Advances in research on ORC turbomachinery

Power plants operating according to the Rankine thermodynamic cycle, but using an organic substance as a working fluid in place of water, are termed organic Rankine cycle (ORC) power plants.The selection of the working fluid is a degree of freedom of design that allows the optimization of the system depending on the temperature of the thermal energy source, which can vary approximately between 100 C and 600 C, and the capacity, which can range from tens of kW to tens of MW (for more details, see e.g., Colonna et al. (2015)).ORC power plants are suitable for the conversion of renewable energy sources into electricity.The technology is mature if the power output is large and the maximum cycle temperature is below 350 C.Its application for the exploitation of geothermal reservoirs is growing at a fast pace around the world.Many biomass-fired ORC power plants have been deployed in Europe since the early 2000s.Relevant opportunities exist if the conversion of otherwise wasted thermal energy from industrial installations is considered, and the potential is even larger if the recovery from propulsion engines is included, see, e.g., the recent report of the Knowledge Center for Organic Rankine Cycle technology (KCORC) (Astolfi et al., 2025).KCORC is the not-for-profit association that organizes every two years the International Seminar on Organic Rankine Cycle Power Systems since 2011.The 7th edition of the seminar was held in Seville, Spain, from September 4th to 6th, 2023.The event gathered 191 participants from more than 30 countries, with representatives from industry, research institutes, and universities, and with a significant presence of young professionals.This special issue of the Journal of the Global Power and Propulsion Society is a collection of four articles whose content was first presented at the seminar in Seville, and it is focused on ORC turbomachinery research.The efficiency of the supersonic expansion of organic fluid vapor into the stator of ORC turbines is critical for the performance of the turbine, therefore the article (Oliveti et al., 2025) is of particular interest as it documents for the first time the accurate characterization of such flow field, whereby the thermodynamic state of the working fluid at the inlet of the stator is highly nonideal.A blow-down facility, the TROVA at Politecnico di Milano, was used to generate a flow of hexamethyldisiloxane (MM) vapor through a planar cascade.The variation of the static pressure in the streamwise flow direction along the blade channels is measured together with the total pressure loss across the cascade.Schlieren imaging was also performed during each experiment.The experimental data are compared to the results of CFD simulations and are in good agreement.The article (Zakeralhoseini and Schiffmann, 2025) documents a method for obtaining novel design maps, namely efficiency contour plots as a function of specific speed and specific diameter, for centrifugal