'Geothermal' comes from the Greek words 'geo' (earth) and 'therme' (heat). It means 'the heat from the earth'. The heat in the Earth's interior originated from its formation as part of the Solar System from dust and gas over 4 billion years ago. It was then very hot but over time that heat energy is radiated into space. However within the Earth are radioactive ements that decay and 'top up' the heat energy content of the Earth.

Geothermal resources range from shallow ground to hot water and rock several miles below Earth's surface, and even farther down to the extremely high temperatures of molten rock called magma

Old Faithful Geyser in Yellowstone National Park

Domestic Power from Geothermal Heat Pumps - Use of Shallow Ground Energy to Heat and Cool Buildings

Earth's energy can be converted into heat and electricity. The three technology categories are geothermal heat pumps, direct-use applications, and power plants.

In winter, water circulating inside a sealed underground loop system absorbs heat from the earth and carries it to the geothermal unit. Here it is compressed to a higher temperature and sent as warm air to your indoor system for circulation throughout the home.

In the summer, the system reverses and expels heat from your home into the cooler earth via the loop system. This heat exchange process is not only natural, but is a natural and highly efficient way to create a comfortable climate in the home. There are several loop options that can be employed. They can either be vertical, horizontal, or submerged in a nearby pond or lake. In many cases, a water well can be used instead. The best application will depend on the characteristics of the property.

Direct-Use Piped Hot Water

Hot water near Earth's surface can be piped directly into facilities and used to heat buildings, grow plants in greenhouses, warm water for fish farming, and to pasteurize milk. Some cities in the USA pipe the hot water under roads and sidewalks to melt snow. District heating applications use networks of piped hot water to heat buildings in whole communities.

Power Plants Generate Electricity from Geothermal Reservoirs

Wells (a mile or more in depth) can be drilled into underground reservoirs to tap steam and very hot water. The pressure from the steam generated by this drives turbines that drive electricity generators. Three types of power plants are operating today:

• Dry steam plants, which directly use geothermal steam to turn turbines;
• Flash steam plants, which pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines; and
• Binary-cycle plants, which 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 steam, which then drives the turbines.
  

Domestiv geothermal energy use:

Geothermal Powerplants

Links:

http://geothermal.marin.org/

http://www.eere.energy.gov/RE/geothermal.html

http://www.eere.energy.gov/geothermal/