An analytical and experimental investigation was performed on a heat removal process of the thermal energy storage (TES) capsule, using gelled Glauber's salt. Transient heat flux at the capsule wall was measured for various cooling conditions. In cases where the initial temperature was lower than the saturation temperature of the phase change material (PCM), numerical analyses were performed, approximating the process by a relaxation phenomenon where crystals grew around crystal nuclei. The results of the numerical solution agreed very well with the experimental results.