Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil that has been cryogenically cooled to a temperature below its superconducting critical temperature. This use of superconducting coils to store magnetic energy was invented by M. Ferrier in 1970. A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is energized, the current will not decay and the magnetic energy can be stored indefinitely. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2–3% energy loss in each direction. SMES loses the least electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%. Due to the energy requirements of refrigeration and the high cost of superconducting wire, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality.
Advantages over other energy storage methods There are several reasons for using superconducting magnetic energy storage instead of other energy storage methods. The most important advantage of SMES is that the time delay during charge and discharge is quite short. Power is available almost instantaneously and very high power output can be provided for a brief period of time. Other energy storage methods, such as pumped hydro or compressed air, have a substantial time delay associated with the energy conversion of stored mechanical energy back into electricity. Thus if demand is immediate, SMES is a viable option. Another advantage is that the loss of power is less than other storage methods because electric currents encounter no resistance. Additionally the main parts in a SMES are motionless, which results in high reliability.
Current use There are several small SMES units available for commercial use and several larger test bed projects. Several 1 MW·h units are used for power quality control in installations around the world, especially to provide power quality at manufacturing plants requiring ultra-clean power, such as microchip fabrication facilities. These facilities have also been used to provide grid stability in distribution systems. SMES is also used in utility applications. In northern Wisconsin, a string of distributed SMES units were deployed to enhance stability of a transmission loop. The transmission line is subject to large, sudden load changes due to the operation of a paper mill, with the potential for uncontrolled fluctuations and voltage collapse. The Engineering Test Model is a large SMES with a capacity of approximately 20 MW·h, capable of providing 40 MW of power for 30 minutes or 10 MW of power for 2 hours.
System architecture A SMES system typically consists of four parts
Superconducting magnet and supporting structure
This system includes the superconducting coil, a magnet and the coil protection. Here the energy is stored by disconnecting the coil from the larger system and then using electromagnetic induction from the magnet to induce a current in the superconducting coil. This coil then preserves the current until the coil is reconnected to the larger system, after which the coil partly or fully discharges.
Refrigeration system
The refrigeration system maintains the superconducting state of the coil by cooling the coil to the operating temperature.
Power conditioning system
The power conditioning system typically contains a power conversion system that converts DC to AC current and the other way around.
Control system
The control system monitors the power demand of the grid and controls the power flow from and to the coil. The control system also manages the condition of the SMES coil by controlling the refrigerator.
Working principle As a consequence of Faraday's law of induction, any loop of wire that generates a changing magnetic field in time, also generates an electric field. This process takes energy out of the wire through the electromotive force (EMF). EMF is defined as electromagnetic work done on a unit charge when it has traveled one round of a conductive loop. The energy could now be seen as stored in the electric field. This process uses energy from the wire with power equal to the electric potential times the total charge divided by time. Where ℰ is the voltage or EMF. By defining the power we can calculate the work that is needed to create such an electric field. Due to energy conservation this amount of work also has to be equal to the energy stored in the field.
P = Q E / t {\displaystyle P=Q{\mathcal {E}}/t}
This formula can be rewritten in the easier to measure variable of electric current by the substitution.
P = Q E / t = I E , {\displaystyle P=Q{\mathcal {E}}/t=I{\mathcal {E}},}
where I is the electric current in Ampere. The EMF ℰ is an inductance and can thus be rewritten as:
E = L d I d t {\displaystyle {\mathcal {E}}=L{\frac {dI}{dt}}}
Substitution now gives:
P = I L d I d t , {\displaystyle P=IL{\frac {dI}{dt}},}
where L is just a linearity constant called the inductance measured in Henry. Now that the power is found, all that is left to do is fill in the work equation to find the work.
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