A large portion of the world's installed geothermal generation capacity is located in North America. The United States has 3,086 MW of installed geothermal capacity and is the world's leading geothermal electricity generator. Mexico ranks fourth with 958 MW and new projects all across the region are underway.
A few new high-temperature fields are under development: Blue Mountain in Nevada, which recently came online, is one such project. Additionally, much of the new geothermal power generation that has come online in the past 10 years has been from expansions of geothermal fields already connected to power grids. And while the largest high-temperature geothermal complex in the world, the Geysers in California, boasts 1,517 MW of installed capacity, there is quite a bit happening on the lower end of the temperature scale.
Existing geothermal energy fields that have more untapped energy potential in leasable areas can be hard to come by, but this doesn't hamper the growth of geothermal energy projects. The reason, some experts say, is that low-temperature geothermal projects are becoming more economical through units such as organic rankine cycles (ORCs) and through new technologies like co-production from oil and gas wells.
Low-temperature geothermal projects are defined as projects that use water temps of up to 300 F to produce power and/or those that have output capacities of just a few megawatts. The U.S. Geological Survey has identified more than 120,000 MW of untapped potential at these temperatures, some of which are considered conventional hydrothermal operations, where shallow geothermal fields are used because the deeper field lacks permeability.
Recent low-temperature developments have also been made in oil and gas coproduction projects, generating electricity from the warm wastewater byproduct at oil and gas wells-wells that have already been drilled, eliminating the costs of one huge step of the process.
(Direct use geothermal and heat pump applications can also be classified as low-temperature geothermal projects. But they are generally used for heating and cooling applications as opposed to power generation.)
Low-temperature geothermal power projects have been in existence in the U.S. at least since 1984 when Ormat Technologies deployed a 700 kW net geothermal power unit in Nevada, the first commercial geothermal power plant in the state.
Mexico and Canada
Mexico has four large geothermal plants in operation and is considered a major geothermal player. The country has plans to expand existing projects and start new ones in the future. A low- to intermediate-temperature 300 kW binary unit operated in Maguarichic until the national grid reached the village three years ago, according to the Mexican Geothermal Association. Mexican state power company CFE acquired two similar plants to provide electricity to pumps systems at the Los Humeros and Las Tres V√≠rgenes geothermal fields using residual high-temperature brines.
A 2009 article in Geotermia magazine by Eduardo R. Iglesias and Rodolfo J. Torres, entitled First assessment of low- to medium-temperature geothermal reserves in 20 Mexican states, gives an assessment of low-to-medium geothermal reserves in Mexico and their aggregate value on a state-by-state basis. It said the most likely reservoir temperatures range between 60 to 180 C (140 to 356 F) with a mean of 111 C (232 F). "Such massive amounts of recoverable energy-and the associated temperatures-are potentially important for the economic development of nearby localities and the nation," the authors wrote.
Although Canada has no current installed geothermal power, two low-temperature geothermal projects are in progress, according to the Canadian Geothermal Energy Association. The Alberta Energy Research Institute recently approved "The Borealis Project" for C$2.6 million. The ADK/Borealis Geothermal Demonstration Project will deliver a minimum of about 1 MWe of electrical power and about 1 MWth of direct heat to a community in northern Canada. Another project was set up to research coproduction from oil and gas wells is located in the Canadian foothills.
ORCs are often used in low- to moderate-temperature systems. With a range between 200 F and 350 F, they are often used when brine is not sufficient for flashing the steam. "It is ideal for vapor power cycles," according to an article by Josh Nordquist of Ormat Technologies in a Geothermal Resource Council publication titled Use of Small, Sub 1 MW Organic Rankine Cycle Power Systems and Low Temperature Resources (Vol. 33, 2009).
An example of how ORCs in low-temperature projects can extend the life of existing resources was recently reported in another part of the world. In Taiwan, the Yilang Chingshui Geothermal Project will use two United Technologies Corp. (UTC) PureCycle systems to generate electrical power at an existing geothermal site for the first time in 25 years. With a capacity of 500 kW (net) using 130 C geothermal resources, power production is expected by the end of the year.
Another application of low-temperature ORC systems is through the use of Solar Ponds. In large lakes with high salt content, much of the salt sinks to the bottom. The upper layers of freshwater act as an insulating blanket and the temperature at the bottom of the pond can reach 90 C. This is a high enough temperature to run an organic rankine cycle engine or Stirling engine, according to SolarThermalMagazine.com. The first solar pond ORC system in the United States was a 100 kW system that supplied process heat to a commercial manufacturer. It was installed in Texas in 1986 and produced at 85 C (185 F), according to Ormat Technologies, which supplied the unit.
Chena Hot Springs, Alaska, is the site of the lowest temperature commercial geothermal plant to date, though test units of 1 to 4 kW have run on lower temperatures. Two 210 kW units using 73.3 C (165 F) geothermal Ô¨Çuid as the heat source were installed at Chena in 2006 by UTC. The systems replaced on-site diesel generation, resulting in substantial cost savings.
Additional projects for analysis and development of low-temperature resources and technology are underway by West Virginia University; the City of Klamath Falls, Ore.; Johnson Controls Inc; and Oasys Water.
The first project to co-produce geothermally heated water from an oil well was a research and development project partnership near Casper, Wyo., between Ormat and the U.S. Department of Energy at the Rocky Mountain Oilfield Testing Center (RMOTC). Before this project, wastewater from oil and gas production wells was typically discarded. The project began in 2008 and produces 217kW at 93.3 C. It could be the first of many of its kind; the technology is also being implemented in Nevada, Mississippi, Louisiana, North Dakota and Texas, opening new areas for increased geothermal development. Just such oil and gas coproduction projects could play a prominent role in the future of low-temperature geothermal.
Prospects, Challenges and Support
A fact sheet from the National Renewable Energy Laboratory explains the potential risks to low-temperature projects: "Low-temperature geothermal resources are more difficult to extract power from since the highest temperature in the power generation cycle has a very strong effect on the overall efficiency." But even though they are extracted with more difficulty, NREL points out that low-temperature resources "are widely available, and power generation and electricity unit installations have doubled in the United States in the last 15 years."
One challenge with smaller-scale geothermal development is in maintaining high turbine efficiency at low costs. But, "If well(s) have already been drilled (as with abandoned oil & gas wells), then the project success probability will increase," said Josh Nordquist of Ormat. New research has shown that much of the 25 billion barrels of geothermally heated "wastewater" produced at oil wells each year in the U.S. is hot enough to produce electricity. This looks good to suppliers. "When considering an organic rankine cycle power system, careful project analysis is required to determine an application can be both technically and economically successful," said Nordquist.
Tax incentives aimed at conventional renewable energy technologies thus far do not extend to co-produced water. Tax credits, carbon credits or other renewable energy benefits would likely be useful in compelling oil and gas producers to consider geothermal coproduction. Continued federal support for all low-temperature projects through both R&D and financial support is needed.
The DOE's Geothermal Technologies Program is one program that does offer federal support for low-temperature technologies, developing new low-temperature and co-production technologies through partnerships in industry and academia. Its goal is the widespread production of low-temperature power by 2020, which DOE hopes to accomplish through support of research and development projects. Supported low-temperature projects beyond ORC units and co-production demos include areas such as improving working fluids and using innovative cooling technologies.
NREL is the DOE lead for low-temperature R&D including oil and gas co-production of electricity, direct use and geothermal heat pumps. Additionally NREL uses research initiatives to define pathways for broad commercial impact of geothermal systems, conducts R&D in advanced power conversion systems, and is involved with systems engineering and integration, according to Dr. Dan Arvizu, laboratory director in an address at a NASEO conference in February. And at the GTP Peer Review in May, Tom Williams of NREL discussed a center for low-temperature geothermal research that will focus on research with quick market impacts.
Oil and Gas Co-production
Aside from funding, there may also be technology-related issues to resolve with oil and gas co-production wells. For example, wells are usually deep and those over 300 F may lose heat to 200 F or so near the surface. Furthermore, the flow rate from these wells can be far lower than conventional geothermal wells. But despite these challenges, many test projects are now underway.
A test plant associated with Southern Methodist University at Brazoria near Houston produced 500 kW from geothermal heat and 500 kW from natural gas for five years at a temperature of 300 F.
A Gulf Coast Green Energy (GCGE) coproduction project at the Denbury oilfields in Laurel, Miss., is replacing Denbury's electric submersible pump with ElectraTherm Green Machines to cut electricity costs by one-third. GCGE has a second 50 kW geothermal natural gas co-production project in Louisiana. And at University of North Dakota, a $1.7 million award through the DOE's Geothermal Technologies Program will help install a geothermal ORC system at another Denbury oilfield.
Universal GeoPower CEO and petroleum geologist George Alcorn Jr. and his business partner, Chris Luchini, a Ph.D physicist will use $1.5 million from a federal stimulus funding award to bring geothermal energy to Liberty County, Texas using existing oil wells. This eliminates the need for investment in drilling, new roads or transmission lines, Alcorn said at a recent Geothermal Energy Association meeting in Washington, D.C.
"The lead-time to revenue generation is about six months, whereas traditional geothermal can take up to five years," he said. "The wells already have known geothermal potential and capital costs are dramatically reduced."
Expanding existing geothermal fields have provided a backbone for geothermal development over past years, paving the way for small-unit expansions of oil and gas fields and new small and low-temperature projects. DOE and NREL are taking the lead in federal initiatives in these areas. Despite challenges, many test projects in oil and gas co-production are underway in the U.S. and have shown positive results. These technologies could be implemented to expand on line capacity in Mexico, develop operating fields in Canada and carry out further geothermal activities worldwide.
If government support is adequate and innovative research continues, low-temperature geothermal units and developments in coproduction projects such as these could open up new areas of North America and the world for geothermal utilization.
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