Off-design simulation and performance of molten salt cavity receivers in solar tower plants under realistic operational modes and control strategies

Solar irradiation is intermittent, but concentrated solar thermal (CST) plants are typically designed and analyzed solely based on their steady design point. Unlike coal power plants, however, CST plants frequently experience thermal loads well above and below their rated design point, leading to off-design operation for much of the operational year. Importantly, if a latent heat thermal energy storage (LHTES) system is employed, the receiver inlet temperature can vary under these conditions. To date, there is a clear lack of knowledge for how to handle off-design conditions in terms of developing appropriate control strategies to maximize the receiver thermal output and its operational region. In this study, a thermal model was developed and validated that is suitable for design/off-design performance analyses of molten salt cavity receivers in both steady state and transient conditions. The study investigated two control strategies - a fixed receiver flow rate (FF) and fixed receiver outlet temperature (FT) - for their off-design performance in each of two off-design operational modes (storage and non-storage). Solar field utilization (SFU) is variable in non-storage mode, but in the storage mode, it is whether variable or fixed at design point (SFU = 1). The feasible operating region in this study refers to the zone restricted by maximum allowable operational parameters defined based on design point analysis, mainly maximum receiver outlet temperature, maximum flow rate, and maximum receiver surface temperature. Through this analysis, it was found that receiver inlet temperatures above the design point (560 K) degrade the receiver performance in both control strategies and under all operational modes. The results also revealed that the maximum allowable receiver inlet temperature that maintains the receiver operation inside the feasible region could not go beyond 700 K or 600 K with the FF and FT strategies (in the storage mode with variable or fixed SFU), respectively. These values also indicate the charging cut-off temperature for the fluid flowing out in LHTES systems. In the non-storage mode, the receiver inlet temperature is remained constant at design point by varying the SFU over the time. While the design point direct normal irradiation (DNI) was 900 W m(-2), the maximum allowable DNI is 700 W m(-2) and 500 W m(-2) with the FF and FT strategies, respectively. These results motivate a hybrid control strategy that switches between the FF and FT strategies to maximize the performance and the number of operational hours of a CST plant during the day. As a final aspect of this study, off-design receiver efficiency correlations are developed that can be used in any simulation environment to accurately predict receiver performance. (C) 2016 Elsevier Ltd. All rights reserved.