Geothermal

Geothermal energy is the heat trapped the rocks and fluid in the Earth’s crust.  This renewable resource can be used to generate electricity or to heat and cool buildings.  Geothermal resources are often classified into three categories:

• High temperature (greater than 150°C or 302°F)
• Moderate temperature (90°C - 150°C or 194 - 302°F) 
• Low temperature (less than 90°C or 194°F)
  

The major benefits of geothermal energy are that the energy can be extracted without burning a fossil fuel and the resource is continuously available – night or day.  Applications of geothermal energy include electric power generation, direct use applications, and ground source heat pumps.

Electric Power Generation
In the US, the most extensive geothermal resources are in the western states.  In this region, deep wells can tap hydrothermal resources that contain steam or hot water that can drive electrical turbines.  Three types of geothermal power plants exist:

• Dry steam plants use geothermal steam directly to turn turbines
• Flash steam plants pull high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines
• Binary-cycle plants pass moderately hot geothermal water by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor and drive the plant’s turbines.
  

Direct Use Applications
Direct use applications make use of the heat in water for a variety tasks.  The water can used to heat buildings, raise plants in greenhouses, heat water for fish farming, dehydrate crops, and contribute to industrial processes, such as dehydrating milk.  The city of Klamath Falls, Oregon uses geothermal energy under roads and sidewalks to melt winter snow.

Ground Source Heat Pumps
The ground just below the surface stays relatively constant year round.  In the winter months, the underground temperature is warmer than the air temperature.  In the summer, the reverse is true – the underground temperature is cooler than the air temperature.  A ground source heat pump can be used to draw heat from the ground in the winter and act as a release for heat in the summer.  All regions of the US – including North Carolina – are suitable for ground source pumps.

A geothermal heat pump consists of three major components: the ground heat exchanger, the heat pump unit, and the air delivery system.  The heat exchanger is a system of pipes – called a loop –buried adjacent to the facility to be heated and cooled. 
The heat exchanger can be buried horizontally, vertically, or in coils.  The heat pump unit circulates a heat transfer fluid, usually water or an antifreeze mixture, through the pipes. This process allows the fluid to absorb or release heat within the ground.  The final component – air delivery system – is the ductwork within the home or facility that moves indoor to and from the heat pump unit. 


A vertical ground source heat pump.
Courtesy of DOE/EERE.


Additional Resources

Geothermal Heat Pump Consortium
The residential information section of this site provides online case studies and brochures, as well as information about geothermal heat pump technologies (called GeoExchange systems on this website), and installation tips.

Geothermal Technologies Program
This website, maintained by the Energy Efficiency and Renewable Energy Division of the Department of Energy, is an excellent resource for information on geothermal energy.

Geothermal Resources Council
The Geothermal Resource Council maintains many informative resources, including an extensive technical library.