SMES stands for Superconducting Magnetic Energy Storage, and is a French invention. The energy is stored via an electric current sent in a coil made of a superconducting wire. Once the coil is short circuited (closed), the current stays forever since there is no loss and produces a magnetic field as in MRI coils. The energy is hence stored in the coil in a magnetic and electric form and can be recovered in a very short amount of time.
This use of superconducting coils to store magnetic energy was invented by M. Ferrier in 1970. The coil must be superconducting, otherwise the energy is dissipated by Joule effect in a few milliseconds. The main characteristics of the SMES are a strong power density, but a moderate energy density, a very high (infinite) number of charge/discharge cycles and a very high productivity in energy conversion, superior to 95%.
Magnetic energy storage in a short-circuited (closed) superconducting coil.
A SMES system is more an impulsive current source than an energy storage device. SMES is hence an excellent solution for non-interruptible power supply systems or some FACTS (Flexible AC Transmission System), which are static equipment to enhance electric networks.
The need of impulsive energy sources for emerging applications such as electromagnetic throwers for military and civilian purposes give SMES other opportunities. SMES is a very convenient solution that has many advantages compared to the conventional solutions, as far as efficiency, lightness and congestion are concerned.
Several SMES systems have proved their performances and operational capacities for powers counted in megawatts and lengths of about a second. They have been used as interruptible sources (“undulators”) for sensitive charges or to stabilize electric networks.
SMES was also the first superconducting application used on an electric network of the BPA (Bonneville Power Administration) in the 80s. These systems can be bought in stores and the feedback from past experience in the United States and Japan is very positive. However, few SMES systems have been sold because of the high initial cost and the competition of more developed technologies. Even though high temperature materials cannot reduce the total cost, they make SMES systems more attractive because they enhance the mass performances of superconducting magnets and reduce the cost of cryogenics (investment and functioning).
