Second-law thermodynamic assessment of cascaded latent-heat stores for pumped-thermal electricity storage

Joule-Brayton cycle-based pumped-thermal electricity storage can develop from a pure electricity-storage function to integrated cooling, heating and power systems. Cascaded latent-heat stores are promising as thermal stores that offer flexible multi-grade heat and cold utilisation. So far, comprehensive investigations of these stores for pumped-thermal electricity storage, particularly through the lens of the second law of thermodynamics, have not been investigated comprehensively. This paper presents a model for cascaded latent-heat stores, and assesses the impacts of tube-side velocity, total stage number and stage area on the exergy performance for the entire store and individual stages. A comparative analysis highlights that the upper limit of roundtrip exergy efficiency in combined heating and power mode surpasses pure electricity-storage mode by 2.1%. Furthermore, cascaded latent-heat stores demonstrate comparable performance to packed-bed and liquid sensible-heat stores in terms of exergy metrics. Notably, in combined heating and power mode, cascaded latent-heat stores improve the upper limit of roundtrip exergy efficiency by 7.5% over packed-bed heat stores. The second-law analysis yields more refined store designs than first-law assessments.