Numerical study of the transient behaviour of a thermal storage module containing phase-change material

In this paper, the transient behaviour of a thermal storage module is studied numerically based on the enthalpy transforming method. The module is composed of a concentric tube, in which the annulus contains the phase-change material (PCM) and the inner tube carries the heat transfer fluid. The full set of governing equations accounting for heat and fluid flow in the inner tube, the change of phase of the PCM in the annulus, and heat conduction in the walls are solved simultaneously as a conjugate problem. A parametric study, based on three different PCMs, one with hypothetical constant properties and the other two with variable properties, is conducted to investigate the effects of outer wall thickness and variable thermo-physical properties of the PCM on the performance of the storage module during the phase-change process. It is observed that property variation effects become more pronounced as the Stefan number increases. Interestingly, more latent heat can be stored at a given time as the thermal diffusivity ratio is increased for both constant and variable property cases. This can prevent the over-sizing of the system based on constant property data. Another feature of considerable interests is the delay in the charging time: the time required for the PCM to store a certain amount of energy when the wall thermal resistance is included. A sample set of predicted data shows an error of at least 22 and/or 7.7 per cent when the property variation and/or wall thermal resistance is neglected, respectively.