Principle

The SMES stores energy in the magnetic field built up by a DC current flowing through the superconducting coil. In a conventional coil made of copper wire the magnetic energy would be rapidly dissipated as heat due to the resistance of the wires. If superconducting wires are used, energy can be stored for a long time.

SMES coils may in principle be made from low (LTS) or high temperature superconductors (HTS). However, in the past only (helium-cooled) LTS have been sufficiently advanced to be used in large-scale SMES applications. With future advances in HTS materials, this will change.

Although the coil itself is based on DC current, the charging and discharging processes require complex AC control circuitry.

Comparison with other storage technologies

Today's SMES technologies are of high-power low-energy type as can be seen in the following Ragone diagram in Figure 1.

Figure 1: Ragone diagram

Source: IZT, data from Schneuwly et al. 2002

Fields of application

Utility sector: Energy storage, increased transmission capacity through enhanced line stability, Voltage and frequency control, sub-synchronous resonance damping etc.

The power and energy characteristics of SMES technologies make them a very difficult candidate for brake energy storage in rail vehicles. This is illustrated in Figure 2. It can be seen that SMES do not meet the requirements of any operation type in railways. Light rail vehicles have the braking characteristics closest to those offered by SMES, but still too far off to make an application realistic in the foreseeable future.

Figure 2: Ragone diagram and charging times (corresponding to braking times of different trains)

Source: IZT, data mainly from: Hentschel et al. 2000