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Sunday, July 22, 2012

Geo-thermal Cooling

It's been extremely hot outside this week, and we're suffering a significant drought in much of the country. The chart above shows the levels of streams compared to their historical levels for this time of month. Though some regions are getting a lot of rain (the deep blue dots), much of the center of the country is experiencing drought conditions reminiscent of those over sixty years ago. The Dustbowl, as the period from 1934-1936 became known, was a drought characterized by Roland Dewing as the "most extreme natural event in 350 years."

Back to the heat, I got thinking about the Underground Heating Exchange Systems (UHES) the Chinese developed a while ago. The idea to pump air underground was first patented in the United States in the wake of the oil crisis in the 1970s. But the Chinese picked up on this technology and highlighted it as part of an integrated agricultural approach to feeding the Cold North East.

When I saw the Chinese paper (republished on the UN FOA website), I thought it was amusing that they'd used clay pipes for their experiment. After all, folks in the US typically use plastic drainage tubing for this sort of heat exchange system.

Then I got thinking last night - a problem with the plastic is getting it to simultaneously drain the water that condenses and keep out roots and pests. Clay pipe didn't sound bad, after all. Then the light bulb went off.

What if I used cinder blocks underground for the heat exchange "tubing?" At $1.65 per linear foot, it is more expensive than the HDPE drainage pipe. But I wouldn't have to worry about water draining through the blocks (it would) and I wouldn't be worried about the "tube" getting crushed.

That took me on another little adventure of the mind, as I tried to figure out how deep to run the "pipes" underground. The Chinese used half a meter, but what if one went deeper?

The ground temperature at 30 feet below the surface is constant - which is why caves are cooler than outside in the summer and warmer than outside in the winter. How far below the surface do I need to go to get to that constant heat, I wondered?

Then I found out something really fun at the Build it Solar website. It turns out the temperature profile of the ground lags the air temperature profile. Air temperatures peak in July (in my area), but the soil temperature peaks in August.

Then comes the cool part. That peak heat migrates slowly down towards the earth's heat skin, so that the date of peak heat level at deep depths lags the temperature of the surface soil by weeks and months.

So not only will deep Underground Heat Exchange Systems access a larger thermal mass, the ground around that thermal mass will actually be heating up when the rest of the world is cooling down.

I'm not sure how practical it is for a home owner to dig down 6 feet or more, but this kind of effort would be within the reach of a minor business. I would love to experiment with this next summer, if I can find a lot where I could dig deep...


  1. Good afternoon Meg, are you thinking a long strip of cinder blocks placed side-on so the holes in the middle match? I could see this buried six feet down with HDPE pipe leading to hole column A; a manifold or U-Turn of flexible ventilation tubing to line up with hole column B at the end of the row; and HDPE pipe out to house or green-house. Put a air mover, like a rug dryer on one end or the other (suck or blow) and cool air out in the Summer and warm air (relatively) in the winter. Length of the blocks and the speed of the air moving through the system should determine temperature on exit.
    It might be easier to make it 4 blocks wide and inter-weave the turn-arounds to decrease the resistance a 180 turn would impose, and make the turn-around pipe wider by several inches to ease the turn resistance also.
    Given enough length and width this could prove to be a significant enough cooling to reduce heat in the Summer, just by blowing the cooler air into the lower parts of the house and forcing warmer air up (and hopefully out). The only mechanical piece would be the blower, cheap enough to replace when broken while the first is being repaired.
    Out here in Western WA, cinder blocks run $1.29 each, and in bulk, could be cheaper. A rectangle of blocks 8 feet long (12 blocks) and 8 feet wide (6 blocks) gives 96 feet of run length at 6 feet deep and covered with dirt packed back in place will yield an average temperature of 54 degrees F or 12.2 degrees C at 6 feet deep, year round. And cost for the blocks would be around $100. Add the piping [to and from cooled cinder blocks] at probably less than $50.00 [depending on length] plus the flexible ventilation tubing at Harbor Freight $43 [16 feet to make the u-turns] and blower 8" at HF for $75. That all adds up to less than $300 [tax included at almost 9% here in WA].
    Being the engineer, Meg, how cold would the air get? Would we see a 10 degree drop? 20 degrees? This particular blower runs at 1400 CF/min or 1575 CF/min. A cheaper blower moves up to 300 CF/min.

  2. Hi Lloyd,

    The great thing about the UHES approach is the additional cooling you get in summer due to the condensation of the water as it goes down into the cold tunnel. The SunnyJohn site goes on at length about how much energy the warm moist air loses when the water vapor condenses. So summer cooling is supposed to be significant.

    I was planning to do the whole thing in cinderblock, with maybe four lines of concrete bricks Teeing off a column of air. I suppose I'll have to draw a picture of what I'm thinking.

    The awesome thing about deep and cinder block is the freedom from worrying about the plastic tubing melting or crushing. If I want to have ducting from my rocket mass heater running around under the garden, I can do so without fear of heat from the rocket mass heater melting the UHES channels.

  3. I'm planning my rocket mass stove, and racking my brain for ways to save money. I agree, the cost of metal vent pipe is too much so I've decided to use cardboard concrete forms. I'll encase them in concrete rather than dirt, and let the first few fires burn the paper forms away. After that I will not have to worry about the cardboard. But I must say I also like your idea of using blocks for the below grade flue section. I might change my mind and go with your idea after all.

    I also like your idea to use concrete block in place of the inner chimney, and the use of sand to fill the blocks at the base. You have found a way to save a great deal of cash and make it easy to service.

    You might enjoy my blog.

    Thanks for the great article and these ideas!

  4. Hi Bob,

    I've gotten a number of comments that the cinder blocks will break down from the heat after some number of burns. But I could easily fit 6 inch diameter black stove pipe up through the cinder blocks I am using (they are the 12 inch by 16 inch size. More expensive than simple cinder block, but much less expensive than insulated steel stove pipe.

    I'm actually leaning towards using solar heating rather than the rocket mass heater, although the rocket stove is there if I want fast heat on an overcast day. My hopes of doing a subterranean system this year have been frustrated by lack of hours in the day. Also, I'm far enough south that my pond fish are surviving without even a greenhouse (though they'd no doubt be happier with warmer water).