Performance testing of a heat pipe PV/T heat pump system under different working modes

Performance testing of a heat pipe PV/T heat pump system under different working modes Abstract The electrical performance of photovoltaic (PV) panel is affected by PV cell temperature. The lower PV working temperature leads to higher electrical efficiency and more power outputs. In this paper, heat pipes are used to absorb heat from PV panel for cooling, meanwhile, the absorbed heat is made full use for producing hot water with the combination of heat pump. A heat pipe PV/thermal (PV/T) heat pump system is proposed in this paper. The performance of the system is tested and the electrical performance between PV/T panel and traditional PV panel is compared under the heating mode and heat charging mode. The results show that the daily average values of electrical efficiency, thermal efficiency, COPth and COPPV/T are 12.2%, 33.9%, 2.78 and 3.40 in heating mode respectively, while the values are 12.9%, 25.3%, 1.96 and 2.52 in heat charging mode. The performance of heating mode is better than that of heat charging mode. The electrical efficiency of PV/T panel is improved relatively by 25.7% and 14.2% compared with traditional PV panel under the heating mode and heat charging mode, respectively. 1 INTRODUCTION In recent years, the development and utilization of solar energy expands rapidly and its cost reduces as well, which makes solar energy playing an important role in global energy market. It is reported that the following several years will be a critical period for solar energy development and the efforts will be focusing on the increase of solar energy share in total energy, the improvement of solar energy technology and the reduce of cost [1]. Solar photovoltaic (PV) power generation can get high-grade power compared with general solar thermal utilization. However, the photoelectric conversion efficiency is generally 12–17%. It is found that the PV electrical efficiency decreases as the working temperature of PV cells increases, and a temperature coefficient could describe the relative change of the electrical efficiency that is associated with the PV cells’ temperature. The temperature coefficient varies from the type of PV cells. Regarding the monocrystalline silicon PV cells, when the PV cells’ temperature is higher than 25°C, the temperature increase of every 1°C results in the reduction of power generation efficiency by 0.5% in relativity [2, 3]. The PV/T structure, combining PV panel with heat collector, has a better energy performance since the heat medium absorbs heat from PV panel and it cools down the PV working temperature. As a result, the electrical efficiency is improved and the absorbed heat is also made full use for hot water or heat supply. Regarding the research on PV/T technology, Ji et al. [4] designed a hybrid PV/T solar system with flat aluminum box as solar heat collector combined with monocrystalline silicon PV panel. The system performance under different water flow rates and different initial water temperatures was investigated. Jing et al. [5] developed a hybrid PV/T system utilizing aluminum square tube, which can effectively reduce the system weight and material costs. Mathematical simulation and experimental testing were conducted to study the effect of solar radiation, water flow rate and surrounding air temperature on system performance. Zhu et al. [6] proposed a novel heat pipe PV/T collector, which could avoid water frozen when used in cold regions compared with the traditional water-type PV/T system. Xu et al. [7] proposed a solar PV/T heat pump system with R134a flowing inside the tubes below PV cells and absorbing solar heat for producing domestic hot water under heat pump circulation. Zhang et al. [8] proposed a novel PV solar-assisted loop heat pipe/heat pump system. It serves as heat pipe PV solar collector when solar energy is sufficient. Otherwise, heat pump will be in operation for heat supply. According to the above-mentioned studies, many researchers were focusing on the water-type and/or direct-expansion PV/T panels, which might lead to water frozen and gas leakage caused by severe cold climate and poor air tightness. A few of them combined the PV panel with heat pipes and studied the energy performance of the novel heat pipe PV/T panel/system, but all the studies were carried out only under the working mode of heat charging. Few studies have been found to investigate the energy performance of heat pipe PV/T system under the working mode of heating and the performance difference between heating and heat charging. In this paper, a heat pipe PV/T heat pump system is designed and the testing rig is constructed as well. The system performance is tested and compared under two working modes, heating mode and heat charging mode, which provides a reference for the operation regulation and performance optimization of the heat pipe PV/T heat pump system. 2 DESCRIPTION OF TESTING RIG The heat pipe solar PV/T heat pump system consists of two parts: heat pipe solar PV/T collection system and heat pump system. The pictures of testing rig and PV/T structure are shown in our previous study [9]. The heat pipe solar PV/T collection system is mainly made up of heat pipe solar PV/T collector, storage tank and circulating water pump. The parameters of PV panel are listed in Table 1. In order to extract heat from PV panel, the evaporation sections of 10 heat pipes (Φ8 × 0.7 × 1300 mm) are pasted on the back surface board with heat conducting silica gel and the heat pipes are wrapped with a thin aluminum sheet to enhance the effect of heat transfer. The condensation sections of heat pipes (Φ14 × 1 × 80 mm) are inserted into the copper sleeve of manifold. The aluminum sheet is covered with a 30 mm thick rubber plastic sponge insulation layer for heat insulation. The PV panels are connected to the off-grid controllers to measure their power outputs. To compare the difference of electrical performance between PV/T panel and traditional PV panel, those two panels connected with inverter controllers are tested under the same condition. Table 1. Parameters of PV panel Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 Table 1. Parameters of PV panel Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 When the heat pipe PV/T collector absorbs solar energy, part of which is converted to electricity and the rest remains in PV panel as thermal energy, resulting in the temperature rise of PV panel. The working fluid in heat pipes absorbs the heat of PV panel and aluminum plate at the evaporation side and evaporates, and then the vapor of working fluid is condensed at the condensation side. The heat is transferred to the circulating water and is further absorbed by R134a at heat pump evaporator. The schematic diagram of heat pipe solar PV/T heat pump system is shown in Figure 1. The heat pump system consists of Danfoss SC10G compressor, evaporator, condenser and capillary tube. The testing instruments are listed in Table 2. Figure 1. View largeDownload slide Diagram of heat pipe PV/T heat pump system. Figure 1. View largeDownload slide Diagram of heat pipe PV/T heat pump system. Table 2. List of testing instruments Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 View Large Table 2. List of testing instruments Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 View Large The heat pipe PV/T heat pump system includes two working modes: heating mode and heat charging mode. When the system is running in heating mode, the valves on both sides of the thermostatic water bath are opened and the valves on both sides of the heat exchange tank are closed. The cooling water absorbs the heat of refrigerant in the condenser and enters the thermostatic water bath. The thermostatic water bath serves as end-user and the outlet water temperature of thermostatic water bath keeps constant. When the system is running in heat charging mode, the valves on both sides of the thermostatic water bath are closed and the valves on both sides of the heat exchange tank are opened. The cooling water absorbs the heat of refrigerant in the condenser and enters the heat exchange tank. 3 PERFORMANCE ASSESSMENT AND EXPERIMENT IMPLEMENTATION 3.1 Performance assessment The main performance evaluation indexes include heat output power, thermal efficiency, electrical output power, electrical efficiency, COPth, and COPPV/T: The heat output power can be expressed by Equation (1): q=cwmw(Tin−Tout) (1) where cw is the specific heat of water (J/(kg°C)), mw is the water mass flow (kg/s), Tin and Tout are the water temperature at the inlet and outlet of the manifold respectively (°C). The thermal efficiency can be expressed by Equation (2): ηth=qG·Ac (2) where G is the solar radiation (W/m2), Ac is the area of collector (m2). The electrical output power can be expressed by Equation (3): qPV=U·I (3) where U is the output voltage (V), I is the output current (A). The electrical efficiency can be expressed by Equation (4): ηPV=qPVG·APV. (4) The coefficient of performance based on thermal can be calculated by Equation (5) COPth=qconN+P (5) where qcon is the heat output at the condenser (W), N is the compressor power input (W), P is the pump power input, according to the flow rate setting, the value is 45 W. To assess the total energy performance of the PV/T system, a term called ‘advanced coefficient of performance’ (COPPV/T), combing thermal and electrical performance, is employed in the study. With the consideration of the different grade types of thermal and electrical energies, the traditional thermal power generation coefficient (ηtp = 0.38) is used for the energy type conversion in calculation [10]. The advanced coefficient of performance can be calculated by Equation (6): COPPV/T=qcon+qpv/ηtpN+P. (6) 3.2 Experiment implementation The experiment is carried out in Beijing University of Civil Engineering and Architecture, a university in the north of China (116.3°E, 39.9°N). The solar collector is installed to be south-facing with a tilt angle of 30°. The testing is conducted under heating mode and heat charging mode from 8:30 a.m. to 16:30 p.m. on 17 and 26 May 2016, respectively, with a data collection interval of 10 min. The daily mean solar radiation intensity and ambient temperature of those 2 days are almost the same, around 700 W/m2 and 31°C, respectively. The initial water temperature in the tank is 24.8°C. During the testing, the water flow rates in evaporator and condenser are both set to be 6 l/min. The inlet water temperature in condenser is 40°C. The capacity of side tank is 120 L. Figures 2 and 3 show the solar radiation and ambient temperature during the testing. Figure 2. View largeDownload slide Variation of solar radiation and ambient temperature on 17 May. Figure 2. View largeDownload slide Variation of solar radiation and ambient temperature on 17 May. Figure 3. View largeDownload slide Variation of solar radiation and ambient temperature on 26 May. Figure 3. View largeDownload slide Variation of solar radiation and ambient temperature on 26 May. 4 RESULTS AND DISCUSSION 4.1 Heating mode Figure 4 shows the variation of heat output power, thermal efficiency and temperature of water in water tank at the evaporation side. In consideration of thermal response time, the thermal performance of system is evaluated as the hourly average values of heat output power and thermal efficiency. It is found that heat output power increases with the increasing solar radiation before 12:00 p.m., reaching the maximum value of 386.3 W, and then the heat output power decreases with the decreasing solar radiation in the afternoon. The average heat output power is 294.8 W during the testing. The thermal efficiency increases gradually and a sharp increase occurs in the later afternoon, with an average of 33.9%. The reason for the significant increase of thermal efficiency can be explained as follows. The solar radiation decreases rapidly in the afternoon, but the ambient temperature is high at 38°C, so the temperature of PV panel remains about 50°C because of less heat loss from PV panel. Meanwhile, since the heat output obtained from PV panel cannot meet the demand of heat pump, it starts to absorb the heat, which is originally stored in water, for heat supply to heat pump. So the water temperature in water tank decreases gradually from 20.8°C to 14.8°C. Since both water temperature and PV panel temperature decrease and the temperature difference fluctuates slightly, the heat obtained from PV panel reduces slightly. The sharp decrease of solar radiation and the slight decrease of heat obtain from PV panel lead to the sharp increase of thermal efficiency in the later afternoon. Figure 4. View largeDownload slide Variation of heat output power, thermal efficiency and temperature of water in water tank at the evaporation side. Figure 4. View largeDownload slide Variation of heat output power, thermal efficiency and temperature of water in water tank at the evaporation side. Figure 5 shows the variation of electrical output power, electrical efficiency, PV/T panel surface temperature and PV panel surface temperature. It is found that the average PV/T panel surface temperature of 46.4°C is lower than the average PV panel surface temperature of 53.6°C. The electrical output power of PV/T system and PV system increase before noon and reach the maximum values of 128.0 W and 122.0 W, respectively, and then decrease in the afternoon. The average electrical output power is 103.9 W in PV/T system and 86.9 W in PV system, respectively, with 19.5% improved. The electrical efficiencies of both PV/T system and PV system are relatively stable and fluctuate a little during the testing. The average electrical efficiency of PV/T system is 12.2%, 2.5% higher than that of PV system of 9.7%. The power generation of PV/T system is 3.0 MJ, 25% higher than that of PV system of 2.4 MJ (shown in Figure 6). The electrical performance of PV/T system is significantly better than that of PV system. The reason for the electrical performance improvement is because a cooling effect of PV cells is obtained for PV/T system and the electrical efficiency increases correspondingly. Figure 5. View largeDownload slide Variation of electrical output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 5. View largeDownload slide Variation of electrical output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 6. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. Figure 6. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. The heat pump performance, such as condensation capacity, compressor power, thermal and advanced coefficient of performance (COPth and COPPV/T), is presented in Figure 7. It is obvious that the fluctuation of compressor power is very small. The compressor power remains about 460 W. The condensation capacity decreases in the afternoon due to the decreasing water temperature in water tank. The average condensation capacity is 1290 W. The COPth fluctuates with the variation of condensation capacity, with an average value of 2.78. The COPPV/T increases with the increasing solar radiation before 12:30 p.m. and reaches the maximum value of 3.7, and then decreases with the decreasing solar radiation in the afternoon. The average COPPV/T is 3.4. The COPth and COPPV/T are lower than the expected values, and the reason is that the color of backboard is white, leading to lower absorptivity compared with dark color backboard. The coefficient of performance can be improved by using a dark color backboard with high absorptivity and decreasing the packing factor at the same time. Figure 7. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. Figure 7. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. 4.2 Heat charging mode Figure 8 shows the variation of heat output power and thermal efficiency in heat charging mode. It is found that heat output power increases before noon, reaching the maximum value of 329.7 W, and decreases in the afternoon. The average heat output power is 230.6 W during the testing. The thermal efficiency increases all the time, with an average of 25.3%, and the reason for the increasing trend is similar as that of heating mode. Figure 8. View largeDownload slide Variation of heat output power and thermal efficiency. Figure 8. View largeDownload slide Variation of heat output power and thermal efficiency. Figure 9 shows the variation of electrical output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature in heat charging mode. It is found that the average PV panel surface temperature is 50.4°C, 9.4% lower than that of PV/T panel surface temperature of 55.6°C. The average electrical output power is 112.9 W in PV/T system and 101.5 W in PV system, respectively, with 11.2% improved. The electrical efficiency of PV/T system is higher than that of PV system. The average electrical efficiency of PV/T system is 13.8%, 1.0% higher than that of PV system of 12.8%. The power generation of PV/T system is 3.3 MJ, 10% higher than that of PV system of 3.0 MJ (shown in Figure 10). It is obvious that the electrical performance improvement in heating mode is more noticeable than that in heat charging mode. Figure 9. View largeDownload slide Variation of heat output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 9. View largeDownload slide Variation of heat output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 10. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. Figure 10. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. Figure 11 shows the variation of water temperatures at condenser side and evaporator side. It is found that the water temperature at evaporator side decreases in the morning, reaching the minimum value of 15.6°C, and increases up to 19.6°C eventually. While the water temperature at condenser side increases all the time, from 26.3°C to 73.5°C. The reasons can be explained as follows. In the early morning of the testing, heat pump has a good performance due to low initial water temperature and condensing temperature. While the solar radiation is relatively low at this time and the heat obtained from PV panel cannot meet the demand of heat pump, and it starts to absorb the heat originally stored in water at evaporator side, therefore the water temperature at evaporator side decreases gradually. With the increasing water temperature at condenser side, the heat demand of heat pump decreases and the water temperature at evaporator side increases as a result. Figure 11. View largeDownload slide Variation of water temperatures at condenser side and evaporator side. Figure 11. View largeDownload slide Variation of water temperatures at condenser side and evaporator side. Figure 12 shows the variation of condenser capacity, compressor power, COPth and COPPV/T of heat pump. It is found that the compressor power increases while the condensation capacity, COPth and COPPV/T decreases with the increasing water temperature at condenser side. The average condensation capacity and compressor power are 1093.3 W and 578 W, respectively. The average COPth and COPPV/T are 1.96 and 2.52. The performance of heat pump reduces due to the increasing condensing temperature of heat pump. Figure 12. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. Figure 12. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. 5 CONCLUSIONS In this paper, a heat pipe PV/T heat pump system is constructed. The system performance is tested and compared under two different working modes. The main conclusions are as follows: Compared with the traditional water-type and/or direct-expansion type PV/T system, the heat pipe PV/T heat pump system could avoid many existing problems. Meanwhile, the study on the system performance under two different working modes, heating mode and heat charging mode, provides a reference for the system operation regulation and performance optimization. The average electrical efficiency, thermal efficiency, COPth and COPPV/T are 12.2%, 33.9%, 2.78 and 3.40 under heating mode, while the above performance parameters are 12.9%, 25.3%, 1.96 and 2.52 under heat charging mode. The performance of heating mode is better than that of heat charging mode. Compared with PV panel without cooling effect, the electrical output power and the electrical efficiency of PV/T system are improved relatively by 19.5% and 25.7%, respectively under the heating mode, while the above performance parameters are improved relatively by 11.2% and 14.2% respectively under the heat charging mode. The COPth and COPPV/T are lower than the expected values. Changing the packing factor of PV panel could adjust the thermo-electric output ratio, and the decrease of packing factor of PV panel leads to the improvement of thermal performance and COP but the decrease of the power generation of PV/T system. REFERENCES 1 Chinese National Energy Board . Solar energy development plan during the ‘13th Five-Year’ . Sol Energy 2016 ; 12 : 744 . (in Chinese). 2 Radziemska E . The effect of temperature on the power drop in crystalline silicon solar cells . Renewable Energy 2014 ; 28 : 1 – 12 . Google Scholar CrossRef Search ADS 3 Skoplaki E , Palyvos JA . On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations . Sol Energy 2009 ; 83 : 614 – 24 . Google Scholar CrossRef Search ADS 4 Ji J , Lu J , He W , et al. . Experiment investigation on a novel solar photovoltaic thermal system with flat aluminous box as collector . Acta Energize Solaris Sinica 2006 ; 27 : 765 – 73 . 5 Jing S , Zhu Z , Wang W , et al. . Investigation on photovoltaic/thermal solar system utilizing aluminum square tube . Acta Energiae Solaris Sinica 2014 ; 9 : 1639 – 45 . 6 Zhu H , Pei G , Fu H , et al. . Comparative research between two different heat pipe spaces PV/T systems . Acta Energiae Solaris Sinica 2013 ; 34 : 1172 – 6 . 7 Xu G , Zhang X , Deng S . Experimental study on the operating characteristics of a novel low-concentrating solar photovoltaic thermal integrated heat pump water heating system . Appl Therm Eng 2010 ; 31 : 3689 – 95 . Google Scholar CrossRef Search ADS 8 Zhang L , Pei G , Zhang T , et al. . A new photovoltaic solar-assisted loop heat pipe/heat-pump system . Journal of Chemical Industry and Engineering 2014 ; 65 : 3228 – 36 . (in Chinese). 9 Chen H , Zhang L , Jie P , et al. . Performance study of heat-pipe solar photovoltaic/thermal heat pump system . Appl Energy 2017 ; 190 : 960 – 80 . Google Scholar CrossRef Search ADS 10 Huang BJ , Lin TH , Hung WC , et al. . Performance evaluation of solar photovoltaic/thermal systems . Sol Energy 2001 ; 70 : 443 – 8 . Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Low-Carbon Technologies Oxford University Press

Performance testing of a heat pipe PV/T heat pump system under different working modes

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Abstract

Abstract The electrical performance of photovoltaic (PV) panel is affected by PV cell temperature. The lower PV working temperature leads to higher electrical efficiency and more power outputs. In this paper, heat pipes are used to absorb heat from PV panel for cooling, meanwhile, the absorbed heat is made full use for producing hot water with the combination of heat pump. A heat pipe PV/thermal (PV/T) heat pump system is proposed in this paper. The performance of the system is tested and the electrical performance between PV/T panel and traditional PV panel is compared under the heating mode and heat charging mode. The results show that the daily average values of electrical efficiency, thermal efficiency, COPth and COPPV/T are 12.2%, 33.9%, 2.78 and 3.40 in heating mode respectively, while the values are 12.9%, 25.3%, 1.96 and 2.52 in heat charging mode. The performance of heating mode is better than that of heat charging mode. The electrical efficiency of PV/T panel is improved relatively by 25.7% and 14.2% compared with traditional PV panel under the heating mode and heat charging mode, respectively. 1 INTRODUCTION In recent years, the development and utilization of solar energy expands rapidly and its cost reduces as well, which makes solar energy playing an important role in global energy market. It is reported that the following several years will be a critical period for solar energy development and the efforts will be focusing on the increase of solar energy share in total energy, the improvement of solar energy technology and the reduce of cost [1]. Solar photovoltaic (PV) power generation can get high-grade power compared with general solar thermal utilization. However, the photoelectric conversion efficiency is generally 12–17%. It is found that the PV electrical efficiency decreases as the working temperature of PV cells increases, and a temperature coefficient could describe the relative change of the electrical efficiency that is associated with the PV cells’ temperature. The temperature coefficient varies from the type of PV cells. Regarding the monocrystalline silicon PV cells, when the PV cells’ temperature is higher than 25°C, the temperature increase of every 1°C results in the reduction of power generation efficiency by 0.5% in relativity [2, 3]. The PV/T structure, combining PV panel with heat collector, has a better energy performance since the heat medium absorbs heat from PV panel and it cools down the PV working temperature. As a result, the electrical efficiency is improved and the absorbed heat is also made full use for hot water or heat supply. Regarding the research on PV/T technology, Ji et al. [4] designed a hybrid PV/T solar system with flat aluminum box as solar heat collector combined with monocrystalline silicon PV panel. The system performance under different water flow rates and different initial water temperatures was investigated. Jing et al. [5] developed a hybrid PV/T system utilizing aluminum square tube, which can effectively reduce the system weight and material costs. Mathematical simulation and experimental testing were conducted to study the effect of solar radiation, water flow rate and surrounding air temperature on system performance. Zhu et al. [6] proposed a novel heat pipe PV/T collector, which could avoid water frozen when used in cold regions compared with the traditional water-type PV/T system. Xu et al. [7] proposed a solar PV/T heat pump system with R134a flowing inside the tubes below PV cells and absorbing solar heat for producing domestic hot water under heat pump circulation. Zhang et al. [8] proposed a novel PV solar-assisted loop heat pipe/heat pump system. It serves as heat pipe PV solar collector when solar energy is sufficient. Otherwise, heat pump will be in operation for heat supply. According to the above-mentioned studies, many researchers were focusing on the water-type and/or direct-expansion PV/T panels, which might lead to water frozen and gas leakage caused by severe cold climate and poor air tightness. A few of them combined the PV panel with heat pipes and studied the energy performance of the novel heat pipe PV/T panel/system, but all the studies were carried out only under the working mode of heat charging. Few studies have been found to investigate the energy performance of heat pipe PV/T system under the working mode of heating and the performance difference between heating and heat charging. In this paper, a heat pipe PV/T heat pump system is designed and the testing rig is constructed as well. The system performance is tested and compared under two working modes, heating mode and heat charging mode, which provides a reference for the operation regulation and performance optimization of the heat pipe PV/T heat pump system. 2 DESCRIPTION OF TESTING RIG The heat pipe solar PV/T heat pump system consists of two parts: heat pipe solar PV/T collection system and heat pump system. The pictures of testing rig and PV/T structure are shown in our previous study [9]. The heat pipe solar PV/T collection system is mainly made up of heat pipe solar PV/T collector, storage tank and circulating water pump. The parameters of PV panel are listed in Table 1. In order to extract heat from PV panel, the evaporation sections of 10 heat pipes (Φ8 × 0.7 × 1300 mm) are pasted on the back surface board with heat conducting silica gel and the heat pipes are wrapped with a thin aluminum sheet to enhance the effect of heat transfer. The condensation sections of heat pipes (Φ14 × 1 × 80 mm) are inserted into the copper sleeve of manifold. The aluminum sheet is covered with a 30 mm thick rubber plastic sponge insulation layer for heat insulation. The PV panels are connected to the off-grid controllers to measure their power outputs. To compare the difference of electrical performance between PV/T panel and traditional PV panel, those two panels connected with inverter controllers are tested under the same condition. Table 1. Parameters of PV panel Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 Table 1. Parameters of PV panel Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 Item Parameter Model number YL200P-23b Material Polycrystalline silicon Size (length × width × height) 1310 mm × 990 mm × 40 mm Open-circuit voltage 31.0 V Short-circuit current 8.73 A Maximum power output 200 W Packing factor 0.95 When the heat pipe PV/T collector absorbs solar energy, part of which is converted to electricity and the rest remains in PV panel as thermal energy, resulting in the temperature rise of PV panel. The working fluid in heat pipes absorbs the heat of PV panel and aluminum plate at the evaporation side and evaporates, and then the vapor of working fluid is condensed at the condensation side. The heat is transferred to the circulating water and is further absorbed by R134a at heat pump evaporator. The schematic diagram of heat pipe solar PV/T heat pump system is shown in Figure 1. The heat pump system consists of Danfoss SC10G compressor, evaporator, condenser and capillary tube. The testing instruments are listed in Table 2. Figure 1. View largeDownload slide Diagram of heat pipe PV/T heat pump system. Figure 1. View largeDownload slide Diagram of heat pipe PV/T heat pump system. Table 2. List of testing instruments Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 View Large Table 2. List of testing instruments Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 Item Model Number Testing accuracy (%) Pyranometer TBQ-2-B 1 ±2.0 Electromagnetic flow meter SE115MM 2 ±0.5 Platinum resistance temperature sensor WZP-01 17 ±0.5 DC current sensor WBI022F21 2 ±1.0 DC voltage sensor WBV342U01-S 2 ±0.2 Power sensor WBP112S41 1 ±0.5 Data logger Agilent34972A 1 View Large The heat pipe PV/T heat pump system includes two working modes: heating mode and heat charging mode. When the system is running in heating mode, the valves on both sides of the thermostatic water bath are opened and the valves on both sides of the heat exchange tank are closed. The cooling water absorbs the heat of refrigerant in the condenser and enters the thermostatic water bath. The thermostatic water bath serves as end-user and the outlet water temperature of thermostatic water bath keeps constant. When the system is running in heat charging mode, the valves on both sides of the thermostatic water bath are closed and the valves on both sides of the heat exchange tank are opened. The cooling water absorbs the heat of refrigerant in the condenser and enters the heat exchange tank. 3 PERFORMANCE ASSESSMENT AND EXPERIMENT IMPLEMENTATION 3.1 Performance assessment The main performance evaluation indexes include heat output power, thermal efficiency, electrical output power, electrical efficiency, COPth, and COPPV/T: The heat output power can be expressed by Equation (1): q=cwmw(Tin−Tout) (1) where cw is the specific heat of water (J/(kg°C)), mw is the water mass flow (kg/s), Tin and Tout are the water temperature at the inlet and outlet of the manifold respectively (°C). The thermal efficiency can be expressed by Equation (2): ηth=qG·Ac (2) where G is the solar radiation (W/m2), Ac is the area of collector (m2). The electrical output power can be expressed by Equation (3): qPV=U·I (3) where U is the output voltage (V), I is the output current (A). The electrical efficiency can be expressed by Equation (4): ηPV=qPVG·APV. (4) The coefficient of performance based on thermal can be calculated by Equation (5) COPth=qconN+P (5) where qcon is the heat output at the condenser (W), N is the compressor power input (W), P is the pump power input, according to the flow rate setting, the value is 45 W. To assess the total energy performance of the PV/T system, a term called ‘advanced coefficient of performance’ (COPPV/T), combing thermal and electrical performance, is employed in the study. With the consideration of the different grade types of thermal and electrical energies, the traditional thermal power generation coefficient (ηtp = 0.38) is used for the energy type conversion in calculation [10]. The advanced coefficient of performance can be calculated by Equation (6): COPPV/T=qcon+qpv/ηtpN+P. (6) 3.2 Experiment implementation The experiment is carried out in Beijing University of Civil Engineering and Architecture, a university in the north of China (116.3°E, 39.9°N). The solar collector is installed to be south-facing with a tilt angle of 30°. The testing is conducted under heating mode and heat charging mode from 8:30 a.m. to 16:30 p.m. on 17 and 26 May 2016, respectively, with a data collection interval of 10 min. The daily mean solar radiation intensity and ambient temperature of those 2 days are almost the same, around 700 W/m2 and 31°C, respectively. The initial water temperature in the tank is 24.8°C. During the testing, the water flow rates in evaporator and condenser are both set to be 6 l/min. The inlet water temperature in condenser is 40°C. The capacity of side tank is 120 L. Figures 2 and 3 show the solar radiation and ambient temperature during the testing. Figure 2. View largeDownload slide Variation of solar radiation and ambient temperature on 17 May. Figure 2. View largeDownload slide Variation of solar radiation and ambient temperature on 17 May. Figure 3. View largeDownload slide Variation of solar radiation and ambient temperature on 26 May. Figure 3. View largeDownload slide Variation of solar radiation and ambient temperature on 26 May. 4 RESULTS AND DISCUSSION 4.1 Heating mode Figure 4 shows the variation of heat output power, thermal efficiency and temperature of water in water tank at the evaporation side. In consideration of thermal response time, the thermal performance of system is evaluated as the hourly average values of heat output power and thermal efficiency. It is found that heat output power increases with the increasing solar radiation before 12:00 p.m., reaching the maximum value of 386.3 W, and then the heat output power decreases with the decreasing solar radiation in the afternoon. The average heat output power is 294.8 W during the testing. The thermal efficiency increases gradually and a sharp increase occurs in the later afternoon, with an average of 33.9%. The reason for the significant increase of thermal efficiency can be explained as follows. The solar radiation decreases rapidly in the afternoon, but the ambient temperature is high at 38°C, so the temperature of PV panel remains about 50°C because of less heat loss from PV panel. Meanwhile, since the heat output obtained from PV panel cannot meet the demand of heat pump, it starts to absorb the heat, which is originally stored in water, for heat supply to heat pump. So the water temperature in water tank decreases gradually from 20.8°C to 14.8°C. Since both water temperature and PV panel temperature decrease and the temperature difference fluctuates slightly, the heat obtained from PV panel reduces slightly. The sharp decrease of solar radiation and the slight decrease of heat obtain from PV panel lead to the sharp increase of thermal efficiency in the later afternoon. Figure 4. View largeDownload slide Variation of heat output power, thermal efficiency and temperature of water in water tank at the evaporation side. Figure 4. View largeDownload slide Variation of heat output power, thermal efficiency and temperature of water in water tank at the evaporation side. Figure 5 shows the variation of electrical output power, electrical efficiency, PV/T panel surface temperature and PV panel surface temperature. It is found that the average PV/T panel surface temperature of 46.4°C is lower than the average PV panel surface temperature of 53.6°C. The electrical output power of PV/T system and PV system increase before noon and reach the maximum values of 128.0 W and 122.0 W, respectively, and then decrease in the afternoon. The average electrical output power is 103.9 W in PV/T system and 86.9 W in PV system, respectively, with 19.5% improved. The electrical efficiencies of both PV/T system and PV system are relatively stable and fluctuate a little during the testing. The average electrical efficiency of PV/T system is 12.2%, 2.5% higher than that of PV system of 9.7%. The power generation of PV/T system is 3.0 MJ, 25% higher than that of PV system of 2.4 MJ (shown in Figure 6). The electrical performance of PV/T system is significantly better than that of PV system. The reason for the electrical performance improvement is because a cooling effect of PV cells is obtained for PV/T system and the electrical efficiency increases correspondingly. Figure 5. View largeDownload slide Variation of electrical output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 5. View largeDownload slide Variation of electrical output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 6. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. Figure 6. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. The heat pump performance, such as condensation capacity, compressor power, thermal and advanced coefficient of performance (COPth and COPPV/T), is presented in Figure 7. It is obvious that the fluctuation of compressor power is very small. The compressor power remains about 460 W. The condensation capacity decreases in the afternoon due to the decreasing water temperature in water tank. The average condensation capacity is 1290 W. The COPth fluctuates with the variation of condensation capacity, with an average value of 2.78. The COPPV/T increases with the increasing solar radiation before 12:30 p.m. and reaches the maximum value of 3.7, and then decreases with the decreasing solar radiation in the afternoon. The average COPPV/T is 3.4. The COPth and COPPV/T are lower than the expected values, and the reason is that the color of backboard is white, leading to lower absorptivity compared with dark color backboard. The coefficient of performance can be improved by using a dark color backboard with high absorptivity and decreasing the packing factor at the same time. Figure 7. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. Figure 7. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. 4.2 Heat charging mode Figure 8 shows the variation of heat output power and thermal efficiency in heat charging mode. It is found that heat output power increases before noon, reaching the maximum value of 329.7 W, and decreases in the afternoon. The average heat output power is 230.6 W during the testing. The thermal efficiency increases all the time, with an average of 25.3%, and the reason for the increasing trend is similar as that of heating mode. Figure 8. View largeDownload slide Variation of heat output power and thermal efficiency. Figure 8. View largeDownload slide Variation of heat output power and thermal efficiency. Figure 9 shows the variation of electrical output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature in heat charging mode. It is found that the average PV panel surface temperature is 50.4°C, 9.4% lower than that of PV/T panel surface temperature of 55.6°C. The average electrical output power is 112.9 W in PV/T system and 101.5 W in PV system, respectively, with 11.2% improved. The electrical efficiency of PV/T system is higher than that of PV system. The average electrical efficiency of PV/T system is 13.8%, 1.0% higher than that of PV system of 12.8%. The power generation of PV/T system is 3.3 MJ, 10% higher than that of PV system of 3.0 MJ (shown in Figure 10). It is obvious that the electrical performance improvement in heating mode is more noticeable than that in heat charging mode. Figure 9. View largeDownload slide Variation of heat output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 9. View largeDownload slide Variation of heat output power, electrical efficiency, PV panel surface temperature and PV/T panel surface temperature. Figure 10. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. Figure 10. View largeDownload slide Comparison of electrical performance between PV/T system and PV system. Figure 11 shows the variation of water temperatures at condenser side and evaporator side. It is found that the water temperature at evaporator side decreases in the morning, reaching the minimum value of 15.6°C, and increases up to 19.6°C eventually. While the water temperature at condenser side increases all the time, from 26.3°C to 73.5°C. The reasons can be explained as follows. In the early morning of the testing, heat pump has a good performance due to low initial water temperature and condensing temperature. While the solar radiation is relatively low at this time and the heat obtained from PV panel cannot meet the demand of heat pump, and it starts to absorb the heat originally stored in water at evaporator side, therefore the water temperature at evaporator side decreases gradually. With the increasing water temperature at condenser side, the heat demand of heat pump decreases and the water temperature at evaporator side increases as a result. Figure 11. View largeDownload slide Variation of water temperatures at condenser side and evaporator side. Figure 11. View largeDownload slide Variation of water temperatures at condenser side and evaporator side. Figure 12 shows the variation of condenser capacity, compressor power, COPth and COPPV/T of heat pump. It is found that the compressor power increases while the condensation capacity, COPth and COPPV/T decreases with the increasing water temperature at condenser side. The average condensation capacity and compressor power are 1093.3 W and 578 W, respectively. The average COPth and COPPV/T are 1.96 and 2.52. The performance of heat pump reduces due to the increasing condensing temperature of heat pump. Figure 12. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. Figure 12. View largeDownload slide Variation of condenser capacity, compressor power, COPth and COPPV/T. 5 CONCLUSIONS In this paper, a heat pipe PV/T heat pump system is constructed. The system performance is tested and compared under two different working modes. The main conclusions are as follows: Compared with the traditional water-type and/or direct-expansion type PV/T system, the heat pipe PV/T heat pump system could avoid many existing problems. Meanwhile, the study on the system performance under two different working modes, heating mode and heat charging mode, provides a reference for the system operation regulation and performance optimization. The average electrical efficiency, thermal efficiency, COPth and COPPV/T are 12.2%, 33.9%, 2.78 and 3.40 under heating mode, while the above performance parameters are 12.9%, 25.3%, 1.96 and 2.52 under heat charging mode. The performance of heating mode is better than that of heat charging mode. Compared with PV panel without cooling effect, the electrical output power and the electrical efficiency of PV/T system are improved relatively by 19.5% and 25.7%, respectively under the heating mode, while the above performance parameters are improved relatively by 11.2% and 14.2% respectively under the heat charging mode. The COPth and COPPV/T are lower than the expected values. Changing the packing factor of PV panel could adjust the thermo-electric output ratio, and the decrease of packing factor of PV panel leads to the improvement of thermal performance and COP but the decrease of the power generation of PV/T system. REFERENCES 1 Chinese National Energy Board . Solar energy development plan during the ‘13th Five-Year’ . Sol Energy 2016 ; 12 : 744 . (in Chinese). 2 Radziemska E . The effect of temperature on the power drop in crystalline silicon solar cells . Renewable Energy 2014 ; 28 : 1 – 12 . Google Scholar CrossRef Search ADS 3 Skoplaki E , Palyvos JA . On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations . Sol Energy 2009 ; 83 : 614 – 24 . Google Scholar CrossRef Search ADS 4 Ji J , Lu J , He W , et al. . Experiment investigation on a novel solar photovoltaic thermal system with flat aluminous box as collector . Acta Energize Solaris Sinica 2006 ; 27 : 765 – 73 . 5 Jing S , Zhu Z , Wang W , et al. . Investigation on photovoltaic/thermal solar system utilizing aluminum square tube . Acta Energiae Solaris Sinica 2014 ; 9 : 1639 – 45 . 6 Zhu H , Pei G , Fu H , et al. . Comparative research between two different heat pipe spaces PV/T systems . Acta Energiae Solaris Sinica 2013 ; 34 : 1172 – 6 . 7 Xu G , Zhang X , Deng S . Experimental study on the operating characteristics of a novel low-concentrating solar photovoltaic thermal integrated heat pump water heating system . Appl Therm Eng 2010 ; 31 : 3689 – 95 . Google Scholar CrossRef Search ADS 8 Zhang L , Pei G , Zhang T , et al. . A new photovoltaic solar-assisted loop heat pipe/heat-pump system . Journal of Chemical Industry and Engineering 2014 ; 65 : 3228 – 36 . (in Chinese). 9 Chen H , Zhang L , Jie P , et al. . Performance study of heat-pipe solar photovoltaic/thermal heat pump system . Appl Energy 2017 ; 190 : 960 – 80 . Google Scholar CrossRef Search ADS 10 Huang BJ , Lin TH , Hung WC , et al. . Performance evaluation of solar photovoltaic/thermal systems . Sol Energy 2001 ; 70 : 443 – 8 . Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 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International Journal of Low-Carbon TechnologiesOxford University Press

Published: Mar 24, 2018

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