Design of Compact Radial Turboexpanders for sCO2 Power Systems

The development of novel conversion technologies for recovering waste heat is one of the technical goals of the European Commission, considering the large amount of thermal energy discharged by industrial processes in Europe. The recently launched EU-H2020 project CO2OLHEAT aims at contributing to industrial waste heat recovery by developing a novel sCO2 power system of 2 MW power capacity. Considering the wide range of conditions featuring waste heat recovery applications (in terms of both flue gas temperature and flow rate), as well as the flexibility of operation required by such installations, a simple recuperative cycle was selected for the CO2OLHEAT plant. The system, however, features a relatively non-conventional multi-shaft configuration, composed of a radial-inflow turboexpander, which drives the near-critical compressor, and by an axial-flow turbine to generate the 2 MW output power. The present study focuses on the design of the radial turbines of the CO2OLHEAT turbo-expander. Due to the low volumetric flow rate, the turbine features small size and, therefore, significant aerodynamic challenges have to be considered in the design process. A preliminary design of the machine was performed by applying a mean-line approach, then a conceptual design of the bladings and of the meridional channels was performed by resorting to industrial in-house database and criteria. Once the turbine design was finalized, the aerodynamics and performance of the machine were evaluated by applying two different computational fluid dynamic simulation tools, which exhibit remarkable agreement. One CFD tool was then to investigate and quantify the impact of wheel tip clearance and surface roughness, as well as to quantify the deviation with respect to mean-line predictions. Relevant quantitative data are extracted as well as aerodynamic indications are inferred, with the aim of establishing a reference for the future design of compact sCO2 radial turbines.