Solar at Home

Solar at Home

The trials, tribulations and rewards of going solar

Is a geothermal heat pump right for you?


Geothermal heat pump with installer Patrick RyanI've tried it all: caulking cracks, blowing in insulation, replacing drafty windows and—I'm especially proud of this one—installing a mail-slot cover so airtight it could be used in a space shuttle docking module. Yet my home heating bill remains an object of fear and loathing. After years of trying low-tech solutions, I'm drawn to a high-tech solution, and the one I keep hearing about is the geothermal heat pump.

Heat pumps, of whatever variety, give you the giddy feeling of breaking the laws of physics. The gas boiler in my basement is 80 percent efficient; burning gas unavoidably heats the exhaust gases as well as the water for the radiators. An electric heater is almost 100 percent efficient. But a heat pump can be more than 100 percent efficient. A 1000-watt electric heater emits 3400 BTU of heat in an hour, but the same amount of electricity, used to run a heat pump, might transfer 15000 BTU of heat into the house.

The trick is that a heat pump moves heat rather than generating it. It's basically just an air-conditioner or refrigerator. During summer, it cools the house; during winter, it runs in reverse, warming the house by refrigerating the outdoors. The outdoors barely notices (unless you're in an urban area thick with heat pumps)—it's like spitting in the sea, which scarcely affects the sea, but does affect the spitter. No matter how cold it gets out there, there's always some heat to harvest, although the task gets harder the colder it gets. By avoiding the production of new heat, the pump evades the naive efficiency limit. It still takes energy to run, but the energy is used mechanically (to circulate and compress a refrigerant) rather than thermally.

Most heat pumps scavenge heat from the outside air. A geothermal one gets it from the ground. In fact, the word "geothermal" is deceptive. These devices don't tap into geysers or volcanic hotspots. At the depths we're talking about, the ground is heated not by Earth's interior but by the sun. "This energy is just solar energy—it's just another form of it," explains Patrick Ryan of Ryan Energy Technologies in Union, N.J. A better term is "ground-source" heat pump, but "geothermal" has stuck because it sounds so much sexier.

As sunlight warms the surface, heat diffuses downward—a slow process that mutes the daily and seasonal temperature variations. In dry soil, the annual temperature swings are halved with every three feet of depth, so the subsurface temperature quickly approaches the average surface temperature, around 55 degrees Fahrenheit here in New Jersey. That's already pretty close to a typical thermostat setting, so a ground-source heat pump has less work to do than an air-source pump does and therefore consumes a half or third as much energy.

Two weekends ago, Ryan showed me one of his projects in Whitehouse Station, in the rural western part of N.J. (Yes, there is a rural western part of New Jersey.) The system consists of a horizontal lacework of pipes buried eight feet under a large meadow where the owners are now planting an apple orchard. Water circulates through them and reaches a temperature of 45 degrees. It doesn't get all the way up to the ground temperature of 55 degrees, because it is flowing too quickly to come to equilibrium with the ground.

Flowing into the basement, the water enters a heat exchanger, where the water pipe wraps around a refrigerant line. The water heats up the refrigerant, cools off to 40 degrees, and flows back outside to warm up again. The pump mechanism then compresses the refrigerant, raising its temperature without adding any heat. Finally, a fan blows air through the refrigerant coil and into the ducts of the house. Another coil preheats domestic hot water, saving money on that, too. In the summer, it is the air that warms the refrigerant that warms the water, sucking heat out of the house and pushing it into the ground. To watch Ryan explaining the system, see the camera-phone video I've posted to YouTube.

Unfortunately, the economics aren't quite as wondrous as the technology. In 2009 Ryan gave me an estimate for my house. Our property is too small for a horizontal loop, so it would have taken three 400-foot wells down into the Triassic-era sandstone under our house. Another complication was that we'd have had to replace our steam heating with forced air. Heat pumps don't do steam: the temperature difference is too large. The total cost was so outlandish that I use it to this day as a party joke: $70,000, plus the costs of acquiring the permits and repaving our driveway after the drillers had dug it up. A state rebate would have covered $10,000, and a Federal tax credit a third of the remainder, but no bake sale was going to cover this one.

What's more, it was hard to know how much I'd save. Although heat pumps use less energy, they use electrical energy, which, BTU for BTU, is several times more expensive than natural gas. A rough estimate for my net savings was a third, in which case the payback period would have been longer than my life expectancy. The heat-pump maker ClimateMaster has another savings-calculator here.

Others confirm that retrofitting an old house for geothermal heat makes sense only under limited circumstances. Tom Mandel in Teaneck, N.J., one of the first homeowners in the state to give it a go, estimates he paid $55,000 and saves $400 a month. So he doubts he'll ever recoup the expense. He adds that his house takes longer to heat up than before and requires an extra boost from an electric heater when the outside temperature drops below 20 degrees.

Geothermal can make sense for new construction, especially in areas where there's no gas hookup. Alan Sexstone, who responded to a Twitter request I put out for geothermal users, says he decided on a geothermal heat pump when he and his wife built a house south of Morgantown, W.Va., 11 years ago. The system comprises three 100-foot wells and cost about $10,000 to install—about twice as much as propane or electric heat, but low enough that the system will almost certainly pay for itself. Like Mandel, Sexstone says the system works well down to about 20 degrees, below which he needs to supplement it with a wood stove.

Another homeowner I heard from through Twitter built a house two years ago near Boulder. He paid $50,000 (before tax credits) for his geothermal system and estimates he saves $1000 a year. In his case, the savings partly reflect the lower effective electricity rate he gets because of his solar array.

There is irony here, because geothermal should actually make more sense for old construction than for new. New houses tend to be better insulated, which reduces their heating costs and hence the potential for savings. But this fact has to be set against the extra costs of retrofitting and the larger size of the average new house.

Even If I had the money, I'd probably be better using it to button up my whole house as tight as that docking module. After all, the ultimate way to heat your house is not to heat it. Some superinsulated houses stay warm by the body heat of their inhabitants. That's hard to pull off with an old house like ours, but in an upcoming post I'll tell the story of a nearby homeowner who has nearly done it.

I also plan to revisit the topic of geothermal heat pumps (and of space shuttles—I'm going down to the Endeavour launch next week), so please let me know your experiences here or on Twitter.

Geothermal installer Patrick Ryan. Photo by George Musser.

The views expressed are those of the author and are not necessarily those of Scientific American.

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