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! in sunny Ag Zone 4.
Supplementary heating and fan cooling costs are often
eliminated!
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 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 coarse
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 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 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 5 times an hour.
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 every 10-20 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 only the above ground air chills enough to require heat input at night. We
take the chill off 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 approach the low limit you set. By circulating
the now cool air underground and then back into the space, the
air extracts heat from the soil 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 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
5 times an hour with each tube having air moving through it at only 2-4
feet/sec. You do not move the air through one long tube... many
short ones!
- Some 55 gallon plastic drums and 12" to 14" 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 about the same length to divide the amount
of air evenly that each gets 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.
- 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 duct booster fan for every 1200 square feet
of greenhouse. Check www.Grainger.com
for inline fans 6" to 12" - see what they look like here.
The fans should be sized to move all the air in the greenhouse underground
five times per hour. The 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
calculator) and dividing that number by 12. You then must go back and calculate to make
sure that with that amount of air being moved, you aren't exceeding more
than 4 feet per sec air speed in EACH tube. You have to use a smaller fan
or put in more tubes to arrive at that goal.
- 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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times since February 1999
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