## 5 Power Generation Performance Loss Factor of the BIPV System
### Dust and Soiling  
This BIPV system has been operated without any cleaning since its completion in November 2009. Figure 8 shows the precipitation and snowfall during the operating period (November 2009-February 2013) of the BIPV system. It is expected that there was a cleaning effect by rainfall and snowfall the operation period. However, this study focused on the effects of forcible cleaning of the BIPV system operated for 40 years.  
The effect of cleaning was analyzed to determine the impact of dust and soiling on the power generation performance degradation of the PV system. The monitoring data used to analyze the impacts on the dust and soiling-utilized part of the data from February to March 2013. On 5 March 2013, the four arrays were cleaned to analyze the effects of cleaning. The index used for the effect analysis was the array conversion efficiency. The conversion efficiency of the PV array is affected by various environmental factors, such as the solar spectrum, changes in the incident angle of the Sun, and the solar radiation intensity. In this study, to minimize the influence of factors other than the cleaning effect, the data was measured for one hour when the solar irradiation intensity was highest among the data measured before and after cleaning at each installation position. Table 5 lists the representative days and time selected for the analysis. Figure 9 shows the irradiation and module temperature for each array during the analysis period (1 h).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{3}{*}{**Analysis Time**} & \multicolumn{8}{c}{**Analysis Date**} \\ \cline{2-10}  & \multicolumn{4}{c}{**Before Cleaning**} & \multicolumn{4}{c}{**After Cleaning**} \\ \cline{2-10}  & **1** & **2** & **3** & **4** & **1** & **2** & **3** & **4** \\ \hline HR 0\({}^{\circ}\) & 12:00 & 2/20 & 2/27 & 3/02 & 3/03 & 3/05 & 3/09 & 3/10 & 3/11 \\ SI 30\({}^{\circ}\) & 12:00 & 2/23 & 2/27 & 3/02 & 3/03 & 3/05 & 3/10 & 3/11 & 3/15 \\ SV 90\({}^{\circ}\) & 12:00 & 2/23 & 2/24 & 2/27 & 3/03 & 3/05 & 3/10 & 3/11 & - \\ WV 90\({}^{\circ}\) & 16:00 & 2/21 & 2/23 & 3/01 & 3/02 & 3/05 & 3/15 & 3/16 & - \\ \hline \hline \end{tabular}
\end{table}
Table 5: Analysis date and time about array cleaning effect.  
Figure 8: Precipitation and snowfall during operating periodsFigure 10 and Table 6 present the power generation efficiency before and after cleaning under each installation condition. An analysis of the changes in PV array efficiency due to cleaning under each installation condition showed no significant difference in the vertical planes. The PV array efficiencies of SV 30\({}^{\circ}\) before and after cleaning were 4.5% and 4.6%, respectively. The PV array efficiency of WV 30\({}^{\circ}\) increased from 3.5% to 3.7%, showing an approximate 5.7% improvement rate for the power generation efficiency, compared to the value before cleaning. On the other hand, the PV array efficiency of the HR 0\({}^{\circ}\) and SI 30\({}^{\circ}\) was improved by 23.1% and 14.3%, respectively, compared to the values before cleaning. These results show that continuous maintenance (ex. cleaning) is required to improve the power generation performance in the case of PV systems installed on the horizontal and inclined planes.