A thermal energy storage system including an enclosure having an internal volume. An incompressible phase change material (PCM) is provided within the internal volume of the enclosure, where the PCM contracts into a solid state when its temperature falls below a certain temperature and expands into a liquid state when its temperature goes above the certain temperature. An elastic bladder is positioned adjacent to the PCM within the internal volume of the enclosure and is filled with a compressible material, where the PCM pushes against the bladder when it is expanded to the liquid state and causes the compressible material to be compressed within the bladder and the enclosure.