Research on the Operating Performance of a Combined Heat and Power System Integrated with Solar PV/T and Air-Source Heat Pump in Residential Buildings

Abstract

Global building energy consumption is significantly increasing. Utilizing renewable energy sources may be an effective approach to achieving low-carbon and energy-efficient buildings. A combined system incorporating solar photovoltaic–thermal (PV/T) components with an air-source heat pump (ASHP) was studied for simultaneous heating and power generation in a real residential building. The back panel of the PV/T component featured a novel polygonal Freon circulation channel design. A prototype of the combined heating and power supply system was constructed and tested in Fuzhou City, China. The results indicate that the average coefficient of performance (COP) of the system is 4.66 when the ASHP operates independently. When the PV/T component is integrated with the ASHP, the average COP increases to 5.37. On sunny days, the daily average thermal output of 32 PV/T components reaches 24 kW, while the daily average electricity generation is 64 kW·h. On cloudy days, the average daily power generation is 15.6 kW·h; however, the residual power stored in the battery from the previous day could be utilized to ensure the energy demand in the system. Compared to conventional photovoltaic (PV) systems, the overall energy utilization efficiency improves from 5.68% to 17.76%. The hot water temperature stored in the tank can reach 46.8 °C, satisfying typical household hot water requirements. In comparison to standard PV modules, the system achieves an average cooling efficiency of 45.02%. The variation rate of the system’s thermal loss coefficient is relatively low at 5.07%. The optimal water tank capacity for the system is determined to be 450 L. This system demonstrates significant potential for providing efficient combined heat and power supply for buildings, offering considerable economic and environmental benefits, thereby serving as a reference for the future development of low-carbon and energy-saving building technologies.

1. Introduction

The primary current applications of solar energy include photovoltaic (PV) systems and solar thermal technologies. However, they encounter challenges, including a low power generation efficiency and limited energy utilization. PV power generation alone usually has an optical conversion efficiency of 16–20% [1]. Studies have shown that the output power of solar silicon cells decreases as the panel temperature rises. For every 1 °C increase, the power generation efficiency drops by approximately 0.3% [2]. Solar photovoltaic/thermal (PV/T) technology is used to effectively recover the excess heat, improve the power generation efficiency while obtaining thermal energy, and achieve the comprehensive utilization of solar energy.

The energy consumption of building operations in China is approximately 8–10 billion tons of standard coal, accounting for about 25% of the total social energy consumption. The pollutant emissions are approximately more than 20% [3]. The development of PV- and PV/T-coupled heat pump technology for the energy supply enables buildings to operate in a low-carbon and energy-efficient manner throughout their entire life-cycle. In the past decade, some scholars have begun to focus on the development of solar PV- and PV/T-integrated systems and applications in buildings. Li JY et al. [4] proposed a new type of PV energy-saving window to reduce the heating load. Li S et al. [5] conducted simulation and experimental research on the different capacities and start-up times of water heaters in a PV heat pump, and developed a real-time comprehensive control system. Choi Y et al. [6] proposed a model that can be integrated with the inner ventilation system, which increased the PV panel’s power generation efficiency by up to 5.89%. Shao NN et al. [7] proposed a novel PV/T roof system; compared to the conventional roof, the PV/T roof reduced the heat gain by 44% and increased the power generation efficiency by about 10% during summer. Zhou C et al. [8] conducted experimental research on the summer cooling characteristics of a single-stage compression PV/T system, and the results showed that the cooling performance was about 20% higher than the traditional ones. Wang G et al. [9] simulated a solar–air composite heat pump system. They presented that, when the environmental temperature was 14.6–17.2 °C and the irradiance was 550–797 W/m², the average power generation efficiency of the system was 13.91%, the average heat collection efficiency was 41.14%, and the COP was 2.29. Recently, some researchers have also carried out experimental studies on solar PV/T heat and power generation systems. For instance, Hou LS et al. [10] studied the influence of water usage patterns on the performance of PV/T hot water systems and suggested measures such as using a dual water tank to improve the systematic adaptability. Zhang B et al. [11] developed a heat pipe PV/T model and optimized the water tank capacity, achieving a maximum overall efficiency of 67.5%. These simulated and experimental studies all indicate that the solar PV/T-integrated heat pump systems can promote comprehensive performance in heat and power generation [12,13,14]. However, the cooling medium used in PV/T back panels is predominantly based on non-phase-change fluids, and related experiments conducted primarily in laboratory settings have not yet been applied in real buildings.

Meanwhile, some scholars have also conducted experiments on the bodies of PV and PV/T solar panels. Wang WK et al. [15] evaluated the traditional PV and thermal–catalytic TC-PV wall. The structure analysis indicated that, when the solar radiation intensity was 400 W/m² and the thickness of the air channel was 0.05 m, it could achieve the best energy efficiency. Li JP et al. [16] studied the operational performance of different flow sections of flat plate micro-heat-pipe PV/Ts and found the performance differences affected by the flow sections. Saeed Aghakhani et al. [17] adopted a V-shaped porous copper plate and controlled airflow through the water pipes in a reciprocating device. They demonstrated that higher heat flux and a lower flow rate could enhance the thermal efficiency, increasing it from 0.3% to over 1%. Aguilar J et al. [18] conducted an actual assessment of the PV/T solar panels for commercial sale, focusing on their thermal performance under different water inlet temperatures and flows, and confirmed the applicability of PV/T solar panels in practical use.

In summary, current PV/T studies are predominantly based on experiments and rarely implemented in real buildings. In addition, the cooling medium used in the back panel of the PV/T is typically a non-phase-change fluid such as water or an ethylene glycol solution. Moreover, the channel layout of the PV/T back panel also plays a significant role in influencing the system’s operational performance. This study adopted a novel fluorine cycle polygonal channel PV/T panel-coupled ASHP system, and its operational performance was studied in a real residential building. Meanwhile, the system was used to compare and analyze the surface temperature variations, cooling efficiency, and power output between PV/T and PV panels under identical conditions, thereby validating the overall performance advantages of the PV/T system. This new system may have the potential to achieve low-carbon and energy-efficient buildings in the future.

2. System Principles and Design

2.1. System Basic Principles

The test site is a single residential building in Fuzhou City, Southern China, with a building area of 496.8 m². The experiment was conducted in winter, so indoor heating/cooling was not necessary. A combined system incorporating solar photovoltaic–thermal (PV/T) components with an ASHP was studied for simultaneous heating and power generation in this residential building.

According to the relevant standards [19,20], the hot water heating index for residential buildings is set at 15 W/m², the electricity load index for the residential building area is 20 W/m², and the surplus coefficient for residential heat and electricity loads is set at 1.1. The required hot water supply heat load for this building is 8.19 kW, and the electricity load is 10.89 kW. This co-generation system mainly consists of four parts, including ASHPs, water heating tanks, water storage tanks, and PV/T components. The back panel of the PV/T component adopts a polygonal Freon circulation channel design, and the refrigerant R134a is used. The heat is carried away through the refrigerant inside of the PV/T panels; then, it is compressed into a high-temperature and pressure liquid by the compressor. It transfers the heat to the water through the heat exchanges. By combining the PV/T component and the ASHP, low-grade thermal energy can be converted into high-grade thermal energy. The electricity generated is led out through the electrodes of the PV module, converted into alternating current by the inverter, and supplied to the building. The test bench uses 32 direct expansion solar PV/T components and the evaporator of the ASHP in parallel. The condenser side is connected in series with one water heating tank, and three water storage tanks are also connected in series simultaneously. Each water tank has a volume of 450 L, and a single-stage scroll variable-frequency compressor is used to achieve the operation of the heat pump system. To compare the performance differences between the PV/T components and traditional PV components, PV components with the same power generation capacity are arranged.

The PV/T heat pump system operates in heating mode, and the back panel absorbs the heat from the upper solar PV modules, thereby improving the thermal efficiency of the heat pump system. Meanwhile, it cools the PV panels to ensure the optimal temperature for power generation on the surface, thereby enhancing the overall utilization efficiency of solar energy. During the night or in rainy weather, the ASHP operates independently. The system’s vortex compressor operates in a variable frequency mode and can still heat the water to the ideal temperature. The system generates electricity through the PV panels due to the photoelectric effect of solar radiation. The electricity produced by the system is stored in the battery to achieve efficient combined heat and power supply. The system principle is shown in Figure 1. The photos of test site is shown in Figure 2.

2.2. System Configuration

The PV/T components were fabricated in accordance with the manufacturing process Standard GB/T39857-2021 [21]. They were made using the blow-in process, and welded together through heating and rolling. Then they were expanded with compressed air to form the refrigerant flow channels, achieving the effect of a low refrigerant flow resistance and uniform temperature distribution. The ASHP evaporator was connected in parallel with the PV/T back panel evaporator, and then passed through equipment such as compressors, condensers, hot water tanks, and water storage tanks to produce water for daily use. The main parameters of the test bench are shown in

Table 1. Main parameters of the test bench.

Component	Criteria	Value
PV module	PV module area, APV/m2	1.85
	PV module rated power, PPV/kW	0.45
	Number of PV modules/n	4
PV/T module	PV/T module area, APV/T/m2	1.85
	PV/T-integrated module power, PPV/T/kW	0.45
	PV/T-integrated module thermal power, Ph/kW	0.75
	The number of PV/T components/n	32
	Type of battery cells	Single crystal
	PVT size/mm
	Battery cell size/mm	182 × 182
	The voltage at the maximum power point, Vmpp/V	42.11
	The current at the maximum power point, Impp/A	3.074
Compressor	Rated power, Pcomp/kW	2.5
	Rated frequency, qcomp/Hz	50
	Rated voltage, Ucomp/V	220
	Maximum allowable pressure (Pmax/MPa)	4.2
	Allowable overpressure for inlet measurement (Pin/MPa)	1.5
Water tank	Volume/L	450
	Count/n	4
Electronic expansion valve	Nominal cooling capacity, Pex/kW	6.24
Refrigerant	/	R134a

.

The main experimental equipment is shown in

Table 2. Basic information of measuring instruments.

Test Instrument	Instrument Picture	Instrument Model	Environmental Parameter	Range	Precision
Agilent data acquisition instrument		Agilent 34972A/San Jose, CA, USA.	/	/	/
Flowmeter		T3-25/Wuhu, China	Water flow rate	0~50 (L/min)	±0.5%
Solar irradiance meter		YGC-TBQ/Wuhan, China.	Solar irradiance	1–2000 W/m2	±1.0%
Thermocouple		WRN-230/Shanghai, China.	Measure the temperature of the PV panel	−20~350 (°C)	±0.5%

.

The detailed parameters of the heat pump are described in

Table 3. Detailed parameters of the heat pump.

Parameters	Value	Parameters	Value
Rated Voltage	220 V	Rated Input Power	2.5 kW
Rated Frequency	50 Hz	Maximum Input Power	3.6 kW
Rated Input Current	11.4 A	Maximum Input Current	16 A
Rated Pressure on Water Side	0.7 MPa	Mass	115 kg
Water Pressure Drop of Heat Exchanger	≤30 kPa	Allowable Working Overpressure on Suction Side	1.5 MPa
Maximum Allowable Pressure on High/Low-Pressure Side	4.2 MPa	Allowable Working Overpressure on Discharge Side	4.2 MPa
Noise	≤53 dB (A)	Model Number	KSD95 EPA

.

2.3. System Operation Mode and Control Strategy

(1)

Low-demand mode: The hot water tank prioritizes supplying water to users, with any excess hot water being stored in the reserve tank.

(2)

High-demand mode: The hot water tank works in conjunction with the reserve hot water tank to provide a continuous water supply.

(3)

Water temperature maintenance mechanism: When the water temperature in the reserve hot water tank falls below the lower threshold, the water is recirculated to the hot water tank via a pump for reheating.

The system control strategy is shown in

Table 4. System control strategy.

Categories of Control	Determining Factors	Operating Conditions	Trigger Conditions	Numeric Parameters
Compressor start–stop control	Heat the water in the water tank/surface temperature of the PV/T plate	Compressor starts	The water temperature in the water tank is below the lower limit or the temperature of the PV/T panel is above the upper limit	≤40 °C/≥35 °C
		Compressor stops	The water temperature in the tank is above the upper limit and the temperature of the PV/T panel is below the lower limit	≥55 °C/≤15 °C
System operation control	Solar irradiance	PV/T heat pump mode	Abundant sunlight	≥200 W/m2
		ASHP mode	No sunlight	≤50 W/m2
		Dual-source operation mode	Insufficient light	≥50 W/m2 ≤200 W/m2

.

3. System Performance Evaluation Indicators

3.1. Photothermal Efficiency

The instantaneous thermal conversion efficiency (η) of the solar collector is defined as the quotient of the useful thermal energy harvested by the collector to the total incident solar flux projected on its aperture surface. This metric is mathematically expressed as [22]

$${\eta }_{\mathrm{t}\mathrm{h}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{t}\mathrm{h}}}{\mathrm{G}·{\mathrm{A}}_{\mathrm{c}}}}={\displaystyle \frac{{\mathrm{c}}_{\mathrm{p}}{\mathrm{m}}_0({\mathrm{T}}_0-{\mathrm{T}}_{\mathrm{i}})}{\mathrm{G}·{\mathrm{A}}_{\mathrm{c}}}}$$

(1)

where Q_th—heat gain of the collector, kJ; G—solar irradiance, W/m²; A_c—collector’s light-collecting area, m²; c_p—specific heat capacity of water, J/kg·°C; m₀—mass flow rate, kg/m³; T_i and T₀—inlet and outlet water temperatures of the collector, °C, respectively.

• The daily average photothermal efficiency of the collector [22] is

$${\eta }_{\mathrm{t}\mathrm{h},\mathrm{a}}={\displaystyle \frac{{\mathrm{c}}_{\mathrm{p}}\mathrm{M}({\mathrm{T}}_{\mathrm{w}}^{″}-{\mathrm{T}}_{\mathrm{w}}^{\prime })}{(\sum \mathrm{G}·\Delta \mathrm{t}){\mathrm{A}}_{\mathrm{c}}}}$$

(2)

where M—the mass of water in the water tank, in kg; T″_w and T′_w—the final temperature and the initial temperature of the water tank, in °C; △t—the data acquisition interval, in s.

3.2. Electrical Efficiency

The instantaneous electrical efficiency of the collector is defined as the ratio of the electrical power output of the PV cells to the solar irradiance received by the cells. The calculation formula is given by [22]

$${\eta }_{\mathrm{p}\mathrm{v}}={\displaystyle \frac{{\mathrm{E}}_{\mathrm{p}\mathrm{v}}}{{\mathrm{G}·\mathrm{A}}_{\mathrm{p}\mathrm{v}}}}={\displaystyle \frac{\mathrm{U}\mathrm{I}}{\mathrm{G}·{\mathrm{A}}_{\mathrm{p}\mathrm{v}}}}$$

(3)

where E_pv—output power, measured in watts (W); U and I—output voltage and current of the PV cell, respectively; A_pv—total area of the PV cell, measured in square meters (m²).

$${\eta }_{\mathrm{p}\mathrm{v},\mathrm{a}}={\displaystyle \frac{\sum {\mathrm{E}}_{\mathrm{p}\mathrm{v}}·\Delta \mathrm{t}}{\left(\sum \mathrm{G}·\Delta \mathrm{t}\right){\mathrm{A}}_{\mathrm{p}\mathrm{v}}}}$$

(4)

3.3. System Total Efficiency

According to the first law of thermodynamics, the total energy of the system remains conserved. Therefore, the total energy utilization efficiency of the system [23] can be expressed as

$${\eta }_0={\eta }_{\mathrm{p}\mathrm{v},\mathrm{a}}+{\eta }_{\mathrm{t}\mathrm{h},\mathrm{a}}$$

(5)

Based on the second law of thermodynamics, electrical energy is considered higher-grade energy compared to thermal energy. To evaluate the solar energy utilization efficiency of PV/T, the concept of comprehensive PV/T efficiency is introduced. By converting electrical energy into thermal energy equivalents, the total system efficiency can be calculated [24].

$${\eta }_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={\eta }_{\mathrm{t}\mathrm{h},\mathrm{a}}+\mathsf{\zeta }{\displaystyle \frac{{\eta }_{\mathrm{p}\mathrm{v},\mathrm{a}}}{{\eta }_{\mathrm{p}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}}}$$

(6)

where ζ = A_PV/A_C; ${\eta }_{\mathrm{p}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}$ represents the power generation efficiency of conventional thermal power plants.

3.4. Heat Loss Coefficient

The thermal loss coefficient of the solar collector can be calculated using the empirical formula proposed by KLEIN [25], as shown in Equations (7)–(9).

$${\mathrm{U}}_{\mathrm{L}}={\mathrm{U}}_{\mathrm{t}}+{\displaystyle \frac{\mathsf{\lambda }}{\mathrm{d}}}+{\displaystyle \frac{\mathsf{\lambda }{\mathrm{A}}_{\mathrm{e}}}{\mathrm{d}{\mathrm{A}}_{\mathrm{c}}}}$$

(7)

$${\mathrm{U}}_{\mathrm{t}}={\displaystyle \frac{1}{\frac{\mathrm{N}}{\frac{520(1-0.000051{\mathsf{\beta }}^2)}{{\mathrm{T}}_{\mathrm{p},\mathrm{m}}}(\frac{{\mathrm{T}}_{\mathrm{p},\mathrm{m}}-{\mathrm{T}}_{\mathrm{a}}}{\mathrm{N}+\mathrm{f}}{)}^{0.43(1-\frac{100}{{\mathrm{T}}_{\mathrm{p},\mathrm{m}}})}}+\frac{1}{{\mathrm{h}}_{\mathrm{w}}}}}+{\displaystyle \frac{\mathsf{\sigma }({\mathrm{T}}_{\mathrm{p},\mathrm{m}}+{\mathrm{T}}_{\mathrm{a}}\left)\right({\mathrm{T}}_{\mathrm{p},\mathrm{m}}^2+{\mathrm{T}}_{\mathrm{a}}^2)}{\frac{\mathrm{q}}{{\mathsf{\epsilon }}_{\mathrm{p}}+0.00591\mathrm{N}{\mathrm{h}}_{\mathrm{w}}}+\frac{2\mathrm{N}+\mathrm{f}-1+0.122{\mathsf{\epsilon }}_{\mathrm{p}}}{{\mathsf{\epsilon }}_{\mathrm{c}}}-\mathrm{N}}}$$

(8)

$$\mathrm{f}=\left(1+0.0892{\mathrm{h}}_{\mathrm{w}}-0.116 6{\mathrm{h}}_{\mathrm{w}}{\mathsf{\epsilon }}_{\mathrm{p}}\right)\left(1+0.07866\mathrm{N}\right)$$

(9)

where U_L is the total heat loss coefficient, W/(m²·K); U_t is the heat loss coefficient around the periphery, W/(m²·K); N is the number of glass layers; T_a is the ambient temperature, °C; h_w is the convective heat transfer coefficient between the ambient air and the glass cover plate, W/(m²·K); ε_p is the emissivity of the PV/T module; ε_c is the emissivity of the glass; β is the inclination angle of the PV/T water heater; λ is the thermal conductivity of the bottom insulation material, W/(m·K); d is the thickness of the insulation material, m; A_e/A_c is the ratio of the total area of the four side walls of the PV/T water heater to the collector area.

4. Results and Discussion

The testing phase was conducted over the period from 1 December 2024 to 28 February 2025. Hourly average temperature data were selected to analyze the performance of the combined heat and power system during the test. Variations in the solar irradiance and outdoor temperature during the tests are illustrated in Figure 3. The average outdoor temperature during the testing period was 18.1 °C, with a maximum of 22.9 °C and a minimum of 4.4 °C. The average solar irradiance was 561.2 W/m², with a maximum of 668 W/m² and a minimum 400 W/m². In the Fuzhou area, a day is generally classified as cloudy when the solar irradiance is below 500 W/m², with a total of 16 days. To compare and analyze the system’s operational performance, a typical day analysis was introduced in this study, including sunny and cloudy days. The average solar irradiance on a sunny day was 582 W/m², while on a cloudy day it was 454 W/m².

4.1. Power Generation Performance

The two primary parameters influencing the system’s power generation are the surface temperature of the PV/T panels and solar irradiance. As shown in Figure 4, the power output of the PV/T modules increases with rising solar irradiance, stabilizing at 0.42 kW when irradiance reaches sufficient levels.

The curves of power generation and consumption are illustrated in Figure 5. During typical days (sunny and cloudy days), the photovoltaic power peaks at around 13:00. On sunny days, the max power output of the 32 PV/T panels reaches 6.88 kW. Under effective solar irradiance, the average hourly power generation is 12.8 kW·h, resulting in a total daily power generation of 64 kW·h (based on 5 h effective sunlight). The system employed a variable-frequency scroll compressor with a rated power of 2.5 kW, consuming 22.5 kW·h of electricity daily. Each PV/T panel was equipped with a 1 kW·h battery. It can ensure that the system power generation meets its own operational demands while providing surplus electricity for the building. On cloudy days, the average daily power generation was 15.6 kW·h. The cloudy days in this region were typically non-consecutive, and the residual power stored in the battery from the previous day could be utilized to ensure the energy demand in the system.

The PV conversion efficiency of the PV/T system is shown in Figure 6. During testing, the efficiency ranged from 12.19% to 16.35%, with an average of 15.16% on sunny days. In comparison, conventional PV systems under similar conditions exhibit an overall electrical efficiency of approximately 10.1%, indicating that the PV/T system achieves a 4.73% improvement in the average efficiency. This enhancement is attributed to the heat dissipation mechanism of the PV/T modules: a portion of the thermal energy is absorbed by the refrigerant in the back solar panel, reducing the surface temperature of the PV components and thereby improving the power generation efficiency. On cloudy days, the average conversion efficiency of the PV/T system was 2.61% because of the lower solar irradiance.

Overall, the system’s power generation is mainly influenced by the panel surface temperature and solar irradiance. The power generation increases as the irradiance increases, and the maximum power generation is 0.42 kW. The total power generation of the 32 panels fluctuates and averages 64 kW·h per day on sunny days, which is sufficient for the system’s operation and basic power supply to the building. The average power generation efficiency of the PV/T system is 15.16%. Under cloudy conditions, the power generation efficiency is only 2.61% and the total power generation is 15.6 kW·h. However, the residual power stored in the battery from the previous day could be utilized to ensure the energy demand in the system.

4.2. Thermal Performance

Figure 7 illustrates the temperature variation in the heating water tank and thermal storage tanks during a typical day. As shown, the maximum temperature of the heating water tank reaches 56.3 °C, with a daily average temperature of 46.2 °C. The daily average temperatures of the three thermal storage tanks are 45.2 °C, 47.1 °C, and 48.2 °C, respectively. The test results demonstrate that the system can fully meet the building users’ daily hot water demands. As shown in the schematic diagram, the system water tank is connected in series. The hot water in the heating water tank can be directly supplied to the users or it can flow to heating water tank 1 to replenish the water. The hot water in storage water tank 1 will be replenished to storage water tank 2 when it reaches the standard, and storage water tank 3 will also complete the water replenishment process.

Form the figure, the heating regulation can be seen. At 8:00–14:00, the system switches to the PV/T operation mode, heating the water in the hot water tank. The water temperature rises from 35.6 °C to 56.9 °C, and then the system stops heating. At 14:00–17:00, the temperature of the hot water tank gradually decreases from 56.9 °C to 55.1 °C. At 17:00–21:00, the household users use hot water. The water temperature drops from 55.1 °C to 39.2 °C. At 21:00–3:00, the ASHP mode operates to heat the water in the hot water tank. The water temperature rises from 39.2 °C to 55.6 °C at 3:00–8:00. From 5:00 to 8:00, it reaches the peak period for hot water users. As fresh water is injected into the water tank, the water temperature drops sharply.

The real-time thermal efficiency variation curve of the PV/T collector during daytime operation is presented in Figure 8. In the initial operation phase, the thermal efficiency fluctuates around 18.34%, primarily due to relatively low outdoor temperatures and suboptimal solar irradiance. As outdoor temperatures rise and the solar irradiance increases, the system’s thermal efficiency gradually improves, peaking at 23.95% at 12:00. Subsequently, the thermal efficiency declines. The average daytime thermal efficiency of the system is 21.58%.

Figure 9 shows the relationship between the heat loss coefficient of the PV/T system and the water flow rate. As shown, the heat loss coefficient gradually decreases with an increasing water flow rate. When the water flow rate rises from 0.15 L/min to 0.68 L/min, the heat loss coefficient decreases from 5.92 W/(m²·K) to 5.62 W/(m²·K), representing a reduction of 5.07%.

In this project, the water tank was filled with 300 L, 350 L, 400 L, and 450 L of water for heating, and the temperature rise during a typical day was recorded as shown in Figure 10. As the injected water volume increased, the time required to raise the water temperature from 21.56 °C to 55.00 °C was 2.06 h, 2.68 h, 3.26 h, and 3.89 h, respectively. The increased water volume effectively removes heat from the PV/T module surfaces, lowering their temperature and improving the power generation efficiency. Considering both practical application requirements and system performance optimization, the optimal injected water volume for the tank is determined to be 450 L.

In summary, the average thermal efficiency of the system is 21.58%, and the heat loss coefficient decreases as the flow rate increases. The average temperature of the hot water tank in the system is 46.2 °C, while the average temperatures of the storage hot water tanks are 45.2 °C, 47.1 °C, and 48.2 °C, respectively. The temperatures can meet the daily hot water demands. The heating time of the water tanks varies with different water volumes. Increasing the water volume can improve the power generation efficiency. The optimal injection volume of the system is 450 L.

4.3. Energy Efficiency

During the operation of the PV/T and ASHP coupling system, the maximum COP of the system was 5.92 and the minimum value was 4.55. The average COP on sunny days was 5.68 and the average COP was 4.76 on cloudy days; when the ASHP operated alone, the average COP of the system was 4.66, and the energy efficiency ratio curve during system operation is shown in Figure 11.

4.4. PV/T vs. PV Comparison

4.4.1. Panel Surface Temperature

For the comparative analysis, a parallel-connected PV module with an identical power generation capacity was integrated into the system. As shown in Figure 12, the PV/T module maintained a lower panel surface temperature on both sunny and cloudy days. Additionally, the temperature difference between the PV/T and PV modules was greater on sunny days. The average surface temperature of the PV/T module was 17.76 °C, while the conventional PV module exhibited a higher average temperature of 30.51 °C on sunny days. The diurnal temperature differential fluctuated between 8.7 °C and 19.2 °C, demonstrating irradiance-dependent variation characteristics. During peak solar irradiance periods (11:00–13:00), the temperature difference between the two PV modules fluctuated within 14.86–16.51 °C. Subsequent temperature decreases in both modules were closely associated with declining solar irradiance levels, and the thermal differential between them remained basically constant throughout the test period.

Figure 13 presents the hourly cooling efficiency variations in the PV/T module compared with a conventional PV module on sunny and cloudy days. The results demonstrate significantly lower cooling efficiency in the morning than in the afternoon, with a mid-day peak observed. Specifically, the cooling efficiency increased from 19.39% at 8:00 to 44.46% by 11:30, followed by a further rise to 67.1% at 14:00. Subsequently, it decreased to 41.2% at 16:00, yielding an average cooling efficiency of 45.02% throughout the testing period. This phenomenon can be attributed to the elevated surface temperature of the conventional PV module under intense mid-day solar irradiance, whereas the PV/T system effectively reduced the module surface temperature, thereby enhancing the cooling efficiency. Generally, the percentage of cooling efficiency is greater on the typical sunny day due to the stronger solar irradiance and panel surface temperature.

4.4.2. Power Generation Capacity and Efficiency

Figure 14 illustrates the power generation variations in the PV/T and PV systems during typical days. Owing to the distinct surface temperatures of these panels, the systems exhibited different power outputs: the PV/T system achieved an average daily power generation of 0.4 kW on sunny days, while the conventional PV system averaged 0.26 kW. The average power outputs were 0.13 kW for the PV/T system and 0.074 kW for the PV system, respectively, on cloudy days. For most sunny weather conditions, when the panel temperature difference reached approximately 18.5 °C, the power generation difference between the two systems was 0.15 kW at 11:30–14:00. Similarly, during 14:30–16:00, a temperature differential of 14.6 °C corresponded to a power output difference of 0.12 kW. Notably, the disparity in power generation gradually diminished as the solar irradiance decreased, reflecting a direct correlation between thermal gradients and electrical performance under varying irradiance conditions.

Figure 15 demonstrates the variations in the power generation efficiency between the PV/T and PV systems. During a typical sunny day, the PV/T system exhibited a power generation efficiency fluctuation between 12.19% and 16.35%, with a daily average efficiency of 15.16%. In contrast, the conventional PV system showed efficiency variations ranging from 8.56% to 11.18%, achieving a daily average efficiency of 10.1%. This indicates a 4.73% enhancement in the daily average efficiency for the PV/T system. Both systems attained their peak efficiency at 12:00–14:00, stabilizing at approximately 15.36% and 11.56% for the PV/T and PV systems, respectively. During a typical cloudy day, both the PV/T and PV systems presented a lower and close power generation efficiency.

4.4.3. Total Efficiency Analysis

Figure 16 illustrates the total energy utilization efficiency of the PV/T system (incorporating both electrical and thermal energy outputs) and the conventional PV system. The results demonstrate the superior energy utilization efficiency of the PV/T system compared to the PV system. Specifically, the PV/T system achieved a maximum efficiency of 36.81% around 12:40, while the PV system peaked at 23.57% near 12:00. With the solar irradiance increase, both systems presented an improved energy utilization efficiency, accompanied by an expanding efficiency gap that peaked at 17.76%. During the afternoon period of declining solar irradiance, the energy utilization rates of both systems progressively decreased, maintaining a consistent downward trend in alignment with the reduction in irradiance.

In summary, the PV/T module maintained a lower panel surface temperature on both sunny and cloudy days. The temperature difference between the PV/T and PV modules was greater on sunny days. Additionally, the cooling efficiency percentage was greater on the typical sunny day due to the stronger solar irradiance and panel surface temperature. The daily average power output of the PV/T system was 0.4 kW, and the traditional PV system was 0.26 kW on most sunny weather days. The power generation efficiency of the PV/T system was 4.73% higher than the traditional PV system. The energy utilization efficiency of the PV/T system is significantly better than that of the PV system, with the maximum difference being 17.76%.

4.5. Impact of Water Injection Flow Rate on Thermal Power Efficiency

Figure 17 depicts the variations in photothermal and photoelectric efficiencies of the PV/T system with an increasing water flow rate. As the flow rate increases from 0.15 L/min to 0.68 L/min, both the photothermal and photoelectric efficiencies show an upward trend; specifically, the photothermal efficiency rises from 23.6% to 62.8%, whereas the photoelectric efficiency increases from 13.9% to 16.2%. This phenomenon can be attributed to the enhanced convective heat transfer at higher flow rates, which effectively extracts heat from the PV/T panel surface, thereby reducing its temperature. The accelerated heat removal by the coolant not only elevates photothermal efficiency but also stabilizes panel temperature, contributing to the enhancement of photoelectric efficiency. Notably, the photothermal efficiency exhibits a significant enhancement of 36.2% across the tested flow rate range, markedly surpassing the 2.3% improvement observed in photoelectric efficiency. These results underscore the critical role of optimized coolant flow in balancing the thermal management and energy conversion performance in PV/T systems.

4.6. System Pressure Analysis

Figure 18 displays the variations in the compressor pressure ratio and the dimensionless pressure loss coefficient of the PV/T module. The compressor pressure ratio serves as a critical indicator for the compressor efficiency, where an excessively high ratio signifies an increased power consumption, worse operational performance, and lower system efficiency. On both sunny and cloudy days, the compressor pressure ratio presented a predominantly linear upward trend. On sunny days, the maximum value reached 3.16 and the average value reached 2.57, which indicated a sustained high-efficiency. The maximum value was 4.51 and the average value was 3.64 on cloudy days, which was higher than that on sunny days. This phenomenon can be primarily attributed to the fact that, under the same compressor exhaust pressure in both operating conditions, the higher surface temperature of the solar panel on sunny days results in a higher evaporation temperature and pressure. Consequently, this leads to an increase in the compressor’s suction pressure and a lower compression ratio on sunny days.

The dimensionless pressure loss coefficient reflects the flow characteristics of the heat pump working fluid in the PV/T panel. The difference in the dimensionless pressure loss coefficient between sunny and cloudy days was relatively close, and it was a little higher on sunny days than that on cloudy days. It remained within 0.029–0.047 during operation, with an average value of 0.041 and 0.038, respectively, on sunny and cloudy days. This low value confirms the favorable uniform flow distribution and minimal flow resistance of the heat pump working fluid within the PV/T module’s back panel.

Figure 19 illustrates the variations in the inlet and outlet pressures of the evaporator. As shown, the evaporator’s pressure fluctuations exhibit synchronization with solar irradiance variations. Taking the inlet pressure on sunny days as an example, it increased from 530 kPa at 08:00 to 720 kPa at 10:25, followed by minor fluctuations around 720 kPa between 10:25 and 13:30. This increase in pressure corresponds to the evaporator’s enhanced heat absorption under intensified solar irradiance, which necessitates higher evaporation pressures to increase the working fluid’s mass flow rate for effective heat dissipation. Subsequently, from 13:30 to 16:00, the inlet pressure decreased from 740 kPa to 600 kPa, influenced by rising ambient temperatures. Notably, under comparable solar irradiance conditions, the afternoon evaporation pressures remained consistently higher than those in the morning.

The average compression ratio of the compressor is 2.57 and 3.64, respectively, indicating a high system efficiency state. The dimensionless pressure loss coefficient of the PV/T module ranges from 0.029 to 0.047, suggesting uniform fluid flow and low resistance. The pressure fluctuations at the inlet and outlet of the evaporator are synchronized with the variations in the solar radiation intensity. The evaporation pressure in the afternoon is higher than in the morning. It reflects the thermodynamic coupling between the environmental temperature and system operational characteristics.

This research on the polygonal fluorine circulation flow channel PV/T-coupled ASHP system was mainly conducted in Fuzhou, a hot summer and warm winter area in China. Through this research, it has been found that the system has excellent operational performance. However, for buildings in hot summer and cold winter climate areas and cold climate areas, the outdoor temperature is relatively low in winter, the solar irradiance is weak, and the effective irradiation duration is short. The applicability of the system is still uncertain, and requires further research and technical improvement. For example, ASHP-enhanced vapor injection technology or PV/T-coupled soil-source heat pump cross-seasonal energy storage technology can be adopted. Thus, a solar PV/T dual-source heat pump system for residential buildings in all climates will be further developed so to create energy-efficient and low-carbon future buildings.

5. Conclusions

A real application test was conducted in a residential building for a dual-source system integrating solar PV/T modules and an ASHP. The system adopted novel fluorine cycle polygonal channel PV/T panels. Through operational analysis, the following key conclusions are drawn:

(1)

Regarding the power generation performance, the PV/T system demonstrated superior thermal conductivity, effectively dissipating residual heat accumulated from solar radiation, thereby enhancing the power generation efficiency by an average of 4.73% compared to conventional PV panels. The PV/T-coupled ASHP system achieved an energy utilization rate 5.68–17.76% higher than traditional PV systems, confirming its high efficacy in real buildings in this climate zone. On cloudy days, although the average daily power generation was 15.6 kW·h, the residual power stored in the battery from the previous day could be utilized to ensure the energy demand in the system.

(2)

Regarding the heat collection performance, the system produced hot water without electrical reheating, and could maintain an average daily storage tank temperature of 46.8 °C, which satisfies domestic hot water demands. The coupled system presented a coefficient of performance (COP) of 5.68 on sunny days and 4.76 on cloudy days, with an average COP of 5.37. This indicates a 15.24% improvement in comprehensive performance compared to the standalone ASHP operation.

(3)

For the test typical day, when the system water flow rate was 0.7 L/min, the system’s thermal and PV efficiency was the best. The rate of change in the system’s heat loss coefficient was 5.07%, which is relatively low. The best water volume for the system’s water tank was 450 L. The average pressure ratio of the system was 2.57, and the average dimensionless pressure loss coefficient was 0.039, which indicates the coolant uniformity of the PV/T back panel in most weather conditions.

(4)

The solar PV/T-coupled ASHP system outperformed traditional PV and ASHP systems in photothermal efficiency, photoelectric efficiency, and overall energy efficiency. These results highlight its promising application for future low-carbon building technologies. Currently, this coupled PV/T system performs well in winter buildings in the hot and humid climate zone of Fuzhou. However, its applicability in cold regions will be further studied.