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Research and DevelopmentNREL's R&D efforts are described below. After each project summary, the principal investigators are named, with links to biographies and e-mail. Also provided are links to more information or documents regarding the research. The documents are available as Adobe Acrobats PDF's. Download Adobe Reader. NREL's quest to lower the costs of producing geothermal energy has led to five specific areas of research concentration:
Additional projects, not related to power plant efficiency, are:
Program ContactsU.S. Department of Energy National Renewable Energy Laboratory
DOE Geothermal Energy Program, Condensation of Mixed Working Fluids
Studies at NREL, Idaho National Laboratory (INL), and elsewhere have shown that using working fluid mixtures in binary-cycle geothermal power plants can reduce thermodynamic inefficiencies in the boiler and condenser, thereby improving overall plant efficiency. However, very little data are available on the heat transfer characteristics of mixed working fluids undergoing condensation. NREL is performing laboratory experiments to study this behavior and generate heat transfer correlations that can be used in equipment design. Our work focuses on water-ammonia, a potential mixed working fluid that has particular application in certain advanced binary power cycles, such as the Kalina cycle.
Contact: Gerald Nix gerald_nix@nrel.gov
Heat exchangers used at geothermal power plants are often exposed to highly fouling (by scale buildup) or corrosive brine. Such fouling can significantly decrease thermal conductivity. Materials used in heat exchangers include expensive stainless steels, nickel-based alloys, and titanium. This project is developing polymer-based coatings, suitable for a geothermal environment, that can be applied to inexpensive carbon steel. The coatings provide corrosion and scaling resistance as good as or better than that demonstrated by conventional, high-cost materials. Our work entails developing coatings and methods of application, lined heat exchanger design, and field tests. The work is being accomplished by NREL, Brookhaven National Laboratory (BNL) in New York, and industry. BNL is developing inexpensive, durable, thermally conductive liner systems using polyphenylene sulphide (PPS) and fillers, such as silicon carbide, that can protect mild (ordinary) steel from corrosion. BNL also produces lined tubes that are field-tested at a geothermal power plant in a scaled-down heat exchanger apparatus. BNL then analyzes the structure and chemical composition of the scale that forms in the tube. NREL builds and maintains the test equipment, conducts the field experiments, analyzes the data on heat transfer through the lined tubes, and evaluates the overall performance of the lined tubes. Industry partners include CalEnergy Operating Company, which supplied a test site in the Salton Sea Known Geothermal Resource Area in southern California; Mammoth Pacific LP, which is currently evaluating the coatings in a long-term test in a high-temperature geofluid environment in northern California; Steamboat Geothermal, which will be testing coated "coupons" (small squares of test material) in Nevada; Thermochem, which tested coatings at a Blundell, Utah, flash plant; and Bob Curran & Sons Corp., which is commercializing the coatings technology and making it available to clients in a variety of industries. Contact: Keith Gawlik, keith_gawlik@nrel.gov
NREL has succeeded in developing and licensing an advanced direct-contact condenser for water-cooled power plants. We are now turning our attention to the need for improved air-cooled condensers used in many binary-cycle geothermal power plants. Because of their low operating temperatures, geothermal power plants reject approximately 90% or more of the geothermal heat energy to the ambient air. The power output from some plants can vary by about 1% for every degree of change in the condensing (air) temperature. Improved heat rejection can have a significant impact on overall plant performance. This is especially true in binary plant applications, many of which use air-cooled condensers due to the lack of make-up water. Because binary plants use lower-quality hydrothermal resources, the performance of these plants is more sensitive to the air temperature and condensing temperature changes. The air-cooled condensers use large airflow rates, requiring significant fan power—on the order of 10% of the plant output. Better performing air-cooled condensers will improve plant output, lower costs, and reduce the parasitic fan power requirement. NREL and INL are investigating ways to improve the heat transfer effectiveness of air-cooled condensers. The NREL concept involves the use of perforated fins with all of the air flowing through the perforations. Tests of two prototypes at NREL and associated computer modeling indicated that 30 to 40% more heat transfer could be obtained for the same fan power. We are now working with Super Radiator Coils of Richmond, Virginia to fabricate and test additional prototypes. NREL is also investigating ways to combine some degree of evaporative cooling with air cooling during hot weather when air-cooled condenser performance drops (and when grid electricity is highly valued for meeting air conditioning loads). We performed an analysis comparing the cost and performance of four different means for augmenting air cooling with evaporative cooling. We have also provided technical support to Mammoth Pacific in their program to install and test evaporative enhancement systems at the Mammoth Lakes geothermal power plant. Contact: Chuck Kutscher, chuck_kutscher@nrel.gov Alternative Non-Condensable Gas Removal Methods At geothermal plants, noncondensable gases accumulating in the condenser can decrease heat transfer and raise turbine backpressure, thereby lowering turbine performance. Typically, either steam jet ejectors or vacuum pumps are used to remove these gases. The former method wastes valuable plant steam, and the latter robs plant electricity. For example, in the 330-MW Tiwi Geothermal Field in the Philippines, roughly 20% of the steam is used to remove noncondensables. This not only causes net power loss, but as the reservoir pressure eventually drops, the steam ejectors lose their efficiency. We compared a number of alternative gas-removal methods: five vacuum system configurations that use the conventional approach of evacuating gas/vapor mixtures from the power plant condenser system, and a system for physical separation of steam and gases upstream of the power turbine:
This study's specific objectives were to:
Conclusions from the final report: Two gas-removal options appear to offer profitable economic potential. The hybrid vacuum system configurations and the reboiler process yield positive net present-value results over wide-ranging gas concentrations. The hybrid options look favorable for both low-temperature and high-temperature resource applications. The reboiler looks profitable for low-temperature resource applications for gas levels above about 20,000 parts per million by volume (ppmv). It might also become economically feasible in high-temperature power plant systems if adopting the reboiler allows offsetting costs to be captured, for example, by eliminating the need for gas abatement systems or by allowing the substitution of lower-cost direct-contact condensers instead of shell-and-tube condensers. A vacuum system configuration using a three-stage turbocompressor battery may be profitable for low-temperature resources, but the results also show that a hybrid system is more profitable. The biphase eductor alternative cannot be recommended for commercialization now. All cases for the biphase eductor covered in this study showed negative net present values. The cause of the projected losses is a combination of low compression efficiencies and high capital costs. Complete Final Report: Contact: Chuck Kutscher, chuck_kutscher@nrel.gov Field Verification of Small-Scale Geothermal Power Plants The primary objectives of this research and development project are (1) to determine and validate, in various locations, the performance and operational characteristics of small-scale geothermal electric power plants and (2) to determine their ability to provide distributed power to increase their use in the western United States. "Small-scale" geothermal power plants are considered to be those that have approximate net electrical outputs of between 300 kW and 1 MW. Their modular designs are easily transportable and allow a plant to be built for a small up-front cost, and they can be automated and run without a full-time operator. NREL staff selected the most promising geothermal sites from the Geo-Heat Center's 271-site survey of geothermal resources in western states. For each site, we ran NREL's Cycle Analysis Software Tool (CAST) to analyze performance of a binary-cycle geothermal power plant operated in each of two modes: directly using the geothermal fluid resource, and using it in series with a direct-use application. The project is conducted under a cost-sharing contract with three phases. Phase I includes permitting, zoning, and other regulatory requirements; preliminary design; well development (if necessary); resource characterization; performance monitoring and evaluation plan; market development; and Phase I presentation. Phase II includes financing, detailed plant design, construction management plan, permitting, and Phase II presentation. Phase III includes power plant construction, instrumentation, startup/checkout, operation and performance monitoring and evaluation, and information dissemination. Awards have been made for Phase I, and those activities are underway. Contact: Chuck Kutscher, chuck_kutscher@nrel.gov Other Projects:Geothermal Facility Siting Issues A workshop on issues relative to geothermal facility siting at Federal lands was convened at NREL on November 14-16, 2000. The participants agreed that significant facility siting issues are impeding development of geothermal energy resources in the United States. They recommended several actions: development of a national policy on renewable energy, coordination among Federal agencies involved in geothermal permitting processes, and establishment of a National Geothermal Coordinating Committee. Contact: Gerald Nix, gerald_nix@nrel.gov International Market Assessment This NREL study stemmed from the fact that opportunities for small geothermal projects exist in many areas of the developing world. The study examined the feasibility of geothermal power plants with less than 5 MW of capacity to supply electricity in remote applications in Latin America, the Caribbean, and the Philippines. The study shows that small geothermal projects face challenges such as the high fixed costs of exploration and drilling, the relatively high transaction costs of obtaining project finance, and the difficulty of establishing and supporting an operation and maintenance infrastructure in remote areas. In spite of these difficulties, the costs of small geothermal generation are in the same range as competitor technologies for rural electricity markets, including diesel generation. Existing mini-grids present opportunities for small geothermal power plants to supplement or displace diesel generation. Government actions could help improve the economics for geothermal technologies, for example, by bundling small projects together. A needed next step is to study the coincidence of near-surface geothermal resources and new electricity demand in key developing countries. By combining electricity generation with industrial process heat needs or water purification, small-scale geothermal plants could become especially attractive. Publications Last updated: | | |||||||||
