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Session: Parallel Session: Small-capacity systems
Session starts: Friday 23 September, 14:00
Presentation starts: 14:40
Room: Auditorium

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Emiliano Casati (Process and Energy Department, Delft University of Technology, Politecnico di Milano, LFM Group)
Matteo Pini (Politecnico di Milano - LFM Group)
Giacomo Persico (Politecnico di Milano - LFM Group)
Andrea Spinelli (Politecnico di Milano - LFM Group)
Vincenzo Dossena (Politecnico di Milano - LFM Group)

Abstract:
Nowadays there is a general agreement that a substantial increase in the share of energy produced in a decentralized way is a desirable perspective for future sustainable societies. As a consequence, small-scale power plants are candidates to play a relevant role in the future distributed energy scenario [1]. Among the technologies that are suitable for high-efficiency conversion of thermal power into electricity in the small to medium power range, Organic Rankine Cycle turbogenerators stand out in terms of reliability and cost-effectiveness. It is well known that, as the size of a power plant reduces, the cost of the conversion engine (typically a turbo-expander) represents an increasing part of the total investment. The use of organic compounds as working fluids leads to relatively simple plant configurations and to design compact and reliable turbines. In common ORC applications the turbine is inherently characterized by a low enthalpy drop that is usually disposed in a low number of stages (even a single stage for radial turbines, or a few stages for axial turbines). These two peculiar aspects result in a very high stage expansion ratios, that, combined with the typical low speed of sound of organic fluids, induces strong supersonic phenomena. As a consequence the turbine maximum efficiencies are in the range of 80-85 % for the largest units [2]. The development of turbines with better performances in terms of efficiency and controllability is therefore a key-theme in the field of ORC: aim of the present work is the critical evaluation of a Ljungstrom-like multi-stage centrifugal turbine whose architecture could have substantial advantages over traditional solutions [3]. Due to the absence of experience and specific literature, a wide-spectrum design procedure has been conceived. A lumped parameter code has been initially developed, on the basis of initial project assumptions, machine design criteria, such as the Mach number at outlet section, and available models for losses evaluation. The results of the 0-D code are then verified with the new throughflow solver of zFlow, a quasi 3-D CFD-based code for turbomachinery applications coupled with the FluidProp package for accurate properties calculation [4]. The calculation method is proven to be a valuable tool to efficiently select a small number of preliminary machine’s designs for the proposed architecture as well as for axial machines, where consolidated experience and experimental data are not available. Furthermore this represent an effective way to determine a baseline configuration for subsequent more detailed optimizations. The overall procedure is finally applied to a 100 kW multi-stage turbine using the siloxane D4 as working fluid. The results are extensively discussed by a comparison with available data from existing machines. REFERENCES [1] U.S. Dept. of Energy, “The Potential Benefits of Distributed Generation and Rate-Related Issues That May Impede its Expansion”, Sect. 1817 - Energy Policy Act 2005. [2] E. Macchi, “Design criteria for turbines operating with fluids having a low speed of sound”, lecture series n. 100 on closed-cycle gas turbines, Von Karman Inst. For Fluid Dynamics,1977. [3] E.p. Coomes, D.G.Wilson et al., “Design of a high-power-density Ljungstrom turbine using potassium as a working fluid”, Proc. Intersociety Energy Conv. Eng. - 1986. [4] G. Persico, S. Rebay, C. Osnaghi, “A novel package for turbomachinery throughflow analysis”, European Turbomachinery Conference 2011