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.