Original Article

Vertical bifacial PV systems: irradiance modeling and performance analysis of a lightweight system for flat roofs

¹ Department of Solar Power Systems, Institute for Energy Technology, Kjeller, Norway
² Over Easy Solar, Oslo, Norway

^* e-mail: mari.ogaard@ife.no

Received: 30 September 2023
Accepted: 5 February 2024
Published online: 11 April 2024

Abstract

Vertical bifacial photovoltaic (PV) systems are gaining interest as they can enable deployment of PV in locations with grid or area limitations. Over Easy Solar has developed a lightweight design for vertical bifacial systems for flat roofs employing small modules with the height of one cell. To model the expected output of these type of systems can, however, be challenging, as it is uncertain if conventional models will give accurate results for vertical bifacial PV. The irradiance conditions are different, and there can be other loss or gain mechanisms that are prominent in these types of systems compared to more conventional PV systems. In this study we assess the use of regular transposition modeling for plane of array irradiance modeling for vertical bifacial PV, and we evaluate the performance of Over Easy Solar pilot installations in Norway to identify prominent loss mechanisms. The results are relevant for most vertical bifacial systems. With regular transposition modeling plane of array irradiance is overestimated by less than 1%, but we find that accuracy of albedo input and choice of sky diffuse model impact modeling accuracy. Irradiance losses such as shading are not considered in the modeling. We calculate a median heat transfer coefficient of 55 W/m²K, indicating high heat transfer and low thermal losses. High annual plane-of-array insolation, module bifaciality, interrow shading, reflection losses caused by high angle of incidence of the direct irradiance, and snow also have significant impact on the overall performance.