At present, geothermal power accounts for a only a small percentage of the electricity generated worldwide. As a better understanding of geothermal energy and ways of harnessing it are realized more and more electricity can be generated in this manner.
Geothermal power refers to any of a number of processes that use the earth's heat as a source of energy. The processes are generally classified into five categories: Earth Energy, Hot Dry Rock Energy, Geopressurized Energy, Magma Energy and Hydrothermal. The first two of these sources of energy are merely are result of temperature increasing with depth in the earth and can be harnessed anywhere. However, with current technologies only a minimal amount of heat can be harnessed in these processes and only with a substantial cost. Earth energy is the heat of the earth at very shallow depths and can be used very efficiently for small scale heating and cooling using heat pumps. Hot dry rock energy is obtained at depths of 5 to 10 miles and produces significantly more heat but at the cost of drilling several miles into the earth as opposed to a few feet. (5).
The other three sources of energy are from high concentrations of heat in various regions, such as geysers, steam vents and other forms of volcanism. These sources usually provide high temperature steam that can be used to run turbines to power electrical generators. Steam at anywhere above 100°C can be used to generate electricity. At temperatures above 175°C the steam is used directly to power the turbine with efficiency and power increases at higher temperature. At temperatures below 175°C the heat is transferred to a secondary fluid (low boiling) which is then runs the turbine to generate electricity.
Geothermal power plants in the United States produced close to 15 million Megawatthours of electricity in 1995 (6). The largest obstacle in increasing the feasibility of geothermal power is in finding more efficient methods of tapping into the earth and harnessing its heat. At the moment, only a select few areas can be used to provide geothermal power. The cost of drilling deep into the earth in other areas and in circulating water and steam through those energy deep energy reserves is more than is deemed reasonably as compared with other energy sources. Funding into geothermal technologies has been reduced over the years along with funding for other renewable energy sources as the prices of oil and coal became more affordable.
Still, with better and better understanding of Geothermal processes the time may soon come when it can be harnessed anywhere on earth and used as a clean source of energy and electricity. Advocates state that geothermal is a clean, inextinguishable resource. Because the steam in the earth is already in a form capable of driving turbines for direct conversion into electricity there are no greenhouse gases or combustion by-products to handle. There are some environmental questions concerning the effect of drilling wholes into the earth and releasing large amounts of energy that had been previously contained. Some gases that are present in the earth are released during construction of geothermal plants. The other current problem is that only a small fraction of the heat released in the geothermal wells is actually captured and converted to electricity.