Thermal Diffusivity Measurements of Phase Change Materials in the Liquid Phase for Latent Thermal Energy Storage

Latent Thermal Energy Storage (LTES) systems utilize phase change enthalpies of different phase transitions. Phase Change Materials (PCM) which are used in LTES systems promise higher energy densities based on higher enthalpy changes during phase transitions. Due to that, a detailed knowledge of the thermophysical properties, especially about the thermal transport is needed to identify the actual energy density but also the thermal transport properties of the observed storage material. As already described in [1] a disc type heat flow Differential Scanning Calorimeter (hf-DSC) is a useful method for measuring specific heat capacity cp(T), phase transition enthalpies ΔHt and cycling stability of PCMs. In the liquid phase, the Laser Flash (LFA) method showed higher uncertainties in the evaluat-ed thermal conductivities λ(T) compared to the Transient Hot Bridge (THB) measurements. Due to that, a detailed investigation of the applicability of the used LFA liquid sample holder system and the influ-ence on the measured thermal diffusivity a(T) in the LFA experiment was conducted by the means of a numerical simulation using Finite Element (FE) calculations. This shown FE model in figure 1 was validated with LFA experiments. The results show that the tem-perature gradient at the edge of the sample differs from the gradient in the centre and propagates to an inhomogeneous temperature field at the top face of the cover steel plate. Due to the thermal mass of the PEEK ring as well as the steel support, heat of the light pulse is consumed by this component lead-ing to lower temperatures at the edge of the sample and the upper cover steel plate. The results have shown that λ(T) of the sample and the focus of the detector have a crucial impact on the measured thermal diffusivity a(T) in the LFA experiment and can lead to a misinterpretation of the actual a(T) of the sample.