Experimental Study of Photovoltaic-thermal Integrated Heat Pump Building Heating System

Photovoltaic-thermal (PV/T) systems and hydronic radiant floor heating systems can enhance energy efficiency and economic viability. This study investigates their integration through experimental research and designs an experiment to test their performance under typical winter conditions. The heat pump system achieves an average COP of 5.66 and an average photovoltaic power output of 107.5 W. The underfloor heating system quickly reaches and maintains 20.47°C with a water supply temperature of 50°C.


Introduction
PV/T systems combine solar radiation for thermal and electrical energy with applications in heating, power generation, cooling, and building integration.Advancements in photovoltaic and solar thermal technologies expand their usage.PV/T systems are crucial for renewable energy solutions in construction, industry, and agriculture [1,2].
Hydronic radiant floor heating systems, known for their comfort, energy efficiency, and space-saving benefits, are widely used for space heating [3].Integrating PV/T heat pump water heating systems with radiant floor heating enhances energy utilization.Combining the heat production of PV/T systems with the even heat distribution of radiant floor systems, buildings can achieve optimal energy efficiency and thermal comfort [4,5].Some studies have already combined radiant floor heating systems with solar thermal systems.For example, Qimiciren et al. [6] performed experiments to evaluate the practical effect of using solar/air source heat pumps for floor heating in cold environments, specifically in barns.Abdellatif [7] developed a combined solar collector-floor heating system using TRNSYS and evaluated its efficiency.
This paper focuses on the integration of hydronic radiant floor heating systems with PV/T heat pump water-heating systems.It explores the synergistic effects, economic viability, and energy utilization efficiency of the combined system through experimental research.

Experimental System
A solar PV/T direct-driven dual-source heat pump water heating system was constructed in Jiangning District, Nanjing City (31°83' N, 118°77' E), as shown in Figure 1.Six photovoltaic panels with an area of 1.85 m² are placed on the top of the container.Heat pipes are laid underneath the photovoltaic panels, and the heated water is pumped into a water tank arranged inside the container after being heated by a heat pump.The water medium is circulated by a water pump.PV panels generate electricity, and the collector converts solar energy into heat.The heat pump raises the water temperature before it is sent to the storage tank.Hot water from the tank is pumped to the underfloor heating pipes, evenly distributing heat indoors.The system stops heating once the desired temperature is reached and continues circulating the remaining hot water until the lower temperature limit is reached, at which point heating resumes.The indoor floor area of the radiant floor heating experiment is 13.5 m².Parameters of the main components in the heat pump system are as shown in Table 1.

Experimental Setup
The experimental test mainly includes the measurement of environmental operating conditions, heat pump performance parameters, and PV module electrical output parameters.
A YokoGawa GX10 portable data logger is used for data collection, with a sampling interval of 1 second.Environmental temperature is measured with thermocouples, with one end placed in a shaded and sheltered location for ambient temperature measurement.The other end is connected to the YokoGawa GX portable data logger.Solar irradiance is measured using a solar radiometer, adjusted to match the tilt angles of the PV modules and PV/T collectors.The output cable of the solar radiometer is connected to the YokoGawa GX portable data logger.
Temperature and pressure sensors are placed at various locations along the heat pump circulation pipeline.Flow rate measurement points are installed on the hot water circulation pipeline and the PV/T panels.On the backside of the PV/T modules, five temperature measurement points are evenly distributed, and the average temperature of these points represents the module's temperature.
K-type thermocouples are used for temperature measurements within the heat pump system, with a measurement range of -60°C to 250°C.The uncertainty of the thermocouple measurements is ±0.1°C.
During the experiment, the pressure at the compressor inlet and outlet is measured using pressure sensors, providing a 4-20 mA current signal.
A PV controller with constant voltage tracking control capability is used to stabilize the PV module's output voltage.
The ground heat flux density is measured using a heat flux meter.Multiple sets of temperature measurement points are placed at different positions and heights inside the room to measure the average indoor temperature.

Experimental Equipment
The experimental equipment and parameters are listed in Table 2.

The power output
The electrical output power of the photovoltaic panel can be determined by measuring the output voltage and output current of the cell and then calculating their product:    (1) where  is the output voltage, V;  is the output current, A.

Coefficient of Performance (COP) of the Heat Pump
The COP can be a measure of the system's efficiency, indicating the amount of heat energy output provided by the heat pump system per unit of electrical energy input.It can be calculated by dividing the heat output by the power consumption: where Q w is the heat output; N com is the power consumption of the compressor.

Average Heat Power of the Underfloor Heating System
The average heat power of the underfloor heating system is a measure of the efficiency of heating the indoor air by the ground: (3) where q ave is the average heat flux density on the ground surface and A f is the area of the ground.

Performance Analysis of the PV System
The operational performance of the solar PV/thermal direct-driven dual-source heat pump hot water system was tested under sunny winter conditions in Nanjing.The experiment started at 9:46 AM and ended at 4:40 PM.The initial temperature of the water tank was 11.6°C, and the target heating temperature was set at 50°C.
The solar radiation intensity and ambient temperature during the experiment are shown in Figure 2. The minimum solar radiation intensity was 83.2 W/m 2 , and the maximum was 942.8 W/m 2 , exhibiting an overall upward trend followed by a downward trend.The average solar radiation intensity throughout the experiment was 634.7 W/m 2 .The minimum ambient temperature during the experiment was 6.6°C.
The maximum was 18.4°C.The average ambient temperature was 13.6°C.The trend of ambient temperature change was similar to that of solar radiation intensity, showing an overall upward trend followed by a downward trend.The PV/T system produces hot water exceeding 50°C, suitable for domestic water heating.The variation of power and temperature over time in radiant floor heating system is shown in Figure 5.When supplied to the indoor radiant floor heating system during winter, it initially provides an average power output of around 2800 W. After 3 hours, the average power gradually decreases, stabilizing at approximately 1700 W after 12 hours.The indoor temperature rapidly rises from 3°C to 20°C within 5 hours, then slowly increases to around 22°C and remains stable.

Conclusions
The conclusions drawn from the investigation of the solar PV/T integrated heat pump hot water system combined with the indoor radiant floor heating system under winter conditions are as follows:  The solar PV/T integrated heat pump system operated for 414 minutes, with an average ambient temperature of 13.6°C and an average tilted solar radiation intensity of 634.7 W. The average power output of the PV/T components was 107.5 W.  COP initially increases rapidly and then gradually decreases over time.The average COP is 5.66. When the indoor radiant floor heating system with an area of 13.5 m² supplied water at 50°C, the indoor average temperature quickly reached 20°C within 5 hours of heating.The average power varies over time and stabilizes at around 1700 W.

Figure 1 .
Figure 1.Top view of the PV/T experimental system.

Figure 2 .
Figure 2. Solar Radiation Intensity and Ambient Temperature over Time Figure 3 illustrates the variation of average water temperature in the tank and average power output per unit area of the PV/T solar panel.The water temperature in the tank gradually rose from an initial 11.6°C, reaching 52.1°C at the end.The power output per unit area followed a similar trend to the inclined surface solar radiation intensity, peaking at 178.4 W/m 2 and averaging 121.7 W/m 2 .The variation of COP with time is shown in Figure 4. COP initially increases rapidly from 7.09 to 11.58 and gradually decreases over time.The average COP is 5.66.

Figure 3 .
Figure 3. Water Temperature and PV/T Power Output Variation with Time.

Figure 4 .
Figure 4. Variation of COP with time.

Figure 5 .
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Table 1 .
Parameters of the main components in the heat pump system

Table 2 .
Accuracy and Measurement Range of Measuring Instruments