A thermal energy storage apparatus is disclosed. The apparatus may include a base and fluid flow plates which cooperate with the base to define a cavity; a phase change material contained within the cavity; an extendable extension spring at least partially contained within the phase change material; and end plates which cooperate with the fluid flow plates to define fluid flow channels. The apparatus may include a housing that holds a heat exchanger and phase change material. Inlet and outlet ports allow for the ingress and egress of a heat exchange fluid into the fluid flow channels or heat exchanger. In operation, the extension of the extendable extension spring induces solidification of at least a portion of the phase change material from a supercooled liquid state to a solid state, releasing thermal energy, allowing for the transfer of thermal energy across the fluid flow plates or heat exchanger from the phase change material to the heat exchange fluid.