Analysis of natural convection on enhanced solar latent-thermal energy storage

Resumen

In this paper, a numerical analysis of the transient heat transfer phenomena during latent thermalenergy storage (LTES) in a PCM container is accomplished. A shell and tube heat exchanger withseven finned tubes and a total shell volume of 5 liters is modeled and meshed using structuredhexahedral elements. In order to reproduce the conjugate heat transfer between the heat carrier fluidand the PCM, the inner tube volume, tube wall and fins were also meshed, considering properrefinement close to the fluid walls and across the regions of high temperature gradients in the solidvolumes. A solidification and melting model is implemented in the shell side to properly reproducethe melting of the PCM during the thermal energy storage process. In order to evaluate the heattransfer from the external finned tube surface to the PCM, the eventual existence of buoyancyinduced fluid motion in the shell side is scrutinized through the implementation of temperaturedependent properties. Correlations for density and viscosity of paraffin wax available in the openliterature were considered, and the impact of the vertical/horizontal orientation of the accumulator isanalyzed. As the LTES system is envisaged for its connection with a solar collectors field, a realisticmodel of the coupling between solar radiation and thermal energy storage is considered: the heattransfer fluid inlet and outlet boundary conditions are coupled with the solar collector area thatprovides the fluid with thermal energy, considering the dynamics of a solar irradiance curve. Thisallows for quantifying the effect of a given solar irradiance curve on the melting dynamics and theappearance of natural convection in the PCM side. The impact of this modelling approach on theevaluation of thermal resistances and computation of charging speed is quantified, using as referencea pure heat conduction melting scenario.