Superconducting Magnets for Grid-Scale Storage
Magnetic-field energy storage could have unique advantages, but scaling up will be a challenge.
Superconducting magnetic energy storage (SMES) has long been pursued as a large-scale technology because it offers instantaneous energy discharge and a theoretically infinite number of recharge cycles. Until recently, however, the material costs for SMES devices have been prohibitively high for all but very small applications. Now a project funded by the U.S. Department of Energy (DOE) could pave the way for SMES technology that offers megawatt hours of energy storage. Such capacity is becoming increasingly necessary for electricity grids that need to balance the intermittency of renewable energy sources.
At a DOE Advanced Research Projects Agency for Energy (ARPA-E) conference in Washington, D.C. on March 2, Swiss-based engineering firm ABB outlined plans for a 3.3 kilowatt-hour proof-of-concept SMES prototype. The device will store electricity in the form of a magnetic field generated by direct current circulated through superconducting wires. The geometry of the superconducting coils creates a highly contained electromagnetic field, but relatively little energy is needed to sustain the field. The energy is released by discharging the coils.
ABB is collaborating with superconducting wire manufacturer SuperPower, Brookhaven National Laboratory, and the University of Houston as part of the $4.2 million ARPA-E grant. The group’s ultimate goal is to develop a 1-to-2-megawatt-hour commercial-scale device that is cost-competitive with lead-acid batteries, says ABB project manager V.R. Ramanan.
Matching the price of lead-acid batteries would make SMES systems less expensive than flywheels but more expensive than pumped hydro or compressed air, according to a recent study by the Electric Power Research Institute. Pumped hydro, which stores energy by pumping water uphill, and compressed air, which stores energy in the form of air compressed in underground caverns, are the two leading methods for storing energy on a large scale today. These approaches are, however, limited to areas with lakes or other reservoirs at high elevations or with underground caverns.
A key advantage that SMES has over other energy-storage technologies is its ability to rapidly release stored energy. “It can go from full charge to full discharge—no other technology can do that,” says Cesar Luongo the senior magnet-division coordinator for the International Thermonuclear Experimental Reactor project in Cadarache, France, who is not involved with the project.
Rapid discharge makes SMES attractive for quickly stabilizing high-voltage transmission lines during periods of heavy use. Crucially, ABB is developing electrical switches that would allow SMES systems to release their energy gradually, over up to an hour, to help compensate for drops in output from renewable energy sources such as wind and solar.
Luongo says that to compete with lead-acid batteries and other technologies, SMES systems may need to be significantly larger than the 1-to-2-megawatt-hour devices envisioned by ABB. They may need to offer tens of megawatt hours of storage, Luongo says, “and the cheaper the other technologies get, the farther out that crossover point gets.”
Steven Minnihan, an analyst at Lux Research, says SMES devices should last longer than flywheels or batteries, because they have no moving parts. But, he says, the material costs remain high. “The real benefit is its substantial life over batteries and flywheels, but I don’t think it’s the most cost-effective technology,” Minnihan says.
SMES systems have about the same life expectancy as pumped hydro and compressed air systems: 10-20 years, as opposed to 1-10 years for batteries and 8-12 years for flywheels, Minnihan says. He has reservations about ABB’s approach because it requires large quantities of high-temperature superconducting wires that at today’s prices would make grid-scale SMES systems prohibitively expensive.
Although the cost of superconducting wire has dropped significantly in recent years, Ramanan admits that it would need to fall by another 300 percent for SMES to be competitive with other grid-scale energy-storage technologies. He says reducing wire cost is a significant technical challenge but adds that taking on such challenges is the purpose of projects funded by ARPA-E. “If we didn’t think this had potential, we would not have gone after this,” he says.