## 2 Materials and Methods  
### Experimental Setup  
All tests were carried out at the SolarTech\({}^{\text{LAB}}\) of the Politecnico di Milano (45.503 N, 9.1566 E). Milan is located in a region with humid subtropical climates, or Cfa, according to the Koppen climate classification. This type of climate is characterized by long hot and humid summers and foggy winters. The test facility is equipped with a meteorological station that records ambient temperatures and humidity, wind speed/direction, global horizontal irradiance (GHI), diffuse horizontal irradiance (DHI), and global tilted irradiance (GTI) at 30\({}^{\circ}\). In the test facility, three PV modules were set up under the same conditions (tilt 30\({}^{\circ}\) and azimuth 6.5 E, being 0\({}^{\circ}\) south), with a GCH of 17 cm. Two of the modules were conventional monofacial PV devices (henceforth referred to as PV1 and PV2) and the third one of bifacial technology (bPV). In the ground floor surrounding bPV, a high reflecting aluminum foil was placed to increase the incident irradiation on the secondary face of bPV. Aluminum foil albedo is in the range of 0.71-0.78 and has low dependence on solar elevation [31]. The experimental setup is shown in Figure 1.  
Module PV1 was manufactured by sunpower with a peak power of 327 W (under standard test conditions, STC: 1000 W/m\({}^{2}\); AM 1.5 spectrum; and module temperature of 25 \({}^{\circ}\)C) [32]; PV2 was produced by Aleo and is able to generate up to 305 under identical conditions [33], whereas bPV (a 345-Watt module, with glass/glass frameless structure) was fabricated by 3 SUN, and its reported bifaciality (\(\varphi\)) is over 85% [34]. The main characteristics of the modules (PV1, PV2, and bPV) are summarized in Table 1. Apart from some construction features and nominal operating cell temperature (NOCT), it includes a short-circuit current (\(I_{sc}\)), open-circuit voltage (\(V_{oc}\)), and maximum power (\(P_{mp}\) and its components \(I_{mp}\) and \(V_{mp}\)), as well as the thermal coefficients of \(I_{sc}\), \(V_{oc}\), and \(P_{mp}\) (\(\alpha\), \(\beta\), and \(\gamma\), respectively). During a 5-day campaign, the _I-V_ curves of the 3 modules were traced in central day hours. For each condition, 5 curves were obtained per device using an _I-V_ tracer manufactured by TeamWare (the process to obtain the five curves takes up to 1.5 min). Additionally, the temperature of each module was measured using type K thermocouples and recorded with a data logger Elo3305, manufactured by LSI. The main characteristics of the equipment used are summarized in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline
**Manufacturer** & **Sunpower** & **Aleo** & **3 SUN** \\
**Model** & **SPR E20 327 COM** & **S59 305** & **35 Dual 72N** \\
**Short Name** & **PV1** & **PV2** & **bPV** \\ \hline \multicolumn{4}{c}{Mechanical} \\ \hline Length \(\times\) width [mm] & \(1558\times 1046\) & \(1660\times 990\) & \(1983\times 998\) \\ Cell material & m-Si & m-Si & m-Si \\ Cell size [mm] & \(125\times 125\) & \(156.75\times 156.75\) & \(156.75\times 156.75\) \\ Number of cells (\(N_{c}\)) & 96 & 60 & 72 * \\ \hline \multicolumn{4}{c}{Electric} \\ \hline \(P_{mp}\) [W] & 327 (+5/\(-\)3 W) & 305 (\(\pm\)5 W) & 345 (+3\%) \\ \(V_{mp}\) [V] & 54.7 & 31.4 & 39.3 \\ \(I_{mp}\) [A] & 5.98 & 9.72 & 8.78 \\ \(V_{oc}\) [V] & 64.9 & 39.6 & 47.9 \\ \(I_{sc}\) [A] & 6.46 & 10.06 & 9.18 \\ \(\beta\) [mV/\({}^{\circ}\)C] & \(-176.6\) & \(-114.84\) & \(-143.7\) \\ (\%/\({}^{\circ}\)C) & \((-0.272)\) & \((-0.29)\) & \((-0.30)\) \\ \(\alpha\) [mA/\({}^{\circ}\)C] & 2.6 & 5.03 & 4.4 \\ (\%/\({}^{\circ}\)C) & \((+0.04)\) & \((+0.05)\) & \((+0.048)\) \\ \(\gamma\) [\%/\({}^{\circ}\)C] & \(-0.35\) & \(-0.4\) & \(-0.38\) \\ \hline \multicolumn{4}{c}{NOCT [\({}^{\circ}\)C]} & \(45\pm 2\) & \(48\pm 2\) & \(44\pm 2\) \\ \hline \multicolumn{4}{l}{*Per face.} \\ \end{tabular}
\end{table} 
Table 1: Main PV module characteristics. Figure 1: Experimental setup at SolarTech\({}^{\text{LAB}}\) (45.503 N, 9.1566 E).