Comparison between enthalpy-porosity and anisotropic methods for phase change materials in domestic water systems

This study presents a numerical investigation of the thermal behaviour of phase change materials (PCMs) during melting and solidification in latent thermal energy storage systems. The system under analysis is a finned tube thermal storage unit for domestic hot water applications, where the PCM is used to enhance storage capacity and improve the overall thermal response of the device. The primary objective is to develop a simplified modelling strategy based on anisotropic effective thermal properties, proposed as an alternative to the conventional enthalpy-porosity method. In the traditional approach, the PCM and metallic fins are modelled explicitly as separate domains. However, given the very small fin thickness relative to the overall computational domain, this strategy requires a highly refined mesh and leads to significant computational cost. In the proposed model, the PCM and fins are treated as a single equivalent solid domain characterised by anisotropic thermal conductivity. Rather than using the solidification and melting model available in Ansys Fluent, the phase change process is represented through an effective specific heat capacity defined over the melting temperature range. This formulation incorporates the latent heat contribution directly into the energy equation, while avoiding the explicit resolution of liquid motion within the PCM. The reference geometry features a compact finned structure with a PCM layer thickness of 1.405 mm, a fin thickness of 0.095 mm, and a total fin pitch of 1.5 mm. Copper pipes are embedded within the equivalent PCM fin region and modelled as solid domains. Transient simulations are performed for both melting and solidification processes. The results demonstrate that the proposed approach reduces computational time from approximately one week to a few hours, while maintaining a physically consistent description of the system's thermal behaviour. Overall, the developed model offers a simplified yet physically meaningful framework for the numerical analysis of fin enhanced PCM thermal storage systems. By replacing the fully resolved PCM fin geometry with an equivalent anisotropic solid domain and incorporating latent heat through an effective heat capacity formulation, the method substantially reduces simulation complexity. Consequently, it represents a potentially valuable tool for the preliminary design and performance evaluation of PCM based thermal storage devices, particularly when both computational efficiency and a reliable representation of the phase change process are required.