Direct air geothermal
by Tim
(Virginia)
Can one create a geothermal direct air system where underground tubing heats up air which can then be used to keep a small indoor space warm?
Is it feasible to have a geothermal direct air system that just takes some of the inside air and pushes it with a fan through a system of hundreds of feet of tubing in the ground, with the air returning to the house either cooler or warmer, depending on the season. I have a small weekend place that I like to keep at 40 degrees or so in the winter when I am away and would like to have a little cooler on hot summer days, so I don't need much variation from the 50 to 60 degree temperature of the ground. It would seem like such a direct air system would be a lot cheaper and simpler than a heat-pump based geothermal system.
Answer from Green Energy Efficient Homes
The problem with a geothermal direct air system is that the amount of heat exchange taking place between the geothermal source and the transport medium is a factor of two variables: the specific heat of the transport medium, and the density of the transport medium.
The specific heat of a substance is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The specific heat of water is 4.186 joule/gram. The specific heat of dry air at 20 degrees Celsius is about 1.005 joule/gram, so air is only able to absorb about one quarter as much heat per unit of weight.
The density of the substance also plays a role. The density of water is 1 by definition (1 cubic centimeter of water weighs 1 gram). The density of air at 20 degrees Celsius and 101.325 kPa pressure is 1.2041 kg/m3, which works out to 1.204 grams per liter, or 0.001204 grams per cubic centimeter.
As you can see, air has a density about one thousandth that of water. As a result, water is 4,000 times more efficient at absorbing heat from the walls of geothermal pipes running underground, than air is. You would therefore need 4,000 times as much piping to get the same amount of heat transer.
Another factor to consider is the risk of radon contamination in a direct air geothermal system. A typical underground geothermal system is a closed loop system, meaning the medium (water or ethylene glycol) in the geothermal loop is used for heat exchange with a heat pump and is pumped back into the ground. I recall from my years living in rural Ohio in the 1980's that radioactive radon gas is a serious concern even in regular old basements, where it can leak from the ground through basement walls and into the living space. Imagine how much more radon gas would enter the living area if you had a large network of pipes running underground that you circulate household air through. At the very least it would be important to install radon measurement devices in the system to ensure you were not exposing yourself to dangerous levels of radon gas.
But the bottom line in my view is that you would be spending a lot more money building much more underground piping, for much less energy return on investment (EROI) than if you were to install a regular geothermal system.
Of course, if your weekend place happens to sit atop a large cavern, you could basically tap into that steady-temperature air for free - but again the radon gas level would need to be assessed.
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