Geothermal cooling with cost estimate

So, I�m sure you�ve heard of the concept of geothermal cooling or heating, and I�m sure the following isn�t any new idea of mine alone.

Well, as you may be aware, the temperature of ground stays fairly constant throughout the year (depending on latitude). I�ve read the numbers vary from 45 to 75 degrees Fahrenheit. For the sake of this article, I�ll just say 60 degrees.

Now, heating and cooling make for large costs, and I am theoretically planning to move to North Carolina, where cooling costs could be significant. One number from HowStuffWorks.com about how air conditioners work states that you need about 30 BTU�s per square foot, so a 1000 square foot house needs 30000 BTU�s which translates to around 3500 watts (or $.28/hour at $.08 per kilowatt). 800 cooling hours per summer would then translate to over $200 of cooling costs. I don�t know if there is any truth in those numbers, but when you consider that a refrigerator consumes so much energy, then an air conditioner must be worse.

So it would be nice to capture this 60-degree coolness that is hanging around for free underground. One solution is to build a basement or underground house and live there, but that is too much effort and has a ton of drawbacks. The other way is to build a geothermal cooling system.

I decided to do a little reading up on this, and of course it�s already been done. You take some plastic tubing, run it into the ground and run it back up. You pump in hot air and out comes cool air. Simple enough. However, there are problems with this method, specifically that the tube gets wet, mouldy, and invites various creatures and pests to live in it. So that method was abandoned. Now, my idea has probably been tried and done before, but I just haven�t seen mention of it� So here it comes�

Basically, you pump water down through the tube instead of air. Water transmits heat better than air anyway (which is colder: sticking your arm into 0 degree air or into 35 degree water?). So basically, you have a large basin of water, then you pump the water down to about 6 or 7 feet underground. There it should take a relatively longish route so that it has time to cool off. Then the water comes back up and goes through a radiator to transfer the coolness of the water to the air. Then the water is returned to the basin and goes through the system again.

So what do you need? A pump to move the water around, many feet of copper piping, a car radiator, a holding tank for the water, and a fan. You could also add antifreeze to the system to avoid it freezing in winter and to increase conductivity.

So how much should such a system cost? Well, you could buy a new radiator for a few hundred dollars, but if you go to eBay, you can get one for around $30 (used and non-leaky). You can get a well pump on eBay for around $50. This would still take some consideration. You might actually need only a very, very weak pump. Well pumps are designed to bring water from underground to ground-level. This system is designed to circulate water. For every gallon that gets pumped up, one gallon goes down. Ignoring friction and temperature changes, circulating the water should take no work (once the water starts moving in a circle, it should never stop). However, friction cannot be ignored, so you need a certain amount of energy for that, and the temperature also presents a problem. Hot water rises, so the cold water would tend to move to the bottom and the hot water to the top, which is the opposite of what we want. So the pump would need a certain amount of power, but from my estimations, very little. A fan can be bought at Walmart for $10. The holding tank should also be cheap, if at all needed. I figure it should be used to allow a little buffer for expansion and contraction in the system due to temperature changes. A standard car expansion tank should do the job for $10 used on eBay.

So far we�re up to around $100. Now we need plumbing. At Lowe�s you can get 50 feet of flexible 1/2 inch copper piping for $36. That should actually suffice. We need about 10 feet for each travel direction (4 feet from the unit to the ground and 6 feet underground), which would leave us 30 feet of travelling room underground. This would give the water enough time to change temperature significantly.

So now it�s time for some calculations. First, how much water does the system hold? Well, the copper pipe holds:
(.5*.5)*pi*(50*12)=471 cubic inches.
One gallon is 231 cubic inches, so the pipe hold just about 2 gallons. The rest of the system holds about two gallons, depending on the size of the radiator and tank. So we�ll have about a four-gallon system. Just a random number, I�d say the water should stay a total of 1 minute in the 30-foot section at the bottom (just a random number, but it seems good). The 30 foot section holds 1.2 gallons, so the thing should pump 1.2 gallons per minute (GPM). A 3 GPM pump I saw on eBay goes at 12 volts and 5 amps, which according to Ohm�s law takes 60 watts. So 20 watts per GPM, so we�ll need 24 watts for the pump.

I made some cooling calculations, but they weren�t that positive. I�d have to see how the cooling works in real life.

Now, the goal is to save money. So, we actually don�t want to pay money. When is it hot? During the day, and specifically when it is sunny. So, you can get solar power during this time. Less sun means less power, but also less heat to cool offer. So, we�ll need around 100 watts of solar power. A pound of solar cells yield 75W, so we�ll take around 2 pounds. 3 pounds of broken solar cells goes for around $60 on eBay (they can be soldered together and they will work).

So for a total of around $250, you can maybe build the entire system including power source. After that it should be basically free (basic maintenance would be required, I suppose).

Well, I should have that implemented in Summer 2007