## 4 Processing of Experimental Data
### 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.  
Figure 6: Variation Curve of Irradiance with Suction and Discharge Temperatures.  
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 \({}^{\circ}\)C, while for the PV back panel, it is 9.89-32.53 \({}^{\circ}\)C, with average temperatures of 12.41 \({}^{\circ}\)C and 21.21 \({}^{\circ}\)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 \({}^{\circ}\)C, while for the PV back panel, it is 10.39-34.63 \({}^{\circ}\)C, with average values of 13.64 \({}^{\circ}\)C and 22.51 \({}^{\circ}\)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 \({}^{\circ}\)C and 27.04 \({}^{\circ}\)C, respectively, while under overcast conditions, the theoretical values of the lowest and highest temperatures are 10.72 \({}^{\circ}\)C and 34.94 \({}^{\circ}\)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 \({}^{\circ}\)C and 0.016 K/W\(\cdot\)m\({}^{-2}\), respectively; under overcast conditions, they are 8.31 \({}^{\circ}\)C and 0.0118 K/W\(\cdot\)m\({}^{-2}\), respectively; and under cloudy conditions, they are 8.62 \({}^{\circ}\)C and 0.0119 K/W\(\cdot\)m\({}^{-2}\), respectively.  
Comparing sunny conditions (510 W\(\cdot\)m\({}^{-2}\)) with overcast conditions (369 W\(\cdot\)m\({}^{-2}\)) and cloudy conditions (483 W\(\cdot\)m\({}^{-2}\)) at different solar radiation intensities, the intercepts of the instantaneous PVT back panel temperature curves decrease by 2.13 \({}^{\circ}\)C and 2.44 \({}^{\circ}\)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\(\cdot\)m\({}^{-2}\).  
Figure 7: Variation Curve of Irradiance and Panel Temperatures.