Enhancing performance of Rankine Carnot battery through cascaded latent heat storage with non-uniform volume allocation

Cascaded latent heat storage (CLHS) presents significant advantages in Rankine Carnot battery (RCB) systems by minimizing exergy losses in heat transfer processes. Nevertheless, the configuration of CLHS volume within RCB remains ambiguous. The current parametric analyses of CLHS primarily address heat storage characteristics without adequately assessing their impact on RCB performance. This study employs numerical simulations to evaluate an RCB system incorporating a three-stage CLHS module (CL-RCB). Two non-uniform volume allocation methods of the CLHS considering phase change in heat transfer fluids are proposed. Among them, the power-based scheme proves superior, yielding enhanced temperature uniformity and higher roundtrip efficiency compared to alternative methods. The three-stage CL-RCB with the power-based scheme improves roundtrip and exergy efficiencies by 27.9 % and 14.0 % compared to the single-stage RCB. Furthermore, the effects of CLHS parameters, such as physical properties of phase change material (PCM) and heat transfer enhancement structures, on the system performance are analysed. It is observed that using PCMs with large latent heat and increasing the utilization temperature range of PCMs both effectively raise the energy density but reduce the temperature uniformity of CLHS, leading to decreased system roundtrip efficiency. This underscores the necessity of balancing higher latent heat and wider temperature utilization against potential efficiency losses. Additionally, reducing the volume fraction of heat storage region and applying copper foam are effective in enhancing heat transfer and improving temperature uniformity in CLHS, which ultimately enhances roundtrip efficiency. These findings offer valuable guidelines for optimizing RCB system performance.