Photovoltaic Thermal Heat Pump Assessment for Power and Domestic Hot Water Generation

Abstract

The efficient utilization of solar energy significantly contributes to energy efficiency in buildings. Solar photovoltaic thermal (PVT) heat pumps, a hybrid of photovoltaic and solar-assisted heat pumps, have demonstrated a significant development trend due to their multi-generational capacity for heating, power, and cooling with reliable operational performance. This research work presents and investigates a single-stage compression PVT heat pump system, along with the operation principle of the system’s heating and power co-generation throughout the winter and transitional season. The construction of the testing facility, data reduction, error analysis, and performance evaluation indices of the system are all explained theoretically. A continuous experiment research project focusing on system heating and power performance was carried out in Dalian during the transition season (November in this study) and winter season (December in this study) as part of our investigation into the potential uses for space heating, residential hot water, and power supply in northern China. The findings of the experimental research demonstrate that the proposed system can generate electricity and heat at high efficiency during the winter and transitional seasons, with long-term stable performance. The system’s average heating COP_t is 5 during the transitional season and 4.4 during the winter season. Meanwhile, the average photovoltaic power efficiency under both weather conditions is 11.9% and 10.2%, with a peak value of 15.7% and 12.0%, respectively. Additionally, the system compression ratio’s variation range is 2 to 3.88, which is lower than the standard heat pump system. As a result, the entire system heating operating process remains constant.

1. Introduction

Energy and environmental crises significantly affect the national economy, people’s livelihoods, and national security. Building energy consumption is included in the social energy consumption structure, which accounts for almost 40% of total energy consumption [1,2]. The Chinese government proposed the Plan for Clean Winter Heating in Northern China (2017–2021) in response to the high consumption of coal for winter heating and the issue of significant air pollution emissions, with the goal of advancing clean heating technology through initiatives like “replacing coal with natural gas” and “replacing coal with electricity”. The excessive adoption, however, has severely constrained the natural gas supply. Meanwhile, rising electricity costs have made it challenging to expand electric heating systems. As a result, solar energy, especially photovoltaic thermal (PVT) heat pumps, is frequently used to boost building energy savings. The improvement in system efficiency and the development of a PVT unit structure are two areas where numerous studies have been published in the literature.

Papers [3,4] presented the demonstration and validation of a PVT heat pump with cold buffer storage, a domestic hot water tank, and tests to look into performance of the system. A direct expansion heat pump with a PVT solar collector acting as the system’s evaporator was discovered in a model study [5,6]. Their study showed that the PVT heat pump was more efficient than an air-to-liquid heat pump and less expensive than a ground-source heat-exchanger-based liquid-to-liquid heat pump to have monthly average electrical, thermal, and COP values of 0.155, 0.752, and 5.35 in the UK. In contrast, in Tibet, the numbers were 0.104, 0.764, and 7.6. A heat-pipe solar-PVT-heat-pump-integrated PVT collector with a heat pump was proposed [7] to improve efficiency with the mutually reinforcing effect. For a domestic hot water system, a distinctive cascade structure combining PVT solar collectors and heat pumps was presented [8]. The PVT cascade heat pump system delivers an annual thermal energy production improvement of between 37% and 68% when compared to an evacuated tube heating system. Roshanzadeh et al. [9] suggested an innovative evacuated hybrid solar photovoltaic–thermal (PVT) system to generate electricity and domestic hot water simultaneously while emitting less greenhouse gases. Fu et al. [10] carried out a comparative study on PVT solar water heating systems with directly coupled photovoltaic pumps, traditional pumps, and natural circulation. By using TRNSYS simulation, Noro and Lazzarin investigated the viability of PVT heat pump hybrid technology in various climates [11]. Prediction control methodology was developed for the design of a PVT solar-assisted heat pump water heating system that takes self-sufficiency into account and demonstrates power autonomy with better performance [12,13]. Vaishak and Bhale [14] analyzed the refrigerant-based PVT heat pump experimentally under exposed weather conditions during the transition season with average electrical efficiency of 15.20% and system COP of 2.96. Mi et al. [15,16] examined a district heating system’s economic performance, energy efficiency, and use of a photovoltaic thermal heat pump, as well as the engineering system’s operational performance forecast approach. Bandaru et al. [17] reviewed the photovoltaic thermal technology for residential applications including performance indicators, progress, and opportunities. Diwania et al. [18] reviewed photovoltaic thermal (PVT) technology with an emphasis on the applications and its advancement.

As a crucial component of the PVT heat pump system, the PVT unit enables multi-generation and energy conversion. Through study optimization, the PVT unit’s performance and efficiency have both dramatically improved. For improved performance, a revised collector/evaporator was constructed using multi-port flat extruded aluminum tubes rather than round copper tubes [19]. Basalike et al. [20] presented research into the roll-bond solar thermal condenser unit’s heat transmission properties in the ambient cold of the night sky. A solar hybrid PVT collector and their combination with direct expansion heat pumps was studied in [21], to test the 1-year field operation performance. When Mohanraj et al. [22] used a PVT heat pump system with a photovoltaic–thermal evaporator in both circular and triangular tube configurations, they found that, in contrast to the circular tube configuration, the triangular tube design improved the heat pump’s essential energy performance parameters. With one-way configurations, hexagon-grid coupled fluid channels performed well in terms of temperature uniformity, hydraulic behavior, thermal efficiency, and electrical efficiency [23,24]. Rijvers et al. [25] performed a numerical analysis of a hybrid photovoltaic thermal (PVT) collector and heat pump residential energy system, and the results revealed that the system’s overall performance varied depending on the type of PVT collector, indicating that the PVT collector was the primary piece of equipment determining the system performance. Investigations were also performed on how operational and weather factors affected the performance of PVT components [26]. The function of thermoelectric generators in hybrid PVT systems was also examined by Babu and Ponnambalam [27]. Artificial intelligence neural network modeling [28,29] was adopted to study integrated PVT heat pump systems. An overview of solar systems and how they work in conjunction with heat pumps is also provided [30]. Additionally, a lot of study was carried out on how to integrate photovoltaic thermal systems with buildings [31,32,33].

Main issues with solar PVT heat pump technology have been the subject of considerable research, including theoretical analyses, simulation, and experimental validation. However, there are many issues that need to be resolved, particularly with regard to heating the water and the space in buildings during the winter and transitional seasons. In particular, it is imperative to look into the continuous operation test in natural conditions and the reasonable performance evaluation under such working situations, giving important reference values for the technical application of the system in heating areas.

To summarize, this work proposes a single-stage compression PVT heat pump operated with a unique roll-bond unit, with multi-energy generation of heating, cooling, and power. During the transition and winter seasons in Dalian, a continuous experimental study was conducted under natural weather circumstances. To evaluate the proposed system’s potential for use in northern China, particular focus was placed on the heating and electricity co-generation performance. Theoretically, the system’s functioning principles and performance evaluation indicators are presented. The results of the experiment on system co-generation performance and operation features are detailed. In Section 2, the roll-bond-PVT (RB-PVT) and the roll-bond-PVT heat pump (RB-PVTHP) system are presented. The basis of experimental research is covered in Section 3, along with the development of a test platform, a performance evaluation strategy, data reduction, and an error analysis. The experiment’s co-generation performance and operating characteristics throughout the transition and winter seasons are covered in Section 4. Finally, in Section 5, the conclusions are presented.

2. Description of the Proposed PVT Heat Pump

A PVT heat pump integrated with the refrigerant-based PVT unit is proposed based on earlier research on the photovoltaic thermal comprehensive utilization technology and the direct expansion solar heat pump (DX-SHP) technology is shown in Figure 1. Photovoltaic cells were added to the refrigerant-based solar collector surface, which worked an evaporator and a converter of solar radiation into electricity. Through the employment of a four-way reversing valve system, the roll-bond unit operated as the condenser throughout the summer to create space cooling and facilitate the changeover from the heat pump cycle to the refrigeration cycle. A substantial amount of thermal energy was accumulated on the solar panels throughout the photovoltaic effect process, which drastically reduced the PV efficiency. The refrigerant utilized as the working fluid in the solar PVT heat pump system, however, is able to absorb the thermal energy produced independently with the photovoltaic panels. The lower-grade heat source of the heat pump system might be the absorbed energy along with the solar radiative energy and the convective thermal energy with air. In order to considerably increase the overall usage of energy efficiency, the PV panel surface temperature was lowered while power generation performance was enhanced. In the authors’ previous study, a roll-bond photovoltaic thermal heat pump multi-energy generation system was proposed [34,35].

Buildings’ needs for space heating and domestic hot water are increased throughout the winter and transitional seasons. The solar PVT heat pump system operates in the heating mode when solar radiation levels are high to supply buildings with electricity and space heating throughout the winter. When the PVT unit functions as an evaporator, heat is mostly absorbed with the refrigerant through the thermal conduction of the photovoltaic panel. In addition, solar radiation and convection from the environment also contribute to heat absorption during the normal working conditions. Under these three heat absorption processes, the multi-heat transmission mechanism of radiation, conduction, and convection is the main way to generate thermal energy. Figure 2 presents the layout of the studied RB-PVT unit.

3. Establishment of Test Platform

The test platform consisted of parts including four units of RB-PVT, a single heat pump (1HP), thermal storage with embedded high-efficiency heat exchanger coil (150 L) with 10 mm insulation, and two micro-inverters typed EVT500 to convert DC into AC and simultaneously integrated into the national grid.

Table 1. Components and specification of RB-PVT.

Name of Component	Associated Parameters
RB-PVT	Size: 1560 mm × 780 mm × 30 mm Single unit area: 1.2 m2 Orientation of installation: South Angle of installation tilt: 40° (the latitude of the experiment location is 39°) Connection: 32 monocrystalline cells in series Coverage ratio: 64% Structure: PV module + EVA sheet + RB heat exchanger Rated photovoltaic power: 125 W Rated photovoltaic efficiency: 16% Rated heating power: 800 W Rated evaporation pressure: 0.9 MPa Rated evaporation temperature: 15 °C
Heat pump unit	1HP compressor Rated power consumption: 0.735 kW
Micro-inverter	Power input range: (180~370 W) × 2 Voltage input range: 18~54 V Voltage range for maximum power point tracking: 24~45 V Conversion efficiency: 95.6%

shows the various components of the RB-PVT.

The monitoring system recorded data on compressor power consumption, PV power generation, temperature, pressure, and external meteorological conditions. Real-time outdoor meteorological data were monitored using the PC-4 meteorological environment monitoring system (purchased from Jinzhou Sunshine weather technology Co., Ltd., Jinzhou, Liaoning Province, China). In order to track the intensity of solar radiation on the inclination surface, a TBQ-2 pyranometer (purchased from Jinzhou Sunshine weather technology Co., Ltd., Jinzhou, Liaoning Province, China) (accuracy: 5%) was put on the inclined plane of the RB-PVT unit. A Pt100 surface mount temperature sensor with an accuracy of 0.2 °C and a CYYZ11-H pressure sensor with an accuracy of 0.25% were employed at the heat pump system’s refrigerant condition monitoring locations. The ZDR-20 temperature logger (purchased from Hangzhou Zeda Instrument Co., Ltd., Hangzhou, Zhejiang Province, China) (accuracy: 0.5 °C) was used to record the water’s temperature in the thermal storage tank. The photovoltaic power generation was monitored with the micro-inverter and monitor. A smart meter (accuracy: 1%) was used to measure the compressor’s energy consumption as well. The entire experimental data were collected using the KEITHLEY 2700 multi-function data collection (purchased from Tektronix Company, Beaverton, OR, USA). Figure 3 displays the installation of the measurement points, the essential experiment components, and the system schematic diagram. Figure 4 depicts the experimental system’s component connection diagram.

3.1. Performance Evaluation Indices

The electrical, thermal, heating, and overall efficiencies of the PVT heat pump system are assessed in this paper using a number of performance metrics. The electrical efficiency of the PVT heat pump [36] unit could be calculated as follows:

$${\eta }_{\mathrm{e}}=\frac{3.6\cdot {10}^6\cdot Q_{\mathrm{e}}}{A_{\mathrm{c}}\cdot I_{\mathrm{t}}\cdot t}=\frac{{\displaystyle \sum _{i=1}^n}{P}_{PVi}}{{\displaystyle \sum _{i=1}^n}(A_{\mathrm{PVT}}\cdot I_{\mathrm{t}})i}$$

(1)

The numerator is the PV electrical power, while the denominator is the total solar radiation energy received by RB-PVT units, which equals to the module effective solar collection areas multiplied by the solar radiation intensity. The thermal efficiency follows as

$${\eta }_{\mathrm{t}}=\frac{Q_{\mathrm{t}}-Q_{\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}}}{A_c\cdot I_{\mathrm{t}}\cdot t}=\frac{C_{\mathrm{p},\mathrm{f}}\cdot \rho \cdot V\cdot (T_{\mathrm{final}}-T_{\mathrm{initial}})-Q_{\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}}}{{\displaystyle \sum _{i=1}^n}(A_{\mathrm{PVT}}\cdot I_{\mathrm{t}}\cdot t)i}$$

(2)

The total solar radiation energy received by RB-PVT units is represented with the denominator, while the total thermal energy produced using the RB-PVT heat pump system is represented with the numerator. It is worth it to mention that the primary energy conservation is used in this study [35], to calculate the overall efficiency of the combined photovoltaic and thermal system,

$${\eta }_{\mathrm{t},\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{l}}={\eta }_{\mathrm{e}}/{\eta }_{\mathrm{power}}+{\eta }_{\mathrm{t}}=\frac{{\displaystyle \sum _{i=1}^n}{P}_{PVi}}{{\eta }_{\mathrm{power}}\cdot {\displaystyle \sum _{i=1}^n}(A_{\mathrm{PVT}}\cdot I_{\mathrm{t}})i}+\frac{C_{\mathrm{p},\mathrm{f}}\cdot \rho \cdot V\cdot (T_{\mathrm{final}}-T_{\mathrm{initial}})-Q_{\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}}}{{\displaystyle \sum _{i=1}^n}(A_{\mathrm{PVT}}\cdot I_{\mathrm{t}}\cdot t)i}$$

(3)

In traditional power plants, ${\eta }_{\mathrm{power}}$ is power generation efficiency having a reference value of 0.38 [37].

Furthermore, the system’s heating COP_t can be computed as the following [35]:

$${\mathrm{C}\mathrm{O}\mathrm{P}}_{\mathrm{t}}=\frac{Q_{\mathrm{t}}}{Q_{\mathrm{comp}}}=\frac{C_{\mathrm{p},\mathrm{f}}\cdot \rho \cdot V\cdot (T_{\mathrm{final}}-T_{\mathrm{initial}})}{Q_{\mathrm{comp}}}$$

(4)

Furthermore, the total energy production and the thermal energy generated may be the same. Therefore, the overall COP_overall [35] follows as

$${\mathrm{C}\mathrm{O}\mathrm{P}}_{\mathrm{overall}}=\frac{Q_{\mathrm{t}}}{Q_{\mathrm{comp}}-3.6\cdot {10}^6\cdot Q_{\mathrm{e},\mathrm{test}}}=\frac{C_{\mathrm{p},\mathrm{f}}\cdot \rho \cdot V\cdot (T_{\mathrm{final}}-T_{\mathrm{initial}})}{Q_{\mathrm{comp}}-3.6\cdot {10}^6\cdot Q_{\mathrm{e},\mathrm{test}}}$$

(5)

3.2. Data Reduction and Error Analysis

The reduced data were subjected to an error analysis using the error propagation approach. The greatest relative error was 2.274%, which satisfied the testing precision requirement for unit thermal efficiency. The same procedure was used to calculate the maximum relative error of the other evaluation indices. Because all of the numbers were less than 5%, the experimental design approach was viable.

4. Results and Discussion

In Dalian, the commonly defined winter seasons included December, January, February, and March, while the transition seasons included April and May in spring as well as October and November in autumn. In this study, the continuous experimental investigation during the transition season was conducted in November, while December was considered for the winter season. The co-generation mode experiment ran from 8:00 a.m. to 16:00 during the day.

4.1. Co-Generation Performance during Transition Season

The weather during the 1-week continuous test period in the transition season included typical seasonal climate characteristics. The external meteorological parameter variation during a 1-week continuous test period in the transition season is shown in Figure 5.

The ambient temperature was between 10 °C and 18 °C during the day and 5 °C and 10 °C at night. Average daytime and overnight temperatures were 13 °C and 6 °C, respectively. The intensity of solar radiation in the parabolic form altered within a day. During the bright days of the transitional season in Dalian, the sky was cleaner and less foggy, which led to less fluctuation in the amount of solar radiation. Compared to the summer’s cloud-induced oscillation and fluctuation phenomenon, the weather during the transitional season was more stable. The solar radiation intensity throughout the test period varied from 600 to 1000 W/m² in bright conditions and from 500 to 800 W/m² in overcast settings, with a maximum value of 1080 W/m². Additionally, the relative humidity and wind speed were observed. Results of the tests revealed that the weather in Dalian throughout the test period of the transitional seasons had the normal weather characteristics and adhered to the statistical rule of yearly values in the city.

Figure 6 shows the PV power generation during testing. The photovoltaic output fluctuated in response to changes in solar radiation intensity, which followed a parabolic pattern. The duration of daylight was reduced in contrast to summer, resulting in a drop in power generating time. Daytime photovoltaic power peaked at a level comparable to summertime. On cloudy or rainy days, the value was about 300 W and 1.3 kWh, respectively, which showed a substantial decrease from that under sunny conditions. On sunny days, the peak value could reach 450 W, and the cumulative power generation for 1 day was about 1.7 kWh.

Overall, as shown in Figure 7, the heating power and COP_t declined dramatically as the system condensation temperature increased. Due to the variable solar radiation intensity over the working period, the results showed a changing state. Because of this, the system’s heating power and COP_t were high at first but steadily dropped as it was put to use. During the 1-day testing process, multiple peak values were observed, as shown in Figure 7. This is because the experiment’s objective was to use a 150 L heat storage tank to provide domestic hot water at a temperature of 55 °C. After the setup, the test was conducted again using a different water tank to examine the heating system’s efficiency throughout the day. As a result, multiple test steps were included in the 1-day test process. The system heating performance at mid-day was much better than in the morning and afternoon during the 1-day test process, demonstrating that solar radiation intensity and ambient temperature were critical factors affecting the system’s heating performance. By analyzing the test data of daytime heating conditions in the transition season, the average heating power of the system was about 3.2 kW, with a peak value of 6 kW. The average and peak value of COP_t was about 5 and 9.45, respectively.

In Figure 8, the photovoltaic power generation performance of the test system is depicted during the course of a typical day during the transition season. The variation in solar radiation intensity caused a 15 min delay before the photovoltaic power transformed into a parabolic form. With an average value of 220 W between 9:00 a.m. and 14:00, the system’s maximum solar power could reach 330 W. Due to the PVT unit being sheltered by the building to the west, the photovoltaic output abruptly decreased after 13:00. The photovoltaic power of the system was consequently low in the afternoon. In total, 1.7 kWh of total power was produced with the system during the course of the 1-day test period. From 9:00 a.m. until 14:00, the photovoltaic efficiency likewise fluctuated in a parabolic pattern, with an average value of 11.9%. At noon, the greatest figure was roughly 15.7%.

Combined Figure 9 and Figure 10 show the heating performance of the proposed system during a transition period. A typical operation process in the morning was selected and the ambient temperature was about 15 °C, and the solar radiation intensity showed an increasing trend. The 150 L thermal storage tank’s starting water temperature was 16 °C. The water needed roughly 120 min to warm up to a temperature of 56 °C. The average heating power of the system was 3.5 kW, the cumulative heat was 7 kWh (25.1 MJ), and the average COP_t was 6. On the whole, this change is mainly influenced by the condensation temperature. The heating COP_t could reach the maximum value of 7.8 after 10 min of running. With the running of the system, the water was heated and the condensation temperature increased, causing a reduction in the COP_t gradually. However, this process was also accompanied by a rise in the amount of solar radiation; therefore, the system’s heating capacity was high. The compressor only used a small amount of energy, so the COP_t drop was likewise minimal.

Throughout the system’s 120 min operation, the heating power experienced significant fluctuations. As determined previously, the heating power should gradually rise as solar radiation intensity rises. However, while the system ran, the condensation temperature rose, which led to a steady loss of system heating power. As a result, the state of fluctuation was caused by the varying levels of influence that solar radiation intensity and condensation temperature had on the heating power. Prior to the system shutting down, the maximum heating power was visible. Although it was not the real value at the time, the value at this point represented the value in a state of fluctuation. Meanwhile, the maximum heating power should appear after 10 min of a system running, and the maximum value was 4.2 kW. The system built up heat, and the water temperature in the thermal storage tank rose steadily while the system ran until it shut off. In addition, albeit the increase was slight, the power consumption of the heat pump unit gradually grew. The total power used for the entire operation was 1.44 kWh, with an average power usage of 0.72 kW. The trial results showed that the system’s photovoltaic power generation and power consumption were about equal throughout the daytime hours of the transition season, enabling it to achieve its self-sufficiency goal.

4.2. System Operation Characteristics during Transition Season

The system compression ratio, evaporation temperature and pressure, and condensation temperature and pressure hourly variation graphs are shown in Figure 11. The evaporation temperature and pressure were constant throughout one heating cycle. The thermal energy absorbed by the PVT unit rose in tandem with the increase in solar radiation intensity, causing the evaporation temperature and pressure to marginally increase with system operation. The average evaporation temperature was 6.5 °C, which was nearly 10 °C lower than the average ambient temperature, making the heat exchange between the PVT unit and the ambient air more favorable. The average evaporation pressure was about 0.6 MPa and the PVT unit worked stably during the whole operation process. The condensation temperature and pressure increased significantly with the system running, which increased from 24 °C to 60 °C. The water temperature was 56.2 °C at the time, and the temperature difference between the interior and outside of the heat exchange coil was 4 °C to 7 °C throughout the process. The condensation pressure steadily rose from 1.0 MPa to 2.4 MPa. Additionally, the compression ratio of the heat pump unit increased gradually throughout system operation, increasing from 2 to 3.61, which was less than that of a typical heat pump system. A high condensation temperature and a low compression ratio helped the RB-PVTHP system run steadily and continuously over an extended period of time.

Figure 12 presents the change in condensing and evaporating temperature. In one heating cycle, the inlet temperature of the evaporator was relatively constant, with a slight increase in the solar radiation intensity, which ranged from 4 °C to 10 °C. The increase in solar radiation energy received by the PVT unit and the process of refrigerant heat transfer in the flow path caused the output temperature of the evaporator to trend upward for the first 20 min before declining. The evaporator’s output temperature ranged from 16 to 22 °C. The condenser’s inlet and outlet temperatures continued to rise as well as the water temperature in the thermal storage tank over the course of this condensation operation. The temperature differential between the condenser’s intake and outlet temperatures was maintained at roughly 30 °C once the system became stable, with the exception of the significant rise in outlet temperature during the first 15 min of operation.

Figure 13 depicts the trend of the system heating power, COP_t, and power consumption with condensation temperature during the transitional season. As the system continued to run, the water’s temperature gradually rose from 16.3 °C to 56.2 °C. As a result, the condensation temperature rose from 23.5 °C to 59.1 °C. During this heating process, the system power consumption increased from 0.5 kW to 1.0 kW with the rise in condensation temperature and pressure. After approximately 10 min of operation, the system heating power and COP_t achieved their maximum values. A reduction in COP_t occurred along with an increase in condensation temperature and power use after that, as a result of the interplay between the increasing solar radiation intensity and condensation temperature. The heating power and COP_t variation ranges, respectively, were 2.7 kW to 4.2 kW and 4.8 to 7.7 kW from the perspective of the entire operation cycle.

The comparison of the surface temperature distribution of PVT units between working and non-working conditions in the transition season can be seen in Figure 14, with the ambient temperature as the reference value. During the system heating operation, the PVT unit operated as an evaporator, absorbing heat to maintain a low evaporation temperature that was lower than the ambient temperature. The average surface temperatures on the front and back were, respectively, about 8.5 °C and 7.1 °C. The temperature on the backside was slightly lower than on the front side because the backside did not receive direct solar radiation and the refrigerant directly evaporated in the heat exchanger plate there. The typical front side and backside surface temperatures were about 33.8 °C and 32.5 °C, respectively, while the system was not in use. Meanwhile, the temperature outside ranged from 14.6 °C to 16.4 °C, with an average of 15.2 °C. According to the compared data, the PVT unit’s surface temperature during the working phase was significantly lower than it was during the non-working process. As a consequence, the system’s heating procedure could efficiently increase the PVT unit’s solar power generating performance throughout the transition season.

4.3. Co-Generation Performance during Winter Season

Throughout the winter, the system ran from 8:30 a.m. to 16:30 p.m. using the same co-generation of power and heat as it did in a transition season. Figure 15 displays the differences in the ambient meteorological parameters during the winter 1-week test period.

The winter climate in Dalian is characterized by sparse rain and snow, and dry cold with the wind. Due to the long-term control of the cold air mass from Northeast China, the air is dry and the ambient temperature is low. During the experiment period, the ambient temperature was mainly distributed between −8 °C and 8 °C, and the temperature difference between day and night was small. With observation of the meteorological parameters, the test period’s fluctuation trend in meteorological data was compatible with Dalian’s statistical rule for meteorological change through time, and had the typical weather features of the annual values.

The changing trend of ambient temperature and total sun irradiation on the sloped surface of the PVT unit installation throughout the winter test is depicted in Figure 15. The outside temperature ranged from 0 °C to 6 °C during the day and −6 °C to 2 °C at night. Total solar irradiance on the unit’s sloped surface shifted to a parabolic shape during the day. The total solar irradiation fluctuated due to the severe wind and cloud cover. The peak value of solar radiation intensity during sunny days could reach 1100 W/m² at mid-day, whereas on windy and rainy days, it was around 400 W/m² to 700 W/m².

The system’s photovoltaic power in the winter was often lower than in the transition season. The peak power of the system could reach 340 W in sunny conditions, and the cumulative power generation was around 1.2 kWh per day, according to Figure 16. Solar power generation on cloudy days was significantly less consistent and fluctuated greatly, with these two numbers being about 250 W and 0.8 kWh per day, respectively.

With the increase in condensation temperature, the heating power and COP_t decreased gradually as shown in Figure 17. The heating performance was higher at the beginning of the system operation and then decreased with the system running. The experiment operation strategy in the winter season was the same as that in the transition season. The heating performance of the system at mid-day was much better than that in the morning and afternoon, indicating that the solar radiation intensity and ambient temperature were the main factors affecting the system heating performance in winter. The average and maximum values of the system heating power were 2.4 and 4.5 kW, respectively, while the average and maximum values for the heating COP_t were, respectively, 4.4 and 8.75.

Figure 18 displays the photovoltaic power generating performance of the test system over the winter. As shown in the graph, the system’s photovoltaic power varied with solar radiation intensity and marginally with cloud cover. From 9:00 a.m. to 14:00, the peak and average values were 270 W and 170 W, respectively. Due to the limitations of meteorological circumstances as well as the site conditions of PVT unit shielding, the photovoltaic power declined substantially with fluctuation after 12:30. The cumulative power generation was about 1.3 kWh in 1 day. However, the photovoltaic efficiency of the system was poor in the winter. The PVT unit was unable to operate in a stable and efficient range, mostly because of the low ambient temperature and erratic solar radiation. The average photovoltaic efficiency between 9:00 a.m. and 14:00 was 10.2%, while the maximum efficiency was roughly 12.0%, according to calculation data.

The heating performance of the test system under meteorological conditions in winter is shown in Figure 19 and Figure 20. A typical test period in a winter morning was selected for the analysis, in which the weather conditions were cloudy to sunny, with a relatively stable ambient temperature of about 1 °C. Meanwhile, the solar radiation intensity first fluctuated and then increased gradually. The thermal storage tank’s original water temperature was 12 °C. According to test results, the system could heat 150 L of water in the tank from its beginning temperature to 56 °C during the course of a 180 min heating operating cycle. Average heating power, cumulative heat, and COP_t were, respectively, 2.6 kW, 7.84 kWh (28.2 MJ), and 4.5. After 10 min of system operation, the system COP_t’s variation trend reaches its peak value of 5.3. When the system is operating for around 10 min, the maximum heating output is 3.2 kW. The thermal storage tank’s total heat production and water temperature increased gradually as the system operated. Although the growth rate was faster, the heat pump unit’s power usage was slightly lower than it was during the transition season. The heat pump used 0.67 kW of power on average. The total amount of energy used was 2 kWh. The results of the tests showed that the system’s solar power production during the day was insufficient to meet the demand for heating power, and an additional grid power supply was needed to fulfil the system’s power consumption.

4.4. System Operation Characteristics during Winter Season

Figure 21 displays the system’s evaporation temperature and pressure, condensation temperature and pressure, and compression ratio fluctuation curve during a typical heating operation during the day in the winter. The overall trend showed that the evaporation temperature gradually grew from −7.2 °C to 7.6 °C and the evaporation pressure gradually increased from 0.38 MPa to 0.69 MPa along with the rise in condensation temperature. In the first half of the system operation, the evaporation temperature was lower than the ambient temperature, and the PVT unit could absorb heat from the air. In the next half of the system operation, the evaporation temperature gradually rose to higher than the ambient temperature, which referred to the PVT unit that would release heat into the environment. The backside of the PVT unit could be insulated to reduce heat loss. Both the condensation temperature and pressure increased with the operation of the system, and the condensation temperature rose gradually from 16.5 °C at the beginning of the system operation to 57.8 °C. The thermal storage tank’s water temperature was 56.4 °C at this time, and the heat exchange coil’s internal and external temperatures varied by 2 to 5 °C. The transition season’s operating conditions, which steadily grew from 0.8 MPa at the start of the system’s operation to 2.0 MPa, were higher than the condensation pressure. Additionally, as the condensation and evaporation pressures changed, so did the heat pump system’s compression ratio. The primary cause is the variation in solar radiation intensity. The value raised gradually from 2.36 to 3.88, and the heat pump system’s whole heating process was fairly stable.

Figure 22 depicts the hourly fluctuation curve of the condenser and evaporator temperature in the winter heating mode. The heat pump unit could operate safely and steadily because the evaporator’s intake and output maintained a superheat temperature of 8 °C to 10 °C. When the system was operating, the condenser’s inlet temperature increased along with the water temperature in the heat storage tank through the coil heat exchanger and the condenser’s outlet temperature. In the first 30 min of the system’s early stage of operation, the condenser’s inlet temperature increased fast; after that, the system’s operation tended to be stable. The condenser’s inlet and outlet temperatures were kept roughly 30 to 35 °C apart.

Figure 23 depicts the variation curve of the test system’s heating output, COP_t, and power use at the condensation temperature in the winter heating mode. In one heating operation cycle, the thermal storage tank’s water temperature grew gradually from 11.6 °C to 56.4 °C, while the condensation temperature increased gradually from 16.5 °C to 57.8 °C. As the condensation temperature increases during this process, the system’s power usage gradually increased from 0.32 kW to 1.01 kW, with an average consumption of 0.67 kW. The first half of the system’s operation saw a dramatic fluctuation in heating power due to the solar radiation’s intense swings in intensity. The system’s second half of operation saw an increase in condensation temperature and solar radiation intensity. With an average value of 2.6 kW under the combined side effects of both, the system heating power fluctuated little and remained relatively stable. After operating for 10 min, the system-heating COP_t reached its maximum value of 5.3 before declining in response to increasing condensing temperatures and power usage. Throughout the whole operation cycle, the COP_t fluctuated between 3.94 and 5.3, with an average value of 4.5.

Figure 24 compares the surface temperature distribution of PVT units in working and non-working conditions during the winter. It was decided to use the outside temperature as the reference. According to the aforementioned research, the system’s evaporation temperature was lower than the ambient temperature in the first half of operation, allowing it to collect heat from the air. Meanwhile, it was greater than the ambient temperature in the second half of the operation, which would dissipate heat into the environment. The average surface temperature of the front and the rear surface of the PVT unit was 0.63 °C and −0.19 °C, respectively, which was lower than the average ambient temperature of 1.66 °C. The PVT unit absorbed heat from the environment using convective heat transfer through its surfaces. On the other hand, while the system was not working, the front and rear surface temperature ranges were 22.1 °C to 25.5 °C and 20.0 °C to 25.1 °C separately, with the average value of 23.7 °C and 22.6 °C. The comparison results revealed that the system’s operation in the winter could be caused by the PVT unit’s low surface temperature, which caused dew and frost on the surface, shading the solar cells and reducing power generation. Simultaneously, the surface temperature strayed from the optimal working temperature for solar power generation, reducing photovoltaic performance. The performance of solar power generation in the winter might be improved by adding a thermal insulation layer to the back of the PVT unit and promptly cleaning the dew and frost cover layer from the front of the PVT unit.

5. Conclusions

This study proposes an RB-PVT-driven multi-energy-generating solar photovoltaic thermal heat pump system. In this work, an experimental approach was used to explore the operation and performance characteristics of the residential hot water and electricity co-generation system throughout the transition and winter seasons. An experimental examination that was conducted in Dalian under real-world weather conditions led to the following conclusions.

Changes in solar radiation intensity and system condensation temperature had the most effects on heating efficiency during the transition season. The system’s COP_t and average heating power were about 3.2 kW and 5, respectively. Typically, 150 L of water was heated from 16 °C to 56 °C in around 120 min, generating 7 kWh of thermal energy (25.1 MJ). The system compression ratio rose from 2 to 3.6 at this stage, which was less than the figure for the typical heat pump system. The RB-PVTHP system was able to operate stably and continuously for a longer amount of time thanks to a high condensation temperature and a low compression ratio. Additionally, on sunny days, the recorded solar power efficiency ranged from a peak of 15.7% to an average of 11.9%.

The heating power and COP_t progressively declined as the condensation temperature rose as a result of the varying solar radiation intensity during the winter. Around 2.4 kW and 4.4 were the average heating power and COP_t, respectively. In total, 150 L of water was heated from 12 °C to 56 °C in around 180 min, generating 7.84 kWh of total thermal energy (28.2 MJ). While the system compression ratio grew from 2.36 to 3.88, the heat pump system’s overall heating process remained largely unchanged. At roughly 10.2% and 12.0%, respectively, the system’s average and peak photovoltaic power efficiencies were likewise rather high.

According to the findings, a PVT heat pump system with an RB-PVT unit co-generates electricity and domestic hot water with high efficiency and long-term stable operation during the transition and winter seasons. The experimental investigation’s results further show that the proposed technology has potential use in northern China. Building integrated photovoltaic thermal applications will be possible with this technology, ranging from modest to large applications. Research is required to take structural obstructions, service obstructions, shading obstructions, building power consumption, and targeted building demand into account for further sizing and installation of PVT heat pump applications.