KEYWORDS: Solar concentrators, Prototyping, Electrical efficiency, Solar cells, Solar energy, Dispersion, Fresnel lenses, Temperature metrology, Sun, Optics manufacturing
Most cost-effective concentrated photovoltaics (CPV) systems are based on an optical train comprising two stages, the first being a Fresnel lens. Among them, the Fresnel-Köhler (FK) concentrator stands out owing to both performance and practical reasons. We describe the experimental measurements procedure for FK concentrator modules. This procedure includes three main types of measurements: electrical efficiency, acceptance angle, and irradiance uniformity at the solar cell plane. We have collected here the performance features of two different FK prototypes (ranging different f -numbers, concentration ratios, and cell sizes). The electrical efficiencies measured in both prototypes are high and fit well with the models, achieving values up to 32.7% (temperature corrected, and with no antireflective coating on SOE or POE surfaces) in the best case. The measured angular transmission curves show large acceptance angles, again perfectly matching the expected values [measured concentration acceptance product (CAP) values over 0.56]. The irradiance pattern on the cell (obtained with a digital camera) shows an almost perfectly uniform distribution, as predicted by raytrace simulations. All these excellent on-sun results confirm the FK concentrator as a potentially cost-effective solution for the CPV market.
Metal grid lines are a vital element in multijunction solar cells in order to take out from the cell the generated
photocurrent. Nevertheless all this implies certain shadowing factor and thus certain reflectivity on cells surface that
lowers its light absorption. This reflectivity produces a loss in electrical efficiency and thus a loss in global energy
production for CPV systems. We present here an optical design for recovering this portion of reflected light, and thus
leading to a system efficiency increase. This new design is based on an external confinement cavity, an optical element
able to redirect the light reflected by the cell towards its surface again. It has been possible thanks to the recent invention
of the advanced Köhler concentrators by LPI, likely to integrate one of these cavities easily. We have proven the
excellent performance of these cavities integrated in this kind of CPV modules offering outstanding results: 33.2%
module electrical efficiency @Tcell=25ºC and relative efficiency and Isc gains of over 6%.
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