A worldwide uptick in enthusiasm for power generation from renewable sources has focused a new spotlight on energy storage technology. This has become an essential part of any sustainable and dependable renewable energy deployment because of the stochastic nature of popular renewable energy sources like wind and solar. A superconducting magnetic energy system (SMES) is a promising new technology for such application.
The theory of SMES's functioning is based on the superconductivity of certain materials. When cooled to a certain critical temperature, certain materials display a phenomenon known as superconductivity, in which both their electrical resistance and magnetic field dissipation are reduced to zero. The energy in SMES devices is preserved as a DC magnetic field, which is produced by a current running along the superconductors.
History of SMES
Ferrier first suggested the idea of SMES in 1969. The first such device was developed in 1971 thanks to studies conducted at the University of Wisconsin. In the late 1990s, the first high-temperature superconductors (HTS) were introduced, and the first commercially available HTS-SMES of any scale was manufactured by American Superconductors in 1997. After that, it was integrated with Germany's main power grid.
What happens to the conductors at cryogenic temperature?
At cryogenic (very low) temperatures, the current-carrying conductor transforms into a superconductor with essentially zero resistive losses while it produces a magnetic field. As long as this is the case, a coil's current can theoretically run forever. The time constant of a coil — t = L/R, where L and R are the inductance and resistance — provides more proof of this. R tends to zero as t grows closer and closer to infinity. SMES works because current flows in a superconductor even after the voltage across it is removed. When chilled below its critical superconducting temperature, a superconducting coil exhibits very low (or no) resistance. Since this is the case, it will continue to conduct electricity.
How does the SMES system work?
As mentioned above, the SMES technology uses a superconducting coil to convert electrical energy into a magnetic form for storage. A power conversion/conditioning system acts as a bridge between the SMES and the main power grid during integration. However, if there is a DC-bus in the microgrid, a bidirectional DC-DC converter or some other similar converters can be used to directly link the SMES system.
The figure below depicts the main parts of a standard SMES system, which include a cryogenic system, superconducting coil, protective system and control system. The superconducting coil stores the energy and is essentially the brain of the SMES system. Because the cryogenic refrigerator system keeps the coil cold enough to keep its superconducting state, the coil has zero losses and resistance. This coil may be manufactured from superconducting materials like mercury or niobium-titanium. The irregular actions in the SMES unit are safeguarded by the protection system, while the control system links grid power requirements with SMES coil power flows.
Source: N. Mughees
When the coil is charged or uncharged, electricity flows through it. For storing energy, the power conditioning system must provide a positive voltage across the coil during the charging phase, when the current flows exclusively in one direction. To discharge the coil, the power conditioning system is reconfigured to act as a load across the coil, producing a negative voltage. The reaction time of SMES systems is high. It only takes a few seconds to go from charging to discharging.
General SMES performance
| Cycle efficiency | 97% |
| General Power Capacity | 100 kW to 10 MW |
| Cycle Lifetime | No degradation |
| Discharging | ≥ mins-hrs |
| Reaction time | 5 ms |
| Carbon footprint | Environment friendly |
| Technical lifetime | 30 years |
Benefits of SMES
- Fast millisecond-scale responses are possible thanks to electrical energy's direct storage.
- It is more effective than other energy storage systems since it does not have any moving parts and the current in the superconducting coil encounters almost little resistance.
- Up to 98% efficiency is possible with the SMES.
- The SMES lifetime is seldom affected by the number of charges/discharge cycles it undergoes.
- Standard structural materials are used.
- Extremely reliable as it can take as much load as the circuitry can handle.
- Highly adaptable for hybridization with any other large-capacity energy storage device to boost both the systems' performance.
Applications of SMES systems
Plug-in hybrid electric vehicles, contingency systems, microgrids, renewable energy sources like wind energy and photovoltaic (PV) systems, and DC and AC power systems are just a few of the many places where SMES devices are put to use. Grid-connected renewable energy systems like solar PV and wind energy conversion systems have shown that SMES is a sustainable and competitive option for applications like reducing output power fluctuations, reactive power compensation, controlling frequencies, boosting transient stability, uninterruptable power supply, and enhancing power quality. SMES devices can be employed in places where pumped hydro storage or compressed air energy storage would be impractical.
Future of SMES systems
Ongoing research seeks to enhance the efficacy, expand storage capacity and decrease the operating costs of SMES systems. The expenditure of keeping conductors cool is real. If this expense could be avoided by switching to a superconductor that operates at room temperature or even close to room temperature, the SMES system would be more feasible and effective. This may be accomplished by switching to a different type of superconducting material with a higher critical temperature, improving the cooling systems, and rearranging the magnetic coils.
[Learn more about superconductors and superconducting materials on GlobalSpec]
Conclusion
SMES has been shown to be effective in energy storage due to its high energy density and fast response, which makes it an ideal solution for large-scale renewable energy deployments. It is an efficient way to store renewable energy as it allows for fast charging and discharging of stored energy. It requires minimal maintenance and is reliable, meaning it can be used in the long term without the need for regular upkeep. As a result, SMESs are efficient and sustainable energy storage systems, as well as being cost-effective and reliable.
