Optimization of Bifacial Photovoltaics by Optical Systems Feasibility Study

Abstract

Bifacial PV cells allow light to reach the cell from the rear, achieving rear side efficiencies from 60% to over 95%. The predicted annual bifacial gain in irradiance (rear / front) can be 5% to 30% depending mainly on the reflectivity of the ground (albedo). It can be increased by optical systems redirecting diffuse and direct solar radiation to the rear of the solar cell without solar tracking. The optical systems can work by the physical effects of refraction or reflection. Beside reflective gratings refractive prismatic systems can be applied, e.g., as micro-structures, which originally were developed to improve daylighting of deep rooms. Applications and production related variations are described. The arrangement of the optical system and PV module can vary according to the orientation and setup of the module, which may be in open spaces or integrated into buildings. The advantage of bifacial over monofacial cells is expressed by the "bifacial gain". This is the additional part of the total energy that a bifacial PV system produces in comparison to a monofacial system with the same orientation and size. This additional yield is given in the literature as 5% to 30%, depending on the reflectivity of the ground and the cell type. For the redirection of solar radiation by micro-structured systems on HIT bifacial solar cells a bifacial gain of 51% to 68% can be assumed. These preliminary results have to be confirmed by tests.