Once the superconducting coil is energized, the current will not decay and the magnetic energy can be stored indefinitely.
In northern Wisconsin, a string of distributed SMES units were deployed to enhance stability of a transmission loop.
[6] 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.
EMF is defined as electromagnetic work done on a unit charge when it has traveled one round of a conductive loop.
For wires that are looped multiple times the inductance L increases, as L is simply defined as the ratio between the voltage and rate of change of the current.
Among them, the strain tolerance is crucial not because of any electrical effect, but because it determines how much structural material is needed to keep the SMES from breaking.
Toroidal geometry can help to lessen the external magnetic forces and therefore reduces the size of mechanical support needed.
Also, due to the low external magnetic field, toroidal SMES can be located near a utility or customer load.
In toroidal SMES, the coil is always under compression by the outer hoops and two disks, one of which is on the top and the other is on the bottom to avoid breakage.
The older large SMES concepts usually featured a low aspect ratio solenoid approximately 100 m in diameter buried in earth.
At the low extreme of size is the concept of micro-SMES solenoids, for energy storage range near 1 MJ.
AC losses depend on the design of the conductor, the duty cycle of the device and the power rating.
The refrigeration requirements for HTSC and low-temperature superconductor (LTSC) toroidal coils for the baseline temperatures of 77 K, 20 K, and 4.2 K, increases in that order.
Other components, such as vacuum vessel insulation, has been shown to be a small part compared to the large coil cost.
To gain some insight into costs consider a breakdown by major components of both HTSC and LTSC coils corresponding to three typical stored energy levels, 2, 20 and 200 MW·h.
Thus, in the very large cases, the HTSC cost can not be offset by simply reducing the coil size at a higher magnetic field.
An increase in peak magnetic field yields a reduction in both volume (higher energy density) and cost (reduced conductor length).
Smaller volume means higher energy density and cost is reduced due to the decrease of the conductor length.
The limit to which the field can be increased is usually not economic but physical and it relates to the impossibility of bringing the inner legs of the toroid any closer together and still leave room for the bucking cylinder.
Superconductor development efforts focus on increasing Jc and strain range and on reducing the wire manufacturing cost.
[11] Using these systems makes it possible for conventional generating units to operate at a constant output that is more efficient and convenient.
[12] However, when the power imbalance between supply and demand lasts for a long time, the SMES may get completely discharged.
These launchers can be realised by the use of the quick release capability and the high power density of the SMES system.
Recent development of HTS wire made of YBCO with a superconducting transition temperature of around 90 K shows promise.Typically, the higher the superconducting transition temperature, the higher the maximum current density the superconductor can sustain before Cooper pair breakdown.
This higher critical current will raise the energy storage quadratically, which may make SMES and other industrial applications of superconductors cost-effective.
A robust mechanical structure is usually required to contain the very large Lorentz forces generated by and on the magnet coils.
To achieve commercially useful levels of storage, around 5 GW·h (18 TJ), a SMES installation would need a loop of around 800 m. This is traditionally pictured as a circle, though in practice it could be more like a rounded rectangle.
Much research has focused on layer deposit techniques, applying a thin film of material onto a stable substrate, but this is currently only suitable for small-scale electrical circuits.
Until room-temperature superconductors are found, the 800 m loop of wire would have to be contained within a vacuum flask of liquid nitrogen.
Several issues at the onset of the technology have hindered its proliferation: These still pose problems for superconducting applications but are improving over time.