The design of broadband, wide-angle interference filters for solar concentrating systems

The optical performance of spectrally selective interference filters for wide-angle solar applications is addressed. A radially variable interference filter is proposed that can maintain acceptable efficiency over its entire surface in solar concentrating systems with a wide field of view. By applying ray tracing methods and a mapping technique, the angle at which the greatest proportion of energy is incident can be determined at as a function of position on the filter surface. The resulting angular distribution is used to design the interference filter with a tapered thickness profile for optimum performance according to the changing mean angle. The method has been applied for the optimisation of a spectral beam splitter in a hybrid photovoltaic/thermal central receiver system. Two types of interference filters are compared: the discrete multilayer and the rugate filter. Results for bandpass and bandstop configurations are presented, and the performance and practical issues associated with each solution are discussed.