Exergy analysis of solar central receivers

Abstract

A high-resolution method to analyse the exergy of the SPT external tubular receivers is presented, examining the different heat transfer process involved individually. This sheds light on the role that each irreversibility source plays in the outcome, aiding in the receiver design and the facility location selection. The exergy efficiency is around 32% in the base configuration. Besides the exergy loss in the heliostat field, over 40%, it is found that the biggest exergy destruction cause are the radiation emissions and absorptions in the tube outer surface, around 17%. From the remaining ones, the greatest are the exergy destructed in the HTF and the one escaping to the ambient (over 4% each). Then, the exergy balance for a variety of strategies and ambient conditions is performed: optical properties of the tubes coating, peak and flat aiming strategies, DNI and ambient temperature. The heliostat field exergy loss rate only varies when changing the aiming. However, the emission and absorption losses and the ones in the HTF suffer the greater modifications with all the parameters studied. The impact of the optical properties degradation, 1% descent in the efficiency per 5% degradation, would advise repainting works in order to avoid greater exergy destruction. The surroundings temperature modification impacts considerably the exergy efficiency, showing the suitability of locations with low ambient temperature and a moderate DNI: descends of over 0.35% occur every 5 °C increase of the temperature for a fixed DNI.