Superconducting magnetic energy storage
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Superconducting Magnetic Energy Storage (SMES) uses the ability of certain materials to conduct electricity without resistance (superconductivity) to store electrical power.
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Technology
A common design of a SMES installation would consist of a coil of superconducting wire buried underground, with power conditioning equipment connecting the coil to the electricity distribution grid.
Application
In principle there is no reason why SMES couldn't be used on a very small scale in place of conventional batteries. In practice the relatively low energy density possible, exacting cooling requirements, and high cost, mean that near-term applications are likely to be limited to power grid applications.
Problems
Size - To achieve commercially useful levels of storage, around 3,600 GJ, a SMES installation would need a loop of around 100 miles (160 km). This is traditionally pictured as a circle, though in practice it could be more like a rounded rectangle. In either case it would require access to a significant amount of land to house the installation, and to contain the health effects noted below.
Manufacturing - There are two manufacturing issues around SMES. The first is the fabrication of bulk cable suitable to carry the current. Most of the superconducting materials found to date are relatively delicate ceramics, making it difficult to use established techniques to draw extended lengths of superconducting wire. Much research has focussed on layer deposit techniques, applying a thin film of material onto a stable substrate, but this is currently only suitable for small-scale electrical circuits.
The second problem is the infrastructure required for an installation. Until room-temperature superconductors are found, the 100 mile (160 km) loop of wire would have to be contained within a vacuum flask of liquid nitrogen. This in turn would require stable support, most commonly envisioned by burying the installation.
Critical current - In general power systems look to maximize the current they are able to handle. This makes any losses due to inefficiences in the system relatively insignificant. Unfortunately the superconducting properties of most materials break down as current increases, at a level known as the critical current. Current materials struggle, therefore, to carry sufficient current to make a commercial storage facility economically viable.
Critical Magnetic Field - Related to critical current, there is a similar limitation to superconductivity linked to the magnetic field induced in the wire, and this too is a factor at commercial storage levels
Health effects - The biggest concern with SMES, beyond possible accidents such as a break in the containment of liquid nitrogen, is the very large magnetic fields that would be created by a commercial installation, which would dwarf the magnetic field of the Earth. Little is know about the long term effects of exposure to such fields, so any installation is likely to require a significant buffer zone around and above it to protect humans and wildlife.