Effects of integration mode of the molten salt heat storage system and its hot storage temperature on the flexibility of a subcritical coal-fired power plant

Abstract

To match the fast-developing renewable energy, it is necessary to improve the flexibility of the existing coal-fired power plants to maintain power grid stability, and integrating heat storage systems to coal-fired power plants is an easy and prospective solution. Four integration modes were proposed by extracting the main or reheated steam to store energy and looping steam back to the low-pressure turbine or condenser. Peak shaving performances under various cases were evaluated based on the first and second laws of thermodynamics. Simulation results showed that the mode of extracting reheated steam and looping steam back to the low-pressure turbine (mode RtLP) is optimal for economy, with the highest round-trip efficiency of 36.25 % and a low discharging time of 0.11 h. From the point of flexibility, discharging time under the mode of extracting main steam and looping steam back to the condenser (mode MtCON) increases to 0.48 h, over 4 times of the one achieved under mode RtLP, while the equivalent round-trip efficiency under mode MtCON is nearly 10 % lower. The minimum power load is achieved under mode MtCON by decreasing power load from 30 % to 14.23 %. Furthermore, peak shaving performance of the integrated system under mode MtCON is enhanced by reducing the hot storage temperature of molten salt. As the hot storage temperature declines from 480 °C to the critical one of 309 °C, the discharging time and round-trip efficiency increase by nearly 2 times and 1.7 times, respectively. This work is expected to provide meaningful guidance for the efficient integration between coal-fired power plant and molten salt heat storage system to achieve the targeted benefits.