Experimental and numerical study of combining encapsulated phase change material to sensible heat storage material in one-tank pilot scale thermal energy storage

International audience ; This work presents the design, experimental and numerical results related to a pilot-scale one-tank (thermocline) thermal energy storage (TES) combining latent and sensible heat storage materials with synthetic oil as heat transfer fluid (HTF). The layer of phase change material (PCM) is used to enhance the thermal performance of the TES. Alumina spheres are used as sensible heat storage materials, while the PCM is NaNO3 (sodium nitrate). The PCM is encapsulated inside 140 stainless steel tubes. The volume of the PCM represents 5.5% of the storage volume. NaNO3 was found safe to use with the synthetic oil at the designated operating conditions in case of leaks in the tank. The influence of the phase change material on HTF temperatures were observed experimentally during charge, discharge and stand-by. A numerical model that couples two one-dimensional (1D) physical methods is developed. It simulates natural convection within the PCM capsules by modifying the thermal conductivity of the material. The enthalpy porosity method is applied to simulate the phase changing behavior and a single equation estimates the liquid fraction of the PCM at each time step. The model is validated from the experimental results. This validation reveals that there is still a high fraction of unsolidified PCM in the tubes during discharge, which indicates that the performance of the combined TES solution is limited by heat transfer within the encapsulation tubes.