The paper aims to give an overview over the keystones of design of the turbomachine for a supercritical CO₂ (sCO2) Brayton cycle. The described turbomachine is developed as part of a demonstration cycle on a laboratory scale with a low through flow. Therefore, the turbomachine is small and operates at high rotational speed. To give an overview on the development, the paper is divided into two parts regarding the aerodynamic and mechanical design. The aerodynamic design includes a detailed description on the steps from choosing an appropriate rotational speed to the design of the compressor impeller. For setting the rotational speed, the expected high windage losses are evaluated considering the reachable efficiencies of the compressor. The final impeller design includes a description of the blading development together with the final geometry parameters and calculated performance. The mechanical analysis shows the important considerations for building a turbomachine with integrated design of the three major components: turbine, alternator, and compressor (TAC). It includes different manufacturing techniques of the impellers, the bearing strategy, the sealing components, and the cooling of the generator utilizing the compressor leakage. Concluding the final design of the TAC is shown and future work on the machine is introduced.

This paper presents experimental data on a centrifugal compressor operated with CO2. Temperature and pressure at the inlet of the compressor are varied to cover the supercritical region from the liquid-like to the gas-like region. In addition, inlet conditions in the two-phase region are also included. Thus, the experimental test campaign considers thermodynamic conditions relevant for the future energy conversion with sCO₂-Joule cycles. Experimental results are presented as compressor pressure ratio vs inlet mass flow rate at different rotational speeds and throttle positions. Reliable conclusions can be drawn from the experimental results since the reproducibility of the measurements has been demonstrated by conducting experiments in two different test rigs, and measurement uncertainties are reported. The entire compressor geometry is disclosed in a data repository, including CAD models, and input files suitable for mean-line and grid generation programs. Thus, the experimental results are exploitable by the scientific community and pave the road for validated analysis and design tools in the context of the sCO₂-Joule cycle. The presented results open the possibility to estimate uncertainties of analysis and design tools with little effort since geometry information can be quickly integrated. The experimental data is already used in this paper to obtain the accuracy of a CFD code and a mean-line program for sCO₂. In addition to quantifying uncertainties, the results presented can be used to identify shortcomings of existing tools. This can be an essential step in the exploration of the sCO₂-Joule cycle.

This paper aims to give an understanding of an effect which stabilizes the inlet conditions of compressors for supercritical CO₂ (sCO2) operating close to the critical point. The effect was observed during testing of the turbomachine within the sCO2-HeRo project, and is caused by the sCO2 real gas properties close to the pseudocritical line. Under theoretical consideration, strong gradients in the fluid properties around this line—dependent on the static temperature and pressure of sCO2—can result in strong variation of compressor performance and finally lead to unstable cycle behavior. However, this paper demonstrates reduced gradients in density at the compressor inlet when varying the cooling power and taking advantage of a stabilizing effect. The applicable range and the significance of this stabilizing effect depended on the cooler inlet temperature and pressure, and was used to evaluate the relevance for individual cycles. Controlling the cooling power and the measurement of the inlet density allowed control of the compressor inlet conditions equally well, independent of the operating point, even close to the critical point.

In the sCO2-HeRo project, the Chair of Turbomachinery at the University of Duisburg-Essen developed, built and tested a turbomachine with an integral design in which the compressor, generator and turbine are housed in a single hermetic casing. However, ball bearings limited operation because their lubricants were incompatible with supercritical CO₂ (sCO₂) and they had to operate in gaseous CO₂ instead. To overcome this problem, the turbomachine was redesigned built and tested in the sCO2-4-NPP project. Instead of ball bearings, magnetic bearings are now used to operate the turbomachine with the entire rotor in sCO₂. This paper presents the revised design, focusing on the usage of magnetic bearings. It also investigates whether the sCO₂ limits the operating range. Test runs show that increasing the density and rotational speed results in greater deflection of the rotor and greater forces on the bearings. Measurements are also analyzed with respect to influence of the density increase on the destabilizing forces in the rotor–stator cavities. The conclusion is that for the operation of the turbomachine, the control parameters of the magnetic bearings must be adjusted not only to the rotor speed, but also to the fluid density. This enabled successful operation of the turbomachine, which reached a speed of about 40,000 rpm during initial tests in CO₂.

The paper aims to improve the understanding of the dependency of compressor inlet conditions close to the critical point in supercritical CO 2 (sCO 2 ) cycles on different volumetric cycle designs. The compressor inlet conditions are fixed by the specific static outlet enthalpy of the main...