Geothermal energy is classed as renewable energy. Renewable energy is generally described as energy obtained from sources that are essentially inexhaustible, in contrast to fossil fuels, of which there is a finite supply.

The term geothermal energy is often used to indicate that part of the earth's heat that may be recovered and utilised. Heat transferred from the earth's molten core to underground deposits of dry steam (steam with no water droplets), wet steam (a mixture of steam and water droplets), hot water, or rocks lying fairly close to the earth's surface. It is also generated locally within the earth's crust from the natural decay of the radiogenic elements that occur in rocks, and in certain granites they can be concentrated such that there is a marked elevation in the local surface heat flow.

GEOTHERMAL SYSTEMS

The characteristics of geothermal systems vary widely, but three components are essential:

• a subsurface heat source
• fluid to transport the heat
• faults, fractures or permeability within sub-surface rocks that allow the heated fluid to flow from the heat source to the surface

The amount of heat being generated by the earth (heat flow) is one of the key factors that determine the temperature gradient at any location. The other major factor is the thermal conductivity of the crustal rocks, which controls how well they trap the generated heat. High heat flow will result in a higher temperature gradient, while an insulating blanket of sedimentary rocks over the heat source will trap that heat. Some rocks make better insulators than others, but in general, fine grained sedimentary rocks such as shale are better insulators than sandstone. The highest thermal gradients are therefore found in regions with both high heat flow and low thermal conductivity.

THE BENEFITS OF GEOTHERMAL ENERGY

When properly developed and managed, geothermal systems are a clean, abundant, and reliable source of renewable energy. Use of geothermal energy for electricity generation or for direct use conserves non-renewable and more polluting resources. It is uniquely reliable, with conventional geothermal energy plants typically achieving much higher load factors compared to typical load factors for hydro and wind power plants. Geothermal energy is effectively a renewable resource that does not consume any fuel or produce significant carbon dioxide emissions.