Numerical analysis on thermal behavior of cascaded thermal storage units using multichannel flat tubes

Developing high-performance latent heat storage units is an effective means of efficiently using solar energy and achieving energy savings and emission reductions. A two-stage phase change thermal storage (PCTS) unit model incorporating multichannel flat tubes (MCFTs) and compact fins is established. A numerical simulation study is conducted by comparing it with a single-stage thermal storage unit (STSU), focusing on the alteration of heat transfer fluid (HTF) inlet temperature and volume flow rate to analyze the heat transfer performance of the cascaded thermal storage unit (CTSU). Moreover, the assessment of CTSU entails the evaluation of phase change material (PCM) melting time, charging capacity, charging efficiency, and exergy efficiency under typical operating conditions for various PCM length ratios and different PCM melting temperature differences. The PCM melting time inside CTSU is reduced by 25 % compared to STSU under typical operating conditions of a temperature of 70 °C and a volume flow rate of 1.5 L/min, while the charging efficiency is increased by 1.09 %. For a PCM length ratio of 1:3, the CTSU demonstrates excellent time matching and achieves a maximum charging efficiency of 84.56 %. When the temperature differential of PCM melting is 7 °C in the CTSU, the PCM completes melting in 26 min, reaching a maximum charging efficiency of 86.10 %.