This paper proposes an indoor procedure based on charge-coupled device camera measurements to characterize the non-uniform light patterns produced by optical systems used in concentration photovoltaic (CPV) systems. These irradiance patterns are reproduced on CPV solar cells for their characterization at concentrated irradiances by using a concentrator cell tester and placing high-resolution masks over the cells. Measured losses based on the masks method are compared with losses in concentrator optical systems measured by using the Helios 3198 solar simulator for CPV modules.
The irradiance and spectral distribution cast on the cell by a concentrating photovoltaic system, typically made up of a primary Fresnel lens and a secondary stage optical element, is dependent on many factors, and these distributions in turn influence the electrical performance of the cell. In this paper, the effect of spatial and spectral non-uniform irradiance distribution on multi-junction solar cell performance was analyzed using an integrated approach. Irradiance and spectral distributions were obtained by means of ray-tracing simulation and by direct imaging at a range of cell-to-lens distances. At the same positions, I-V curves were measured and compared in order to evaluate non-uniformity effects on cell performance. The procedure was applied to three different optical systems comprised a Fresnel lens with a secondary optical element consisting of either a pyramid, a dome, or a bare cell.
The effect of soiling in flat PV modules has been already studied, causing a reduction of the electrical output of 4% on average. For CPV's, as far as soiling produces light scattering at the optical collector surface, the scattered rays should be definitively lost because they cannot be focused onto the receivers again. While the theoretical study becomes difficult because soiling is variable at different sites, it becomes easier to begin the monitoring of the real field performance of concentrators and then raise the following question: how much does the soiling affect to PV concentrators in comparison with flat panels?' The answers allow to predict the PV concentrator electrical performance and to establish a pattern of cleaning frequency. Some experiments have been conducted at the IES-UPM and CSES-ANU sites, consisting in linear reflective concentration systems, a point focus refractive concentrator and a flat module. All the systems have been measured when soiled and then after cleaning, achieving different increases of 7 S c-I n general, results show that CPV systems are more sensitive to soiling than flat panels, accumulating losses in 7 S c of about 14% on average in three different tests conducted at IES-UPM and CSES-ANU test sites in Madrid (Spain) and Canberra (Australia). Some concentrators can reach losses up to 26% when the system is soiled for 4 months of exposure.
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