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Superconducting magnetic energy storage (SMES) is unique among the technologies proposed for diurnal energy storage for the electric utilities in that there is no conversion of the electrical energy, which is stored directly as a circulating current in a large superconducting magnet, into another energy form such as mechanical, thermal, or chemical. Thus one advantage of SMES is the inherent high storage efficiency that is possible because energy conversion processes are avoided. The actual round-trip efficiency of a large unit is expected to be 90 percent or greater. The fast response (< 100 ms) of the system to power demand means that a diurnal storage unit can also function as a swing generator or provide system stabilization. The major components of a SMES system are a large superconducting coil cooled by liquid helium, an ac-to-dc convertor, and a refrigerator that maintains the temperature of the helium coolant. This paper describes the design and functions of these and other components of an engineering reference design for a 1-GWh SMES unit. Also included is a sketch of the historical development of superconductivity, which was first discovered in 1911, and SMES, which was first proposed as a method of diurnal storage in 1969.