Optimization of stationary nonimaging reflectors for tubular evacuated receivers aligned north-south

Abstract

Several manufacturers are developing solar collectors with tubular evacuated receivers aligned north-south. Adding low-concentration, wide-acceptance-angle reflectors to such tubes allows greater tube spacings, reducing the number of tubes per area of collector. It also improves collector efficiency, particularly for conditions of high ΔT I , such as high temperatures or low light levels. This detailed study optimizes the reflector design for maximum daily energy collection and includes the effects of reflection losses, reflector-receiver alignment errors, variation of selective surface absorptance with angle of incidence on the receiver, and losses through the gap between the receiver and the reflector. Three general conclusions have been reached: The use of optimized nonimaging reflectors with tubular evacuated receivers will increase the energy collection efficiency—particularly for high-temperature and harsh environment conditions. Wide-acceptance-angle reflectors are forgiving to receiver-reflector alignment errors. It is neither necessary nor desirable to design reflectors for undersize receivers in order to compensate for misalignments that result from manufacturing tolerances. The daily energy collection of collectors using these reflectors having acceptance half-angles in a range. near 60° is not a sensitive function of the acceptance angle. Manufacturers' final reflector design decisions will probably be based on technical considerations related to fabrication and assembly techniques and possibly on market-related considerations such as collector appearance.