PrintGenerating Electricity from Geothermal Energy
Remember how your basic steam turbine works in a power plant that uses fossil fuels: Fuel is burned to heat water in a boiler, to create steam. The steam is used to drive a turbine, which generates electricity. What if you could get all that steam without burning a single ounce of coal, oil or natural gas? That is the appeal of geothermal electricity production. In certain locations (primarily near active or recently active volcanoes) there are very hot rocks deep under the earth’s surface. In these "geothermal" regions, the temperature may rise by 40-50°C every kilometer of depth, so just 3 km, the temperature could be 120 to 150°C, well above the boiling point for water. The rocks in these regions will typically have pore spaces filled with water, and the water may still be in the form of liquid water since the pressure is so high down there (in some very hot areas, the water is actually in the form of steam trapped in the rocks). If you drill a deep well into one of these "geothermal reservoirs", the water will rise up and as it approaches the surface, the pressure decreases and it turns to steam. This steam can then be used to drive a turbine that is attached to a generator to make electricity. In some regards, this is very much like a coal or natural gas electrical plant, except that with geothermal, no fossil fuels are burned, which means no carbon emissions.
There are three basic types of geothermal power plants, depending on the type of hydrothermal reservoir:
- Dry steam plants, which draw steam directly from deep underground (a la Old Faithful);
- Flash steam plants, which draw hot water under high pressure up towards the surface. As the pressure decreases, the water boils, which generates steam to power the turbine;
- Binary steam plants, which utilize hot water (perhaps around 150 degrees Celsius) to vaporize another fluid (one with a lower boiling point). This hot vapor then drives the turbine, generating electricity.
The oldest geothermal plant (1904) in the world is Lardarello, in Italy, which is a dry steam plant. The Geysers, in California, is the largest geothermal installation in the world and the only accessible dry-steam area in the United States (other than Old Faithful and the rest of Yellowstone, which is off-limits). Most modern geothermal plants are “closed-loop” systems, which means that the water (or steam) brought up from the surface is re-injected back into the earth, as shown in the figure below. If the water is not replaced, then eventually, the geothermal reservoir will dry up and cease function.
The image is a diagram titled "Binary Cycle Power Plant," illustrating the operation of a binary cycle geothermal power plant.
- Production Well: On the left side of the diagram, there is a vertical pipe labeled "Production well" which goes down into the ground through various rock layers. This well extracts hot geothermal fluid from beneath the Earth's surface.
- Heat Exchanger: The hot geothermal fluid from the production well is directed into a heat exchanger, represented by a rectangular component with internal structures suggesting heat transfer. Inside the heat exchanger, the geothermal fluid transfers its heat to a secondary working fluid without mixing with it.
- Turbine: Above the heat exchanger, there is a turbine, depicted with blades inside a cylindrical housing. The secondary working fluid, now heated, expands and turns the turbine, converting thermal energy into mechanical energy.
- Generator: Connected to the turbine is a cylindrical component labeled "Generator." The mechanical energy from the turbine is converted into electrical energy by the generator.
- Injection Well: After passing through the heat exchanger, the cooled geothermal fluid is returned to the Earth through another vertical pipe labeled "Injection well," which also goes down through the rock layers.
- Load: On the right side of the diagram, the electrical energy generated is shown being used to power a light bulb, representing the "Load" or the end use of the generated electricity.
- Flow of Fluids: The flow of the geothermal fluid is indicated by arrows, showing movement from the production well, through the heat exchanger, and back into the injection well. The flow of the secondary working fluid is shown entering the turbine.
The diagram uses simple, clear lines and labels to illustrate the process of converting geothermal heat into electricity using a binary cycle system, where the geothermal fluid and the working fluid do not mix.