## Superconducting Magnetic Energy Storage

Energy stored in magnetic fields.

### Background

Superconducting Magnetic Energy Storage (SMES) is a method of energy storage based on the fact that a current will continue to flow in a superconductor even after the voltage across it has been removed. When the superconductor coil is cooled below its superconducting critical temperature it has negligible resistance, hence current will continue to flow (even after a voltage source is disconnected). The energy is stored in the form of a magnetic field generated by the current in the superconducting coil. It can be released by discharging the coil. The coils are usually made of niobiumtitane (NbTi) filaments which has a critical temperature of around 9K. As SMES stores electrical current the only conversion involved with the process is the conversion from AC to DC. Hence the efficiencies of SMES systems are very high. SMES can switch from full discharge to full charge very quickly and visa versa. It has negligible deterioration due to cycling. However, SMES has a high self-discharge rate due to the energy expenditure of cooling via cryogenic liquid and mechanical stability problems. The magnetic energy stored in a conducting coil is given by:

$E = \frac{1}{2} L I^2$

Where $I$ is the current in amperes and $L$ is the inductance in henries.

Figure: Illustration of an SMES system

The SMES concept started with the idea of very large plants with capacities of GWdays, that were intended for diurnal load levelling [1]. However, with the advancement of superconductor technology, notably the increase in Tc (the critical temperature of the superconducting transition), recent development has mostly been on smaller scale applications and systems up to 10 MW are commercially available. There is some very recent research on SMES for grid scale applications. The U.S. Department of Energy Advanced Research Projects Agency for Energy (ARPA-E) has awarded a $4.2 million grant to Swiss-based engineering firm ABB to create a 3.3 kilowatt-hour proof-of-concept SMES prototype. ABB is collaborating with superconducting wire manufacturer SuperPower, Brookhaven National Laboratory, and the University of Houston. The group's ultimate goal is to develop a 1-2 MWh commercial-scale device that is cost-competitive with lead-acid batteries. ### Characteristics and Applications Due to its very high cycling capacity and high efficiency over short time periods SMES is very well suited to high power short duration applications. They are used in many voltage stability and power quality applications, for example to provide very clean power in microchip manufacture. On-site SMES is suitable to mitigate the negative impacts of renewable energy in power quality related issues, especially with power converters – needed for solar photovoltaic and some wind farms – and wind power oscillations and flicker. The biggest problem with SMES at present is the very high capital costs of the cooling units required, which use either liquid helium at 4.2K or super-fluid helium at 1.8K. ### Summary of characteristics  Typical Capacity Typical Power Efficiency (%) Storage Duration$/kWh \$/kW Lifespan Cycling capacity Up to 20 MWh Up to 40 MW >95 [2] milliseconds – mins 1000-10000 [2,3] 200 - 400 [2,3] 20+ years Very High

Table: SMES characteristics