Thermal energy storage in subsurface soils can produce both inexpensive capacity and storage timescales of the order of a year. In concept, storing excess ambient or solar heat in summer for future winter use and winter “cold” for summer air conditioning can provide essentially zero-carbon space heating and cooling. An innovative ground coupling using a reversible (pump-assisted) thermosiphon with its high heat flux characteristics, intrinsic to two-phase heat pipes, as an inground heat exchanger is proposed and its performance is evaluated in a series of lab-scale experiments. Extraction and injection of heat from/into the water-saturated sand with a single thermosiphon unit representing a cell in an array of thermosiphons is modeled. These results demonstrate that near freezing point of water, due to weak or no natural convection, heat transfer is mainly due to conduction. Also, due to low energy input requirement for pumping working fluid and high heat transfer potential of the reversible thermosiphon, seasonal thermal energy or “cold” storage can be provided for low energy air conditioning applications.

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