Like hydroelectric power, geothermal energy can be extremely reliable and cost-effective. It is a well-established technology that uses several different methods to harvest heat from underneath the earth’s surface. As with many other forms of renewable energy, geothermal energy plants must be located in appropriate areas. In this case, potential sites include areas with volcanic activity, tectonic shifting, major hot springs or geysers, where the earth’s heat is very near the surface. In the United States, most geothermal resources are located in the western portion of the country, along the numerous fault lines on the western seaboard and in the Rocky Mountains. The U.S. is a world leader in geothermal energy, with nearly 3,000 megawatts of installed capacity, accounting for 35% of world capacity. (Nonetheless, geothermal generation is a very minor percentage of total electric power in the U.S. As of 2005, geothermal power accounted for a mere 0.35% of the country’s total power generation.) In many parts of the U.S., smaller geothermal resources are used to heat buildings or to provide commercial quantities of hot water, but are not used to generate electricity.
This may change with the development of up to 74 new projects mostly in the western U.S. that could more than double capacity. Backed by federal tax credits, utility companies are looking to geothermal as a greater power source. The Massachusetts Institute of Technology conducted a 2007 study that concluded that as much as 100,000 megawatts of U.S. power generation could come from geothermal resources by 2050.
There are two predominant techniques for generating geothermal electricity, depending on the type of heat resource: flash steam and binary cycle. High temperature locations can be tapped directly, using steam coming out of the ground to drive a turbine in a technique known as flash steam generation. This is the most common plant type in use. Where lower-temperature geothermal sources are tapped, steam is used to heat another liquid with a lower boiling point, which then drives the turbine. This technique is known as binary cycle generation. The drawback of binary cycle generation is that it is much less efficient than flash generation. Engineers have also begun combining flash and binary generation, which together increase the efficiency of a plant.
Iceland is a respected leader field in geothermal and hydroelectric power. Even though the country's capacity for both is less than that of some other countries, the low-population island nation of Iceland supplies more than 50% of its energy needs with geothermal energy and another 17% by hydroelectric. Generating such a massive amount of energy with these sources is made possible by the island nation’s incredible natural resources, but was brought to bear by a concentrated effort by the government and the people.
Technology developed at Los Alamos National Labs (LANL) in New Mexico may create new opportunities for the utilization of geothermal plants. In a 26-year-long project, LANL was able to develop the tools necessary to harvest heat from almost anywhere on earth. Called Hot Dry Rock Geothermal Energy Technology (HDR), the technique drills holes into the ground until they reach rock that is suitably hot. Then, pipes are installed in a closed loop. Water is pumped down one pipe, where it is heated to extraordinarily high temperatures. The resulting steam shoots up to the surface. This steam drives a turbine that powers an electric generating plant. As the steam cools, it returns to liquid state and is pumped back into the ground. There are several development projects in place around the world, including Japan, France and Australia.
Another new technique makes it possible to produce power from hot springs that were previously thought too cool to efficiently use for geothermal efforts. The Chena hot springs in Alaska average about 109 degrees Fahrenheit, but the springs’ owners and engineering conglomerate United Technologies (
www.utc.com) have devised a method using a refrigerant called R134a to drive turbines. Water from the hot springs is used to heat R134a, which has a relatively low boiling point. A gas similar to steam is produced, which drives the turbines. Cooler temperatures yield smaller amounts of gas, so the designers of the Chena plant compensated by slashing operating costs. Mass-produced air conditioner parts were substituted for standard geothermal components, a scheme that will likely be adopted by geothermal plants the world over.
Meanwhile, the Iceland Deep Drilling Project (IDDP), funded by a consortium of three Icelandic energy companies, is tapping extremely hot steam in an existing geothermal well at depths of up to 2.5 miles, which is close enough to the Earth’s layer of magma to produce steam of over 1,100 degrees Fahrenheit. The drilling and collecting equipment necessary is more expensive than standard geothermal machinery, due to the higher pressures and temperatures found at great depths. However, proponents of the project believe that the extra costs (which might double or triple) will be easily regained because the amount of electricity produced is expected to multiply by as much as 10 times. The IDDP was in testing stages as of the end of 2007; however, there are plans to drill the first deep hole in Northeastern Iceland in 2008, with two more planned for 2009 and 2010 respectively.
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