Geothermal energy refers to heat energy stored in the ground. Heat is supplied to the ground from two sources namely the hot core of the planet and the sun. It can be classified as either 'deep' or 'shallow' depending on the depths involved.
The centre of the earth is approximately 4,200 degrees celsius. Some of this heat is produced from the geological process which helped to form the earth 4.5 billion years ago, but most of this heat is provided from the decay of radioactive isotopes. The majority of this heat arrives at the surface of the earth at too low a temperature to be used for heating or power generation activities. This deep geothermal energy can only be accessed when it arrives at the earth's surface through geological processes such as through fault lines on the earth’s crust (or areas of volcanic activity) or by drilling through the surface to access it. The Geothermal Association of Ireland are very active in this area, go to www.geothermalassociation.ie.
The second source of heat in the ground is from radiation from the sun. Solar thermal radiation is absorbed by the surface of the earth each day. This energy can be regarded as stored energy which stays relatively warm throughout the year. This heat can then be extracted by using a ground source heat pump for example. See Geothermal Maps.