In this article we are going to look closer at the technology behind geothermal electricity, or in other words, how we can convert geothermal energy to electrical energy.
There is currently only 10,715 MW geothermal power installed across 24 different countries (May 2010), just a tiny fraction of the World’s consumption of electricity. The upper estimate of our geothermal resources reveals that the total potential is more than adequate to supply humanity with energy.
How is Geothermal Energy Converted to Electricity?
There are several different main types of geothermal plants:
- Dry steam
- Flash steam
- Binary cycle
What these types of geothermal power plants all have in common is that they use steam turbines to generate electricity. This approach is very similar to other thermal power plants using other sources of energy than geothermal.
Water or working fluid is heated (or used directly incase of geothermal dry steam power plants), and then sent through a steam turbine where the thermal energy (heat) is converted to electricity with a generator through a phenomenon called electromagnetic induction. The next step in the cycle is cooling the fluid and sending it back to the heat source.
Water that has been seeping into the underground over time has gained heat energy from the geothermal reservoirs. There no need for additional heating, as you would expect with other thermal power plants. Heating boilers are not present in geothermal steam power plants and no heating fuel is used.
Production wells (red on the illustrations) are used to lead hot water/steam from the reservoirs and into the power plant.
Rock catchers are in place to make sure that only hot fluids is sent to the turbine. Rocks can cause great damage to steam turbines.
Injection wells (blue on the illustrations) ensure that the water that is drawn up from the production wells returns to the geothermal reservoir where it regains the thermal energy (heat) that we have used to generate electricity.
Depending on the state of the water (liquid or vapor) and its temperature, different types of power plants are used for different geothermal reservoirs. Most geothermal power plants extract water, in its vapor or liquid form, from the reservoirs somewhere in the temperature-range 100-320°C (220-600°F).
This type of geothermal power plant was named dry steam since water water that is extracted from the underground reservoirs has to be in its gaseous form (water-vapor).
Geothermal steam of at least 150°C (300°F) is extracted from the reservoirs through the production wells (as we would do with all geothermal power plant types), but is then sent directly to the turbine. Geothermal reservoirs that can be exploited by geothermal dry steam power plants are rare.
Dry steam is the oldest geothermal power plant type. The first one was constructed in Larderello, Italy, in 1904. The Geysers, 22 geothermal power plants located in California, is the only example of geothermal dry steam power plants in the United States.
Geothermal flash steam power plants uses water at temperatures of at least 182°C (360°F). The term flash steam refers the process where high-pressure hot water is flashed (vaporized) into steam inside a flash tank by lowering the pressure. This steam is then used to drive around turbines.
Flash steam is today’s most common power plant type. The first geothermal power plant that used flash steam technology was the Wairakei Power station in New Zealand, which was built already in 1958:
The binary cycle power plant has one major advantage over flash steam and dry steam power plants: The water-temperature can be as low as 57°C (135°F).
By using a working fluid (binary fluid) with a much lower boiling temperature than water, thermal energy in the reservoir water flashes the working fluid into steam, which then is used to generate electricity with the turbine. The water coming from the geothermal reservoirs through the production wells is never in direct contact with the working fluid. After the some of its thermal energy is transferred to the working fluid with a heat exchanger, the water is sent back to the reservoir through the injection wells where it regains it’s thermal energy.
These power plants have a thermal efficiency rate of only 10-13%. However, geothermal binary cycle power plants enable us, through lowering temperature requirements, to harness geothermal energy from reservoirs that with a dry- or a flash steam power plant wouldn’t be possible.
First successful geothermal binary cycle project took place in Russia in 1967.
Cogeneration (Combined Heat and Power)
Depending on what type of geothermal power plant, location and various other factors, the thermal efficiency rate is not more than 10-23%. Technically, low efficiency rates do not affect operational costs of a geothermal power plant, as it would with power plants that are reliant on fuels to heat a working fluid.
Electricity generation does suffer from low thermal efficiency rates, but the byproducts, exhaust heat and warm water, have many useful purposes. By not only generating power, but also taking advantage of the thermal energy in the byproducts, overall energy efficiency increases. This is what we call geothermal cogeneration or combined heat and power (CHP). Here are some good examples of this:
- District heating
- Timber mills
- Hot springs and bathing facilities
- Snow and ice melting
- Desalination (processes that remove salt and other minerals from saline water)
- Various other industrial processes
How is geothermal energy transported? It is not a surprise that the electricity that is generated with geothermal power plants is transported in the same way as you would with any other power plant (or a wind or solar farm for that matter): Voltage is increased to minimize losses and the current is sent onto the electrical grid. Transporting heat over long distances, as you would with CHP, requires a heavily insulated piping system, which is a significant addition to costs.
Above is a picture of Blue Lagoon geothermal spa that uses warm wastewater from Svartsengi Power Station in the background.