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 external seed crystal reservoir trigger assembly at least partially positioned 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, actuation of the external seed crystal reservoir trigger assembly to an open state 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.