Numerical Analysis of Reversible Radial-Flow Turbomachinery for Energy Storage Applications

Decarbonization of energy systems is expected to require significant amounts of energy storage. Among the different technologies studied, thermo-mechanical storage is being considered for long duration storage due to its low cost and limited usage of critical materials. Based on the experience of pumped hydro plants, significant cost savings can be achieved if a single stage reversible compressor-turbine is used for both the charging and discharging of the storage. Due to the modest (<4.5) pressure ratio involved, low temperature adiabatic compressed air energy storage and pumped thermal energy storage are considered as candidate applications for reversible turbomachinery.
This paper presents a numerical analysis of an existing high efficiency centrifugal compressor operating both as a compressor and a turbine using mean-line analysis and steady-state simulations with Reynolds-Averaged Navier-Stokes equations in ANSYS CFX. After validation against experimental data in compressor operating mode, the turbine mode was tested by changing only the inlet and outlet boundary conditions and reversing the direction of rotation. Subsequently, a new stage was designed using conventional procedures developed for compressors and evaluated using computational fluid dynamics simulations.
The results indicate that for an up-scaled version of the existing compressor it is possible to reach total-to-total polytropic efficiencies exceeding 86.7 % as a compressor and 92.1 % as a turbine, which is comparable with the efficiencies of dedicated machines. The new design reaches a polytropic efficiency of 89.4 % as compressor and 90.4 % as turbine. In both cases the numbers obtained do not include a volute or diffuser cone. Furthermore, the results indicate that there is a compromise between the optimal design for the compressor and turbine mode.