Demonstration of molded glass primary optics for high-efficiency micro-concentrator photovoltaics

Micro-concentrator photovoltaics (micro-CPV) is a branch of the CPV technology where the size of the solar cell is reduced below the millimeter range to permit novel manufacturing methods for cost reductions and maintaining high efficiencies by using high performance optical materials. The reduction in size of all the components demands a very good optical quality of the primary lenses to achieve high concentrations and high optical efficiencies. PMMA or silicon-on-glass are the most used materials for CPV primary optics, but PMMA reliability against UV weathering is questionable and silicone has a large coefficient of thermal expansion (CTE) mismatch with respect to glass, which introduces a large thermal sensitivity in the optical efficiency. In contrast, direct glass molding is a promising alternative due to benefits such as a high Abbe number (lower dispersion losses), great mechanical properties, low CTE, low thermal coefficient of the refractive index (dn/dT), and a proven outdoor reliability with very little degradation. Furthermore, glass can generally be manufactured with high throughput, straightforward, and cost-effective compression or roller molding processes. In this work we present a collaboration with the company Holophane S.A.S. in France that resulted in the manufacturing of several micro-CPV lens arrays of plano-convex lenses with a design tailored to the requirements imposed by the manual compression molding process. A comprehensive experimental characterization has been carried out based on surface metrology and functional measurements with a collimated-light solar simulator, and the results are compared with ray-tracing simulations. The measured optical efficiency at the design concentration of 178X is 79.2 ± 2.5% for the whole array with a best lens efficiency of 82.2%. Furthermore, the best performing array has been used to assemble a demonstrator mono-module, which has been characterized outdoors. The outdoor characterization shows very good thermal and spectral stability of the lens array with an I–V curve fill factor of 84.8 ± 1.9% and an electrical efficiency of 27.2 ± 1.8%. The experience gained with these prototypes will lead to further optimization in the manufacturing process to increase the optical performance, and to a larger scale manufacturing for micro-CPV modules. This works paves the way for the development of high-efficiency robust glass-based micro-CPV modules able to generate maximum energy yield per area in space-constrained applications.