What are Geothermal Power Plants?

There are three geothermal power plant technologies being used to convert hydrothermal fluids to electricity. The conversion technologies are dry steam, flash, and binary cycle. The type of conversion used depends on the state of the fluid (whether steam or water) and its temperature. Dry steam power plants systems were the first type of geothermal power generation plants built. They use the steam from the geothermal reservoir as it comes from wells, and route it directly through turbine/generator units to produce electricity. Flash steam plants are the most common type of geothermal power generation plants in operation today. They use water at temperatures greater than 360°F (182°C) that is pumped under high pressure to the generation equipment at the surface. Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units.

Types of Geothermal Power Plants

Dry Steam Power Plants

Photo of dry steam power plants.

Dry steam power plants in California.

This is the earliest form of geothermal power plant, which directs steam into turbines to produce electricity. Excess heat from the production well is channeled back into the reservoir via an injection well. This type of generator was first used in 1904, to generate electricity in Lardarello, Italy, where it still stands today, fully operational. The United States have also built dry steam power plants, including those in Northern California geysers.

Steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. (Also eliminating the need to transport and store fuels!) This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904, and is still very effective. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal power. These plants emit only excess steam and very minor amounts of gases.

Illustration of a Dry Steam Power Plant -  Geothermal steam comes up from the reservoir through a production well.   The steam spins a turbine, which in turn spins a generator that creates  electricity.  Excess steam condenses to water, which is put back into  the reservoir via an injection well.

Flash Steam Power Plants

Hot springs above 1750ºC may is used to power Flash Steam Power Plants. These hot fluids are channeled to a low pressure flash tank, magnifying its steam formation. This flash steam is then used to power turbines, activating the generator to produce electricity. Excess heat is returned to the reservoir by means of an injector well. One example of a flash steam power plant is the Cal-Energy Navy I, located in Coso Geothermal Field, California.

Hydrothermal fluids above 360°F (182°C) can be used in flash plants to make electricity. Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator. If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy.

Illustration of a Flash Steam Power Plant -  Pressurized geothermal hot water comes up from the reservoir through a  production well.  The water enters a flash tank where it depressurizes  and flashes to steam.  The steam then spins the turbine, which in turn  spins a geneator that creates electricity.  Excess steam condenses to  water, which is put back into the reservoir via an injection well.

Binary-Cycle Power Plants

This type of power plant use a completely different method compared with the above systems, where the steam from production wells does not directly come into contact with the turbines. Steam is used to heat working fluids in the heat exchanger, which then generates flash steam. This steam is then used to power the turbines and generator to produce electricity. Steam from the heat exchanger is what’s called Binary / Secondary Fluid. This is a closed loop system, where no excess heat is released into the air.

BCPP is able to be operated in low temperatures, between 90-1750ºC. One example of this technology is the Mammoth Pacific Binary Geo-Thermal Power Plants in Casa Diablo geothermal field. This technology is a glimpse of future geothermal technology, one that will be used in the future.

The Agency For the Assessment and Application Technology (BPPT) has built a prototype 2KW binary cycle power plant with hydrocarbon as its primary fluid. BPPT has also planned to develop small scale power plants between 2010-2014 which includes a 1 MW binary cycle power plant (targeted for 2014) through a 2 KW prototype (2008) and 100 KW pilot project (2012), and the development of condensing turbine power plant technology with a capacity of 2-5 MW (2011 and 2013).

Most geothermal areas contain moderate-temperature water (below 400°F). Energy is extracted from these fluids in binary-cycle power plants. Hot geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines. Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.

Illustration of a Binary Cycle Power Plant  - Illustration of a binary-cycle power plant.  Geothermal hot water  comes up from the reservoir through a production well.  The hot water  passes by a heat exchanger that is connected to a tank containing a  secondary hydrocarbon fluid.  The hot water heats the fluid, which turns  to vapor.  The vapor spins a turbine, which in turn spins a generator  that creates electricity.  The hot water continues back into the  reservoir via an injection well.  This closed-loop system produces no  emissions.

The Future of Geothermal Electricity

Steam and hot water reservoirs are just a small part of the geothermal resource. The Earth's magma and hot dry rock will provide cheap, clean, and almost unlimited energy as soon as we develop the technology to use them. In the meantime, because they're so abundant, moderate-temperature sites running binary-cycle power plants will be the most common electricity producers.

Before geothermal electricity can be considered a key element of the U.S. energy infrastructure, it must become cost-competitive with traditional forms of energy. The U.S. Department of Energy is working with the geothermal industry to achieve $0.03 to $0.05 per kilowatt-hour. We believe the result will be about 15,000 megawatts of new capacity within the next decade.