Figs and the 'Solar Geothermal Greenhouse'

ingevald(5)October 30, 2008

This is a followup note to my original posting regarding the growing of fresh figs for market in Missouri. I have not had time to process all of the information from the visit nor have I yet had time to research the topic in great depth. I will do my best to explain my observations from my short but impressive visit.

The first of his two 30x100Â greenhouses is filled with figs. It is heated with vegetable oil and is cooled by the raising of a curtain on both sides of the greenhouse (cross ventilation) with an electric motor that runs off of a battery that is charged by a solar panel. The motor drives a worm gear that pulls a cable and pulley system that raises the side curtains (both sides) of the greenhouse, allowing for cross ventilation. Â Â It cools down adequately on hot days.

    His "solar geothermal" hoophouse contains about %25 figs and the rest are vegetables and strawberries. This house is cooled and heated by taking advantage of the thermal mass under the greenhouse - a subterranean heating and cooling system. The earth under the greenhouse was excavated to install two heat exchange systems - two is needed to maintain maximum efficiency. Each system is composed of 16 rows of 4" flexible perforated tubes, spaced 1 foot apart. The 4" pipes connect to a larger pipe (15"??). The two larger pipes connect to a 20" vertical pipe for input and output. (see my basic drawing)

    Air is blown down one 20" pipe and comes out the opposite corner.  During the day the fans run, pushing the the warm and humid air through the pipes.   Mold, fungus, and mildew does not form in the tubes because it is an aerobic system. The nearly continuous air flow plus the perforated pipes contribute to the health of the system.  The perforated drainage pipe allows moisture and heat to transfer between the soil and the pipe.   This also warms up the soil which is good for the roots, especially when front and back end seasonal temperatures are cold.    (He planted some fig cuttings this spring and they are already making figs on their 5-6' stems! See the photo below)

  At nighttime the fans can run and the heat that was stored in the ground becomes available to warm the greenhouse. Warm moist air is produced by the system at night.

There is also a thermostatically controlled exhaust fan mounted on the back wall to pull out excessive heat during the day.  The fans that blow air through the underground grid are also controlled by a thermostat.     I am not sure of the exact temperature levels that are set for day and night time running cycles. (He has a goal to someday run these fans off of solar electricity if I remember correctly)

The coldest that it got last winter was about 2 below zero F. Â Â Â The geothermal system kept the greenhouse around 43 deg during this coldest time. Â That is what really impressed me! Â Â It is like having a Mediterranean microclimate all year.

    The outside of the greenhouse has a thick blue styrofoam insulation skirt that extends about 2 feet outward on the ground level, then extends about 5 feet into the ground. This is more important in the winter time, keeping the frozen ground at some distance from the edge of the greenhouse.   He has a double layer of plastic that is inflated, especially in the winter months.  This adds some insulation value.

The minimum sized greenhouse that will will function this way is 1000 sq feet. Small houses end up with more exposed surface area relative to the amount of underground heat storage.

If you are serious about this type of greenhouse - shop around. There are huge differences in price with little difference in quality in some cases. Also, thoroughly research your plan. Fossil fuel heated greenhouses are wasteful and expensive to run - this alternative design really saves a lot of money and energy in the long term.

This first photo shows the dense fig tree growth in his first greenhouse.

The second photo shows the outside of the first greenhouse with the movable curtain to allow ventilation.

The third photo shows one of two exit vents from the underground system Clean dry fresh air!

The fourth photo show an input fan next to a bed of strawberries.

The fifth photo shows the first years growth from cuttings - now five to seven feet tall and ripening figs. The warm soil makes them grow fast.

The last photo shows the outside of the first greenhouse with the ventilation curtain open. There is a forest of Brunswicks growing outside.

Here are some websites that might shed additional light on Solar Greenhouses.

A) Build it Solar -

B) Permaculture Greenhouse -

C) Calculations -

D) SunnyJohn - interesting array of info -

E) Columbia Tribune - another article about IvanÂs greenhouses -


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Wow Ingevald, great information. I was going to install a green house on my property but I may just wait to install a system like this.

Great info and writeup!

    Bookmark   October 30, 2008 at 1:32AM
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It is a great post.
I had a greenhouse not for a hobb but you have a very ingenious and nice greenhouse.
Many thanks for your post.

    Bookmark   October 30, 2008 at 2:36PM
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I like the idea of a solar green house or geo thermal greenhouse. Excellent posting. Thanks.

    Bookmark   October 30, 2008 at 8:45PM
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Actually, after mulling this over, I have come to an initial conclusion that this design may not be optimal for heat transfer, but I am rusty at my thermodynamics.

If you consider this like a water system the furthest vertical pipes, when heat is being stored (fan direction in) will not be storing the optimal amount of heat because of reduced air flow to the ducting at the opposite of the fan. The same applies when the system is reversed and air is pumped out. Air supply may be preferential at the pipes closes to the fan, which were the pipes that stored the least. I guess I am saying that air pressure in all the vertical pipes will not be for the most part equal which means this system might not be taking advantage of the its heat storage capability. Unless the system can maintain a positive pressure, there will be preferential flow.

Here is a document that I found:

Here is a pic of most common HVAC designs:

    Bookmark   October 31, 2008 at 11:10AM
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Thanks for putting some thought into this concept. I am not able to completely explain all of what is going on with this system - I haven't had the time to dig as deeply as I would like.
Your comment about the positive pressure made me think about this some more. I asked Ivan about the flow and whether or not the air flows equally through all of the tubes. I was concerned about the possibility of a preferential flow thereby bypassing a percentage of of the available heat transfer surface. I have no evidence at this point in time to say one way or another. I remember Ivan or possibly the representative at SunnyJohn saying that if the pipes are the same diameter and same length (running between the two larger [15"] perpendicular pipes - the air would flow equally.
So, this is a good question and one that is critical to get the most out of this type of system. The system on Ivan's greenhouse apparently does function satisfactorily. If there is a way to improve it - that would also be nice to know. Thanks again for your thoughts.
Oh, one other comment - I remember hearing from Ivan that the volume of air pushed by the fan was an important factor. I don't know the numbers on this, but there is an optimum amount. Too much or not enough air flow will adversely affect the maximum potential of the heat transfer from the pipes to the soil.


    Bookmark   October 31, 2008 at 2:12PM
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When you say that he has two grids. Does this mean he has one deeper than the other one on top of it? Or does he have two side by side? It seems like one grid would work how did he come up that two would be needed?

I may try something like this myself but finding information on it is very hard and every system is unique.
What I would like to know is how much space can one of these 4 inch pipes heat depending on length and air speed? Does anyone have this info floating around?

    Bookmark   November 14, 2010 at 5:44PM
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Great thread. The key issue is the static pressure the fan can generate. If it generates sufficient pressure the airflow through all 4" tubes would be the same. That plus the Cu'/min will define the fan and enable the system. a 50' 4" tube will undoubtedly have turbulence to the flow so a high static pressure will be needed. Static pressure needed is proportional to the 4th power of the radius in laminar flow and to the 5th power of the radius in turbulent flow. The fan will not be cheap.

As for how the two are arranged, since each system is 30'x50' and the greenhouse is 30'x100', I imagine they're end to end. Making 1 system 30x100' would probably require a fan 4x as powerful and 4-16x as expensive. (Just a guess)

    Bookmark   October 19, 2013 at 7:36PM
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