Discharging via the low-pressure section of the thermal power plant: a new integrated Carnot battery concept and the performance evaluation

The inferior discharging performance severely limits the efficiency enhancement of the Carnot batteries using heat pump and organic Rankine cycle (HP-ORC). This work proposed a new Carnot battery concept thermally assisted by thermal power plant, which discharges through the low-pressure section of steam turbine instead of ORC. Its energy-conversion characteristics and operational performance are evaluated based on the typical coal-fired power plant (CFPP) and Carnot battery cases. As the results show: (1) The round-trip efficiency of the proposed system (82.30–133.32%) is significantly higher than the HP-ORC (47.27–64.27%) within similar temperature ranges, attributed to the higher discharge efficiency (19.40%). Thereby, carbon reduction of CFPP and energy storage density are also improved. (2) The energy-level promotion of the discharging working fluids is the thermodynamic reason for the better discharging performance, affected by the redistribution of steam fluxes in turbine and the pressure ratio variation of each turbine-stage group. (3) The key constraints are identified to determine the operating scenarios for variable boundary conditions. (4) Improving the hot-tank temperature can almost promote all the performance indicators, different from the dilemma of HP-ORC between efficiency and other indicators; besides, the proposed concept is more suitable for the power plants not capable of extremely-low-load operation.