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2 Geothermal Resources Council Transactions, Vol. 28, August 29 - September 1, 2004 Designing Binary Cycles for Mixed Fluids John Brugman Director of Technology Bibb and Associates, Inc., Pasadena, CA Keywords Binary cycle, power generation, mixed working fluids, heat transfer, Empire KGRA ABSTRACT For over 30 years research into the use of mixtures of hydrocarbons as working fluids has shown that the use of these substances can increase the efficiency of power generation from geothermal resources. Mixed fluids offer several advantages over single component fluids including the ability to be tailored to the specific needs of a given resource and the flexibility to adapt to changing resource and ambient conditions. The world s first commercial power plant designed to use a mixed hydrocarbon working fluid is now under development. This paper reviews the history of mixed-fluid binary cycle development and design considerations specific to the use of mixed-fluid cycles. Introduction Figure 1. Schematic of Geothermal Binary Cycle Plant. Perhaps the first practical commercial use of the organic ranking cycle was in Frank W. Olfelt used naphtha as the working fluid for a marine engine that powered a small launch. These launches were very popular for about 20 years until the development of the marine internal combustion engine. In 1967, a 340 kw freon based experimental geothermal binary cycle plant was built at Paratunka in Kamchatka (Schubin, 1967). The first commercial binary cycle geothermal plant to be operated in the U.S. was a 10 MW plant built by Magma Energy at East Mesa, California in In 1985, the world's first air-cooled binary cycle geothermal power plant was commissioned at Mammoth Lakes, California. Over the years, numerous binary cycle geothermal power plants have been built throughout the world. All of these facilities have used single component working fluids. However, for more than 30 years research into the use of mixtures of components as working fluids has been ongoing. In 1971 the Ben Holt Co. (now Bibb and Associates, Inc) began to develop advanced binary cycles that, among other things, utilized mixed working fluids. In 1975, U.S. patent number 3,893,299 was granted to A. J. L. Hutchinson and Douglas Cortez (Hutchinson and Cortez, 1975) who were both employees of the Ben Holt Co. for an advanced binary cycle that used a mixed hydrocarbon working fluid (Hutchinson, 1977). Since that time, considerable research has been done by Bibb and the National Laboratories (Mines and Bliem, 1988) to optimize the use of mixed working fluids for geothermal power generation. This work will soon culminate with the construction of the world s first binary cycle plant designed to work with a mixed hydrocarbon working fluid. 519

3 Brugman Process Considerations The basic binary cycle geothermal power plant is shown in Figure 1. Based on the rankine cycle, the process consists of a working fluid pump, a preheater/vaporizer, an expansion turbine, and a condenser. There are two basic types of binary cycle plant. In the supercritical cycle, the vaporizer pressure is above or close to the critical pressure of the working fluid. The fluid changes from liquid to vapor in a smooth continuous fashion without the occurrence of boiling or the formation of two discrete simultaneous phases. The power plants at Mammoth Lakes, California are representative of this type of plant. The second type of binary cycle plant is a subcritical cycle where the working fluid vaporizes at a pressure enough below its critical pressure so that significant boiling occurs. This process is usually better suited to lower temperature resources. The existing power plants at Empire, Nevada are of the subcritical type. The working fluid and turbine inlet conditions are usually determined by the resource characteristics and the site conditions. As a general rule for hydrocarbon working fluids, the hotter the geothermal resource, the higher the molecular weight of the working fluid. However, condensing conditions must also be considered. It is usually undesirable to have a working fluid condense at a pressure lower than ambient atmospheric pressure in order to avoid the leakage of air into the system. The presence of noncondensible air in the condenser will increase the condensing pressure and may pose a hazard in the presence of a flammable working fluid. Currently, three hydrocarbons are used in operating binary plants. These are commercial grades of isobutane, pentane, and isopentane. Depending on the circumstances given above, one of these fluids is selected and then the operating conditions are optimized to give the best performance for the resource and site. Figure 2 is a cycle diagram of a typical subcritical binary cycle using commercial isobutane as a working fluid. The vaporizer outlet pressure is 300 psia and the corresponding Figure 2. Thermodynamic Cycle Diagram of a Single Component Binary Cycle. vapor temperature is 250 F. The geothermal fluid is available at 290 F and cooling water is available at about 68 F. This results in a condensing pressure of 78 psia. The geothermal fluid is returned at a temperature of 167 F. If a higher vaporizer pressure were chosen, the cycle thermal efficiency would increase but, since the vaporizer temperature would also rise, the geothermal fluid reject temperature would also increase resulting in less heat recovered from a given amount of geothermal fluid. In this case, the loss of heat input to the cycle is not compensated by the increased cycle efficiency and so the net output would decrease. It may occur that, for a given set of resource and site conditions, none of the three hydrocarbons is really optimal. This problem can be overcome by mixing two or more hydrocarbons together to create a working fluid tailored to the specific needs of a particular project. Figure 3 is the cycle diagram of a subcritical mixed fluid binary cycle. In this case, the working fluid is a mixture of isobutane and heptane. The vaporizer outlet pressure is 270 psia and the corresponding vapor temperature is 240 F. The cooling water is available at about 68 F which results in a condensing pressure of 61 psia. The geothermal fluid is returned at a temperature of 168 F. This cycle will produce more net output than the best pure fluid cycle for these conditions. Mixed working fluids offer several advantages over single component working fluids. They can be tailored to match the existing resource and ambient conditions. They also provide operational flexibility. As conditions change from season to season or as the resources changes over time, the mixture composition can be varied accordingly. Mixed fluid cycles are intrinsically more efficient than single fluid cycles, particularly in subcritical applications. This is due to more effective heat transfer. Since mixed fluids boil and condense over a temperature range, they match the heat source and heat rejection heat curves better. For example, a mixed fluid can be condensed at a lower temperature (and therefore a lower pressure) than constant boiling fluids (see Figures 2 and 3). There are some potential disadvantages to mixed fluids as well. The principal one is the possibility of a lower heat transfer coefficient for mixed fluids compared to pure fluids. This has been seen in laboratory experiments (Tleimat, et al, 1980) but not confirmed in commercial operations. Since single component working fluids are not "pure" either, there may be little difference in a practical sense. The other possible disadvantage involves maintaining the mixture composition at a given value. If the fugitive hydrocarbon losses or thermal degradation rates are different for the constituent components, the composition may change slowly over time. Design Considerations In most respects, a mixed fluid binary cycle plant, whether supercritical or sub- 520

4 Brugman Commercialization Figure 3. Thermodynamic Cycle Diagram of a Mixed Fluid Binary Cycle. critical, will be designed the same as a single fluid plant. Mixed fluids behave the same as single component fluids. Therefore, the turbine, pumps, rejection equipment, auxiliary systems, etc. are no different than in current commercial plants. There are, however, two main differences. The most important difference involves heat transfer. Single component cycles exhibit isothermal boiling and condensing whereas mixed fluids boil and condense over a temperature range that depends on the composition of the fluid. This effect is particularly important for subcritical cycles. It is current commercial practice to use kettle type reboilers to vaporize single component working fluids baby. This type of heat exchangers is appropriate for constant boiling fluids. However for mixed fluids, this type of heat exchanger is not recommended. Reboilers exhibit considerable internal recirculation so that the bulk fluid in the kettle stays at one temperature. This defeats the advantage of non-isothermal boiling since the bulk fluid would be at the outlet temperature. Therefore, for mixed fluids, a counterflow heat exchanger is preferred. The second difference is less significant. There should be separate storage for each component of the mixture so that the composition can be easily adjusted as needed. If seasonal composition changes are anticipated, then it may also be advisable to provide additional storage for the off-season blend. After over 30 years of research and study, plans are currently underway to build the world's first commercial scale mixed hydrocarbon binary cycle geothermal power plant. As a result of cooperation between the National Renewable Energy Laboratory (NREL) and Empire Energy, Inc., A nominal 1 MW mixed fluid binary cycle plant will be built at Empire, Nevada. This plan will utilize a 290 F geothermal fluid. It will be integrated with the existing infrastructure at the site and serve as a test bed for mixed fluid research. In addition to the normal power plant instrumentation and controls, there will be additional instrumentation to allow detailed monitoring of plant performance. Both single component fluids (commercial isobutane) and mixed fluids will be used so that valid performance comparisons can be made. Further details will be announced as the project proceeds. Dedication This paper is dedicated to the memory of Ben Holt ( ). Ben was a true visionary who, more than 30 years ago, saw the potential benefits of the geothermal binary cycle. He pioneered in numerous technological advances including the use of mixed fluids. He generously contributed his time, talent, and treasure to the advancement of the geothermal industry. References Hutchinson, A.J.L. and Cortez, D.H., U.S. Patent 3,893,299, Geothermal Heat Recovery by Multiple Flashing. Hutchinson, A.J.L., U.S. Patent 4,057,964, Working Fluids and Systems for Recovering Geothermal Waste Heat. Mines, G.L. and Bliem, C.J., 1988, Improving the Efficiency of Binary Cycles. Geothermal Program Review VI Proceedings, U.S. DOE, p Schubin, B.F., 1967, Experimental Freon Geothermal Power Station. Elekticheskiyestantsii, n. 5, p

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