Study of vessel shape effect on charge/discharge rates of a silicon-based LHTES system

This work presents a validated computational fluid dynamics (CFD) model able to simulate the solidification/melting process of high energy density phase-change materials (PCM), such as silicon. Storage devices utilizing such PCMs can be used in power-to-heat-to-power (P2H2P) systems, to store heat in high temperature and use it for electricity production. Prior to manufacturing such a device, there is a necessity to scrutinize the complex heat transfer mechanisms occurring during the phase change of the contained PCM and indicate a flexible vessel design that enables quick charge/discharge rates and low heat losses during storage period. The CFD model, which is developed in Fluent v17.0 platform, combines the volume of fluid method with the enthalpy porosity approach and an adaptive local grid refinement method, able to achieve a sharp gas/PCM and solid/liquid PCM interface. In contrast to analytical or other CFD models available in the literature, this one takes into account the PCM volume change during solidification/melting, possible dendrites formation and buoyancy-driven natural convection effect on heat transfer mechanisms. Numerical results unveil that the system charge/discharge rates are highly dependent on the PCM permeability and vessel design. Between five shapes investigated, i.e. cylinder, cube, truncated cone, sphere and cut-off sphere, of volume V_vessel=3.75E-03 m³, the truncated cone is the most suitable solution for a P2H2P application, considering design flexibility, melting rates and heat losses. Actually, the PCM melting time achieved with the cone is 21% quicker compared to sphere and only 6% slower compared to cube; the latter is the optimum geometry, as concerns charge rates, but it should be avoided because of its excess lateral losses during storage period. During discharging, when heat is extracted from the vessel bottom, the truncated cone results in the lowest solidification rates, almost 50% lower than the cut-off sphere, which is the optimum shape in terms of discharge rates. Nevertheless, the truncated-cone discharge rates can be further improved by altering its height, without increasing its volume. In practice, the current model is of high accuracy and can give optimization guidelines for the design of such and similar LHTES systems.