Handbook of Energy Storage for Transmission or ... - W2agz.com

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Figure 2
Artist concept of a diurnal SMES system that is constructed underground
System Components
The peak power capacity and the maximum stored energy in a SMES system are determined by
application and site-specific requirements. Once these values are set, a system can be designed
with adequate margin to provide the required energy on demand. It is apparent from Figures 1
and 2 that SMES units have been proposed over a wide range of power capacities (1 to
1000 MW) and energy storage ratings (0.3 to 10,000,000 kWh). Independent of capacity and
size, however, a SMES system always includes a superconducting coil, a refrigerator, a power
conversion system (PCS), and a control system as shown in Figure 3. Each of these components
is discussed in this section. A description of the magnetic basis for the energy storage of SMES
systems and a discussion of mechanical support for the superconducting coils are included in the
Appendix. These are included to give additional insight to those interested in some of the details
of SMES technology.
The Coil And The Superconductor
The superconducting coil, the heart of the SMES system, stores energy in the magnetic field
generated by a circulating current. Since the coil is an inductor, the stored energy is proportional
to the square of the current, as described by the familiar equation:
1 2
E = LI ,
2
where L is the inductance of the coil, I is the current, and E is the stored energy.
The total stored energy, or the level of charge, can be found from the above equation and the
current in the coil. The maximum stored energy, however, is determined by two factors.
• The size and geometry of the coil, which determines the inductance. The larger the coil the
greater the stored energy.
• The characteristics of the conductor, which determines the maximum current.
Superconductors carry substantial currents in high magnetic fields. For example, at 5 T,
which is 100,000 times greater than the earth’s field, practical superconductors can carry
currents of 300,000 A/cm 2 .
SMES Page 2