Solar Air Conditioning - Geothermal Cooling

Geothermal Cooling

Earth sheltering or Earth cooling tubes can take advantage of the ambient temperature of the Earth to reduce or eliminate conventional air conditioning requirements. In many climates where the majority of humans live, they can greatly reduce the build up of undesirable summer heat, and also help remove heat from the interior of the building. They increase construction cost, but reduce or eliminate the cost of conventional air conditioning equipment.

Earth cooling tubes are not cost effective in hot humid tropical environments where the ambient Earth temperature approaches human temperature comfort zone. A solar chimney or photovoltaic-powered fan can be used to exhaust undesired heat and draw in cooler, dehumidified air that has passed by ambient Earth temperature surfaces. Control of humidity and condensation are important design issues.

A geothermal heat pump uses ambient Earth temperature to improve SEER for heat and cooling. A deep well recirculates water to extract ambient Earth temperature (typically at 2 gallons of water per ton per minute). These "open loop" systems were the most common in early systems, however water quality could cause damage to the coils in the heat pump and shorten the life of the equipment. Another method is a closed loop system, in which a loop of tubing is run down a well or wells, or in trenches in the lawn, to cool an intermediate fluid. When wells are used, they are back-filled with Bentonite or another grout material to ensure good thermal conductivity to the earth.

In the past the fluid of choice was a 50/50 mixture of propylene glycol because it is non-toxic unlike ethylene glycol (which is used in car radiators). Propylene glycol is viscus, and would eventually gum up some parts in the loop(s), so it has fallen out of favor. Today,the most common transfer agent is a mixture of water and ethyl alcohol (ethanol).

Ambient earth temperature is much lower than peak summer air temperature, and much higher than the lowest extreme winter air temperature. Water is 25 times more thermally conductive than air, so it is much more efficient than an outside air heat pump, (which become less effective when the outside temperature drops in Winter).

The same type of geothermal well can be used without a heat pump but with greatly diminished results. Ambient Earth temperature water is pumped through a shrouded radiator (like an automobile radiator). Air is blown across the radiator, which cools without a compressor-based air conditioner. Photovoltaic solar electric panels produce electricity for the water pump and fan—eliminating conventional air-conditioning utility bills. This concept is cost-effective, as long as the location has ambient Earth temperature below the human thermal comfort zone. (Not the tropics)