Superconducting Magnetic Energy Storage Systems (SMES)

Energy storage is a process that takes place with devices or physical media that store energy so that they can use it efficiently later.

Energy storage systems can be divided into mechanical, electrical, chemical and thermal. One of the modern energy storage technologies is the SMES systems — superconducting magnetic energy storage (superconducting magnetic energy storage systems).

Superconducting magnetic energy storage (SMES) systems store energy in a magnetic field created by a direct current flow in a superconducting coil that has been cryogenically cooled to a temperature below its critical superconducting temperature. When the superconducting coil is charged, the current does not decrease and the magnetic energy can be stored indefinitely. The stored energy can be returned to the grid by discharging the coil.

The superconducting magnetic energy storage system is based on a magnetic field generated by the flow of direct current in a superconducting coil.

The superconducting coil is continuously cryogenically cooled, so as a result it is constantly below the critical temperature, i.e. superconductor… In addition to the coil, the SMES system includes a cryogenic refrigerator as well as an air conditioning system.

The conclusion is that a charged coil in a superconducting state is capable of sustaining a continuous current by itself, so that the magnetic field of a given current can store the energy stored in it for an infinitely long time.

The energy stored in the superconducting coil can, if necessary, be supplied to the network during the discharge of such a coil. To convert DC power into AC power, inverters, and for charging the coil from the network — rectifiers or AC-DC converters.

In the course of highly efficient conversion of energy in one direction or another, losses in SME represent a maximum of 3%, but the most important thing here is that in the process of energy storage by this method, the losses are the least inherent in any of the currently known methods for energy storage and storage. The overall minimum efficiency of SMEs is 95%.

Due to the high cost of superconducting materials and taking into account the fact that cooling also requires energy costs, SMES systems are currently used only where it is necessary to store energy for a short time and at the same time improve the quality of the power supply. That is, they are traditionally used only in cases of urgent need.

The SME system consists of the following components:

• superconducting coil,
• Cryostat and vacuum system,
• Cooling system,
• Energy conversion system,
• Control device.

The main advantages of SME systems are obvious. First of all, it is an extremely short time during which the superconducting coil is able to accept or give up the energy stored in its magnetic field. In this way, it is possible not only to obtain colossal instantaneous discharge forces, but also to recharge the superconducting coil with a minimal time delay.

If we compare the SME with compressed air storage systems, with flywheels and hydraulic accumulators, then the latter are characterized by a colossal delay during the conversion of electricity into mechanical and vice versa (see — Flywheel energy storage).

The absence of moving parts is another important advantage of SMES systems, which increases their reliability. And, of course, due to the absence of active resistance in a superconductor, storage losses here are minimal. The specific energy of SMES is usually between 1 and 10 Wh/kg.

1 MWh SMES are used worldwide to improve power quality where needed, such as microelectronics factories that require the highest quality power.

In addition, SMEs are also useful in utilities. So, in one of the states of the USA there is a paper factory, which during its operation can cause strong surges in power lines. Today, the factory's power line is equipped with a whole chain of SMES modules that guarantee the stability of the power grid. An SMES module with a capacity of 20 MWh can sustainably provide 10 MW for two hours or all 40 MW for half an hour.

The amount of energy stored by a superconducting coil can be calculated using the following formula (where L is inductance, E is energy, I is current):

From the point of view of the structural configuration of the superconducting coil, it is very important that it is resistant to deformation, has minimal indicators of thermal expansion and contraction, and also has a low sensitivity to the Lorentz force, which inevitably arises during the operation of the installation (The most important laws of electrodynamics). All this is important in order to prevent the destruction of the winding at the stage of calculating the properties and the amount of construction materials of the installation.

For small systems, an overall strain rate of 0.3% is considered acceptable. In addition, the toroidal geometry of the coil contributes to the reduction of external magnetic forces, which makes it possible to reduce the cost of the supporting structure, and also allows the installation to be placed close to the load objects.

If the SMES installation is small, then a solenoid coil may also be suitable, which does not require a special support structure, unlike a toroid. However, it should be noted that the toroidal coil needs press hoops and discs, especially when it comes to a rather energy-intensive structure.

As noted above, a cooled superconductor refrigerator continuously requires energy to operate, which of course reduces the overall efficiency of the SMES.

So, the thermal loads that must be taken into account when designing the installation include: thermal conductivity of the supporting structure, thermal radiation from the side of the heated surfaces, joule losses in wires through which charging and discharging currents flow, as well as losses in the fridge while working.

But although these losses are generally proportional to the nominal power of the installation, the advantage of SMES systems is that with an increase in energy capacity of 100 times, cooling costs increase only 20 times. In addition, for high-temperature superconductors, the cooling savings are greater than when using low-temperature superconductors.

It appears that a superconducting energy storage system based on a high-temperature superconductor is less demanding on cooling and therefore should cost less.

In practice, however, this is not the case, as the total cost of the installation infrastructure usually exceeds the cost of the superconductor, and the coils of high-temperature superconductors are up to 4 times more expensive than the coils of low-temperature superconductors.

In addition, the limiting current density for high-temperature superconductors is lower than for low-temperature ones, this applies to operating magnetic fields in the range of 5 to 10 T.

So to get batteries with the same inductance, more high-temperature superconducting wires are needed. And if the energy consumption of the installation is about 200 MWh, then the low-temperature superconductor (conductor) will turn out to be ten times more expensive.

In addition, one of the key cost factors is this: the cost of the refrigerator is in any case so low that reducing the cooling energy by using high-temperature superconductors gives a very low percentage saving.

It is possible to reduce the volume and increase the energy density stored in the SMES by increasing the peak operating magnetic field, which will lead to both a reduction in wire length and a reduction in overall cost. The optimal value is considered to be a peak magnetic field of about 7 T.

Of course, if the field is increased beyond the optimum, further reductions in volume are possible with a minimal increase in cost. But the field induction limit is usually physically limited, due to the impossibility of bringing the internal parts of the toroid together while still leaving room for the compensating cylinder.

Superconducting material remains a key issue in creating cost-effective and efficient installations for SMEs. The efforts of developers today are aimed at increasing the critical current and the range of deformation of superconducting materials, as well as reducing the cost of their production.

Summarizing the technical difficulties on the way to the widespread introduction of SME systems, the following can be clearly distinguished. The need for a solid mechanical support capable of withstanding the significant Lorentz force generated in the coil.

The need for a large piece of land, since an SME installation, for example with a capacity of 5 GWh, will contain a superconducting circuit (circular or rectangular) of about 600 meters in length. In addition, the vacuum container of liquid nitrogen (600 meters long) surrounding the superconductor must be located underground and reliable support must be provided.

The next obstacle is the brittleness of superconducting high-temperature ceramics, which makes it difficult to draw wires for high currents.The critical magnetic field that destroys superconductivity is also an obstacle to increasing the specific energy intensity of SMES. NS has a critical current problem for the same reason.