## 3 Optical Model Comparison and Evaluation of Influences on the Incident Irradiance on the PV Module
### Ground and Nearby Surfaces' Reflectivity Influence  
The reflectivity \(\alpha\), also called the albedo coefficient of a material, is the ratio of the amount of radiation reflected by a surface to the total irradiance incident on that surface. Each surface is characterized by a specific \(\alpha\), which can be further considered as a function of the wavelength and the angle at which the light hits the surface. In this field, however, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tilt Angle (\({}^{\circ}\))** & **Weather Cond.** & \
\begin{tabular}{c} **Max. Irradiance** \\ **( kWh/m\({}^{2}\))** \\ \end{tabular} &
\begin{tabular}{c} **Total Insolation** \\ **( kWh/m\({}^{2}\)day)** \\ \end{tabular} \\ \hline
57 & Sunny & 1032 & 6.84 \\
42 & Sunny & 993 & 6.42 \\
27 & Sunny & 862 & 5.53 \\
0 & Sunny & 560 & 3.41 \\
57 & Cloudy & 229 & 1.01 \\
42 & Cloudy & 247 & 1.08 \\
27 & Cloudy & 271 & 1.20 \\
0 & Cloudy & 288 & 1.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Maximum irradiance and daily total insolation for different tilt angles and weather conditions. Retrieved from [66]. Figure 9: Schematic view of the three light components incident on the PV module: direct (yellow), diffuse (blue), and reflected (orange). surfaces are considered to be Lambertian, so \(\alpha\) is constant regardless of wavelength or angle of incidence. Some guideline values are given in Table 3.  
For the case of bi-PVs, reflected radiation plays a crucial role, as the major contributions of the component \(G_{\text{rear}}\) come from reflections from nearby surfaces. These can be the ground, other PV modules, and the mounting structure. However, in evaluating the energy performance of bi-PVs, as highlighted in [31; 33; 72; 73; 74], the ground beneath the module is the most significant surface. Both [75; 76] demonstrated the occurrence of a linear relationship between the ground albedo coefficient and \(G_{\text{rear}}\). Furthermore, [PERSON] et al. reported in [77] the measurements of the ratio \(G_{\text{rear}}/G_{\text{front}}\) under different ground albedo conditions. At the same \(G_{\text{front}}\), it was observed that the average of \(G_{\text{rear}}/G_{\text{front}}\) increased from 16.2% to 35.3% when the albedo was increased from 0.21 to 0.79. However, an important effect to consider is the ground shading caused by adjunct PV modules, usually called the edge effect. To investigate it, it is first necessary to define two parameters: the ground coverage ratio (\(GCR\)) and the bifacial gain (\(BG\)). The first defines the ratio between the utilized ground surface and the available ground surface, and the second defines the energy yield variation between bi-PVs and mo-PVs, the latter unaffected by ground reflection/shading. The two parameters are calculated as  
\[GCR=CW/pitch \tag{8}\]  
\[BG=100\ \frac{y_{\text{bi}}-y_{\text{mo}}}{y_{\text{mo}}} \tag{9}\]  
where \(CW\) is the longest dimension of the PV module, the pitch is the distance between two equal points of adjacent strings, and \(y_{\text{bi}}\) and \(y_{\text{mo}}\) the energy yield of the bifacial and monofacial modules, respectively. The combination of the two permits the evaluation of the impact of ground shaded by other PV modules. [PERSON] et al. in [26] simulated the influence of the PV plant configuration (i.e., module height, number of modules per string, number of strings, and \(GCR\)) on the \(BG\). It was observed that a higher number of strings and modules per string and lower height all led to lower \(BG\)s. In particular, when the height was increased from 0.5 m to 2 m, the central module of a single seven-module string increased its \(BG\) by 8%. On the other hand, as the number of modules per string increased from one to seven, the central module \(BG\) of a single string at height 1 m decreased by 2.7%. In parallel, [PERSON] et al. in [75] obtained the dependencies of \(NU\) and \(G_{\text{rear}}\) by the same variables as in [26].