Superconducting magnetic energy storage

Superconductivity offers, in principle, the ideal way of storing electric power. The storage system comprises an electromagnetic coil of superconducting material which is kept extremely cold. Off-peak electricity is converted to DC and fed into the storage ring, and there it stays, ready to be retrieved as required. Provided the system is kept below a certain temperature, electricity stored in the ring will remain there indefinitely without loss.

The key to the superconducting magnetic energy storage device is a class of materials called superconductors. Superconductors undergo a fundamental change in their physical properties below a certain temperature called the transition temperature which is a characteristic of each material. When a material is cooled below its transition temperature it becomes superconducting. In this state it has zero electrical resistance. This means that it will conduct a current with zero energy loss.

Unfortunately the best superconducting materials only undergo this transition at below 20°K (—253°C). Temperatures this low can only be maintained by cooling the superconducting coil with liquid hydrogen or liquid helium, in either case an expensive process.

In recent years scientists have discovered materials that become superconducting at relatively high temperatures, temperatures accessible by cooling with liquid nitrogen. (Liquid nitrogen boils at 98°K, — 175°C.) Most of these materials have proved to be rather brittle ceramics which are difficult to work but techniques are being found to exploit them. This is helping make superconductivity more economically attractive for a range of utility applications including storage.

Superconductors store DC current without loss but losses occur in converting the off-peak AC current to DC and then back to AC when required. The round trip efficiency is around 90%. A superconducting magnetic storage device can respond extremely quickly, delivering its rated power in about 20 ms.

A number of small superconducting storage rings have been built for use as power-conditioning systems. One of 10 MW capacity has been tested on a utility system in the USA where its primary role was to improve transmission system stability. Such systems are extremely expensive.

The unit cost of storing power in a superconducting ring decreases as the size of the plant increases so large storage devices would be preferred for utility applications. The superconducting ring for a 5000-MW device would be roughly 1600 m in diameter. The magnetic fields associated with such a device would be enormous and it would have to be built into rock to ensure it did not collapse under the force generated.

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