical Performance 14 With Cooling 13 Without Cooling 12 11 10 9 8 35 40 45 50 55 60 65 70 Temperature (C) Figure 6.14 Electrical efficiency as a function of PV temperature at irradiation at 1000W/m2 78 14 Electrical Efficiency (%) With Cooling Without Cooling 13 12 11 10 9 8 30 32 34 36 38 40 42 44 46 Temperature (C) Figure 6.15 Electrical efficiency as a function of PV temperature at irradiation at 250W/m2 In Figs 6.14 and 6.15 show that the variation of electrical efficiency of the PV module with the operating temperature. It can be seen that the electrical efficiency of the PV module is significantly affected by the operating temperature under high irradiation conditions. However, under low irradiation conditions as shown in Fig 6.15, cooling mechanism is insignificant in affecting the electrical efficiency of PV module. It is probably due to the low operating temperature of PV module under low irradiation. Hence, the cooling mechanism cannot significantly affect the electrical efficiency of PV module. High irradiation will incur high operating temperature of PV module, this has been shown at Fig 6.14. Without applying the cooling mechanism on the PV module, the temperature of PV module can attain to around 68℃ and electrical efficiency of the PV module is around 8.6%. If the PV module cooled by air, the 79 electrical efficiency of PV module can be boosted to around 13% and the operating temperature is only 36℃. From the result of experiment, it strongly proved that the cooling mechanism can greatly help increase the electrical efficiency of PV module by reducing the operating temperature. Furthermore, the high operating temperature can also reduce the lifespan of PV module by degrading the material of module. Zondag and Helden [60] showed that the temperature of PV module can reach 120℃ when the flow fails in the collector. The thermal cycling tests also showed that the EVA inside the PV module may have a risk in delamination when operating temperatures of PV module over 135℃. Fig 6.14 also provides an indicative trend in the relation of electrical efficiency and operating temperature. A linear equation obtained from the Fig 6.14: η el = 0.1577 − 0.0009Tpanel (6.1) The theoretical efficiency of PV module can be obtained from the Eq 5-4 From the theoretical deduction, the electrical efficiency of the module can be written as the equation below: ηel = 0.1664 − 0.0007Tpanel (6.2) 80 Electrical Efficiency (%) 15 Experimental Result Theoretical Result y = -0.07x + 16.64 14 13 12 11 y = -0.09x + 15.77 10 9 8 30 35 40 45 50 55 60 65 70 75 Temperature (C) Figure 6.16 Comparison between theoretical and experimental results Based on experimental data as shown in Fig 6.16, showed that the theoretical electrical efficiency is about 1 to 2% higher than experimental electrical efficiency. This discrepancy can be attributed to the connection of module to module will incur the electrical efficiency drop. Tonui et al [57] reported that the linear correlation between the electrical efficiency and the module temperature which obtained from the experiment is: ηel = 0.147 − 0.0008Tpanel (6.3) Tripanagnostopoulos [49] also presented another experimental result on correlation between the electrical efficiency and module temperature and the linear equation is given by: 81 η el = 0.166 − 0.0001Tpanel From the (6.4) electrical efficiency correlation of the work of Tonui, Tripanagnostopoulos and author, it can be concluded that the increase of temperature of PV module can reduce the electrical efficiency. However, the variation of the constant term of each equation may be attributed to the different model of PV module and therefore the performances of the PV module are also different to each other. The comparison of these three models also showed that under the same increment of temperature the reduction of electrical efficiency of Tripanagnostopoulos’s experiment is lower than other two cases. 13.00 Electrical Efficiency (%) 12.50 12.00 11.50 Irradiation=1000W/m2 11.00 Irradiation=900W/m2 10.50 Irradiation=500W/m2 10.00 Irradiation=700W/m2 9.50 9.00 8.50 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Flow Rate (kg/s) Figure 6.17 Influence of flow rate on electrical efficiency 82 0.18 This shows that the reduction of electrical efficiency of Tripanagnostopoulos’s experiment will be lower than those for two other experiments under the same irradiation. By varying the flow rate, the electrical efficiencies of PV module are also investigated in this study. The effect of varying the flow rate on PV electrical efficiency is presented in Fig 6.17 and the trend is similar to shown in Fig 6.13 The electrical efficiency of the PV module increases with the flow rate until the flow rate reaches 0.05 to 0.055 kg/s. The electrical efficiency of PV module will be maintained at a fixed value after the flow rate at 0.055 kg/s. This could be explained associated with the thermal efficiency of collector. When the flow rate increases to around 0.05 kg/s, the thermal efficiency of the collector will also be maintained at certain level. In other words, the heat which extracted by the flow has come to a saturated level and it can no longer be increased by increasing the flow rate. Thus, the electrical efficiency of PV module will also be maintained at a fixed value after the flow rate at 0.05 kg/s. Fig 6.17 also showed that the electrical efficiency of PV module at low irradiation will be higher than the high irradiation. It is highly likely that the operating temperature of PV module is much higher in the condition of high irradiation. Hegazy [35] reported that four types of PV/T air system were investigated in a comparative study and the electrical efficiency of those systems will come to 83 maximum value at the flow rate at 0.045kg/s and the electrical efficiency will also keep constant after this flow rate. The discrepancy between the author and Hegazy’s study could be attributed to the difference of configuration for both systems. Hegazy utilized a double pass flow design to investigate the electrical efficiency. Therefore, the air flow has sufficient time to contact with the module and module can be effectively cooled. Electrical efficiency of the module will be increased due to the effective cooling. For that reason, the flow rate of Hegazy’s experiment will be lower than author to attain the maximum electrical efficiency in experiment. Mass Flow Rate= 0.0389 kg/s Electrical Efficiency (%) 13.2 Mass Flow Rate= 0.0676 kg/s 13 Mass Flow Rate =0.0783kg/s 12.8 12.6 12.4 12.2 12 11.8 11.6 11.4 11.2 0 1 2 3 4 5 6 7 8 9 10 Temperature Difference (C) Figure 6.18 Influence of temperature difference (To-Ti) on electrical efficiency for different flow rate The electrical efficiency of PV module decreases when the temperature difference of the inlet and outlet flow increases. Fig 6.18 shows that the electrical efficiency of 84 the PV module significantly decreases with the increase of temperature difference over the inlet and outlet flow. This may be explained by the high temperature gradient, which causes the occurrence of hot spots in the PV module. Therefore, the overall electrical efficiency was decreased due to the local high temperature spot emerges in the PV module. The inlet and outlet flow temperature difference should be controlled in an optimum range to ensure that the electrical efficiency of the PV module can still be maintained at desired output value. Temperature of the PV module also affects the PV electrical power output and the results are also shown in Fig 6.19. Under the solar irradiation at 1000W/m2, there is a decrease of 0.69% of electrical power output for every Celsius degree increase. However, a decrease of electrical output power by 0.65%/K has been reported by Radziemska [61]. The decrease in insolation at 800W/m2, 600W/m2 and 400W/m2 are 0.47%/K, 0.39%/K and 0.34%/K, respectively. The trend could be explained from the derivation below: It may be assumed that the relation between the solar irradiation and temperature of PV module to be linear, as also observed from Fig. 6.3. Therefore, the solar radiation will be linearly proportional to the temperature of the module. Let assume, 85 G = FT − C (6.5) where G is the solar irradiation, T is the temperature of PV module, F and C are the coefficient. To find the relation between the PV power output and module temperature can be started from Eq 5-4 ηe = ηo [1 − β (Tc − To )] ηe = Pel AG (6.6) Substituting the Eqs 5-4 to 6-6, the equation becomes: Pel = AGηo [1 − β (T − To )] (6.7) The details of derivation are presented in Appendix. After arranging the equation, it can be written in the form: Pel = ( FTo − FTo β − C ) + ( F + C β )[T − To ] − F β (T − To ) 2 Let 86 (6.8) T ' = T − To (6.9) Substituting Eq (6.9) into the equation (6. 8) yields, P = Aηo [(T '− F + C β 2 F + C β 2 FTo − C − F β To ) +( ) + ] 2F β 2F β Fβ (6.10) In order to simplify the Eq (6.10), let: F + C β 2 FTo − C − F β To ) + Fβ 2F β (6.11) F + Cβ 2F β (6.12) Z =( L= Substituting Eq (6.11) and Eq (6.12) to Eq (6.10) yields, Pel = −[T '− L]2 + Z (6.13) Substituting T1 and T2 to equation 6.13 Pel1 = −[T1 '− L ]2 + Z (6.14) And 87 Pel 2 = −[T2 '− L]2 + Z (6.15) Let Eq (6.14) subtract Eq (6.15) Pel1 − Pel 2 = −[T1 '− L]2 + Z + [T2 '− L]2 − Z (6.16) Percentage of Electrical power decrease over the temperature difference T1 and T2 Pel1 − Pel 2 (T2 '+ T1 '− 2 L)(T2 '− T1 ') Pel1 Z − (T1 '− L) 2 = T1 − T2 T1 − T2 (6.17) Electrical output decrease over temperature increase Pel1 − Pel 2 Pel1 (T '+ T1 '− 2 L) =− 2 T1 − T2 Z − (T1 '− L) 2 (6.18) Thee derivation above, it shows that when temperature T2 is much larger than T1 then the electrical output power percentage decreases significantly for every Celsius degree. This situation could happen in high irradiation under with and without cooling as under the high irradiation temperature of PV module can be very high without any cooling mechanism. However, if the air is circulated at the back of PV module, the 88 temperature of PV module can be reduced. It means that under the same solar irradiation, the temperature of PV module with and without cooling could be varied a lot. Eq 6.18 shows good agreement with the experimental data which shown in Fig 6.19. At low solar irradiation, the temperature of T1 and T2 cannot be varied a lot and therefore the electrical output power percentage is lower than the situation in high solar irradiation. Irradiation= 1000W/m2 PV Power Output (W) 250 Irradiation= 800W/m2 Irradiation= 600W/m2 200 Irradiation= 400W/m2 150 100 50 0 30 35 40 45 50 55 Temperature (C) 60 65 Irradiation 6.0 1000 PV Current 5.5 500 3.0 400 2.5 300 2.0 200 1.5 100 1.0 15:38 3.5 14:42 4.0 600 13:46 700 12:50 4.5 11:54 5.0 800 10:48 900 PV Current (A) 1100 9:47 Irradiation (W/m2) Figure 6.19 PV electrical power output under different solar radiation Time Figure 6.20 Solar radiation of the entire day and the corresponded PV current due to the solar radiation (23 September 2009) 89 PV Voltage 32 1000 Irradiation 32 2 900 32 800 32 700 32 600 31 500 31 16:20 16:02 15:44 15:26 15:08 14:50 14:32 14:14 13:56 13:38 13:20 13:02 12:44 12:26 12:08 11:50 200 11:32 31 11:14 300 10:46 31 10:23 400 10:05 31 9:47 PV Voltage (V) 1100 Irradiation (W/m ) 33 Time Figure 6.21 Solar irradiation and the PV Voltage for the entire day (23 September 2009) Fig 6.20 shows the relation between the irradiation and PV current output. This figure also provides important information about the stand alone system. Stand alone systems need to use the battery bank to store the electricity generated by the PV module during operation. This figure shows that the PV current generated corresponds to the solar irradiation. Fig 6.21 also shows the relation of irradiation and PV voltage. Some minor fluctuations are seen in this figure but the overall trend of the PV voltage is well corresponded to the irradiation. However, when the battery bank of stand-alone PV systems are fully charged, the PV current and PV voltage of the system do not correspond to the irradiation. Figs 6.22 and 6.23 display the results when the battery banks are fully charged. Once the battery bank of the stand alone systems are fully charged, the voltage of the PV module becomes constant at, around 38 V. This could be 90 because the PV current cannot flow through the external load but the electron-hole pair 39 900 38 800 37 700 36 600 35 Irradiation 500 34 PV Voltage 17:23 17:02 16:41 16:20 15:59 15:38 15:17 14:56 14:35 14:14 13:53 13:32 13:11 12:50 12:29 12:08 11:47 11:26 32 11:05 300 10:44 33 10:23 400 10:02 PV Voltage (V) 1000 9:41 2 Irradiation (W/m ) continues to generate voltage by the Photovoltaic effect. Time Figure 6.22 Solar radiation and the PV Voltage for the entire day (8 June 2009) 3 Irradiation 3 PV Current 800 3 2 700 2 600 2 500 PV Current (A) 2 Irradiation (W/m ) 900 2 400 2 17:23 17:01 16:39 16:17 15:55 15:33 15:11 14:49 14:27 14:05 13:43 13:21 12:59 12:37 12:15 11:53 11:31 11:09 10:47 10:25 10:03 1 9:41 300 Time Figure 6.23 Solar radiation of the entire day and the corresponded PV current due to the solar radiation (8 June 2009) 91 Thus, the voltage in the PN junction will increase to abnormal level. Fig 6.21 indicated that the PV voltage is around 32 V when the battery banks of the stand alone system are partially discharged. In other words, the PV current which generated by the Photovoltaic effect can flow through the external load and the electricity can be stored by the battery bank. Fig 6.23 provides useful information to the stand alone system, as it can tell whether the battery bank of the system is fully charged. 26.2 26.1 26.0 25.8 25.7 25.6 25.5 Blower Voltage 25.4 Battery Voltage 25.3 16:22 16:02 15:42 15:22 15:02 14:42 14:22 14:02 13:42 13:22 13:02 12:42 12:22 12:02 11:42 11:22 10:52 10:27 10:07 25.2 9:47 Voltage (V) 25.9 Time Figure 6.24 Battery and blower voltage of partially discharged battery bank (23 September 2009) 92 31 30 Voltage (V) 30 29 29 Battery Voltage 28 Blower Voltage 28 27 17:11 16:53 16:35 16:17 15:59 15:41 15:23 15:05 14:47 14:29 14:11 13:53 13:35 13:17 12:59 12:41 12:23 12:05 11:47 11:29 11:11 10:53 10:35 9:59 10:17 9:41 27 Time Figure 6.25 Battery and blower voltage of fully charged battery bank (8 June 2009) Figs 6.24 and 6.25 are the battery and blower voltage in fully charged and partially discharged conditions, respectively. The voltage of the blower was almost identical to the battery voltage as the blower was hooked up to the battery directly. In the partially discharged case, the voltage of battery will vary with the state of battery storage. The battery voltage will keep constant all the way when it is in fully charged condition. The battery voltage will be around 25 to 26 V when it is under the partially discharged condition. However, if the battery bank is always hooked up with the panel without any discharging mechanism, the battery bank may come to a saturated condition and the battery voltage will be around 30. 5 V. Gassing problems might occur if the battery continues to be charged after being fully charged as during the charging process of a fully charged battery, hydrogen and oxygen are released. Therefore, to avoid the hydrogen explosion hazard, the battery should be kept in a well ventilated area. 93 PV Current (A) 6.00 5.50 Fully Charged Battery 5.00 Partially Discharged Battery 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 500 550 600 650 700 750 2 800 850 900 950 1000 Irradiation (W/m ) Figure 6.26 PV current generated by module in case: (a) partially discharged battery and (b) fully charged battery Fig 6.26 indicates that, the current which generated by the PV module is linearly proportional to the solar radiation when the battery bank was not at the condition of fully charged. However, if the battery bank was under fully charged condition, the PV current will keep constant at 1.6 A, regardless of the increase of solar irradiation. For every 100 W/m2 of solar irradiation increment, the PV current output from the module will increase 0.56 A. It can be seen that when the solar irradiation reaches 1000 W/m2, the PV current output can attain to around 5.6 A. By partially discharging the battery bank can ensure that the PV module working properly and effectively. 94 Battery Partially Discharged 14 Battery Fully Charged Electrical Efficiency (%) 13 12 11 10 9 8 7 6 5 16:22 16:02 15:42 15:22 15:02 14:42 14:22 14:02 13:42 13:22 13:02 12:42 12:22 12:02 11:42 11:22 10:52 10:27 10:07 9:47 4 Time Figure 6.27 Electrical Efficiency of fully charged and partially discharged at the similar meteorological condition The electrical efficiencies of stand alone system which in battery bank fully charged and partially discharged condition are presented in Fig 6.27. The electrical efficiency of the system at the fully charged battery condition will be lower than that in the usual condition and this may be observed in Fig 6.27. The electrical efficiencies of PV module are similar in the initial condition, once the battery bank is fully charged by the PV module; the electrical efficiency of the PV module will drastically drop. The electrical efficiency of PV module can decrease to 4.6% in the condition of fully charged of battery bank. However, if the battery bank of the system is partially discharged, the electrical efficiency may be able to reach 13.4%. This significant difference has been found from the experiment and it also states the importance of discharging battery regularly to ensure that the PV module can always be operated effectively. 95 35.00 31.20 Total Solar Power 30.00 31.00 Energy (MJ) Electrical Energy 25.00 22.70 Thermal Energy 20.00 17.10 16.40 13.70 15.00 10.90 10.60 10.00 6.18 4.37 3.24 5.00 1.39 3.26 1.11 2.30 0.00 1 2 3 4 Days 5 Figure 6.28 Input solar radiation and thermal and electrical energy production over five days 25.00 Electrical Energy Energy (MJ) 20.00 Thermal Energy Total Energy Gain 15.00 10.00 5.00 0.00 1 2 3 4 5 Days Figure 6.29 Electrical and thermal energy and the total energy gain over the five days 96 Thermal Efficiency 70.00% Electrical Efficiency 60.00% Efficiency 50.00% 40.00% 30.00% 5 5 .1 7 % 5 2 .5 6 % 4 5 .1 1 % 4 7 .9 5 % 4 1 .2 3 % 20.00% 10.00% 1 0 .1 5 % 1 0 .3 8 % 1 2 1 0 .4 7 % 1 0 .5 2 % 1 0 .1 7 % 3 4 5 0.00% Days Figure 6.30 Comparison of thermal and electrical efficiency over 5 days. The solar power input over the five days (from 22 September to 26 September) is displayed in Fig 6.28. The electrical and thermal energy produced by the system are also given. This provides a good estimation of how much energy can be generated by using this experimental set-up. Application wise, it can also show that how much electrical energy can be withdrawn from the PV module to be utilised in household application. Fig 6.28 also shows the incident solar power for those five days which means that according to the solar power input information, the energy power output of thermal and electrical can be estimated. Fig 6.29 show that a large amount of thermal energy was generated during the operation of PV system, and this also shows that the thermal energy can be utilised in other aspects like, drying the food product or using as a heater in temperate zone country instead of exhausting the hot air to the surrounding. Due to the meteorological 97 condition, the solar radiation at Day 3 was much lower than the rest of days and this was also reflected on the energy output in thermal and electrical aspect. Fig 6.29 shows that the peak of total energy output occurs on the second day and fourth days. This can be attributed to the meteorological conditions on those days. The ambient temperature of those two days was relatively high and the solar radiation was also very intense. Therefore, it can be concluded that under the proper function of the system, the output energy can be generated proportional to the solar power input. The efficiency of the system shown in Fig 6.30 indicates that the electrical efficiency seems to be more stable than the thermal efficiency. The average electrical efficiency range is around 10.1% to 10.9%. However, the thermal efficiencies of the system are around 40% higher than the electrical efficiency of the system. From the graph, it can be easily seen that the thermal efficiency fluctuates significantly, unlike electrical efficiency. The reason could be that the thermal efficiency is a function not only of solar irradiation but also of the ambient temperature, heat losses to the surrounding and other meteorological parameters. Due to those factors, the variation of the thermal efficiency of the system is understandable. The total efficiency of the system is around 55% to 65%. It can be concluded that the overall efficiency of the PV/T system is much higher than the PV system. This is also implied that the PV/T system can adequately harness the solar energy. 98 6.3 Simulation heat transfer on a single cell under the meteorological condition on 23 September 2009 This simulation was done using a commercial finite element software-COMSOL MULTIPHYSICS. In this simulation, the meteorological data of 23 September 2009 were used to simulate the operation of PV system under the cooling condition. The purpose of this simulation is to investigate the temperature profile of PV module under the solar irradiation at 23 September 2009 and the experimental data are used to verify the simulation result. A good agreement between the simulation and experimental results has been shown in the Fig 6.31. Some deviations are presented from 12.00 pm to 2 pm. This may be attributed to the variation of the irradiation data and it can be seen at Fig 6. 20. This figure shows the temperature profile of the back of the PV module. However, Fig 6.32 shows the temperature profile of the front glass of PV module. The discrepancy of simulation and experimental data are significant in this figure and this is because that the wind speed of the simulation is assumed to be constant but in the real meteorological condition the wind speed is varied from time to time and it is also very difficult to use a polynomial equation to represent. For that reason, the temperature profile of the front glass of panel is quite different that from the experiment but the trend according to the solar irradiation is still in agreement. 99 56 54 Temperature (C) 52 50 48 46 44 Experiment 42 Simulation 40 38 9:47 10:21 10:51 11:19 11:58 12:04 13:20 13:52 15:42 16:02 16:25 Time Figure 6.31 A comparison of simulation and experiment in the temperature profile of the back of PV module 54 Temperature (C) 52 50 48 46 Experiment Simulation 44 42 40 9:47 10:21 10:51 11:19 11:58 12:04 13:20 13:52 15:42 16:02 16:25 Time Figure 6.32 A comparison of simulation and experiment in the temperature profile of the front of PV module 100 Figure 6.33 Temperature contour of the PV cell at 1:30 pm (highest solar radiation on that day) Temperature profile of PV module is displayed in Fig 6.33. The maximum temperature of the module occurrs at the silicon cell. This is attributed to the high absorption of silicon cell in solar irradiation. Temperature of Tedlar (backsheet) is higher than front glass of PV module, this can be explained that the tedlar is closer to the silicon cell compared to the front glass even though the thermal diffusivity of the glass is higher than Tedlar. In addition, from the analysis in chapter 5, it is showed that the solar irradiation with the wavelength more than 1.1μm will transmit the silicon solar cell and absorbed by the Tedlar (backsheet). Therefore, the simulation results show that the temperature at the back of PV module is higher than that of the front glass. 101 Figure 6.34 Temperature gradient of the PV module at 1:30 pm The temperature gradient over the module is also investigated in this simulation. The results of simulation are presented in Fig 6. 34. The maximum and minimum temperature gradient of the PV module occurs in the material of Tedlar and silicon solar cell, respectively. According to the heat diffusion equation Eq 5-19, the thermal diffusivity is inversely proportional to the temperature gradient. Table 5-1 shows the thermal diffusivity of each layer of material inside the PV module. The thermal diffusivity of silicon solar cell is the highest among the materials and it means that the material of large thermal diffusivity will respond quickly to change in its thermal environment. For that reason, the temperature gradient of the silicon solar cell is the lowest among the materials. However, the thermal diffusivity of Tedlar is the lowest among the materials, by using the Eq 5-19, it is understandable that the temperature 102 gradient of this material is the highest due to the low thermal diffusivity in the denominator of the equation. This simulation has accurately predicted the behaviour of the PV module under the meteorological condition at 23 September 2009. This model can be used to simulate different type of meteorological conditions to predict the temperature profile of PV module. However, the meteorological condition, like solar irradiation, ambient temperature and wind speed, must be able to use a polynomial equation to represent it otherwise the simulation result might have a significant discrepancy with the experimental outcome. This is because that the simulation model is created by using a transient equation and the ambient temperature and solar irradiation is varied from time to time, therefore, if those meteorological conditions cannot be represented as a polynomial equation, the discrepancy between the experiment and simulation would be very significant. 103 CHAPTER 7 CONCLUSION The performance of a PV/T system has been successfully determined in an experimental study. The result of the heat transfer simulation of the silicon solar cell was also in good agreement with the experimental results. The impact of the cooling on the PV module has been thoroughly discussed in the thesis. The PV module temperature is a function of conversion efficiency, which can severely adversely affect the electrical performance of the PV system. The electrical efficiency of PV module at 68℃ is around 8.6%. Therefore, the pay back period of the overall system needs to be extended and the degradation of the PV materials could also happen due to the high operating temperature. The effects of cooling and non-cooling on the PV module operating temperature are clearly presented in this thesis. By using the active cooling technique, the experimental result has shown the significant improvement on the electrical efficiency of PV module, the electrical efficiency of PV module can be maintained at around 13%. The optimum flow rate to enhance the heat transfer from the PV module to air is also found in this study to be around 0.05 kg/s. The thermal performance of the PV/T system is also computed and it shows that large amount of thermal energy is collected instead of being dissipated to 104 the environment or trapped in the PV module. The total energy efficiency of the system can reach 65 %. In other words, there will be 65 % of solar irradiation converted into the usable energy through the PV/T system of this study. Furthermore, the uniform flow field which presented in the experiment also presented a minor temperature difference over the PV module. It can be observed in the layout of experimental result of Chapter 6. Reducing the temperature difference over the different PV module can also help to increase the entire system electrical efficiency. In short, the flow field of cooling medium is also a factor to get an efficient PV/T system. It then seems the advantageous to combine the PV module with the collector. The high efficiency of the combined system can shorten the payback period of the entire system. The cost of adding the collector to the PV module is not very significant compared to the price of PV module itself. Therefore, the PV/T system is worth developing in the industry. 105 CHAPTER 8 RECOMMENDATION Some methods of increasing the performance of the PV/T system are recommended in this chapter. The output power could then be increased and the payback period shortened. Therefore, the modification should not significantly increase the cost of the entire system, otherwise the pay back period might need to be extended. The methods introduced below are of low cost but has significant impact on the electrical efficiency. For the PV cell, the electron in the PV material can only be knocked into higher energy state by a photon of certain wavelength which corresponds to the band gap of the PV material. For silicon cell, the photon of wavelength above 1.1 μm is unable to knock the electron from valence band to conduction band and that part of energy will be converted into phonon and increases the temperature of PV module. The radiation of wavelength above 1.1 μm can be addressed as infrared radiation. However, according to some research, it was found that water can be effectively used to absorb the thermal energy from the IR band but allows transmission of the visible spectrum [62] most useful for the PV operation. Fig 8.1 shows that the absorption coefficient of water in visible band is very low compared to that in the infrared band. This adequately indicates that the water effectively absorbs energy in 106 the infrared band and this there is too energy to knock the electron from valence band to conduction band. Therefore the contribution of phonons from the infrared band can be reduced and temperature of PV module can also be maintained. Figure 8.1 Spectral absorption coefficient of pure water (solid line) and of pure sea water (dotted line) as a function of wavelength. In order to investigate this phenomenon, a design has been proposed as shown in Fig 8.2. According to that design, the water can absorb the infrared in the sunlight and let the visible band pass through the water without any interference. The temperature of water which absorbs the infrared will increase and it can be utilised as household application. The thermal energy from the infrared can be recovered instead of throwing to the environment and thereby the total efficiency of the system will increase significantly. 107 Figure 8.2 Transparent water passage in front of the PV panel to pre-filter the solar irradiation before it strikes the solar cell. Another method to boost the overall efficiency of the PV module involves a plane reflector to augment the aperture area of the PV module. 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E.Radziemska, The effect of temperature on the power drop in crystalline silicon solar cells, Renewable energy, 28, pp.1-12. 2003. 62. Mobley C. D., Optical Properties of Water, in Handbook of Optics, Second Edition, M. Bass, editor, 1994, McGraw-Hill, Inc. 116 Appendix A Manufacturer’s Specifications A.1 Neste Advanced Power System Solar Modules A.2 FirstPower Technology Deep Cycle Gel Batteries A.3 Sanyo Denki DC Fan 117 Fig A.1. Specifications of PV modules 118 Fig A.2. Specifications of solar batteries 119 Fig A.3. Specifications of DC fan (model no. 109E2024MH002) 120 Appendix B Calibration of T-type thermocouple Master Thermometer (℃) Thermocouple 1 80 70 y = 0.9946x + 0.8102 60 R2 = 0.9997 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 T1 (℃) Thermocouple 2 70 y = 0.9946x + 0.6602 60 R2 = 0.9997 50 (℃) Master Thermometer 80 40 30 20 10 0 0 10 20 30 40 T2 (℃) 121 50 60 70 80 Master Thermometer (℃) Thermocouple 3 80 70 y = 0.9922x + 0.5936 60 R2 = 0.9996 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 T3 (℃) Master Thermometer (℃) Thermocouple 4 80 70 y = 0.9898x + 0.6075 60 R2 = 0.9996 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 T4 (℃) Master Thermometer (℃) Thermocouple-Inlet 80 70 y = 0.9869x + 0.8089 60 R2 = 0.9997 50 40 30 20 10 0 0 10 20 30 40 50 Inlet Temperature (℃) 122 60 70 80 Master Thermometer (℃) Thermocouple-Outlet 80 70 y = 0.9929x + 0.3123 60 R2 = 0.9995 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 Outlet Temperature (℃) Master Thermometer (℃) Thermocouple 7 80 70 y = 0.9844x + 0.5501 60 R2 = 0.9995 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 T7 (℃) Master Thermometer (℃) Thermocouple 8 80 70 y = 0.9873x + 0.4387 60 R2 = 0.9996 50 40 30 20 10 0 0 10 20 30 40 T8 (℃) 123 50 60 70 80 Master Thermometer (℃) Thermocouple 9 80 70 y = 0.9887x + 0.3807 60 R2 = 0.9994 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 60 70 80 60 70 80 T9 (℃) Master Thermometer (℃) Thermocouple 10 80 70 y = 0.9799x + 0.6747 60 R2 = 0.9996 50 40 30 20 10 0 0 10 20 30 40 50 T10 (℃) Master Thermometer (℃) Thermocouple 11 80 70 y = 0.9881x + 0.5503 60 R2 = 0.9994 50 40 30 20 10 0 0 10 20 30 40 T11 (℃) 124 50 Master Thermometer (℃) Thermocouple 12 80 70 y = 0.9902x + 0.3977 60 R2 = 0.9993 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 60 70 80 T12 (℃) Master Thermometer (℃) Thermocouple 13 80 70 60 y = 0.9935x + 0.2027 50 R2 = 0.999 40 30 20 10 0 0 10 20 30 40 50 T13 (℃) Master Thermometer (℃) Thermocouple 14 80 70 y = 0.9919x + 0.2999 60 R2 = 0.9993 50 40 30 20 10 0 0 10 20 30 40 T14 (℃) 125 50 60 70 80 Appendix C Derivation of the Result P = Aηo ( FT − C )[1 − β (T − To )] = Aηo [ FT − C − FT β (T − To ) + C β (T − To )] = Aηo [ F (T − To ) + FTo − C − F β (T − To ) 2 − F β To + C β (T − To )] = Aηo [− F β (T − To ) 2 + ( F + C β )(T − To ) + ( FTo − C − F β To )] = Aηo [−(T − To ) 2 + Let ( FTo − C − F β To ) (F + Cβ ) (T − To ) + ] Fβ Fβ T ' = T − To P = Aηo [(T '− F + C β 2 F + C β 2 FTo − C − F β To ) +( ) + ] Fβ 2F β 2F β 126 Appendix D Progress Log of Simulation Progress - Solve Problem: Mon Dec 07 00:58:44 CST 2009 fem.sol=femtime(fem, ... 'solcomp',{'T'}, ... 'outcomp',{'T'}, ... 'tlist',[0:1:2331], ... 'tout','tlist'); -------------------------------------------------------------------------------Number of degrees of freedom solved for: 1675 Symmetric matrices found. Format not changed since UMFPACK uses unsymmetric storage. Step 1 2 3 4 5 6 7 8 9 10 11 27 Time 0.001 0.003 0.007 0.015 0.031 0.063 0.127 0.255 0.511 1 out 1.023 2 out 2.047 3 out 4 out Res 3 5 7 9 11 13 15 17 19 2331 out Jac Sol Order Tfail NLfail 2 3 1 0 2 5 2 0 3 7 1 0 4 9 1 0 5 11 1 0 6 13 1 0 7 15 1 0 8 17 1 0 9 19 1 0 0 0 0 0 0 0 0 0 0 21 10 21 1 0 0 23 11 23 1 0 0 39 22 39 3 0 0 Time-stepping completed. 127 [...]... electrical efficiency of the PV cell is significantly affected by the operating temperature The electrical efficiency of PV cell linearly decreases when the operating temperature increases, which is an advantage of the PVT system 1.2.4 Photovoltaic Thermal (PV/ T) System A photovoltaic/ thermal hybrid system (or PVT system) is a combination of photovoltaic and solar thermal system The PVT system can produce... temperature profile of the back of PV module 100 Figure 6.32 A comparison of simulation and experiment in the temperature profile of the front of PV module 100 Figure 6.33 Temperature contour of the PV cell at 1:30 pm (highest solar radiation on that day) 101 Figure 6.34 Temperature gradient of the PV module at 1:30 pm 102 Figure 8.1 Spectral absorption coefficient of pure water (solid line) and of pure... Cross section view of velocity contour of manifold design 71 Figure 6.8 Top view of the pressure contour of manifold design 72 Figure 6.9 Temperature profile of the front glass of module 73 Figure 6.10 Temperature profile of inlet and outlet flow 74 Figure 6.11 Variation of temperature difference (To-Ti) with incident radiation for flow rate 0.0389 kg/s and 0.0932 kg/s 75 Figure 6.12 Thermal efficiency... in an increase of COP from 2 to 3 under low irradiance conditions However, the COP of the system can attain a value of 6 when it is under high irradiance Zondag et al [22] and Jong [23] have conducted a series of comparison between different types of PV/T design and different types of thermal systems Those experiments generally investigated the covered and uncovered PV/T and thermal system with and... 2.5 shows that thermal exergy of the coverless PV/T was the lowest amongst the system considered The latter may be due to heat losses from the top of the device Figure 2.5 Monthly changes of available energy gain by exergetic evaluation on thermal [17] 12 The thermal exergy with monthly changes is presented in Fig 2.5.Flow rate affects the performance of PV/T system since the increase of water velocity... The PVT system refers to a system that extracts heat from the panel with using heat transfer fluid, usually water or air and sometimes both There are several reasons which motivate the development of the PV/T system One of the main reasons is that PV/T system can provide higher efficiency than individual PV and thermal collector system With increased the efficiency, the payback period of the system. .. heat pump The high thermal efficiency of the system is because that the inflow air is always kept in low temperature However, the net electrical efficiency of the system turns into negative because of the energy consumption of the heat pumps Currently, PV/T systems are always installed for residential use In order to investigate the actual condition of the residential building, the PV/T systems were installed... availability of the roof top space per house The disadvantage of this system is that the shading angle of PVT collector must be smaller than the conventional solar thermal collector because of the shading effect 2.2 Air cooled PV/T The first PV/T air facility was built in 1973 at the University of Delaware This PV/T air facility was called as ‘Solar House’ and the air collectors were integrated in the roof top... different kinds of configurations developed to test the overall performance of the combined system Numerical simulation of PV/T systems has also provided more detailed information on the performance of the system 2.1 Water cooled PV/T Figure 2.1 Water PV/T collector [15] 7 Figure 2.2 Water and air mixed-type PV/T collectors [5] Figure 2.1 shows the common configurations of current PV/T systems in use... of the crystalline silicon is 43% They also proposed that the system can also be utilized for pre-heating of hot water for residents in that building Furthermore, the systems can also provide cooling for the building with absorption of heat by the wall of building reduced during the operation of PVT system It was concluded that the hybrid system has potential to be widely advocated in a sub-tropical ... advantage of the PVT system 1.2.4 Photovoltaic Thermal (PV/ T) System A photovoltaic/ thermal hybrid system (or PVT system) is a combination of photovoltaic and solar thermal system The PVT system can produce... investigate the thermal and electrical performances of the Photovoltaic thermal system This system was built on the roof top of EA building at the National University of Singapore The photograph of the... Calibration of T-type thermocouple 121 Appendix C Derivation of the result 126 Appendix D Process log of Simulation 127 iv Summary This thesis discusses aspects of a photovoltaic/ thermal (PV/ T) system