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Energy 195 (2020) 116959
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Experimental investigation on the performance of direct-expansion roof-PV/T heat pump system Nina Shao, Liangdong Ma*, Jili Zhang Institute of Building Energy, Dalian University of Technology, Dalian, 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2019 Received in revised form 7 January 2020 Accepted 11 January 2020 Available online 14 January 2020
Although the integration of photovoltaic-thermal (PV/T) heat pump systems on buildings can assist in reducing the electricity demand of high-energy consuming buildings, the large occupation area of PV/T modules hinders its popularization. To overcome this problem, a novel PV/T evaporation roof coupled with a heat pump system composed of electronic expansion valves, a condenser and a compressor is proposed, which serves as an electric generator, evaporator for the heat pump system and the external surface of a building roof. This paper investigates the actual operating performance of the directexpansion roof-PV/T heat pump system under different seasonal conditions. The structure and design of PV/T evaporation roof and the whole system are first presented, followed by the construction of an experimental platform and finally the operation of the system is analyzed through performance evaluation indices and field-testing. The experimental results indicated that the system performed better in the summer than in the winter, with the electrical efficiency, thermal efficiency and overall efficiency averaging at 11.23%, 64.25% and 83.32%, respectively. The average COP was 5.9, with a maximum value of 8.9. Additionally, it took 2 h to heat the water within a 1.5 m3 heat storage tank from 25 C to 60 C, which was twice as fast as under winter conditions. Meanwhile, a significant decrease in system performance resulting from changes in environmental parameters in the winter was observed. The average electrical efficiency, thermal efficiency, and overall efficiency were 11.67%, 60.17% and 78.84%, respectively, while the average and maximum COP were 3.7 and 5.24 respectively. © 2020 Elsevier Ltd. All rights reserved.
Keywords: PV/T heat pump PV/T evaporation roof Building integrated PV/T System performance COP
1. Introduction With the growing concern for the prevailing energy crisis and the development of the photovoltaic (PV) industry, buildings integrated with PV modules to achieve energy self-sufficiency have become the trend for the future [1]. PV modules are usually installed on the roof or facade of a building, which convert solar energy into electricity supplied directly to the electrical equipment or to feed into the grid. However, over 80% of the solar energy that strikes the module surfaces is converted into thermal energy, which reduces the electrical efficiency due to the increase in temperature of the PV cell. To improve the electrical and comprehensive utilization efficiency of solar energy, the concept of photovoltaic/thermal (PV/T) was proposed [2]. The system incorporates an additional heat exchanging channel at the back of the PV modules, which absorbs and utilizes heat released by the modules. The PV/T system
* Corresponding author. E-mail address: [email protected] (L. Ma). https://doi.org/10.1016/j.energy.2020.116959 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
achieves dual utilization of solar energy and increases greater thermal and PV efficiency per unit solar radiation striking area. Furthermore, these innovative systems have the potential to greatly promote the utilization and development of renewable solar energy sources [3]. The PV/T system can be divided in to three types, depending on the operating fluid of the heat exchanger, which are air, water, or refrigerant. Air-based PV/T systems have a lower freezing point and leakage risk, as well as reduced installation and maintenance costs [4]. Owing to these advantages, air-based systems have been extensively used to supply hot air and provide heating and dehumidification while simultaneously supplying electricity. However, due to the small thermal mass of air the resulting efficiency is often quite low. To improve efficiency, many investigations have been carried out, including establishing the heat transfer model [5e7], optimizing the air flow configurations [8], changing the material of the cover layer [9], evaluating performance [10], and assessing economy [11], among others. Meanwhile, water-based PV/T systems benefit from the large thermal mass of water to cool the PV
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N. Shao et al. / Energy 195 (2020) 116959
modules. The flow of water through the heat exchanging unit provides cooling to the PV modules, which can help improve the electrical efficiency and the heated water can also be utilized to supply domestic hot water, as well as solar-assisted heating and cooling. A series of studies involving theoretical and experimental analyses have been put forward in this field, including the establishment of theoretical models and energy flow analysis [12], analysis of the influence of different factors on the electrical and thermal efficiency [13], performance evaluation and technoeconomic analysis [14], as well as measures to enhance the indirect evaporative cooling effect [15]. Although water-based PV/T systems provide improvements to the comprehensive rate of utilization of solar energy, several drawbacks also exist in practical operation, such as leakage, freezing in cold environments and high cost. In addition, as the water needs to be heated to elevated temperatures in order to be effective, the cooling effect on PV modules and subsequent increase in electrical efficiency are often inadequate [16,17]. Refrigerant-based PV/T systems usually combine a PV/T collector with a solar-assisted heat pump. The systems are divided into direct-expansion and indirect-expansion solar-assisted heat pump systems, depending on whether the PV/ T collector and heat pump system work collaboratively or independently. In comparison with air-based PV/T and water-based PV/ T systems, refrigerant-based PV/T systems have higher electrical and thermal efficiency, and are able to meet almost all the energy requirements of a building, including power supply, heating, cooling, domestic hot water and other energy needs. Therefore, refrigerant-based PV/T systems have the greatest potential to serve as the next-generation of solar energy harvesting systems. In recent years, exploration into the refrigerant-based PV/T system has covered multiple aspects. Ji et al. [18,19] introduced a direct-expansion PV solar-assisted heat pump (PV-SAHP), which adopted a PV/T collector as the evaporator of the heat pump system. In their studies, the heat transfer process was analyzed and numerical simulations were also performed based on the distributed parameters approach. The performance of the PV evaporator in response to different environmental conditions was investigated empirically. The results showed that the PV/T-SAHP had a better coefficient of performance and electrical efficiency than the separate PV/T collector. The theory surrounding an indirect-expansion heat pump system was analyzed by Vallati et al. [20], in which a PV/T collector was used as the heat source of the heat pump system. The performance of the system was compared under different operating conditions. The results indicated that the heating effect of the system was significantly influenced by the weather conditions, and the heating supplied by the system could cover up to 70% of the heating demand when operating under high solar radiation and outdoor air temperature. Zhou et al. [21] successfully developed a novel parallel-laid PV/micro-channel-evaporation module with a solar-driven, direct-expansion heat pump system to heat a 150 m2 flat. In addition, the evaporator of a heat pump system was also integrated with the building roof, introducing the concept of a PV/e roof [22]. The impacts of different parameters on the performance of the PV/e roof were analyzed, and the electrical and thermal efficiencies of the PV/e heat pump and the PV heat pump system were compared theoretically. This work was the first to achieve the incorporation of the PV/T heat pump system with the external surface of building. A series of experiments were conducted by Kong et al. [23] under different conditions to obtain the optimal control strategies of a heat pump system. The results proved that a direct-expansion solar-assisted heat pump system can effectively regulate the extent of superheating to the range of 5e10 C. Additionally, to reduce exergy losses and improve system performance, studies employing a variable capacity compressor and an electrical expansion valve in solar-assisted heat pump systems have also
been carried out [24,25]. Investigations into the heat transfer processes of PV-walls and PV-roofs [26e28] reveal that although the use of such systems help to save energy during the summer by reducing the cooling load, the heating load increases during the winter season. The research into PV/T systems to date have greatly improved the comprehensive utilization efficiency of solar energy and encouraged the development of renewable energy technologies to some extent. In spite of these advantages, the practical applications of PV/T modules are often impeded due to the need for relatively large amounts of land occupation in order to sufficiently meet the energy demands of end-users, hindering the popularization of solar energy in the market. Therefore, to make the most of land resources and accelerate the promotion of solar energy, it is of significant benefit to combine the PV/T modules with the external surface of the building. At present, most studies in the existing literature focus on air-based or water-based PV/T systems integrated with the building wall or roof, while few combine the external surface of the building with refrigerant-based PV/T systems such as higherefficiency PV/T solar-assisted heat pump systems. In this paper, a direct-expansion PV/T solar-assisted heat pump system is integrated with the building roof, to design and build a novel PV/T evaporation roof. The PV/T evaporation roof simultaneously acts as the roof surface, electrical generator and evaporator for a heat pump system, realizing the dual utilization of solar thermal and photoelectric energy to generate and supply electricity and heat to the building. The structure of the PV/T evaporation roof and the working principle of the roof-PV/T heat pump system are introduced in detail. Then, the system is assessed through evaluation indices and the operating performances of the system under different seasons are evaluated empirically. This work provides an insight into the actual operating characteristics of such systems and is expected to promote the development of buildings integrated with PV/T systems.
2. Construction of an experimental direct-expansion roof-PV/ T heat pump system 2.1. Working principle The direct-expansion roof-PV/T heat pump system consists of four main components: (1) a variable frequency scroll compressor, (2) a condenser that acts as heat exchanger between the refrigerant and the cooling medium (water), (3) electronic expansion valves for throttling and reducing pressure and (4) a PV/T evaporation roof acting as an evaporator, which is composed of multiple PV/T modules. The system also includes a water tank as the energy storing device and a power inverter. A schematic diagram of the system is outlined in Fig. 1. The working principle of the system is as follows: when the liquid refrigerant absorbs heat as it passes through the evaporator roof, becoming a saturated or superheated vapor. Then, the vapor enters the compressor where it is pressurized to a hightemperature, high-pressure and superheated vapor. Afterward, the vapor is directed to the condenser, where it releases heat to the cooling medium (water) during the condensation process. When heated to elevated temperatures, the cooling medium can be used as a direct supply of domestic hot water or to provide heating to the building. Following condensation, the refrigerant is then delivered to the electrical expansion valve, where it experiences a significant fall in both pressure and temperature. The low-pressure, cooled refrigerant is subsequently re-circulated to the evaporation roof for the next cycle of heat absorption.
N. Shao et al. / Energy 195 (2020) 116959
water tank
3
condenser compressor
receiver
electronic expansion valve
filter
gas-liquid separator
PV/T module
flowmeter
valve
temperature sensor
water pump
pressure sensor
AC 380V inverter
Power transducer Fig. 1. Schematic diagram of direct-expansion roof-PV/T heat pump system.
2.2. Design of the PV/T evaporation roof The PV/T evaporation roof integrates the evaporator of a PV/T heat pump system with the roof of a building, as shown in Fig. 2. The main PV/T module combines multiple functions into one system acting not only as the evaporator in a heat pump system, but also the generator for heating and electric power. Additionally, the PV/T module also serves as the external surface of the roof offering resistance to water, wind and other environmental conditions. In the PV/T evaporation roof, the PV/T module is fixed to the original building roof by steel frames. Refrigerant pipes are located between the steel frames and connected with evaporation coils using a flexible connection to facilitate construction and to avoid leakage. The supply pipe feeds a gas-liquid refrigerant to the evaporation coil, while the return pipe removes the vapor refrigerant from the coil. To enhance the heat transfer performance of the refrigerant, a cavity rather than insulating material is set between the PV/T module and the steel frames. The PV/T module comprises of three main layers from top to bottom: (1) a glass cover to protect the PV cells from water vapor and to reduce the convective heat loss between the module and the
surroundings, (2) a PV layer to generate electricity, consisting of an EVA (ethylene-vinyl-acetate) layer, a PV cell and KPE (PVDF-PETEVA, composed of three polymer films, PVDF (polyvinylidene fluoride), PET (polyethylene terephthalate) film and EVA). The PV cell plays an important role in converting short-wave solar radiation to electricity, while EVA acts as a medium of adhesion, protection, and heat conduction, and KPE serves to isolate electricity and enhance absorption capacity [29], (3) a back layer to act as an evaporator in heat pump system, consisting of an evaporation pipe and a roll-bond aluminum plate. The refrigerant absorbs heat and undergoes phase change in the evaporation pipe, while the rollbond aluminum plate helps enhance heat transfer and protects the whole module. In the manufacture of the PV/T module, the three main layers are tightly pressed together with a laminating machine, and the layer of heat-conducting EVA rather than air enables efficient heat transfer from the upper layer to the refrigerant. The structure of the PV/T module is illustrated in Fig. 3. To enhance the heat transfer effect, the refrigerant flow path at the back layer of the module is configured as a serpentine. The refrigerant flows into the module through one inlet and then divides into double-tube, along with the flow path gradually divides into eight-tube and finally flows out of the module through two outlets. The PV/T module was 1.56 m long and 0.78 m wide, each containing 32 polycrystalline silicon solar cells with dimensions of
Glass cover PV/T component Cavity Horizontal steel frame Original roof Roof beam
a. Elevation view of PV/T roof
b. Appearance drawing of PV/T roof
Fig. 2. Schematic diagram of PV/T roof.
Evaporation pipe
PV cell
Roll-bond aluminium plate Fig. 3. Structure of PV/T module.
4
N. Shao et al. / Energy 195 (2020) 116959
0.156 m 0.156 m. The flow path of the refrigerant and the layout of the PV cells are illustrated in Fig. 4, and the detailed performance parameters of each PV/T module are presented in Table 1. Additionally, an L-shaped downward folding edge was incorporated around the module to improve its strength and resistance to deformation, as well as facilitate its installation on to the building roof. 2.3. Determination of the system component parameters The PV/T modules were configured as eight rows and six columns on the entire roof of a building which had the dimensions of 12.5 m in length and 6.5 m in width. The relatively large number of PV/T modules prevents the uniform flow of refrigerant through each module due to the pipe and unit resistances, which directly influence the heat transfer and reduce system efficiency. Therefore, each column of PV/T modules was configured with an electronic expansion valve to mitigate these effects. The choice of refrigerant should comprehensively consider factors such as environmental impacts, chemical stability, availability, and performance characteristics. In this investigation, the widely used, environmentally friendly refrigerant R134a was chosen as the operating fluid. Furthermore, as the electricity and heat production of the system varies with the ambient environment, a variable frequency compressor with a maximum power of 15 HP was selected to accommodate the varying power output. The condenser was composed of a heat exchanging plate, with an area of 7.7 m2. A 1.5 m3 heat storing water tank and water circulation pump were used to receive and store thermal energy from the refrigerant during the condensation process. Finally, an inverter was connected to the system to convert the direct current generated by the PV cell to a 380 V alternating current which was fed into the municipal grid. Fig. 5 shows an image of the experimental setup, and the details of the experimental apparatus in the system are listed in Table 2. 2.4. Test parameters The experimental system involved various measurement instruments and sensors, which were used to measure outdoor meteorological parameters, temperature, pressure, flow rate, energy consumption and power generation. A PC-4 meteorological environment monitoring system was used to monitor the outdoor meteorological parameters, including the ambient temperature, humidity, velocity, and solar radiation. The solar radiation striking on the roof was measured by a TBQ-2 pyranometer. In the heat pump system, a PT100 temperature sensor and pressure sensor were used to measure the temperature and pressure, while the flow rates of the refrigerant and water were measured by the turbine flowmeter and mass flowmeter, respectively. The power generation and consumptions of the compressor and water circulation pump were recorded by a power transducer, and all of the data collected were transferred to a computer through a KEITHLEY 2700 multi-
Table 1 Heating and power generation performance of a single PV/T module (test conditions: solar radiation intensity G ¼ 1000 W/m2, ambient temperature Ta ¼ 25 C). Q0heat (W)
P0e (MPa)
T0e ( C)
T0s-h ( C)
E0 (W)
h0e,PV/T (%)
I0mp (A)
U0mp (V)
I0sc (A)
U0oc (V)
800
0.9
15
5
125
16
6.41
18.9
6.95
20.5
Note: Q0heat, P0e , T0e , T0s-h, E0, h0e,PV/T, I0mp, U0mp, I0sc, U0oc in this table represent the heat production, evaporating pressure, evaporating temperature, superheating temperature, electricity production, electrical efficiency, current of maximum power, voltage of maximum power, short circuit current and the open circuit voltage under standard test conditions, respectively.
function data collector at 1 min data collection intervals. The specific locations of the measuring points in the experimental platform are shown in Fig. 1, and the specifications of the test instruments and sensors are detailed in Table 3. All instruments and sensors were calibrated and tested prior to the experiment in accordance with the test specification.
3. Performance evaluation method As the outputs of the direct-expansion roof-PV/T heat pump system are electricity and heat, the performance of the system is evaluated by the electrical, thermal and overall efficiencies, and the coefficient of performance (COP). The average electrical efficiency is the ratio of the electricity produced by the PV/T module to the intensity of solar radiation striking on the roof, which can be expressed as the formula in Eq. (1) [30]:
he;PV=T ¼ E
. G , APV=T
(1)
where E is the electricity output [W], G is the solar radiation [W$m2] and APV/T is the surface area of the PV/T module [m2]. The average thermal efficiency is defined as the ratio of the heat produced by the heat pump system to the intensity of solar radiation striking on the PV/T roof, given by Eq. (2) [30]:
ht;PV=T ¼ Qr;heat
. . G , APV=T ¼ Mr ðhin hout Þ G , APV=T
where Qr, heat is the heat released by the refrigerant in the condenser [W], expressed by the product of mass flow rate and the enthalpy difference of the refrigerant when it flows through the condenser. Mr is the refrigerant mass flow rate [kg$s1], and hin and hout are inlet and outlet enthalpies of the refrigerant respectively [J$kg1$K1]. The overall efficiency is the combined efficiency of the electrical and thermal properties. Taking into consideration the higher quality of electrical power than thermal power, the overall efficiency is not simply a sum of the efficiencies, but rather expressed as in Eq. (3) [30]:
.
.
hoverall ¼ he;PV=T he þ ht;PV=T ¼ he;PV=T 0:38 þ ht;PV=T
a. Refrigerant flow path
b. Dist ribution of photovoltaic cell
Fig. 4. Design diagram of the PV/T module.
(2)
(3)
where he is the power generation efficiency of conventional power plants with a value of 0.38. The COP is the ratio of thermal energy stored in the water tank to the energy consumption of the compressor and water circulation pump, which is written as in Eq. (4) [21]:
N. Shao et al. / Energy 195 (2020) 116959
5
Inverter
Heat storage tank
PV/T evaporation roof
Condenser
Compressor Liquid orage tank a
c
b
a. PV/T evaporation roof
b. heat pump units
d
c. heat storage tank
d. inverter
Fig. 5. Main experimental equipment of the system.
Table 2 Details of the experimental apparatus. Name
Module
Country
Compressor Condenser Electronic expansion valves PV/T module Water tank Water circulation pump Inverter
Copeland: ZR190KCE-TFD Zhejiang Shengsong: B3-052 Sanhua: DPF(Ts1) 2.4C Homemade Homemade Lingming: 40SG6-20 Sungrow: SG10KTL-EC
America China China China China China China
experimental data in this investigation, error analyses were conducted based on the error propagation method [31]. Following calculation, it was found that the relative errors of electrical efficiency, thermal efficiency, overall efficiency and COP were 0.76%, 1.84%, 1.10%, 3.21% respectively, which are all within the 5% stipulated limit of precision. The results of the error analysis indicate that the experimental design process is feasible and the precision of each sensor meets the requirements of accuracy. 4. Results and discussions
COP ¼ Qw;heat
Wcomp þ Wpump ¼ Mr ðhin hout Þ Wcomp þ Wpump ¼ cw rw Vw Tw;in Tw;out Wcomp þ Wpump (4)
where Qw, heat is the heat absorbed by water [W], equal to the heat released by the refrigerant in the condenser, cw, rw, Vw are the specific heat capacity [J$kg1$K1], density [kg$m3], and volume of water [m3], respectively. Tw,in and Tw,out represent the respective inlet and outlet temperatures of the water tank [ C] while Wcomp and Wpump are the power consumptions of the compressor and circulating water pump [W], respectively. In quantitative analysis, empirical data need to meet a certain level of accuracy as inaccurate results may lead to incorrect scientific conclusions. Thus, to evaluate the reliability of the
To study the performance of the direct-expansion roof-PV/T heat pump system, the experiments were conducted in both the summer and winter seasons in Dalian, China (coordinates N38.9 , E121.44 ). The summer experiments were conducted from 25eAugust 27, 2018, while the winter experiments were carried out on 19, 20 and 24 December 2018. The experimental process began at approximately 9:00, when the PV/T heat pump system was switched, and until the water of the heat storage tank was heated to 60 C or exhaust temperature of compressor reached 105 C. The initial water temperatures of the heat storage tank in the summer and winter experiments were about 25 C and 20 C, respectively. The experimental results are presented in Table 4 while Fig. 6 shows the outdoor meteorological parameters during the experiments. To analyze the experimental performance of the system in detail, the experiment was performed on days on which the weather was representative of the average weather of the different seasons. As solar radiation has a direct influence on the system’s electrical and thermal performance, more emphasis was given to
Table 3 Specifications of the test instruments. Name
Module
Country
Range
Accuracy
Meteorological environment monitoring system
Jinzhou Sunshine: PC-4
China
Pyranometer Temperature sensor Pressure sensor Turbine flowmeter Refrigerant mass flowmeter
Jinzhou Sunshine: TBQ-2 Heraeus: PT100 Suzhou Xusheng: PCM 300 Dalian Jiesite: JSTLWGY-32A Dalian Jiesite: TC-E015000LL1HS20Y0MY Suzhou Taihua: TED-3WBF1 Keithley: 2700
China Germany China China China
0e2000 W m2 40e80 C, 0e70 m s1 0e2000 W m2 50e100 C 0e4 MPa 1.5e15 m3 h1 0e1200 kg h1 0e300 kg h1 0e34.7 kW e
±2% ±0.4 C ±0.1 m s1 ±2% ±0.2% ±0.5% ±0.5% ±0.2%
Power transducer Multi-function data collector
China America
±0.5% e
6
N. Shao et al. / Energy 195 (2020) 116959
Table 4 Experimental results in summer and winter seasons. Date Summer
Winter
Aug.25 Aug.26 Aug.27 Dec.19 Dec.20 Dec.24
G (W$m2)
Ta ( C)
TPV/T ( C)
Tw ( C)
Heating Time
735.28 729.65 900.74 521.2 580.6 581.7
27.88 29.63 26.86 9.2 8 3.9
33.58 32.9 33.74 11.77 11.41 8.22
25e60 26e62 25e60 19e60 19e60 20e60
9:05e11:05 9:05e11:15 9:05e11:00 9:30e13:25 9:20e13:10 9:10e13:15
he,PV/T
ht,PV/T
hoverall
(%)
(%)
(%)
11.34 11.25 11.13 10.45 10.53 10.45
78.69 81.35 67.31 58.04 62.26 52.39
108.54 110.96 93.89 85.53 89.97 79.9
COP 6.03 5.76 5.9 3.85 3.8 3.46
Note: Ta, TPV/T, Tw represent the temperatures of the ambient, PV/T component and water in the heat storage tank, respectively.
switching on the heat pump (Fig. 9) due to the continuous absorption of solar radiation without heat output. Thus, once the heat pump begins to operate, the large amounts of heat of the PV/T evaporation roof are absorbed by the refrigerant, generating a significant amount of thermal energy. During the operation of the heat pump system, the average thermal energy produced in the summer and winter were 544 and 311 W m2, respectively. Since the amounts of thermal energy produced were different between the two seasons, the duration of time taken to heat the water within the 1.5 m3 heat storage tank from 25 C to the end temperature of 60 C also differed seasonally, which was 125 min in the summer and 200 min in the winter, respectively. The electrical, thermal and overall efficiencies of the heat pump system shown in Fig. 10 suggest that the electrical efficiency was less affected by changes in solar radiation and ambient temperature due to its relative stability. The average electrical efficiency in the summer and winter was 11.22% and 10.58%, respectively. In contrast, the thermal and overall efficiencies were observed to be high at low levels of solar radiation and reducing with increasing levels of the solar radiation. These observations are consistent with those found in the literature [21], where elevated temperatures of the PV/T evaporation roof resulting from greater intensity of solar radiation lead to the reductions in thermal efficiency and overall efficiency. Under typical weather conditions, the average thermal efficiency and overall efficiency were 81% and 111% in the summer, and 58% and 86% in the winter respectively. The system operating parameters of evaporating temperature, condensing temperature, evaporating pressure, condensing pressure and the flow rate of refrigerant are presented in Figs. 11 and 12. The results show an increase in the condensing temperature and
8
40
Temperature /
30
Ambient temperature Velocity Solar radiation
7
5 4
10
3
800 600 400
2
0
1
- 10
1000
6
Winter
20
1200
Velocity / m/s
Summer
Solar radiation / W/m2
the solar intensity of each day during experiment scheduling. The intensity of solar radiation is closely related to the cloud coverage of the sky, which tends to be high in summer (opaque sky cover is 64%) and relatively low in winter (opaque sky cover is 36%) over the test site. A day with high cloud cover on 26 August in the summer and low cloud cover on 19 December in the winter were chosen to perform the experimental testing. Unless specified otherwise, the following analyses are based on experiments in typical weather conditions. The variation of power generation in typical weather conditions presented in Fig. 7 indicate that the changes in power generation had a similar trend to the intensity of solar radiation. The solar radiation was observed to vary markedly in the summer due to the clouds blocking out the sun while varying slowly in winter. The power generated increased with the intensity of solar radiation, reaching at maximum value at the peak of solar radiation while gradually decreasing with reduced solar radiation intensity. In the typical experimental conditions, the average power generated in the summer and winter experiments were 82 and 54 W m2, respectively. Neglecting the small difference in electrical efficiency (shown in Fig. 10), the seasonal differences in solar radiation is the main reason for the large difference in power generation during the winter and summer. An analysis of the variation of heat generation under typical weather conditions illustrated in Fig. 8 also reveals a trend that corresponds to the changes in solar radiation. Large amounts of heat were generated at the initial stages of the experiment, reaching a maximum value shortly after the heat pump system was switched on. These observations are clearly attributed to the elevated temperatures of the PV/T evaporation roof prior to
12:00
Aug.25
0:00
12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 Dec.19 Dec.20 Aug.27 Aug.26
12:00
Dec.24
Experiment time in summer and winter Fig. 6. Solar radiation, ambient temperature and velocity curves in summer and winter.
0:00
0
200 0
N. Shao et al. / Energy 195 (2020) 116959
7
40 Power generation Ambient temperature Solar radiation
1400
30
1000
3000 20 2000
10
1000
Solar radiation / W/m2
1200
Temperature /
Power generation / W
4000
800 600 400 200
0
10:00
11:00
Summer
10:05
11:05
12:05
13:05
0
0
Winter
Time
Fig. 7. Power generation curve with changing solar radiation and ambient temperature.
Heating generation / kW
35 30 25
70
1400
60
1200
50
1000
40
20 30
15
20
10 5 0
10:00
11:00
Summer
10:05
11:05
12:05
13:05
800 600 400
10
200
0
0
Solar radiation / W/m2
Heating generation Water temperature Ambient temperature Solar radiation
Temperature /
40
Winter
Time
Fig. 8. Heating generation curve with the change of radiation and ambient temperature.
70
30
60
30 20
10
1000
10
Operational process 0
1200
50 40
20
1400
9:00
10:00
Summer
11:00
Operational process 9:00
Time
10:00
11:00
12:00
13:00
Solar radiation / W/m2
Heating generation Ambient temperature PVT roof temperature Solar radiation
Temperature /
Heating generation / kW
40
800 600 400
0
200
-10
0
Winter
Fig. 9. Temperature of the PV/T-roof with changing solar radiation and operational process.
N. Shao et al. / Energy 195 (2020) 116959
200
40 Electrical efficiency Thermal efficiency Overall efficiency Ambient temperature Solar radiation
1000 30 800
120 20 80
600 400
10
40
0
Temperature /
Efficiency / %
160
1200
Solar radiation / W/m2
8
200
10:00
11:00
Summer
10:05
Time
11:05
12:05
13:05
0
0
Winter
Fig. 10. System efficiency with changes in solar radiation and ambient temperature.
Water temperature Evaporating temperature Condensing temperature Flow rate of refrigerant Ambient temperature Solar radiation
Temperature /
60
40
800
1200
700
1000
600
800
500 20 400 0
-20
10:00
11:00
Summer
10:05
Time
11:05
12:05
13:05
Solar radiation / W/m2
80
seen to rise continuously with increasing water temperature of the heat storage tank, while the COP is observed to decrease. In the summer, the COP was observed to decrease from 8.6 to 3.8 when the input power of the compressor increased from 3930 W to 8690 W, and decreased from 6.3 to 2.4 when the input power of compressor increased from 3510 W to 8070 W in the winter. The average COP values under typical summer and winter operating conditions were 5.8 and 3.9, respectively, while the average COP during the whole experiment in the summer and winter was 5.9 and 3.7, respectively. The values of COP under different operating conditions are tabulated in Table 4. In the direct-expansion roof-PV/T heat pump system, the power consumption is a sum of two components: the power consumption of the compressor and the water circulation pump. The electrical power produced by the PV cells and power consumed by the heat pump system are compared in Fig. 14, showing that the power consumed by the water circulation pump was relatively stable, with summer and winter average values of 883 W and 890 W, respectively. Meanwhile, the power consumed by the compressor varied with the temperature of the water in heat storage water tank, rising with increasing water temperature. In the summer, the compressor consumption increased from 3930 W to 8690 W when water
Flow rate / kg/h
pressure over time, which is due to the rising temperatures of the water within the heat storage tank as more heat is accumulated. The temperature difference between the refrigerant and the cooling medium (water) in the heat exchange process was relatively stable, at 8.6 C in typical summer operating conditions and 4.8 C in typical winter operating conditions. When the water temperature reached 60 C, the condensing temperatures in the summer and winter were recorded to reach 69 C and 64 C, respectively. At this instance, the condensing pressure also reached peak values of 2.07 MPa in the summer and 1.85 MPa in the winter. Meanwhile, the evaporating temperature and evaporating pressure were relatively stable, with average values of 20.9 C and 0.59 MPa in the summer and 0 C and 0.29 MPa in the winter. However, in the initial stages of the heat pump system operation, the evaporating temperature and pressure were very high and dropped sharply as the system continued to operate. The reasons for these observations is akin to those for the high heat generation in the early stages of operation, which are both caused by the large amounts of heat of the hot PV/T evaporation roof absorbed by the refrigerant at the beginning of the experiment. The variations of COP and input power of the compressor over time are shown in Fig. 13. The input power of the compressor is
600 400
300
200
200
0
Winter
Fig. 11. Variation of evaporating temperature, condensing temperature and flow rate of refrigerant.
N. Shao et al. / Energy 195 (2020) 116959
9
Pressure / MPa
2.0
80
700
70
Flow rate / kg/h
Evaporating pressure Condensing pressure Flow rate of refrigerant Water temperature
800
600 1.5
60 50
500 1.0
40
400
Temperature /
2.5
30 0.5
0.0
300
10:00
11:00
Summer
10:05
11:05
12:05
20
200
13:05
10
Winter
Time
Fig. 12. Variation of evaporating pressure and condensing pressure.
COP Water temperature Input power of compressor
8
70
10000
60
9000 8000
COP
6 40 4 30 2
0
Temperature /
50
20
10:00
11:00
Summer
10:05
11:05
12:05
13:05
10
7000 6000
Input power / W
10
5000 4000 3000
Winter
Time
Fig. 13. Variation of COP.
Water pump power consumption Compressor power consumption Cumulative power consumption
Water temperature Power generation Cumulative power generation
8
400 80 350 70 300
6
50 40
4
30 2
0
6:30 8:00 9:30 11:00 12:30 14:00 15:30 17:00
Summer
Time
--
8:30 10:00 11:30 13:00 14:30 16:00
Winter
Fig. 14. Contrast of consumed power and generated power.
Temperature /
Power / kW
60
250 200 150 100
20
50
10
0
Cumulative Power / kW
10
10
N. Shao et al. / Energy 195 (2020) 116959
temperature ranged from 26 C to 62 C, and 3510 We8070 W when water temperature ranged from 19 C to 60 C in the winter. The cumulative power consumption during system operation in the summer and winter were 45.9 kWh and 73.4 kWh, respectively, while the total power generation over the entire day in the summer and winter were 72.8 kWh and 35.3 kWh, respectively. The differing seasonal intensity of solar radiation mean that while the power produced by the system can meet the power consumption of the system itself in the summer, an external supply of electricity is still needed in the winter. Although the generated power was lower than the power consumed in winter, the total power consumption of the direct-expansion roof-PV/T heat pump system was reduced by 48% in comparison with the common heat pump system which consumes electricity but does not produce electricity. In the summer, the energy saving effect of the direct-expansion roof-PV/T heat pump system was more significant, with the energy saving rate as high as 159%.
5. Conclusions This paper presented an integrated PV/T evaporation roof and direct-expansion heat pump system along with the system’s design and construction, performance evaluating indices and experimental results under the climate conditions of Dalian, China. The actual operating performance of the direct-expansion roof-PV/T heat pump system was tested under different seasonal weather conditions, and the key findings are outlined below: The electrical efficiency remained relatively stable (11.22% in summer and 10.58% in winter), as it was less affected by environmental factors, while the thermal and overall efficiencies were negatively correlated with the variation of solar radiation (81% and 111% in summer, 58% and 86% in winter, respectively). Although increasing levels of solar radiation does not improve the electrical efficiency, greater amounts of electricity and heat are generated despite reduced thermal efficiency and overall efficiencies. The overall results reveal that the condensing pressure, condensing temperature and compressor input power all show a rising trend with the increase of temperature of water in the heat storage tank, while the COP decreases. As long-term exposure to high temperature and pressure may shorten the service life of the equipment, high water temperatures are not conducive to the improvement of system performance and the long-term operation of the system. Therefore, the temperature of the water in the heat storage tank should be controlled within a certain range during operation. Furthermore, the power generated by the PV/T modules in the summer exceeds the electrical energy consumption the system, and the excess electricity can be used by other electrical equipment. However, the power generated in the winter is insufficient due to the seasonal low levels of solar radiation. These findings suggest that the system may be a feasible installation on low-energy buildings in areas rich in solar radiation throughout the year, if energy self-sufficiency is to be achieved. The above conclusions indicate that although the directexpansion roof-PV/T heat pump system has good application prospects in northern China, even better results may be attained in southern China. The system provides a solution to the challenges of energy shortage and environmental pollution to some extent, while simultaneously overcoming the drawback of significant space occupation of traditional PV/T modules in practical applications.
The results of this investigation will greatly promote the development of the PV/T heat pump system and accelerate the widespread construction of buildings integrated with PV/T systems. Acknowledgements This work is supported by Natural Science Foundation of China (grant numbers 51676026). References [1] Schoen TJN. Building-integrated PV installation in The Netherlands: examples and operational experiences. Sol Energy 2001;70:467e77. [2] Chow TT. A review on photovoltaic/thermal hybrid solar technology. Appl Energy 2010;87:365e79. [3] Zhang XX, Zhao XD, Smith S, Xu JH, Yu XT. Review of R&D progress and practical application of the solar photovoltaic/thermal (PV/T) technologies. Renew Sustain Energy Rev 2012;16(1):599e617. [4] Tyagi VV, Kaushik SC, Tyagi SK. Advancement in solar photovoltaic/thermal (PV/T) hybrid collector technology. Renew Sustain Energy Rev 2012;16: 1383e98. [5] Poulek V, Matuska T, Libra M, Kachalouski E, Sedla cek J. Influence of increased temperature on energy production of roof integrated PV panels. Energy Build 2018;166:418e25. [6] Brinkworth BJ. Estimation of flow and heat transfer for the design of PV cooling ducts. Sol Energy 2000;60:413e20. [7] Athienitis AK, Barone G, Buonomano A, Palombo A. Assessing active and passive effects of façade building integrated photovoltaics/thermal systems: dynamic modelling and simulation. Appl Energy 2018;209:355e82. [8] Wu SY, Wang T, Xiao L, Shen ZG. Effect of cooling channel position on heat transfer characteristics and thermoelectric performance of air-cooled PV/T system. Sol Energy 2019;180:489e500. [9] Kamthania D, Sujata S, Tiwari GN. Performance evaluation of a hybrid photovoltaic thermal double pass facade for space heating. Energy Build 2011;43(9):2274e81. [10] Tripathy M, Sadhu PK, Panda SK. A critical review on building integrated photovoltaic products and their applications. Renew Sustain Energy Rev 2016;61:451e65. [11] Tongsopit S, Junlakarn S, Wibulpolprasert W, Chaianong A, Kokchang P, Hoang NV. The economics of solar PV self-consumption in Thailand. Renew Energy 2019;138:395e408. [12] Chow TT. Performance analysis of photovoltaic-thermal collector by explicit dynamic model. Sol Energy 2003;75:143e52. [13] Aste N, Pero CD, Leonforte F. Water PV/T collectors performance comparison. Energy Procedia 2017;105:961e6. [14] Buker MS, Mempouo B, Riffat SB. Performance evaluation and technoeconomic analysis of a novel building integrated PV/T roof collector: an experimental validation. Energy Build 2014;76:164e75. [15] Buker MS, Mempouo B, Riffat SB. Experimental investigation of a building integrated photovoltaic/thermal roof collector combined with a liquid desiccant enhanced indirect evaporative cooling system. Energy Convers Manag 2015;101:239e54. [16] Chaturvedi SK, Shen JY. Thermal performance of a direct expansion solarassisted heat pump. Sol Energy 1984;33:155e62. [17] Gagliano A, Tina GM, Aneli S, Ni zeti c S. Comparative assessments of the performance of PV/T and conventional solar plants. J Clean Prod 2019;219: 304e15. [18] Ji J, Pei G, Chow TT, Liu KL, He HF, Lu JP, Han CW. Experimental study of photovoltaic solar assisted heat pump system. Sol Energy 2008;82:43e52. [19] Ji J, He HF, Chow TT, Pei G, He W, Liu KL. Distributed dynamic modeling and experimental study of PV evaporator in a PV/T solar-assisted heat pump. Int J Heat Mass Transf 2009;52:1365e73. P, Colucci C, Mauri L, de Lieto Vollaro R, Taler J. Energy analysis [20] Vallati A, Ocłon of a thermal system composed by a heat pump coupled with a PV/T solar collector. Energy 2019;174:91e6. [21] Zhou JZ, Zhao XD, Ma XL, Qiu ZZ, Ji J, Du ZY, Yu M. Experimental investigation of a solar driven direct-expansion heat pump system employing the novel PV/ micro-channels-evaporator modules. Appl Energy 2016;178:484e95. [22] Zhao XD, Zhang XX, Riffat SB, Su YX. Theoretical study of the performance of a novel PV/e roof module for heat pump operation. Energy Convers Manag 2011;52:603e14. [23] Kong XQ, Jiang KL, Dong SD, Li Y, Li JB. Control strategy and experimental analysis of a direct-expansion solar-assisted heat pump water heater with R134a. Energy 2018;145:17e24. [24] Kong XQ, Sun PL, Li Y, Jiang KL, Dong SD. Experimental studies of a variable capacity direct-expansion solar-assisted heat pump water heater in autumn
N. Shao et al. / Energy 195 (2020) 116959 and winter conditions. Sol Energy 2018;170:352e7. [25] Kong XQ, Li Y, Lin L, Yang YG. Modeling evaluation of a direct-expansion solarassisted heat pump water heater using R410A. Int J Refrig 2017;76:136e46. [26] Yang HX, Ji J. Study on the heat gain of a PV-wall. Acta Energiae Solaris Sin 1999;20(3):270e3 [in Chinese)]. [27] Wang Y, Wang DJ, Liu YF. Study on comprehensive energy-saving of shading and photovoltaics of roof added PV module. Energy Procedia 2017;132: 598e603. [28] Kapsalis V, Karamanis D. On the effect of roof added photovoltaics on
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building’s energy demand. Energy Build 2015;108:195e204. [29] Yang HX, Lv L, Peng JQ, Zhou W. Application of solar energy technologies in building. second ed. Beijing: China construction industry press; 2015. [30] Shao NN, Ma LD, Zhang JL. Experimental study on electrical and thermal performance and heat transfer characteristic of PV/T roof in summer. Appl Therm Eng 2019;162:114267. [31] Liu ZX, Wang L. Experimental design and data processing. second ed. Beijing: Chemical Industry press; 2015.
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Energy journal homepage: www.elsevier.com/locate/energy
Experimental investigation on the performance of direct-expansion roof-PV/T heat pump system Nina Shao, Liangdong Ma*, Jili Zhang Institute of Building Energy, Dalian University of Technology, Dalian, 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2019 Received in revised form 7 January 2020 Accepted 11 January 2020 Available online 14 January 2020
Although the integration of photovoltaic-thermal (PV/T) heat pump systems on buildings can assist in reducing the electricity demand of high-energy consuming buildings, the large occupation area of PV/T modules hinders its popularization. To overcome this problem, a novel PV/T evaporation roof coupled with a heat pump system composed of electronic expansion valves, a condenser and a compressor is proposed, which serves as an electric generator, evaporator for the heat pump system and the external surface of a building roof. This paper investigates the actual operating performance of the directexpansion roof-PV/T heat pump system under different seasonal conditions. The structure and design of PV/T evaporation roof and the whole system are first presented, followed by the construction of an experimental platform and finally the operation of the system is analyzed through performance evaluation indices and field-testing. The experimental results indicated that the system performed better in the summer than in the winter, with the electrical efficiency, thermal efficiency and overall efficiency averaging at 11.23%, 64.25% and 83.32%, respectively. The average COP was 5.9, with a maximum value of 8.9. Additionally, it took 2 h to heat the water within a 1.5 m3 heat storage tank from 25 C to 60 C, which was twice as fast as under winter conditions. Meanwhile, a significant decrease in system performance resulting from changes in environmental parameters in the winter was observed. The average electrical efficiency, thermal efficiency, and overall efficiency were 11.67%, 60.17% and 78.84%, respectively, while the average and maximum COP were 3.7 and 5.24 respectively. © 2020 Elsevier Ltd. All rights reserved.
Keywords: PV/T heat pump PV/T evaporation roof Building integrated PV/T System performance COP
1. Introduction With the growing concern for the prevailing energy crisis and the development of the photovoltaic (PV) industry, buildings integrated with PV modules to achieve energy self-sufficiency have become the trend for the future [1]. PV modules are usually installed on the roof or facade of a building, which convert solar energy into electricity supplied directly to the electrical equipment or to feed into the grid. However, over 80% of the solar energy that strikes the module surfaces is converted into thermal energy, which reduces the electrical efficiency due to the increase in temperature of the PV cell. To improve the electrical and comprehensive utilization efficiency of solar energy, the concept of photovoltaic/thermal (PV/T) was proposed [2]. The system incorporates an additional heat exchanging channel at the back of the PV modules, which absorbs and utilizes heat released by the modules. The PV/T system
* Corresponding author. E-mail address: [email protected] (L. Ma). https://doi.org/10.1016/j.energy.2020.116959 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
achieves dual utilization of solar energy and increases greater thermal and PV efficiency per unit solar radiation striking area. Furthermore, these innovative systems have the potential to greatly promote the utilization and development of renewable solar energy sources [3]. The PV/T system can be divided in to three types, depending on the operating fluid of the heat exchanger, which are air, water, or refrigerant. Air-based PV/T systems have a lower freezing point and leakage risk, as well as reduced installation and maintenance costs [4]. Owing to these advantages, air-based systems have been extensively used to supply hot air and provide heating and dehumidification while simultaneously supplying electricity. However, due to the small thermal mass of air the resulting efficiency is often quite low. To improve efficiency, many investigations have been carried out, including establishing the heat transfer model [5e7], optimizing the air flow configurations [8], changing the material of the cover layer [9], evaluating performance [10], and assessing economy [11], among others. Meanwhile, water-based PV/T systems benefit from the large thermal mass of water to cool the PV
2
N. Shao et al. / Energy 195 (2020) 116959
modules. The flow of water through the heat exchanging unit provides cooling to the PV modules, which can help improve the electrical efficiency and the heated water can also be utilized to supply domestic hot water, as well as solar-assisted heating and cooling. A series of studies involving theoretical and experimental analyses have been put forward in this field, including the establishment of theoretical models and energy flow analysis [12], analysis of the influence of different factors on the electrical and thermal efficiency [13], performance evaluation and technoeconomic analysis [14], as well as measures to enhance the indirect evaporative cooling effect [15]. Although water-based PV/T systems provide improvements to the comprehensive rate of utilization of solar energy, several drawbacks also exist in practical operation, such as leakage, freezing in cold environments and high cost. In addition, as the water needs to be heated to elevated temperatures in order to be effective, the cooling effect on PV modules and subsequent increase in electrical efficiency are often inadequate [16,17]. Refrigerant-based PV/T systems usually combine a PV/T collector with a solar-assisted heat pump. The systems are divided into direct-expansion and indirect-expansion solar-assisted heat pump systems, depending on whether the PV/ T collector and heat pump system work collaboratively or independently. In comparison with air-based PV/T and water-based PV/ T systems, refrigerant-based PV/T systems have higher electrical and thermal efficiency, and are able to meet almost all the energy requirements of a building, including power supply, heating, cooling, domestic hot water and other energy needs. Therefore, refrigerant-based PV/T systems have the greatest potential to serve as the next-generation of solar energy harvesting systems. In recent years, exploration into the refrigerant-based PV/T system has covered multiple aspects. Ji et al. [18,19] introduced a direct-expansion PV solar-assisted heat pump (PV-SAHP), which adopted a PV/T collector as the evaporator of the heat pump system. In their studies, the heat transfer process was analyzed and numerical simulations were also performed based on the distributed parameters approach. The performance of the PV evaporator in response to different environmental conditions was investigated empirically. The results showed that the PV/T-SAHP had a better coefficient of performance and electrical efficiency than the separate PV/T collector. The theory surrounding an indirect-expansion heat pump system was analyzed by Vallati et al. [20], in which a PV/T collector was used as the heat source of the heat pump system. The performance of the system was compared under different operating conditions. The results indicated that the heating effect of the system was significantly influenced by the weather conditions, and the heating supplied by the system could cover up to 70% of the heating demand when operating under high solar radiation and outdoor air temperature. Zhou et al. [21] successfully developed a novel parallel-laid PV/micro-channel-evaporation module with a solar-driven, direct-expansion heat pump system to heat a 150 m2 flat. In addition, the evaporator of a heat pump system was also integrated with the building roof, introducing the concept of a PV/e roof [22]. The impacts of different parameters on the performance of the PV/e roof were analyzed, and the electrical and thermal efficiencies of the PV/e heat pump and the PV heat pump system were compared theoretically. This work was the first to achieve the incorporation of the PV/T heat pump system with the external surface of building. A series of experiments were conducted by Kong et al. [23] under different conditions to obtain the optimal control strategies of a heat pump system. The results proved that a direct-expansion solar-assisted heat pump system can effectively regulate the extent of superheating to the range of 5e10 C. Additionally, to reduce exergy losses and improve system performance, studies employing a variable capacity compressor and an electrical expansion valve in solar-assisted heat pump systems have also
been carried out [24,25]. Investigations into the heat transfer processes of PV-walls and PV-roofs [26e28] reveal that although the use of such systems help to save energy during the summer by reducing the cooling load, the heating load increases during the winter season. The research into PV/T systems to date have greatly improved the comprehensive utilization efficiency of solar energy and encouraged the development of renewable energy technologies to some extent. In spite of these advantages, the practical applications of PV/T modules are often impeded due to the need for relatively large amounts of land occupation in order to sufficiently meet the energy demands of end-users, hindering the popularization of solar energy in the market. Therefore, to make the most of land resources and accelerate the promotion of solar energy, it is of significant benefit to combine the PV/T modules with the external surface of the building. At present, most studies in the existing literature focus on air-based or water-based PV/T systems integrated with the building wall or roof, while few combine the external surface of the building with refrigerant-based PV/T systems such as higherefficiency PV/T solar-assisted heat pump systems. In this paper, a direct-expansion PV/T solar-assisted heat pump system is integrated with the building roof, to design and build a novel PV/T evaporation roof. The PV/T evaporation roof simultaneously acts as the roof surface, electrical generator and evaporator for a heat pump system, realizing the dual utilization of solar thermal and photoelectric energy to generate and supply electricity and heat to the building. The structure of the PV/T evaporation roof and the working principle of the roof-PV/T heat pump system are introduced in detail. Then, the system is assessed through evaluation indices and the operating performances of the system under different seasons are evaluated empirically. This work provides an insight into the actual operating characteristics of such systems and is expected to promote the development of buildings integrated with PV/T systems.
2. Construction of an experimental direct-expansion roof-PV/ T heat pump system 2.1. Working principle The direct-expansion roof-PV/T heat pump system consists of four main components: (1) a variable frequency scroll compressor, (2) a condenser that acts as heat exchanger between the refrigerant and the cooling medium (water), (3) electronic expansion valves for throttling and reducing pressure and (4) a PV/T evaporation roof acting as an evaporator, which is composed of multiple PV/T modules. The system also includes a water tank as the energy storing device and a power inverter. A schematic diagram of the system is outlined in Fig. 1. The working principle of the system is as follows: when the liquid refrigerant absorbs heat as it passes through the evaporator roof, becoming a saturated or superheated vapor. Then, the vapor enters the compressor where it is pressurized to a hightemperature, high-pressure and superheated vapor. Afterward, the vapor is directed to the condenser, where it releases heat to the cooling medium (water) during the condensation process. When heated to elevated temperatures, the cooling medium can be used as a direct supply of domestic hot water or to provide heating to the building. Following condensation, the refrigerant is then delivered to the electrical expansion valve, where it experiences a significant fall in both pressure and temperature. The low-pressure, cooled refrigerant is subsequently re-circulated to the evaporation roof for the next cycle of heat absorption.
N. Shao et al. / Energy 195 (2020) 116959
water tank
3
condenser compressor
receiver
electronic expansion valve
filter
gas-liquid separator
PV/T module
flowmeter
valve
temperature sensor
water pump
pressure sensor
AC 380V inverter
Power transducer Fig. 1. Schematic diagram of direct-expansion roof-PV/T heat pump system.
2.2. Design of the PV/T evaporation roof The PV/T evaporation roof integrates the evaporator of a PV/T heat pump system with the roof of a building, as shown in Fig. 2. The main PV/T module combines multiple functions into one system acting not only as the evaporator in a heat pump system, but also the generator for heating and electric power. Additionally, the PV/T module also serves as the external surface of the roof offering resistance to water, wind and other environmental conditions. In the PV/T evaporation roof, the PV/T module is fixed to the original building roof by steel frames. Refrigerant pipes are located between the steel frames and connected with evaporation coils using a flexible connection to facilitate construction and to avoid leakage. The supply pipe feeds a gas-liquid refrigerant to the evaporation coil, while the return pipe removes the vapor refrigerant from the coil. To enhance the heat transfer performance of the refrigerant, a cavity rather than insulating material is set between the PV/T module and the steel frames. The PV/T module comprises of three main layers from top to bottom: (1) a glass cover to protect the PV cells from water vapor and to reduce the convective heat loss between the module and the
surroundings, (2) a PV layer to generate electricity, consisting of an EVA (ethylene-vinyl-acetate) layer, a PV cell and KPE (PVDF-PETEVA, composed of three polymer films, PVDF (polyvinylidene fluoride), PET (polyethylene terephthalate) film and EVA). The PV cell plays an important role in converting short-wave solar radiation to electricity, while EVA acts as a medium of adhesion, protection, and heat conduction, and KPE serves to isolate electricity and enhance absorption capacity [29], (3) a back layer to act as an evaporator in heat pump system, consisting of an evaporation pipe and a roll-bond aluminum plate. The refrigerant absorbs heat and undergoes phase change in the evaporation pipe, while the rollbond aluminum plate helps enhance heat transfer and protects the whole module. In the manufacture of the PV/T module, the three main layers are tightly pressed together with a laminating machine, and the layer of heat-conducting EVA rather than air enables efficient heat transfer from the upper layer to the refrigerant. The structure of the PV/T module is illustrated in Fig. 3. To enhance the heat transfer effect, the refrigerant flow path at the back layer of the module is configured as a serpentine. The refrigerant flows into the module through one inlet and then divides into double-tube, along with the flow path gradually divides into eight-tube and finally flows out of the module through two outlets. The PV/T module was 1.56 m long and 0.78 m wide, each containing 32 polycrystalline silicon solar cells with dimensions of
Glass cover PV/T component Cavity Horizontal steel frame Original roof Roof beam
a. Elevation view of PV/T roof
b. Appearance drawing of PV/T roof
Fig. 2. Schematic diagram of PV/T roof.
Evaporation pipe
PV cell
Roll-bond aluminium plate Fig. 3. Structure of PV/T module.
4
N. Shao et al. / Energy 195 (2020) 116959
0.156 m 0.156 m. The flow path of the refrigerant and the layout of the PV cells are illustrated in Fig. 4, and the detailed performance parameters of each PV/T module are presented in Table 1. Additionally, an L-shaped downward folding edge was incorporated around the module to improve its strength and resistance to deformation, as well as facilitate its installation on to the building roof. 2.3. Determination of the system component parameters The PV/T modules were configured as eight rows and six columns on the entire roof of a building which had the dimensions of 12.5 m in length and 6.5 m in width. The relatively large number of PV/T modules prevents the uniform flow of refrigerant through each module due to the pipe and unit resistances, which directly influence the heat transfer and reduce system efficiency. Therefore, each column of PV/T modules was configured with an electronic expansion valve to mitigate these effects. The choice of refrigerant should comprehensively consider factors such as environmental impacts, chemical stability, availability, and performance characteristics. In this investigation, the widely used, environmentally friendly refrigerant R134a was chosen as the operating fluid. Furthermore, as the electricity and heat production of the system varies with the ambient environment, a variable frequency compressor with a maximum power of 15 HP was selected to accommodate the varying power output. The condenser was composed of a heat exchanging plate, with an area of 7.7 m2. A 1.5 m3 heat storing water tank and water circulation pump were used to receive and store thermal energy from the refrigerant during the condensation process. Finally, an inverter was connected to the system to convert the direct current generated by the PV cell to a 380 V alternating current which was fed into the municipal grid. Fig. 5 shows an image of the experimental setup, and the details of the experimental apparatus in the system are listed in Table 2. 2.4. Test parameters The experimental system involved various measurement instruments and sensors, which were used to measure outdoor meteorological parameters, temperature, pressure, flow rate, energy consumption and power generation. A PC-4 meteorological environment monitoring system was used to monitor the outdoor meteorological parameters, including the ambient temperature, humidity, velocity, and solar radiation. The solar radiation striking on the roof was measured by a TBQ-2 pyranometer. In the heat pump system, a PT100 temperature sensor and pressure sensor were used to measure the temperature and pressure, while the flow rates of the refrigerant and water were measured by the turbine flowmeter and mass flowmeter, respectively. The power generation and consumptions of the compressor and water circulation pump were recorded by a power transducer, and all of the data collected were transferred to a computer through a KEITHLEY 2700 multi-
Table 1 Heating and power generation performance of a single PV/T module (test conditions: solar radiation intensity G ¼ 1000 W/m2, ambient temperature Ta ¼ 25 C). Q0heat (W)
P0e (MPa)
T0e ( C)
T0s-h ( C)
E0 (W)
h0e,PV/T (%)
I0mp (A)
U0mp (V)
I0sc (A)
U0oc (V)
800
0.9
15
5
125
16
6.41
18.9
6.95
20.5
Note: Q0heat, P0e , T0e , T0s-h, E0, h0e,PV/T, I0mp, U0mp, I0sc, U0oc in this table represent the heat production, evaporating pressure, evaporating temperature, superheating temperature, electricity production, electrical efficiency, current of maximum power, voltage of maximum power, short circuit current and the open circuit voltage under standard test conditions, respectively.
function data collector at 1 min data collection intervals. The specific locations of the measuring points in the experimental platform are shown in Fig. 1, and the specifications of the test instruments and sensors are detailed in Table 3. All instruments and sensors were calibrated and tested prior to the experiment in accordance with the test specification.
3. Performance evaluation method As the outputs of the direct-expansion roof-PV/T heat pump system are electricity and heat, the performance of the system is evaluated by the electrical, thermal and overall efficiencies, and the coefficient of performance (COP). The average electrical efficiency is the ratio of the electricity produced by the PV/T module to the intensity of solar radiation striking on the roof, which can be expressed as the formula in Eq. (1) [30]:
he;PV=T ¼ E
. G , APV=T
(1)
where E is the electricity output [W], G is the solar radiation [W$m2] and APV/T is the surface area of the PV/T module [m2]. The average thermal efficiency is defined as the ratio of the heat produced by the heat pump system to the intensity of solar radiation striking on the PV/T roof, given by Eq. (2) [30]:
ht;PV=T ¼ Qr;heat
. . G , APV=T ¼ Mr ðhin hout Þ G , APV=T
where Qr, heat is the heat released by the refrigerant in the condenser [W], expressed by the product of mass flow rate and the enthalpy difference of the refrigerant when it flows through the condenser. Mr is the refrigerant mass flow rate [kg$s1], and hin and hout are inlet and outlet enthalpies of the refrigerant respectively [J$kg1$K1]. The overall efficiency is the combined efficiency of the electrical and thermal properties. Taking into consideration the higher quality of electrical power than thermal power, the overall efficiency is not simply a sum of the efficiencies, but rather expressed as in Eq. (3) [30]:
.
.
hoverall ¼ he;PV=T he þ ht;PV=T ¼ he;PV=T 0:38 þ ht;PV=T
a. Refrigerant flow path
b. Dist ribution of photovoltaic cell
Fig. 4. Design diagram of the PV/T module.
(2)
(3)
where he is the power generation efficiency of conventional power plants with a value of 0.38. The COP is the ratio of thermal energy stored in the water tank to the energy consumption of the compressor and water circulation pump, which is written as in Eq. (4) [21]:
N. Shao et al. / Energy 195 (2020) 116959
5
Inverter
Heat storage tank
PV/T evaporation roof
Condenser
Compressor Liquid orage tank a
c
b
a. PV/T evaporation roof
b. heat pump units
d
c. heat storage tank
d. inverter
Fig. 5. Main experimental equipment of the system.
Table 2 Details of the experimental apparatus. Name
Module
Country
Compressor Condenser Electronic expansion valves PV/T module Water tank Water circulation pump Inverter
Copeland: ZR190KCE-TFD Zhejiang Shengsong: B3-052 Sanhua: DPF(Ts1) 2.4C Homemade Homemade Lingming: 40SG6-20 Sungrow: SG10KTL-EC
America China China China China China China
experimental data in this investigation, error analyses were conducted based on the error propagation method [31]. Following calculation, it was found that the relative errors of electrical efficiency, thermal efficiency, overall efficiency and COP were 0.76%, 1.84%, 1.10%, 3.21% respectively, which are all within the 5% stipulated limit of precision. The results of the error analysis indicate that the experimental design process is feasible and the precision of each sensor meets the requirements of accuracy. 4. Results and discussions
COP ¼ Qw;heat
Wcomp þ Wpump ¼ Mr ðhin hout Þ Wcomp þ Wpump ¼ cw rw Vw Tw;in Tw;out Wcomp þ Wpump (4)
where Qw, heat is the heat absorbed by water [W], equal to the heat released by the refrigerant in the condenser, cw, rw, Vw are the specific heat capacity [J$kg1$K1], density [kg$m3], and volume of water [m3], respectively. Tw,in and Tw,out represent the respective inlet and outlet temperatures of the water tank [ C] while Wcomp and Wpump are the power consumptions of the compressor and circulating water pump [W], respectively. In quantitative analysis, empirical data need to meet a certain level of accuracy as inaccurate results may lead to incorrect scientific conclusions. Thus, to evaluate the reliability of the
To study the performance of the direct-expansion roof-PV/T heat pump system, the experiments were conducted in both the summer and winter seasons in Dalian, China (coordinates N38.9 , E121.44 ). The summer experiments were conducted from 25eAugust 27, 2018, while the winter experiments were carried out on 19, 20 and 24 December 2018. The experimental process began at approximately 9:00, when the PV/T heat pump system was switched, and until the water of the heat storage tank was heated to 60 C or exhaust temperature of compressor reached 105 C. The initial water temperatures of the heat storage tank in the summer and winter experiments were about 25 C and 20 C, respectively. The experimental results are presented in Table 4 while Fig. 6 shows the outdoor meteorological parameters during the experiments. To analyze the experimental performance of the system in detail, the experiment was performed on days on which the weather was representative of the average weather of the different seasons. As solar radiation has a direct influence on the system’s electrical and thermal performance, more emphasis was given to
Table 3 Specifications of the test instruments. Name
Module
Country
Range
Accuracy
Meteorological environment monitoring system
Jinzhou Sunshine: PC-4
China
Pyranometer Temperature sensor Pressure sensor Turbine flowmeter Refrigerant mass flowmeter
Jinzhou Sunshine: TBQ-2 Heraeus: PT100 Suzhou Xusheng: PCM 300 Dalian Jiesite: JSTLWGY-32A Dalian Jiesite: TC-E015000LL1HS20Y0MY Suzhou Taihua: TED-3WBF1 Keithley: 2700
China Germany China China China
0e2000 W m2 40e80 C, 0e70 m s1 0e2000 W m2 50e100 C 0e4 MPa 1.5e15 m3 h1 0e1200 kg h1 0e300 kg h1 0e34.7 kW e
±2% ±0.4 C ±0.1 m s1 ±2% ±0.2% ±0.5% ±0.5% ±0.2%
Power transducer Multi-function data collector
China America
±0.5% e
6
N. Shao et al. / Energy 195 (2020) 116959
Table 4 Experimental results in summer and winter seasons. Date Summer
Winter
Aug.25 Aug.26 Aug.27 Dec.19 Dec.20 Dec.24
G (W$m2)
Ta ( C)
TPV/T ( C)
Tw ( C)
Heating Time
735.28 729.65 900.74 521.2 580.6 581.7
27.88 29.63 26.86 9.2 8 3.9
33.58 32.9 33.74 11.77 11.41 8.22
25e60 26e62 25e60 19e60 19e60 20e60
9:05e11:05 9:05e11:15 9:05e11:00 9:30e13:25 9:20e13:10 9:10e13:15
he,PV/T
ht,PV/T
hoverall
(%)
(%)
(%)
11.34 11.25 11.13 10.45 10.53 10.45
78.69 81.35 67.31 58.04 62.26 52.39
108.54 110.96 93.89 85.53 89.97 79.9
COP 6.03 5.76 5.9 3.85 3.8 3.46
Note: Ta, TPV/T, Tw represent the temperatures of the ambient, PV/T component and water in the heat storage tank, respectively.
switching on the heat pump (Fig. 9) due to the continuous absorption of solar radiation without heat output. Thus, once the heat pump begins to operate, the large amounts of heat of the PV/T evaporation roof are absorbed by the refrigerant, generating a significant amount of thermal energy. During the operation of the heat pump system, the average thermal energy produced in the summer and winter were 544 and 311 W m2, respectively. Since the amounts of thermal energy produced were different between the two seasons, the duration of time taken to heat the water within the 1.5 m3 heat storage tank from 25 C to the end temperature of 60 C also differed seasonally, which was 125 min in the summer and 200 min in the winter, respectively. The electrical, thermal and overall efficiencies of the heat pump system shown in Fig. 10 suggest that the electrical efficiency was less affected by changes in solar radiation and ambient temperature due to its relative stability. The average electrical efficiency in the summer and winter was 11.22% and 10.58%, respectively. In contrast, the thermal and overall efficiencies were observed to be high at low levels of solar radiation and reducing with increasing levels of the solar radiation. These observations are consistent with those found in the literature [21], where elevated temperatures of the PV/T evaporation roof resulting from greater intensity of solar radiation lead to the reductions in thermal efficiency and overall efficiency. Under typical weather conditions, the average thermal efficiency and overall efficiency were 81% and 111% in the summer, and 58% and 86% in the winter respectively. The system operating parameters of evaporating temperature, condensing temperature, evaporating pressure, condensing pressure and the flow rate of refrigerant are presented in Figs. 11 and 12. The results show an increase in the condensing temperature and
8
40
Temperature /
30
Ambient temperature Velocity Solar radiation
7
5 4
10
3
800 600 400
2
0
1
- 10
1000
6
Winter
20
1200
Velocity / m/s
Summer
Solar radiation / W/m2
the solar intensity of each day during experiment scheduling. The intensity of solar radiation is closely related to the cloud coverage of the sky, which tends to be high in summer (opaque sky cover is 64%) and relatively low in winter (opaque sky cover is 36%) over the test site. A day with high cloud cover on 26 August in the summer and low cloud cover on 19 December in the winter were chosen to perform the experimental testing. Unless specified otherwise, the following analyses are based on experiments in typical weather conditions. The variation of power generation in typical weather conditions presented in Fig. 7 indicate that the changes in power generation had a similar trend to the intensity of solar radiation. The solar radiation was observed to vary markedly in the summer due to the clouds blocking out the sun while varying slowly in winter. The power generated increased with the intensity of solar radiation, reaching at maximum value at the peak of solar radiation while gradually decreasing with reduced solar radiation intensity. In the typical experimental conditions, the average power generated in the summer and winter experiments were 82 and 54 W m2, respectively. Neglecting the small difference in electrical efficiency (shown in Fig. 10), the seasonal differences in solar radiation is the main reason for the large difference in power generation during the winter and summer. An analysis of the variation of heat generation under typical weather conditions illustrated in Fig. 8 also reveals a trend that corresponds to the changes in solar radiation. Large amounts of heat were generated at the initial stages of the experiment, reaching a maximum value shortly after the heat pump system was switched on. These observations are clearly attributed to the elevated temperatures of the PV/T evaporation roof prior to
12:00
Aug.25
0:00
12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 Dec.19 Dec.20 Aug.27 Aug.26
12:00
Dec.24
Experiment time in summer and winter Fig. 6. Solar radiation, ambient temperature and velocity curves in summer and winter.
0:00
0
200 0
N. Shao et al. / Energy 195 (2020) 116959
7
40 Power generation Ambient temperature Solar radiation
1400
30
1000
3000 20 2000
10
1000
Solar radiation / W/m2
1200
Temperature /
Power generation / W
4000
800 600 400 200
0
10:00
11:00
Summer
10:05
11:05
12:05
13:05
0
0
Winter
Time
Fig. 7. Power generation curve with changing solar radiation and ambient temperature.
Heating generation / kW
35 30 25
70
1400
60
1200
50
1000
40
20 30
15
20
10 5 0
10:00
11:00
Summer
10:05
11:05
12:05
13:05
800 600 400
10
200
0
0
Solar radiation / W/m2
Heating generation Water temperature Ambient temperature Solar radiation
Temperature /
40
Winter
Time
Fig. 8. Heating generation curve with the change of radiation and ambient temperature.
70
30
60
30 20
10
1000
10
Operational process 0
1200
50 40
20
1400
9:00
10:00
Summer
11:00
Operational process 9:00
Time
10:00
11:00
12:00
13:00
Solar radiation / W/m2
Heating generation Ambient temperature PVT roof temperature Solar radiation
Temperature /
Heating generation / kW
40
800 600 400
0
200
-10
0
Winter
Fig. 9. Temperature of the PV/T-roof with changing solar radiation and operational process.
N. Shao et al. / Energy 195 (2020) 116959
200
40 Electrical efficiency Thermal efficiency Overall efficiency Ambient temperature Solar radiation
1000 30 800
120 20 80
600 400
10
40
0
Temperature /
Efficiency / %
160
1200
Solar radiation / W/m2
8
200
10:00
11:00
Summer
10:05
Time
11:05
12:05
13:05
0
0
Winter
Fig. 10. System efficiency with changes in solar radiation and ambient temperature.
Water temperature Evaporating temperature Condensing temperature Flow rate of refrigerant Ambient temperature Solar radiation
Temperature /
60
40
800
1200
700
1000
600
800
500 20 400 0
-20
10:00
11:00
Summer
10:05
Time
11:05
12:05
13:05
Solar radiation / W/m2
80
seen to rise continuously with increasing water temperature of the heat storage tank, while the COP is observed to decrease. In the summer, the COP was observed to decrease from 8.6 to 3.8 when the input power of the compressor increased from 3930 W to 8690 W, and decreased from 6.3 to 2.4 when the input power of compressor increased from 3510 W to 8070 W in the winter. The average COP values under typical summer and winter operating conditions were 5.8 and 3.9, respectively, while the average COP during the whole experiment in the summer and winter was 5.9 and 3.7, respectively. The values of COP under different operating conditions are tabulated in Table 4. In the direct-expansion roof-PV/T heat pump system, the power consumption is a sum of two components: the power consumption of the compressor and the water circulation pump. The electrical power produced by the PV cells and power consumed by the heat pump system are compared in Fig. 14, showing that the power consumed by the water circulation pump was relatively stable, with summer and winter average values of 883 W and 890 W, respectively. Meanwhile, the power consumed by the compressor varied with the temperature of the water in heat storage water tank, rising with increasing water temperature. In the summer, the compressor consumption increased from 3930 W to 8690 W when water
Flow rate / kg/h
pressure over time, which is due to the rising temperatures of the water within the heat storage tank as more heat is accumulated. The temperature difference between the refrigerant and the cooling medium (water) in the heat exchange process was relatively stable, at 8.6 C in typical summer operating conditions and 4.8 C in typical winter operating conditions. When the water temperature reached 60 C, the condensing temperatures in the summer and winter were recorded to reach 69 C and 64 C, respectively. At this instance, the condensing pressure also reached peak values of 2.07 MPa in the summer and 1.85 MPa in the winter. Meanwhile, the evaporating temperature and evaporating pressure were relatively stable, with average values of 20.9 C and 0.59 MPa in the summer and 0 C and 0.29 MPa in the winter. However, in the initial stages of the heat pump system operation, the evaporating temperature and pressure were very high and dropped sharply as the system continued to operate. The reasons for these observations is akin to those for the high heat generation in the early stages of operation, which are both caused by the large amounts of heat of the hot PV/T evaporation roof absorbed by the refrigerant at the beginning of the experiment. The variations of COP and input power of the compressor over time are shown in Fig. 13. The input power of the compressor is
600 400
300
200
200
0
Winter
Fig. 11. Variation of evaporating temperature, condensing temperature and flow rate of refrigerant.
N. Shao et al. / Energy 195 (2020) 116959
9
Pressure / MPa
2.0
80
700
70
Flow rate / kg/h
Evaporating pressure Condensing pressure Flow rate of refrigerant Water temperature
800
600 1.5
60 50
500 1.0
40
400
Temperature /
2.5
30 0.5
0.0
300
10:00
11:00
Summer
10:05
11:05
12:05
20
200
13:05
10
Winter
Time
Fig. 12. Variation of evaporating pressure and condensing pressure.
COP Water temperature Input power of compressor
8
70
10000
60
9000 8000
COP
6 40 4 30 2
0
Temperature /
50
20
10:00
11:00
Summer
10:05
11:05
12:05
13:05
10
7000 6000
Input power / W
10
5000 4000 3000
Winter
Time
Fig. 13. Variation of COP.
Water pump power consumption Compressor power consumption Cumulative power consumption
Water temperature Power generation Cumulative power generation
8
400 80 350 70 300
6
50 40
4
30 2
0
6:30 8:00 9:30 11:00 12:30 14:00 15:30 17:00
Summer
Time
--
8:30 10:00 11:30 13:00 14:30 16:00
Winter
Fig. 14. Contrast of consumed power and generated power.
Temperature /
Power / kW
60
250 200 150 100
20
50
10
0
Cumulative Power / kW
10
10
N. Shao et al. / Energy 195 (2020) 116959
temperature ranged from 26 C to 62 C, and 3510 We8070 W when water temperature ranged from 19 C to 60 C in the winter. The cumulative power consumption during system operation in the summer and winter were 45.9 kWh and 73.4 kWh, respectively, while the total power generation over the entire day in the summer and winter were 72.8 kWh and 35.3 kWh, respectively. The differing seasonal intensity of solar radiation mean that while the power produced by the system can meet the power consumption of the system itself in the summer, an external supply of electricity is still needed in the winter. Although the generated power was lower than the power consumed in winter, the total power consumption of the direct-expansion roof-PV/T heat pump system was reduced by 48% in comparison with the common heat pump system which consumes electricity but does not produce electricity. In the summer, the energy saving effect of the direct-expansion roof-PV/T heat pump system was more significant, with the energy saving rate as high as 159%.
5. Conclusions This paper presented an integrated PV/T evaporation roof and direct-expansion heat pump system along with the system’s design and construction, performance evaluating indices and experimental results under the climate conditions of Dalian, China. The actual operating performance of the direct-expansion roof-PV/T heat pump system was tested under different seasonal weather conditions, and the key findings are outlined below: The electrical efficiency remained relatively stable (11.22% in summer and 10.58% in winter), as it was less affected by environmental factors, while the thermal and overall efficiencies were negatively correlated with the variation of solar radiation (81% and 111% in summer, 58% and 86% in winter, respectively). Although increasing levels of solar radiation does not improve the electrical efficiency, greater amounts of electricity and heat are generated despite reduced thermal efficiency and overall efficiencies. The overall results reveal that the condensing pressure, condensing temperature and compressor input power all show a rising trend with the increase of temperature of water in the heat storage tank, while the COP decreases. As long-term exposure to high temperature and pressure may shorten the service life of the equipment, high water temperatures are not conducive to the improvement of system performance and the long-term operation of the system. Therefore, the temperature of the water in the heat storage tank should be controlled within a certain range during operation. Furthermore, the power generated by the PV/T modules in the summer exceeds the electrical energy consumption the system, and the excess electricity can be used by other electrical equipment. However, the power generated in the winter is insufficient due to the seasonal low levels of solar radiation. These findings suggest that the system may be a feasible installation on low-energy buildings in areas rich in solar radiation throughout the year, if energy self-sufficiency is to be achieved. The above conclusions indicate that although the directexpansion roof-PV/T heat pump system has good application prospects in northern China, even better results may be attained in southern China. The system provides a solution to the challenges of energy shortage and environmental pollution to some extent, while simultaneously overcoming the drawback of significant space occupation of traditional PV/T modules in practical applications.
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