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Session: Parallel Session: Systems Design, Optimization and Applications III
Session starts: Friday 23 September, 11:20
Presentation starts: 11:40
Room: Senaatszaal

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Florian Heberle (Universität Bayreuth, Lehrstuhl für Technische Thermodynamik und Transportprozesse (LTTT))
Dieter Brüggemann (Universität Bayreuth, Lehrstuhl für Technische Thermodynamik und Transportprozesse (LTTT))

Abstract:
Optimal operating parameters for geothermal applications of the Organic Rankine Cycle (ORC) are identified under exergoeconomic criteria. For typical geothermal conditions in Germany the use of the zeotropic mixture isobutane/isopentane as a working fluid is evaluated compared to the pure components. Therefore the minimum temperature difference in the evaporator and condenser is varied to figure out the minimal specific costs of electricity generation. In addition, exergetic variables, like second law efficiency and irreversibilities of each component, are calculated. The purchase equipment costs are determined as a function of heat surface area and power of the turbine or pump. The costs of the fuel consist of costs associated with the exploration of the geothermal resource and operating and maintenance costs of the borehole pump. The results of the exergetic analysis show that second law efficiency using the zeotropic mixture increases up to 15 % compared to the pure components. Due to a better glide matching the irreversibilities in the condenser decrease significantly for those mixture compositions, where temperature glide at phase change and temperature difference of the cooling water are equal. In general the condenser shows the highest required surface area in consequence of a low logarithmic mean temperature difference and a high amount of transferred heat. In case of fluid mixtures a reduction of heat transfer coefficients due to additional mass transfer effects leads to a higher surface area of the condenser and evaporator compared to pure fluids. Therefore the total purchase equipment costs increase in the range of 5 % to 30 %. In case of pure working fluids isobutane leads to slightly lower specific costs of electricity than isopentane. In both considered geothermal case studies, the most suitable concept under exergoeconomic criteria is the choice of the fluid mixture as a working fluid. The investigations show that the use of an isobutane/isopentane mixture is a promising optimization strategy for low-temperature geothermal applications. Related with high exploration costs, the efficiency increase overcompensates the additional heat transfer areas. The calculations point out that different fluids should be selected, if a variable minimum temperature difference in the condenser and evaporator is assumed.