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Some of the 750,000 solar panels of the Bighorn Solar project in Pueblo are shown in this Oct. 21, 2021 photo. The Evraz Steel Mill (shown at the top of the frame) will purchase the energy generated by the array to power a new rail mill currently under construction. (Mike Sweeney, Special to The Colorado Sun)

Xcel Energy plans to build a 10-megawatt battery array the size of a football field near its Comanche coal-fired power complex in Pueblo to store days’ worth of clean solar and wind electricity, a move hailed as a key step in renewable energy evolution. 

Xcel’s battery complex, to be matched by a similar facility at a Minnesota power plant, will use iron-air battery technology housed inside hundreds of shed-sized containers at Comanche, which must be retired from burning coal by 2030. Clean energy advocates say large, innovative battery storage must accompany utility-scale wind and solar growth in order to provide grid backup during outages, storms or unfavorable generating weather. 

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Colorado laws target 80% of utility generation, previously the largest emitter of greenhouse gases that cause climate change, to come from clean, renewable sources by 2030. Federal and state energy policies call for storage growth to help replace retired fossil fuel plants run by coal or natural gas that delivered a steady stream of power on demand.  

Costs of storage and other renewable projects are defrayed by tax credits from the 2022 Inflation Reduction Act and other grant sources.

“This is an exciting new frontier for energy storage in Colorado,” said Mike Kruger, president and CEO of the Colorado Solar and Storage Association. “This announcement goes to show that when there is clear policy, American companies can innovate to meet the electric power sector’s needs.”

Xcel Energy is partnering with Form Energy, a maker of massive battery arrays, for a renewable energy storage project at the Comanche Generating Station in Pueblo. This rendering shows a larger Form storage project, but the layout in Pueblo will be similar. (Courtesy of Form Energy Inc.)

Xcel has an agreement with storage developer Form Energy for the Colorado and Minnesota systems. The systems are expected to go online in 2025.

“This is really a game changer in battery technology, but it’s also a game changer in how we would be able to use wind and solar,” said Robert Kenney, Xcel’s Colorado president.

Xcel currently buys power from the 300-megawatt Bighorn solar project between Comanche and the Evraz steel plant, that delivers power to Evraz. The utility has also contracted for hundreds more megawatts of solar power from farms under construction elsewhere in Pueblo County, which sits in Colorado’s belt of prime solar and wind conditions for generation. 

Unlike other more exotic technologies that depend on precious metals like lithium or molten salt, iron-air batteries use common materials to store and release power.

“When discharging, the battery breathes in oxygen from the air and converts iron metal to rust. While charging, an electrical current converts the rust back to iron and the battery breathes out oxygen,” according to Scientific American. Companies like Form have improved the efficiency of the iron-air concept, which has been around for decades. 

Storage complexes will allow Xcel to “provide reliable and affordable electric service to our customers well into the future,” Xcel Energy Chairman Bob Frenzel said. The multistate utility plans to generate all electricity carbon-free by 2050.

Xcel has been developing multiple wind and solar projects to fill projected gaps in Colorado generation demand in coming years, as it gradually retires all of its coal-fired plants in the state. The Comanche 3 unit, which has often been out of commission with a series of operation failures, went offline again on a scorching day in August. The cut to Xcel’s available supply forced the company to lock the thermostats of 22,000 customers participating in its AC Rewards program, and limited their air conditioning use.

Once charged with off-peak electricity from renewable sources, the battery arrays can discharge back onto the grid for up to 100 hours. 

The iron-air battery is a “pilot project,” and one of a few promising technologies Xcel is studying for storage, Kenney said. Battery arrays made with lithium ion technology feature expensive materials and can only store power for four to six hours of release, making them impractical as a utility backup, he said. 

The pilot project size of 10 megawatts at Comanche would be enough to power 2,000 homes for up to four or five days, he said. With federal construction credits, the iron-air battery arrays are cost competitive with power from conventional energy plants, Kenney said.

“From a technological standpoint, this is a very significant project,” he said.

Michael Booth

Michael Booth is a Colorado Sun reporter covering health, health policy and the environment. Email: Twitter: @MBoothDenver