# Towards Improving the Durability and Overall Performance of PV-ETICS by Application of a PCM Layer
## 4 Results and Discussion  
The first step in the PCM layer design was calculation of the amount of solar energy that needs to be stored. For this purpose, the solar energy incident on the facade was calculated during the time when the PV panel temperature exceeds 80 \({}^{\circ}\)C. Using a temperature-dependent model of electricity generation, the values of energy production were calculated. The obtained results were also correlated with the instantaneous values of PV panel temperature, and the values representative of temperatures above 80 \({}^{\circ}\)C were used for further analysis. It was assumed that part of the solar energy incident on the facade will be transferred into electricity, while the rest will be transferred into thermal energy, causing the PV temperature rise. Since the overheating of the panels above 80 \({}^{\circ}\)C is destructive, it was assumed that the PCM layer should be designed to store this excess energy. Calculation of the average annual productivity of the flexible PV panels used with ETICS revealed that this productivity is at the level of approximately 5%, which means that 95% of the solar energy affecting the facade should be stored (Figure 6). The values of the amount of energy that should be stored in the PCM layer significantly varied for the selected locations. For 13 locations, the values of energy did not exceed 20 kW h/m\({}^{2}\)/a, while four of them had values above 50 kW h/m\({}^{2}\)/a. The highest need for solar energy storage was registered in Madrid, Lisbon, Rome and Bucharest, which were also characterized by the highest solar irradiance availability at the horizontal surface. Figure 5: Maximum values of temperature on the external surface of ETICS and PV-ETICS for analysed locations. Analysis of the daily values of the amount of solar energy to be absorbed in the PCM layer revealed that for most locations the problem of PV panel overheating lasts for less than 20 days during the year, and despite high maximum values for most of the time, calculated values oscillate on the lower levels (Figure 7). Because of this, it was assumed that the thickness of the PCM layer should be designed for the amount of material needed to absorb the average value of solar energy. The daily values were used in the further analysis to ensure the daily cycle of energy absorption and release (active thermal capacity). Due to the stated goal of the PCM layer design, which was protection from overheating over 80 \({}^{\circ}\)C, suitable paraffin wax was selected. Based on the properties of the selected material (RT80 HC)--density of 880 kg/m\({}^{3}\), and latent heat of 220 kJ/kg, and a phase change temperature range between 70 \({}^{\circ}\)C and 85 \({}^{\circ}\)C-- the average daily value of the solar energy that should be stored in the material as well as its weight and thickness was calculated (Figure 8). The obtained values of the layer thickness varied from 1.2 cm for  
Figure 6: Solar energy incident on the facade, calculated as the potential to be absorbed in the PCM layer. Figure 7: Daily maximum and average values of the solar energy to be absorbed in the PCM layer and number of overheating days. Minsk up to 4.4 cm for Madrid. All values were assumed as possible to be applied from a technological point of view and were used in further calculations. It was assumed that PCM will be applied in a shape stabilized layer. In the case of the En-ActivETICS model, the conductivity of PCM was assumed to be constant in both solid and liquid states. The conductivity of the material was 0.14 W/m K, and the material was in a direct thermal contact with the PV panel. The PCM can be used in the form of a metal container, which was proposed by the authors of [10].