## 4 Optimal Tilt Angle Dependencies
### Optimal Tilt Angle Dependence on the Ground Albedo  
Herein, \(G_{\text{eq}}\), \(G_{\text{front}}\), and \(G_{\text{rear}}\), were obtained as a function of the ground albedo coefficient \(\alpha\). The operating conditions consisted of clear-sky weather, obtained through the [PERSON] clear-sky model [80], a module height of 2 m, and a ground albedo of 0.75. In Figure 10(a) are shown the effective annual insolation received by the PV module surfaces and the normalized annual insolation, as a function of \(\gamma\). The normalization was performed using the maximum obtainable value for the same component. In these conditions, \(\gamma_{\text{opt}}\) was equal to 45\({}^{\circ}\). More in-depth, it showed how the equivalent annual energy insolation did not change significantly in the range of variations in \(\gamma\) between 40\({}^{\circ}\) and 50\({}^{\circ}\). Indeed, comparing \(G_{\text{eq}}\) at 40\({}^{\circ}\) and 50\({}^{\circ}\) with \(G_{\text{eq}}\) at 45\({}^{\circ}\), the deviations were 6.68 kWh/m\({}^{2}\) per year (i.e., 0.20%) and 7.26 kWh/m\({}^{2}\) per year (i.e., 0.22%), respectively. Similar observations could be made if the ground albedo was set to 0.1. For this second case, the results, reported in Figure 10(b), showed \(\gamma_{\text{opt}}=40^{\circ}\). Thus, by decreasing the ground-albedo coefficient from 0.75 to 0.1, \(\gamma_{\text{opt}}\) decreased from 45\({}^{\circ}\) to 40\({}^{\circ}\). Furthermore, it was possible to scout the entire range of ground-albedo coefficients. In Figure 12 are reported the \(G_{\text{rear}}\) and \(G_{\text{front}}\) magnitudes at \(\gamma_{\text{opt}}\), the optimal tilt angle for the singular insolation components (i.e., \(\gamma_{\text{opt,f}}\) for \(G_{\text{front}}\), \(\gamma_{\text{opt,r}}\) for \(G_{\text{rear}}\), and \(\gamma_{\text{opt}}\) for \(G_{\text{eq}}\)), and the bifacial gain. It is possible to distinguish multiple behaviors. For \(\alpha>0.25\), the \(\gamma_{\text{opt,f}}\) angle tended to increase while the \(\gamma_{\text{opt,r}}\) angle tended to decrease. This indicates that to maximize the energy harvested, one should try to maximize the module's exposure relative to the ground surface. This trend demonstrates that the radiation reflected from the ground has an appreciable impact on both \(G_{\text{rear}}\) and \(G_{\text{front}}\). When \(\alpha\) increased from 0.10 to 0.75, \(G_{\text{front}}\) found its maximum in correspondence with a tilt angle increasing from 40\({}^{\circ}\) to 55\({}^{\circ}\). This is attributable to the fact that the \(G_{\text{front}}^{\text{ref}}\) increase compensates the decrease in \(G_{\text{front}}^{\text{beam}}\), given by a sub-optimal orientation. It is possible to observe how \(G_{\text{rear}}\) presents a different trend. When \(\alpha=0\), \(G_{\text{rear}}\) found its maximum at a tilt equal to 90\({}^{\circ}\), while at \(\alpha=0.75\), the maximum moved, corresponding to a tilt of 0\({}^{\circ}\). This was due to a respective increased contribution of \(G_{\text{rear}}^{\text{ref}}\) in \(G\), whose contribution changed from a negligible to major. At low values of \(\alpha\), \(G_{\text{rear}}^{\text{ref}}\) was negligible, and the main contribution of \(G_{\text{rear}}\) was \(G_{\text{rear}}^{\text{beam}}\). Therefore, the latter being visible only when the Sun's azimuth deviated more than 90\({}^{\circ}\) from the bi-PV azimuth, and the rear face was towards the Sun, \(G_{\text{rear}}\) was maximized when \(\gamma\) was 90\({}^{\circ}\). Besides the approach, to determine \(\gamma_{\text{opt}}\), it is important to consider the magnitude of each light component magnitude. The results showed that \(\gamma_{\text{opt}}\) was influenced by \(\gamma_{\text{opt,r}}\) only for \(\alpha>0.5\), when the maximum \(G_{\text{rear}}\) magnitude was 37% of the maximum \(G_{\text{front}}\) magnitude.