A concentrator photovoltaic system based on branched planar waveguides

Abstract

We propose a concentrator photovoltaic system based on a planar waveguide. Here, the waveguide has one stem at one end and the other end is divided into multiple branches. A right-angle prism is attached to each end of the branches. A lens-array is stacked on the waveguide such that each prism is placed near a focal point of a corresponding lens. Its 45-degree slope leads the focused sun light into the waveguide via total internal reflection. The light propagates inside the waveguide and its intensity increases at each branching point. A solar cell is coupled to the end of the stem for photoelectric conversion. The branched portion can be either straight or curved. In both cases, according to our ray tracing simulations, the light loss inside the waveguide becomes negligible when we set the focal length of the lens larger than a certain value. For example, this value is 300mm for a 5mm-thick, 150mm-long straight waveguide coupled to a lens-array with a lens diameter of 90mm. This number is reduced to 220mm for a curved waveguide. It is further reduced to 100mm when we assume 100% reflection at the 45-degree slope. In these cases, the efficiency defined as the ratio of the optical power exiting the waveguide to one entering the lens-array is close to 87%. The major loss mechanism is the Fresnel reflections at the lens surfaces (8%) and the prism surfaces (3%). The rest is mostly due to the absorption by the material assumed for the waveguide (PMMA) (1-2%).