Thermodynamic investigation of latent-heat stores for pumped-thermal energy storage

As a large-scale energy storage technology, pumped-thermal energy storage uses thermodynamic cycles and thermal stores to achieve energy storage and release. In this paper, we explore the thermodynamic feasibility and potential of exploiting cascaded latent-heat stores in Joule-Brayton cycle-based pumped-thermal energy storage systems. A thermodynamic model of cascaded latent-heat stores is developed, and the effects of the heat store arrangement (i.e., total stage number and stage area) and fluid velocity in the thermal store tubes as key parameters that affect the heat storage and release rates, as well as the roundtrip efficiency, are evaluated. A pure electricity-storage mode and a combined heating and power mode are proposed and investigated, which allows such technologies to transform from a pure electricity storage system to an energy management system supplying power and multi-grade thermal and cold energy, and also to integrate with external waste heat and/or cold sources. Results show that the roundtrip efficiency of cascaded latent-heat stores is higher in the combined heating and power mode than in the pure electricity-storage mode, and that roundtrip efficiencies ranging from 62 % to 100 % can be achieved in the combined heating and power mode, accompanied by a corresponding pressure loss gradient ranging from 10 Pa/m to 2270 Pa/m. A comparison with packed-bed and liquid sensible-heat stores is also performed, and the results indicate that if these can be well designed, cascaded latent-heat stores can deliver comparable performance in terms of the total heat storage and release rates, roundtrip efficiency and flow resistance loss. Therefore, it is concluded that cascaded latent-heat stores can be considered for use in Joule-Brayton cycle-based pumped-thermal energy storage systems aimed at intelligent energy management for the provision of power and multi-grade heat and cold, if the costs can justify this decision.