Advanced Binary Geothermal Power Plancts Working Fluid Property Determination and Heat Exchanger Design

The performance of binary geothermal power plants can be improved through the proper choice of a working fluid, and optimization of component designs and operating conditions. This paper reviews the investigations at the Idaho National Engineering Laboratory (INEL) which are examining binary cycle performance improvements: for moderate temperature (350 to 400 F) resources with emphasis on how the improvements may be integrated into design of binary cycles. These investigations are examining performance improvements resulting from the supercritical vaporization of mixed hydrocarbon working fluids and achieving countercurrent integral condensation with these fluids, as well as the modification of the turbine inlet state points to achieve supersaturated turbine vapor expansions. For resources where the brine outlet temperature is restricted, the use of turbine exhaust recuperators is examined. The baseline plant used to determine improvements in plant performance (characterized by the increase in the net brine effectiveness, watt-hours per pound of brine) in these studies operates at conditions similar to the 45 MW Heber binary plant. Through the selection of the optimum working fluids and operating conditions, achieving countercurrent integral condensation, and allowing supersaturated vapor expansions in the turbine, the performance of the binary cycle (the net brine effectiveness) can be improved by 25 to 30% relative to the baseline plant. The design of these supercritical Rankine-cycle (Binary) power plants for geothermal resources requires information about the potential working fluids used in the cycle. In addition, methods to design the various components, (e.g., heat exchangers, pumps, turbines) are needed. This paper limits its view of component design methods to the heat exchangers in binary power plants. The design of pumps and, turbines for these working fluids presents no new problems to the turbine manufacturer. However, additional work is proceeding at the Heat Cycle Research Facility to explore metastable expansions within turbines. This work, when completed, should allow the designer more flexibility in the state point selection in the design of these cycles which will potentially increase the system performance. The paper explores the different systems of thermodynamic and transport properties for mixtures of hydrocarbons. Methods include a computer program EXCST developed at the National Bureau of Standards in Boulder, as well as some of the thermodynamic models available in the chemical process simulation code, ASPEN, which was originally developed by the Department of Energy. The heat exchanger design methodology and computer programs of Heat Transfer Research, Inc. (HTRI) have been used because they represent data which is used throughout the industry by A & E firms as well as most heat exchanger manufacturers. For most cases, some modification of the computer results are necessary for supercritical heater design. When condensation takes place on the inside of enhanced tubes, new methods beyond HTRI's present state are necessary. The paper will discuss both of these modifications