Small cooling system

[RoySalisbury]

I have recently built a small observatory out at a remote location. It is entirely solar powered but does not produce enough power for A/C or heat. Since it is located in the AZ desert, heat is a big issue. I'd like to be able to maintain about a 10-15 deg F below ambient temperature inside the building.

The area of the building that I want to cool is about 200 sq ft and is insulated on all walls and roof but not the floor. The other 200 sq ft is NOT insulated in any way (but is completely enclosed).

So my initial thought was a simple geothermal exchange system. I dig a trench about 5 ft deep long enough to route about 100' of 1" tube in. I fill that tube with water and using a small pump (powered by the solar array and/or battery bank) to circulate the water. Then I run the water through a coil of copper tubing mounted to a small fan. The air goes over the tubing and blows cool air.

I'm guessing this is all standard stuff. My question is, will it actually work? If so, how well? Will it get my 10-15 deg margin that I am looking for? Also, how much water needs circulated in the tube so that the that exchange actually works. And what size tubing (both underground and above ground copper coil) would be correct?

Any suggestions that anyone can provide would be great. I plan on doing this all DIY, so the easier the better.

Thanks!

Roy

[myocardia]

Hello, Roy. Welcome to Nature2Energy. I haven't heard of anyone trying to do a geothermal exchange system themselves, at least the way that you're attempting it. I have heard of an awful lot of people on solar power using 5" or 6" PVC buried 6 or so feet underground, with a fan pushing air through it. It gives most of the temperature benefits of an underground house (which would be absolutely perfect for your location), without the underground house.

Most people these days are lazy though, and just use an air-to-air mini split heat pump. A 9000 BTU heat pump uses a continuous 300 watts, when run on low power mode. Of course, a 16" house fan uses a whole lot less than that, and that's all that is really needed for the first setup I mentioned.

[RoySalisbury]

The air exchange sounds easier to do, but more expensive for the pipes. Any idea how much is needed? Are we talking 100' of 6" pipe, or more? Any links you can provide of people describing how they have done it? I'm not looking for commercial options, but actual real people that have done it and know how it works in the real world (not a sales pitch by someone looking to make a $$).

Thanks!

Roy

[myocardia]

There is no commercial way to do what I described, but only because nobody is making a "kit" yet. As far as "how much pipe is needed", I couldn't tell you. I'm sure you'd have to hire a civil engineer to tell you, since not only will the square footage of the house/building matter, but also how insulated it happens to be, along with the temperatures involved. I can pretty much guarantee you that it's going to be different in every location, and even different for different buildings in the same location, exactly the same as with air conditioners.

I can tell you this much, though: the more cooling you want or need, the more pipe you will have to buy. I'd personally buy about 3-400 feet of 4 or 5" pipe, if I were going to attempt it. Larger pipe would require less length, though. Maybe 150-200' of 6", but that's only a guess. Realize though that you want the cheapest PVC you can buy. You aren't needing Schedule 80 here.

[RoySalisbury]

Ok.. thanks for the info you have provided. I'll keep looking around for more info on how it works and to build it. I can only imagine that the more pipe you lay, the better the output. I'm assuming that there is not a point of negative return on the cooling, so if I was to go "over kill" on the length of pipe I would not hurt anything.

Roy

[myocardia]

As far as using too much pipe, the only problem would be the wasted money and wasted digging. You would of course be prepared for an extra hot, 125°F Bullhead City, AZ day, though. Realize though that longer lengths of smaller pipe will remove more heat than shorter lengths of larger diameter pipe.* It will because it will be exposed to more ~50° earth. Later, when I have the time, I'll see if I can find someone bragging about theirs on a forum, with pictures.

*making 4" pipe probably the best, with 5" not too far behind it in performance. I'll have to do some digging for performance numbers, if they exist.

[RoySalisbury]

I'm going to have to rent a backhoe or trencher for some other projects, and will have it for a few days. Depending on the depth the trencher will go, doing extra length would not be that big of an issue. However, if I go the backhoe route, then it will take a lot more time. I have unlimited space to dig into (2.5 acres) so if longer length means smaller tube (and hence lower cost) then I just need to find the "break even" cost point on length vs diameter

Roy

[Nicolas_Thomas]

Geothermal heating and cooling is cost effective because it uses energy so efficiently. This makes it very environmentally friendly, too. Geothermal heating systems are quiet and work best in moderate climates.