Optical Design and Characterization of Solar Concentrators for Photovoltaics

Abstract: Stationary solar energy concentrators are a promising option for decreasing the price of photovoltaic electricity. This thesis studies stationary concentrators in PV/Thermal applications. The studied systems are parabolic troughs intended for building integration. The first chapters briefly explain the optics of solar energy concentrators. The theoretical maximum concentration ratios of two and three dimensional systems were derived using the concept of étendue conservation and a review of current concentrators was presented. An asymmetrically truncated compound parabolic concentrator, CPC, for flat roof integration was characterized as an example of a stationary concentrator. The current-voltage characteristics were measured, the optical efficiency was calculated, and the annual thermal and electrical output were simulated. This was done for two systems, one with aluminium reflectors, and one with aluminium laminated steel reflectors. The output estimates show no difference in annual output between the two materials. It was estimated that the bifacial system would produce 37% more electricity than a reference mounted in the plane of the concentrator aperture. The estimated annual output of thermal energy was 145 kWh/m2 of hot water at 50°C. It was concluded that the system should have cells facing both the front and back reflectors to maximize the system performance. The actual output of stationary concentrators with standard photovoltaic cells is often lower than what could be expected. This is due to the highly non-uniform irradiation distribution on the cells created by the parabolic reflectors. Three microstructured reflectors in asymmetric CPC troughs were evaluated using ray tracing in an attempt to homogenize the irradiation on the cells. The simulations show that all three proposed structures reduce the peak irradiance with only small reductions in the optical efficiency. The microstructured reflectors were shown to increase the concentration ratio of the troughs which gives higher flexibility in designing new concentrators. The structured reflector troughs had a high optical efficiency when the sun was in the meridian plane, but the structures lowered the efficiency outside this plane. This was due to the disruption of the translational symmetry when the microstructured reflectors were introduced. It was concluded that structured reflectors are used at their largest benefit if they are applied in new concentrators designed for structures. For the existing designs, only a small input increase can be expected when structured reflectors are used. A new biaxial model for the incidence angle dependent optical efficiency of concentrators was presented. It is valid for translationally symmetric concentrators, flat plate collectors, and planar photovoltaic modules. It models the reflector and the cover glazing separately, and these model functions are multiplied at each angle of incidence to form the system model. The optical efficiency of the reflector was modelled as a function of the transverse angle of incidence. Existing models tend to overestimate the optical losses of the cover glazing, and the proposed model addressed this problem by modelling the optical efficiency of the glazing as a function of the true angle of incidence. The biaxial model was used to estimate the annual output of electricity from a concentrator and the estimates were compared with measurements during two summer months. The comparison showed that the proposed model is a good tool for estimating the output of photovoltaic concentrators.