Electrical Power Generation

Cost-Benefit Analysis for Geothermal Systems

 

Geothermal energy currently produces more than 8,000 megawatts of electricity worldwide. The United States produces over 2,200 MW. California and Nevada account for over 90 percent of the US capacity for geothermal electricity. Geothermal power currently provides about 8 percent of California's total electrical demand. There are various development plans to add more than 500 MW of US geothermal electrical capacity over the next decade. The cost of geothermal electricity in the U.S. ranges from less than $.03 to $.08 per kilowatt-hour. The lowest-cost geothermal producers sell power for $.015 per kilowatt hour. The Geysers in northern California sells power at $.03 to $.035 per kilowatt hour. Many geothermal power plants built today are economic at about $.09 per kilowatt hour.

Geothermal production costs depend on reservoir depths, water temperatures, chemical composition of fluids, locations, and a variety of operational and maintenance considerations. The economics of geothermal power can be improved through co-production of goods and services from high-temperature geothermal brine. Combining power generation with direct use (space heating, pools) is the most common economic improvement to project economics.The geothermal industry, with assistance from the U.S. Department of Energy, is working to lower geothermal electricity to $.03/kW hour. It is anticipated that costs in this range will result in about 15,000 MW of new capacity installed by U.S. companies in the ensuing decade. Large areas in the western United States have significant potential for new, or expanded, geothermal production. Geothermal power plants are also extremely reliable and usually operate more than 95 percent of the time, with some plants over 99 percent. This compares to 60-70 percent operational times for coal and nuclear power plants.

The current geothermal power plants are of two general types, either steam driven or binary power plants. Steam plants are the largest and most cost-effective systems when the water temperature is above 175 C. The water is expanded to steam and passed through a turbine to generate electricity. For steam-dominant systems (like Geysers), the steam is used directly, for hot water-dominant systems the water is flashed to steam in a low pressure tank and then used to drive a turbine. For lower temperature resources (below 350 F), flash plants are inefficient and heat is better processed by a secondary fluid. These are called Binary Power plants. The volatile working fluid (usually isobutene or isopentane) vaporizes at lower temperatures than water. This vapor is then passed through the turbine to generate electricity. These plants usually have higher equipment costs, but they are much more applicable to small scale plants, and since the geothermal fluid does not vaporize they often have fewer problems with saline geothermal solutions.

Small scale, binary, power plants have several distinct advantages for use with known Montana geothermal resources. Most importantly, water temperatures of less than 350 F can be effectively used in binary power plants. Also binary plants are small enough to locate near urban, or recreational areas with little notice. Binary plants can also be of modular design so that a small plant investment can be increased after time. It is also relatively easy to design the plants to operate automatically, thus considerable reducing operational costs. Finally, binary plants are well suited for a secondary, or metered, electrical source, which can obtain retail, rather than wholesale, electrical rates.

Binary power plant assessments for Montana resources assume that moderate geothermal fluid temperatures are available at reasonable or shallow depths with low operation costs. Most systems for these resources rely on air-cooled binary cycle power plants (Organic Rankine Cycle) that can be located near the geothermal well site.

Recent advances in binary power technologies are illustrated at the new Chena, Alaska binary power plant. The Chena geothermal power plant came online in July 2006, and is the lowest temperature geothermal power resource in the world. The geothermal water at Chena is only 165 F. As at all binary plants, a secondary fluid, with a lower boiling point is used to generate steam.  Heat from the geothermal water causes the secondary fluid to flash to vapor which then drives the turbine.  Because this is a closed loop system, virtually nothing is emitted to the atmosphere. UTC and Carrier refrigeration have expanded on the use of temperature differences between condenser and evaporator as the driving force. At Chena there is an abundant supply of 40-50°F cooling water, allowing for a working temperature range of 120°F. Combined, this temperature difference with off-shelf power equipment allowed for an economical plant construction.