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| Frances Li | francesl@usc.edu | Undeclared Engineering |
Geothermal: Power Plants
There are three different power plants. Wells that are more than a mile deep are drilled to underground reservoirs to tap steam and scorching water to power turbines, that drive electricity generators. The three types of geothermal power plants are dry steam, flash steam, and binary cycle.
Dry steam power plants uses the steam directly from the geothermal reservoir to run the turbines. There are only a few reservoirs that produce mostly steam. The steam shoots through a rock-catcher and into a turbine. The turbines spin to drive the generator that produces the electricity. This is the oldest type of a geothermal plant and it was first used in Lardarello Italy in 1904. The Geysers is the largest dry-steam power plant that is able to supply electricity to a city the size of San Francisco.
Most geothermal plants use the flash steam system. When hot water from the production wells is passed through separators. Flash steam is produced when the water it is transfered from high the high pressure of the deep reservoir. The fluid is sprayed into a tank that is held at lower pressure that the fluid, causing the fluid to rapidly vaporize to steam. The steam powers the turbine generators. Hydrothermal fluids have to be above 400 F. Left over liquid can be flashed again to create more electricity. After the steam runs through the turbines, it is condensed and injected back down into the reservoir.
Binary-cycle power plants use moderately hot geothermal water and another chemically safe secondary liquid. This power plant has moderate temperature waters below 400F. The heated water goes through a heat exchanger that heats the second liquid, a working fluid, turning it into steam to run the turbines. The binary liquid is condensed and again tunneled through the exchanger to be reused. The flashed secondary liquid causes the turbines to spin and run the generator to create electricity. The binary liquid is usually isobutane or isopentane. Binary power plants use water more efficiently. The close looped system does not have any or little emissions.
These geothermal technologies are used only in small portions of the nation. Earth heat is readily available miles in the earth, but it is a matter of tapping earth’s heat. We are in need of technologies to tap this heat effectively. Currently, there are technology that is being developed to drill into the dry rock that is heated by the molten magma. Cold water would be tunneled through the hot rock and drawn from another well.
Larderello, Italy has been using geothermal energy since 1913. Wairakei, New Zealand had integrated the systems since 1958, and the Geysers have been up and running since 1960.
Cost per kWh
Currently, the lowest cost of geothermal electricity is about $0.015 per kilo-Watt hour. The Geysers in California has sold their power for $0.03 to $0.035 per kWh. A power plant that is built today would require about $0.05 per kWh. The cost of energy is also dependent on other factors such as supply and demand. Geothermal energy production accounts $1.5 billion per year of the energy industry. The geothermal energy and the United States Department of Energy has been trying to reach $0.03 per kWh.
The process of building a geothermal power plant is a long and expensive process. First, the wells have to be drilled, and the pipeline must be constructed. Several wells have to be drilled, and most are slant or advanced directional to have less impact on the land. Several wells can be build on the same plot of land. A resource analysis of the drilling information is done. Then the design of the plant is considered and completed with the final field development. The initial cost for the field and power plant is about $2000 per installed kWh in the United States. It costs about $3000 to $5000 per kWe for a small power plant, and $1500 to $2500/kWe for a larger plant. This of course depends on the temperature and chemistry of the resource, which contributes to the efficiency of power plant. Operating and maintenance costs go from $0.015 to $0.045 per kWh, depending on the contract price for the electricity. Most geothermal plants are able to run for 90 percent of the time because if it runs even longer, maintenance and operation prices could increase. If the energy is more expensive, then the running efficiency of the plant can cover operating and maintenance costs.