Radial outflow turbines for Organic Rankine Cycle expanders

Among the turbine technologies for medium to large size Organic Rankine Cycle (ORC) systems, the radial outflow turbine represents the most recent development. This chapter gives an overview of the technology in terms of history, theoretical basis, and technical solutions. A brief history of the radial outflow turbine is presented. Starting from the radial outflow single-rotating steam turbine designed and applied in 1884 by Sir. Charles Parsons, the chapter then looks at the development of this technology by Birger and Fredrik Ljungström, who patented their turbine in 1908. In respect to other turbines their configuration had a unique feature; a multi-stage counter-rotating centrifugal steam turbine, in which two discs facing each other are connected with two independent shafts, with crowns of blades installed alternately on both of them. Thanks to this unique design, the turbine was used mainly for power generation but it also found application in railway engineering because of its favorable mass-to-power ratio and is the basis of designs used today. A detailed description of the Radial Outflow Turbine (ROT) basics is carried out in comparison to the more common axial and radial inflow arrangements, describing the main features of the ORC radial outflow turbine through mechanical, thermodynamic, and fluid-dynamic fundamentals. A comparison of the presented ROT with an axial ORC turbine in an overhung configuration directly coupled with a generator is presented. This analysis highlights differences in fluid-dynamic losses and can explain the higher efficiency of ROTs compared with the axial overhung turbines. Finally, the typical configurations allowed by the radial outflow technology are presented.