The issue
Geothermal energy is a form of renewable energy that uses geothermal heat to generate electricity. Geothermal energy is renewable because the earth’s core generates a nearly unlimited amount of heat.
U.S. Geothermal Resources
Source: National Renewable Energy Laboratory
Geothermal energy has been used by utilities in the Unites States since 1960 when Pacific Gas and Electric built the first large-scale geothermal electric generating plant, an 11-MW dry steam geothermal facility near San Francisco, California.
Types of geothermal power plants
Geothermal power plants are generally subdivided into three types: dry steam plants, flash plants and binary cycle plants. The plant type selection is based on the temperature available in the well fields.
Dry steam plants route steam directly from their geothermal source to the turbine to generate electricity. This is the oldest form of geothermal power with its first use dating back to Lardello, Italy in 1904.
Flash plants use water in a liquid form but at higher temperatures, typically 347° to 572°F. Because these plants are dealing with a liquid, they do not have geothermal steam available and must create it. These plants create steam by routing the water to a facility where the pressure is lowered so that the fluid “flashes” into steam and is then used to spin the turbine.
Binary cycle plants generate electricity from the lowest range of water temperatures, 194° to 347°F. Binary cycle plants use two liquids, one from the geothermal source to generate heat and one to spin the turbine. These plants generate electricity by routing the geothermal water through a closed loop heat exchanger to heat a working fluid into steam which is then used to spin the turbine.
Net Generation from Geothermal Plants
Source: U.S. Energy Information Administration
Low temperature geothermal resources represent the most abundant geothermal resource and, therefore, binary cycle plants are the type of plant with the greatest opportunity for additional development. In addition, future plants may incorporate some means of harnessing the heat of the earth’s hot dry rock and magma as a heat source to generate steam.
Potential issues
The primary potential issue with geothermal power is identifying and developing a suitable location that will produce geothermal power at a cost comparable to that of fossil generated power. In addition to the need to find sites with the necessary amount of geothermal heat, sites that do not have a geothermal water source must supply the water themselves. Geothermal plants can conserve water by using a closed loop process or, in cases where geothermal water is present, by re-injecting that water back into the reservoir at a depth below groundwater levels.
Current usage and projects
As of April 2010, geothermal power plants accounted for 3,086.6 MW of installed capacity. In 2009, 176 MW of capacity was brought online.
The Geothermal Energy Association’s 2010 US Geothermal Power Production and Development Update report identifies projects currently under development in states where AEP operates. Louisiana currently has two projects totaling 5.3 MW of capacity in the early stages of development. Texas currently has a small 0.4 MW capacity project in the early stages of development.
Geothermal Capacity Online April, 2010
Source: Geothermal Energy Association
Geothermal Capacity Additions
Source: Geothermal Energy Association
