## 4 Processing of Experimental Data
### 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 \({}^{\circ}\)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\(\cdot\)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 \({}^{\circ}\)C, the radiation intensity and COP are higher by 165.08 W\(\cdot\)m\({}^{-2}\) and 0.21, respectively, with a COP increase of 8.05%. Meanwhile, at an average ambient temperature of 16.45 \({}^{\circ}\)C,  
Figure 14: Variation Curve of Solar Radiation Intensity and Photovoltaic Efficiency of PVT/PV Panels under Sunny Conditions.  
the radiation intensity and COP are higher by 171.37 W\(\cdot\)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 Figures 15 and 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\(\cdot\)m\({}^{-2}\), respectively, while in the afternoon, they are 2.05809 and 0.00133 W\(\cdot\)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\(\cdot\)m\({}^{-2}\), respectively, and in the afternoon, they are 1.87055 and 0.00155 W\(\cdot\)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.  
Figure 15: Variation Curves of Solar Radiation Intensity and System COP: (**a**) Overcast, (**b**) Cloudy, (**c**) Sunny.