Geothermal to Heat a Greenhouse
Filed under Ask a Geothermal Expert
geothermal heating for a greenhouse
Q: I have a dry water well borehole that I’d like to use for geothermal / geoexchange to heat my greenhouse.
A: Your borehole can heat and cool a greenhouse of 1,000 to 2,000 sq ft depending on what temperatures you want to maintain. With the geothermal / geoexchange system you can keep the greenhouse mostly “closed”, meaning when it’s sunny, instead of opening the vents and dumping the precious heat, you will cool the plant zone instead and move the heat into the borehole. That night when you want to heat, the warmed borehole will return the day’s heat into the greenhouse. This is where you save lots of money because you’re using the day’s earlier solar gain instead of buying the heat energy from your favourite utility. 
This system can be taken a giant step forward yet by changing the typical outdated heating methods to heating the plant zone instead of the whole space. You’ll need an air handler (a box with a fan and some coils in it) and some plastic ducting that will blow the conditioned air into the plant zone instead of the ceiling space. This will give you much improved plant yields and further energy savings. In essence you are abandoning the upper area of the greenhouse that is normally heated (uselessly) and you’re only heating/cooling/(de)humidifying the place that matters: the plant zone.
We can design and build the geothermal / geoexchange system for greenhouse heating in-house and we have partners who specialize in this new air handling technology.
Hoping this sheds some “warmth” on the subject!
We have a small college and will soon be putting in 20,000+ sqft of double skin poly greenhouse (from dismantling one in a nearby town). Our campus buildings are 25,000sqft. We have 14 acres of land (soil is clay-loam). We also have a 3 million BTU coal furnace (for backup, and natural gas for secondary backup) and coal is free from a mine 25 minutes south of here for the cost of loading and hauling. We can obtain 6″ steel pipe (in odd lengths) at $2.50/ft and have all the furnaces and fans we can use for the greenhouses. We also have about 3,000+ feet of 3″ steel pipe donated to us. We are looking for an effective way of turning the summer heat and long sunshine hours (2450 annually) here into a heat source for the campus buildings and greenhouse. What do you suggest? We think air to ground is probably most likely as it takes less plumbing, however the amount of summer heat is substantial and we think it will take a lot of holes in the ground to absorb that much heat… probably beyond the actual area of the greenhouse.
We are toying with getting a boring machine to put in vertical holes, but need some advice.
Hi Alan, the problem with getting cheap recycled material is that it’s almost never appropriate for the application. You can do a closed loop, full heat-recovery system in your greenhouse but it will require much drilling and than only worthwhile if your ground has no advective flow.
That means that if there is water flowing through your ground, it will carry the stored heat away.
The whole point is that you want to absorb the excess heat in the daytime, store it in the ground, and extract it at night. The same goes for the summer/winter cycle. Plants in greenhouses have critical temperature requirements, not just to grow, but to avoid fungi and disease. If you want to experiment, go ahead; if you want to produce, you have to spend money on the appropriate technology to make money from your crops.