Geothermal energy is literally the heat from inside the Earth. The core, 4,000 miles deep, is the hottest part of the planet, while temperatures get lower nearer to the Earth's surface. The most shallow ground -- up to ten feet beneath the surface -- stays at 50 to 60 degrees Fahrenheit and is actually not heated geothermally, but by solar energy. Heat pump systems are nonetheless referred to as either "geothermal" or "ground source." Cooler than above-ground temperatures in summer and warmer in winter, the earth is used to heat and cool buildings.
Geothermal Heat Pump Basics
There are three main parts to a geothermal heat system: the air delivery system, the heat exchanger and the pump mechanism. Geothermal heat pumps have been used in the U.S. since the 1940s and are more efficient than the alternative air-source heat pumps, because they draw on the constant earth temperatures instead of the more changeable outdoor air conditions. Geothermal pumps are between 300- and 600-percent efficient on cold winter nights compared to a maximum air source pump efficiency of 250 percent. (The heat pump actually transfers more energy through heat than it uses in electricity, which is what gives it more than 100-percent efficiency.) The heat exchanger is a set of pipes buried in shallow ground near the house or other building. In winter, heat from the exchange is pumped into the house; in summer the heat is extracted and pumped back into the heat exchanger.
There are more advantages to geothermal heating and cooling systems than negatives; the primary obstacle for most consumers is the initial financial outlay. Aside from cost, geothermal energy "could prove to be the most benign energy resource of all, and the most abundant," according to consumer group Energy Consumers Edge. The Environmental Protection Agency reports that geothermal heating and cooling systems are the most cost-effective, energy-efficient and environmentally beneficial systems available (see Resources).
The installation price of geothermal heating and cooling systems are significantly higher than air source systems. The Department of Energy maintains that costs are recouped through energy savings. This can take two years, ten years or even longer, however, depending on the individual system and household needs. The cost for some homes is prohibitively high: George Musser of "Scientific American" magazine estimated a cost of $70,000 in 2009 to change his home to a geothermal system. Although a state rebate would have delivered $10,000 toward the project and a Federal tax credit another $20,000, the out-of-pocket cost would still have been substantial, at $40,000. Depending on how much energy bills are reduced through geothermal systems, it's possible that such high costs are never recouped.
The Disadvantages of Retrofits
Geothermal systems are mainly installed within the building; the only part that's outside is the ground loop that is piped into the house. When installing a system in an older home, it disrupts the landscaping, leading to extra restorative costs. If horizontal pipes are used, 4-foot-deep trenches are laid 5 feet apart, so homeowners must be prepared for disruption and take care to safeguard tree roots and landscaping where necessary.
- U.S. Department of Energy: Geothermal Heat Pumps
- Renewable Energy World: Geothermal Energy
- Energy Consumers Edge: Pros and Cons of Geothermal Energy
- Connecticut General Assembly; Pros and Cons of Geothermal Systems; Kevin E. McCarthy; September 2008
- Renewable Energy World: Geothermal Heat Pumps
- "Old House Online"; Geothermal and Heat Pump Systems; Brian D. Coleman
- Photo Credit Comstock/Comstock/Getty Images
- Alliant Energy: Geoexchange Heating and Cooling Systems -- Fascinating Facts
- "Scientific American"; Is a Geothermal Heat Pump Right for You?; George Musser; April 2011
- The Geothermal Exchange Organization: A Paradigm for GeoExchange Retrofits
- U.S. Department of Energy: Selecting and Installing a Geothermal Heat Pump System
- e2: Geothermal Energy