Pressure-compensated thermal energy storage module

A thermal energy storage system comprising:

• at least one thermal energy storage module including an enclosure having an internal volume, an incompressible phase change material (PCM) provided within the internal volume of the enclosure, said PCM contracting into a solid state when its temperature falls below a certain temperature and expanding into a liquid state when its temperature goes above the certain temperature, and an annular elastic bladder completely positioned around the PCM within the internal volume of the enclosure and being filled with a compressible material, wherein the PCM pushes against the bladder when it is expanded to the liquid state, which causes the compressible material to be compressed within the bladder and push the bladder against the enclosure;
• and
• a reservoir containing a volume of the compressible material, wherein the compressible material in the bladder is in fluid communication with the compressible material in the reservoir.