Dr. Rainer Kurz and Dr. Klaus Brun

Principal @ RKSBenergy LLC, and Global Director R&D @ Ebara Elliott Energy

The use of Hydrogen as an energy carrier is one of the concepts to reduce carbon emissions while maintaining energy security. Hydrogen will have to be compressed to be useful in many applications, involving transport, storage and usage. The size of hydrogen volume will require centrifugal compressors as the compressors of choice. Hydrogen used as an energy carrier is relatively pure, and thus has a very low mole weight and a high heat capacity. Thus, it requires significantly more work (or enthalpy rise) to achieve a certain pressure ratio compared to other gases like natural gas or CO2. To achieve a sufficiently high pressure ratio with Hydrogen, the choice is to either have a large number of compressor stages, or to increase the tip speed of the impellers. A large number of stages may be commercially unattractive, while an increase in tip velocity increases the stress levels in the impeller. 

The successful and commercially viable use of centrifugal compressors for hydrogen compression requires the capability to operate at high tip velocities, which in turn lead to high stress levels in the impellers. Higher tip velocity can be achieved by combinations of aerodynamic mechanical design (low stress, clearance control, high work coefficient), and materials. Open faced impellers allow higher tip velocities than shrouded impellers, but may incur efficiency penalties due to clearance losses. Impellers with radially ending blades offer potentially lower stress levels, but suffer from lower efficiency and less operating range. Combinations of 1 or 2 open faced impellers and the remained with shrouded, lower diameter impellers may be a valid compromise. This is already common practice in integrally geared compressors, and has been used in some in-line compressors. Attaching impellers to the shaft at high stress levels can be accommodated by modular rotors, or rotors where the impellers and the shaft are machined as a single piece from a forging. Material choices, in particular the move to composite materials may allow to stay with shrouded impellers and traditional backsweep while still allowing for high tip speeds. These materials, however, have not been employed in commercial use yet.