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In our part of New England, the temperature of the soil is about 56 F year 'round. That is good for heating (in a modest way: bringing unheated garage up toward 56 F) and great for cooling in the summer. When we re-did the patio back in 2008, I used the opportunity to bury pipe. 250' coils of PVC are pretty cheap, and they're really tough! I made "lazy loops," partly overlapping, out of three coils. After they tamped down the stone dust where the pavers were going, this is what the yard looked like. The coils are under the soil in just about the entire picture. I lay the loops of pipe something like this, about 4' down. (It was tidier in reality, of course, than these touch-pad-drawn curlicues suggest.) I put in three separate circuits as a hedge against the possibility that any one fails. Of course, with even modest internal pressure, the consequence of leaks is pretty small... you could sill use the circuit. Blockage, on the other hand, might be more of a problem; how to clear it without risking more damage? Anyway, knock wood. The two pictures above are of the heat exchanger - mark I. Two old Ford F-150 radiators and a box fan sit in a washing machine flood pan; the cool water comes from under the back yard, branches through the two radiators, and returns to the back yard. The fan pushes room air through the radiators and that air comes out cooled. Schematic of the cooling set-up: I haven't quantified the system yet... except to know that 80 F air coming in one side of the heat exchanger comes out at about 68 F. Very nice! It seems that for about 150W of pumping and fanning, we're getting a 15 F drop in temperature for the air that passes through. How much air per unit time is that? I don't yet know... Next step: quantify! Then make improvement decisions after that. I think also that while I have it right for the cool water to flow into the radiators in parallel, I think I don't have it right that the air should pass through them in series. More later... when I can back up these hunches with some numbers. Once the cost of energy includes more of the externalities, I'm sure all houses will be built with buried loops in the near future... this is way more efficient cooling than compressor-driven A/C. And that's only the beginning... temperature difference can be used for all kinds of good things.

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