Assessment of different control strategies to manage cloud-induced transients in central receiver systems using molten salts

Abstract

This paper presents an assessment of different control strategies to be applied in central receiver systems operating with molten salts. The objective is to identify the best control scheme to achieve a threefold objective: (i) to ensure that the outlet temperature of the receiver remains at the highest possible value, (ii) to guarantee that the temperature limit of the working fluid is not exceeded, and (iii) to minimize the temperature gradients induced in the pipes. The analysis is performed in a reference plant whose main specifications are taken from Gemasolar, located in Fuentes de Andalucía (Seville, Spain). The model of the solar receiver and the configuration of the clouds have been presented in previous works by the authors. This work moves one step forward, focusing on the thermal performance of the receiver under different flow management strategies. Two classical control schemes are adopted in this work: feed-forward and feed-back. The latter, improved with a variable under-relaxation factor K, is found to be the best choice in order to guarantee safe operation. It yields a lower maximum temperature gradient than the feed-forward scheme and it also ensures that the outlet temperature is always within the limits that avoid degradation of the working fluid (600ºC). Regarding the worst cloud scenario, Total Shadow results to be the most demanding case requiring complete shut-down of the pumping station, at which point the power plant operates on the storage system only.