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This paper presents and analyzes a novel fossil-fuel-free trans-critical energy storage system that uses CO2 as the working fluid in a closed loop shuttled between two saline aquifers or caverns at different depths, one a low-pressure reservoir, and the other a high-pressure reservoir. Thermal energy storage and a heat pump are adopted to eliminate the need for external natural gas for heating the CO2 entering the energy recovery turbines. We carefully analyze the energy storage and recovery processes to reveal the actual efficiency of the system. We also highlight thermodynamic and sensitivity analyses of the performance of this fossil-fuel-free trans-critical energy storage system based on a steady-state mathematical method. It is found that the fossilfuel-free trans-critical CO2 energy storage system has good comprehensive thermodynamic performance. The exergy efficiency, round-trip efficiency, and energy storage efficiency are 67.89%, 66% and 58.41%, and the energy generated of per unit storage volume is 2.12 kW⋅h/m 3 , and the main contribution to exergy destruction is the turbine re-heater, from which we can quantify how performance can be improved. Moreover, with a relatively higher energy storage and recovery pressure and lower pressure in the low-pressure reservoir, this novel system shows promising performance.

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