September 28, 2011

MIT Technology Review

New Battery Could Be Just What the Grid Ordered

A Pittsburgh company says its battery has the long life and cheap cost needed to be practical for energy storage.

Utilities need cheap, long-lasting ways to store the excess energy produced by power plants, especially as intermittent power from solar and wind farms is added to the mix. Unfortunately, the batteries available for grid-level storage are either too expensive or don't last for the thousands of cycles needed to make them cost-effective.

A new battery developed by Aquion Energy in Pittsburgh uses simple chemistry—a water-based electrolyte and abundant materials such as sodium and manganese—and is expected to cost $300 for a kilowatt-hour of storage capacity, less than a third of what it would cost to use lithium-ion batteries. Third-party tests have shown that Aquion's battery can last for over 5,000 charge-discharge cycles and has an efficiency of over 85 percent.

The company has now received $30 million in venture capital to step up manufacturing of its sodium-ion batteries. The new technology could be the cheapest way to store large amounts of energy for the power grid using batteries, says Jay Whitacre, the company's founder and chief technology officer.

Aquion's battery uses an activated carbon anode and a sodium- and manganese-based cathode. A water-based electrolyte carries sodium ions between the two electrodes while charging and discharging. The principle is similar to lithium-ion, but sodium ions are more abundant and hence cheaper to use. Compared to solvent-based electrolytes, the aqueous electrolyte is also easier to work with and cheaper. Even better, the materials are nontoxic and the battery is 100 percent recyclable, Whitacre says.

Grid-scale trials of the technology are next. Aquion has started shipping pre-production battery prototypes to off-grid solar power companies. Next month, a 1,000-volt module will go to KEMA, a Dutch energy consulting and testing outfit, which has a facility outside Philadelphia.

Utilities use stored energy to meet electrical demand during peak usage periods, a practice called peak shaving, which helps keep the grid reliable and efficient and electricity prices low. Whitacre says Aquion's battery is designed for these grid applications. "It's very well-suited for off-grid solar and wind support, and also for peak shaving," he says. "It's two very different applications, and our battery has been shown to be effective in both."

John Miller, an electrochemical capacitor expert and president of consulting firm JME in Shaker Heights, Ohio, says Aquion's battery could be the cheapest of the various battery technologies vying to provide grid storage. He compares it to today's most common grid storage technology, pumped hydro, which accounts for 95 percent of utility-scale energy storage. Pumped hydro involves moving water to an elevation when electricity demand is low, and releasing that water through turbines during peak periods. It is, however, limited by geology and space, and pumped hydro systems take many years and millions of dollars to build. Utilities are now starting to look at batteries because they can be delivered in months and, in principle, can be sited anywhere.

"Lead-acid is even too expensive," Miller says. "Aquion's technology is getting to the range of pumped hydro in cost, which is two cents per kilowatt-hour [over the system's lifetime]. They're unique. I would say it's very promising for grid storage."

So far, no available technology meets all grid energy storage requirements, says Haresh Kamath, a program manager for energy storage at the Electric Power Research Institute. "Each technology has a different sweet spot" in terms of cost, safety, reliability, lifespan, and efficiency, he says.

Some power companies use lead-acid batteries and sodium-sulfur batteries for grid storage. Lead-acid batteries are cheap but only last for 500-1000 cycles, while sodium-sulfur batteries are costly at $1,000 a kilowatt-hour. Other technologies on the horizon—lithium-ion, above ground compressed air storage, and flow batteries—remain expensive and unproven.

Grid-storage battery technology also "has to be plug-and-play, and not require extensive installation," says Ali Nourai, an executive consultant at KEMA. Aquion's batteries may have the disadvantage of being as large and heavy as lead-acid batteries, Nourai says, but their low cost and long cycle life make up for that. "The biggest barrier to grid storage is cost, and Aquion has an upper hand there," he says. "People will tolerate low efficiency and high weight if the price is right."

Kamath says that the sodium-ion battery is an interesting new technology, but grid-scale demonstrations will tell whether it has what utilities are looking for. "More than any other, this is a very early stage technology, and we don't know what it's capable of," he says. "Based on principle, it looks very promising, and that's why a lot of folks in this industry are excited about this. But it remains to be seen if the promises are actually played out."

Whitacre has ambitious plans for Aquion, though. The company is making 35 watt-hour units that are modular and stackable at its research and development facility. Next year, the company wants to produce multiple megawatt-hours worth of batteries at this facility, launch its first commercial product, and break ground on a 500-megawatt-hour capacity factory.