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# Geothermal Regenerative Heat Exchanger

Improved "Earth Tube"
 (+1) [vote for, against]

This idea is a minor improvement in a relatively old idea.

Basically, an earth tube system consists of a long buried pipe, typically buried about 10 feet (3 meters) underground. A fan forces air through this pipe, and into a building.

For this idea, we'll first increase the depth to 20 feet, and use an open loop system (fresh air coming from outside the house, not recirculating), and calculate the yearly average air temperature outdoors (and call this temperature T).

Next (and this is the really important part), whenever the air temperature outdoors is above T, we blow the air through the pipe in one direction, and whenever the outside temperature is below T, we blow the air through the pipe in the opposite direction.

That's it!

It's using dirt in the ground for thermal mass, and it's a regenerative heat exchanger. Thus, the name.

As with any regenerative heat exchanger, sending cold air in results in hot air coming out, and sending hot air in results in cold air coming out. So in summer, when the air outside is hot, we will be getting (free!) cold air for our building, and in winter, when the air outside is cold, will will be getting (free!) hot air for our building.

 — goldbb, Sep 07 2013

What aspect of this is the original invention?
 — rcarty, Sep 07 2013

No radioactive magma at all? Hardly working on bun accrual at all...
 — not_morrison_rm, Sep 07 2013

I think a few details are missing. Where's the input to the house ?
 — FlyingToaster, Sep 07 2013

Air is a terrible heat conductor and I suspect 20 feet is not a sufficient depth to extract a useful heating source from the earth. Could be wrong!
 — daseva, Sep 07 2013

I think I get it. You're setting up a temperature gradient in the pipe, right? I imagine you'll need a H shaped topology, where the horizontal represents the storage tube, the top halves of the verticals lead to the external air, and the bottom halves lead to the house. The part you've left out is that the house draws hot or cold air from these as required. Right?
 — spidermother, Sep 07 2013

 That's what I got, too.

 In effect, in summer you're pumping heat into the earth around the "hot" end (where it's stored for winter), and extracting coolth from the cold end.

 In winter, you're pumping coolth into the earth around the "cold" end (where it's stored for summer), and extracting warmth from the warm end.

I think this makes sense; I'm not sure you need the "H" geometry, if the length of the pipe is sufficient.
 — MaxwellBuchanan, Sep 07 2013

The "H" would allow charging to be independent of consumption. Otherwise, heating or cooling of the house would be governed by the outside temperature - the tail would wag the dog.
 — spidermother, Sep 07 2013

 "Geothermal" typically refers to pipes that circulate a conductive fluid from a climate controlled environent to some depth within the earth, using the earth's ambient temperature as a source of relative cold in the summer and hot in the winter (Think: oceans). There is no need for atmospheric exhaust. A simple loop topology will do, unless I'm severely missing something.

If the depth of the tube is sufficient, there is no need to change current directions as the idea implies. And, air sucks at transferring heat. Again, having trouble understanding the novelty here.
 — daseva, Sep 07 2013

 rcarty, the novel part of the idea is the switching directions of air flow.

 daseva, the word geothermal has at it's roots, geo (meaning earth) and thermal (meaning heat).

 There's no specific requirement that a closed loop be used, or that the intake come from a climate controlled environment -- while there are many such systems, there also exist systems which have an intake from the atmosphere outside of the house.

 Also, there's no specific requirement that the fluid coming out of the system match the earth's ambient temperature -- while existing systems aim for this, my idea aims to do BETTER than that.

 Existing systems get tepid fluid from the geothermal system all year round -- in winter this is warm //relative to the atmosphere//, and in summer this is cold //relative to the atmosphere//, but it's actually roughly the same //absolute// temperature all of the time.

 My idea, on the other hand would get actual WARM (summer-like) air out of the system in winter, and actual COLD (winter-like) air out of the system in summer.

 I'm not sure what you mean by "atmospheric exhaust"... the system's "exhaust" is inside the building.

As for the purpose of changing the direction of air flow, spidermother and MB understood it correctly.
 — goldbb, Sep 14 2013

 The root idea here is a "ground loop" heat pump, which is an absolutely standard way to get extremely efficient climate control by using the stable ground temperature. This substitutes for the negatively correlated air temperature (hot when you need cooling, cold when you need heating).

 It can have problems in an environment where the majority of the system requirements are either heating or cooling, as the ground temperature can be affected.

 This, however, turns it into a stored energy version, which is also a known approach. The problem is that the volume of earth needed is so large that it is typically not viable to do it on a seasonal basis.

Where it may be useable is to increase the heat exchange with the environment in the above mentioned unbalanced states, allowing the ground loop version to be used in a wider range of environments.
 — MechE, Sep 14 2013

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