Overview of Geothermal energy

Low Temperature Geothermal

When the term "geothermal" is used in the context of energy the term tends to engender images of shooting geysers and bubbling lava.  Certainly this type of energy generation known as "High Temperature Geothermal" exists in many parts of the U.S. and is discussed in detail in our section on Geothermal Plants.  However, for most homeowners the most exciting story when it comes to renewable energy is low temperature geothermal using a type of heating system called a ground source heat pump. This type of heating leverages the fact that the temperature of the earth in your front and back yard remains very stable throughout the year at between 45° - 75° degrees Fahrenheit.  By leveraging these constant temperatures it is possible to heat and cool your home geothermally at substantially less cost than you probably do today.  In an era where natural gas and heating oil prices have gone through the roof, a ground source heat pump is something that just about every homeowner ought to consider.

Geothermal energy is a bi-directional energy source that can be used for both heating and cooling.  When using geothermal energy for cooling  in the summer heat pumps are used to extract the hotter air from the house so that it can be cooled in the earth. In the winter the outside air is colder than the ground temperature and so the heat exchanger in the heat pump can move the heat from the ground into the house or building.

Ground source heat pump systems can save you substantial money on your home heating and cooling bill.  Most studies indicate that geothermal heating and cooling can lead to savings between 25%-50% over conventional heating and cooling systems. While they do have an upfront cost, the payback period is relatively short, between 2-10 years depending upon the installation. Given recent changes in heating oil and natural gas prices the savings are likely to be even greater in the future and payback periods even shorter.  For more information on costs check out the Typical Costs section. Also, there are federal tax credits available for geothermal heating and many states provide incentives as well.  You can find information on incentives in our Incentives section.

In addition to lower cost, ground source heat pumps offer a number of other advantages over a conventional furnace.  Ground source heat pumps run on electricity.  This means they are exceptionally safe.  There is no combustion and no combustion gases such as methane are released.  This means it is not only safer for you, it is better for the environment as well.  Heat pumps also tend to provide a more even heat than traditional furnaces and usually run much quieter. 

High Temperature Geothermal

The direct use of geothermal heating has been practiced for thousands of years, and direct use continues today.  Many parts of the world, particularly those in the regions which border the earth's tectonic plates (sometimes called the ring of fire) have strong access to geothermal sources close to the surface.  In the United States there are extensive geothermal sources in the west and northwest.  These geothermal sources can be tapped directly or indirectly and then used for heating buildings, streets or districts.  The heat is also used in heating spas and pools, in aquaculture and greenhouses, and in various industrial applications.  For information on direct uses of geothermal heating look at our section on Geothermal Heating in the Geothermal Plants section.

Another use of geothermal energy is electric power generation.  Electric power generation using geothermal resources may take place in several different types of plants, but most rely on the process of piping water or steam into a plant from an underground heat source.  The steam is used to run a conventional steam turbine when then generates the electricity.  Because there is no need to use natural gas or oil to heat the water and create steam, these types of plants can be extremely efficient and can generate enough electricity to feed thousands and sometimes tens of thousands of homes. Moreover they do this without adding any hydrocarbons or pollutants to the atmosphere. For information on electric generation with geothermal look at our section on Geothermal Electric in the Geothermal Plants section.

Direct access to geothermal sources and electrical generation through tapping geothermal sources are often referred to as High Temperature Geothermal in that they are leveraging extremely high temperature underground sources.  These sources are usually only found in specific geological areas adjacent to the earth's tectonic plates. However,  it is also possible to develop useful energy by leveraging the differential that exists between air temperature and ground temperature near the surface.  These types of Low Temperature Geothermal systems are called Geo-exchange systems, and constitute a third category of geothermal energy use.

High temperature geothermal energy systems require significant physical infrastructure and therefore are generally restricted to large heating or electric plants.  However, low temperature geothermal exchange systems using heat pumps are much simpler and can be used by most home or small business owners. They are becoming increasingly common throughout the United States and Europe and are one of the most promising renewable energy approaches.