Advancing Flywheels with HTSC Bearings
High temperature superconducting bearings are actively being researched at the Center for Electromechanics. This high temperature superconductor (HTSC) is a radioactively treated component made up of yttrium barium copper oxide (YBCO), a chemical compound that occurs naturally in the environment. The research revolved around HTSC bearings are for flywheel energy storage, which works by accelerating a flywheel to a very high speed and maintaining the energy in the system as rotational energy.
The purpose of researching HTSC bearing in relation to flywheel energy storage is an effort to reduce spinning losses and expand flywheel applications for longer term utility grid storage applications. The basic principles behind HTSC bearing lie in the stable levitation of a permanent magnet above a bulk superconductor. This stable levitation is possible due to induced pinning currents within the superconductor which resist changes in the magnetic fields and produce restoring forces on the levitated permanent magnet. For a ring shape permanent magnet, with a consistent magnetic field around the circumference, the magnet can spin freely with nearly zero losses.
Reduced Order Model Development
The physics behind modeling the interaction between a permanent magnet and bulk superconductor are quite involved, due to the non-linear losses which are responsible for trapped fields and hysteretic force-displacement profiles. To date, proposed reduced order models either ignore these losses or only provide quasi-static solutions. Finite element methods (FEM) are the surest way to include all losses and physical effects, but the 3-D FEM required to estimate translational stiffness and coupling forces is computationally intensive, and can take days to solve on a standard desktop.
Researchers at the Center for Electromechanics, under funding from GCEP, are currently working on developing reduced order models of the permanent magnet - HTSC dynamic interaction which will maintain the nonlinear dynamics, but greatly reduce computation time, and allow trade studies to be quickly performed for sizing HTSC bearing systems without requiring computationally expensive FEM analysis.
Vertical Test Scenario
In this vertical test scenario, a magnet is dropped from a known height, and bounces until it comes to rest (due to friction from the guide rod) at an equilibrium height over the bulk HTSC
The reduced order models have been compared to FEM results as shown below. Here, the force-displacement profile of a permanent magnet passing through a HTSC ring is considered.
The FEM model with 12,000 elements…took over 48 hours to solve…the reduced order model developed by CEM provided a solution in under a minute, while maintaining all the nonlinear losses.
The FEM method required over 12,000 elements, and took over 48 hours to solver, considering all the non-linear losses within the bulk HTSC, whereas the reduced order model provided a solution in under a minute, while maintaining all the nonlinear losses. The improvement in calculation speed also allows the reduced order method to be incorporated into larger dynamic system models, such as a rotor-dynamic model, to provide dynamic response for bearing designs.
Currently, the CEM team is continuing development of the reduced order method for accurately modeling a horizontal testing scenario.