OPTIMAL SCHEDULING OF PARABOLIC HELIOSTATS AIM TARGETS IN A MINI-TOWER SOLAR CONCENTRATOR SYSTEM

摘要

Solar tower with heliostat mirrors is an established technology for utility-scale solar energy harvesting. The setup has several advantages such as the capability to reach high temperature, modularity and ease of maintenance of the heliostats, containment of the high temperature zone, as well as overall low cost per harvested energy. Downscaling to medium and small scale applications is a desirable goal in order to attract more users of the technology. However, the downscaling often does not turn out economically feasible while using flat mirror heliostats, which are the norm in utility-scale systems. This is mainly due to the need to preserve the number (typically several hundred) of mirrors in order to maintain the solar concentration ratio. Use of parabolic mirrors instead can significantly reduce the required number of mirrors for smaller scale systems, but comes with new challenges. Unlike flat mirrors, the effective focal length of parabolic mirrors changes with the incidence angle causing undesirable variations in the concentration ratio and/or flux distribution at the receiver. To overcome this issue, we propose adjustment of the aim targets of the heliostat mirrors. Instead of aiming at the center of the receiver, aim targets are set as design variables and optimized to reduce undesirable peaks in the flux distribution. A special implementation of genetic algorithm is developed and applied to a case study of a nominal lOkW solar concentrator. Results of the study show significant improvement in flux distribution.