Performance model for two-stage optical concentrators for solar thermal applications

Abstract A performance model has been developed for evaluating benefits associated with the addition of a nonimaging secondary concentrator to a conventional paraboloidal solar dish. The model uses a Monte Carlo ray-trace procedure to determine the focal plane distribution as a function of optical parameters and, by evaluating the trade-off between thermal losses and optical gain, calculates the corresponding optimized concentration and thermal efficiency as a function of temperature, both with and without the secondary. These comparative optimizations, carried out over a wide range of design parameters, show that the efficiency of a two-stage concentrator is always greater than that of a single stage if all other design parameters are the same. For example, for a reference design corresponding to a dish with a focal length to diameter ratio of 0.6 and a characteristic slope error of 5 milliradians operated at a receiver temperature of 1000°C, the optimized efficiency with a secondary is 0.70 compared to 0.59 for the primary alone. At fixed focal ratio, the relative performance advantage with a secondary increases, if either the temperature or the primary slope error or both, are increased, whereas it decreases if they are decreased. However, the advantage remains significant at temperatures above 400°C, even in the “high performance limit” of slope errors

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