SMES

SMES: Superconducting Magnetic Energy Storage


What is SMES

Electric power has always been available on instant demand with a high degree of reliability, flexibility, and control; but on the other hand, it has never been easy to store it in large quantities in a cost-effective and environmentally-sound basis. Environmental limits on generation and the advent of renewable sources, deregulation and competition, and the increasing complexity of transmission systems, all point towards the need for a reliable and cost-effective means to store and retrieve electric power. The ability to store electricity in bulk would greatly improve the generating asset utilization factor for the most efficient units, and add a whole new dimension to the dispatch mix. In the transmission arena, the complexities of power purchase and distribution in the deregulated market could be ameliorated by the utilization of even short-term storage.

In SMES, electric energy is stored by circulating a current in a superconducting coil, or inductor. Because no conversion of energy to other forms is involved (e.g., mechanical or chemical), round-trip efficiency can be very high. SMES can respond very rapidly to dump or absorb power from the grid, limited only by the switching time of the solid-state components doing the DC/AC conversion and connecting the coil to the grid.

Principles of Operation

As an energy storage device, SMES is a relatively simple concept. It stores electric energy in the magnetic field generated by DC current flowing through a coiled wire. If the coil were wound using a conventional wire such as copper, the magnetic energy would be dissipated as heat due to the wire's resistance to the flow of current. However, if the wire is superconducting (no resistance), then energy can be stored in a "persistent" mode, virtually indefinitely, until required.

SMES coils vary in size depending in the energy they store. The superconductor of choice for this application is a niobium-titanium alloy which needs to be kept at liquid helium temperature in order to superconduct. ‘High-temperature’ superconductors, those that operate at liquid nitrogen temperature or above, are not as technically advanced at this point to be considered for a large-scale application such as this. Once SMES has established a foothold in the utility market based on conventional superconductors, high-temperature superconductors can be introduced to reduce capital and operating costs once their physical characteristics have improved, and the manufacturing processes are more mature.

The coil is a DC device, yet charge and discharge are usually accomplished through an AC utility grid. A power conditioning system (PCS) is required at the coil/grid interface. The PCS uses standard solid-state DC/AC converters as well as other filtering and control circuitry. In addition to the superconducting coil and the PCS, the only other major elements comprising a SMES plant are the cryogenic refrigerator and systems (including cryostat). A standard switchyard connects the coil/PCS to the utility grid.

SMES Applications

The applications of SEMS in the utility sector go well beyond energy storage. Some of these applications are listed below:

Many transmission lines are limited in power capacity by stability considerations, even more so as transmission networks become heavily interconnected. Because of its fast response to inject or absorb power, a SEMS plant can significantly enhance the stability of a transmission line. Lines could operate at higher capacity and still be stable in the face of transient events such as the sudden loss of generating capacity or loads (i.e., during an earthquake). This is of significant benefit to the utilities considering the difficulties of acquiring and permitting new rights of way for transmission lines. Transmission line stability enhancement is likely to be the first commercial application of SEMS in the utility sector, at least in the United States.

Cable-in-Conduit Conductor

The foundation of the specific SMES design of my work is the Cable-in-Conduit Conductor (CICC) concept. In the CICC design, the superconducting cable is placed inside a conduit (or jacket) filled with helium and so the conductor is not only the main electrical path, but it is also the helium containment element. This combination of functions allows for more flexibilty in the magnet design, with the ensuing potential for simplification and lower cost. The SMES-CICC was a pioneering design not only in size and rated current, but also in the use of segregated cooling channel inside the conduit. The CICC concept is widely used in superconducting magnets designed and built for other large-scale applications, most notably fusion.

The CICC for the SMES-ETM was rated at 200 kA (at 1.8K and 5T). This conductor was tested twice, first during the 1988-90 period, and again in 1993 during a second test phase. The conductor reached rated current without training during both tests. In September of 1993, the conductor reached a world-record 303kA along the 4.5T load line (1.8K).
200kA SMES-CICC Superconductor