## 1 Introduction  
The exploitation of solar radiation is fundamental for the transition from a fossil fuels-based technology to renewable ones. A photovoltaic (PV) panel allows a clean energy production transforming the radiation incident into electricity [1, 2]. Nowadays, typical commercial PV panels, composed of silicon, have a photovoltaic conversion efficiency ranging from 15% to 20%. Modern OPV (organic photovoltaic) modules can reach a higher performance [3, 4]. The remaining share of energy is lost in the environment as heat. In addition, the just-defined yield refers to the standard test conditions (STC), namely, an incident radiation of 1000 W/m\({}^{2}\), an air mass of 1.5 and a cell temperature of 25 \({}^{\circ}\)C. This latter parameter is very influential because as the temperature increases, the efficiency decreases [5]. Quantitatively, it depends on the typology of the panel, for instance, for the prevalent mono crystalline-type, a power reduction of 0.5%/\({}^{\circ}\)C has been observed. As a result, in warm periods and climates, the yield, which is already low under standard test conditions, can be significantly reduced. Accordingly, it is important to maintain the operating temperature as low as possible to maximize the efficiency of the module. Over the years several strategies have been adopted to limit this phenomenon, with the aim of cooling the module. A macro subdivision can be an active and passive system [6, 7]. An example of a passive system is natural air circulation. The main advantage is the low cost and lack of maintenance. On the other hand, they suffer from a low heat transfer rate, a low thermal conductivity and heat capacity of air, a dependence on wind direction and speed and, for these reasons, they can offer a limited temperature reduction, similar is the case of cooling via heat pipes. Active cooling systems [8, 9], such as forced air circulation,hydraulic or thermoelectric cooling, have higher heat transfer rates compared to natural air circulation air and independence on wind direction and speed [10]. As disadvantages, they have high initial maintenance costs, an electricity consumption associated with an electricity consumption, and they are noisy systems. Recently a new passive solution has been developed as a cooling system of modules, namely, through the exploitation of phase change materials (PCM) [11; 12; 13]. These materials are proving to be a good application [14]. PCMs use latent heat to absorb a large amount of energy and keep the panel cool. Dissimilar to sensitive heat, which is associated with an increase in temperature, the phase change occurs at a nearly constant temperature. PCMs absorb a large amount of heat during the solid-liquid phase change, and this allows to have smaller sizes with equal absorbed heat. In addition, reaching a lower temperature, self-discharge thermal losses are reduced, compared to fluids such as water, or air that can reach higher temperatures. In addition, maintenance is not usually required and there is no associated power consumption, as there are no fluids to be handled. The advantages of the application of the change phase materials in the back side of a photovoltaic module have already been studied. [PERSON] et al. [15] investigated the effects of the tilt angle of a PV panel from 0\({}^{\circ}\) to 90\({}^{\circ}\) obtaining a minor cell temperature in a range of 0.4% to 12%. [PERSON] [16] conducted an experimental investigation combining two different PCMs achieving the highest temperature reduction. [PERSON] et al. [17] experimentally obtained a reduction of 3.8 \({}^{\circ}\)C of the panel with an increase in the open-circuit voltage of 4.4%, at an incident radiation of 1000 W/m\({}^{2}\). [PERSON] et al. [18] monitored over a one-year period the performance of a PV panel integrated with phase change materials. The annual electrical energy yield increased by 5.9% in hot climatic conditions.