Subterranean Heating and Cooling System
solar heated greenhouses)
reprinted with permission of
John Cruickshank, Going Concerns Unlimited
What does "UACT"
stand for? (Pronounced "you act")
Underground Air Circulation Tubing (for a SHCS - 4" thin
wall, corrugated, perforated polyethylene drainage line - commonly called ADS or Advanced Drainage System)
put, how does the SHCS work?
To put it in a nutshell, hot moist daytime air of the greenhouse is circulated
with a fan through underground tubing and then back into the greenhouse cooled and dryer. The result is a greenhouse cooler
in the day, soil that is very warm, and a greenhouse that is much, much warmer at night because of the high soil temperatures.
It makes perfect sense, but we never discovered how great it would work until we did it and noticed that because the air is
so moist and it is dropped in temperature so much, that the phenomenon of dewpoint occurs. Because of this it is "acting"
as a refrigerator using phase change of water rather than freon.
What is the actual
system cycle that is working to our advantage?
First up, how does one cool air? What is going on, actually?
From what I can see here, what we are in a position to do here is entirely based on the fact that it is a living, breathing,
transpiring greenhouse we are working with. In other words, we are looking at a solar powered vaporizing machine.
The plants and soil give up their water as vapor, and to do that they must have some heat (Btu’s - British Thermal
Units) to do it. Some Btu’s for direct evaporation (the main function of the moist soil exposed to the sun and warm
air), some Btu’s for powering the transpiration of the plants (chemical photosynthesis AND physical transpiration create
vapor from solar gain - both processes use up Btu’s). So, we have this air that has used up most of the Btu’s
from the sun to create vapor. If we don't remove those Btu’s, then we have to remove the air. Old school says remove
the air. Now, if its 30c outside, and your house is at 35-40c, old school logic doesn't pay off much - the most cooling
that is physically possible is to get it to 30c, right. Not much good considering that if it's going to take huge fans
in an enclosed space to change that air out every minute or so. Or you have to build a house that is virtually open to the
outside. Any way, maybe that is what will work best for cooling, I don't know yet. That would depend on whether a totally
open greenhouse works for your production plans or not. If it doesn't then we are looking at huge fans if we go old school.
So let’s look at "new school" approach. First, the golden rule when moving heat. If there is no difference
in temperatures to take advantage of, there will be no heat transfer. Heat only moves if there is a "hot" to "cold"
pathway. In other words, to move heat you need to have a heat "sink". Old school knew that, so everybody started
cramming cold sinks into the space, using up expensive floor space in the process - trombe walls, concrete floors, rock storage,
water barrels, etc, etc... With very little consideration of the fact that most houses are sitting on literally tons of heat
sink in the form of subsoil.