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2 Geothermal Energy R&D Program jj I IJ Greg. Mines Idaho National Engineering YCE INVESTIGATIONS I c I il Y i i KEY WORDS Binary power cycles, working fluid mixtures PROJECT OBJECTIVE The objective of this project is to develop more efficient power cycles for the liquid-dominated, moderate-temperature hydrothermal resource. The developmeat of more efficient power cycles will lead to a greater utilization of the energy contained in the hydrothermal resource and lower cost the cost of the electrical power produced. APPROACH Project analytical studies identified significant performance improvements possible with supercritical cycles which utilized working fluid mixtures. At the optimized conditions, the defined supercritical cycle was projected to provide a 20% improvement in performance for a 180 C resource. In order to determine both the validity of the assumptions made in these performance projections and to assess the adequacy of existing technology, field investigations were planned and conducted. Investigators utilized the data collected during the testing to validate assumptions and to define and develop the technology base required to design and build a commercial facility which would realize the projected performance improvements. PROJECT STATUS Background Studies have shown that for the low- to moderate-temperature, liquid-dominated hydrothermal resources, binary power cycle technology has a performance advantage over flash-steam technology (in terms of power produced per unit mass of brine). Commercial facilities presently utilizing binary cycle technology operate at subcritical pressures With single component working fluids. To improve the cycle performance and efficiency, project investigators have focused on reducing the thermodynamic irreversibility associated with the heat transfer processes for the condensation of the working fluid, as well as its vaporization. The resulting analytical studies showed that a cycle where rking fluid mixture of hydrocarbons is vaporized at supercritical pressures will result in reduced cycle irreversibilities, increased efficiencies and performance, and lo

3 CONVERSION TECHNO OGY U.S. Department of Energy The supercritical power cycle utilizing mixed working fluids has been the focus of the project investigations for the past several years. Field studies related to this cycle have been conducted at the Heat Cycle Research Facility. This facility is a small binary power plant operated in California's Imperial Valley, with components and an operating mode similar to those that would be found in a commercial facility incorporating the identified cycle concepts. The data collected during the operation of this facility is used by researchers to determine the validity of the assumptions used in making the projection of a 20% improvement in performance, and to evaluate the adequacy of the available technology to design a commercial facility to incorporate the identified cycle concepts. The field investigations in this research area have been completed, and a final report is being prepared. Research Results To achieve the projected improvements in performance from using the supercritical cycle with mixed hydrocarbon working fluids, the heat exchanger components must be designed to provide counter-current flow paths and to achieve integral phase changes (vaporization and condensing). Both were assumed in the analytical studies, and are critical to achieving the stated performance improvements. The objective of the field investigations was to determine whether these assumptions were valid, and to assess the adequacy of the existing design methods for heat exchangers to achieve these processes. The data collected has confirmed that the heat exchanger designs utilized at the Heat Cycle Research Facility achieved both the counter-current flow paths, as well as the integral phase changes. The analysis of the data also confirmed that the existing heat exchanger design codes were adequate for the design of the supercritical heater and vaporizer for the mixed hydrocarbon working fluids, as well as the pure fluids; the design codes would be slightly conservative in predicting the heat exchanger area. The data showed that the vertical condenser, with in-tube condensing, achieved both the counter-current flow paths and the integral condensation of the working fluid mixtures. The analysis of the data showed that the existing heat exchanger design codes would conservatively (slightly) predict the required heat exchanger area with the condenser oriented vertically. Although deficiencies in the design codes were found at other condenser orientations, project investigators were able to define schemes for modeling the condenser which resulted in predicted heat exchanger areas which agreed closely with the actual heat exchanger area. As a result of these investigations, the project has been able to confirm that the projected 20% performance improvement can be achieved and has defined the technology required to design the components necessary to attain this improvement. The project has examined where additional cycle performance improvements might be attained. These studies revealed that the performance of the supercritical cycle utilizing the mixed working 264

4 Geothermal Energy R&D Program ; fluid was approaching the performance of an idealized cycle operating under practical equipment constraints. While some small additional improvement was possible, the supercritical cycle performance was closely approaching the thermodynamic maximum possible. Plans The final report on the supercritical cycle investigations will be issued in the summer of REFERENCES 1. C. J. Bliem and G. Mines, Advanced Binarv Geothermal Power Plants -- imits of Performance, EGG-EP-9207, January Demuth, O.J., "Analysis of Mixed Hydrocarbon Binary Thermodynamic Cycles for Moderate Temperature Geothermal Resources", EGG-GTH-5753, February Bliem, C.J., "Preliminary Performance Estimates of Binary Geothermal Cycles Using Mixed- Halocarbon Working Fluids", EGG-EP-73 12, July Demuth, O.J. and RJ. Kochan, "Analysis of Mixed Hydrocarbon Binary Thermodynamic Cycles for Moderate Temperature Geothermal Resources Using Regenerative Techniques", EGG- GTH-5710, November Demuth, 0. J., "Effects of Vaporizer and Evaporative-Condenser Size on Geofluid Effectiveness and Cost of Electricity for Geothermal Binary Power Plants," EGG-GTH-6376, E G & G Idaho, Inc., October Bliem, C.J., "Economic Impact of Heater and Condenser Size in Geothermal Binary Cycles," i EGG-ERTP-10598, December d -.. CONTACTS DOE Program Manager: Raymond asala Geothermal Division, EE-122 US. Department of Energy 1000 Independence Avenue, SW Id Washington, DC 20585, Phone: (202) Fax: (202) c 265

5 U.S. Department of Energy Principal Investigator: Greg Mines Idaho National Engineering aboratory P.O. Box 1625 Idaho Falls, ID Phone: (208) Fax: (208)