Thermal performance analysis and optimization of multiple stage latent heat storage unit based on entransy theory

•The applicability of entransy theory on phase change process is discussed.•The equivalent specific heat capacity of PCM is more practical due to piecewise fitting.•The entransy balance equation for packed bed with spherical capsules is derived and simplified.•The storage performance is optimized by the entransy dissipation based thermal resistance. In this paper, a one dimensional transient model for HTF and PCM capsules is developed to predict the cold storage performance of a cascaded cold storage unit. The heat conduction in PCM capsules is considered by applying equivalent specific heat method. Then, the application of entransy theory on phase change process is discussed. The thermal resistance based upon entransy dissipation is also derived as a criterion to optimize the system. Finally, the optimization on stage numbers, solidification temperature offset between first and last layer and dimensionless capsule diameter are carried out. The results show that the entransy theory applies to analyze the thermal performance during phase change process. In addition, the utilization of multiple stage could make the phase change process faster and more uniform. The optimized parameters including stage number of 3–6, solidification temperature offset of 2.5 °C and the dimensionless capsule diameter of 0.05–0.1 are recommended. With the optimized structure, the integral average equivalent thermal resistance can be reduced by 75.7%.