Spectral splitting strategy and optical model for the development of a concentrating hybrid PV/T collector

Abstract

The efficient utilization of the solar spectrum has been intensively addressed by solar researchers in the last decades. For instance, many efforts have been made to improve the efficiency of PV cells and to allow their operation at high temperatures and concentration ratios (CR). On the other hand, solar thermal collectors are generally mature technologies which can be selected according to the outlet temperature required. The study of hybrid photovoltaic/thermal (PV/T) collectors using beam splitting has gained particular interest since the photovoltaic cells can be thermally decoupled from the thermal receiver allowing operation at significantly different temperatures; in addition, the PV receiver is illuminated only with the region of the solar spectrum that matches well with its spectral response curve. In this work, a general methodology for the determination of the optimal spectral splitting parameters in hybrid PV/T collectors is presented. The method is applied in the design of a novel PV/T collector which is also modeled in detail using the ray tracing software Zemax 12. The optimal wavelengths for the spectrum division in this collector correspond to 732 and 1067nm, which represents the region of the spectrum that should be directed to the PV cells. Moreover, the method indicates that under this particular partition, 47% more power can be delivered from the collector in relation to a concentrating PV stand-alone system under the same CR. The design of the beam splitting devices was addressed by using SiNx and SiO2 as high and low refractive index materials respectively, in a multilayer thin film filter profile that was included in the ray tracing model. Finally, the ray tracing model suggests that the optical efficiency of the collector is approximately 65% in real operational scenarios and that the concentrated light can be successfully split independently of the light angle of incidence.