Geothermal energy refers to the process of taking heat from deep underground and using it to generate steam and from that electricity.
The best way to describe the Geothermal companies is they are Green. The energy is considered renewable because the heat probably comes from the friction that grinds tectonic plates together and forms volcanoes.
As many areas in the Nevada and California area have large heat reservoirs close to the surface it makes it very economically feasible to generate electricity at completive rates.
In this authors opinion investing in well managed geothermal companies is a good 5 year strategy as the electric car market will lead to people seeking greener electricity sources and wind power will only go so far.
Among renewable energy sources, geothermal is one of the most reliable. Energy is available 24/7 with no natural interruptions. One caveat is that pumping water into the earth to generate steam from hot rocks has been associated with earthquakes in some areas.
Enhanced geothermal systems (EGS)
According to an article in Economist magazine,  Enhanced geothermal systems (EGS) is an advanced geothermal technology with potential to improve efficiency by 38% or more. Essentially, fracking technology is used to break rocks underground and improve heat transfer efficiency. Alta Rock Energy, a start up funded by Khosla Ventures, is setting up to exploit the technology. They claim 6 to 10 times more power than from traditional geothermal fields.
Geothermal Energy Companies
Ormat Technologies Inc.
See the wikipedia article here: 
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