Sunday, July 22, 2012
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...