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Annual Simulation Results for an Air-Cooled Binary Cycle Employing Flash Cooling Enhancement

Background. 85-90% of geothermal heat is rejected Plant power increases ~1.5% of rated power for every 1?F drop in condenser temperature Analysis and field experiments have shown value of summertime evaporative enhancement of air-cooled plants Analysis by Mines showed potential for flashing a portion of the brine to provide water for evaporative enhancement.

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Annual Simulation Results for an Air-Cooled Binary Cycle Employing Flash Cooling Enhancement

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    1. Annual Simulation Results for an Air-Cooled Binary Cycle Employing Flash Cooling Enhancement Geothermal Resources Council Annual Meeting September 13, 2006

    2. Background 85-90% of geothermal heat is rejected Plant power increases ~1.5% of rated power for every 1ºF drop in condenser temperature Analysis and field experiments have shown value of summertime evaporative enhancement of air-cooled plants Analysis by Mines showed potential for flashing a portion of the brine to provide water for evaporative enhancement

    3. Objective To perform detailed hour-by-hour simulation of air-cooled cycle with flash-supplied cooling water using two types of evaporative enhancement: spray nozzles and evaporative media

    4. Approach Engineering Equation Solver (EES) model of system with built-in capital cost equations Excel front end to perform annual hour-by-hour simulations and annual costs 3 resource temperatures: 270°F, 300°F, and 330°F Reno weather data (TMY2) Assumed fixed electricity rate Assumed negligible NCG content in brine Power cycle costs from chemical engineering literature and Aspen ICARUS; evap. cooling costs from Kutscher and Costenaro (2002)

    5. Assumptions 10 MW plant at 300°F resource Brine flow rate of 2 x 106 lb/hr Constant turbine inlet pressure and working fluid flow rate Isobutane working fluid Flash operation from May 15 to October 15 55% fan efficiency Constant 80% pump and turbine efficiencies Price of electricity = $0.08/kWh, 12% discount rate

    6. Spray Nozzles 300 psig 70% evaporation efficiency 80% saturation efficiency DRIFdek® mist eliminator

    7. Munters Packing 8” of CELdek® Munters data for saturation efficiency and pressure drop

    8. Optimization Hybrid design optimized for average ambient conditions from May 15 to Oct. 15 (64°F dry bulb, 40% RH) Varied heat exchanger pinch points (preheater and condenser), superheat, flash pressure, steam condenser area, sub-cooler area Maximum brine routed to flash tank: Pturb-inlet sets T sat-working fluid ,Tsat-steam > Tsat of working fluid, Tsat-steam sets Pflash tank Minimized $/kW Annual simulations done with optimized design, fixed heat exchangers

    9. EES Model

    11. Heat Transfer Coefficients

    12. Analysis Results

    13. Net Power Output

    14. Effects of Hybrid Design on Net Power Evaporative cooling lowers sink temperature Additional heat exchange area (steam condenser and subcooler)

    15. Flash Steam Production

    16. Avg. Monthly Power 330°F Resource

    17. Avg. Monthly Power 300°F Resource

    18. Avg. Monthly Power 270°F Resource

    19. Levelized Cost of Electricity

    20. Impact of Cooling Water

    21. Impact of Water on Performance of 3 Hybrid Systems at 300°F Resource

    22. Average Monthly Net Power for Alternative Cooling Water Scenarios (300°F Resource)

    23. Conclusions Flashing a portion of the brine for evaporative pre-cooling of air produces limited cooling water, lowers binary-cycle efficiency, incurs parasitic power penalty and cost of additional heat exchange area Performance is about the same or worse than simple binary; levelized electricity cost is higher Spray nozzles perform better than Munters due to lower fan power and cost Obtaining water for cooling via other means (e.g., direct use of brine or reverse osmosis) is attractive

    24. Acknowledgements

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