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What is Geothermal?

Definition ~ Geothermal Power Plants ~ Enhanced Geothermal Systems (EGS) ~ Direct Use and Geothermal Heat Pumps ~ Videos

U.S. State-by-State Guides to Geothermal Energy Potential

The Geothermal Resources Council (GRC), Geothermal Energy Association (GEA), and Geothermal Exchange Organization (GEO) have released guidance for U.S. states on meeting new clean energy standards from the U.S. Environmental Protection Agency (EPA).

The free state-by-state guides walk through the benefits and uses of three major types of geothermal applications:

  • Power generation,
  • Direct use
  • Heat pumps

More information.........

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)

Enhanced Geothermal Systems (EGS)

Enhanced Geothermal Systems (EGS), also sometimes called engineered geothermal systems, offer great potential for dramatically expanding the use of geothermal energy.

The EGS concept is to extract heat by creating a subsurface fracture system to which water can be added through injection wells. Creating an enhanced, or engineered, geothermal system requires improving the natural permeability of rock. Rocks are permeable due to minute fractures and pore spaces between mineral grains. Injected water is heated by contact with the rock and returns to the surface through production wells, as in naturally occurring hydrothermal systems. EGS are reservoirs created to improve the economics of resources without adequate water and/or permeability.

More information from DOE/EERE/GTO

Direct Use and Geothermal Heat Pumps

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

Direct Use

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.

Direct, or non-electric, use of geothermal energy refers to the use of the energy for both heatng and cooling applications. Fluids with temperatures of <300°F, adequate for direct use, are available throughout much of the United States. Direct use of geothermal energy in homes and commercial operations is much less expensive than using traditional fuels; savings can be as much as 80%!

Furthermore, direct-use applications such as fish farms, greenhouses, microbreweries, fruit and vegetable drying, spas, pulp and paper processing, and lumber drying offer attractve and innovative opportunities for local businesses and entrepreneurs.


  • Jobs Boost. Direct-use geothermal energy projects leverage existing workforces and companies within the state. Their simple design and construction from off-the-shelf parts can utilize local engineering firms, geologists, drilling operators, construction trades, pipefitters, technicians, and welders. A rough prediction of potential job opportunites created by installing direct-use systems may be 3 temporary jobs per MWth during construction, with 1 full-time job per MWth for ongoing operation.
  • Economy Boost. Geothermal heated facilities have the potential to stimulate economies through increased tax revenues, the creation of new businesses and local jobs, tourism, agriculture, and enhanced community involvement.
  • Locally Produced. Directly using geothermal energy in homes and commercial operations, such as food production from local agriculture, can offset imported energy, keeping jobs, dollars, and other benefits in local communities.
  • Carbon Emission Reduction. Geothermal direct-use projects produce near-zero emissions. Depending on the existing heating fuels being offset, this may result in annual emissions reductions of anywhere between 1,700 tons (if offsettng natural gas) to 9,300 tons (if offsettng electricity) of CO2 saved per MWth of installed geothermal direct-use capacity.
  • Flexible Heating Systems. Applications of geothermal direct use may include district heating, snow melting, spas and pools, agriculture, food processing, and other uses. Within a single system these diverse applications can be “cascaded” and work together in the most efficient way possible to ensure the maximum benefit and lowest costs possible from direct-use systems.
  • Reliable and Sustainable Heat Source. Geothermal heating projects last for decades—typically 25 years or more— providing reliable energy at a low, stable price. This can provide price certainty and insulate consumers (and the economy) from more unpredictable fluctuations in fossil fuel prices.

Heat Pumps

Geothermal heat pumps use the natural insulating properties of the earth from just a few feet underground to as much as several hundred feet deep, offering a unique and highly efficient renewable energy technology for heating and cooling.

Most work by circulating water in a closed system through a “loop field” installed horizontally or vertically in the ground adjacent to or even beneath a building. Heat is taken from the building and transferred to the ground in the summer.

The system is reversible, and heat is taken from the ground and used in the building in the winter. The system only moves heat, which is much more efficient than using a fuel or electricity to create heat.

Geothermal heat pumps can support space heating and cooling needs in almost any part of the country.


  • Economic. On average, a typical home of 2000 square feet will require 4 tons of heating and cooling capacity with an average system installation cost between $5,000 and $7,500 per ton.
  • Energy Efficient. Geothermal heat pumps use 25% to 50% less energy than conventional heating or cooling systems.
  • Carbon Emissions Reduction. One ton (12,000 BTU/hr) of GHP capacity over a 20 year operating cycle avoids 21 metric tons of CO2 emissions. So a typical home system can avoid 80-100 metric tons of CO2 emissions.
  • Improved Indoor Air Quality & Safety. There is no combustion in a geothermal heat pump; therefore there is no chance of carbon-monoxide poisoning. By adding high-efficiency air cleaners with geothermal, these systems can improve inside air quality.
  • Locally Produced. Everywhere. Unlike other geothermal technologies, heat pumps are not limited by geography or geology. They can be installed in most locations in any of the 50 states or territories of the U.S.
  • Sustainable Investment. The lifespan of a geothermal system is usually greater than 24 years. A conventional furnace will last 7-10 years with regular maintenance. The ground loop of the geothermal system has a warranty of 50 years. These loops are made up of high-density polyethylene pipe, the same pipe which is used in city gas lines.
  • Quiet Operation. Unlike air conditioners, there is no outdoor unit. Geothermal units are very smooth and quiet in operation.

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)

Energy 101: Geothermal Heat Pumps (Video: DOE)


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