Performance research of the PVT-coupled water loop heat pump system

Based on one public office building retrofit project in Tianjin, this paper describes the construction of a PVT coupled with a water-to-water heat pump system to test its performance in cold regions. The system’s performance was evaluated under winter conditions, including the temperature changes of the photovoltaic panel and water tank with respect to the irradiation time, the instantaneous power generation of PVT and PV systems, the daily power generation efficiency, the heat collection efficiency of PVT system, and the COP of the system. The results show that under one day’s solar radiation, the mean temperature of the PVT and PV systems can reach 16.1°C, and the PVT power efficiency is 2.2% higher than the PV. The PVT system has a daily average power generation efficiency of about 10.4%, a heat collection efficiency of 42.2%, and a daily power generation of 194.4 kWh. The maximum temperature of the storage water tank reached 46.4°C, and the mean COP for the whole day was 6.86, meeting the actual heating requirements.


Introduction
In response to the climate crisis brought about by energy shortages and environmental pollution, China has proposed the "3060" dual-carbon target [1] .With the gradual expansion of solar energy systems and the rapid upgrading of related technologies and industries, solar energy has become an important emerging industry in many countries.However, in the process of utilizing solar energy, the power generation efficiency decreases with the increase in temperature.According to research, for each 1°C increase in the working temperature of the battery, the efficiency of photovoltaic components decreases by 0.4~0.5% [2] .Compared with traditional photovoltaic systems and solar thermal systems, the use of photovoltaic/thermal (PVT) systems can obtain thermal benefits and reduce the temperature of photovoltaic cells, thereby improving the electrical efficiency of the system.In addition, the heat pump, as a highly efficient energy-saving device, converts low-grade energy (such as air) into high-grade energy by injecting a small amount of high-grade energy (such as electricity) [3] .The PV/T module, coupled with a heat pump, can be applied to the energy production of domestic hot water, heating, and cooling for public buildings.This is of great significance for accelerating the decarbonization process in the construction industry.
In recent years, scholars have conducted extensive research on PVT-coupled heat pump systems.Zhang [4] conducted experimental tests on the direct expansion PVT heat pump hot water system based on the blown film components.The test results showed that the system had a COP of about 3.2 under winter conditions.Zhou et al. [5] studied the co-generation performance of one direct expansion solar PVT heat pump system.Yao et al. [6] constructed an experimental platform for a direct expansion solar PVT heat pump system and analyzed its system performance and decarbonization potential.
Based on the above analysis, current research on PVT-coupled heat pump systems is mostly focused on system configuration and component structure optimization while lacking analysis of the system's operating performance under different conditions in different regions.This hinders the demonstration of the system's performance and its widespread application.
This study focuses on a typical carbon-negative industrial park in Tianjin, China.It investigates the electric and thermal performance of a PVT-coupled water source heat pump heating and cooling system under various operating conditions.The data from an actual retrofit project is used to analyze the system's performance.The study primarily investigates the heating performance of the system in extremely cold regions.It examines the changes in the temperatures of the PV panel and the water tank over time.The study also compares the instantaneous power generation, daily power generation efficiency, and thermal efficiency of the PVT and the PV module.The results can serve as a reference for future applications of PVT-coupled water source heat pump systems in cold regions.

Analysis of heating status quo in office buildings in cold regions
In the city of Tianjin, which belongs to a cold region, there are mainly five forms of central heating sources: thermal power plants, gas boiler rooms, coal-fired boiler rooms, municipal heating, and ground-source heat pumps.Among them, public institutions, including office buildings, cover an area of 119 million square meters, accounting for 23% of the central heating area.The energy structure of Tianjin's heating sources is shown in the figure, with gas heating having the largest proportion, reaching 40% of the total heating area, followed by co-generation, accounting for 37% of the heating area.Coal-fired heating ranks third, while geothermal and other energy sources account for the smallest proportion.The consumption of natural gas for central heating is 1.4 billion cubic meters, accounting for 27.5% of the total gas consumption [7] .Figure 1.Heating heat source structure of office building in Tianjin In recent years, there have been significant changes in the energy structure of heating projects in Tianjin.The municipal government has been committed to improving the economy and people's livelihoods.It has implemented a series of transformation measures, gradually replacing coal-fired boilers with gas and co-generation, leading to the clean and intelligent development of the heating industry in Tianjin, which has seen rapid and positive growth.Based on this, this paper proposes a PVT-coupled water loop heat pump system and analyzes the corresponding operating data.

System installation location
The project was installed in Tianjin, China, which is one of the Class 2 regions with abundant solar energy resources in China.Therefore, the project has the potential to fully utilize solar energy resources to achieve the goal of transforming the factory area into a carbon-negative one.The focus of the transformation is on the envelope structure of the main building and the energy system.The envelope structure transformation includes measures such as adding external insulation materials and replacing high-performance doors and windows.The transformation of the energy system involves the addition of an intelligent energy management system, distributed photovoltaic and photovoltaic/thermal energy storage equipment, etc.The transformed energy system adopts the technical measures of air-source heat pumps and PV/T-coupled water loop heat pumps, effectively solving the problem of high energy consumption of heating in the main building.At the same time, the intelligent control platform provides real-time data monitoring and analysis, realizing the clean and intelligent heating mode.

System device design and component parameters
This paper presents an experimental design of a PVT-coupled water source heat pump system.The system consists of a PV carport, PVT circulating pump, water source heat pump, and terminal pump, with the electrical performance parameters of the PV components detailed in Table 1.As shown in Figures 2 and 3, the PV carport is arranged in an east-west orientation, comprising 20 PV modules and 80 PVT modules, with the PV and PVT panels placed vertically with the same inclination angle, and the water cycle between the PVT modules and the water storage tank.Measured parameters include the temperature of the PV module, the inlet and outlet temperatures of the PVT module, and the temperature of the water tank.By comparing the operating parameters of the system and using experimental data to quantitatively evaluate the advantages of PVT over PV systems, and calculating the efficiency based on the measured data, the performance of the system was analyzed.This experimental design helps to evaluate the performance of the PVT system under different conditions and provides a reference to optimize the system design.

System performance evaluation indicators
To analyze the thermal and optical performance of the PVT system, the system evaluation criteria of PVT are established based on the photoelectric and thermal efficiency.
The term "power generation efficiency" is defined as the ratio of the electricity generated by a photovoltaic system to the solar radiation absorbed by the photovoltaic panels: in the equation, Apv represents the area of the PV module in square meters (㎡); G denotes the solar irradiance in watts per square meter (W/㎡); U represents the PV voltage in volts (V); and I represents the PV current in amperes (A).
The thermal efficiency is defined as the ratio of the heating absorbed by the working fluid in the PVT collector during steady operation to the solar radiation received on the surface of the PV module: = in the equation, Qcoll is the thermal collection power of the collector in watts (W), and Acoll is the area of the collector in square meters (㎡).COP is the ratio of the heat gained from the PVT system to the total power consumed: in the equation, Qheat, j represents the heat gained by the hot water within the time step in watts (W) and Wcomp, j represents the power consumed by the compressor within the time step in watts (W).In this study, COP refers to the average value during the heating cycle.mw represents the mass of water in the water tank in kilograms (kg), cw represents the specific heat capacity of water in joules per kilogram per Kelvin (J/(kg•K)), Tw, final and Tw, initial represent the water temperatures at the end and beginning of the time step in the water tank in degrees Celsius (℃).Thermal energy and electrical energy are forms of energy with different qualities.In order to properly evaluate system performance, it is beneficial to consider the corresponding thermal energy required to generate an equivalent amount of electrical energy as the energy benefit of the system.Huang et al. [9] , through theoretical analysis, proposed the concept of the PV/T collector's overall efficiency (also known as primary energy saving rate).This metric serves as an evaluation method for comparing the energy grade differences between thermal energy and electrical energy, and it can be calculated as follows: in the equation, ηpower is the power generation efficiency of conventional power plants, typically assumed to be 38% [10,11] .This metric takes into account the quantity of both thermal and electrical performance, providing a more accurate reflection of the PV/T system's ability to convert solar into both thermal and electricity.

Test results and analysis
On April 1, 2023, from 8:00 to 17:00, the system performance experiment was conducted with the aim of investigating the variations in PVT and PV back-panel temperatures, as well as the water tank temperature, following the activation of the PVT circulation pump.The experiment also recorded fluctuations in environmental parameters.During the experiment, the ambient temperature fluctuated within the range of 16°C to 21°C, while the solar radiation intensity varied between 156 W/m 2 and 834 W/m 2 , reaching its peak around noon.The objective of this experiment was to gain a deeper understanding of the system's performance during actual operation and to provide empirical data for subsequent system optimization.These results can serve as a reference for practical engineering applications.
Figure 4. Environmental parameters and system plate temperature changes From Figure 4, it can be observed that between 8:00 and 9:00, before the PV/T circulation pump was activated, the east-west PV and PV/T system back-panel temperatures increased at a relatively high rate.However, after the PV/T circulation pump was activated at around 9:00 AM, the rate of temperature increases significantly decreased for the east-west PV/T system back-panel temperatures.Subsequently, the rate of temperature increases slightly increased.This can be attributed to the stabilization of the heat pump operation after a certain period of time, leading to a weakening cooling effect.Additionally, as the solar radiation intensity continued to increase, both the east-west PV and PV/T system back-panel temperatures steadily rose.They reached their peak around 1:00 PM, with the eastward PV/T back-panel temperature reaching 42.799°C, the westward PV/T back-panel temperature reaching 42°C, the eastward PV back-panel temperature reaching 51.799°C, and the westward PV back-panel temperature reaching 50.6°C.The average temperature difference between the two systems was 11.26°C.After 1:00 PM, the east-west PV and PV/T system back-panel temperatures began to exhibit varying degrees of decline, which can be attributed to the decrease in solar radiation intensity and the shading effect caused by the shadow of the main building on the photovoltaic canopy.During the testing period, the average panel temperature for the eastward PV system was 39.7°C, for the westward PV system was 38.9°C, for the eastward PV/T system was 23.6°C, and for the westward PV/T system was 23.2°C.
The comparison of power generation and efficiency between the PVT and the PV system is illustrated in Figure 5.After activating the PV/T circulation system, the instantaneous power of the PV/T system exceeded the PV system due to the cooling effect of the heat pump.The maximum power of the PVT system was 4898 W, with an average instantaneous power of 2762 W. In contrast, the maximum power of the PV system was 4301 W, with an average instantaneous power of 2222 W. The difference in instantaneous power between the two systems reached a maximum of 1116 W.During the testing period, the average power generation efficiency of the PVT system and the PV system was 10.4% and 8.2%, respectively.The PV/T system exhibited a 14.58% improvement in power generation efficiency compared to the PV system.The variations of inlet and outlet water temperatures and efficiencies of the east-west oriented PV/T system are shown in Figure 6.The trends of the overall efficiency and heat collection efficiency are consistent, exhibiting an initial decline followed by an increase.This is because when the heat pump starts operating, the refrigerant initially absorbs heat from the heating collect plate at a low water temperature at the inlet of the PVT system.At the same time, the heat is also extracted from the environment, leading to a higher heat collection efficiency.At noon, both the heat collection efficiency and overall efficiency of the PVT system reach their peak for the day.Based on data analysis, the average heating collection efficiency of the PVT system is 42.21%, and the average overall efficiency is 52.61%.In summary, the daily average overall efficiency of the system exceeds 50.00%, indicating that the system possesses a certain level of competitiveness in the market.To visually illustrate the performance differences between the PVT system and the PV system, the experimental data is summarized, and the performance comparison is presented in Table 2 7.The system's COP reaches its peak around noon, with a value of 16.91.The average COP throughout the day is 6.86.

Conclusion
This paper presents a solar PVT coupled water-to-water heat pump system and tests its actual operational performance.The main research findings are as follows: (1) Solar irradiance is the primary factor influencing power generation.Compared to traditional PV systems, the average temperature of the PVT system is reduced by 16.1℃, resulting in an increase of 19.5% in power generation and 2.2% in power generation efficiency compared to non-cooled photovoltaic modules.The daily average power generation efficiency of the PVT system is 10.4%.
(2) During the testing period, the PVT system generated 194.4 kWh of electricity, while the PV system generated 23.6 kWh.Due to the difference in installed capacity between the PVT system and the PV system, under the same operating conditions, the PV/T system generated an additional 6.25 kWh of electricity compared to the PV system.
(3) The mean thermal efficiency of the system is 42.2%, and the average overall efficiency can reach 52.6%.The average coefficient of performance is 6.86, indicating good operational performance of the system.These findings provide a reference for the design of similar PV/T-coupled water-heat pump systems in cold regions.

Figure 2 .
Figure 2. Photovoltaic carport site drawing and construction drawing

Figure 5 .
Figure 5.Comparison of power generation and power generation efficiency between PV/T system and PV system

Figure 6 .
Figure 6.PV/T system efficiency and collector inlet and outlet temperature changes

Figure 7 .
Figure 7. Changes in COP over time

Table 1 .
PV component electrical performance parameters

Table 2 .
. Performance comparison of PV/T system and PV system During the full-day testing experiment, the variations of COP (Coefficient of Performance) and water temperature in the full heating mode are shown in Figure