Performance and economic analysis of steam extraction for energy storage to molten salt with coupled ejector and thermal power units

Abstract

A new thermal power unit peaking system coupled with thermal energy storage and steam ejector was proposed, which is proved to be technically and economically feasible based on the simulation of a 600 MW thermal power unit. Results show that the percentage of exergy losses in the retrofitted system is in the order of condenser, turbine and thermal energy storage system, with the exergy losses in the heat release process accounting for approximately 70 % of the cycle. The condenser and evaporator corresponding to the storage and heat processes account for 60 % of the total exergy losses in thermal energy storage system. The retrofitted system has a maximum cycle efficiency of 70–80 % with low and peak modulation rates of 16.5 % and 11.7 %. Extraction of main steam dominates the peaking rate and cycling efficiency compared to extraction of reheat steam. Increasing the main steam pressure at the ejector inlet increases the low peaking rates by 1.5 %, while reducing the molten salt flow rate per unit peaking depth by 10 t/h, which is essential for the stable operation of the retrofitted system. The economic analysis of the retrofitted system shows that the system operating time is the main factor affecting the payback period compared to the interest rate and starts to be profitable in 3.8 years, with later profitability of up to RMB 25 million/year. Further analysis reveals that two different extractions of reheat steam with the same extraction of main steam will achieve the same payback period, which offers a variety of possibilities for practical operational scenarios. In addition, when the extraction of main steam is <250 t/h, the payback time of the system will exceed 15 years, at which point it will be challenging to meet the economic viability of the retrofitted system.