Xcel Energy will partner with Form Energy to install a battery in Becker that would store electricity for considerably longer periods of time than current grid batteries.

Xcel also on Thursday reported a 19% increase in fourth quarter profits over a year earlier.

Xcel and Massachusetts-based Form Energy said the Minneapolis-based utility would deploy batteries in Becker and in Pueblo, Colo., the sites of Xcel coal plants that are slated to close.

The projects would be among the first for Form Energy and its promising battery technology.

Both Xcel batteries, which are subject to regulatory approval, are expected to come online as early as 2025. Xcel did not disclose costs.

Form's iron air battery would store electricity for 100 hours, compared with the four-hour standard of most lithium-ion grid batteries. The mineral at the heart of Form's technology is iron.

"As we build more renewable energy into our systems, our partnership with Form Energy opens the door to significantly improve how we deliver carbon-free energy," Xcel CEO Bob Frenzel said in a press statement.

Form is expected to roll out its first pilot project in 2024 for Great River Energy, a Maple Grove-based wholesale electricity cooperative. It will be located in Cambridge, Minn., and Great River will consider a larger Form battery depending on the pilot plant's performance.

The pilot battery will be able to store 1.5 megawatts of electricity, compared with 10 megawatts for each of the two Xcel batteries. Form's 10-megawatt battery discharges 1,000 megawatt hours of electricity.

That's relatively small. But if batteries like Form's succeed, they could have higher capacities — and they would be multiplied across the grid.

Batteries of all kinds — but particularly those with long duration storage — are critical to building a carbon-free electrical grid. Wind and solar power are inherently variable. Batteries can store power and discharge it later to even out electricity flows.

In a statement, Xcel said that Form's battery "will strengthen the grid against normal day-to-day, week-to-week, and season-to-season weather variability, in addition to extreme weather events including severe winter storms and polar vortex events."

Today's most common grid batteries are based on lithium-ion technology. Lithium-ion batteries can be built to store 100 hours of power like Form's battery, but the economics don't work.

A lithium-ion grid battery currently costs about $300 per kilowatt hour to build compared with less than $20 per kilowatt hour for Form Energy's battery, said Mateo Jaramillo, the company's CEO and co-founder.

Form Energy, which counts Bill Gates among its investors, is building a $760 million manufacturing plant in West Virginia that is expected to open in late 2024.

Aside from its deal with Xcel, Form is also working with Georgia Power on a 5- to 15-megawatt battery.

Form Energy's use of iron is a potential advantage, since the mineral is far less scarce than lithium and usually presents fewer environmental obstacles to mining. Form will use "direct-reduced iron" (DRI) as a feedstock.

Minnesota's Iron Range is gearing up to produce more iron ore pellets aimed specifically at DRI furnaces. "We certainly hope we will be able to use domestic iron," Jaramillo said, though the company is still developing iron sourcing plans.

Xcel is Minnesota's largest electricity provider and second largest natural gas utility.

The battery announcement was included in Xcel's discussion of fourth quarter results. The company, which also operates in Wisconsin, the Dakotas, Colorado and three other states, reported quarterly profits of $379 million, or 69 cents per share, up from $315 million, or 58 cents per share.

Xcel's earnings topped analysts' estimates by 1 cent per share. The company reported fourth quarter revenue of $4.05 billion, up from $3.6 billion a year ago.