There are two basic types of geothermal power plants currently in use, steam plants and binary plants. Steam plants use the hot water and very hot steam (with temperatures over 300F) to power a turbine that feeds a generator directly (if this is a "dry steam" principle) or they can depressurize the very high-pressure and temperature water to create steam ("flash steam" principle). Currently steam plants can generate electricity for about 4 cents to 6 cents per kWh.
Binary plants have one advantage over steam plants, namely that they can operate at locations with lower temperatures ((100 F to 300 F), which makes them available at much more places than steam plants. This is also the reason why there are much more binary plants than steam plants Hot water passes through a heat exchanger along with a working fluid that has a lower boiling point (such as isobutene). The working fluid vaporizes, moving the turbines and powering the generators.
As said before geothermal energy can also be used a direct heat source. If we take a look at Iceland we can see that all the houses in Iceland are heated with water from hot springs, and Iceland currently generates more than half its energy from geothermal sources.
We should also mention underground heat pumps that are also a form of geothermal energy. Underground heat pumps have very interesting working principle, namely they work like refrigerators in reverse. Heat pumps use pipes (that circulate a refrigerant through a cycle of condensing and evaporating between two heat exchangers. One heat exchanger evaporates at low pressure and absorbs heat. A compressor pushes the refrigerant vapour, now at high pressure, to other coil where the refrigerant condenses and releases the heat. Unlike refrigerators and air conditioners, heat pumps can be used for both heating and cooling.
Geothermal energy is definitely one of the energy sources we should be using more in years to come. Of course, in order for this energy source to become cost competitive there are still lot of researches needed to decrease currently high installation costs, and to find methods to improve efficiency. Given that is environmentally friendly energy source we should be definitely working more on methods to exploit it as much as possible. Many positive examples have so far proved that this energy source has huge potential and it would really be a shame if we won't do anything to expand its use, especially now when world seeks for alternative energy sources to combat climate change and pollution.
Geothermal energy overview:
1. Geothermal energy - Quick overview
2. Geothermal energy - How to use
3. Geothermal energy - Advantages and disadvantages (previous)
4. Geothermal energy - Technology and power plants (current)