Energy yield measurement of an elevated PV system on a white flat roof and a performance comparison of monofacial and bifacial modules

Bifacial solar panels installed on flat rooftop of industrial buildings are an effective way to boost the yield. The albedo of the flat roof determines to great extent the bifacial gain. However, because it changes due to soiling and moss growth, the power output of bifacial PV installations over time is difficult to calculate. For the purpose of evaluating and rating the effect of bifacial PV rooftop systems, a small plant with 20 modules including dummies was installed. To guarantee an optimized output, a commercial bifacial PV module supporting construction was placed on a flat roof that was painted white. By using an east-west orientation, an optimum for the use of available roof space was reached. The yield for each module was determined with power optimizers and the extracted data, provided by the monitoring portal, were analysed. The data were collected for each day over a time of one year. Three different modules, both east and west orientated, were investigated. The bifacial module with a transparent backsheet compared to a module with the same bifacial cells but with a black backsheet, showed initially a benefit of 17% for the east and 15% for the west orientated panels. After one year of operation, a measured benefit of 7% for the east and 5% for the west orientated panels remained due to albedo loss because of moss growth and pollution. The use of panels with 92% bifaciality resulted in a higher yield of up to 3% compared to panels with 70% bifaciality.