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Session: Poster session & Sponsor Exhibition
Session starts: Monday 07 October, 14:00

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Wei Liu (Technische Universität München)
Dominik Meinel (Technische Universität München)
Christoph Wieland (Technische Universität München)
Hartmut Spliethoff (Technische Universität München)

Abstract:
Working fluids, i.e. different refrigerants, with a variety of thermodynamic properties are of great interest for Organic Rankine Cycles (ORC) operating with different low temperature heat sources. However, most of the currently used working fluids have destructive effects on the environment, e.g. depletion of the ozone layer or global warming. To reduce the enviromental effects, refrigerants have been progressed from Chlorofluorocarbons (CFCs), Hydrochlorofluorocarbons (HCFCs) to Hydrofluorocarbons (HFCs) in the past decades. Hydrofluoroolefins (HFOs), i.e. derivatives of propene, have emerged in recent years as the fourth generation of refrigerants which are considered to be one of the most promising replacements for third generation refrigerants like HFCs as it possesses considerably lower effects on the environment. This work will present a study concerning the thermodynamic performances of eight different HFOs as working fluids in ORC applications. The thermodynamic properties of the HFOs are estimated using the Peng-Robinson equation of state in combination with the group contribution method. Simulations are carried out in Matlab, in which the self-developed calling functions are added for calculation of thermodynamic parameters. In this study the heat source is represented by geothermal brine. The temperature region around 140 \degree C is of especial interest for Germany, since these are common temperatures obtained in the Molasse-basin in Germany. On this account, the inlet temperature of the heat source is varied from 120 to 200 \degree C, while the operating system pressure of the ORC-system is increased from 15 to 30 bar. At a constant system pressure the system efficiency is strongly increasing for low geothermal temperatures and changes only slightly beyond a specific temperature. This temperature indicates the point, where the pinch-point changes from evaporator to preheater. These points are determined for the investigated ORC fluids at corresponding system pressures. Depending on the level of the inlet temperature of the heat source the operating pressure influences the system efficiencies in different ways. Taking R1225yeE as an example, at a lower inlet temperature (<=130 \degree C) the system efficiencies reach maxima at intermediate operating pressures (between 15 and 17.5 bar). On the other hand, at a higher inlet temperature (> 130 \degree C) the system efficiencies increase monotonically with increasing system pressures.