Optimized strategy for upgrading single-stage to cascaded latent heat thermal energy storage devices: Structural design and phase change temperature matching

Cascaded latent heat thermal energy storage (CLHTES) devices offer high heat transfer efficiency and storage capacity. However, existing studies mainly focus on structural design or parameter optimization for fixed phase change materials (PCMs) combination, with limited guidance for transitioning from single-stage to cascaded systems. Based on the heat transfer characteristics of a single-stage latent heat thermal energy storage (SLHTES) device, this study proposes a novel structural design method for a three-stage CLHTES and systematically analyzes phase change temperature gradients to guide the selection of the remaining composite phase change materials (CPCMs) and maximize thermal performance. Results show that positive temperature gradients promote charging but hinder discharging, while discharge-oriented temperature matching significantly enhances useful energy output and efficiency. Temperature gradients exhibit dual effects: a low gradient (ΔTm ≤ 20 K) increases useful energy and energy efficiency by 20.7 % and 12.9 %, respectively, whereas a high gradient (ΔTm ≥ 40 K) improves thermal uniformity and shortens cycle time by ∼30 %. A new performance index, TPI, is introduced to guide CPCMs matching. Compared with a single-stage system, the proposed cascaded design reduces charging-discharging time and improves useful energy output and energy efficiency by 25.5 %, 12.5 %, and 12 %; relative to an equally divided cascaded system, improvements of 3.6 %, 5.8 %, and 2 % are achieved. This work provides design guidance for upgrading single-stage systems to cascaded structures.