One embodiment of the invention is a thermosiphon vessel for storing seasonal environment energy in the ground. The thermosiphon includes: a fluid vessel comprising a heat exchange tube, configured to contain a working fluid, and further configured to receive a fluid inlet and a fluid outlet; a wicking material arranged within the fluid vessel and configured to retain liquid-state working fluid substantially adjacent an inner wall of the heat exchange tube; a fluid level sensor arranged within the fluid vessel and configured to determine the level of liquid-state working fluid therein; and a pump arranged within the fluid vessel and configured to maintain the level of liquid-state working fluid below a threshold level within the fluid vessel. The pump is operable between an energy capture mode to absorb thermal energy from an above-ground heat source and an energy release mode to release thermal energy to an above-ground heat sink.