Mathematical modelling of a latent heat storage: Influence of PCM thermal conductivity and enthalpy-temperature relationship

Mathematical modelling of latent heat storage units is useful to calculate their thermal performance for integration in larger systems. This requires the input of thermophysical properties of the phase change material (PCM) used, but data provided by manufacturers or published in the literature is often incomplete and/or varies between sources. However, it is not clear to what extent an accurate knowledge of these properties, including their phase dependency, is necessary to obtain reliable calculations, especially when air is the heat transfer fluid. This study presents a numerical model for a latent heat storage unit containing PCM in rectangular slabs. The model was validated against own measurements and then employed to assess the effect of the assumed thermal conductivity values and the enthalpy curve shape on calculation accuracy depending on factors such as slab thickness and length, air velocity, PCM type and inlet air temperature. Results show that for thin slabs and/or low air velocity, accurate calculations can be obtained with a constant thermal conductivity within the solid-liquid range, irrespective of the operating conditions and material properties tested. As slab thickness and air velocity increase, conductivity gradually becomes more important. For non-isothermal phase change, the assumption of a linear enthalpy curve is sufficient to obtain accurate performance calculations under the conditions tested.