## 2 Materials and Methods  
A common monocrystalline PV panel was experimentally compared to an enhanced panel, integrated in the rear part with graphene/fins/phase change material. A monocrystalline panel (650 \(\times\) 505 mm) was chosen. The rated power was 40 W, with 17.8 V module voltage and 2.3 A nominal current, while the open-circuit voltage and the short-circuit current were 21.3 V and 2.7 A, respectively. Paraffin wax was chosen as phase change material (PCM), the RT35 HC, produced by the Rubitherm company (Germany). The most important properties are reported in Table 1.  
The first enhancement proposed to increase the low thermal conductivity of PCM was the addition of graphene powder, in a percentage of 2 wt%. It consisted of nanoparticles (average diameter 50 \(\upmu\)m and thickness 3 nm), in appearance such as a powder, with high thermal conductivity (between 3000 and 5000 W/m\({}^{\circ}\)C). Graphene was characterized by low density and chemical activity, large specific surface area (300-500 m\({}^{2}\)/g) and high infrared emissivity (0.95). The purpose was to distribute graphene powder evenly in the PCM matrix and increase its thermal conductivity. The preparation of the PCM/graphene sample consisted of different steps (Figure 1). First, the PCM was completely melted and then the graphene powder was added. The mixture is mechanically stirred for 3 h at a temperature of around 70 \({}^{\circ}\)C. This ensured uniformity of the composite. Afterwards, the mixture underwent a further treatment by an ultrasonic processor for 30 min at 40 \({}^{\circ}\)C for appropriate dispersion of molecules. \begin{table}
\begin{tabular}{l l} \hline \hline
**Characteristics** & **RT 35 HC** \\ \hline Melting area (\({}^{\circ}\)C) & 34–36 peak 35 \\ Congealing area (\({}^{\circ}\)C) & 36–34 peak 35 \\ Volume expansion (\%) & 12 \\ Latent heat (kJ/kg) & 240 \\ Heat conductivity (W/m\({}^{2}\) K) & 0.2 \\ Specific heat capacity (kJ/kg K) & 2 \\ Liquid density (kg/l) & 0.77 \\ Solid density (kg/l) & 0.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of PCMs used. The second enhancement concerned the implementation of aluminum fins. This way, the contact area between the PCM and the PV panel was greatly increased, and they worked as \"thermal bridges\" between the PV module and the PCM-graphene mixture. Fins were obtained from cutting and bending of strips from a thin aluminum sheet glued to the back surface of the panel. The containment box was composed of plexiglass to ensure transparency and to visualize the melting process. The external dimensions of the box were 65 x 50 x 2.5 cm, while the internal ones were 64 \(\times\) 49 \(\times\) 2.0 cm. Once the PCM/graphene preparation was completed, the liquid mixture was poured in the plexiglass box and then it was closed with the panel where the fins were glued. The proposed system appeared as in Figure 2. A set of thermocouples (red bullets in figure) were mounted throughout the system to monitor the temperatures. One was mounted frontally on the PV panel and another on the back, glued on the aluminum sheet. Two thermocouples were placed in the PCM, at 7 mm from each other, to monitor the heat flow and phase change process of the PCM; the last one, on the rear surface of the plexiglass. Thermocouples were T type (copper-constantan) with a sensitivity of 48.2 \(\upmu\)V/\({}^{\circ}\)C and a measurement range from \(-\)200 \({}^{\circ}\)C to 400 \({}^{\circ}\)C. The global solar radiation was monitored by a pyranometer, model DPA/ESR 154, with a range of measure from 0 to 2000 W/ m\({}^{2}\), a sensitivity of 10.88 \(\upmu\)V/Wm\({}^{-2}\) and a linearity of 0.75%. Two sets of measures were organized. The first one was performed indoors under a solar simulator [34], useful to allow repeatable tests under the same conditions (such as the incident radiation and the ambient temperature). The solar simulator consisted of  
Figure 1: Preparation of the PCM/graphene mixture. Figure 2: A schematic section of the proposed PV–PCM panel.