A Fundamental Look at Energy Storage

Limitations integral to energy storage technologies are fundamentally constitutive in nature. For example, flywheel energy storage is limited by the flywheel’s ultimate yield stress. If a flywheel is designed to operate below the first bending mode, the energy per unit volume for a cylindrical flywheel is dictated entirely by the material yield stress, Poisson ratio, and the safety margin of operation below ultimate yield. Yield stress measurements on carbon nonotubes suggest that these materials have the capacity to improve the energy density of flywheels an order of magnitude over the best carbon composite materials used today. Both high and low temperature superconducting magnetic energy storage (SMES) is limited by the quench or irreversible field level above which the medium becomes non-superconducting. The more promising media such as LTS Ni3Sn and HTS trapped field magnets also suffer the problem of sustaining the mechanical stresses commensurate with high magnetic fields. With capacitive energy storage, the limitation is dielectric constant, and dielectric breakdown strength. Batteries are limited by the half cell potential, and the area required for coulomb exchange at the electrodes. The following table summarizes the points.

Naval and Air Force applications place a high priority on volume. Applying these principles, it is possible to generate a comparison chart for various energy storage technologies.

This information was presented at the October Electric Energy Storage – Applications and Technology Conference. More complete information is in the paper, “A fundamental look at energy storage focusing primarily on flywheels and superconducting magnetic energy storage” by K. Davey and R. Hebner.

For further information, please contact Kent Davey.

As advances are made in the power density in motors and generators, the electrical insulation is operated at new levels of combined electrical, mechanical, and thermal stresses. In addition, new materials are becoming available with different properties. To aid in system engineering, staff from the Center for Electromechanics are developing new approaches to qualify insulating materials for novel applications. As part of that work, partial discharge measurements are being investigated as a tool to predict safe operating levels. Specifically, partial discharges were used to investigate the effect of the electrode surface roughness in an epoxy insulated system. A paper describing the work titled, “Surface finish effects on partial discharge with embedded electrodes” by A. Wilder and R. Hebner was presented at the 2003 Conference on Electrical Insulation and Dielectric Phenomena in October 2003.

For further information, contact Aleta Wilder.

Locomotive Flywheel

To permit high speed passenger service on rail systems that have not been electrified, the Center for Electromechanics is designing, building the components for, and planning to test a hybrid, high-speed locomotive. R. Thelen, J. Herbst, and M. Caprio presented a paper describing this system titled, “A 2 MW Flywheel for Hybrid Locomotive Power” at the IEEE Semiannual Vehicular Technology Conference. Key elements of the system are a 100 kW-hr flywheel capable of providing 2 MW for three minutes, a 3 MW turbine-alternator combination designed for 15,000 rpm operation without speed-reducing gears, and the required power electronics and controls.

For further information, please contact Bob Thelen.

Flywheels offer an attractive alternative to batteries in hybrid vehicles. They have a very long life compared to batteries in deep discharge applications. This long life comes in a package that can be smaller than a battery and has fewer environmental issues related to manufacture and disposal. A new paper, presented at October IEEE Semiannual Vehicular Technology Conference, shows that flywheel integration in a vehicle favors motor designs with good performance toward the high-end of the torque-speed motor characteristics. This behavior creates constraints on the motor design such that permanent-magnet, brushless, direct-current motors are likely to be poorly suited for this application while induction and switched-reluctance machines are likely to be good candidates. The paper written by Y. Gao, S. Gay, and M. Ehsani of Texas A&M University and R. Thelen and R. Hebner of CEM is titled, ”Flywheel Electric Motor/Generator Characterization for Hybrid Vehicles”.

For further information, contact Bob Thelen.