Design and Performance Analysis of Flexibility Promotion Schemes of a Coal-Fired Power Unit Based on Molten Salt Heat Storage and Steam Compression

Abstract

The flexibility of conventional coal-fired units is limited, especially at low loads, making it challenging to adapt to the fluctuation in power generation caused by large-scale wind and solar power stations in the grid. Molten salt heat storage system (MSHSS) is widely considered to expand the output range of coal-fired power units through taking heat from the extraction steam of the steam turbine when less power is required by the grid. In this paper, two types of MSHSS coupling with coal-fired units are analyzed, including four specific schemes. Scheme 1 and Scheme 2 utilize Hitec and Hitec+LiNaK carbonates to store the sensible and latent heat of the extracted main steam, as well as part of the sensible heat of the condensate water. Scheme 3 and Scheme 4 are proposed to further improve the flexibility of the power plant through extracting and compressing the steam at the outlet of the intermediate-pressure turbine (IPT) using a two-stage compressor. Due to the considerable power consumption by the compressors, the minimum power output of the unit is lowered, and its flexibility is increased. Thermodynamic and economic analyses are performed, and the results indicate that Scheme 2 achieves the highest round-trip efficiency (RTE) of 70.40%. whereas when applying Scheme 4, the unit demonstrates the greatest flexibility. When the boiler load is 75%, 50%, and 40%, the minimum output can be reduced from 750MW, 500MW, and 400MW to 233.09MW, 286.90MW, and 294.21MW, respectively. At an electricity price of 45.00 $/MWh, Scheme 1 has the lowest additional expense, with a net present value of 168.69 MM$, and a levelized cost of supply (LCOS) is 56.10 $/MWh for MSHSS.

Keywords: Molten salt heat storage, Coal-fired power plant, Peak shaving capacity, Compressed steam energy storage, Thermodynamic analysis