## 3 Results and Discussion
### Bifacial Comparison (Module bPV): Mono vs. Bifacial Performance  
In comparison to other PV modules (PV1 and PV2), the bPV module exhibits a superior power capacity, achieving up to a +7.8% increase under the test conditions being analyzed (refer to Figure 6a and Table 3). It is imperative to discern whether this enhancement arises primarily from the secondary PV face or if variances among manufacturers contribute to a fraction of it. This comparison essentially evaluates bPV against itself, considering both mono- and bifacial configurations.  
_I-V_ curves' maximum power points of bPV (in monofacial operation) were derived utilizing two distinct methods outlined in the preceding section. Initially, employing Equations (1)-(4), the baseline curve was adjusted to encompass all bPV G-T tracing conditions (as shown in Figure 5), and the MPP was then identified. The second method was then applied using (5) under identical conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **PV1** & **PV2** & **bPV** & **PV1-bPV** & **PV2-bPV** \\ \hline Max [\({}^{\circ}\)C] & 47.6 & 46.6 & 47.4 & 0.2 & \(-\)0.8 \\ Median [\({}^{\circ}\)C] & 39.7 & 36.7 & 36.5 & 3.2 & 0.2 \\ Mean [\({}^{\circ}\)C] & 36.9 & 34.8 & 34.9 & 2.0 & \(-\)0.1 \\ Normal lowest * [\({}^{\circ}\)C] & 27.0 & 25.2 & 25.1 & 1.9 & 0.1 \\ \hline \hline \end{tabular}  
* Defined as \(Q1-1.5(Q3-Q1)\), where \(Q1\) and \(Q3\) are the first and third quartile, respectively.  
\end{table}
Table 4: Some statistical differences in temperature operation.  
Figure 6: Performance comparison of modules PV1, PV2, and bPV: maximum power (**a**), short\(-\)circuit current (**b**), open\(-\)circuit voltage (**c**), and cell temperature (**d**). Linear regressions are included in each legend.  
Both the measured and estimated maximum power outputs of bPV are depicted in Figure 7, as well as the disparity between the two methods. Both approaches yielded comparable (Figure 7a) and consistent (Figure 7b) outcomes. During days characterized by clear-sky conditions and minimal cloud cover, the additional power output ranged from 1 W to 7 W. However, it notably increased on overcast days (see Figure 5), reaching values within the range of 7 W to 15 W. This observed phenomenon is attributed to a significant reduction in back-side irradiance non-uniformity under high diffuse fraction conditions [10], consequently diminishing the current mismatch on the secondary face and augmenting its power output.  
The bifacial power gain (BPG) can be defined (7) to quantify bifacial contribution:  
\[BPG=\frac{P_{mp,b}}{P_{mp,m}}-1 \tag{7}\]  
where the \(b\) and \(m\) subscripts distinguish between bi- and monofacial maximum power, respectively.  
When BPG is calculated (7), for clear or marginally covered days, its value is close to 2% (means of 1.7% and 1.9%, depending on the monofacial estimation method, see Figure 8a) but can reach 4%. However, in overcast scenarios, it can be as high as 15%, even under non-favorable test conditions. This is clearly evidenced if BPG is plotted against diffuse fraction (see Figure 8b), where the two clusters are observed. In Figure 8b, the diffuse fraction was calculated on the horizontal plant, that is, directly from measured data (DHI/GHI).  
Figure 7: Bifacial–monofacial MPP comparison of bPV module. Recorded MPP of bPV and estimated monofacial by two different methods (**a**) and estimated bifacial contribution (**b**). Estimation methods: Procedure 4 of IEC 60891 [35] (red crosses) and (5) (black squares).