Homopolar Generator?
Keith Lofstrom suggested tossing the rigid body structure of a flywheel. He proposed two D-magnets separated by a useful distance, with actively controlled attractive levitation, be applied to warp the path of a steel ribbon 180 degrees at each end of a racetrack path. The materials are thus not stressed so as to approach mechanical failure.
The velocity proposed for application within a vacuum was 11 kilometers per second. Magnetic levitation suspended the weight of the moving mass with low loss of energy. Linear motors magnetically coupled to the ribbon provided for two way transformation between electrical and kinetic energy.
He proposed initial efforts toward storing sufficient energy to load level around peak demand. A projection suggested servicing a peak market could pay for a properly scaled system in three months.