Subterranean Heating & Cooling System (Page 2) Problem was, how to interface it (one method is embodied in the construction details of the SHCS). When the initial thought to use the soil as a heat sink was floating around in the industry, most all engineering types were thinking of conducting the heat of air into the soil. They were just expanding on the original concept of water barrels, rocks and mass in the space. Just let them heat up in the sun, and that would suck up the heat. So we have this thing called conduction as the old school design goal. Choose a good heat sink, and conduct the heat to it. Now, what was missing in this picture was that we are not looking at the ACTUAL heat gain engine - the vapor cycle. The air doesn't actually absorb many Btu’s, it is the water that heats up and vaporizes, and the Btu’s to do that are absorbed into the system as "heat of evaporation". So, looking at conduction transfers we're missing the real point. There was some use in including conductive transfers, but not enough to ever justify taking up space in the greenhouse. What we actually need to do here is get back those Btu’s used in the "heat of evaporation". Now the interesting point here is that physics law tells us that it takes 500 times more Btu’s to vaporize water then it does to raise its temperature one deg F. And it tells us that if you condense water out of the air, you are getting back the "heat of evaporation" as "warm water". So we are looking at a potential heat transfer here that is 500 times more efficient than conduction transfers alone! Of course, that number is never actually met in a system that can be practically set up, but even if there is 10 times more heat transfer, it's still money in the bank. So.... after that bit of long winded pre-amble, let's move on. If we are to zero in on using the "heat of evaporation" gains in reverse, to take advantage of them, we have to somehow condense the water. So we get into another concept - dew point. When water vapor reaches dewpoint, it condenses as a liquid, and it releases all the energy took to cross over the evaporation set point. So what does it take to reach dewpoint? The answer is that we need to introduce enough conductive heat loss to the air so that the vapor temps are lowered so that the air saturates - goes to the highest level of humidity it can physically maintain. Then we take it just 1 deg further. Bam! The vapor condenses, and all the heat of evaporation is released, and it rains warm rain. Now look at this - the dewpoint is dependent on the humidity of the air AND it's temperature. The higher the humidity, the less conductive loss is needed to get there. Hot and steamy - sounds like a greenhouse full of plants doesn't it? So that means the conductive loss we need is minimal. Now for some rules of thumb - dropping normal humidity air by 30 deg F will bring it to dewpoint. (The cold sweaty glass of water experience) In a greenhouse high in humidity, that figure likely drops to 20 deg F. So how do you drop the temperature of vapor saturated greenhouse air by 20 deg F.? Well you are going to need something at least 20 F. deg's lower, that is for sure. And what is the most logical source of this kind of heat sink. Of course it’s the soil - it seldom gets above 60 deg's F on its own. If it is at 60 deg F., then 90 deg F. saturated air should be able to get to dewpoint without too much trouble. So that is the "number" to work with - max soil temp of 60 - 70 deg F., saturated air temps in the 80 to 90 deg. F. range. Sounds like a normal working greenhouse to me! Now, how do we get all that air intimate with the soil so that conduction losses bring us to dewpoint? Enter the SHCS. How much soil to interface is the next question. So some more physics... It takes one btu to raise the temp of one pound of water (close to the same for soil mass) one deg. F. Loose, moist soil weighs in at about 76 lbs/cu.ft., so a yard of it is about 2052 lbs. So, it takes about 2000 Btu’s to raise one yard of soil one deg. In other words, for every square yard of surface, the current SHCS specs (3 layer matrix interfacing with about 3 feet of soil) can suck up 2000 Btu’s or about 150 Btu’s per square foot for EACH single degree change. A 20x30 foot greenhouse (600 sqft) would have to absorb 90000 Btu’s before the soil temp raises one degree! Just in case you need a gauge, a normal household water heater uses 30k Btu’s per hour - this represents the same heat transfer as you'd see in a gas fired water heater running for three hours. Now, experience has shown that the soil temp raises at least 20 deg. above normal in a summer season using the SHCS. That represents 1.8 million btu's of accumulated heat tied up in the soil. Looking at cooling only, that translates to 1.8 million btu's of cooling effect. (not to mention the obvious perk of having all the water recycled back into the soil! Next