Performance study of a supercritical carbon dioxide energy storage system with non-uniform graded compression heat recovery

Compressed energy storage systems play a crucial role in the widespread adoption of renewable energy, effectively addressing the unpredictability and intermittency of renewable energy. Among these systems, compressed supercritical carbon dioxide systems represent a novel category within the realm of energy storage solutions. To enhance the utilization of low-quality compression heat, this study introduces an approach involving staggered correspondence between compression heat and regenerative heat utilization. Building upon this concept, a coupled heat pump-type energy storage system is developed without needing an external heat source. A nonuniform compression and expansion ratio optimization strategy is presented, based on the staggered heat utilization method. This method considers the utilization of compression heat in graded stages. On this basis, from the thermodynamic parameter perspective of the stage design, compression and expansion ratio, the thermodynamic characteristics are analyzed using round-trip and thermal efficiency as key metrics. Results indicate that optimal compression and expansion ratios should prioritize first-stage compression and second-stage expansion. Notably, implementing a first-stage compression ratio of 4.5 and a first-stage expansion ratio of 1.5 respectively enhances round-trip efficiency by 10.2 % relative to the initial design conditions (69.0 %) and the maximum efficiency up to 80.1 %.