CFK: a Fresnel–Köhler concentrator with an external confinement cavity
In a concentrator photovoltaic (CPV) module, the solar cell surface reflects a non-negligible portion of the incoming light, leading to a loss in module efficiency. The Fresnel–Köhler with an external confinement cavity (CFK) is a novel optical CPV concentrator designed to recover this portion of the reflected light. The design is based on an external confinement cavity, an optical element able to redirect the light reflected by the cell surface towards its surface again. Its integration into a CPV module is possible, thanks to the recent invention of advanced Köhler concentrators by LPI. This strategy, based on light recovery, leads to a significant increase in electrical efficiency. We have tested the excellent performance of these cavities by means of integrating one of them into an FK concentrator and manufacturing a proof-of-concept prototype. The measured results are outstanding: a relative electrical efficiency and Isc gains of up to 6% when comparing both with and without cavity designs, and a 33.2% of CFK module electrical efficiency (@Tcell = 25 °C) using a 38.5% nominal efficiency cell (without anti-reflection coatings on the optics).