A fast numerical model to calculate the thermal performance of a heat exchanger with rectangular PCM storage

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

The use of phase change materials (PCM) in heat exchangers for energy storage and release is under research. CFD modelling is extremely expensive regarding computational resources and time. For example, a PCM heat exchanger that would be used in a domestic heating system could reach a size of 0.8×0.6×0.6 m3, which is a lot to apply CFD commercial software. Nowadays, there is also a lack of knowledge about the thermal performance of these systems during the loading and unloading processes. Besides, many phase change substances can be used. This work presents an analysis of the thermal performance of a paraffin that would be used in a PCM heat exchanger of a heating system, being the operating temperatures of the heating fluid around 80 ºC at the supply and 30 ºC at the return. A fast model has been developed. This model takes into account the results of a CFD commercial model, but the knowledge of this model was summarized into useful polynomial equations. These equations, coupled with finite volume equations for the aluminium covering of the PCM and for the heating fluid, were implemented in a Matlab R2012a program and applied to a module consisting of two parallel flat plates separated by 30 mm thickness of PCM, which would be used in a heat exchanger. Two different spatial positions of the module, horizontal and vertical, and two phase change processes, melting and solidification, were studied. The horizontal position is faster than the vertical one, the melting time being nearly twice with respect to the vertical position. The results of the fast model were compared satisfactorily with the results of a full CFD model solved in Ansys Fluent 14.5. For the same boundary conditions in both models, the computational time diminishes from several weeks to few days.