Thermodynamic Analysis of a Cascaded Latent Heat Store in a Pumped Thermal Electricity Storage System

In this paper, the feasibility of replacing sensible-heat, packed-bed stores with cascaded latent-heat stores in pumped thermal electricity storage systems is explored through thermodynamic optimization based on exergy and entropy generation. The effects of the total stage number and of the NTU on the temperature distributions, energy, exergy and entropy generation in each stage, and over the cascaded store during the heat charging and discharging processes are discussed. The optimal outlet and melting temperatures of each stage during the heat charging process are both found to follow a geometric progression along the length of the store. It is found that the first few storage stages play a more important role in terms of overall heat transfer, exergy storage and release in both the heat charging and discharging processes. During the heat charging process, the total exergy storage rate increases and approaches its maximum, while the total entropy generation rate decreases and stabilizes as the total stage number and NTU increase. During the heat discharging process, the total heat transfer and total exergy release rates both increase as the total stage number and NTU increase, while the total entropy generation rate decreases. The highest roundtrip energy and exergy efficiencies are 94%-98% and 88%-95% for the investigated NTUs, respectively. The highest roundtrip energy and exergy efficiencies are 73%-98% and 60%-95% for the investigated total stage numbers.