Design Study of a Transcritical CO2 Bladeless Expander Prototype

The global demand for efficient, cost-effective, low-maintenance, and environmentally sustainable technologies is rising. Tesla or bladeless turbines present a promising solution for small-scale power generation or energy harvesting. These turbines utilize a peculiar boundary layer operating principle, harnessing the distinctive thermodynamic properties of the working fluid. This working principle is particularly suitable for the expansion of transcritical or supercritical CO2 in low-capacity applications (e.g. > 1MW); such conditions are indeed characterized by very low volumetric flows, which would drive the designers towards volumetric expanders, affected by vibration and efficiency issues exacerbated by the very high-pressure levels. Bladeless turbines may represent an interesting alternative, providing an effective, scalable, and cost-effective solution in such a small-scale application. This paper focuses on the design and development of innovative bladeless turbines utilizing transcritical CO2 as the working fluid within a CO2-based Pumped Thermal Energy Storage cycle. The bladeless turbine is employed during the charge cycle, where the expander volumetric flow is minimum (cold expansion). The initial design of the CO2 turbine was analyzed using tailored 0D and 1D tools and subsequently verified by high-fidelity 3D CFD simulations. The turbine prototype is designed to operate at 15,000 rpm with a mass flow rate of 0.15 kg/s, delivering an estimated power output of 3.1 kW. During this design, optimal disk gap and diameter ratio have been considered for optimal performance. Without considering rotor leakage and end-wall losses, the analytical and numerical results show good agreement, with acceptable variation. These results indicate that the transcritical CO2 bladeless turbine is capable of achieving around 50% isentropic efficiency (without assembly and generator losses), despite the very small size.