What geothermal power is about

The word geothermal comes from the Greek words “geo” earth and “therme” heat. It means the heat of the earth and is naturally occurring. Today, there is about 9,000 MW of geothermal electricity generating capacity installed world-wide. Geothermal power is a clean and renewable energy source.

The geothermal energy is derived from the heat contained in the earth’s core. In regions where the earth's crust is thin or fractured, or where magma bodies are close to the surface (like Iceland or New Zealand), there are high temperature gradients (this means that the temperature increases rapidly as we go deeper into the Earth). Often, in these areas, deep faults, rock fractures and pores allow groundwater to percolate towards the heat source and become hot. Some of this hot geothermal water travels back to the surface and appears as hot springs, mud pools, geysers, or fumaroles.



Two broad categories of geothermal resources:
 

  • High temperature, usually magmatic-related resources. These higher temperature geothermal areas are close to the edges of continental plates. These have temperatures of 200°C to 350°C at economically-drillable depth. They are of limited occurrence, and form individual geothermal systems of up to 50 square km in area. The vast majority of the world’s geothermal generation capacity of approximately 9,000 MW is based on these kinds of resource.
  • Low to moderate temperature resources, which are common and not related to volcanic systems. These have only average temperature gradients and range from 80°C to 140°C, depending on depth. Examples include the Molasse Basin in Germany and Great Artesian Basin in Australia.  Some estimates suggest that the global geothermal resource in this temperature range is as much as three times larger than the resource above 200°C.




    • The wide range of other uses for geothermal energy can be divided into direct use and electricity generation. Both uses require drilling of special wells to intersect the deep geothermal resources.

    Direct use involves using geothermal heat directly for heating of buildings, industrial processes, domestic heating, greenhouses, aquaculture, public baths and pools. The simplest geothermal power plants use only the separated steam from geothermal steamfields, passing it through a condensing steam turbine.

    More sophisticated power plants use the separated brine (the hot water), which may still be at a temperature in excess of 130°C, to boil a secondary working fluid to drive other smaller turbines in a closed cycle. This is usually referred to as a binary plant. Kalina cycle is a type of binary plant.

    Whichever plant type is selected, the relatively low steam temperatures and pressures mean that the efficiency of conversion of heat to electricity is low compared to fossil-fuel fired plants. On the other hand, geothermal resources, where properly managed, have low carbon emissions and are classified as renewable.