Experimental testing of SiNx/SiO2 thin film filters for a concentrating solar hybrid PV/T collector

Achieving high temperature thermal outputs from concentrating photovoltaic/thermal (PV/T) systems presents a challenge in that the performance of the PV cells declines with increasing temperature. Spectral beam splitting is an attractive approach to address this conflict by thermally decoupling the PV and thermal receivers, allowing the PV cells to operate at low temperature and the thermal receiver to operate at high temperature. In this study, SiNx/SiO2 multilayer thin film filters were designed and fabricated to act as beam splitting devices in a 10 sun, linear Fresnel mirror-based, concentrating PV/T solar collector. In this collector, reflected light is directed to a silicon PV cell whilst the transmitted light is directed to a thermal receiver. Plasma-enhanced chemical vapor deposition (PECVD) was used to fabricate the filters which were designed to obtain maximum hybrid output. The resulting devices have high reflectance (greater than 95%) for light between 713 and 1067 nm and high transmittance (greater than 90%) for sunlight outside that reflection window. The concentration of process gases in the PECVD reactor was varied in order to reduce undesired absorption at short wavelengths –lower than 650 nm– by the SiNx layers. Indoor testing was carried out for the filters in a system which consists of a Si PV cell, a thermal sensor, and a solid-state plasma light source (6500 K black body spectrum). This study tested filter performance for various angles of incidence (AOI) between 20 and 45°. The experimental results indicate that the PV cells, illuminated with the reflected light from the filters, operate on average at 9.2% absolute higher efficiency than the same cells without the filter. Furthermore, for the best filter, in terms of relative percentage, the measured hybrid output (weighted by a worth factor of electrical vs. thermal energy) is ∼9% higher than the electrical output of a PV cell stand-alone system exposed to the same light source. This paper represents the first study of a hybrid PV/T solar collector using SiNx/SiO2 thin film filters and demonstrates the feasibility of such systems. This study also indicates that this type of system can utilize 85.6% of the incoming solar spectrum based on the measured optical properties of the filters.