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14:00   Turbo expanders II
Chair: Prof. Giacomo Persico
20 mins
Luca G. Xodo, Claudio Spadacini, Marco Astolfi, Ennio Macchi
Abstract: In the wide range of applications of the ORC, the medium-high enthalpy heat recovery applications are acquiring a growing importance, as energy efficiency and primary energy savings are becoming a focus for most of the industrial world. Medium-high enthalpy applications are defined as those applications in which the waste heat is available at a considerable peak temperature (higher than 250 °C), e.g. industrial waste heat from glass mills, steel mills or cement factories and simple cycle power production applications such as gas/oil/biogas engines and gas turbines. The ORC becomes an option in specific applications: medium size, need for remote control and isolated environment; nevertheless the overall heat recovery efficiency is the first parameter to consider in order to give a competitive solution to the market. Overall heat recovery efficiency can be defined as the maximum amount of high-valued energy, in this case electrical power, can be obtained from the available heat source. This means a right balance between thermal power recovery, lowering the minimum temperature to which the gas should be cooled down, and the cycle conversion efficiency. In fact, once the heat sink temperature level has been set, the ORC efficiency, together with the theoretical Lorenz efficiency, is bound to the average temperature level of the heat source. For a supercritical or a subcritical superheated cycle once the maximum temperature is selected the other optimization variable is the operating pressure of the cycle which directly affects both the enthalpy drop and the volume flow ratio along the expansion. In particular the total volume flow ratio is another limit that has to be carefully considered and it is strictly related to the thermodynamic properties of the fluid. Organic fluids commonly used for medium high enthalpy applications are long chain alkanes, perflourated fluids and siloxanes, which are characterized by an high molecular weight and an high molecular complexity. For a fixed turbine pressure ratio this kind of fluids shows a small enthalpy drop along the expansion which allow designing the turbine with just few stages (at limit just one) with small load coefficients, low peripheral speed and low mechanical stresses. However the design of this kind of turbine is not trivial: several difficulties arise because of the low speed of sound (high Mach numbers) and because the whole volume flow ratio has to be managed in a little number of stages. This last aspect entails a challenging design of turbine blades and big flaring angles are commonly used with the formation of radial fluxes in blade channels and reduction of the stage efficiency. Economic concerns suggest to limit the number of stages to at maximum 3-4 and so any technological solution able to guarantee an high efficiency with a limited number of stages is crucial in ORC field. The use of a radial outflow turbine might solve most of the over mentioned problems. In fact increasing the blade mean diameter along the expansion allows to obtain a better and easier design of turbine blades which can have small flaring angles and they don’t need to be twisted in the last turbine stages. Where this kind of advanced turbine is adopted, it is possible to relax the constrain related to the maximum volume ratio along the expansion, to increase the maximum operating pressure and to reach an higher efficiency. The study compares the application of a 3-stage axial turbine and a 3+ stages radial outflow turbine on a case study of heat recovery from industrial process, comparing isentropic efficiency, overall cycle efficiency and operational parameters of the different configurations. Finally an analysis of turbine design is presented in order to appreciate the advantages of the proposed solution.
20 mins
Jos van Buijtenen, Quirijn Eppinga, Stefano Ganassin
Abstract: A novel Organic Rankine Cycle (ORC) plant has been developed by Triogen B.V. of The Netherlands. The ORC system is based on a high temperature resistant hydro-carbon as a working fluid, hence suitable for direct use of intermediate temperature heat sources from 350 °C and above. The core of the unit consists of a combined turbine – generator – pump: the High Speed Turbo-Generator (HTG). Thanks to the use of a high speed generator (25.000 rpm), the turbine and pump could be laid out at their optimum specific speed, leading to high internal efficiencies. Moreover, this concept allowed for a seal-less design: there are no shaft seals necessary, and the only connections between the internals and the outside world are flanged connections for the working fluid to enter and exit the HTG and the well-insulated electric cables. Lubrication of bearings and cooling of the generator is taken care of by the working fluid itself, so there is no need for lub-oil and subsequent system. Therefore the unit can be considered to be completely hermetic. Twenty units are now in commercial operation in different applications, while there are more than 10 units on order. Heat sources vary from exhaust gasses of gas- and diesel engines to landfill gas combustion and wood firing as well as industrial waste heat. This paper will report on the current operating experience (more than 200.000 accumulated hours). Moreover, the unique features of the design, such as the hermetically closed turbo-generator, the cycle design and the balance of plant will be highlighted in view of the applications. Units are being built as a standard package for 90 to 165 kWe, being adapted to the heat source by sizing the evaporator.
20 mins
David Japikse, Francis Di Bella, Maxwell Hurgin, Keith Patch
Abstract: Concepts NREC has completed its design and development of an integrated high speed axial turbine and permanent magnet generator. The Turbine –Generator Unit (T.G.U.) is ideal for ORC applications for its compactness, reliability, and versatility. The 20,000 rpm TGU does not use a shaft seal or gearbox and the generator is evaporatively cooled using the ORC working fluid. The TGU is designed with the intent of having the same turbine housing and generator used with a range of operating ORC fluid operating pressures and temperatures to enable its wide spread use in a variety of ORC heat recovery applications. The rotor and nozzle stators are changed to aerodynamically optimize the performance of the turbine at different ORC fluid operating conditions and flow rates. The output voltage of the generator can range from 300 to 480 Vac and at 50 hz and 60 hz. This facilitates its use in waste heat recovery applications throughout the world. This technical paper will review the mechanical and electrical design features of the turbine-generator unit.
20 mins
David Pasquale, Matteo Pini, Giacomo Persico, Antonio Ghidoni, Stefano Rebay
Abstract: During the last decade, Organic Rankine Cycle (ORC) turbogenerators have become very attractive for the conversion of low-temperature thermal energy sources in the small-to-medium power range. Complex gasdynamic phenomena and strong real-gas effects in the thermodynamic behavior of the working fluid usually characterize the thermo-fluid-dynamics of ORC turboexpanders. The use of Computational Fluid Dynamics (CFD) codes coupled with accurate thermo-physical property models is crucial to correctly model the flow expansion and therefore to achieve high-efficiency ORC turboexpanders. The design of ORC turbines is particularly challenging for small power output machines (up to a few hundreds of kWe); in these applications compactness is crucial and single-stage turbines represent a typical solution. As a result, fully supersonic flow conditions are typically adopted in these machines, and dedicated design techniques must be applied to avoid the onset of strong shocks in the stator-rotor gap. In the present work an automated procedure to design single-stage centripetal turbines for ORC systems is presented. The design methodology involves a two-step procedure coupled to a global optimization strategy in order to define the optimal degree of reaction, size and the flow path of the machine. First the mean-line code zTurbo and a Genetic Algorithm (GA) are used to perform a preliminary design of the machine. The mean flow surfaces of the selected configurations are then optimized by means of the CFD-based throughflow solver TzFlow and a metamodel-assisted GA. This latter design strategy was successfully applied to the design of an axial compressor. Both numerical codes are coupled with the most accurate equations of state for the thermophysical description of the fluid. The overall procedure is applied to design two turbines with a power output of about 50 and 250 kW. The results are extensively discussed.