Reviving SMES, One Niche at a Time

Conceptual design of a HTS SMES device. Image: Energy to Power Solutions

Conceptual design of a HTS SMES device. Image: Energy to Power Solutions

Many projects in applied superconductivity hold great promise for the energy sector, but have failed to catch on due to cost, competition from established systems, perceived risk and other factors. Among them is Superconducting Magnetic Energy Storage (SMES), a system that stores energy in the form of a magnetic field. Storing energy in a superconducting magnet is not a new idea, but Tallahassee-based Energy to Power Solutions (e2P) may have just provided a refresh that will garner renewed attention. The company specializes in niche applications, and included in its recent designs are prototype SMES devices for ARPA-E and the US Air Force that demonstrate far greater energy storage capacities than previously achieved.

“I think things in superconductivity are going to stay in the niche market until they can expand elsewhere because people have to see it working in certain applications, niche applications, before they are going to feel comfortable adopting it,” says Dr. Christopher Rey, e2P’s founder and president. “And that’s something that my company focuses on, niche special applications where superconductivity can offer a premium for performance. In this case, the premium in performance is space savings, weight savings, safety savings and power delivery.”

Rey, a CSA Director and Boom awardee, says that e2P has developed a way of enhancing the amount of current that high temperature superconductors can carry. The method is proprietary, but the results are tangible. “I think we can build a power delivery device that’s superior to even ultracapacitors in terms of power density and in terms of being able to scale to higher powers with fairly straightforward cryogenic logistics.”

A SMES device can be charged and discharged rapidly without degradation or loss of system life, providing relatively high power delivery. Dr. Roger Boom, mentor to many at the University of Wisconsin-Madison, was one of the first proponents of SMES, back in the 1970s and 80s when the systems used low temperature superconductors (LTS) such as niobium-titanium (NbTi). These early SMES devices had low energy storage densities and were often large, difficult to transport and expensive to build.

The discovery of high temperature superconductors (HTS) allowed SMES to obtain higher fields and increased the amount of energy a system could store. Energy stored in a magnet is proportional to the square of the field. Switching from an LTS NbTi with a magnetic field of 7 Tesla to an HTS like Y-Ba-Cu-O (YBCO) at 14 Tesla, for example, would result in four times the energy storage. HTS also allowed engineers to simplify the cryogenics, developing SMES systems to operate with cryocoolers at higher temperatures and reduce the overall size and cost.

The e2P team is looking to take these advancements even further. A future HTS SMES device could be made small enough to transport by aircraft or rail, according to Rey, and could be factory built at far lower cost in quantities sufficient to take advantage of economies of scale. The cost of such devices, however, would remain a premium. “SMES devices are not low cost energy storage devices,” he says. “A battery is a low cost energy storage device, but batteries are not very good power delivery devices. You can store a lot of energy in a battery, but you cannot pull it out quickly or too often.”

Rey says that SMES would work well on transportable craft, such as electric powered aircraft or military delivery systems where engineers want to safely achieve high power delivery that can be repeatedly cycled. His cautionary takeaway, however, is that HTS SMES will remain a specialized niched application for quite some time. “But as we find these applications where its performance is superior to conventional technology,” he says, “it will help pave the way for more typical, recognized energy storage devices. We kind of need to get our foot in the door before we’re going to see more applications.”

The e2P prototype uses YBCO and a commercial cryocooler for cooling. The design team has tested a nominal 75 kilojoule unit at 77 K and plans in the next phase of the project to scale up to ~150-200 kilojoules and down towards 20 K. Rey says that he watches the HTS market and research closely, looking for performance advancements in existing, practical superconductors that may hint at lower cost and/or higher performing wire. “I want to emphasize to the materials community to keep struggling and making material better and producing lower cost superconductors,” he says, “That’s going to help everybody in the community.”

After the next round of testing is complete, Rey hopes the Air Force, which is funding the project, will decide to move on to aircraft trials. He acknowledges here a degree of uncertainty, but it is a playing field he’s comfortable navigating. “Superconductivity is hard to introduce where existing lower cost conventional technology exists, but find an application where performance is the primary driver and you’ll have a great market opportunity.” ■