- Email: [email protected]
heat pump system
Applied Thermal Engineering 31 (2011) 4132e4138
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on a hybrid photovoltaic/heat pump system Hongbing Chena, b, Saffa B. Riffata, *, Yu Fua a b
Institute of Sustainable Energy Technology, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2011 Accepted 17 August 2011 Available online 26 August 2011
Several studies have found that the decrease of photovoltaic (PV) cell temperature would increase the solar-to-electricity conversion efficiency. Different working fluids such as air and water have been used for the cooling of PV modules, but the improvement in energy performance has been found to be small. In this paper, R134a refrigerant was employed to cool the PV modules. With its low evaporating temperature, it was expected to achieve better cooling effect and electrical performance of the PV modules than using air and water working fluids. An experimental rig of a hybrid micro PV panel-based heat pump system was constructed for the performance testing in a laboratory at the University of Nottingham. A small PV panel was made of 6 glass vacuum tube e PV module e aluminium sheet e cooper tube (GPAC) sandwiches connected in series, acting as the evaporator. This was coupled with a small heat pump system. The glass vacuum tubes reduced the heat loss from the PV panel to the ambient, resulting in the improvement of thermal performance. Three testing modes were proposed to investigate the effect of solar radiation, condenser water flow rate and condenser water supply temperature on energy performance. The testing results showed that an averaged COP reached 3.8, 4.3 and 4.0 under the three testing modes with variable radiation, condenser water supply water temperature and water flow rate, respectively, but this could be much higher for a large capacity heat pump system using large PV panels on building roofs. The COP increased with the increasing solar radiation, but decreased with the increasing condenser water supply temperature and water flow rate. The electrical efficiency of PV panel was improved by up to 1.9% based on a reference PV efficiency of 3.9%, compared with that without cooling. The condenser water supply temperature and water flow rate had little effect on the electrical performance. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Photovoltaic panel Electrical efficiency Heat pump system Coefficient of performance
1. Introduction Photovoltaic (PV) technology has been widely used for generating electricity. However, PV modules have low solar-to-electricity conversion efficiency, less than 20% for commercial PV products [1]. That means the rest 80% of the absorbed energy is dumped to the PV modules as waste heat. Many studies [2,3] were carried out to investigate the effect of PV cell temperature on the electrical efficiency of PV modules and found that the reduction of PV cell temperature would increase the electrical efficiency. Researchers proposed using air and water as the working fluids to cool PV modules. The heat extracted from the PV modules by air and water was also used for space heating and domestic hot water supply.
* Corresponding author. Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom. Tel.: þ44 115 9513158; fax: þ44 115 9513159. E-mail address: [email protected] (S.B. Riffat). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.08.027
Tonui et al. [4] proposed a suspended thin flat metallic sheet at the middle or fins at the back wall of an air duct as heat transfer augmentations in an air-cooled photovoltaic/thermal (PV/T) solar collector to improve its overall performance. Kumar and Rosen [5] investigated the performance of a double pass PV/T solar air heater with and without fins. The study showed that the extended fin area reduced the cell temperature considerably, from 82 C to 66 C, which led to a significant improvement on the electrical and thermal efficiencies. In New Delhi, Dubey et al. [6] presented a theoretical and experimental investigation on a PV/T solar water heater with different PV module coverage on the absorber. The study indicated that there was a significant increase in the instantaneous efficiency from 33% to 64% due to increase in glazing area. He et al. [7] found that the primary-energy saving efficiency of a hybrid PV/T solar system under natural circulation of water was about 60e75%, much higher than that of an individual PV plate and a traditional solar collector. Tiwari et al. [8] carried out a performance evaluation of hybrid PV/T water/air heating system using four different configurations and found that an overall efficiency of the system for summer
H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138
and winter conditions was about 65% and 77%, respectively. Chow et al. [9] proposed a new design of thermal absorber with flat-box structure. They found that the new type collector had an annual average thermal efficiency of 38.1%. A new type of PV/T collector with dual heat extraction operation, either with water or with air circulation was developed by Tripanagnostopoulos [10]. Chow et al. [11] investigated a photovoltaic-integrated solar heat pump system in Hong Kong based on a dynamic simulation model. The simulation results indicated that the proposed system with R134a was able to achieve a yearly-average COP of 5.93 and PV output efficiency of 12.1%. Ji et al. [12,13] studied a direct-expansion evaporator (PV evaporator) with sheet-and-tube design used in a PV/T solar-assisted heat pump. They found the PV efficiency and thermal efficiency were above 12% and 50% during the testing period. In another experimental study in Hefei, China, Ji et al. [14] presented an average COP and PV efficiency of 5.4 and 13.7%. Xu et al. [15] carried out an experimental study on a novel low-concentrating solar PV/T integrated heat pump water heating system and achieved an averaged COP of 4.8 for heating water from 30 C to 70 C on a sunny summer day, with an output electrical efficiency of 17.5%, 1.36 times higher than that of the same PV system without cooling. Most of the above studies were using air and water for the cooling of PV modules and found water was better than air in terms of PV cooling and energy performance. A few studies proposed the direct-expansion flat plat PV evaporator/heat pump system using a refrigerant for cooling PV modules. But the heat loss from the front surface of PV panel to the ambient could reduce the thermal performance when the ambient temperature is relatively low in winter. In this study, the refrigerant R134a was employed for the cooling of PV modules insulated by glass vacuum tubes. The PV panel, acting as the evaporator, was coupled with a heat pump system. The small portion of the absorbed solar energy was converted to electricity. The rest energy was converted to waste heat, extracted by the refrigerant at the back surface of the PV panel. After the Rankine refrigeration cycle operation, the heat was released later at the condenser. With the low evaporating temperature, it was expected to achieve better cooling effect and better electrical performance of the PV modules. Meanwhile, the glass vacuum tube type PV panel reduced the heat loss to the ambient, compared with the conventional flat plate type PV panel, resulting in the improvement of thermal performance. This paper investigates experimental work on a hybrid glass vacuum tube type PV panel-based heat pump system.
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2. Description of testing rig 2.1. Heat pump system The schematic diagram of a hybrid PV panel-based heat pump system is shown in Fig.1. The heat pump system was made up of four main components: glass vacuum tube type PV panel (evaporator), compressor, water-cooled condenser and thermostatic expansion valve. Receiver and filter were installed at the location between condenser and expansion valve. Since the testing was carried out in a laboratory, the PV panel was limited to a small size, and thus the heat capacity of PV panel was low. A small capacity Danfoss household compressor was employed to match the PV panel. Fig. 1 shows the refrigeration cycle and water circulation system. The refrigerant R134a was used for the heat pump system. The PV modules were connected to a designed electronic circuit for power testing. When the heat pump system is in operation, the refrigerant mixture (liquid and gas) with low temperature enters the PV evaporator, and it extracts the solar heat to evaporate and becomes the superheated refrigerant gas at the exit of the PV evaporator. Then, the superheated refrigerant gas enters the compressor where the refrigerant pressure and temperature are lifted up. After that, the refrigerant gas enters the water-cooled condenser, it releases heat energy to water and condenses to liquid during the process. Finally, the refrigerant liquid with high pressure enters the expansion valve where the pressure drops sharply, and then enters the PV evaporator as a mixture and continues another cycle operation. The water is supplied to the condenser by the pump and absorbs heat from the refrigerant gas. Then, it enters the radiator and releases heat to the ambient. The water supply temperature to the condenser is automatically controlled by the controller for the cooling fan. When the supply temperature is higher than the setting point (T þ 0.2 C), the cooling fan starts to run and the temperature decreases. The cooling fan is switched off when the temperature is lower than the setting point (T 0.2 C). 2.2. PV panel The glass vacuum tube type PV panel is made of 6 glass vacuum tube e PV module e aluminium sheet e cooper tube (GPAC) sandwiches connected in series. The testing rig of the PV panel is shown in Fig. 2 and Fig. 3 shows a cross-sectional view of a GPAC sandwich. The aluminium sheet (1 mm thick) was adhered on the
Fig. 1. The schematic diagram of the hybrid PV panel-based heat pump system.
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H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138
Fig. 2. Testing rig of the PV panel.
back of the PV module (3 mm thick) with thermal glue for good thermal conductance. The u-shape copper tube (6 mm diameter) was tightly wrapped with the aluminium sheet at two side-ends along the cooper tube under precise pressure control to provide a good contact between the aluminium sheet and the copper tube. This enables a good heat transfer from the aluminium sheet to the refrigerant. The copper tube pitch was 40 mm. The GPAC sandwich was insulated with a transparent glass vacuum tube (58 mm internal diameter). The total aperture area and PV cell area were 0.42 m2 and 0.294 m2, respectively. The PV cells are made of amorphous silicon. The 6 PV modules in parallel are connected to an electronic circuit for power testing
(shown in Fig. 4). The main components of the electronic circuit include rheostat, current shunt, resistances and data logger. The rheostat (0w250 U) is regulated to measure the power output under different load. The current shunt is used to measure the circuit current. Since the voltage of PV modules is about 10 V, which is out of the voltage range of data logger (2.5 V), two resistances (100 kU and 10 kU) in series are connected to the rheostat in parallel for the testing of voltage output. The power output can be calculated with the tested voltage and current outputs. Under the radiation of 1000 W/m2 and the ambient temperature of 25 C, the PV panel has an open circuit voltage of 11.28 V and a short circuit current of 0.97 A. The peak power output is 8.03 W with an electrical efficiency of 3.9%. The list of the testing devices is shown in Table 1. 3. Performance assessment and experiment implementation 3.1. Performance assessment
Glass vacuum tube The electrical efficiency of the PV modules is given by
PV module
Aluminium sheet Copper tube
Fig. 3. Cross-sectional view of a GPAC sandwich.
Fig. 4. The schematic diagram of the electronic circuit for power testing.
H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138 Table 1 List of the testing devices.
COP
4
Adhesive thermocouple type K (RS 621-2287, UK) PT100 temperature sensor (RS 611-8264, UK)
5
6 7
Data logger (DT500 DataTaker, UK) Calibrated flow indicator (RS 186-0049, UK)
PV evaporator
500 PV evaporator inlet and outlet; Condenser inlet and outlet PV evaporator
6 6
sa sr bc 1 ð1 bc ÞRg
(3)
where Qout is the heat output of the heat pump system; Win is the compressor power input.
Qout ¼ mw Cpw ðTout Tin Þ
(4)
where mw is the water mass flow; Cpw is the specific heat of water; Tin and Tout are the water temperature at the inlet and outlet of the condenser, respectively. 3.2. Experiment implementation The testing was carried out at a laboratory of the University of Nottingham. The solar radiation was simulated by a radiation simulating panel, which was made up of 12 well-distributed tungsten halogen floodlights with 500 W for each one. The radiation can be adjusted to any requested value by a control box. The condenser Table 2 List of the testing modes.
10, 10, 10, 10 30
B C
590 w 630
0 0
200
400
600
800
0 1000
2
Radiation (W/m )
water flow was regulated by a control valve. The condenser water supply temperature was controlled by a control panel with manual setting. The effect of radiation, condenser water flow and condenser water supply temperature on the energy performance of the hybrid heat pump system was investigated under different testing modes. Three testing modes are listed in Table 2.
4. Results and discussions 4.1. The effect of solar radiation
in
100
(2)
COP ¼ Qout=W
200 400 600 800 600
300
Fig. 5. Variation of COP, condenser capacity and compressor power with radiation.
where sa is the transmittance of glass vacuum tube considering only absorptance loss and sr is the transmittance of glass vacuum tube considering only reflection loss; bc is the absorptance of PV cells; Rg is the diffuse reflectance of glass vacuum tube. The energy performance of the heat pump system is assessed by the coefficient of performance (COP), which is given by,
A
2
Water pipe after pump
(1)
Radiation (W/m2)
400
200
PV evaporator inlet and outlet; Condenser refrigerant inlet and outlet; Condenser water inlet and outlet
1 1
3
1
where E is the electricity generation; G is the solar radiation; Ac is the area of PV cells; (bs)c is the effective absorptance of PV cells, given by [16]
Mode
600
4
Compressor input (AC)
E Ac ðbsÞc G
ðbsÞc ¼
700
Ambient temperature ( C)
Condenser water flow (L/min)
Condenser water supply temperature ( C)
21.7 2
2
35
The testing on the effect of solar radiation on energy performance of the hybrid heat pump system was carried out under the testing mode A. During the testing, the ambient temperature, condenser water flow rate and condenser water supply temperature were kept constant. Fig. 5 shows the variation of COP, condenser heat capacity and compressor power input under different radiation. It can be seen from Fig. 5 that the COP and condenser heat capacity increased with the increasing radiation. At the radiation of 200 W/m2, the COP was 2.9 and the condenser heat capacity was 345 W. As the radiation increased to 800 W/m2, the COP and condenser heat capacity rose to 4.6 and 582 W, respectively. The COP was not as high as expected due to the low capacity compressor. It can also be seen from Fig. 5 that the compressor power decreased slightly from 126.8 W to 120.9 W with the increasing radiation from 200 W/m2 to 800 W/m2. This is because the increasing radiation leads to the increase of evaporating temperature and pressure, which reduces the compression ratio, and thus reduces the compressor power input. Fig. 6 shows the variation of electrical efficiency and PV power output under different radiation. It can be seen that the electrical Electrical efficiency
PV power 7
5
6
4
5 3
4
2
3 2
1
1
0 22.5 2
2
23 1.3
1, 2, 3, 4, 5
25, 30, 35, 40, 45 35
0
200
400
600
800
0 1000
2
Radiation (W/m ) Fig. 6. Variation of electrical efficiency and PV power with radiation.
Power (W)
3
Compressor power
Power (W)
Pyranometer (Kipp & Zonen 1 CMP11, Netherlands) Plug-in power & energy 1 monitor (Energy Monitor World, UK) Pressure transducer (RS 461-341, UK) 4
2
hc ¼
Quantity Location
Efficiency (%)
1
Condenser capacity
5
COP
Item Device
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H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138
with cooling
PV power
Electrical efficiency
Radiation
6
800
4
5
700
Power (W) Efficiency (%)
Electrical efficiency (%)
without cooling 5
3 2 1
4
600
3 500
2
400
1
0
0 0
200
400
600
800
1000
Radiation (W/m 2 )
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300 20
25
30
35
40
45
50
Radiation (W/m 2 ) Fig. 7. Comparison of electrical efficiency with and without cooling.
Fig. 9. Variation of electrical efficiency and PV power with condenser water supply temperature.
efficiency and PV power output increased with the increasing radiation. At the radiation of 200 W/m2, the electrical efficiency and PV power were 0.5% and 0.2 W, respectively. As the radiation increased to 800 W/m2, the electrical efficiency and PV power rose to 4.6% and 6.1 W, respectively. Before the construction of testing rig, the electrical performance of PV modules without cooling was tested for the investigation on the performance improvement made by refrigerant cooling. Fig. 7 shows the variation of electrical efficiency for the PV modules with and without cooling under different radiation. From Fig. 7, it can be seen that the electrical efficiency with refrigerant cooling was higher than that without cooling and the electrical efficiency improvement made by refrigerant cooling increased with the increasing radiation. At the radiation of 200 W/m2, the electrical efficiency was 0.21% without cooling and 0.5% with refrigerant cooling, respectively, with 0.29% improved. At the radiation of 800 W/m2, the electrical efficiency was 2.7% without cooling and 4.6% with refrigerant cooling, respectively, with 1.9% improved. 4.2. The effect of condenser water supply temperature The testing on the effect of condenser water supply temperature on energy performance of the hybrid heat pump system was carried out under the testing mode B. During the testing, the radiation, ambient temperature and condenser water flow rate were kept constant. Fig. 8 shows the variation of COP, condenser capacity and compressor power input under different condenser water supply temperature. It can be seen from Fig. 8 that the COP and condenser heat capacity decreased with the increasing condenser water supply temperature. At the condenser water Condenser capacity
The testing on the effect of condenser water flow on energy performance of the hybrid heat pump system was carried out under
Compressor power COP
800
6
3
400 300
2
200 1
100
0
0 20
25
30
35
40
45
50
Condenser water supply temperature ( Fig. 8. Variation of COP, condenser capacity and compressor power with condenser water supply temperature.
Power (W) Efficiency (%)
500
Compressor power 1500
6
Power (W)
600 4
Condenser capacity
7
700
5 COP
4.3. The effect of condenser water flow
1200
5 4
900
3
600
Power (W)
COP
supply temperature of 25 C, the COP and condenser heat capacity were 5.2 and 586 W, respectively. As the condenser water supply temperature increased to 45 C, the COP and condenser heat capacity dropped to 3.2 and 495 W, respectively. From Fig. 8, it can also be found that the compressor power input rose from 111.8 W to 153.9 W as the condenser water supply temperature increased from 25 C to 45 C. This is because that the increase of condenser water supply temperature leads to a higher condensing pressure, and thus, a higher compression ratio and higher compressor power input. When the condenser water supply temperature increased, the temperature difference between the refrigerant and water at the condenser decreased. Therefore, the condenser heat capacity dropped. Meanwhile, the compressor power input went up, so the COP dropped more sharply than the condenser heat capacity. Fig. 9 shows the variation of electrical efficiency and PV power output under different condenser water supply temperature. From Fig. 9, it can be seen that the radiation varied within 600 30 W/m2 during the testing mode B. The electrical efficiency and PV power output were mainly affected by the radiation instead of the condenser water supply temperature. The minimum radiation 573.6 W/m2 at the condenser water supply temperature of 30 C led to the minimum electrical efficiency of 4.77% and the minimum PV power output of 4.5 W, while the maximum radiation 626.5 W/m2 at the condenser water supply temperature of 45 C led to the maximum electrical efficiency of 4.86% and the maximum PV power output of 5.0 W.
2 300
1 0
0 0
1
2 3 4 Condenser water flow (L/min)
5
6
Fig. 10. Variation of COP, condenser capacity and compressor power with condenser water flow.
H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138
PV power
Electrical efficiency
dropped from 5.2 to 3.2, responding to the condenser supply temperature from 25 C to 45 C, at the radiation of w600 W/ m2 and condenser water flow rate of 2 L/min. The condenser water supply temperature had little effect on the PV power output and electrical efficiency. The COP of the heat pump system decreased with the increasing condenser water flow rate. The COP dropped from 6.7 to 2.8, responding to the condenser water flow rate from 1 L/min to 5 L/min, at the radiation of w600 W/m2 and condenser water supply temperature of 35 C. The condenser water flow rate had little effect on the PV power output and electrical efficiency.
Radia tion
5
800
Power (W) Efficiency (%)
600 500
3
400 2
300 200
1
Radiation (W/m 2 )
700 4
100 0
0 0
1
2 3 4 5 Condenser supply water flow (L/min)
6
Fig. 11. Variation of electrical efficiency and PV power with condenser water flow.
the testing mode C. During the testing, the radiation, ambient temperature and condenser water supply temperature were kept constant. From Fig. 10, it can be seen that the COP and condenser heat capacity dropped sharply as the condenser water flow increased from 1 L/min to 2 L/min, and then the drop tended to be smooth as the condenser water flow continued to increase from 2 L/ min to 5 L/min. At the condenser water flow of 1 L/min, the COP and condenser heat capacity were 6.7 and 916 W, respectively. As the condenser water flow increased to 5 L/min, the COP and condenser heat capacity dropped to 2.8 and 352 W, respectively. It can also be found that the compressor power input decreased slightly from 137.4 W to 125 W with the increasing condenser water flow. Fig. 11 shows the variation of electrical efficiency and PV power output under different condenser water flow. From Fig. 11, it can be seen that the radiation varied within 590w630 W/m2 during the testing mode C and had an important impact on the electrical efficiency and PV power output instead of the condenser water flow. The minimum radiation 598.1 W/m2 at the condenser water flow of 2 L/min led to the minimum electrical efficiency of 4.2% and the minimum PV power output of 4.1 W, while the maximum radiation 622.8 W/m2 at the condenser water supply temperature of 5 L/min led to the maximum electrical efficiency of 4.5% and the maximum PV power output of 4.6 W.
5. Conclusions The paper presents experimental study on the energy performance of a hybrid PV/heat pump system. The hybrid glass vacuum tube type PV panel-based heat pump system was constructed for the testing in a laboratory at the University of Nottingham. Three testing modes were proposed to investigate the effect of radiation, condenser water flow rate and condenser water supply temperature on energy performance of the heat pump. The study can be concluded that: The COP of the heat pump system increased with the increasing radiation. The COP varied from 2.9 to 4.6, responding to the radiation from 200 W/m2 to 800 W/m2, at the constant condenser water flow rate of 2 L/min and water supply temperature of 35 C. The COP was not as high as expected due to the low capacity compressor, selected to match the small size and low capacity of PV panel limited indoors. The PV power output and electrical efficiency also increased with the increasing radiation. The electrical efficiency of PV panel in the heat pump system was improved by up to 1.9%, compared with that without cooling. The COP of the heat pump system decreased with the increasing condenser water supply temperature. The COP
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Acknowledgements The work of this paper was fully supported by Marie Curie Incoming International Fellowship (No. PIIF-GA-2009-234843). Nomenclature
A Cp E G m R Q T W
area (m2) specific heat at constant pressure (J kg1 K1) photovoltaic power per unit surface area (W m2) solar irradiance (W m2) mass flow rate (kg s1) reflectance heat capacity (W) temperature (K) power (W)
Greek
b h s bs
absorptance efficiency transmittance effective absorptance
Subscripts c solar cells g glass vacuum tube in input, inlet out output, outlet w water References [1] PVT Roadmap e a European guide for the development and market introduction of PV-thermal technology, available at www.pvtforum.org/index.html with last (accessed 11.05.11). [2] E. Skoplaki, J.A. Palyvos, On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations, Solar Energy 83 (2009) 614e624. [3] Y.B. Assoa, C. Menezo, G. Fraisse, R. Yezou, J. Brau, Study of a new concept of photovoltaic-thermal hybrid collector, Solar Energy 81 (2007) 1132e1143. [4] J.K. Tonui, Y. Tripanagnostopoulos, Improved PV/T solar collectors with heat extraction by forced or natural air circulation, Renewable Energy 32 (2007) 623e637. [5] R. Kumar, M.A. Rosen, Performance evaluation of a double pass PV/T solar air heater with and without fins, Applied Thermal Engineering 31 (2011) 1402e1410. [6] S. Dubey, G.N. Tiwari, Thermal modeling of a combined system of photovoltaic thermal (PV/T) solar water heater, Solar Energy 82 (2008) 602e612. [7] W. He, Y. Zhang, J. Ji, Comparative experiment study on photovoltaic and thermal solar system under natural circulation of water, Applied Thermal Engineering 31 (16) (2011) 3369e3376. [8] A. Tiwari, M.S. Sodha, Performance evaluation of hybrid PV/thermal water/ air heating system: a parametric study, Renewable Energy 31 (2006) 2460e2474.
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[9] T. Chow, W. He, J. Ji, A.L.S. Chan, Performance evaluation of photovoltaicethermosyphon system for subtropical climate application, Solar Energy 81 (2007) 123e130. [10] Y. Tripanagnostopoulos, Aspects and improvements of hybrid photovoltaic/ thermal solar energy systems, Solar Energy 81 (2007) 1117e1131. [11] T. Chow, K. Fong, G. Pei, J. Ji, W. He, Potential use of photovoltaic-integrated solar heat pump system in Hong Kong, Applied Thermal Engineering 30 (2010) 1066e1072. [12] J. Ji, H. He, T. Chow, G. Pei, W. He, K. Liu, Distributed dynamic modeling and experimental study of PV evaporator in a PV/T solar-assisted heat pump, International Journal of Heat and Mass Transfer 52 (2009) 1365e1373.
[13] J. Ji, K. Liu, T. Chow, G. Pei, W. He, H. He, Performance analysis of a photovoltaic heat pump, Applied Energy 85 (2008) 680e693. [14] J. Ji, G. Pei, T. Chow, K. Liu, H. He, J. Lu, C. Han, Experimental study of photovoltaic solar assisted heat pump system, Solar Energy 82 (2008) 43e52. [15] G. Xu, X. Zhang, S. Deng, Experimental study on the operating characteristics of a novel low-concentrating solar photovoltaic/thermal integrated heat pump water heating system, Applied Thermal Engineering 31 (17e18) (2011) 3689e3695. [16] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, third ed. Wiley, New York, USA, 2006.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on a hybrid photovoltaic/heat pump system Hongbing Chena, b, Saffa B. Riffata, *, Yu Fua a b
Institute of Sustainable Energy Technology, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2011 Accepted 17 August 2011 Available online 26 August 2011
Several studies have found that the decrease of photovoltaic (PV) cell temperature would increase the solar-to-electricity conversion efficiency. Different working fluids such as air and water have been used for the cooling of PV modules, but the improvement in energy performance has been found to be small. In this paper, R134a refrigerant was employed to cool the PV modules. With its low evaporating temperature, it was expected to achieve better cooling effect and electrical performance of the PV modules than using air and water working fluids. An experimental rig of a hybrid micro PV panel-based heat pump system was constructed for the performance testing in a laboratory at the University of Nottingham. A small PV panel was made of 6 glass vacuum tube e PV module e aluminium sheet e cooper tube (GPAC) sandwiches connected in series, acting as the evaporator. This was coupled with a small heat pump system. The glass vacuum tubes reduced the heat loss from the PV panel to the ambient, resulting in the improvement of thermal performance. Three testing modes were proposed to investigate the effect of solar radiation, condenser water flow rate and condenser water supply temperature on energy performance. The testing results showed that an averaged COP reached 3.8, 4.3 and 4.0 under the three testing modes with variable radiation, condenser water supply water temperature and water flow rate, respectively, but this could be much higher for a large capacity heat pump system using large PV panels on building roofs. The COP increased with the increasing solar radiation, but decreased with the increasing condenser water supply temperature and water flow rate. The electrical efficiency of PV panel was improved by up to 1.9% based on a reference PV efficiency of 3.9%, compared with that without cooling. The condenser water supply temperature and water flow rate had little effect on the electrical performance. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Photovoltaic panel Electrical efficiency Heat pump system Coefficient of performance
1. Introduction Photovoltaic (PV) technology has been widely used for generating electricity. However, PV modules have low solar-to-electricity conversion efficiency, less than 20% for commercial PV products [1]. That means the rest 80% of the absorbed energy is dumped to the PV modules as waste heat. Many studies [2,3] were carried out to investigate the effect of PV cell temperature on the electrical efficiency of PV modules and found that the reduction of PV cell temperature would increase the electrical efficiency. Researchers proposed using air and water as the working fluids to cool PV modules. The heat extracted from the PV modules by air and water was also used for space heating and domestic hot water supply.
* Corresponding author. Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom. Tel.: þ44 115 9513158; fax: þ44 115 9513159. E-mail address: [email protected] (S.B. Riffat). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.08.027
Tonui et al. [4] proposed a suspended thin flat metallic sheet at the middle or fins at the back wall of an air duct as heat transfer augmentations in an air-cooled photovoltaic/thermal (PV/T) solar collector to improve its overall performance. Kumar and Rosen [5] investigated the performance of a double pass PV/T solar air heater with and without fins. The study showed that the extended fin area reduced the cell temperature considerably, from 82 C to 66 C, which led to a significant improvement on the electrical and thermal efficiencies. In New Delhi, Dubey et al. [6] presented a theoretical and experimental investigation on a PV/T solar water heater with different PV module coverage on the absorber. The study indicated that there was a significant increase in the instantaneous efficiency from 33% to 64% due to increase in glazing area. He et al. [7] found that the primary-energy saving efficiency of a hybrid PV/T solar system under natural circulation of water was about 60e75%, much higher than that of an individual PV plate and a traditional solar collector. Tiwari et al. [8] carried out a performance evaluation of hybrid PV/T water/air heating system using four different configurations and found that an overall efficiency of the system for summer
H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138
and winter conditions was about 65% and 77%, respectively. Chow et al. [9] proposed a new design of thermal absorber with flat-box structure. They found that the new type collector had an annual average thermal efficiency of 38.1%. A new type of PV/T collector with dual heat extraction operation, either with water or with air circulation was developed by Tripanagnostopoulos [10]. Chow et al. [11] investigated a photovoltaic-integrated solar heat pump system in Hong Kong based on a dynamic simulation model. The simulation results indicated that the proposed system with R134a was able to achieve a yearly-average COP of 5.93 and PV output efficiency of 12.1%. Ji et al. [12,13] studied a direct-expansion evaporator (PV evaporator) with sheet-and-tube design used in a PV/T solar-assisted heat pump. They found the PV efficiency and thermal efficiency were above 12% and 50% during the testing period. In another experimental study in Hefei, China, Ji et al. [14] presented an average COP and PV efficiency of 5.4 and 13.7%. Xu et al. [15] carried out an experimental study on a novel low-concentrating solar PV/T integrated heat pump water heating system and achieved an averaged COP of 4.8 for heating water from 30 C to 70 C on a sunny summer day, with an output electrical efficiency of 17.5%, 1.36 times higher than that of the same PV system without cooling. Most of the above studies were using air and water for the cooling of PV modules and found water was better than air in terms of PV cooling and energy performance. A few studies proposed the direct-expansion flat plat PV evaporator/heat pump system using a refrigerant for cooling PV modules. But the heat loss from the front surface of PV panel to the ambient could reduce the thermal performance when the ambient temperature is relatively low in winter. In this study, the refrigerant R134a was employed for the cooling of PV modules insulated by glass vacuum tubes. The PV panel, acting as the evaporator, was coupled with a heat pump system. The small portion of the absorbed solar energy was converted to electricity. The rest energy was converted to waste heat, extracted by the refrigerant at the back surface of the PV panel. After the Rankine refrigeration cycle operation, the heat was released later at the condenser. With the low evaporating temperature, it was expected to achieve better cooling effect and better electrical performance of the PV modules. Meanwhile, the glass vacuum tube type PV panel reduced the heat loss to the ambient, compared with the conventional flat plate type PV panel, resulting in the improvement of thermal performance. This paper investigates experimental work on a hybrid glass vacuum tube type PV panel-based heat pump system.
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2. Description of testing rig 2.1. Heat pump system The schematic diagram of a hybrid PV panel-based heat pump system is shown in Fig.1. The heat pump system was made up of four main components: glass vacuum tube type PV panel (evaporator), compressor, water-cooled condenser and thermostatic expansion valve. Receiver and filter were installed at the location between condenser and expansion valve. Since the testing was carried out in a laboratory, the PV panel was limited to a small size, and thus the heat capacity of PV panel was low. A small capacity Danfoss household compressor was employed to match the PV panel. Fig. 1 shows the refrigeration cycle and water circulation system. The refrigerant R134a was used for the heat pump system. The PV modules were connected to a designed electronic circuit for power testing. When the heat pump system is in operation, the refrigerant mixture (liquid and gas) with low temperature enters the PV evaporator, and it extracts the solar heat to evaporate and becomes the superheated refrigerant gas at the exit of the PV evaporator. Then, the superheated refrigerant gas enters the compressor where the refrigerant pressure and temperature are lifted up. After that, the refrigerant gas enters the water-cooled condenser, it releases heat energy to water and condenses to liquid during the process. Finally, the refrigerant liquid with high pressure enters the expansion valve where the pressure drops sharply, and then enters the PV evaporator as a mixture and continues another cycle operation. The water is supplied to the condenser by the pump and absorbs heat from the refrigerant gas. Then, it enters the radiator and releases heat to the ambient. The water supply temperature to the condenser is automatically controlled by the controller for the cooling fan. When the supply temperature is higher than the setting point (T þ 0.2 C), the cooling fan starts to run and the temperature decreases. The cooling fan is switched off when the temperature is lower than the setting point (T 0.2 C). 2.2. PV panel The glass vacuum tube type PV panel is made of 6 glass vacuum tube e PV module e aluminium sheet e cooper tube (GPAC) sandwiches connected in series. The testing rig of the PV panel is shown in Fig. 2 and Fig. 3 shows a cross-sectional view of a GPAC sandwich. The aluminium sheet (1 mm thick) was adhered on the
Fig. 1. The schematic diagram of the hybrid PV panel-based heat pump system.
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Fig. 2. Testing rig of the PV panel.
back of the PV module (3 mm thick) with thermal glue for good thermal conductance. The u-shape copper tube (6 mm diameter) was tightly wrapped with the aluminium sheet at two side-ends along the cooper tube under precise pressure control to provide a good contact between the aluminium sheet and the copper tube. This enables a good heat transfer from the aluminium sheet to the refrigerant. The copper tube pitch was 40 mm. The GPAC sandwich was insulated with a transparent glass vacuum tube (58 mm internal diameter). The total aperture area and PV cell area were 0.42 m2 and 0.294 m2, respectively. The PV cells are made of amorphous silicon. The 6 PV modules in parallel are connected to an electronic circuit for power testing
(shown in Fig. 4). The main components of the electronic circuit include rheostat, current shunt, resistances and data logger. The rheostat (0w250 U) is regulated to measure the power output under different load. The current shunt is used to measure the circuit current. Since the voltage of PV modules is about 10 V, which is out of the voltage range of data logger (2.5 V), two resistances (100 kU and 10 kU) in series are connected to the rheostat in parallel for the testing of voltage output. The power output can be calculated with the tested voltage and current outputs. Under the radiation of 1000 W/m2 and the ambient temperature of 25 C, the PV panel has an open circuit voltage of 11.28 V and a short circuit current of 0.97 A. The peak power output is 8.03 W with an electrical efficiency of 3.9%. The list of the testing devices is shown in Table 1. 3. Performance assessment and experiment implementation 3.1. Performance assessment
Glass vacuum tube The electrical efficiency of the PV modules is given by
PV module
Aluminium sheet Copper tube
Fig. 3. Cross-sectional view of a GPAC sandwich.
Fig. 4. The schematic diagram of the electronic circuit for power testing.
H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138 Table 1 List of the testing devices.
COP
4
Adhesive thermocouple type K (RS 621-2287, UK) PT100 temperature sensor (RS 611-8264, UK)
5
6 7
Data logger (DT500 DataTaker, UK) Calibrated flow indicator (RS 186-0049, UK)
PV evaporator
500 PV evaporator inlet and outlet; Condenser inlet and outlet PV evaporator
6 6
sa sr bc 1 ð1 bc ÞRg
(3)
where Qout is the heat output of the heat pump system; Win is the compressor power input.
Qout ¼ mw Cpw ðTout Tin Þ
(4)
where mw is the water mass flow; Cpw is the specific heat of water; Tin and Tout are the water temperature at the inlet and outlet of the condenser, respectively. 3.2. Experiment implementation The testing was carried out at a laboratory of the University of Nottingham. The solar radiation was simulated by a radiation simulating panel, which was made up of 12 well-distributed tungsten halogen floodlights with 500 W for each one. The radiation can be adjusted to any requested value by a control box. The condenser Table 2 List of the testing modes.
10, 10, 10, 10 30
B C
590 w 630
0 0
200
400
600
800
0 1000
2
Radiation (W/m )
water flow was regulated by a control valve. The condenser water supply temperature was controlled by a control panel with manual setting. The effect of radiation, condenser water flow and condenser water supply temperature on the energy performance of the hybrid heat pump system was investigated under different testing modes. Three testing modes are listed in Table 2.
4. Results and discussions 4.1. The effect of solar radiation
in
100
(2)
COP ¼ Qout=W
200 400 600 800 600
300
Fig. 5. Variation of COP, condenser capacity and compressor power with radiation.
where sa is the transmittance of glass vacuum tube considering only absorptance loss and sr is the transmittance of glass vacuum tube considering only reflection loss; bc is the absorptance of PV cells; Rg is the diffuse reflectance of glass vacuum tube. The energy performance of the heat pump system is assessed by the coefficient of performance (COP), which is given by,
A
2
Water pipe after pump
(1)
Radiation (W/m2)
400
200
PV evaporator inlet and outlet; Condenser refrigerant inlet and outlet; Condenser water inlet and outlet
1 1
3
1
where E is the electricity generation; G is the solar radiation; Ac is the area of PV cells; (bs)c is the effective absorptance of PV cells, given by [16]
Mode
600
4
Compressor input (AC)
E Ac ðbsÞc G
ðbsÞc ¼
700
Ambient temperature ( C)
Condenser water flow (L/min)
Condenser water supply temperature ( C)
21.7 2
2
35
The testing on the effect of solar radiation on energy performance of the hybrid heat pump system was carried out under the testing mode A. During the testing, the ambient temperature, condenser water flow rate and condenser water supply temperature were kept constant. Fig. 5 shows the variation of COP, condenser heat capacity and compressor power input under different radiation. It can be seen from Fig. 5 that the COP and condenser heat capacity increased with the increasing radiation. At the radiation of 200 W/m2, the COP was 2.9 and the condenser heat capacity was 345 W. As the radiation increased to 800 W/m2, the COP and condenser heat capacity rose to 4.6 and 582 W, respectively. The COP was not as high as expected due to the low capacity compressor. It can also be seen from Fig. 5 that the compressor power decreased slightly from 126.8 W to 120.9 W with the increasing radiation from 200 W/m2 to 800 W/m2. This is because the increasing radiation leads to the increase of evaporating temperature and pressure, which reduces the compression ratio, and thus reduces the compressor power input. Fig. 6 shows the variation of electrical efficiency and PV power output under different radiation. It can be seen that the electrical Electrical efficiency
PV power 7
5
6
4
5 3
4
2
3 2
1
1
0 22.5 2
2
23 1.3
1, 2, 3, 4, 5
25, 30, 35, 40, 45 35
0
200
400
600
800
0 1000
2
Radiation (W/m ) Fig. 6. Variation of electrical efficiency and PV power with radiation.
Power (W)
3
Compressor power
Power (W)
Pyranometer (Kipp & Zonen 1 CMP11, Netherlands) Plug-in power & energy 1 monitor (Energy Monitor World, UK) Pressure transducer (RS 461-341, UK) 4
2
hc ¼
Quantity Location
Efficiency (%)
1
Condenser capacity
5
COP
Item Device
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with cooling
PV power
Electrical efficiency
Radiation
6
800
4
5
700
Power (W) Efficiency (%)
Electrical efficiency (%)
without cooling 5
3 2 1
4
600
3 500
2
400
1
0
0 0
200
400
600
800
1000
Radiation (W/m 2 )
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300 20
25
30
35
40
45
50
Radiation (W/m 2 ) Fig. 7. Comparison of electrical efficiency with and without cooling.
Fig. 9. Variation of electrical efficiency and PV power with condenser water supply temperature.
efficiency and PV power output increased with the increasing radiation. At the radiation of 200 W/m2, the electrical efficiency and PV power were 0.5% and 0.2 W, respectively. As the radiation increased to 800 W/m2, the electrical efficiency and PV power rose to 4.6% and 6.1 W, respectively. Before the construction of testing rig, the electrical performance of PV modules without cooling was tested for the investigation on the performance improvement made by refrigerant cooling. Fig. 7 shows the variation of electrical efficiency for the PV modules with and without cooling under different radiation. From Fig. 7, it can be seen that the electrical efficiency with refrigerant cooling was higher than that without cooling and the electrical efficiency improvement made by refrigerant cooling increased with the increasing radiation. At the radiation of 200 W/m2, the electrical efficiency was 0.21% without cooling and 0.5% with refrigerant cooling, respectively, with 0.29% improved. At the radiation of 800 W/m2, the electrical efficiency was 2.7% without cooling and 4.6% with refrigerant cooling, respectively, with 1.9% improved. 4.2. The effect of condenser water supply temperature The testing on the effect of condenser water supply temperature on energy performance of the hybrid heat pump system was carried out under the testing mode B. During the testing, the radiation, ambient temperature and condenser water flow rate were kept constant. Fig. 8 shows the variation of COP, condenser capacity and compressor power input under different condenser water supply temperature. It can be seen from Fig. 8 that the COP and condenser heat capacity decreased with the increasing condenser water supply temperature. At the condenser water Condenser capacity
The testing on the effect of condenser water flow on energy performance of the hybrid heat pump system was carried out under
Compressor power COP
800
6
3
400 300
2
200 1
100
0
0 20
25
30
35
40
45
50
Condenser water supply temperature ( Fig. 8. Variation of COP, condenser capacity and compressor power with condenser water supply temperature.
Power (W) Efficiency (%)
500
Compressor power 1500
6
Power (W)
600 4
Condenser capacity
7
700
5 COP
4.3. The effect of condenser water flow
1200
5 4
900
3
600
Power (W)
COP
supply temperature of 25 C, the COP and condenser heat capacity were 5.2 and 586 W, respectively. As the condenser water supply temperature increased to 45 C, the COP and condenser heat capacity dropped to 3.2 and 495 W, respectively. From Fig. 8, it can also be found that the compressor power input rose from 111.8 W to 153.9 W as the condenser water supply temperature increased from 25 C to 45 C. This is because that the increase of condenser water supply temperature leads to a higher condensing pressure, and thus, a higher compression ratio and higher compressor power input. When the condenser water supply temperature increased, the temperature difference between the refrigerant and water at the condenser decreased. Therefore, the condenser heat capacity dropped. Meanwhile, the compressor power input went up, so the COP dropped more sharply than the condenser heat capacity. Fig. 9 shows the variation of electrical efficiency and PV power output under different condenser water supply temperature. From Fig. 9, it can be seen that the radiation varied within 600 30 W/m2 during the testing mode B. The electrical efficiency and PV power output were mainly affected by the radiation instead of the condenser water supply temperature. The minimum radiation 573.6 W/m2 at the condenser water supply temperature of 30 C led to the minimum electrical efficiency of 4.77% and the minimum PV power output of 4.5 W, while the maximum radiation 626.5 W/m2 at the condenser water supply temperature of 45 C led to the maximum electrical efficiency of 4.86% and the maximum PV power output of 5.0 W.
2 300
1 0
0 0
1
2 3 4 Condenser water flow (L/min)
5
6
Fig. 10. Variation of COP, condenser capacity and compressor power with condenser water flow.
H. Chen et al. / Applied Thermal Engineering 31 (2011) 4132e4138
PV power
Electrical efficiency
dropped from 5.2 to 3.2, responding to the condenser supply temperature from 25 C to 45 C, at the radiation of w600 W/ m2 and condenser water flow rate of 2 L/min. The condenser water supply temperature had little effect on the PV power output and electrical efficiency. The COP of the heat pump system decreased with the increasing condenser water flow rate. The COP dropped from 6.7 to 2.8, responding to the condenser water flow rate from 1 L/min to 5 L/min, at the radiation of w600 W/m2 and condenser water supply temperature of 35 C. The condenser water flow rate had little effect on the PV power output and electrical efficiency.
Radia tion
5
800
Power (W) Efficiency (%)
600 500
3
400 2
300 200
1
Radiation (W/m 2 )
700 4
100 0
0 0
1
2 3 4 5 Condenser supply water flow (L/min)
6
Fig. 11. Variation of electrical efficiency and PV power with condenser water flow.
the testing mode C. During the testing, the radiation, ambient temperature and condenser water supply temperature were kept constant. From Fig. 10, it can be seen that the COP and condenser heat capacity dropped sharply as the condenser water flow increased from 1 L/min to 2 L/min, and then the drop tended to be smooth as the condenser water flow continued to increase from 2 L/ min to 5 L/min. At the condenser water flow of 1 L/min, the COP and condenser heat capacity were 6.7 and 916 W, respectively. As the condenser water flow increased to 5 L/min, the COP and condenser heat capacity dropped to 2.8 and 352 W, respectively. It can also be found that the compressor power input decreased slightly from 137.4 W to 125 W with the increasing condenser water flow. Fig. 11 shows the variation of electrical efficiency and PV power output under different condenser water flow. From Fig. 11, it can be seen that the radiation varied within 590w630 W/m2 during the testing mode C and had an important impact on the electrical efficiency and PV power output instead of the condenser water flow. The minimum radiation 598.1 W/m2 at the condenser water flow of 2 L/min led to the minimum electrical efficiency of 4.2% and the minimum PV power output of 4.1 W, while the maximum radiation 622.8 W/m2 at the condenser water supply temperature of 5 L/min led to the maximum electrical efficiency of 4.5% and the maximum PV power output of 4.6 W.
5. Conclusions The paper presents experimental study on the energy performance of a hybrid PV/heat pump system. The hybrid glass vacuum tube type PV panel-based heat pump system was constructed for the testing in a laboratory at the University of Nottingham. Three testing modes were proposed to investigate the effect of radiation, condenser water flow rate and condenser water supply temperature on energy performance of the heat pump. The study can be concluded that: The COP of the heat pump system increased with the increasing radiation. The COP varied from 2.9 to 4.6, responding to the radiation from 200 W/m2 to 800 W/m2, at the constant condenser water flow rate of 2 L/min and water supply temperature of 35 C. The COP was not as high as expected due to the low capacity compressor, selected to match the small size and low capacity of PV panel limited indoors. The PV power output and electrical efficiency also increased with the increasing radiation. The electrical efficiency of PV panel in the heat pump system was improved by up to 1.9%, compared with that without cooling. The COP of the heat pump system decreased with the increasing condenser water supply temperature. The COP
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Acknowledgements The work of this paper was fully supported by Marie Curie Incoming International Fellowship (No. PIIF-GA-2009-234843). Nomenclature
A Cp E G m R Q T W
area (m2) specific heat at constant pressure (J kg1 K1) photovoltaic power per unit surface area (W m2) solar irradiance (W m2) mass flow rate (kg s1) reflectance heat capacity (W) temperature (K) power (W)
Greek
b h s bs
absorptance efficiency transmittance effective absorptance
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