Geothermal energy uses the heat of the earth to provide for direct heat or electricity production. Direct heat geothermal uses low to moderate temperature water to heat structures, grow plants in greenhouses, and in industrial processes such as drying food or fish farming. These systems pump hot water directly into the structures they are warming. Producing electricity from geothermal uses high temperature resources to convert heat into power, though new technologies are emerging that allow lower temperature resources to be utilized in electricity generation.
Currently, three types of geothermal electric generators are in use:
Dry stream power plants use steam emitted directly from geysers or fumaroles to turn turbines and create electricity. These require relatively rare, very hot hydrothermal systems, where almost all the water is in steam form under the surface of the earth. The world’s largest group of geothermal power plants, The Geysers in northern California, are examples of dry stream power plants. The Geysers have a combined 725 MW of operating capacity and there are plans to add an additional 80 MW in the near future.
Flash steam power plants require geothermal fluids in excess of 360°F. These fluids are pumped into a tank held at a very low pressure, causing the fluids to vaporize instantly. The resulting steam is used to drive a generator. Most geothermal power plants in operation today are flash steam power plants.
Binary-cycle power plants use a new technology that requires only moderately hot water. These power plants generate electricity by pumping hot water into a heat exchanger where a fluid with a lower boiling point than water is stored. The hot water causes the other fluid to vaporize and the resulting steam turns a turbine to generate electricity. The fluid is cooled and returned to the heat exchanger, creating a closed system. Many flash steam power plants also employ a binary cycle to capture more energy after the very hot geothermal fluids have condensed and cooled. Since most geothermal resources in the world are low-to-moderate heat, the number of binary-cycle power plants in operation will likely increase in the future.
Geothermal Potential in Alaska
Alaska’s location on the Ring of Fire, a volcanic arc circling the Pacific Ocean, means there are many opportunities for geothermal development in the state. There are over 130 volcanoes and volcanic fields that have been active in Alaska in the last 10,000 years, and an additional 100+ sites where thermal springs and wells have been identified. In a project completed in 1982, the USGS identified four major regions that warranted further study for their geothermal potential. These regions were 1) the Interior Hot Springs, running east-west from Canada’s Yukon Territory to the Seward Peninsula, 2) the Southeast Hot Springs north-east of Ketchikan, 3) the Wrangell Mountains and 4) the Ring of Fire volcanoes on the Aleutian Chain, the Alaska Peninsula, and Mt. Edgecumbe on Kruzof Island. The Interior and the Southeast both have low to moderate geothermal systems with surface expression as hot springs. The Wrangell Mountains and the Ring of Fire are comprised of active volcanoes surrounded by high-temperature hydrothermal systems manifested as hot springs, geysers and fumarole fields.
While some communities in Alaska are considering using geothermal resources for energy production, the greater value to Alaskans from geothermal sources may be direct heat. An Institute of Social and Economic Research (ISER) study showed that the average annual cost for heating a home in Alaska was around $4500 at May 2008 prices. Geothermal direct heat could provide an environmentally sound way to effectively heat many homes in Alaska at a reduced price. While new technologies are making energy production from geothermal sources more economically viable, the greatest challenge to Alaskans is the remote location of much of our geothermal potential.
In July of 2006, Chena Hot Springs Resort near Fairbanks installed a 400kW binary-cycle power generator produced by United Technologies Corporation (UTC) that runs on 165°F water, the lowest temperature for an operating geothermal power plant in the world. At 400 kW, the original $2.1 million project displaces 150,000 gallons of diesel annually and saves over $450,000 a year based on $3.00/gallon fuel prices. An additional 280 kW of generating capacity has been installed since then, but is not yet in operation. In addition to the electric power plant, the Chena Resort uses its geothermal resources for outdoor baths, district heating, swimming pool heating, and to provide heat and carbon dioxide to its greenhouses. The site also demonstrates the use of geothermal energy for refrigeration. The resort installed a 16-ton absorption chiller in 2005 to provide chilling to an outdoor ice museum, which is kept frozen year-round. The chiller uses water from a 165°F well as a heat source, and a 40°F creek as a heat sink. This technology has potential applications in other Alaska communities that could use waste or geothermal heat to provide cooling for fish processing, ice production and community cold storage. Research is ongoing at Chena Hot Springs to determine the full geothermal potential of the area, with the goal of fully powering the resort with clean geothermal power and providing a blueprint for geothermal development projects in other parts of Alaska.
Exploration of geothermal potential is increasing statewide. Drilling and exploration done at Mt. Makushin near Unalaska in the 1980s indicates that tens of megawatts could be generated using geothermal resources. The adoption of binary-cycle power generators has made this project economically feasible and in early 2008 the Alaska Energy Authority gave a matching grant of $1.5 million to the City of Unalaska for further drilling in the area in the summer of 2009. The City of Akutan is planning geophysical and geochemical exploration and possible drilling in the summer of 2009 at the nearby Hot Springs Valley to investigate providing power and heat to the city and a local fish processor. Mt. Spurr, near Anchorage, is being considered for large-scale development to diversify the Railbelt’s energy supply. In 2008, the State of Alaska awarded leases for geothermal exploration at Mt. Spurr to Ormat Technologies, Inc., a worldwide leader in geothermal power plants.