Thermodynamic and advanced exergy analysis of Rankine Carnot battery with cascaded latent heat storage

Cascaded latent heat storage offers significant advantages in Rankine Carnot battery systems by minimizing exergy destruction during heat transfer processes. However, the system performance and the sources of exergy destruction remain unclear, and the configuration of multiple phase change materials remains ambiguous. This paper investigates a Rankine Carnot battery system integrated with cascaded latent heat storage using energy, conventional exergy, and advanced exergy analysis methods. First, a novel phase change material partitioning configuration scheme is proposed, considering the phase change of the working fluid, significantly improving temperature matching within the storage. The number of stages in heat storage devices significantly affects system performance, with roundtrip and exergy efficiencies increasing by 32.2 % and 15.2 % as the number of stages rises from 1 to 4, though further increases have minimal impact. Advanced exergy analysis on a 4-stage system indicates that the endogenous avoidable exergy destruction of the expander, compressor, evaporator, and condenser constitutes 12.4 %, 12.0 %, 10.5 %, and 7.0 % of the total system exergy destruction, respectively, highlighting the priority components for system improvement. Furthermore, raising the heat storage temperature by 45 °C reduces roundtrip efficiency by 26.5 % while increasing power output by 63.2 %. Increasing the heat pump's evaporation temperature by 30 °C improves roundtrip efficiency by 151.1 % but reduces power output by 33.7 %. Reducing the amount of waste heat and surplus electricity results in a significant decrease in the roundtrip efficiency and exergy efficiency, which is mainly caused by the reduction in the isentropic efficiencies of compressor and expander under partial load conditions. The findings on the priority components for system improvement and the configuration of cascaded latent heat storage provide an effective approach for enhancing Rankine Carnot batteries.