Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 688

where L is the inductance of the coil, and I is the current passing through it. Therefore,
material properties are extremely important as temperature, magnetic fi eld, and
current density are pivotal factors in the design of SMES. The overall effi ciency
of SMES is in the region of 90% [13] to 99% [3]. SMES has very fast discharge
times, but only for very short periods of time, usually taking less than 1 min for a
full discharge. Discharging is possible in milliseconds if it is economical to have
a PCS that is capable of supporting this. Storage capacities for SMES can be any-
thing up to 2 MW, although its cycling capability is its main attraction. SMES
devices can run for thousands of charge/discharge cycles without any degradation
to the magnet, giving it a life of 20+ years.
4.8.1 Applications of SMES
Due to the high power capacity and instantaneous discharge rates of SMES, it is
ideal for the industrial power quality market. It protects equipment from rapid
momentary voltage sags, and it stabilises fl uctuations within the entire network
caused by sudden changes in consumer demand levels, lightening strikes or opera-
tion switches. As a result, SMES is a very useful network upgrade solution with
some sources claiming that it can improve the capacity of a local network by up
to 15% [3]. However, due to high energy consumption of the refrigeration system,
SMES is unsuitable for daily cycling applications such as peak reduction, renew-
able applications, and generation and transmission deferral [2].
4.8.2 Cost of SMES
SMES cost approximately $300/kW [2] to $509/kW [3]. It is worth noting that it is
diffi cult to compare the cost of SMES to other storage devices due to its scales and
purpose. In practical terms SMES should be compared to other network upgrade solu-
tions where it is often very competitive or even less costly. Finally, the cost of storing
electricity within a superconductor is expected to decline by almost 30% which could
make SMES an even more attractive option for network improvements [3].
4.8.3 Disadvantages of SMES
The most signifi cant drawback of SMES is its sensitivity to temperature. As dis-
cussed the coil must be maintained at an extremely low temperature in order to
behave like a superconductor. However, a very small change in temperature can
cause the coil to become unstable and lose energy. Also, the refrigeration can
cause parasitic losses within the system. Finally, although the rapid discharge
rates provide some unique applications for SMES, it also limits its applications
signifi cantly. As a result, other multifunctional storage devices such as batteries
are usually more attractive.
4.8.4 Future of SMES
Immediate focus will be in developing small SMES devices in the range of
1-10 MW for the power quality market which has foreseeable commercial
potential. A lot of work is being carried out to reduce the capital and operating
costs of high-temperature SMES devices, as it is expected to be the commer-
cial superconductor of choice once manufacturing processes are more mature,