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hybrid greenhouses: pit, climate battery and geothermal in one structure  RSS feed

 
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My cooperative house based in Salt Lake City is looking to build a greenhouse on my land.  We decided on using a Chinese design for the exterior, with a climate battery, but I keep wondering, there's all these different schools of thought, why not merge the elements that make the most sense from each?  I like the idea of a pit greenhouse especially in our residential neighborhood, so we can grow tall plants without having such a high structure.  I love climate batteries, and geothermal and really want to incorporate them both.  I've poked around the internet for a hybrid greenhouse that uses both geothermal and a climate battery, but cannot find any that have adopted both means of heating.  Is there such a thing?  If not, why doesn't it exist?

Thanks for any and all help.
 
pioneer
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Hi Eric, that sounds like a cool project.  Will you try to grow year round?  Are you in a part of SLC that experiences inversions?  That may be a design consideration vs other mountain west locations with copious sunlight in winter.

Could you further explain what you mean when you say "geothermal"?  Climate batteries seem like geothermal to me.  But then again, so do geothermal heat pumps for houses. 

Welcome to Permies!
 
pollinator
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When you say Chinese style exterior, I think of an insulated vertical North wall, a glazed arch to the South, and an insulated blanket to cover the glazing.
A good  start.

I think insulating the sides of the pit would help prevent the mass of the earth from soaking up all solar gain.
Reflective layers could put the heat and light back towards the floor and plants.

Run corrugated perforated pipes from one side of the pit to another, gathered in plenums at both ends and buried.
Pump air from the highest point in the greenhouse through these pipes to capture heat via condensation.
Build rocket stove, boil water, direct the steam into the network of pipes.
 
pollinator
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Location: Zone 4, SD
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If you have not seen this Alliance, Nebraska greenhouse, you should check out what he has done.  I visited once when there was huge piles of snow on the ground and it was T-Shirt comfortable inside with only a box fan circulating the air from the greenhouse through the ground.

http://www.starherald.com/news/local_news/alliance-greenhouse-uses-geothermal-heat-to-produce-all-yearlong/article_0184cb18-f081-5329-8b54-0e767472216e.html

http://www.citrusinthesnow.com/geothermal-energy.html

 
pollinator
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Good idea, just plan it out to be site specific. can you use the excavated dirt to berm the north side? Is your subsoil stable and dry enough to use vertical holes for heat storage? Likewise with horizontal trenching will it flood with groundwater seasonally? Consider your winter sun angle for how deep in the ground to dig the south wall to avoid shading the plants.
 
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Eric Rich wrote:My cooperative house based in Salt Lake City is looking to build a greenhouse on my land.  We decided on using a Chinese design for the exterior, with a climate battery, but I keep wondering, there's all these different schools of thought, why not merge the elements that make the most sense from each?  I like the idea of a pit greenhouse especially in our residential neighborhood, so we can grow tall plants without having such a high structure.  I love climate batteries, and geothermal and really want to incorporate them both.  I've poked around the internet for a hybrid greenhouse that uses both geothermal and a climate battery, but cannot find any that have adopted both means of heating.  Is there such a thing?  If not, why doesn't it exist?

Thanks for any and all help.



Around here (just north of you, in Idaho), the term geothermal is typically used to describe a means in which to heat floors with hot water. Is that what you are referring to as well?

That said, I have wondered the same thing... using a pit greenhouse with climate battery tech. It might be a good combo in some areas/contexts. Wouldn't be workable with a high water table though, obviously. Excavation isn't cheap either... but, if you had a friend with a backhoe...

I'd build a climate battery greenhouse in a heartbeat if I had room for one. Attended a workshop a few yrs ago and was sold on them. Was picking figs and eating them during breaks taken while pruning a banana tree. At 7,000'! Can't wait to get some property to build my own!

 
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You don't want to use greenhouse as a thermal store.

Thermal storage is proportional to the temperature change.  A store that swings 20 degrees stores twice as much heat as one that swings 10 degrees.

You want even temps for the plants.  Most plants shutdown at temps over 90F, and do little at temps under 45F.

To restrict the swings in temperature you have to use a lot more mass.  Circulate the air through underground pipes.  google shcs greenhouse

You can build a separate heat battery building.  This isn't as bad as it sounds, as you design it to have the maximum temperature swing possible.

Google "Nick Pine" solar heat storage. 
 
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Location: Taos, New Mexico at 7000 ft. - Zone 5
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I think this is a great research question! Sorry, I'm a scientist by training and think this way. I love the idea of trying to synergize methods... very permie-ish, like stacking functions.

I'd look at how each system works separately and then see if they're likely to synergize or compete:

- Pit works by utilizing the relatively stable temperature of the earth
- Geothermal greenhouses work by circulating ever colder nighttime air and putting it into contact with relatively warm, say 50 degree F, earth a minimum of 8' below the ground.
- Climate battery (I spent 2 days a CRMPI with Jerome Osentkowski and saw them work magnificently) has a dual method of working: 1) daytime warm air in the greenhouse is put into contact with cooler soil by circulating it through perforated pipes buried in the soil. Intake is up high where air is warm, return is down low so cooled air (after soil has sucked away some of that heat) can be re-heated. 2) The soil must be damp... water has three times the "heat capacity" of concrete and is superb at storing heat in the soil itself. This is even more ridiculously multifunctional because warmth is delivered directly to the roots!

So... I think the first two of these strategies reinforce eachother... they're using the same approach.

But I worry that mixing the climate battery with geothermal might compete. My logic is all about temps. Somebody already mentioned that heat transfer is proportional to temperature difference. During the daytime, the climate battery and geothermal accomplish the same; they actually cool the air down to prevent crispy leaves. But what happens at night? The climate battery fan continues to run, trying to use heat stored in the soil to warm up the air. I've not done enough research to know what the output temp is at night. But, if you take that heat out of the soil and send it down into the ground which may be colder, at least in the earlier part of the night, that may leave the heat ... in the ground? I suppose if the climate battery outgoing temp is colder than the geothermal air, then at least you'll experience less warming from the earth and the latent heat in the dirt surrounding those deep buried will last longer. The bottom line is that this tries to mix warmed air with warmed air.

I suppose you could just try! And... from an engineering perspective, isn't sometimes simpler more elegant and effective? I think you'd get more bang from your buck by choosing either the climate battery or the geothermal. Both of these systems require separate operating fans driven by electricity. Both require significant labor to install. The geothermal requires either endless hand digging (8 feet down!) or heavy equipment. I had a fantasy at one point about digging a 4 foot pit and digging 4 feet more with a "ditchwitch" to get the 8 feet. But I think that pipe needs to snake around and be long. I have irrigation issues that preclude this, but it might work for you.

I'll leave you with a personal note: I am a workaholic, and I tend to overbuild. Jeez it's exhausting! How nice it would be to put in effort, get decent if not stellar results, and not go crazy. Some refer to this as the "Law of Diminishing Returns".

You can always ask Jerome and his architect partner Michael Thompson. They're at ecosystems-design.com. Good luck and thanks for sharing your question and vision!
 
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Location: Maple City Michigan
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There is such a structure, look inside your head, it's in there somewhere I guarantee it............

  My plan is all the things you described. To further add redundancy for my cold winter nights (zone 6a), I'm going to add a rocket mass heater with the mass being a portion of the "polar" wall and heavy-duty insulated window coverings for nights. All those appropriate technologies together will prove to be a very resilient system. Place the whole thing against the equatorial wall of the house and you can use some of the heat for living quarters plus the produce will be easily accessible............
 
Sherwood Botsford
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Here's the archived web page of the originator of the SCHS system.  https://web.archive.org/web/20110602114926/http://www.sunnyjohn.com:80/indexpages/shcs.htm

https://www.michiganmedicalmarijuana.org/topic/41439-shcs-and-greenhouse-design-faqs/

https://permacultureglobal.org/post_projects/403

http://simplysolar.supporttopics.com/post/subterranean-heating-and-cooling-6679334

***

The key issue here is whether you are looking for diurnal heating/cooling or seasonal.

Dirt runs about 1/3 of the heat capacity of water by volume.  It's a lousy insulator, so stored warmth and coolth tend to spread and blur.

A greenhouse takes about a  24 BTU/degree heating day per square foot per year, based on an effective envelope at R1.  (Double poly is R2, but you have about 1.6 square feet of roof per square foot of floor, plus air exchange.)

So a 30 x 66 foot green house has 2000 square feet.  In my 10,000 degree day climate that will be 2000 * 10,000 * 24 = 480 million BTU.  If you are heating conventionally, this will be about 480 GJ (the way natural gas is sold in Canada.) or 4800 therms (US measure)

Here in Alberta That would be $1500/year at today's prices.


It's not quite that bad.  Even in our winters you get some heat in the winter.  If you can size a system for diurnal exchange you would cut your gas bill to somewhere between 20 and 50% of the numbers above.  so lets look at a typical winter day.


It's cold out there.  It's -40 outside and 60 inside the greenhouse.  100 degree temperature differential.  So 100 * 24 * 2000 = 4,800,000 BTU (1% of our annual heat budget today.)  But we also have 8 hours od daylight.  6 hours of that the sun is high enough to do something. and during those 6 hours we're getting 6 * 600 w/m2 * .32 BTU/watt*sqft*hr = about 1200 BTU.  So on a bitter winter day we are getting half enough sunshine to keep the green house warm.

A cloudy day has the advantage it's not usually as cold, but the sunlight is more dilute.


If the outside temp is only 10 F that gives you a 50 degree temperature difference.  You hold your own.  This is the kind of day the system is made for.

In an Alberta climate you hold your own again if you can make a greenhouse that is R2 instead of R1, but still collects all the sun.  Going to a triple wall polycarbonate gives you effectively R3 for the covering (R2/square foot), but you cut down the sunlight by about 20%

You can get another boost by letting the greenhouse cool more at night.  If you are willing to drop the temp to 40 F the cabbage and kale won't care.  The egg plants will NOT be happy however.  Anyway that gives you another 20%

But -40 days aren't that common.  10F days here are.  So the minimum size of heat storage we want is around 3 million BTU.  This will allow us to average the high and the low on a moderate winter day.


A cubic foot of earth has the same heat capacity as about 20 lbs of water.   If we try to keep the temperature swings in the greenhouse to under 20 degrees , then we need 3 million BTU/20degrees/20 lb = 7,500 cubic feet.  Thats 4 feet deep under the entire greenhouse.

If we multiply that by a factor of 2 we have a safety margin for unrecognized system deficiencies.

If we wanted a seasonal scale system we need to warm up 100 times as much dirt.  That doesn't take 100 times as much pipe.  Since it's seasonal, we can heat or cool on a slower scale, so instead of having pipe every 2 feet in a grid under the green house we can have them every 10 feet, 10 feet down.  For each linear foot of trench you are using 100 cubic feet of earth as thermal mass.  So you need somewhere around 1 to 2 miles of trench.  You don't gain much by having them further apart than they are buried.

Don't think that will fly.

Yes this is vastly over simplified.  You don't need to store all the heat in the summer. It's doesn't need to be a fully seasonal system  Our 7500 cubic foot system worked well down to 10F  Even a modest increase in that gives us storage for a cloudy day after a couple of sunny days.



 
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Eric, did you look into the Alliance, NE geothermal greenhouses that another responder linked?  They grow oranges and tons of other subtropical plants year-round on the Nebraska plain.

I ran across this your post by googling "Taos geothermal greenhouse" because the materials put out by Russ Finch (designer of the Alliance greenhouses) said there was one built in Taos and I am trying to track it down.  I have a building site in Prewitt, NM (90 min W of ABQ just off of I-40, elev. 7200') where I'd like to build, and am trying to locate that Taos greenhouse since I think we'll have a similar environment/similar issues.  If you learn any more about them, I'd love to hear it!
 
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