Optical Design for Stationary Solar Concentrators

University dissertation from Faculty of Engineering, LTH at Lund University

Abstract: Solar electricity is one of the most promising technologies for our future electricity supply. By using concentrators, it is possible to reduce the cost of generating photovoltaic electricity. This thesis discusses how to design stationary low concentrating systems for photovoltaic or PV/Thermal applications. The first chapters briefly explain the optics of solar energy concentrators. The theoretical maximum concentration ratios of two dimensional and three dimensional systems were derived using the concept of étendue conservation and a review of current concentrators was presented. In order to improve existing concentrators, it is important to identify the most significant losses. This was done by characterization of an asymmetrically truncated CPC fitted with standard solar cells. The non uniform irradiance distribution on the cells was identified as the single most important reason for electrical losses. To address the problems of non uniform irradiance distribution, a structured reflector was introduced in the characterized system. The structured reflector created a more homogeneous light distribution on the cells, but because of larger optical losses, it was difficult to show any improved performance. It was expected that the more uniform distribution would improve the annual output, but to what extent was difficult to estimate. A new simulation based method for evaluation of photovoltaic concentrators was therefore developed. It consisted of three steps, optical simulations of the concentrator, electrical simulations to evaluate how the light distribution affected the output, and finally annual simulations to get an estimate of the annual electrical output. Using the new method, two new concentrators were developed. One of the systems was intended for roof integration, and the other for wall integration. Both systems were fitted with structured reflectors. The concentration ratio of both systems was increased compared to their references in order to utilize the optimum potential of the structured reflectors. It was shown that the roof concentrator would yield 191 kWh per m2 solar cells. This was 20% higher than the reference system. The wall concentrator was estimated to generate 213 kWh per m2 solar cells, which was 10% higher than the reference wall concentrator. Measurements on the newly developed roof concentrator showed that the more uniform irradiance distribution and increased concentration ratio increased the electrical output in the meridian plane. However, because of low manufacturing precision it was difficult to demonstrate this for all angles of incidence. The last chapter of the thesis discusses the advantages and disadvantages of possible changes to stationary photovoltaic concentrators. The chapter ends by defining a set of rules on how to design stationary concentrators with standard cells for maximum annual electrical output.