House cooling method (will it work?)

My brother is building a house in Mexico but wanted to minimize his electricity bill. An idea I had was based on root cellars, but I wanted to see if such a thing would actually work, and how much it would cost to have put into a house.

Specifically, you would drive regularly spaced water-filled copper pipes straight down into the earth about ten feet under the house, so that the floor of the house would have the ends of the pipes showing. Then of course you would insulate the outside of the house very well. Ideally this would work to try to equalise the inside of the house with the earth below it, which at 10 feet down is supposedly a good 30-40 degrees cooler than outside.

I suspect you’re more likely to equalize the temperature by heating up the ground connected to those pipes. The ground is very good at insulating from heat, which would mean you’d just warm up the ground around the bottoms of the pipes.

Isn’t that the same thing? Any heat gained by the ground is heat lost from the house.

You’re talking about Geothermal exchange. There are commercial systems available. It’s tricky getting the balance right to have consistent cooling; the local school district has tried it and are finding in most cases they’re having to go back and retrofit more conventional coling systems to adequately cool the schools.

Do you mean a geothermal loop system? I thought those were usually a couple hundred ft deep, not 10.

I’m having a hard time figuring out what the water in the pipes is for. If the bottom of the pipe isn’t sealed, then the water will eventually drain away (or rise, depending on the water table). If the idea is to have the water as a heat transfer fluid, then some sort of method of circulating the fluid would be needed, as well as some heat exchanger. And of course, I suspect that you havn’t priced copper lately.

What you are basically trying to do is make some sort of passive ground source heat pump. Rather than trying to reinvent this device, why not just install one with coils of poly tubing buried on site? Then with a well insulated house that combines other energy efficient construction techniques such as window overhangs and shading (including using vegitation), radiant heat barriers in the roofing, etc., the refrigeration unit could be minimally sized. Splitting up the house into zones could further reduce the cooling costs, at the price of an increased first cost for the equipment.

This is essentially the idea behind a heatpump. I think you’ll need something less passive and more dynamic.

Actually the ground isn’t an especially good insulator, but it’s a hell of a heat sink. This is basically a ground source heat pump, except in reverse, where you use the water as a heat exchange medium between the air in the house and the ground. Since the air has relatively little thermal mass compared to the ground, you can dump a lot of heat energy into the ground without appreciably warming it up.

The problem, however, is making the system sufficiently efficient to be worthwhile. The temperature difference between the water and the ground is going to need to be pretty large to get any useful heat transfer, which means a long loop or a second heat exchanger between the water and the air to “concentrate” the heat. A compressible working fluid cycle will be far more efficient, but then you’re basically getting back to an air conditioner. You’re not going to effectively engineer something like this by rule of thumb, and it’s really not going to be efficient in terms of amortizing the capital cost on anything smaller than a commercial installation.

A better idea (assuming that the house is to be located in an arid desert region with large temperature fluctuations between day and night) is to build a large central thermal mass into the middle of the building. It’ll lose heat at night, and absorb it at day, especially if you have good circulation. Another option is the compressed earth/adobe construction that is or at least was common to the region; the massive earthen walls act as a heat sink. This is labor intensive, of course, and mostly limits you to one story without a significant amount of engineering, but the result can be quite effective (in conjunction with other passive solar and insulative methods) in moderating temperature in arid climates.

Stranger

The soil should be tested to determine the proper depth of the wells or length of the horizontal coil loops. Some soils are so poor in this regard that it makes no sense to use it. If the capacity of the ground loop is not enough, then a cooling tower can be added with little trouble. The interior loads should be easy enough to determine that the room units should be properly sized (of course, the bulding occupancy could change requiring additional cooling).

That is actually a part of the school district’s problem, they didn’t completely take into account all the after-school uses their buildings have (meetings, groups, etc.) so they’ve got more load than anticipated. They geothermal was installed by a professional geothermal company and should be adequately sized but, even with the additional loading, the cooling capacity is not what was expected. I’ve been told it works better at the one school that actually has several more shallow systems than at the schools that have a single, much deeper system.

BTW, if you’re interested, here’s an old article rah-rahing the systems before they started having some problems. The last school mentioned, Menchaca (not Menchaccu), is the one were it works very well.

They sunk mine in 6 feet deep, but it’s several hundred feet of tubing. You can do deep if you want, but that’s considerably more expensive (although my back yard would be looking a heck of a lot better right now).

Sage Rat, electricity in Mexico is very heavily federally subsidized. When I lived in Hermosillo back in 2005, I never had a bill over 1000 pesos, even when the outside temperature never dipped below 100.

It will be a single-story house. What is a “thermal mass” in lay-person speak?

It was more a thought-experiment than something important. And besides energy consumption there’s issues of maintenance (given that it can be slow to get things fixed in Mexico) and the dehydrating effect of standard A/C which can be annoying if you’re running it for a long time.

Thermal mass : It’s a bunch (a mass) of something that gets hot and cold more slowly than the surrounding air. You might think of a rock in a bonfire – the rock doesn’t reach the temperature of the air around the flames for a while, and conversely, it doesn’t cool down for hours after the fire is out. If you get the right size of thermal mass, it warms up during the day (by taking in heat from your house, if that’s where the thermal mass is), and slowly releases that heat overnight, in the reverse of what the sun is doing to the house.

Yep. It’s just a big, dense mass that moderates the radiation of heat. Its primary method of heat transfer is radiation, not convection or conduction, and so it works best when you have dramatic temperature differences between day and night, as in a desert, especially high desert. In a climate where temperatures stay hot or cold through the day/night cycle it isn’t all that effective. It needs to be properly sized, though, and made from materials with the right blackbody properties for a 24 hour cycle. This isn’t something to just guess at, especially since there isn’t much you can do to modify it once the structure is built.

Something else you might look at is a solar chimney. Combined with a evaporative water reservoir this might provide a significant amount of cooling for only moderate additional cost, though again you’re going to have to basically build the house around it, so you’ll want to spend time researching and/or getting the proper engineering analysis done before building.

Stranger