Optical design of stationary solar concentrators for high latitudes

Abstract: The optical design of stationary solar concentrators for high latitudes has beenstudied. Irradiation distribution diagrams, which are tools for the design ofstationary linear concentrators, have been developed. The annual irradiationdistribution in a north-south vertical plane is asymmetric at high latitudes due toreduced irradiation during the winter. This implies that asymmetricalconcentrators are attractive for high latitudes, that a large optimum collector tilt isrequired and that the degree of concentration can be relatively high, compared to low latitude sites. Planar booster reflectors for solar cell modules and corrugatedbooster reflectors for solar collectors are suggested as two promising techniquesfor increasing the annual performance.Collector heat losses were studied in a CPC (Compound Parabolic Concentrator)collector model. It was found that low thermal emittance of the reflectors and theuse of transparent insulation, such as teflon films, in the collector decrease thecollector heat losses. The low heat loss in CPC collectors is demonstrated in astudy of a 13.6 m2 CPC collector with a concentration ratio of 1.53, which showed25% lower heat loss coefficient compared to an equivalent flat plate collector.The complex incidence angle performance of a low-concentrating CPC with flatabsorber was investigated. The result indicates that the use of factorised incidenceangle modifiers will overestimate the annual delivered energy by about 4-5%.The optical performance of CPCs will change if the reflectors are equipped withlinear corrugations since this will increase the acceptance of radiation in themeridian plane. This is less desirable for stationary concentrators but make theseconcentrators interesting as secondary concentrators for two-axis trackingparabolic troughs.