Tapping ground heat in small greenhouse

[PaulK]

Has anyone ever tried to tap geothermic energy on a small and simple scale? I'd like to add a few degrees of warmth to my 16'x24' by 9' high wood frame/plastic enclosed greenhouse over the winter.
We're 35 miles due West of Manhattan. Over the past 5 years, the greenhouse night temperature has not dipped below freezing after Feb 15. Actively decomposing wood chips and leaves [about 2-3"] lightly covered with straw gave me an extra week, i.e. Feb 7th, as last freeze. I'm trying to kick it up a notch by sinking 4' deep 4" diameter PVC filled with coarse sand every 6' to see if it has an impact. If it fails, I'll just pull up the pipe and have better drainage. This is probably a naive and foolish scheme - but the hard-working bacteria and fungi scheme had a surprisingly discernable output.
Any suggestions?
Paul

 

[RoyBrooks]

I have a ground source heat pump which works great here in southern Ontario Canada. When the system was being installed the sales rep talked about trying to get approval to install the heat loop under a septic bed. He said that the bateria would help generate heat and would increase the systems %eff. You could try sinking the tubes around the septic tank if possible to draw off some of the heat generated around it.

 

[NoTrespassing]

I realize it's a lot more work, but putting an earth berm on the north side and both ends seems to increase the growing season by a substantial amount.

Kevin

 

[monkeybreath]

I built a small - about 12' by 16' greenhouse (from a design for a greenhouse built in Canada and for very cold climates) with the sun only able to come in from one side - angled optimally - I then put down about 6" of large rocks (2-3" size) on the ground inside, then in the center working area put bricks down on the rocks (over some sand). I put black plastic barrels someone gave me (probably used about 20 of these - each about 24" high and about 14" in diameter) under the benches I built and which surrounded the greenhouse walls and filled them with water. All of this served to capture the heat from the sun during the day and release it slowly during the night. I don't use it much, but it is not because it doesn't get hot enough. You can hardly stand to go in there during the day and the temperature fluctuation between day and night is greatly reduced. The key is the "heat sink" - having rock and heavy material and water to collect the heat.

 

[Taylortractornut]

My bartenders family is from Minnesota and have a dairy farm up there. They take silage green gound up corn and dump it n a pile with there water troughs around it. It keeps the water thawed all winter. Good green compst shoudl do the same. I built a green house one year nd lefte a truck load of manure piled in the center that kept it warm all year.

 

[MossRoad]

People around here dig a hole about 2 feet deep and fill the bottom foot with horse manure, then a foot of soil. They build a wood frame around it and put an old window on top of it. You can grow cool weather crops like lettuce, carrots, radishes and spinach all winter in it. On really cold days, they throw a tarp over it. Horse manure is pretty safe stuff, because the horse already composted it for you /forums/images/graemlins/laugh.gif

 

[jmc]

Paul,

Thats an interesting question without a simple answer. The heat must be transfered from the deeper ground to your space by either conduction or convection. In either case, the plastic pipe is going to slow it down so you will need more pipe than metal pipe like irrigation tubing.

If the best heat transfer is achieved thru conduction, fill the tubes with the best conductor available. Still, even with solid copper rods, its hard to visualize much heat with the warm ends in only 50 to 60 F ground.

With convection, it seems the best design depends on how fast the heat is conducted into the tube from the ground. If its fast enough, an empty tube would be best for maximum air flow. Otherwise, fill it with a size of gravel to match the air flow through the gravel with the conducted heat transfer between the ground and gravel. With a blind tube, the airflow passage will only be about half of whats available since the other half is the cold air return in a convection loop.

Before going to all that trouble, sink one tube outside and put a container over it with a themometer inside. Put a thermometer in a similiar inverted container next to it (no tube) and compare the temperatures. If the difference is only a few degrees, it probably won't help since there is much more heated space/pipe in your greenhouse.

Is there a way to bury enough pipe horizontally, with both ends vented to the space? Blowing air thru it with a tiny, thermostatically controlled fan would help.

John

 

[PaulK]

Roy,
In the next iteration of the greenhouse, I may abandon my attempt to heat the structure without gas, electric, etc. The use of a fan to conduct ground warmth seems to be the most logical - and proven - way to go. Thanks for the input.
Paul

 

[PaulK]

Many thanks to everyone for their input.
Kevin: You're right about the value of the berm. I have 1-2' of wood chips piled around the edges of the greenhouse. It clearly helps.
Ron: I'm also a big fan of water. I have my central workbench ringed with 100 clear plastic quart water bottles to keep seedlings warm overnight. The 6" of stone on the ground is a new twist for me. I'm not familiar with the properties of a heat sink but I can see the logic of the design. In my case, the ground of the greenhouse is used to grow lettuce and tomatos. I remove half the roof cover and 3 side covers during the growing season. In the fall I close it up and get an extra couple months for the tomatos.
Moss Road: I use that same system for my lettuce. I use hay bales and double 6 mil plastic as a cover. The horse manure is "hotter" - less digested and possessing a better/lower carbon to nitrogen ratio than cow manure - and better for keeping the ground warm. The disadvantage of using hay bales is that even when compressed they're still 12" high. Given the sun's winter angle, you have to make [at a minimum] a 10' wide by 20' long structure to get a 3' wide by 10' long patch of lettuce.
John: You've hit the nail on the head - there's an inherent flaw in my naive assumption that I could get enough throughput via a passive system. I've seen how much tubing has to be burried in order to make such a system work in a house. Whether by conduction or convection, the tapping of heat from the ground is "complex" in that it will need the use of a fan for circulation/movement of air. Relative to other heat sources in place: sunlight, water containers, decomposing organic matter, there doesn't seem to be much to be gained. I have both electric and water [seasonal] in the greenhouse. The reluctance to install a fan is just a quirk, perhaps only temporary, because I view a neighbor's gas-heated greenhouse for her orchids as an irresponsible use of energy.
I have used convection in cooling the greenhouse - so my quest is not abandoned, just given a dose of reality. I cool the greenhouse by a 2" PVC pipe drilled with holes and located along the top ridge of the roof. It is vented outside on each end with a 90 degree elbow about 1' above the roof line. Each end of the pipe has a gate valve - an 8.5' high pain in the neck that has to be accessed infrequently. I have to remove excess heat beginning in mid-March when the temperature a 8' is 100F plus and only 65F in the soil. This scheme works extremely well because of the temperature differential. Far better than any fan. I just need to do some quality thinking an figure how I can do this below the ground.
Again, thanks to all.
Paul

 

[mrcaptainbob]

Geo Thermal heat is an interesting concept. I've seen the horizontal tubes in the pond/lake scheme, as well as the ground heat tubes. You could use coils in the ground filled with a liquid and pump it to a heat exchanger. There's also the vertical tubes. Similar to a water well, they drill them deep enough to get to a water table and use two tubes to flow their 'solution' through. These are around a couple hundred feet deep for whole house use. I would think, that if you could tap a higher water table, say around three or five feet, you could certainly use that as a heat pump. There's a lot of BTU's in that water that would get your greenhouse to where you need it. I'd bet that you could get much of what you'd need by tapping that source.