 The center of the Earth is around 7000 degrees Celsius - easily hot enough to melt rock.  Even a few kilometers down, the temperature can be over 250 degrees Celsius if the Earth's crust is thin.  In general, the temperature rises one degree Celsius for every meters you go down, but this does vary depending on location.  In volcanic areas, molten rock can be very close to the surface.

 Geothermal energy has been used for thousands of years in some countries for cooking and heating.  It is simply power derived from the Earth's internal heat.  This thermal energy is contained in the rock and fluids beneath Earth's crust.  It can be found from shallow ground to several miles below the surface, and even farther down to the extremely hot molten rock called magma.  These underground reservoirs of steam and hot water can be tapped to generate electricity or to heat and cool buildings directly.

 Geothermal electricity is electricity generated from geothermal energy.  According to the Geothermal Energy Association, the heat continuously flowing from the core of the Earth is estimated to be equivalent to 42,000 gigawatts (GW) of power (20+ times today's global electricity generation).  If harnessed properly, geothermal could become a material contributor to global electricity generation.  Geothermal electricity generation is possible by drilling wells to bring to the surface these superheated fluids or steam to drive turbines.

 Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904 in Larderello, Italy. It successfully lit four light bulbs.  Later, in 1911, the world's first commercial geothermal power plant was built there.  Italy was the world's only industrial producer of geothermal electricity until  In 1958, New Zealand became the second major industrial producer of geothermal electricity.  In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States at The Geysers in California.  The binary cycle power plant was first demonstrated in 1967 in Russia and later introduced to the USA in  Worldwide, 11,400 megawatts (MW) of geothermal power is online in 24 countries in 2012.

The world's first Geothermal power station in Larderello, in Southern Tuscany, Italy Prince Piero Ginori Conti with the first geothermal power plant in 1904

 Electricity generation requires high temperature resources that can only come from deep underground.  The heat must be carried to the surface by fluid circulation, either through  Magma conduits  Hot springs  Hydrothermal circulation  Oil wells  Drilled water wells or a combination of these.  This circulation sometimes exists naturally where the crust is thin.  Magma conduits bring heat close to the surface, and hot springs bring the heat to the surface.  If no hot spring is available, a well must be drilled into a hot aquifer.

1. Hot water is pumped from deep underground through a well under high pressure. 2. When the water reaches the surface, the pressure is dropped, which causes the water to turn into steam. 3. The steam spins a turbine, which is connected to a generator that produces electricity. 4. The steam cools off in a cooling tower and condenses back to water. 5. The cooled water is pumped back into the Earth to begin the process again.

Hot Water Steam Turbine Generator Cooling Tower Injection Well

 Dry Steam Power Plants  Flash Steam Power Plants  Binary-Cycle Power Plants

 Dry steam plants are the simplest and oldest design.  They directly use geothermal steam of 150°C or greater to turn turbines.  The Dry Steam technology allows for the steam from a geothermal production well to be fed directly to a steam turbine without a secondary heat exchanger.  The turbine then coverts the change in steam pressure to mechanical rotational energy, which is converted to electrical energy by a generator.

 The boiling point of a fluid increases as its pressure is increased.  Superheated water is liquid water under pressure at a temperature higher than the normal boiling point of 100 °C.  When the pressure is reduced the water flashes to steam.  Superheated water pumped from the ground at temperatures of 175 °C or more.  It can be flashed to steam in a separator or flash tank to drive a turbine directly.  Surplus water from the flash plant is reinjected into the ground.  This is the most common type of plant in operation today.

 Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 57°C.  The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water.  This causes the secondary fluid to flash vaporize, which then drives the turbines.  This is the most common type of geothermal electricity plant being constructed today.  Both Organic Rankine and Kalina cycles are used.  The thermal efficiency of this type plant is typically about 10–13%.

 In ground that is hot but dry, or where water pressure is inadequate, injected fluid can stimulate production.  Developers bore two holes into a candidate site, and fracture the rock between them with explosives or high pressure water.  Water travels through fractures in the rock, capturing the rock's heat until forced out of a second borehole as very hot water.  The water's heat is converted into electricity using either a steam turbine or a binary power plant system.  All of the water, now cooled, is injected back into the ground to heat up again in a closed loop.  This approach is called hot dry rock geothermal energy in Europe.  EGS systems are currently being developed and tested in France, Australia, Japan, Germany, the U.S. and Switzerland.

 Geothermal power is considered to be sustainable because the heat extraction is small compared with the Earth's heat content.  The emission intensity of existing geothermal electric plants is on average 122 kg of CO2 per kilowatt-hour (kW·h) of electricity, about one-eighth of a conventional coal-fired plant.  Unlike solar and wind energy, geothermal energy is always available, 365 days a year.  It's also relatively inexpensive; savings from direct use can be as much as 80% over fossil fuels.  Geothermal plants use 404 square meters per GWh versus 3,632 and 1,335 square meters for coal facilities and wind farms respectively.  They use 20 litres of freshwater per MW·h versus over 1000 litres per MW·h for nuclear, coal, or oil

 Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect.  The power stations do not take up much room, so there is not much impact on the environment.  No fuel is needed.  Once you've built a geothermal power station, the energy is almost free.  It may need a little energy to run a pump, but this can be taken from the energy being generated.

 The big problem is that there are not many places where you can build a geothermal power station.  You need hot rocks of a suitable type, at a depth where we can drill down to them.  The type of rock above is also important, it must be of a type that we can easily drill through.  Sometimes a geothermal site may "run out of steam", perhaps for decades.  Hazardous gases and minerals may come up from underground, and can be difficult to safely dispose of.