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What is Geothermal?

Geothermal Energy is heat (thermal) derived from the earth (geo)

 It is the thermal energy contained in the rock and fluid (that fills the fractures and pores within the rock) in the earth's crust.

Calculations show that the earth, originating from a completely molten state, would have cooled and become completely solid many thousands of years ago without an energy input in addition to that of the sun. It is believed that the ultimate source of geothermal energy is radioactive decay occurring deep within the earth (Burkland, 1973).

In most areas, this heat reaches the surface in a very diffuse state. However, due to a variety of geological processes, some areas, including substantial portions of many USA western states, are underlain by relatively shallow geothermal resources.

The uses to which these resources are applied are also influenced by temperature. The highest temperature resources are generally used only for electric power generation. Current U.S. geothermal electric power generation totals approximately 3,442 MW or about the same as five large nuclear power plants.

The average temperature gradient for planet Earth is 20 ℃ (68 ℉) per kilometer. However, there are many areas around the world where the gradient is higher, the temperature increases at a faster rate with depth below the ground.

With a temperature gradient of between 50 and 100 ℃ geothermal resources are more readily accessible.

  • Above 20 ℃ (68 ℉) geothermal waters can be used for direct uses like greenhouses, aquaculture and district heating.
  • Above 75 ℃ (167 ℉) the water is hot enough to be used for electricity generation using binary cycle technology.
  • Above 160 ℃ (320 ℉) flash steam generation can be used to produce clean, renewable electricity.

With better drilling technology geothermal resources at greater depth and temperature can be reached.

Geothermal Power Plants

Geothermal power plants use hydrothermal resources that have two common ingredients: water (hydro) and heat (thermal). Geothermal plants require high temperature (300°F to 700°F) hydrothermal resources that may come from either dry steam wells or hot water wells. We can use these resources by drilling wells into the Earth and piping the steam or hot water to the surface. Geothermal wells are typically one to two miles deep.

There are three basic types of geothermal power plants:

  • Dry steam plants use steam piped directly from a geothermal reservoir to turn the generator turbines. The first geothermal power plant was built in 1904 in Tuscany, Italy, where natural steam erupted from the Earth.
  • Flash steam plants take high-pressure hot water from deep inside the Earth and convert it to steam to drive the generator turbines. When the steam cools, it condenses to water and is injected back into the ground to be used over and over again. Most geothermal power plants are flash steam plants.
  • Binary cycle power plants transfer the heat from geothermal hot water to another liquid. The heat causes the second liquid to turn to steam which is used to drive a generator turbine.

The current production of geothermal energy from all uses places third among renewables, following hydroelectricity and biomass, and ahead of solar and wind. Despite these impressive statistics, the current level of geothermal use pales in comparison to its potential. The key to wider geothermal use is greater public awareness and technical support. (Information from the US DOE)

Uses for low and moderate temperature resources can be divided into two categories: Direct use and Geothermal heat pumps.

Direct use, as the name implies, involves using the heat in the water directly (without a heat pump or power plant) for such things as heating of buildings, industrial processes, greenhouses, aquaculture (fish farming) and resorts. Direct use projects generally use resource temperatures between 38°C (100°F) to 149°C (300°F). Current U.S. installed capacity of direct use systems totals 470 MW or enough to heat 40,000 average-sized houses.

Geothermal Heat Pumps use the earth or groundwater as a heat source in winter and a heat sink in summer. Using resource temperatures of 4°C (40°F) to 38°C (100°F), the heat pump, a device which moves heat from one place to another, transfers heat from the soil to the house in winter and from the house to the soil in summer. Accurate data is not available on the current number of these systems; however, the rate of installation is thought to be between 10,000 and 40,000 per year. (Information furnished by the Geo-Heat Center)


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