Desarrollo de elementos ópticos innovadores para sistemas de ultra-alta concentración fotovoltaica. Análisis del impacto de la no uniformidad espacial y espectral

Abstract

The main aim of Concentrator Photovoltaics (CPV) is to replace expensive semiconductor material with cheaper optical devices. CPV is capable of using highly efficient multi-junction solar cells (MJSC) that would be very expensive if concentration was not used. These cells are composed of several subcells with different energy absorption bands connected in series, which allows a better exploitation of the solar spectrum. One aspect of concern when CPV systems operate in real conditions is the impact of non-uniform illumination on the solar cell. This lack of uniformity is generated by the use of concentrator optics or other external effects such as partial shading. These can produce differences in the intensity of illumination on the cell surface, Gaussian irradiance profiles or chromatic aberration effects. In this thesis, it is intended to broaden the knowledge referred to the behaviour of photovoltaic devices when they are highly influenced by this phenomenon. For this purpose, an experimental setup was developed to generate and characterise non-uniformity patterns under controlled laboratory conditions, as well as to electrically measure their impact on a MJSC. In this sense, it was possible to quantify the distribution of absolute and spectral irradiance for the range of the top and middle subcells. From the most relevant results, it is worth highlighting the different behaviour shown by the refractive and reflective optics regarding the spatial and spectral non-uniformities. Subsequently, the non-uniformity effects produced by the shading generated on the concentrator optics on different CPV receivers were studied. The obtained results indicated that shading not only contributes to reducing spatial uniformity, but also introduces additional spectral phenomena. In addition, it was experimentally verified that the units incorporating SOE presented a more stable behavior against shading conditions. In parallel, an optical design at ultra-high concentration (UHCPV) was proposed making use of achromatism and compactness provided by the prototype design in order to improve the competitiveness of the technology. The optical system consists of a reflective concentrator based on the Cassegrain-Koehler design that allows concentrating the rays from four independent and symmetrical optical units onto a single solar cell. Different geometric configurations (2000 - 6000x) at UHCPV were analysed using a ray tracing model. Finally, the selected design, at 3015x, was manufactured and experimentally characterised, obtaining an effective concentration of 938 suns with an angular tolerance to misalignment of AA ± 0.30º and an optical efficiency of 31%.