Bifacial photovoltaic module performance in correlation to cloud conditions, sun spectrum and irradiance enhancement

This study presents the performance behavior of 10° tilted, east and west oriented bifacial photovoltaic (PV) modules during irradiance enhancement (IE). The impact of meteorological parameters on the performance of different bifacial photovoltaic module types was determined. The analysis was done time resolved for irradiance enhancement events as well as for the total annual yield in the period 04/2020–06/2021. A cloud classification was performed for clouds that trigger irradiance enhancements. Irradiance enhancements exceeding 1000 ${\mathrm{Wm}}^{-2}$ were found to occur only on days with clouds in Vienna (Austria). For 179 irradiance enhancement situations analyzed, 81% of all enhancements happened for cloudiness greater than 0.4 and still 30% for a cloudiness greater than 0.7, the latter resulting also in single enhancement events greater than 1150 ${\mathrm{Wm}}^{-2}$. Cloud genera preferentially causing irradiance enhancements were identified as Altocumulus and Cumulus clouds. The evaluation of cloud pictures during IE events was done by hand and not automated. By this the position of the sun towards the clouds could be also taken into account. The mechanism of irradiance enhancement compatible to the position of the sun towards the clouds were in accordance with the cloud types Mie-scattering and edge reflections, respectively, or a mix of both. The overall photovoltaic long term performance results showed that the average weighted absolute efficiencies of the bifacial photovoltaic modules were 2%–4% higher than the ones of monofacial standard reference modules. The power output of the bifacial modules was between 17%–24% higher throughout the 15 month period of the investigation compared to monofacial reference modules. This results held independent of orientation while there was a visible seasonal variation, namely 19%–28% more power output in winter, 18%–24% in spring, 16%–25% in summer and 17%–27% in autumn, respectively.