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Choose your green weapon

The warm embrace of the Earth or the power of the sun?

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My cousin Eli once ran a solar company. So I wasn't surprised when he told me I ought to include solar thermal panels in my green dream house.

But my friend Ned is a geologist who drills natural gas wells. "Kenny," he said to me, "forget solar. What you need to do is heat and cool your house with geothermal."

It may have been the first time someone actually explained to me how a geothermal system works; it certainly was the first time I realized the two eco-friendly technologies could be pitted against each other for cost effectiveness and environmental benefits.

So which system makes more sense? And which is more likely to become the standard as energy costs and global temperatures rise?

First, let me dispose of one common confusion: Solar thermal systems aren't the same as solar electricity. Both rely on panels usually installed on a building's roof to catch the sun's energy. The latter uses "photovoltaic" panels made of silicon wafers that convert that energy into electricity.

Solar thermal panels are less high-tech and less expensive. They're basically big cases filled with fat glass tubes or narrow pipes that have liquid inside them. The liquid expands when heated and circulates into the house, where it's usually sent through the water tank, to serve as a heating element. Most of the year, in a place like Atlanta, a good solar thermal system can heat the entire tank, so you end up not having to pay for the gas or electricity that would otherwise be required. Some people also use solar-thermal systems to produce radiant heat, but that adds a good bit of expense.

Geothermal systems are based on just as elegant an idea: They use the Earth's constant temperature, which is somewhere in the high 50s, to warm the house in the winter and cool it in the summer.

The contractor puts a hole at least 300 feet deep in your yard or under the house, and stuffs two pipes, which are connected at both ends to form a loop, into the hole. The pipe is filled with a cooling and heating medium that's pumped through the loop when the system is on. A fan pushes air over the top part of the loop and into the house's heating and air-conditioning ducts. If it's cold outside that heat exchange makes the air warmer (and the stuff in the tube colder); if it's hot outside, the air going through to the ducts gets cooler. And the constantly circulating medium inside the tube either cools off when it goes back underground or brings up more heat.

Geothermal systems can be modified to heat water, also. The idea is to coil the pipe that sucked up all the heat from that hot summer air right through the water heater. But that only works when you're air-conditioning the house.

Obviously the costs and benefits of either system depend on the house. The typical geothermal system might add $15,000 to the cost of a house's heating and air-conditioning system (although the granite formations that lie under much of metro Atlanta can make drilling more expensive). Sounds like a lot, but it's not unusual for geothermal to cut energy costs by $75 a month. If the costs are folded into a mortgage, it could work out so you're actually paying less monthly for your loan than you'd pay at today's prices for the heavily polluting electricity that would otherwise be heating and cooling your house.

Solar hot-water systems can be less expensive, maybe $8,000. But they also give you less in savings. The typical homeowner spends $20 a week on hot water, according to the U.S. Department of Energy.

The bottom line is that solar thermal costs less up front, but geothermal seems more likely to provide a return on your investment. (Solar advocates might argue, and they're welcome to do so on the blog.)

The pity is that Congress allowed a $2,000 tax credit for people who install either clean-energy system to expire. That credit made doing the right thing for our nation's energy future more economically sensible. And Georgia, unlike a lot of states, offers absolutely no incentive of its own for clean energy.

So doing the right thing – even if it makes economic sense in the long run – requires a bit of risk taking, whether you choose solar or geothermal.

Ken Edelstein's My Green Dream House blog can be found at www.mygreendreamhouse.wordpress.com.

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