SHCS: Subterranean Heating and Cooling System

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SHCS (Subterranean Heating and Cooling System) is an innovative approach to optimized solar greenhouse heating and cooling that uses phase-change for heat exchange, just like a refrigerator.

By slowly circulating all of the hot, moist daytime air of the greenhouse down underground where it is always cooler than the greenhouse air, the SHCS design forces the vapor to condense. By doing so, the solar heat as well as the chemical heat from the plant photosynthesis that was required to evaporate the moisture in the first place is forced into the soil. By inducing this "dewpoint" condition in the soil of the greenhouse, the plant roots are always being bathed in warm, moist conditions - the perfect balance for plants and solar greenhouses. The space is heated by the massive amount of radiating solar heat stored in the soil under the greenhouse.

http://hyperphysics.phy-astr.gsu.edu/hb ... phase.html