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# Seasonal Heat Pump

Giant subterranean seasonal heat pump
 (+4, -2) [vote for, against]

Despite global climate change, it still gets pretty cold in the winter and pretty hot in the summer. Some parts of the world see dramatic temperature swings between summer and winter months. Take Nebraska, for instance. I know that Nebraska is not an easy concept to consider, but it is easily 95 degrees some days in the summer there, and just as easily 18 degrees in the winter.

What if we built two humongous subterranean reservoirs (each about the capacity of Lake Tahoe), lined with thermal insulation? In the summer, water (or some other heat-retaining fluid) would be piped through a network or coil of black sunlight-absorbing pipes on the surface of the ground. That water could probably heat up to about 140 degrees before being piped back underground into the "hot" tank.

Similarly, during the winter, water (or some other cold-retaining fluid, preferably with a lower freezing point than water) would be piped through the snow-covered network or coil of pipes, getting the temperature down to 33 degrees (or less). That water would be piped down into the "cold" tank.

Then, you could use those two temperature extremes to make some sort of energy-creating pump, which I suppose would be at peak efficiency during the spring and the fall. Or, at the very least, use the "hot" tank in the winter to heat buildings in the area, and the "cold" tank in the summer for cooling structures.

I'm not sure we have the technology to build adequately insulated reservoirs that could ensnare the heat (or cold) for 8-9 months at a time.

The water would be conserved in this closed system, and the Earth's seasonal variation would be the only "source" of the power system. If you think about it, wind power is nothing more and nothing less than the result of temperature extremes.

I'm sure the environmental impact of building giant insulated reservoirs would be fought tooth and nail by the Sierra Club, but maybe some existing locations could be modified (e.g., empty gas wells in Texas, salt mines under Detroit).

It just doesn't make sense that we expend energy to HEAT our homes in the winter and COOL our homes in the summer, when we just need to time-shift those thermodynamic properties!

 — thekohser, Jun 15 2006

What would you use to insulate this? Don't forget the equation is U*A*dT, and that A is going to be comparatively huge. I don't think this will work for anything short of a complete vacuum layer with insulated supports. Which means the container would have to be thick and rigid.
 — Worldgineer, Jun 15 2006

Can you forget an equation (U*A*dT) that you never even knew? I'm not saying I'm an engineer. I'm just the idea man. And this is a great idea.
 — thekohser, Jun 15 2006

 Tahoo sounds a bit extreme I think.

 Have you ever heard of that system that pumps water through subterranean pipes and then circulates the water through tubes in the floor?

Well they sound pretty good to me, since the ground a few feet down stays the same temperature within about 2 degrees, all-year long. I think it's something like 64 degrees?
 — BJS, Jun 15 2006

 Sorry, [theko], it sounded like an idea an engineer would have. I think I mean that as a compliment. What the equation means is that heat loss from this reservoir will be equal to the U value of the insulation (the conductivity, or how well the material conducts heat) times the surface area of your reservoir times the difference in temperature between the reservoir and the ground (you have to also do the same thing to calculate the heat loss from the ground to the air).

Actually, thinking about this further you could remove the fluid and just run water loops in the ground, storing heat energy there. You'll still lose a lot of heat, but this would be much more practical than storing a fluid. This is a common design for geothermal heat pumps.
 — Worldgineer, Jun 15 2006

// I'm not saying I'm an engineer. I'm just the idea man. And this is a great idea.//
With your permission, or even without it, [thekosher], I will use this phrase in my next company meeting.
 — methinksnot, Jun 15 2006

 Semi-baked centuries ago. In a number of countries they dug deep pits (10 meters or more deep), which were filled with snow and ice in winter. A small insulated building on top of the pit protected the ice from the rays of the sun.

 During the hot summers, these pits stayed cold enough to store fish, meat and other perishables.

Nonetheless, I think this is such a good idea to reimplement in a high-tech (and hugely scaled-up) way, that I'll give you my bun.
 — Forthur, Jun 16 2006

Impractical.
 — James Newton, Jun 16 2006

 Why not just insulate the temperature reservoir with about fifteen feet of dirt and rock? The average air temperature is going to be the same as the groundwater temperature is year around.

In other words, super insulation is pretty much a waste of time and money. Read up on ground-source heat pumps for year-round use. [-]
 — baconbrain, Jun 16 2006

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