Geothermal Background

Geothermal resources are reservoirs of hot water that exist at varying temperatures and depths below the Earth's surface. Wells can be drilled a mile or more into underground reservoirs to tap steam and very hot water that can be brought to the surface for use in a variety of applications, including electricity generation, direct use, and heating and cooling. In the United States, most geothermal reservoirs are located in the western part of the country.

Commercial geothermal power generation is an established industry both in the U.S. and around the world. Italy was the first country to develop geothermal power commercially in 1914 at Larderello. This was followed by plants at Wairakei, New Zealand in 1958 and at the Geysers in California in 1960.

Over the past three decades, geothermal energy production has grown at a significant rate both nationally and internationally. World power capacity from geothermal resources is estimated at over 11 gigawatts, and annual power production from geothermal energy is estimated to be approximately 67,000 GigaWatt-Hours (Gw-h). The U.S. leads the world in geothermal power generation with about 30% of the world’s installed geothermal power capacity.

Generating Electricity with Geothermal Power

There are three general types of geothermal plants used to generate electricity. The type of plant is determined primarily by the nature of the geothermal resource at the site.

Direct steam geothermal plants are used when the geothermal resource produces steam directly from the well. The steam, after passing through separators (which remove small sand and rock particles) is fed to the turbine. These were the earliest types of plants developed in Italy and in the U.S. Direct steam plants in the U.S. have been installed in capacities up to 110 Megawatt (MW) at the Geysers in California. Unfortunately, direct-steam resources are the rarest of the all geothermal resources and exist in only a few places in the world. Direct Steam plants are used only with high-temperature resources.

Flash steam plants are employed in cases where the geothermal resource produces high-temperature hot water or a combination of steam and hot water. The fluid from the well is delivered to a flash tank where a portion of the water flashes to steam and is directed to the turbine. The remaining water is directed to disposal (usually injection). Depending on the temperature of the resource, it may be possible to use two stages of flash tanks. In this case, the water separated at the first stage tank is directed to a second stage flash tank where more (but lower pressure) steam is separated. Remaining water from the second stage tank is then directed to disposal.

Binary plants are used for lower temperature geothermal applications. The name derives from the fact that a second fluid in a closed cycle is used to operate the turbine rather than geothermal steam. Figure 1 presents a simplified diagram of a binary-type geothermal plant. Geothermal fluid is passed through a heat exchanger called a boiler or vaporizer (in some plants, two heat exchangers in series, the first a preheater and the second a vaporizer) where the heat in the geothermal fluid is transferred to the working fluid causing it to boil. Past working fluids in low-temperature binary plants were CFC (Freon type) refrigerants. Alternatively, plants may use hydrocarbons (isobutane, pentane, etc.) of HFC type refrigerants, with the specific fluid chosen to match the geothermal resource temperature.

Figure 1. Binary Geothermal Power Plant

The working fluid vapor is passed to the turbine where its energy content is converted to mechanical energy and delivered through the shaft to the generator. The vapor exits the turbine to the condenser where it is converted back to a liquid. In most plants, cooling water is circulated between the condenser and a cooling tower to eject this heat to the atmosphere. An alternative is to use so called “dry coolers” or air-cooled condensers which eject heat directly to the air without the need for cooling water. This design essentially eliminates any consumptive use of water by the plant for cooling.   Liquid working fluid from the condenser is pumped back to the higher pressure preheater/vaporizer by the feed pump to repeat the cycle.