## 3 The Influence of Soiling on Photovoltaic Energy
### Experimental Assessment of PV Sailing Losses
Particularly a single cypress closely adjacent to the PV system had a remarkable soiling impact. Interestingly, two different soiling patterns caused by pollen were noticed. In the first place, the PV module surfaces underwent uniform soiling after several ra  
Figure 5: General trend of the local total airborne pollen (adapted from [101]). Figure 6: Local airborne pollen (first semester of 2019): comparison of the airborne total pollen with the _Cupressaceae_ and the _Platanus_ sp. contributions. Numerical data gathered from [100]. an inevitable fact which, at some point, would require cleaning the PV modules to reduce the soiling losses. Within this general uniformity, some pollen agglomeration distributed homogeneously over the PV module surfaces was observed, perhaps induced by the condensation and evaporation of water drops, provided that morning dew events were regularly noticed during that rainless period. In the second place, non-uniform soiling was observed just after a light rain event: instead of effectively washing pollen away (neither from the 8\({}^{\circ}\)-tilted nor from the 35\({}^{\circ}\)-tilted module surfaces), 0.7 mm of rainfall dragged pollen downwards and accumulated it along the bottom edge of the PV modules. While this heterogeneous soiling issue was inevitable in the framed multi-crystalline silicon (mc-Si) modules, it could have been prevented in the frameless cadmium telluride (CdTe) modules by modifying the overhanging supporting structure which artificially retained pollen (Figure 7b). In any case, pollen deposition may cause such a non-uniform soiling pattern during a lengthy rainless springtime period without the need of a light rain event [79]. The international standard IEC 61724 [102] defines the soiling ratio (\(SR\)) as the ratio of the actual power output of the PV array under given soiling conditions to the power that would be expected if the PV array were clean and free of soiling. Soiling loss (\(SL\)) and soiling ratio are thus complementary, that is, \(SL\)= \(1-SR\). Overall, during the starting monitoring period (March-June 2019), the 8\({}^{\circ}\)-tilted multicrystalline silicon modules of the PV installation endured an average soiling loss of 1.3% [103], having an impact on the short circuit current rather than on the open-circuit voltage or the fill factor. As well, two graphics containing some initial performance data have been included for description purposes (Figure 8). These graphs compare the performance of two identical uncoated multicrystalline silicon modules installed at a tilt angle of 8\({}^{\circ}\). Using the short-circuit current data provided by an I-V tracer from two adjacent identical PV modules, being one of them soiled (\(I_{\text{SC,solic}}\)) and the other clean (\(I_{\text{SC,clean}}\)), to determine the performance ratio (\(PR\)) and the soiling ratio (\(SR\)) is an accepted practice within the photovoltaic community [104]. Therefore, the performance ratio related to \(I_{\text{SC}}\) of both PV modules, \(PR_{\text{soiled}}\) (%) and \(PR_{\text{clean}}\) (%), can be calculated using the following equation [104]:  
\[PR_{\text{Isc}}\left(\%\right)=\frac{\sum{I_{\text{SC}}/I_{\text{SC,STC}}}}{ \sum{G_{\text{POA}}/1000}}\times 10 \tag{1}\]  
where \(G_{\text{POA}}\) is the irradiance in the plane of the array in watts, and \(I_{\text{SC,STC}}\) is the short-circuit current of the PV module at standard test conditions (i.e., in-plane irradiance 1000 W/m\({}^{2}\), PV cell temperature 25 \({}^{\circ}\)C and reference spectral irradiance distribution as defined in IEC 60904-3 [105]). Hence, the soiling ratio, \(SR\) (%), can be calculated by comparing the soiling ratio of the soiled module with the soiling ration of a clean one [102]:  
\[SR\left(\%\right)=\left(1-\frac{PR_{\text{soiled}}}{PR_{\text{clean}}}\right) \times 100 \tag{2}\]  
Figure 7: CdTe modules soiled by pollen: (**a**) uniform soiling and (**b**) non-uniform soiling.