[home] [Personal Program] [Help]

---

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

---

Theresa Weith (Universität Bayreuth, Zentrum für Energietechnik, Lehrstuhl für Technische Thermodynamik und Transportprozesse)
Dieter Brüggemann (Universität Bayreuth, Zentrum für Energietechnik, Lehrstuhl für Technische Thermodynamik und Transportprozesse)
Andreas P. Weiß (Hochschule Amberg-Weiden, Labor für Strömungsmaschinen)
Gerd Zinn (DEPRAG SCHULZ GMBH u. CO.)

Abstract:
Waste heat of biogas cogeneration units as well as plenty kinds of industrial waste heat provide high potential for power generation by applying Organic Rankine Cycles (ORCs). In this field of application, where high heat source temperatures can occur, conventional ORCs normally comprise an additional thermal oil loop in order to prevent the ORC working fluid from decomposition as well as to avoid self-ignition of the fluid when getting into contact with hot exhaust gas due to leakage. Moreover, when regarding small-scale plants with a power output of less than 30 kWel, scroll or screw expanders are mostly used as an expansion device. The described state-of-the-art systems suffer from several disadvantages, like high investment costs and complex plant design in case of thermal oil loop together with low efficiencies of common expansion devices. Therefore, the present work deals with the development of a 15 kWel ORC plant for waste heat recovery with a direct evaporator and a micro-expansion turbine. Working fluids that come into consideration are preselected by taking into account property data as well as non-thermodynamic issues as for example economic aspects and the fluid’s hazardous potential to health, water and environment. Aside from this, steady-state process simulations have been performed for heat source temperatures in the range of 573 K to 673 K and the effect of fluid specific turbine efficiency on the overall electric efficiency of the ORC has been investigated for three selected fluids. The results show an increase in ORC efficiency as the fluid-specific turbine efficiencies exceed the primarily assumed turbine efficiency of 60 %. For the example of cyclopentane, a relative improvement in overall electric efficiency of up to around 16 % could be observed. Moreover, the maximum of the overall ORC efficiency is shifted towards lower pressures. Turbine optimal design and efficiency strongly depend on several parameters, e.g. operating pressure ratio, volumetric flow ratio or specific speed, which are mainly determined by the applied ORC fluid. Hence, it affects the choice of the proper working fluid as well as the design point of the ORC plant. Based on the theoretical results, cyclopentane was chosen as promising working fluid for the present ORC plant. Process simulations with cyclopentane predict a maximum electric efficiency of 11.65 % for a heat source temperature of 623 K, an upper pressure of 20 bar and an isentropic efficiency of the turbine of 64.3 %. Currently, a pilot plant is under construction consisting of a plate-and-shell heat exchanger for direct evaporation, an axial impulse turbine, a piston diaphragm pump, an air-cooled condenser as well as a gas burner as heat source. The authors gratefully acknowledge financial support by Bayerische Forschungsstiftung.