Thermodynamic design and optimization of pumped thermal electricity storage systems using supercritical carbon dioxide as the working fluid

Pumped thermal electricity storage systems are a potential approach to large-scale energy storage, and supercritical carbon dioxide (SCO₂) is a promising working fluid. Therefore, this study designed a SCO₂ pumped thermal electricity storage system based on the reversible Brayton cycle and clarified the characteristics and restrictions of using SCO₂ as the working fluid. Then, a series of improved variants that included recuperation, the elimination of medium–low temperature heat storage, and recompression were developed in a targeted manner. Roundtrip efficiency and energy density were defined as the performance indicators of different configurations, and multi-parameters were optimized by using the Find minimum of constrained nonlinear multivariable function algorithm. This study focused on the influence of the heat transfer matching of SCO₂ and storage medium on system performance. Results showed that the improved configuration using recompression had the highest roundtrip efficiency of 69.38%, which was 33.48%-pts and 14.11%-pts higher than the roundtrip efficiencies of the basic configuration without recuperation and the configuration with simple recuperation, respectively. Exergy analysis was carried out to quantify the exergy losses of components further. The maximum portion of exergy was destroyed in the heat exchangers between SCO₂ and the storage medium in the nonrecuperative system, and in the recuperators in the recuperative systems. Therefore, improving the heat transfer matching of SCO₂ with the storage medium and recuperator performance is the key to increasing efficiency. Finally, different high temperature heat storage materials for the SCO₂ pumped thermal electricity system were compared, and suggestions for the selection of storage medium were provided.