Record high multi-junction solar cell efficiencies need ultrahigh optical concentrations. There is a vast potential for increasing the optical concentrations of photovoltaic cells in comparison to pushing the practical limit of the number of junctions. The theoretical limit of optical concentration is >40000x and currently our record cell efficiencies are <1000x. Optical designs such as that presented by Ceballos et al. are importantly increasing the understanding of ultrahigh (>1000x) concentrator photovoltaics; addressing novel methods for alignment, acceptance angle, thermal management and overall understandings of optical and system efficiencies in this concentration range. Ceballos et al. couple light from Fresnel lenses into fiber optic receivers; combining light beams onto a single multi-junction solar cell. Ray trace simulations suggest a high optical efficiency of 84% is obtainable for this 4096x geometric concentration system, resulting in an impressive effective concentration of 3468x. Importantly, the acceptance angle of 0.61° is obtained for such a high concentration, which needs to be as high as possible to accommodate manufacturing (inherent uncertainties) and working condition boundaries (solar tracker accuracies). High quality manufacturing and alignment of such prototype technology is a challenge at lab scale, hence the reported experimental results by Ceballos et al. obtain 23.1% optical efficiency and 556x effective concentration but with identified assembly errors.
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