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thermal integrated dual-source heat pump water heating system
Accepted Manuscript Title: Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system Author: Qu Minglu, Chen Jianbo, Nie Linjie, Li Fengshu, Yu Qian, Wang Tan PII: DOI: Reference:
S1359-4311(15)01185-0 http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.126 ATE 7243
To appear in:
Applied Thermal Engineering
Received date: Accepted date:
28-7-2015 9-10-2015
Please cite this article as: Qu Minglu, Chen Jianbo, Nie Linjie, Li Fengshu, Yu Qian, Wang Tan, Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system, Applied Thermal Engineering (2015), http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.126. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system Qu Minglu*1, Chen Jianbo, Nie Linjie, Li Fengshu, Yu Qian, Wang Tan School of Environment & Architecture, University of Shanghai for Science & Technology No.516, Jungong Road, Shanghai, China Highlights
We propose a solar PV/T integrated dual-source heat pump (DSHP) water heater
The system work efficiently in both water-water mode and air-water mode.
We suggest the system adopting air-water mode when solar energy is insufficient.
In Shanghai, the electrical conversion efficiency can be increased by 10.3%.
ABSTRACT This paper proposed a novel solar photovoltaic/thermal (PV/T) integrated dual-source heat pump (DSHP) water heating system. This system used the circulating water in solar photovoltaic thermal system and air source to provide domestic hot water for domestic usage, and meanwhile, decreased the cooling water temperature for the PV/T panel to improve the electrical conversion efficiency. An experimental PV/T integrated DSHP unit was established in Shanghai to examine its performance. The influences of the inlet temperature of the evaporator and the hot water temperature *
Corresponding author. Tel.: +86 13795377789; fax: +86 021 55270686. E-mail address: [email protected] 1
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under water-water mode and the influence of the outdoor temperature under air-water mode on the system performance were studied. Energy analysis was conducted between water-water mode and air-water mode. The cooling effect and photovoltaic efficiency of the PV/T panel were evaluated. The results showed that when water side evaporator was used, the PV/T panel operating temperature can be decreased up to 45°C and the electrical conversion efficiency yielded a surplus of 10.3%. Keywords: Photovoltaic-thermal; Dual-source heat pump; Electrical conversion efficiency; Experimental
1. Introduction Photovoltaic (PV) technology has been widely used for generating electricity. However, the electrical conversion efficiency of PV module is usually less than 20%. Thus more than 50% of the incident solar energy is converted as heat and the temperature of PV module is increased. The photovoltaic/thermal (PV/T) systems produce thermal energy and electricity as total output energy, and the overall efficiency of the hybrid system can be remarkably improved for a given collector-area. It is well known that elevated PV panel temperature may decrease in voltage and power for single- and multi-crystalline silicon solar cells. The PV/T reduces the PV panel temperature by circulating fluid, resulting in higher electrical conversion efficiency [1]. Over the last decades, modeling and testing research on PV/T systems have been performed. Garg and Agarwal [2] established a mathematic model of PV/T system to study its photovoltaic and thermal performances. The influences of solar 2
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cell area and water flow rate on the system performance were studied. Tiwari et al. [3] conducted an exergy analysis of an integrated PV/T solar system, the effect of mass flow rate, length of PV module, heat capacity of water and constant collection temperature on an overall thermal and exergy efficiency were studied. Chaabane et al.[4] compared the electrical performance of concentrating PV and PV/T systems. The results showed that the concentrating PV/T system allowed higher electrical power output and electrical efficiency compared to the concentrating PV system. Buker et al. [5] presented a novel building integrated PV/T roof collector which integrated polyethylene heat exchanger loop underneath PV modules. The thermal and electrical performances, energy and exergy evaluation and techno-economic analysis of the PV/T roof collector under Nottingham’s climate conditions were provided.
Thermal energy from a PV/T system could be used for space heating, domestic water heating and even for air conditioning. A PV/T system can be coupled with a heat pump for continuous space heating [6]. In this way, the operating temperature of the PV panel can be decreased and meanwhile the evaporator temperature of the heat pump can be increased, resulting in higher electrical conversion efficiency for PV panel and higher COP for heat pump system. Consequently, the performances of both subsystems could be enhanced. Xu et al. [7] developed a novel low-concentrating solar PV/T integrated heat pump water heating system. The experimental results showed that the averaged system COP for heating water from 30°C to 70°C on a sunny summer day was 4.8, with an output electrical efficiency of 17.5%, 1.36 times 3
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higher than that of the same PV system without cooling. Chen et al. [8] established a testing rig of a hybrid glass vacuum tube type PV panel-based heat pump system, and studied the effects of radiation, condenser water flow rate and condenser water supply temperature on the energy performance of heat pump. Banister and Collins [9] developed a dual tank SAHP system for domestic hot water heating and its control strategy to minimize electricity consumption. The experimental and theoretical results showed that significant energy savings can be achieved compared to traditional electric or solar domestic hot water system.
Considering no solar energy can be used at night or when the solar irradiance intensity is low during cloudy or rainy days, dual source solar assisted heat pump is designed. The dual source SAHP has two evaporators, e.g., solar-side evaporator and air-side evaporator. The heat pump used either collected solar energy or the energy from the ambient air as the source [10]. Relative experimental and theoretical investigations have been done [11-13]. And the heating performances of series heat pump system, parallel heat pump system and dual source heat pump system were compared. To boost the performances of both the PV/T collector and the heat pump, and provide stable domestic hot water under different weather conditions, the PV/T integrated dual-source heat pump (DSHP) is proposed. However, research on the performance of PV/T integrated DSHP is seldom seen.
In this paper, a novel PV/T integrated DSHP water heating system was proposed. Two 4
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independent evaporators, e.g., air-cooled evaporator and water-cooled evaporator, are included in the novel PV/T integrated DSHP unit. Water-cooled evaporator can provide circulating cooling water for the PV/T panel to decrease its operating temperature, resulting in higher electrical conversion efficiency. On the other hand, the air-cooled evaporator can be used when the solar irradiance intensity is low, such as cloudy or raining day, to provide stable domestic hot water. An experimental setup where the novel PV/T integrated DSHP could be realized was specifically developed. And its performance was examined in Shanghai. Extensive experimental work on its operating characteristics has been carried out and experimental results were analyzed and are presented.
2. Experimental setup
2.1. The PV/T integrated DSHP water heating system The PV/T integrated DSHP water heating system presented here is at Shanghai, China, latitude: 31°14 ʹ N; longitude: 121°29 ʹ E. A schematic diagram of the PV/T integrated DSHP water heating system testing rig is shown in Fig. 1. The basic components include: PV/T panel, compressor with a rated cooling capacity of 2785 W, four-way reversing valve, condenser, air-cooled evaporator, water-cooled evaporator, water pumps, receiver, dryer and other auxiliary components such as solenoid valves. R134a is used as the refrigerant in this heat pump. The specifications of the experimental PV/T integrated DSHP water heating system are illustrated in Table 1. As seen in Fig. 5
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1, the air-cooled evaporator and water-cooled evaporator are connected in parallel in the DSHP system. When the solar energy is adequate, the heat pump adopts the water-cooled evaporator to provide both hot water and electricity by switching on Valve 2 and switching off Valve 1. Meanwhile, the water temperature in the heat storage tank can be reduced, as well as the water temperature flow though the PV/T panels, which cools the PV panel for higher electrical conversion efficiency. In this way, the heat in the heat storage tank can be transport to domestic hot water (DHW) tank through the heat pump cycle. While when the solar energy is insufficient, the heat pump adopts the air-cooled evaporator to provide hot water by switching on Valve 1 and switching off Valve 2.
2.2 Photovoltaic/thermal system The photovoltaic/thermal system mainly consisted of PV/T panels, grid-connected inverter, controller, power distribution cabinet, heat storage tank, air cooler, and water pump. The tilt angle of the PV/T panels was 22°[18,19],which was determined based on the calculation method proposed by Klien[20]. Fig. 2 is a photograph showing seven PV/T panels. Each PV/T panel had a surface area of 1.24 m2 (1550×800 mm). Fig. 3(a) illustrates the cross-sectional view of the sheet-and-tube PV/T panel. The PV module was encapsulated with mono-crystalline Si solar cells, and a copper sheet attached six parallel copper tubes and two main cooper tubes were bonding at the rear surface of the PV module using thermally conductive silicone rubber, as shown in Fig
3(b). An insulation layer of 0.04 m thickness was provided to reduce heat losses 6
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through the back of the panels. The panels were connected to the heat storage tank of capacity 500 L along with a flow meter. Water was circulated between PV/T collectors and heat storage tank using Pump 1. The circulation of water under the PV/T panels helped to extract the excessive heat from the PV module, hence reduced its operating temperature and increased the PV electrical conversion efficiency. Meanwhile, the transferring heat can be utilized for domestic hot water.
The DSHP water heating system was placed in the psychometric room which could provide a stable outdoor condition. Meanwhile, the psychometric room also provided stable cold/hot water to the condenser and the water-cooled evaporator according to the experimental conditions.
Table 1 Specifications of the experimental PV/T integrated DSHP water heating system
3. Experiments
3.1 Experimental conditions In this research, 25°C [14] was the control temperature for the PV/T panel. According to
the related criteria [15-17] and actual application, the experimental study were carried out under three groups of experiments (A, B and C). For the water-cooled evaporator, 7
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the influences of the inlet temperature of the water-cooled evaporator, and the hot water temperature on the operating performance were investigated (Group A and B). The experimental conditions in each group are listed in Table 2. And for the air-cooled evaporator, the influence of the outdoor temperature on operating performance were examined (Group C) was investigated under the outdoor temperature of -7°C, -5°C, 2°C, 7°C, 10°C, 15°C, 20°C, 25°C, 30°C with hot water temperature of 50°C. And reverse cycle defrosting method was adopted when the outdoor air temperature was below 12°C. The defrosting process was initiated when the temperature difference between air and the outdoor coil surface was higher than 12°C and the DSHP water heating system working under heating mode for more than 70 min. The defrosting process was terminated when the surface temperature of an outdoor coil was higher than 16 °C or the defrosting process was lasted for more than 12 mins.
Table 2 The experimental conditions in Group A and Group B
3.2 Test method A data acquisition unit was installed to simultaneously monitor the outdoor weather conditions and system’s operating conditions. Outdoor air temperature, relative humidity (RH) and wind speed were measured by a commercially available weather station, with uncertainty of ±0.5 oC for temperature, ±3% for RH, and±0.1m/s for wind speed measurement. Solar radiation intensity was measured by a Pyranometer 8
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mounted on the surface of solar collectors, with a measuring uncertainty of 2%.
Pre-calibrated T-type thermocouples (of ±0.3 °C accuracy) were used for measuring the refrigerant temperatures. Refrigerant pressures were measured using pressure transmitters with an accuracy of ± 0.3% of full scale reading. The temperature sensors for water at the inlet/outlet of the heat storage tank and DHW tank were of platinum Resistance Temperature Device (RTD) type (PT100, Class A) with a pre-calibrated accuracy of ± 0.1 ºC. The water flow rate was measured by electromagnetic flowmeter (of ±0.3% accuracy). The power consumption of the compressor, water pump and fan were measured using a pulse-width-modulation (PWM) digital power meter with a reported uncertainty of ± 2% of reading. To measure the temperature of the top/rear surface of the PV/T panel, three platinum RTDs (PT100, Class A) were attached
uniformly on the top/rear surface of the PV/T module, and the temperature of the top/rear surface was obtained by averaging the three temperature readings.
Coefficient of the performance (COP) of DSHP water heating system was evaluated by: COP
Qh
(1)
W
where W is the input power to the compressor, water pump, and fan, and Qh is the heating capacity of the DSHP water heating system, can be evaluated by: Q h cm 2 ( t
2o ,
t
)
i2 ,
(2)
where c is the specific heat of water, m2 is the water flow rate in the Pump 2 cycle, t2,o 9
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and t2,i are the outlet and inlet temperature of the condenser in the Pump 2 cycle, respectively.
Cooling capacity of the DSHP water heating system when adopt water-cooled evaporator is evaluated by: Q c cm 1 ( t
1i ,
t
)
o1 ,
(3)
where m1 is the water flow rate of the Pump 1 cycle, t1,o and t1,i are the outlet and inlet temperature of the heat storage tank at the Pump 1 cycle, respectively.
Electrical conversion efficiency was evaluated by: e
Pm
(4)
AG
where Pm is the maximum output power of the PV/T panel, A is the area of the PV/T panel, G is the solar radiation.
3.3. Experimental results 3.3.1 The influences of the inlet temperature of the water-cooled evaporator The comparisons of discharge and suction pressure/temperature variations under different inlet temperature of the water-cooled evaporator in Group A experiments are presented in Figs. 4 and 5. As seen in Fig.4, the discharge and suction pressure increased slightly with the increase of inlet temperature of the water-cooled evaporator. Similar trends can be found in Fig.5. When the inlet temperature was 30 °C, the discharge and suction temperature reached 82 °C and 23.6 °C, respectively. 10
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Fig. 6 shows the comparison of the cooling/heating capacity and the input power under different inlet temperature of the water-cooled evaporator in Group A experiments. The cooling/heating capacity increased with the increase of inlet temperature of the water-cooled evaporator. When the inlet temperature was 30 °C, the cooling capacity and heating capacity were 5.22 kW and 4.61 kW, respectively. When the inlet temperature was 10 °C, the heating capacity was 2.41 kW. The input power also increased with the increase of inlet temperature of the water-cooled evaporator. The results showed that the PV/T integrated DSHP water heater system can run stably under 10~30 °C of inlet temperature of the water-cooled evaporator.
3.3.2 The influences of the hot water temperature Fig. 7 shows the discharge and suction pressure variation comparison under different hot water temperature in Group B experiments. As seen, the higher the hot water temperature, the higher the discharge pressure. When the hot water temperature reached 55 °C, the discharge pressure was 19.7 bar. The suction pressure was stabled at about 3.6 bar under different hot water temperature.
Fig. 8 presents the discharge and suction temperature variations comparison under 11
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different hot water temperature in Group B experiments. Similar trends can be seen as those in Fig. 7. When the hot water temperature reached 55 °C, the discharge pressure was 66.8°C. The suction temperature was stabled at about 13.1°C under different hot water temperatures.
Fig. 9 shows the comparison of the cooling/heating capacity and the input power under different hot water temperature in Group B experiments. The cooling/heating capacity decreased with the increase of hot water temperature. When the hot water temperature was 50 °C, the cooling capacity and heating capacity were 2.85 kW and 3.59 kW, respectively. The input power increased with the the increase of hot water temperature. When the hot water temperature was 55 °C, the input power was 1.07 kW.
3.3.3 Energy efficiency analysis of the DSHP system The system COP comparisons under different inlet temperature of evaporator are shown in Fig. 10. The system COP increased along with the inlet temperature of evaporator. When the inlet temperature of evaporator was 20 °C and the hot water temperature was 50 °C, the COP was 3.6. When the inlet temperature of evaporator was 10 °C and the hot water temperature was 50 °C, the COP was 2.63, which means the PV/T integrated DSHP water heating system can work efficiently. When the inlet 12
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temperature of evaporator was 30 °C and the hot water temperature was 50 °C, the COP reached 5.05, showing good performance of the PV/T integrated DSHP water heating system.
When the system was operating under air-water mode, the influence of the outdoor temperature on operating performance were examined in Group C under the outdoor temperature of -7°C to 30°C with hot water temperature of 50°C. The system COP comparisons under different inlet water temperature/outdoor air temperature are illustrated in Fig. 11. The system COP increased along with the inlet temperature or the outdoor air temperature. When the temperature was higher than 20 °C, the system COP under water-water mode was higher than that in water-air mode. When the temperature was between 10~20°C, the system COPs in the two working conditions were almost the same.
Based on the annual experimental results of the PV/T panels temperature, there were 270 days that the PV/T panels temperature was higher than 25°C in Shanghai, accounting for 80.7% in one year. Therefore, the PV/T integrated DSHP water heating system can work in water-water mode efficiently for 270 days in one year to provide hot water by using solar energy and increase the electrical conversion efficiency by decreasing the temperature of the PV/T panel. While in winter, the outdoor 13
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temperature usually dropped below 10°C in Shanghai, sometimes even lower than 0°C. From the experimental results, the days that the PV/T panels temperature higher than 15°C account for 67.7% in December, 57.2% in January. Therefore, the PV/T integrated DSHP water heating system can work in water-water mode more efficiently than that in water-air mode in winter. Meanwhile, working in water-water mode can avoid frosting and defrosting operation occurring in air-water mode in winter. When the solar energy is insufficient in cloudy or rainy days, the PV/T integrated DSHP water heating system can work in air-water mode to provide continuous hot water efficiently.
3.3.4 Electrical performance Electrical performance of the PV/T integrated DSHP water heating system was experimentally measured on May 15th, 2014, a sunny day in Shanghai, China. The average outdoor air velocity was 1.35 m/s. The PV/T integrated DSHP water heating system was switched on at 7:45. At 11:45, Pump 1 was turned on to cool the PV/T panel, and the circulated water flow rate was 0.75 m3/h. The setting hot water temperature was 50°C.
Fig. 12 shows the water temperatures in DHW tank and heat storage tank. From 7:45 to 11:45, the water temperature in the heat storage tank dropped from 26.8°C to 17.5°C as Pump 1 was off and the PV panel was work without cooling. At 11:45, 14
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Pump 1 was turned on to cool the PV/T panel, the water temperature in heat storage tank increased to 24.0°C at 14:15, and then decreased gradually to 20.2°C. The water temperature in heat storage tank maintained at ~22°C, which guarantee the continuous operation of the DSHP system. The water temperatures in DHW tank increased from 30°C to 50°C in 1.5 hour.
The variations of outdoor temperature, sunlight intensity, the top and rear surface temperature of the PV/T panel with time are shown in Fig. 13. Before Pump 1 was
turned on, the top/rear surface temperature of the PV/T panel was increased with the increasing of the sunlight intensity and the operating time. The top surface temperature of the PV/T panel reached its maximum 69.2°C at 11:00-11:45. At 12:15, the top surface temperature of the PV/T panel decreased from 69.2°C to 46.7°C as Pump 1 was turned on at 11:45. As seen, the rear surface temperature of the PV/T panel was always lower than the top surface temperature as the cooling water was passing through the bottom of the PV/T panel. And with the cooling of the PV/T panel,
the top/rear temperature of the PV/T panel finally dropped to 24°C, decreased by ~45°C, showing that adopting cooling water supplied by DSHP is an effective cooling measure for the PV/T panel.
Fig.14 presents the variations of top surface temperature of the PV/T panel and the electrical conversion efficiency with time. As seen, when the top temperature was 15
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~69.2°C at 11:00-11:45, the electrical conversion efficiency was ~12.18%. And the electrical conversion efficiency began to increase as the top surface temperature of the PV/T panel decreased at 11:45. From 11:45 to 12:15, the electrical conversion efficiency raised to13.44%, increased by 10.3%. From 12:45, the electrical conversion efficiency decreased gradually along with the decreasing of sunlight intensity, as seen in Fig. 13.
4. Conclusions
This paper proposed a novel PV/T integrated DSHP water heating system, and studied the performance of the system. The following conclusions can be drawn: 1) The PV/T integrated DSHP water heating system can provide hot water stably, and reliably under both air-water and water-water condition. 2) Based on the experimental results of energy efficiency analysis, the PV/T integrated DSHP water heating system work efficiently in water-water mode, and when the solar energy is insufficient in cloudy or rainy days, the PV/T integrated DSHP water heating system can be operated in air-water mode. 3) Taking the weather conditions of Shanghai as an example, the electrical conversion efficiency can be increased by 10.3% as the top temperature of the PV/T panel decreased by ~45°C when cooling is provided.
The geographical location, the weather condition the system will have a huge impact on the system performance. Therefore, it is necessary to develop a model for the novel 16
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PV/T integrated DSHP water heating system to investigate its performance under different conditions.
Acknowledgments The authors wish to acknowledge the funding supports from The National Natural Science Foundation of China (Project No.: 51406119), Shanghai Sailing Program of Shanghai Committee of Science and Technology, China (Project No.: 14YF1410000), the Hujiang Foundation of China (Project No.: D14003)
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analysis of integrated photovoltaic thermal solar water heater under constant flow rate and constant collection temperature modes. Appl Energy 2009; 86:2592–2597. [4] Chaabane M, Charfi W, Mhiri H, Bournot P. Performance evaluation of
concentrating solar photovoltaic and photovoltaic thermal systems. Sol Energy 2013; 98:315–321. 17
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[5] Buker MS, Mempouo B, Riffat SB. Performance evaluation and techno-economic
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18
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[14] Wilson E. Theoretical and operational thermal performance of a ‘wet’ crystalline
silicon
PV module
under
Jamaican
conditions.
Renew
Energ
2009;
34:1655-1660. [15] China National Standard GB/T19409-2013. National standard for water (ground)
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commercial & industrial and similar application. Beijing: China Standards Press, 2008. [18] Yue WW. Research on Application features of solar photovoltaic optothermal
integrated components. Mater Dissertation, University of Shanghai for Science and Technology 2013 [in Chinese]. [19] Sun YL, Du XR, Wang XY, Luo L. Calculation of solar radiation and optimum
tilted angle of fixed grid connected solar PV array. Acta Energiae Solaris Sinica 2009, 30: 1597-1601[in Chinese] [20] Klein SA. Calculation of monthly average insolation on tilted surfaces. Solar
Energy 1977, 19(4):325-329
Nomenclature A
Area of the PV/T panel, m2 19
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COP
Coefficient of the performance,-
c
Specific heat of water, kJ/kg·°C
G
Solar radiation, W/m2
m
Water flow rate, kg/s
Pm
The maximum output power of the PV/T panel, W
Qh
Heating capacity, W
Qc
Cooling capacity, W
t
Water temperature, °C
W
Input power to the compressor, water pump, and fan, W
Greek symbols e
Electrical conversion efficiency, %
Subscripts i
Inlet
o
Outlet
1
Pump 1 cycle
2
Pump 2 cycle
20
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Figure captions Fig. 1 Schematic diagram of the experimental prototype of the novel PV/T integrated DSHP water heating system Fig. 2 The photograph showing the PV/T solar collectors mounted Fig. 3(a) Cross-sectional view of PV/T module (b) arrangement of the copper tubes at the rear surface of PV modules Fig. 4 Discharge and suction pressure comparison under different inlet temperature of evaporator Fig. 5 Discharge and suction temperature comparison under different inlet temperature of evaporator Fig. 6 Cooling/heating capacity and input power comparison under different inlet temperature of evaporator Fig. 7 Discharge and suction pressure variation comparison under different hot water temperature Fig. 8 Discharge and suction temperature comparison under different hot water temperature Fig. 9 Cooling/heating capacity and input power comparison under different hot water temperature Fig. 10 System COP comparison under different inlet temperature of evaporator Fig. 11 System COP comparison under different inlet water temperature/outdoor air temperature Fig. 12 The water tank temperature variation with time Fig. 13 The variations of outdoor temperature, sunlight intensity, the top and rear 21
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surface temperature of the PV panel with time Fig. 14 The variations of top surface temperature of the PV panel and the electrical conversion efficiency with time
22
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Table 1 Specifications of the experimental PV/T integrated DSHP water heating system Parameters
Values/ details
Compressor Rated input power
0.895 kW
Rated cooling capacity
2.785 kW
Refrigerant
R134a
Charge amount
1.2 kg
Air-cooled Evaporator Type
Finned tube
Tube external diameter
9 mm
Tube spacing
25.4 mm
Fin pitch
1.8 mm
Air flow rate
1500 m3/h
Water-cooled evaporator Type
Plate heat exchanger
Rated cooling capacity
2.5 kW
Area of heat transfer
0.3 m2
Number of plate
12
Water side condenser Type
Double-pipe heat interchanger
Maximum heat capacity
5.4 kW
23
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Refrigerant outlet tube external diameter
12.7 mm
Water inlet tube external diameter
15.88 mm
TEV Type
Externally equalized
Heat storage tank
500 L
Hot water tank
200 L
24
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Table 2 The experimental conditions in Group A and Group B Group
Inlet
temperature
of
the Hot water temperature (°C)
water-cooled evaporator (°C) A1
10
50
A2
15
50
A3
20
50
A4
25
50
A5
30
50
B1
20
45
B2
20
50
B3
20
55
25
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S1359-4311(15)01185-0 http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.126 ATE 7243
To appear in:
Applied Thermal Engineering
Received date: Accepted date:
28-7-2015 9-10-2015
Please cite this article as: Qu Minglu, Chen Jianbo, Nie Linjie, Li Fengshu, Yu Qian, Wang Tan, Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system, Applied Thermal Engineering (2015), http://dx.doi.org/doi: 10.1016/j.applthermaleng.2015.10.126. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system Qu Minglu*1, Chen Jianbo, Nie Linjie, Li Fengshu, Yu Qian, Wang Tan School of Environment & Architecture, University of Shanghai for Science & Technology No.516, Jungong Road, Shanghai, China Highlights
We propose a solar PV/T integrated dual-source heat pump (DSHP) water heater
The system work efficiently in both water-water mode and air-water mode.
We suggest the system adopting air-water mode when solar energy is insufficient.
In Shanghai, the electrical conversion efficiency can be increased by 10.3%.
ABSTRACT This paper proposed a novel solar photovoltaic/thermal (PV/T) integrated dual-source heat pump (DSHP) water heating system. This system used the circulating water in solar photovoltaic thermal system and air source to provide domestic hot water for domestic usage, and meanwhile, decreased the cooling water temperature for the PV/T panel to improve the electrical conversion efficiency. An experimental PV/T integrated DSHP unit was established in Shanghai to examine its performance. The influences of the inlet temperature of the evaporator and the hot water temperature *
Corresponding author. Tel.: +86 13795377789; fax: +86 021 55270686. E-mail address: [email protected] 1
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under water-water mode and the influence of the outdoor temperature under air-water mode on the system performance were studied. Energy analysis was conducted between water-water mode and air-water mode. The cooling effect and photovoltaic efficiency of the PV/T panel were evaluated. The results showed that when water side evaporator was used, the PV/T panel operating temperature can be decreased up to 45°C and the electrical conversion efficiency yielded a surplus of 10.3%. Keywords: Photovoltaic-thermal; Dual-source heat pump; Electrical conversion efficiency; Experimental
1. Introduction Photovoltaic (PV) technology has been widely used for generating electricity. However, the electrical conversion efficiency of PV module is usually less than 20%. Thus more than 50% of the incident solar energy is converted as heat and the temperature of PV module is increased. The photovoltaic/thermal (PV/T) systems produce thermal energy and electricity as total output energy, and the overall efficiency of the hybrid system can be remarkably improved for a given collector-area. It is well known that elevated PV panel temperature may decrease in voltage and power for single- and multi-crystalline silicon solar cells. The PV/T reduces the PV panel temperature by circulating fluid, resulting in higher electrical conversion efficiency [1]. Over the last decades, modeling and testing research on PV/T systems have been performed. Garg and Agarwal [2] established a mathematic model of PV/T system to study its photovoltaic and thermal performances. The influences of solar 2
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cell area and water flow rate on the system performance were studied. Tiwari et al. [3] conducted an exergy analysis of an integrated PV/T solar system, the effect of mass flow rate, length of PV module, heat capacity of water and constant collection temperature on an overall thermal and exergy efficiency were studied. Chaabane et al.[4] compared the electrical performance of concentrating PV and PV/T systems. The results showed that the concentrating PV/T system allowed higher electrical power output and electrical efficiency compared to the concentrating PV system. Buker et al. [5] presented a novel building integrated PV/T roof collector which integrated polyethylene heat exchanger loop underneath PV modules. The thermal and electrical performances, energy and exergy evaluation and techno-economic analysis of the PV/T roof collector under Nottingham’s climate conditions were provided.
Thermal energy from a PV/T system could be used for space heating, domestic water heating and even for air conditioning. A PV/T system can be coupled with a heat pump for continuous space heating [6]. In this way, the operating temperature of the PV panel can be decreased and meanwhile the evaporator temperature of the heat pump can be increased, resulting in higher electrical conversion efficiency for PV panel and higher COP for heat pump system. Consequently, the performances of both subsystems could be enhanced. Xu et al. [7] developed a novel low-concentrating solar PV/T integrated heat pump water heating system. The experimental results showed that the averaged system COP for heating water from 30°C to 70°C on a sunny summer day was 4.8, with an output electrical efficiency of 17.5%, 1.36 times 3
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higher than that of the same PV system without cooling. Chen et al. [8] established a testing rig of a hybrid glass vacuum tube type PV panel-based heat pump system, and studied the effects of radiation, condenser water flow rate and condenser water supply temperature on the energy performance of heat pump. Banister and Collins [9] developed a dual tank SAHP system for domestic hot water heating and its control strategy to minimize electricity consumption. The experimental and theoretical results showed that significant energy savings can be achieved compared to traditional electric or solar domestic hot water system.
Considering no solar energy can be used at night or when the solar irradiance intensity is low during cloudy or rainy days, dual source solar assisted heat pump is designed. The dual source SAHP has two evaporators, e.g., solar-side evaporator and air-side evaporator. The heat pump used either collected solar energy or the energy from the ambient air as the source [10]. Relative experimental and theoretical investigations have been done [11-13]. And the heating performances of series heat pump system, parallel heat pump system and dual source heat pump system were compared. To boost the performances of both the PV/T collector and the heat pump, and provide stable domestic hot water under different weather conditions, the PV/T integrated dual-source heat pump (DSHP) is proposed. However, research on the performance of PV/T integrated DSHP is seldom seen.
In this paper, a novel PV/T integrated DSHP water heating system was proposed. Two 4
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independent evaporators, e.g., air-cooled evaporator and water-cooled evaporator, are included in the novel PV/T integrated DSHP unit. Water-cooled evaporator can provide circulating cooling water for the PV/T panel to decrease its operating temperature, resulting in higher electrical conversion efficiency. On the other hand, the air-cooled evaporator can be used when the solar irradiance intensity is low, such as cloudy or raining day, to provide stable domestic hot water. An experimental setup where the novel PV/T integrated DSHP could be realized was specifically developed. And its performance was examined in Shanghai. Extensive experimental work on its operating characteristics has been carried out and experimental results were analyzed and are presented.
2. Experimental setup
2.1. The PV/T integrated DSHP water heating system The PV/T integrated DSHP water heating system presented here is at Shanghai, China, latitude: 31°14 ʹ N; longitude: 121°29 ʹ E. A schematic diagram of the PV/T integrated DSHP water heating system testing rig is shown in Fig. 1. The basic components include: PV/T panel, compressor with a rated cooling capacity of 2785 W, four-way reversing valve, condenser, air-cooled evaporator, water-cooled evaporator, water pumps, receiver, dryer and other auxiliary components such as solenoid valves. R134a is used as the refrigerant in this heat pump. The specifications of the experimental PV/T integrated DSHP water heating system are illustrated in Table 1. As seen in Fig. 5
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1, the air-cooled evaporator and water-cooled evaporator are connected in parallel in the DSHP system. When the solar energy is adequate, the heat pump adopts the water-cooled evaporator to provide both hot water and electricity by switching on Valve 2 and switching off Valve 1. Meanwhile, the water temperature in the heat storage tank can be reduced, as well as the water temperature flow though the PV/T panels, which cools the PV panel for higher electrical conversion efficiency. In this way, the heat in the heat storage tank can be transport to domestic hot water (DHW) tank through the heat pump cycle. While when the solar energy is insufficient, the heat pump adopts the air-cooled evaporator to provide hot water by switching on Valve 1 and switching off Valve 2.
2.2 Photovoltaic/thermal system The photovoltaic/thermal system mainly consisted of PV/T panels, grid-connected inverter, controller, power distribution cabinet, heat storage tank, air cooler, and water pump. The tilt angle of the PV/T panels was 22°[18,19],which was determined based on the calculation method proposed by Klien[20]. Fig. 2 is a photograph showing seven PV/T panels. Each PV/T panel had a surface area of 1.24 m2 (1550×800 mm). Fig. 3(a) illustrates the cross-sectional view of the sheet-and-tube PV/T panel. The PV module was encapsulated with mono-crystalline Si solar cells, and a copper sheet attached six parallel copper tubes and two main cooper tubes were bonding at the rear surface of the PV module using thermally conductive silicone rubber, as shown in Fig
3(b). An insulation layer of 0.04 m thickness was provided to reduce heat losses 6
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through the back of the panels. The panels were connected to the heat storage tank of capacity 500 L along with a flow meter. Water was circulated between PV/T collectors and heat storage tank using Pump 1. The circulation of water under the PV/T panels helped to extract the excessive heat from the PV module, hence reduced its operating temperature and increased the PV electrical conversion efficiency. Meanwhile, the transferring heat can be utilized for domestic hot water.
The DSHP water heating system was placed in the psychometric room which could provide a stable outdoor condition. Meanwhile, the psychometric room also provided stable cold/hot water to the condenser and the water-cooled evaporator according to the experimental conditions.
Table 1 Specifications of the experimental PV/T integrated DSHP water heating system
3. Experiments
3.1 Experimental conditions In this research, 25°C [14] was the control temperature for the PV/T panel. According to
the related criteria [15-17] and actual application, the experimental study were carried out under three groups of experiments (A, B and C). For the water-cooled evaporator, 7
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the influences of the inlet temperature of the water-cooled evaporator, and the hot water temperature on the operating performance were investigated (Group A and B). The experimental conditions in each group are listed in Table 2. And for the air-cooled evaporator, the influence of the outdoor temperature on operating performance were examined (Group C) was investigated under the outdoor temperature of -7°C, -5°C, 2°C, 7°C, 10°C, 15°C, 20°C, 25°C, 30°C with hot water temperature of 50°C. And reverse cycle defrosting method was adopted when the outdoor air temperature was below 12°C. The defrosting process was initiated when the temperature difference between air and the outdoor coil surface was higher than 12°C and the DSHP water heating system working under heating mode for more than 70 min. The defrosting process was terminated when the surface temperature of an outdoor coil was higher than 16 °C or the defrosting process was lasted for more than 12 mins.
Table 2 The experimental conditions in Group A and Group B
3.2 Test method A data acquisition unit was installed to simultaneously monitor the outdoor weather conditions and system’s operating conditions. Outdoor air temperature, relative humidity (RH) and wind speed were measured by a commercially available weather station, with uncertainty of ±0.5 oC for temperature, ±3% for RH, and±0.1m/s for wind speed measurement. Solar radiation intensity was measured by a Pyranometer 8
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mounted on the surface of solar collectors, with a measuring uncertainty of 2%.
Pre-calibrated T-type thermocouples (of ±0.3 °C accuracy) were used for measuring the refrigerant temperatures. Refrigerant pressures were measured using pressure transmitters with an accuracy of ± 0.3% of full scale reading. The temperature sensors for water at the inlet/outlet of the heat storage tank and DHW tank were of platinum Resistance Temperature Device (RTD) type (PT100, Class A) with a pre-calibrated accuracy of ± 0.1 ºC. The water flow rate was measured by electromagnetic flowmeter (of ±0.3% accuracy). The power consumption of the compressor, water pump and fan were measured using a pulse-width-modulation (PWM) digital power meter with a reported uncertainty of ± 2% of reading. To measure the temperature of the top/rear surface of the PV/T panel, three platinum RTDs (PT100, Class A) were attached
uniformly on the top/rear surface of the PV/T module, and the temperature of the top/rear surface was obtained by averaging the three temperature readings.
Coefficient of the performance (COP) of DSHP water heating system was evaluated by: COP
Qh
(1)
W
where W is the input power to the compressor, water pump, and fan, and Qh is the heating capacity of the DSHP water heating system, can be evaluated by: Q h cm 2 ( t
2o ,
t
)
i2 ,
(2)
where c is the specific heat of water, m2 is the water flow rate in the Pump 2 cycle, t2,o 9
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and t2,i are the outlet and inlet temperature of the condenser in the Pump 2 cycle, respectively.
Cooling capacity of the DSHP water heating system when adopt water-cooled evaporator is evaluated by: Q c cm 1 ( t
1i ,
t
)
o1 ,
(3)
where m1 is the water flow rate of the Pump 1 cycle, t1,o and t1,i are the outlet and inlet temperature of the heat storage tank at the Pump 1 cycle, respectively.
Electrical conversion efficiency was evaluated by: e
Pm
(4)
AG
where Pm is the maximum output power of the PV/T panel, A is the area of the PV/T panel, G is the solar radiation.
3.3. Experimental results 3.3.1 The influences of the inlet temperature of the water-cooled evaporator The comparisons of discharge and suction pressure/temperature variations under different inlet temperature of the water-cooled evaporator in Group A experiments are presented in Figs. 4 and 5. As seen in Fig.4, the discharge and suction pressure increased slightly with the increase of inlet temperature of the water-cooled evaporator. Similar trends can be found in Fig.5. When the inlet temperature was 30 °C, the discharge and suction temperature reached 82 °C and 23.6 °C, respectively. 10
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Fig. 6 shows the comparison of the cooling/heating capacity and the input power under different inlet temperature of the water-cooled evaporator in Group A experiments. The cooling/heating capacity increased with the increase of inlet temperature of the water-cooled evaporator. When the inlet temperature was 30 °C, the cooling capacity and heating capacity were 5.22 kW and 4.61 kW, respectively. When the inlet temperature was 10 °C, the heating capacity was 2.41 kW. The input power also increased with the increase of inlet temperature of the water-cooled evaporator. The results showed that the PV/T integrated DSHP water heater system can run stably under 10~30 °C of inlet temperature of the water-cooled evaporator.
3.3.2 The influences of the hot water temperature Fig. 7 shows the discharge and suction pressure variation comparison under different hot water temperature in Group B experiments. As seen, the higher the hot water temperature, the higher the discharge pressure. When the hot water temperature reached 55 °C, the discharge pressure was 19.7 bar. The suction pressure was stabled at about 3.6 bar under different hot water temperature.
Fig. 8 presents the discharge and suction temperature variations comparison under 11
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different hot water temperature in Group B experiments. Similar trends can be seen as those in Fig. 7. When the hot water temperature reached 55 °C, the discharge pressure was 66.8°C. The suction temperature was stabled at about 13.1°C under different hot water temperatures.
Fig. 9 shows the comparison of the cooling/heating capacity and the input power under different hot water temperature in Group B experiments. The cooling/heating capacity decreased with the increase of hot water temperature. When the hot water temperature was 50 °C, the cooling capacity and heating capacity were 2.85 kW and 3.59 kW, respectively. The input power increased with the the increase of hot water temperature. When the hot water temperature was 55 °C, the input power was 1.07 kW.
3.3.3 Energy efficiency analysis of the DSHP system The system COP comparisons under different inlet temperature of evaporator are shown in Fig. 10. The system COP increased along with the inlet temperature of evaporator. When the inlet temperature of evaporator was 20 °C and the hot water temperature was 50 °C, the COP was 3.6. When the inlet temperature of evaporator was 10 °C and the hot water temperature was 50 °C, the COP was 2.63, which means the PV/T integrated DSHP water heating system can work efficiently. When the inlet 12
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temperature of evaporator was 30 °C and the hot water temperature was 50 °C, the COP reached 5.05, showing good performance of the PV/T integrated DSHP water heating system.
When the system was operating under air-water mode, the influence of the outdoor temperature on operating performance were examined in Group C under the outdoor temperature of -7°C to 30°C with hot water temperature of 50°C. The system COP comparisons under different inlet water temperature/outdoor air temperature are illustrated in Fig. 11. The system COP increased along with the inlet temperature or the outdoor air temperature. When the temperature was higher than 20 °C, the system COP under water-water mode was higher than that in water-air mode. When the temperature was between 10~20°C, the system COPs in the two working conditions were almost the same.
Based on the annual experimental results of the PV/T panels temperature, there were 270 days that the PV/T panels temperature was higher than 25°C in Shanghai, accounting for 80.7% in one year. Therefore, the PV/T integrated DSHP water heating system can work in water-water mode efficiently for 270 days in one year to provide hot water by using solar energy and increase the electrical conversion efficiency by decreasing the temperature of the PV/T panel. While in winter, the outdoor 13
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temperature usually dropped below 10°C in Shanghai, sometimes even lower than 0°C. From the experimental results, the days that the PV/T panels temperature higher than 15°C account for 67.7% in December, 57.2% in January. Therefore, the PV/T integrated DSHP water heating system can work in water-water mode more efficiently than that in water-air mode in winter. Meanwhile, working in water-water mode can avoid frosting and defrosting operation occurring in air-water mode in winter. When the solar energy is insufficient in cloudy or rainy days, the PV/T integrated DSHP water heating system can work in air-water mode to provide continuous hot water efficiently.
3.3.4 Electrical performance Electrical performance of the PV/T integrated DSHP water heating system was experimentally measured on May 15th, 2014, a sunny day in Shanghai, China. The average outdoor air velocity was 1.35 m/s. The PV/T integrated DSHP water heating system was switched on at 7:45. At 11:45, Pump 1 was turned on to cool the PV/T panel, and the circulated water flow rate was 0.75 m3/h. The setting hot water temperature was 50°C.
Fig. 12 shows the water temperatures in DHW tank and heat storage tank. From 7:45 to 11:45, the water temperature in the heat storage tank dropped from 26.8°C to 17.5°C as Pump 1 was off and the PV panel was work without cooling. At 11:45, 14
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Pump 1 was turned on to cool the PV/T panel, the water temperature in heat storage tank increased to 24.0°C at 14:15, and then decreased gradually to 20.2°C. The water temperature in heat storage tank maintained at ~22°C, which guarantee the continuous operation of the DSHP system. The water temperatures in DHW tank increased from 30°C to 50°C in 1.5 hour.
The variations of outdoor temperature, sunlight intensity, the top and rear surface temperature of the PV/T panel with time are shown in Fig. 13. Before Pump 1 was
turned on, the top/rear surface temperature of the PV/T panel was increased with the increasing of the sunlight intensity and the operating time. The top surface temperature of the PV/T panel reached its maximum 69.2°C at 11:00-11:45. At 12:15, the top surface temperature of the PV/T panel decreased from 69.2°C to 46.7°C as Pump 1 was turned on at 11:45. As seen, the rear surface temperature of the PV/T panel was always lower than the top surface temperature as the cooling water was passing through the bottom of the PV/T panel. And with the cooling of the PV/T panel,
the top/rear temperature of the PV/T panel finally dropped to 24°C, decreased by ~45°C, showing that adopting cooling water supplied by DSHP is an effective cooling measure for the PV/T panel.
Fig.14 presents the variations of top surface temperature of the PV/T panel and the electrical conversion efficiency with time. As seen, when the top temperature was 15
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~69.2°C at 11:00-11:45, the electrical conversion efficiency was ~12.18%. And the electrical conversion efficiency began to increase as the top surface temperature of the PV/T panel decreased at 11:45. From 11:45 to 12:15, the electrical conversion efficiency raised to13.44%, increased by 10.3%. From 12:45, the electrical conversion efficiency decreased gradually along with the decreasing of sunlight intensity, as seen in Fig. 13.
4. Conclusions
This paper proposed a novel PV/T integrated DSHP water heating system, and studied the performance of the system. The following conclusions can be drawn: 1) The PV/T integrated DSHP water heating system can provide hot water stably, and reliably under both air-water and water-water condition. 2) Based on the experimental results of energy efficiency analysis, the PV/T integrated DSHP water heating system work efficiently in water-water mode, and when the solar energy is insufficient in cloudy or rainy days, the PV/T integrated DSHP water heating system can be operated in air-water mode. 3) Taking the weather conditions of Shanghai as an example, the electrical conversion efficiency can be increased by 10.3% as the top temperature of the PV/T panel decreased by ~45°C when cooling is provided.
The geographical location, the weather condition the system will have a huge impact on the system performance. Therefore, it is necessary to develop a model for the novel 16
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PV/T integrated DSHP water heating system to investigate its performance under different conditions.
Acknowledgments The authors wish to acknowledge the funding supports from The National Natural Science Foundation of China (Project No.: 51406119), Shanghai Sailing Program of Shanghai Committee of Science and Technology, China (Project No.: 14YF1410000), the Hujiang Foundation of China (Project No.: D14003)
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[14] Wilson E. Theoretical and operational thermal performance of a ‘wet’ crystalline
silicon
PV module
under
Jamaican
conditions.
Renew
Energ
2009;
34:1655-1660. [15] China National Standard GB/T19409-2013. National standard for water (ground)
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commercial & industrial and similar application. Beijing: China Standards Press, 2008. [18] Yue WW. Research on Application features of solar photovoltaic optothermal
integrated components. Mater Dissertation, University of Shanghai for Science and Technology 2013 [in Chinese]. [19] Sun YL, Du XR, Wang XY, Luo L. Calculation of solar radiation and optimum
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Energy 1977, 19(4):325-329
Nomenclature A
Area of the PV/T panel, m2 19
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COP
Coefficient of the performance,-
c
Specific heat of water, kJ/kg·°C
G
Solar radiation, W/m2
m
Water flow rate, kg/s
Pm
The maximum output power of the PV/T panel, W
Qh
Heating capacity, W
Qc
Cooling capacity, W
t
Water temperature, °C
W
Input power to the compressor, water pump, and fan, W
Greek symbols e
Electrical conversion efficiency, %
Subscripts i
Inlet
o
Outlet
1
Pump 1 cycle
2
Pump 2 cycle
20
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Figure captions Fig. 1 Schematic diagram of the experimental prototype of the novel PV/T integrated DSHP water heating system Fig. 2 The photograph showing the PV/T solar collectors mounted Fig. 3(a) Cross-sectional view of PV/T module (b) arrangement of the copper tubes at the rear surface of PV modules Fig. 4 Discharge and suction pressure comparison under different inlet temperature of evaporator Fig. 5 Discharge and suction temperature comparison under different inlet temperature of evaporator Fig. 6 Cooling/heating capacity and input power comparison under different inlet temperature of evaporator Fig. 7 Discharge and suction pressure variation comparison under different hot water temperature Fig. 8 Discharge and suction temperature comparison under different hot water temperature Fig. 9 Cooling/heating capacity and input power comparison under different hot water temperature Fig. 10 System COP comparison under different inlet temperature of evaporator Fig. 11 System COP comparison under different inlet water temperature/outdoor air temperature Fig. 12 The water tank temperature variation with time Fig. 13 The variations of outdoor temperature, sunlight intensity, the top and rear 21
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surface temperature of the PV panel with time Fig. 14 The variations of top surface temperature of the PV panel and the electrical conversion efficiency with time
22
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Table 1 Specifications of the experimental PV/T integrated DSHP water heating system Parameters
Values/ details
Compressor Rated input power
0.895 kW
Rated cooling capacity
2.785 kW
Refrigerant
R134a
Charge amount
1.2 kg
Air-cooled Evaporator Type
Finned tube
Tube external diameter
9 mm
Tube spacing
25.4 mm
Fin pitch
1.8 mm
Air flow rate
1500 m3/h
Water-cooled evaporator Type
Plate heat exchanger
Rated cooling capacity
2.5 kW
Area of heat transfer
0.3 m2
Number of plate
12
Water side condenser Type
Double-pipe heat interchanger
Maximum heat capacity
5.4 kW
23
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Refrigerant outlet tube external diameter
12.7 mm
Water inlet tube external diameter
15.88 mm
TEV Type
Externally equalized
Heat storage tank
500 L
Hot water tank
200 L
24
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Table 2 The experimental conditions in Group A and Group B Group
Inlet
temperature
of
the Hot water temperature (°C)
water-cooled evaporator (°C) A1
10
50
A2
15
50
A3
20
50
A4
25
50
A5
30
50
B1
20
45
B2
20
50
B3
20
55
25
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