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Subterranean Heating and Cooling System Explained 

Back to Guidestone Distro Greenhouse  |  Back to SHCS Greenhouses 
  

Subterranean Heating and Cooling System - SHCS - is an innovative approach to optimized solar greenhouse heating and cooling design that uses the magic of phase-change for heat exchange.  
This proven system can heat your greenhouse for operating costs for as little as 7 cents per square foot per year. like I've done in sunny Colorado Ag Zone 4. Supplementary heating and fan cooling costs are often eliminated!

Examples of different systems in place |  Examples of plans I can provide  |  SHCS Frequently Asked Questions  | SHCS Design and Payback Calculator  |  Slideshow of recent Install | Videos of latest install | Chinese study with all the Physics Formulas | Slideshow of SHCS owner/builds

    Using research from Canadian government studies, Jerome Osentowski of the Colorado Rocky Mountain Permaculture Institute was one of the first to pioneer this approach.  He has consulted for many more projects, soliciting the technical assistance of Going Concerns Unlimited to optimize the designs for his clients.   Since their collaboration began, the system has gained in popularity and efficiency.  Going Concerns Unlimited can now take the primary design goals and quickly refine them into working drawings for clients.  Initial "test of concept" only requires a floor plan of the greenhouse, a few emails, sometimes a few minutes on the phone, and possibly a small fee for the expense of creating working drawings (only if you really need them.)  Most folks don't need drawings from me, I review and comment on their own sketches - they can build the system right away on their own.   Approximate material cost for a full system is only about $1/sq/ft of greenhouse.  That can be lowered to as low as 50 cents if you have fans, thermostats and barrels on hand.  

Do-it-yourself is the practicum of choice - Going Concerns Unlimited is dedicated to setting the course straight for the solar greenhouse grower interested in optimum yields with the absolute minimum operating inputs and system expense.

Phase Change - The Physics behind the system

The magic of phase change from liquid to vapor and back again drives the SHCS.  The system functions like a simple refrigeration system, moving heat from one place to another. But a typical 1200 square foot greenhouses needs only the equivalent equipment and running costs of a single large household refrigerator!  

By slowly circulating all of the hot, moist daytime air of the greenhouse down underground where it is always cooler than the greenhouse air, the SHCS design forces the vapor to condense.  By doing so, the solar heat as well as the chemical heat from the plant photosynthesis  that was required to evaporate the moisture in the first place is forced into the soil.  The "miracle" is that by inducing temperature change over the phase change barrier we have the potential to harness 5 times the energy normally the case if we simply tried to solar heat objects cluttering up your precious greenhouse floor space.  And by inducing this "dewpoint" condition in the soil of the greenhouse, the plant roots are always being bathed in warm, moist conditions - the perfect balance for plants and solar greenhouses.  The space is heated by the massive amount of radiating solar heat stored in the soil under the greenhouse!  The physics behind this effect is fully explained here. As well, the Chinese have done one very thorough analysis of the SHCS principle.  This document explains their approach, contains ALL of the physics formulas for EVERY heat exchange occurring, whether under or above ground, and all of their payback figures for food production.  If you need a better rendering of the math and calculus figures and symbols in their analysis, download and view the original MS Word Document here instead.  

Heating Storage

A small  fan is turned on by a thermostat set about 20 F deg. higher than the soil temperature (in the 75 deg. F - 80 deg. F range.)  As the sun heats the space, the fan comes on and the solar heated air is moved underground, the entire volume of the space moving underground a minimum of 5 times every hour.  Because the soil is so much cooler than the air, regular heat transfer AND dewpoint conditions created underground cause the hot moist air of the greenhouse to give up some it's daytime heat AND moisture into the subsoil of the greenhouse.  The air goes underground through the tubing system hot and moist, and exists right back into the greenhouse cooler and dryer. Each and every cubic foot of air in the greenhouse makes this route past the plants and then through the soil at least 5 times an hour in order to be cost effective.  

Daytime Cooling

Through regular heat conduction and by inducing a phase change while it is underground, the air returning back into the greenhouse is cooler (usually a 30 degree F. drop!) and  some of the vapor has condensed out into the subsoil.  Being dryer, it has the ability to absorb more moisture and so evaporatively cools the space once again, returning back underground to continue releasing and storing heat and moisture, and then returning to continue the cooling effect at least every 12 minutes.  Some Colorado greenhouses don't even need cooling fans with this system in place! The first system I built here was regularly cooler inside in the midday, midsummer heat than it was outside.  We were amazed to walk from 105 degree F blasting Colorado summer heat into a "cool" 95 degree F  greenhouse - with NO exhaust fans - just the little SHCS fan humming along in the background!  Passive ventilation is all that is usually needed, ending dependence on fresh air fans.

Night Time Space Heating

By putting the sun's excess energy underground each day, the warm soil means that only the above ground air chills enough to require heat input at night.  We take the chill off with the very same fan that is used to store the excess heat during the day.  In this case though, the second thermostat turns the fan on when the air temperatures drop enough to approach the low limit you set.  By circulating the now cool air back underground and then back into the space, the air extracts heat from the soil warmed that day and returns as warm air - exactly the opposite of the heat storage cycle, but using the same fan and one more separate night heating thermostat.   Some Colorado greenhouses can easily maintain a Mediterranean environment all year without any supplementary heat required. Yes! - Figs can be grown in AG Zone 4 with no problems at from with cold.

Equipment and Materials

  • Several hundred feet of thin walled 4" corrugated, perforated, flexible polyethylene drainage tubing (commonly called ADS -Advanced Drainage System)  You will need approximately six linear foot of ADS for every 4 square feet of greenhouse space.  Enough separate tubes must be laid out to result in all the air of the greenhouse moving underground at least 5 times an hour.  You do not move the air through one long tube... many short ones!
  • Some 55 gallon plastic drums and 12" to 18" plastic ADS culverts to plug all the tubes into, one for each fan and one for each exhaust location.  The culverts are optional, you can plug all the tubes into the barrels if the installation scenario permits it. Each tube should be the same length to divide the amount of air that each gets evenly and culvert plenums accomplish this the best. 
  • A jigsaw and/or 3/8" shank flycutter and hand drill to cut the holes in the barrels and culverts to accept the tubes.  You don't need perfect holes... it's all buried after all... so the hole and tubing fit need only be dirt tight so you don't get too much falling into the tubing. You can patch up the gaping mistakes with plastic stuffing if yo have to, an then backfill.
  • at least a 6" wide trencher to install the tubing into trenches if you have trenchable soil, otherwise an excavator contractor to install the tubing by removing the subsoil and then backfilling one layer of tubing at a time.  Not all contractors are the same, so make sure they are aware of the issues.  Show them this video to get the idea across clearly.
  • One inline fan for every 1200 square feet of greenhouse. Check www.Grainger.com for inline fans 6" to 14" - see what they look like here. The fans should be sized to move all the air in the greenhouse underground at least five times per hour.  The minimum Cubic Feet per Minute (cfm)  your fan must move is determined by calculating the entire volume of the greenhouse (basically length x width x height or go here for a free calculator link) and dividing that number by 12. 
  • Two 30-100 deg F adjustable remote sensing heating/cooling line voltage thermostats 
  • A tubing plan you can build from, complete with electrical schematic, typical blower plenum diagram and fan sizing schedule to suit the greenhouse size and layout.  

    Free SHCS Design Calculator 

    Examples of plans you might need are here 

    Videos of my most current Installation of a Backfilled SHCS

     

    • You are free to contact Going Concerns Unlimited for any additional services beyond what these pages have outlined.  Should you prefer a customized schedule, drawings and schematics set, as little as $150 gets can get you one.  All you need to supply is a floor plan and the 3D dimensions of the greenhouse in mind and some time describing your plan so we don't get off on the wrong track. Keep in mind that if you understand the system well enough, you can use the numbers I've provided here and also here to design your own system free of charge!  And if you don't understand it well enough a call or an email should set you straight - no charge! 
    • Additional consulting and installations off site available very occasionally too, inquire for rates.

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Going Concerns Unlimited specializes in designing
  Subterranean Heating and Cooling Systems 
for solar greenhouses. 

John Cruickshank at Going Concerns Unlimited

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