The Effect of Radiation Intensity on the Performance of Direct-Expansion Solar PVT Heat Pump Systems

Abstract

The objective of this study was to investigate the impact of solar radiation intensity on the performance of direct-expansion solar PVT heat pump systems. To this end, an experimental setup was constructed for direct-expansion photovoltaic (PVT) solar heat pump water heating systems and photovoltaic (PV) power generation systems. The system performance and main parameters were analyzed and discussed under different solar radiation intensities. The winter experiments in southern China revealed that the exhaust temperature of the heat pump unit varied considerably under clear conditions, while the back temperature remained stable, fluctuating between approximately −13.5 °C and 24 °C. In contrast, the power generation of PVT panels increased with the increase in radiation intensity, from 78.33 W to 122.68 W, for an increase of 56.6%. Furthermore, the total electricity generation of the PVT panels was higher than that of PV panels, with an increase of 8.7–8.3%. Nevertheless, discrepancies between experimental and theoretical data were observed, particularly under overcast conditions, where the back panel temperature error was pronounced. Additionally, the system exhibited enhanced stability at elevated temperatures in comparable environments, accompanied by an improvement in the system’s coefficient of performance (COP) by 5.67%.

1. Introduction

With the rapid development of the economy and continuous technological progress, the world is facing unprecedented energy and environmental challenges. Solar energy, as a clean and renewable energy solution, is widely regarded as key to addressing climate change and achieving carbon neutrality goals [1]. Meanwhile, heat pump technology, as a method of extracting heat energy from low-temperature heat sources, is also of great significance, as it can obtain a greater quantity or higher quality of heat energy.

Photovoltaic/thermal (PVT) technology has been widely studied since the 1970s. This technology can fully utilize the waste heat from photovoltaic power generation and reduce the temperature of photovoltaic modules, thereby improving photovoltaic efficiency and increasing the overall utilization of solar energy [2]. The combination of PVT and heat pump technology, with its characteristics of temperature matching and cascaded energy utilization, has attracted the attention of many scholars [3,4,5,6]. For example, a study by Sheng Zhang et al. [7] found that a PVT heat pump system can reach a COP of 4.96 and increase the photovoltaic conversion efficiency to 25% when deployed at scale. Chao Zhou and Jian Yao [8,9] respectively investigated the combined heat and power stability of PVT systems in northern China during summer and the potential performance enhancement under cold conditions. Research by Quitiaquez et al. found that when the average solar radiation is 607.5 W/m², the maximum COP of the system can reach 5.75, effectively reducing CO₂ emissions by 1065.6 kg [10]. Furthermore, Du et al.’s experimental study indicated that under typical heating conditions in winter, the heating COP of the PVT air dual-source direct-expansion heat pump system exhibited variations under different modes [2]. Researchers also proposed design methods for direct-expansion PVT modules through theoretical analysis and experimental validation [11], as well as a novel direct-expansion solar-assisted ejector-compression heat pump cycle, which exhibited excellent heating COP and heating capacity under high solar radiation conditions [12]. Additionally, Yao et al.’s simulation study showed that a solar PVT heat pump system combined with built-in phase-change material (PCM) thermal storage could achieve a heating COP of 5.79, 70% higher than that of conventional air conditioning systems [13]. Kong et al. showed that the system performance of a direct-expansion solar-assisted heat pump water heater improved with increasing solar radiation intensity or ambient temperature [14] and developed a variable frequency control method, the validation of which showed an average COP of more than 3.0 for most of the year [15]. Shi et al.’s review emphasized the performance optimization and system configuration diversity of direct-expansion solar-assisted heat pump systems [16]. Rabelo et al.’s experimental analysis showed that for small-scale CO₂ solar-assisted heat pumps, static expansion devices such as capillaries were a cost-effective choice [17]. In this research field, researchers like Antonio Rosato found that hybrid systems had significant advantages in terms of energy, environment, and economy [18]. Additionally, Christopher Pourier’s research indicated that combining ground-source heat pumps with free cooling and adding PVT collectors could improve system efficiency and reduce resource requirements [19].

Nevertheless, despite numerous in-depth studies on PVT heat pump systems [20,21,22,23,24], detailed research on the effect of solar radiation intensity on system performance remains insufficient. The dearth of comprehensive discourse on radiation intensity variables in the extant literature presents a significant research opportunity and impetus for this study.

Therefore, the objective of this study was to analyze in detail the effect of solar radiation intensity on the performance of a direct-expansion solar PVT heat pump system, with particular attention to its specific effects and trends on the back and exhaust temperatures of the heat pump within the system. Additionally, the performance of the direct-expansion solar PVT heat pump system in terms of power generation and power generation efficiency, as well as its changing patterns, was systematically explored in comparison with conventional PV panels. The results of these studies provide a new analysis of PV solar thermal technology in terms of energy conversion efficiency improvement. They also offer more specific technical support and direction for solving energy and environmental challenges.

2. System Design and Working Principle

This paper discusses the direct-expansion solar PVT heat pump system, which synchronizes the output of electrical and thermal energy by efficiently utilizing solar radiant energy, as shown in the experimental principle in Figure 1. The system consists of two subsystems: a photovoltaic power generation subsystem and the heat pump subsystem. The photovoltaic power generation subsystem comprises photovoltaic panels, batteries, and inverters. It primarily converts solar energy directly into electricity, outputs 48 V DC power stored in batteries, and converts it into 220 V, 50 Hz AC power through inverters for supply to the national grid or local use. The heat pump subsystem includes PVT collectors, compressors, condensers, and expansion valves (as shown in Figure 2), using R410a as the circulating refrigerant. The refrigerant evaporates directly in the PVT panels, absorbing heat and removing part of the heat accumulated in the photovoltaic cells. After compression, the refrigerant enters the condenser in the water tank and exchanges heat with the water in the tank. This process not only reduces the temperature of the PVT panels, thereby increasing the photovoltaic conversion efficiency, but also produces hot water by utilizing the heat.

3. Test Bench Introduction

3.1. Experimental Equipment

To investigate the effect of solar radiation on the performance of the direct-expansion solar PVT heat pump system, the experimental platform was set up in Fuzhou City, China, and the research configuration contained 12 PVT panels and four standard PV panels, in addition to key equipment such as batteries, inverters, heat pump components, heating tanks and heat storage tanks, as shown in Figure 2.

The experiment utilized T-type solar PVT panels manufactured by Zhongneng Electric Appliances Co., Ltd., Fuzhou, China (shown in Figure 3) and standard PV panels. Both were mounted on stainless-steel frames facing due south to optimize solar energy absorption efficiency. The parameters related to the PVT collectors are listed in

Table 1. Parameters of PV/T Panels and PV Panels.

Parameters	Value
PV/T Panel Place of origin	Fuzhou City, China
PV/T Panel Brand name	SOLAR MODULE
PV/T Panel Model number	T-type model
PV/T Panel Material	single crystal silicon
PV/T Panel Size/mm	1755 × 1038
PV/T Panels Peak power/pmax (W)	360
PV/T Panel Rated voltage (V)	34
PV/T Panel Rated current (A)	10.6
PV/T Panel Open circuit voltage (V)	40.8
PV/T Panel Short-circuit current (A)	12.7
PV Panel Place of origin	Fuzhou City, China
PV Panel Brand name	SOLAR MODULE
PV Panel Material	single crystal silicon
PV Panel Size/mm	1755 × 1038

.

Compared to traditional PV panels (see Table 1 for relevant parameters), the system used in this study incorporates PVT panels with a T-type evaporator on the backside, cooled by the refrigerant R410a flowing through the backplate. All PVT panels are connected to the heat pump unit in parallel via copper pipes, with black insulation wrapped around the pipes to minimize heat loss.

The heat pump unit used in the experiment is the KSD95 EPA dual heat source unit developed by Zhongneng Electrical Appliances Co., Ltd. (Wenzhou, China). It consists of components such as return pipe, suction pipe, compressor, discharge pipe, condenser, and expansion valve. These components are connected to the heating water tank via copper pipes to facilitate water circulation. The water circulation is powered by an A80/180 M type circulating water pump, located between the outlet of the heating water tank and the inlet of the heat pump unit.

3.2. Experimental Instruments

This study analyzes the performance of a direct-expansion solar PVT heat pump system and its response characteristics to solar radiation intensity. To ensure accurate data collection, a series of precision instruments were employed in the experiment. These instruments include ultrasonic flow meters (China Weihai Tiangang Instrument Co., Ltd., Weihai, China), SBWZP-23 temperature sensors (China Beijing Zhongyi Huashi Technology Co., Ltd., Beijing, China), PT100 temperature sensors (China Shandong Jianda Renke Co., Ltd., Jinan, China), solar irradiance instruments (China Beijing Zhongyi Huashi Technology Co., Ltd., Beijing, China), and a Programmable Logic Controller (PLC) data acquisition module (Fuzhou Zhongneng Electric Power Equipment Co., Ltd., Fuzhou, China). The ultrasonic flow meters determine the mass flow rate of the heat pump outlet water, while the temperature sensors at the heat pump outlet measure the temperature of the heat pump outlet water. PT100 temperature sensors measure the temperature variations on the surfaces of the PVT panels and the back of the PV panels. To ensure uniform temperature measurement, multiple temperature sensors are installed. Four back-mounted PT100 temperature sensors (T20 and T19, T9 and T8) are arranged at the inlet and outlet of two PVT panel components, and 15 back-mounted PT100 temperature sensors (T1-T20, excluding T20, T19, T9, T8, T10) are positioned at different locations on the backplate of the T-shaped PVT panels, as shown in Figure 4.

At the same time, the solar irradiance instrument is installed on the top of the control cabinet to measure solar radiation intensity. To comprehensively analyze the experimental data, the PLC data acquisition module collects signals from all sensors at one-minute intervals throughout the day. Specific instrument parameters and measurement ranges are detailed in

Table 2. Schematic Diagram of the Experimental Setup.

Instrument Name	Specification Model	Measurement Range	Accuracy
Heat Pump Flow Meter	LC82	0.1–5 m3/h	0.2%
Heat Pump Inlet/Outlet Temperature Transducer	SBWZP-230	0–100 °C	1.0%
intensity of solar radiation	RS-GZ-I20-2-65535	0–200,000 Lux	±0.7%
Temperature Sensor	WZPX-145M	0–100 °C	0.2%

.

4. Processing of Experimental Data

To investigate the influence of solar radiation intensity on the performance of a direct-expansion solar heat pump water heater system, this study conducted several sets of experiments in the natural meteorological conditions of Fuzhou, China, from November to January. Seven sets of typical experimental data were selected for discussion and analysis of key operating parameters, including panel temperature, electricity generation, electrical efficiency, COP, back temperature, and exhaust temperature. Throughout the experiment, the temperature of the water tank was maintained at 40 ± 2 °C.

(1)

Ambient temperature

Given the minimal differences in ambient temperature between the various experimental groups, the study selected data from several experiments conducted under the condition of maintaining the ambient temperature at 16 ± 1 °C.

(2)

Solar irradiance

The solar irradiance meter recorded data once every minute; therefore, the calculated solar irradiance I₀ is the average value over the experimental period.

(3)

The mathematical model for the temperature of PVT and PV panels can be expressed as follows [7,21,25,26]:

$$T_{PVT}=30+0.0175\times (I-300)+1.14\times (T_a-25)$$

(1)

$$T_{PV}=T_a+\frac{T_{NOCT}-20}{800}I$$

(2)

In the formula, T_PV represents the temperature of the PV panel, in degrees Celsius (°C).

T_PVT represents the temperature of the PVT panel, in degrees Celsius (°C).

I represents the solar radiation intensity, in watts per square meter (W·m⁻²).

T_NOCT represents the nominal operating cell temperature (NOCT) of the photovoltaic cells, taken as 45 °C.

T_a represents the ambient temperature, in degrees Celsius (°C).

(4)

Power generation of direct-expansion PVT heat pump system [22].

$$P=UI$$

(3)

In the formula, U represents the output voltage of the PVT panel.

I represents the output current of the PVT panel.

(5)

Photovoltaic efficiency of the direct-expansion PVT heat pump system [27]

$${\eta }_{\mathrm{e}}=\frac{UI}{GA}$$

(4)

In the formula, U and I represent the output voltage and current of the PVT panel collector, respectively.

G is the total solar irradiance on the inclined surface, measured in watts per square meter (W·m⁻²).

A is the area of the photovoltaic panel, measured in square meters (m²).

(6)

Theoretical photovoltaic efficiency of the PVT panel [28].

$${\eta }_{\mathrm{th}}=\frac{Q_{\mathrm{e}}}{AG}$$

(5)

In the formula, Q_e represents the output electrical energy, measured in watts (W).

G represents the total radiation intensity incident on the inclined surface, measured in watts per square meter (W·m⁻²).

A represents the area of the photovoltaic panel, measured in square meters W·m⁻²).

(7)

The coefficient of performance for heating system.

$$COP=\frac{Q_h}{W_{com}}$$

(6)

In the formula, Q_h represents the heat output of the system, measured in joules (J).

W_com represents the power consumed by the compressor, measured in joules (J).

5. Results and Discussion

This study conducted experiments during the winter in Fuzhou, focusing on the detailed analysis of the impact of solar radiation intensity on the performance of the direct-expansion solar PVT heat pump system. By comparing the electrical performance data of PV panels and PVT panels of the same specifications, the specific experimental data can be found in

Table 3. Winter Experiment Data Sheet for Fuzhou.

Experiment Date	Weather Condition	Ambient Temperature	Radiation Intensity	Suction Temperature	Discharge Temperature	PVT Panel Temperature	PV Panel Temperature	PVT Generation Efficiency η	PV Generation Efficiency η	COP
		°C	(W/m2)	°C	°C	°C	°C
30 December 2023	Overcast	16.94	369	4.82	86.47	12.68	25.59	19.92%	17.99%	2.44
3 January 2024	Cloudy	16.06	483	4.27	85.66	14.4	21.4	21.12%	19.49%	2.61
4 January 2024	Sunny	16.45	510	7.35	89.95	13.87	22.22	22.37%	20.54%	2.52
25 December 2023	Sunny	11.48	527	3.22	89.06	10.72	19.2	23.47%	22.83%	2.14
28 December 2023	Cloudy	17.39	441	6.99	91.91	15.32	28.03	21.28%	19.45%	2.66
2 January 2024	Cloudy	15.24	486	2.19	84.12	14.82	26.33	22.82%	0.2043	2.53
5 January 2024	Cloudy	16.53	468	3.68	85.7	14.57	27.81	20.64%	0.1872	2.81

.

5.1. Solar Radiation Intensity and Ambient Temperature Variation Curve

Figure 5 depicts the curves of solar radiation intensity and ambient temperature for 7 days, from 25 December 2023 to 28 December 2023, and from 2 January 2024 to 5 January 2024. After system initialization, the Programmable Logic Controller (PLC) automatically collects and stores data every minute. During the seven-day testing period, the system starts at 8:00 a.m. and shuts down at 5:30 p.m. each day. The average solar radiation intensity and ambient temperature are marked in the figure, with (t₀) representing the daily average ambient temperature and (I₀) representing the daily average radiation intensity.

Under sunny conditions, the solar radiation intensity exhibits an arch-shaped variation with an initial increase followed by a decrease, with the system entering a high radiation range (600 W·m⁻²) between 9:50 a.m. and 2:15 p.m. Under overcast conditions, the solar radiation intensity also shows a trend of increase followed by decrease, but the data are more dispersed, influenced by cloud distribution, movement, and wind, resulting in fluctuating radiation intensity, occasionally reaching maximum (1093.62 W·m⁻²) and minimum (296.16 W·m⁻²) values.

Under cloudy conditions, the solar radiation intensity exhibits oscillations and dispersion after reaching a certain value, due to the constantly moving sparse cloud layers causing changes in radiation intensity. During the experiment, the trend of ambient temperature change is similar to that of radiation intensity, both showing an initial increase followed by a decrease, and consistently staying above the initial temperature. The ambient temperature gradually decreases during system operation, reaching approximately 16 °C between 12:30 p.m. and 3:30 p.m., followed by a gradual decrease until system shutdown.

The data indicate that during the period from 9:50 a.m. to 3:30 p.m., both solar radiation intensity and ambient temperature reach relatively high values, with solar radiation intensity reaching 600 W·m⁻² and ambient temperature approximately 16 °C. Under high radiation intensity, the refrigerant can absorb more heat from the sun in the PVT panel, reducing the temperature of the PVT backboard and thereby increasing the system’s electricity generation efficiency. Considering the longer heating time in winter, it is appropriate to expand the time interval to 9:00 a.m. to 6:00 p.m. as the operating time period of the system to achieve better environmental conditions.

5.2. Suction and Discharge Temperatures in Relation to Solar Radiation

The heat pump’s compressor back temperature and discharge temperature are crucial indicators affecting the system’s performance. With the variation in solar radiation intensity, these two temperatures are significantly influenced, thus holding crucial importance for the overall efficiency and safe operation of the heat pump system. Therefore, this section extensively discusses the impact of solar radiation intensity on the compressor back temperature and discharge temperature throughout the operation of the entire system, as illustrated in Figure 6.

In the initial phase of system operation, during the self-equilibrium process, the compressor back and discharge temperatures initially increase significantly. Through analysis of Figure 6, this study found that under conditions with an average ambient temperature of 16.5 ± 0.5 °C and average solar radiation intensities of 369, 468, 483, and 510 W·m⁻², respectively, the average temperature difference between the heat pump compressor discharge and back temperatures ranged from 81.65 to 83.02 °C, with the discharge temperature reaching −11 ± 2 °C at the end of the experiment. As solar radiation intensity increases, the back temperature oscillates upwards, with some data showing refrigeration effects and temperatures below 0 °C.

Under sunny conditions, the solar radiation intensity exhibits a uniform rise followed by a declining trend, and correspondingly, the back and discharge temperatures generally rise before falling. During the experiment, the compressor discharge temperature fluctuates greatly, ranging from 76 to 100 °C, with a maximum temperature difference of 24 °C. Throughout the testing period, the average discharge temperature is 89.95 °C, while the back temperature shows relatively smaller fluctuations compared to the discharge temperature, with a maximum value of 19 °C, a minimum value of −9.5 °C, and an average temperature of 7.35 °C.

Under overcast conditions, the solar radiation intensity fluctuates synchronously with the back temperature, reaching its maximum value (1093.62 W·m⁻²) of the day between 10:22 and 12:04 due to the movement of clouds. During this period, the back temperature also oscillates upwards with the fluctuation of solar radiation, while the discharge temperature remains relatively stable. Compared to overcast conditions, the compressor discharge temperature exhibits a greater magnitude of variation, reaching a maximum value of 110 °C, while the compressor back temperature remains relatively stable, fluctuating between −13.5 and 24 °C.

5.3. Variation of PVT and PV Panel Temperatures with Solar Radiation

The panel temperature is one of the key factors affecting system efficiency and output power. Excessively high panel temperatures not only reduce the power generation efficiency of PVT panels but may also decrease the lifespan of the panels and increase safety risks. Therefore, this section provides an in-depth analysis of the influence of solar radiation intensity on panel temperature. Using clear-sky and overcast conditions as examples, the specific effects on system operating efficiency, as depicted in Figure 7, are discussed.

During the operation of the system, with the gradual increase in solar radiation intensity, the temperatures of both PVT and PV back panels show an oscillating upward trend, with similar temperature fluctuation patterns, overall exhibiting an increasing trend. Under sunny conditions, the variation range of PVT back panel temperature is 4.28–20.54 °C, while for the PV back panel, it is 9.89–32.53 °C, with average temperatures of 12.41 °C and 21.21 °C, respectively. A comparative analysis of PV back panel temperature data reveals a relative decrease of 36.86% and 56.72% in the highest and lowest back panel temperatures, respectively. Under overcast conditions, the variation range of PVT back panel temperature is 3.71–23.58 °C, while for the PV back panel, it is 10.39–34.63 °C, with average values of 13.64 °C and 22.51 °C, respectively. Comparative analysis of PV back panel temperature shows a relative decrease of 31.91% and 64.29% in the highest and lowest back panel temperatures, respectively.

Calculations based on theoretical Formulas (1) and (2) indicate that under sunny conditions, the theoretical values of the lowest and highest temperatures of the PVT panel are 11.88 °C and 27.04 °C, respectively, while under overcast conditions, the theoretical values of the lowest and highest temperatures are 10.72 °C and 34.94 °C, respectively. The average back panel temperature error values are 36.23% and 40.25% for sunny and overcast conditions, respectively. Through comprehensive analysis of experimental results under different conditions, it is observed that there is a certain degree of error between experimental and theoretical data, with larger errors in back panel temperatures observed under overcast conditions.

To further compare the variations in system back panel temperatures under different solar radiation intensity conditions, statistical analysis was conducted on the data in this study, and regression linear curves for each condition were fitted. The intercept and slope of the instantaneous PVT back panel temperature curves are shown in Figure 8. Under clear-sky conditions, the mean values of the intercept and slope of the instantaneous PVT back panel temperature curve are 6.18 °C and 0.016 K/W·m⁻², respectively; under overcast conditions, they are 8.31 °C and 0.0118 K/W·m⁻², respectively; and under cloudy conditions, they are 8.62 °C and 0.0119 K/W·m⁻², respectively.

Comparing sunny conditions (510 W·m⁻²) with overcast conditions (369 W·m⁻²) and cloudy conditions (483 W·m⁻²) at different solar radiation intensities, the intercepts of the instantaneous PVT back panel temperature curves decrease by 2.13 °C and 2.44 °C, respectively, with increasing solar radiation intensity. The slopes of the instantaneous PVT back panel temperature curves are nearly the same, around 0.01 K/W·m⁻².

5.4. Electricity Generation of PVT and PV Panels in Relation to Solar Radiation

The significance of the electricity generation of PVT panels is that it directly reflects the overall performance of the direct-expansion solar PVT heat pump system and visually represents the energy-saving effects of the system. This is crucial for assessing the sustainability and efficiency improvements of the system during its operation.

As shown in Figure 9, through comprehensive data analysis across 4 days under different weather conditions, the electricity generation gradually increases with the increase in radiation intensity, reaching a peak and then stabilizing, followed by a decrease as the radiation weakens. Under conditions with average temperatures of 16 ± 0.5 °C and average radiation intensities of 369 W·m⁻² (overcast), 483 W·m⁻² (cloudy), and 510 W·m⁻² (sunny), the average instantaneous electricity generation of the PVT panels is 78.33 W, 109.83 W, and 122.67 W, respectively. Data analysis indicates a significant positive correlation between the average electricity generation of the PVT panels and solar radiation intensity. As the radiation intensity increases, the electricity generation increases, but the rate of increase gradually slows down, and the system’s response to the increase in radiation tends to saturate.

Under similar environmental temperatures, with the radiation intensity increasing from 369 W·m⁻² to 510 W·m⁻², the instantaneous electricity generation of the PVT panels increases from 78.33 W to 122.68 W, representing a 56.6% increase. This change reflects that under higher solar radiation intensity, the PVT panels absorb more energy, thereby increasing the system’s electricity generation.

Throughout the entire operational period, the total electricity generation of the PVT panels is significantly higher than that of the PV panels, as shown by the time integration analysis from 8:00 to 17:30 (see Figure 10). Under average temperatures of 16 ± 0.5 °C and different radiation intensities, the total electricity generation of the PVT panels is 0.75 kWh, 1.04 kWh, and 1.17 kWh, respectively, while that of the PV panels is 0.69 kWh, 0.98 kWh, and 1.08 kWh, respectively. Detailed experimental data can be found in

Table 4. Experimental Data of Total Electricity Generation under Different Conditions.

Time	Weather Conditions	Radiation Intensity	Total PVT Power Generation (kWh)	Total PV Power Generation (kWh)
12/30	Overcast	369	0.75	0.69
1/3	Cloudy	483	1.04	0.98
1/4	Sunny	510	1.17	1.08
1/5	Cloudy	468	1.01	0.93

.

Under overcast, cloudy, and sunny conditions, the total electricity generation of the PVT panels relative to the PV panels increases by 8.7%, 6.1%, and 8.3%, respectively. The data indicate that in the direct-expansion solar PVT heat pump system, the PVT panels exhibit higher electricity generation under different conditions, especially during the transition from overcast to sunny conditions, where the performance of the PVT panels in electricity generation relative to the PV panels is significantly enhanced. This further confirms the excellent performance of the PVT system in utilizing solar energy.

To provide a more intuitive comparison of the relationship between the instantaneous electricity generation of PVT and PV panels and solar radiation intensity, linear fitting was performed in this study, and the data for average solar radiation intensity and average instantaneous electricity generation under various conditions were obtained (with goodness of fit greater than 0.99). The fitting results show that the intercept and slope of the instantaneous electricity generation curve for PVT panels are −33.79 ± 11.07 and 0.3 ± 0.02, respectively, while those for PV panels are −31.18 ± 9.46 W and 0.28 ± 0.02, respectively. The functional relationships between solar radiation intensity and average instantaneous electricity generation obtained are as follows(As shown in Figure 11):

Y_PVT = 0.3X − 33.79

Y_PV = 0.28X − 31.18

5.5. Relationship between Photovoltaic Efficiency and Solar Irradiance

This study explores the close relationship between photovoltaic efficiency and solar radiation, displaying the instantaneous photovoltaic efficiency curves over operating time in Figure 12, Figure 13 and Figure 14.

In the overcast condition (30 December), as shown in Figure 12, the photovoltaic efficiency gradually increases with the increase in solar radiation intensity, especially after 11:30. Despite the weakening of solar radiation intensity, both the PVT and PV panels exhibit a relatively stable trend in photovoltaic efficiency, with average instantaneous efficiencies reaching 21.03% and 19.32%, respectively, until a sharp decrease in solar radiation occurs around 16:30. Throughout the experiment, the average photovoltaic efficiencies of the PVT and PV panels are 19.69% and 18.03%, respectively, representing an increase of 1.34% and 1.29% compared to the baseline condition.

In the cloudy condition (3 January), as depicted in Figure 13, the photovoltaic efficiency increases with the enhancement of solar radiation intensity until 11:00. However, from 11:00 to 13:30, the emergence of clouds causes fluctuations in solar radiation intensity, thereby affecting the photovoltaic efficiency of both the PVT and PV panels. The average instantaneous photovoltaic efficiencies reach 23.49% and 22.47%, respectively. Overall, the average photovoltaic efficiencies of the PVT and PV panels are 21.12% and 19.49%, respectively, representing an increase of 2.37% and 2.98% compared to the baseline condition.

In the sunny condition (4 January), the photovoltaic efficiency increases with the increase in solar radiation intensity and stabilizes after reaching its peak, then decreases as solar radiation decreases. Both the PVT and PV panels exhibit similar arch-shaped trends in photovoltaic efficiency with solar radiation intensity. The average photovoltaic efficiencies are 22.37% and 20.47%, respectively, representing an increase of 1.9% for the PVT panel compared to the PV panel.

By calculating with theoretical Formula (5), the photovoltaic efficiency of the PVT panel is 19.84%. Compared to the experimental results, its relative increases in partly cloudy, sunny, and partly sunny conditions are 1.19%, 3.65%, and 2.53%, respectively. This comparison further validates the potential of the system in improving solar energy conversion efficiency.

5.6. System COP in Relation to Solar Radiation

The COP is the most important indicator for evaluating the performance of a heat pump system, as it can represent the energy-saving effect of a system’s operation. COP can be calculated using Formula (5), and the time can be divided into morning and afternoon periods based on the maximum solar radiation intensity to find a function suitable for prediction.

In Figure 15, the COP of the direct-expansion solar PVT heat pump system exhibits a close correlation with ambient temperature and solar radiation intensity under three different operating conditions. The data indicate that when the average ambient temperature is 16.94 °C, the stable average value of the system is higher compared to the average value during the experimental period. Under this condition, the radiation intensity and COP are higher by 194.44 W·m⁻² and 0.14, respectively. Additionally, the COP increases by 5.67%. With the change in average ambient temperature, the performance of the system varies under different temperature conditions. When the average ambient temperature is 16.06 °C, the radiation intensity and COP are higher by 165.08 W·m⁻² and 0.21, respectively, with a COP increase of 8.05%. Meanwhile, at an average ambient temperature of 16.45 °C, the radiation intensity and COP are higher by 171.37 W·m⁻² and 0.31, respectively, with a COP increase of 12.30%.

The experimental results demonstrate significant variations in COP under different ambient temperatures, especially at higher temperatures in similar environmental conditions, where the stable average COP of the system is minimally different from the average value during the experimental period, indicating a more stable trend of the system under high-temperature conditions.

Through quantitative analysis based on Figure 15 and Figure 16, it is revealed that there exists a clear linear relationship between the COP of the direct-expansion solar PVT heat pump system and solar radiation intensity, while being influenced by ambient temperature. At higher ambient temperatures, the intercept and slope of the COP curve in the morning are 0.92199 and 0.00268 W·m⁻², respectively, while in the afternoon, they are 2.05809 and 0.00133 W·m⁻². However, at lower ambient temperatures, the intercept and slope of the COP curve in the morning are 1.01663 and 0.00243 W·m⁻², respectively, and in the afternoon, they are 1.87055 and 0.00155 W·m⁻². These data not only demonstrate the dependency of COP on solar radiation intensity but also show the system’s adjustment capability during different time periods and under different ambient temperatures. Particularly, under high-temperature conditions, COP is enhanced by increasing the intercept rather than the slope, while under low-temperature conditions, a similar adjustment pattern is adopted.

6. Conclusions

This study delves into the impact of solar radiation intensity on the direct-expansion solar PVT heat pump system, systematically analyzing the variations in PVT panel temperature, electricity generation, photovoltaic efficiency, and COP through comparison with traditional PV panels. The main conclusions of this paper are as follows:

• Under different weather conditions, the system exhibits varying operational states. On sunny days, the system enters a high radiation range, while under cloudy and overcast conditions, radiation intensity fluctuates significantly. The return and discharge temperatures of the compressor increase with solar radiation, indicating partial cooling effects. In sunny and overcast conditions, the overall discharge temperature of the system initially rises and then falls, while the back temperature fluctuates between −13.5 and 24 °C. Under conditions with an average ambient temperature of 16.5 ± 0.5 °C and an average solar radiation intensity of 510 W·m⁻², the average temperature difference between discharge and return air temperatures ranges from 81.65 to 83.02 °C, with the final discharge temperature reaching −11 ± 2 °C.
• The system demonstrates significant performance superiority over PV panels under sunny, overcast, and cloudy conditions, achieving temperature decreases of 12 °C, 11 °C, and 19 °C, respectively, compared to PV panels. The temperature slope remains consistent at approximately 0.01 K/W·m⁻², indicating the stable and uniform response of the PVT panel to solar radiation. Furthermore, the decrease in temperature curve intercepts from 2.13 °C to 2.44 °C highlights the system’s excellent adaptability to environmental changes. There is a certain degree of discrepancy between experimental and theoretical data, with larger errors observed in the overcast conditions.
• Under different radiation intensities, the PVT system demonstrates higher power generation efficiency, with PVT panel power output increasing with radiation intensity from 78.33 W to 122.68 W, representing a 56.6% increase, significantly higher than that of PV panels. Additionally, the total electricity generation of the PVT panel exceeds that of the PV panel in all conditions, with an increase ranging from 8.7% to 8.3%. The theoretically calculated electrical efficiency is 19.84%, with measured relative increases of 1.19%, 3.65%, and 2.53%, validating its stability and performance advantages.
• Linear fitting results show that the response of the PVT panel to radiation intensity is more sensitive, with intercepts and slopes of −33.79 and 0.3, respectively, superior to that of PV panels.
• The performance of the direct-expansion solar PVT heat pump system is closely related to ambient temperature and solar radiation intensity. Under different conditions, the COP of the system exhibits significant variations. At an average ambient temperature of 16.94 °C, the system’s COP increases by 5.67% compared to the average value, with radiation intensity and COP exceeding 194.44 W·m⁻² and 0.14, respectively. COP shows a linear relationship with solar radiation, with slope differences of 0.00135 and 0.00088 W·m⁻² under high and low temperatures, respectively.

7. Future Research Directions

Based on the current research results, future studies can be deepened based on the following directions:

• Optimization of flow channel design:

The current study indicates the need for further optimization of the PVT backside flow channel design. Future research should focus on the combined effects of the geometric shape, size adjustments, and material selection of the flow channels on the thermal performance and electrical efficiency of the system. By using computational fluid dynamics (CFD) simulations and experimental validation methods, different design schemes can be compared to determine the optimal flow channel configuration.

2.

Thermal stability analysis:

This study shows that there are periodic fluctuations in the temperature of the PVT rear panel, which may affect the stability and long-term operating efficiency of the system. Therefore, future research should focus on developing high-precision thermal models to systematically evaluate the combined effects of ambient temperature, solar irradiation intensity, wind speed, and other factors. In addition, real-time dynamic control strategies to suppress temperature fluctuations can be explored to improve system reliability and lifetime.

3.

System integration and energy efficiency improvement:

Future research can explore the integration design with industrial hybrid heat pumps, investigating the synergistic optimization and integration strategies of the two systems, including the dynamic management and optimal configuration of energy flows to maximize energy efficiency. In addition, evaluating the economic and environmental benefits of such integrated systems in different application scenarios will be an important aspect of future research.

In-depth exploration of these research directions can be expected to provide scientific evidence and technical support for the design and application of PVT systems, thereby promoting the advancement and application of renewable energy technologies.