What’s the most ecologically conscious way to heat and cool your home? While a number of green alternatives exist, geothermal technology is one that’s been gaining ground in recent years. According to the U.S. Department of Energy (DOE), geothermal heat pump installations have seen strong growth over the past 6 years, and, as of 2008, totaled over 1 million nationwide. Approximately 100,000 to 120,000 systems are installed annually in the U.S. in about 1 out of every 38 new U.S. homes.

While the popularity of geothermal heating and cooling may be new, the technology itself has been around since the late 1940’s. Geothermal heat pumps work by exploiting a natural fact: no matter how large the atmospheric temperature fluctuations in different regions of the world, the temperature just a few feet below the earth’s surface remains a steady 45°F (7°C) to 75°F (21°C).

CPUC Geothermal
image via Ram Power

How It Works

The parts of a geothermal heat and cooling system include a heat pump, an air delivery system (i.e., ductwork), and a heat exchanger. The heat exchanger is, essentially, a system of pipes buried in shallow ground. In the winter, when above-ground temperatures drop, the heat pump removes heat from the below-ground air inside the heat exchanger and pumps it into the building via the indoor air delivery system. In the summer, the process is reversed, and the heat pump pulls air from inside the building into the heat exchanger, where heat is removed, thanks to cooler temperatures below-ground. As an added bonus, heat removed from the indoor air during the summer can also be used to provide a free source of hot water.

Different Types of Geothermal Heating and Cooling Systems

Geothermal systems come in four types, each of which are appropriate for different circumstances, based on the climate, soil conditions, available land, and local installation costs. All four approaches are equally appropriate for residential and commercial applications.

Horizontal Geothermal
image via US DOE

The Horizontal type system is generally most cost-effective for residential installations, especially new construction, provided enough land is available. In this system, two pipes are used–one buried at 6 feet and the other at 4 feet, or two pipes placed side-by-side at 5 feet in the ground in a 2-foot-wide trench.

Vertical Geothermal
image via US DOE

The Vertical system, as the name suggests, requires less in the way of acreage and more in terms of trench depth, and is often more appropriate for large commercial buildings. The Vertical system uses two pipes buried 20 feet apart and 100–400 feet deep, connected at the bottom with a U-bend to form a loop. These “vertical loops” are connected with horizontal pipe (i.e., manifold), placed in trenches–often under commercial parking lots–and connected to the heat pump in the building.

A third type, called a Pond or Lake system, makes use of an existing body of water and a water-source heat pump. In this system, a pipe is run underground from the building to the water and coiled into circles at least 8 feet under the surface, in order to prevent freezing.

Pond_Lake Geothermal
image via US DOE

The final type of system, the Open Loop system, uses well or surface body water as the heat exchange fluid. This water circulates directly through the heat pump system, absorbing the temperature below the earth, then returns to the ground through the well, a recharge well, or surface discharge. (This option is practical only where there is an adequate supply of relatively clean water and all local codes and regulations regarding groundwater discharge can be met.)

While there are no areas where geothermal heat pumps won’t work at all, there are places where efficiencies and installation costs make them impractical–for instance, areas with very dry soils, or where the climate is relatively mild and varying, such as coastal California. In the latter type of settings, an air source heat pump (which operates the same way as a ground source heat pump, but without taking air from below the ground) will do just as well.

More information on selecting and sizing a geothermal heating and cooling system is available from the DOE.

Incentives

If you’re currently in the market for a new geothermal heat pump system, the federal government will kick in a 30% Federal Tax credit as part of a credit that also applies to solar technology.  A wide variety of state and local incentives are also available, so it pays to do some research before you calculate costs. While, on average, according to the DOE, a geothermal heat pump system costs about $2,500 per ton of capacity, or roughly $7,500 for a 3-ton unit (a typical residential size),

Geothermal Map US
image via Nevada Geothermal

The Future of Geothermal Heating and Cooling

What will it take for geothermal heating and cooling to gain more widespread acceptance? According to Chris Kielich, spokesperson for the DOE, it’s an issue that rests largely in the costs of adoption and individual contractors’ willingness to learn a new technology.

“Any new building technology that is not a simple replacement has a long uptake due to the decentralized nature of the construction industry,” she told EarthTecling. “Each individual builder has to be willing and able to install the new technology.” Studies on the future of geothermal heating and cooling are available from Navigant and ORNL for those who are curious.

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