If we’re ever to run our electric grid on the on-again, off-again power that wind and sun provide, we’re going to need better batteries. About half a dozen types of batteries are now grid-ready, but a 30-year-old technology known as a flow battery could be the best bargain.
In place of the solid electrodes of a conventional battery, flow batteries use two liquid electrolytes that react when pumped through a cell stack. The battery is broken down into a cell stack and two large electrolyte tanks; as the electrolyte flows past a porous membrane in each cell, ions and electrons flow back and forth, charging or discharging the battery. Recharging simply means putting in fresh electrolyte; to increase the energy storage, you merely need to make the tanks larger.
Such batteries are already used for backup power at factories and cellphone towers. Now manufacturers are attracting millions of dollars in venture capital to develop the batteries for grid-level systems. The U.S. Department of Energy (DOE) has poured in US $31 million in Recovery Act funds to jump-start five utility-scale projects.
Cost is critical for grid storage, and this is where flow batteries deliver. Some zinc-bromine devices in the works could store energy for less than $500 a kilowatt-hour, a third as much as for lithium-ion batteries and about three-quarters as much as for its toughest competitor, the sodium-sulfur battery.
The inherent architecture of flow batteries makes them particularly safe, says Craig Horne, CEO of flow-battery start-up EnerVault, in Sunnyvale, Calif. ”With a flow battery you can have megawatt-hours of energy stored [in the electrolyte tanks], but only a small fraction of the volume is in the stacks at any instant,” Horne says. ”It’s simple to shut off the electrolyte supply.”
There are many chemical reactions around which such a battery can be designed, but those involving vanadium and zinc-bromine are the most familiar. Vanadium systems have been tied to the grid to reduce peak loads and store wind energy, mostly in Japan by Sumitomo Electric Industries. In the United States, ZBB Energy Corp., in Menomonee Falls, Wis., and Premium Power Corp., in North Reading, Mass., sell trailer-transportable zinc-bromine systems that store megawatt-hours of energy. ZBB tested its 2-megawatt-hour system in California for reducing peak loads in 2007, while Premium Power will test seven 2.8-MWh systems for three years with DOE-allocated Recovery Act money.
Many other grid-ready flow-battery systems will be on the market within five years. Beijing-based Prudent Energy is likely to be a major player. And start-ups such as Primus Power, EnerVault, and Deeya Energy are working on newer chemistries.
Sodium-sulfur batteries have one edge over flow batteries: They’ve been tested extensively in the field. The only manufacturer, Tokyo-based NGK Insulators, has sodium-sulfur storage in Japan capable of producing 270 MW. Utilities in the United States have installed 9 MW of capacity, with projects of an equal amount on the way.
However, the advent of the smart grid could make these distinctions moot. Utilities are now looking for a mix of energy storage technologies, flow batteries included.
”The technology is well understood,” says Dan Rastler, an energy-storage expert at the Electric Power Research Institute. ”The biggest challenge is adapting to utility scale and doing complete system integration, which also involves power electronics and controls.” In the next two to three years, he adds, flow-battery technology will get plenty of opportunities to prove its mettle.
About the Author
Prachi Patel, an IEEE Spectrum contributing editor based in Pittsburgh, in February profiled Ronald Thomas, an engineer who studies lightning associated with volcanoes.