Heat contained within the planet causes macrogeological events like earthquakes, volcanoes and tectonic movement. Geothermal energy in the context of this book refers to that small fraction of the Earth's heat which can be converted to useful energy. Most of this heat is generated by decaying radioactive isotopes within the Earth's mantle.
The rate of increase of temperature according to depth in the ground is called the 'geothermal gradient' and averages 2.5 to 3.0°C per 100 m of depth. Modern drilling techniques can penetrate up to 10 km.
Where there are active geothermal areas this gradient can increase by a factor of ten, producing temperatures above 300°C at 500 to 1000 m. This occurs where there is an upward intrusion of high-temperature rocks from the magma belt. In such circumstances a temperature of around 600°C can be expected at depths from 5 to 10 km. This would provide high-pressure steam. However, useful geothermal energy is available at the normal geothermal gradient.
This heat has to be brought to the surface. Geothermal springs do this spontaneously. More often, water has to be injected into the hot, permeable rocks known as the 'thermal reservoir', where it circulates, absorbing heat in the process. If there are several geothermal wells in a vicinity, this is described as a 'thermal field' (see Fig. 4.1).
An advantage of geothermal energy is that it is independent of climate or seasonal/diurnal variation. The capacity factor of geothermal plants is often in excess of 90%, producing energy at a price which is lower than most other renewable technologies.
Was this article helpful?
The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.