Homemade Geosolar
Remember how I was saying I was hoping to get some energy stored in the thermal mass of the garden bed, so that it could slowly release heat at night? Chewing on that problem reminded me of a video by Geoff Lawton done on a greenhouse in Canada. They keep their greenhouse warm enough to grow things during the winter via a method of storing solar heat in the earth.
It’s called a Geo-solar system. It works by taking hot air from the top of the greenhouse and pumping it underground through a horizontal lattice of pipes, where the heat is dissipated and slowly returns up to the surface over time (the designer said the heat from summer usually continued to rise and warm the greenhouse until mid-January.) I don’t have access to enormous insulated concrete pads or industrial solar powered fans, but I still wondered if I could make a small approximation myself.
This is the biggest bed in there – 7.5 feet by about 5 feet. Not too much space to work with, but perhaps enough to provide us with a few fresh salads during the winter.
I began by turning over the soil – I know, I know! I’m a staunch advocate of no-till agriculture. And from now on, it will be no-till. But it was really compacted and there was a serious network of roots I had to take care of in there first. Plus I had a bunch of amendments, mineralized fertilizers, that enormous tub of compost, and several hefty bags full of coffee grounds from my local Starbucks (thanks, Starbucks!) that I wanted to work in. I know these are all problems that can be taken care of perfectly well with no-till and a bit of time, but see… I’ve already planted my peas, and tomorrow I’ll plant my lettuces, spinach, claytonia, and mache. I didn’t have a lot of time.
But listen. While I was turning over the soil, I took the opportunity to sink in this 16’horseshoe of aluminum dryer vent hose, and then a couple sections of perforated plastic drain pipes on top of those.
The aluminum pipes are sunk as deep as my impatient soul could handle digging (so, not very) so as to sink the heat deep in the soil. Eventually a rigid metal pipe will rise 7-8 feet into the air from each of the outlets; one I will paint flat black and the other I will leave reflective, or maybe paint white. I have this notion, based off the chimneys of some complex solar ovens: If the air in the black pipe heats up more than the surrounding greenhouse temperature it will rise. This will cause a vacuum pulling warm air down the other pipe, which should begin a draft of warm greenhouse air under the earth, passively dissipating heat as it moves. There’s probably a reason this won’t work, though, or someone would have done it before – every geosolar system I’ve seen uses solar fans to push hot air down through the pipes from the ceiling. I haven’t found a small (3″) solar-powered fan yet, or a way to hook up a computer fan to a solar panel… But if I have to, I will. I just want to see if maybe I can do this the easy way first. 
I could definitely see this system failing for two reasons: 1) scorching the plant roots if it works too well. After all the pipes are only sunk a couple feet below ground and might raise the soil temps too much. And 2) if water gets into the pipes and pools there and starts mildewing, the air passing over it would fill the greenhouse with a nasty smelling draft all the time. Gross.
Speaking of water, the two perforated drain pipes lie above the aluminum pipes, only a few inches under the soil. Their purpose is to convey rainwater from where it trickles in (I’ve cut a little channel from the gutter output, into the greenhouse) and give it a pathway to travel all the way across under the beds. The water in them should slowly percolate out and saturate the soil, thus giving the bed a fairly even and thorough moisturizing. Now I’m under no impression that this will completely take care of all watering – not by a long shot! But it should help utilize runoff rainwater in a resourceful way.