A fundamental look at energy storage focusing primarily on flywheels and superconducting energy storage

This paper compares energy storage efficiency of Superconducting Energy Storage devices (SMES) with high speed flywheels employing magnetic bearings. Both solid cylinder and shell cylinder flywheels are examined from fundamental physics. Solid cylinder flywheels have a fixed energy density by weight and volume dependent only on the constitutive properties of the flywheel. For a target energy storage, the flywheel’s radius, length, and rotation speed are determined given the governing limitation on hoop stress and the requirement that operation will occur below the first bending mode. No design parameters are open for engineering judgment except the margin of safety. Thus the volume necessary to reach a target energy storage is well defined. The shell cylinder has only the thickness of the shell as an open design variable. The constraint for a SMES system is that the magnetic field density remain below the quench value for the superconductor. This constraint involves the current density, the magnetic field density, and the temperature. A theoretical upper limit can be reached by considering a volume with a B field just under the quench value. In this theoretical upper limit, given the materials available today, the flywheel stores the same energy in a volume 7.4 times smaller than the SMES system even when assuming a 20 T field for the SMES system. Both systems allow for energy to be added and removed rapidly by comparison to battery and capacitive storage, but the flywheel is by far the more efficient choice when examined on a per volume basis.