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Xcel Energy explores menu of storage options for site after coal plants

 by Allen Best

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Molten salt, green hydrogen, biomass gasification—these ideas and more are in play as potential storage media for energy at the coal-fired power plants that Xcel Energy plans to close in the coming decade.

Xcel wants a little help from state legislators in the form of a bill, HB21-1324, which the company says will deliver needed clarification as it examines possible storage technologies that it may submit to the  Public Utilities Commission, the state regulatory body overseeing investor-owned utilities.

In theory, the law could play into whether Xcel puts storage at Hayden, Pueblo, and Brush. It will be closing coal plants at all three locations in the coming decade. But Xcel plans to replace the coal plant at Brush, called Pawnee, with a natural-gas plant. New storage technologies might be possible at Pueblo, although Xcel is already planning substantial placement of battery storage and renewables in that area even as it prepares to close Comanche 1 and 2 in 2022 and 2024.

Hayden stands out because of its vulnerability. It stands not only to lose relatively high-paying jobs but also property tax base when the coal-burning units close in 2027 and 2028.

“We just built a $61 million school project that is going to put a burden on the community if this power plant is unable to transition,” Tim Redmond, a Routt County commissioner and a former mayor of Hayden, told the Colorado House Energy and Environment Committee meeting on May 27.

The power plant provides over one-third of the assessed valuation of the Hayden School District. The same applies to library, cemetery, and other special districts.

Hayden has a new school that is substantially predicated on tax revenues from the coal-fired power plant. Photo/Allen Best

Some technologies Xcel has been talking about might actually add jobs—and not just any jobs. Hayden, located 25 miles west of Steamboat Springs, already has some workers in the tourism economy. But the $95,000-a-year jobs that the power plant has will be preserved or perhaps improved. “We are not going to replace these jobs with service jobs,” Redmond said.

Mathew Mendisco, the town manager in Hayden, told state legislators the community is “very excited about the energy transition” and eager to support “innovative climate-favorable technology.”

(See story, page 5, about the legislative bill).

Alice Jackson, the president of Xcel Energy Colorado, said in February that while closing the coal plants, Xcel intended to stay in Hayden, perhaps using lithium-ion batteries or some other technologies.

Xcel representatives were more precise in a recent meeting with Redmond and other Routt County commissioners. The Steamboat Pilot reported that Jack Ihle, the director of environmental policy for Xcel, identified molten salt storage as the most promising technology. It would allow Xcel to use a turbine and other existing infrastructure. But all options are on the table.

Ihle said Xcel intends to invite proposals from energy technology companies next year. This solicitation for proposals will be issued as part of the company’s energy resource plan for the second half of this decade.

That plan submitted to the Colorado PUC will add a “ton of wind, a ton of solar and a ton of batteries,” Matthew Larson, also an Xcel representative, told state legislators at a committee hearing.

This was published in Big Pivots 39 on June 8, 2021.

Energy storage will be a key challenge for utilities large and small during the coming decade. Most have said they know how to get to 80% emissions-free electricity by 2030 and some say they believe they can get to 90% with existing technology. But everybody agrees some things need to be figured out.

Pumped-storage hydro currently is the single most important storage technology in Colorado, and it’s not new. Xcel’s two 162-megawatt turbines at its Cabin Creek project near Georgetown date to 1967. Only in the last three years have lithium-ion batteries started being used in Colorado, first at United Power’s four-megawatt array along I-25 north of Denver. Other but still small Tesla battery packs have been put into place, with many more to come in the next few years. Xcel has 275 megawatts of battery storage planned in Pueblo and Adams counties. Holy Cross Energy will access batteries near Glenwood Springs, Silt, and Parachute.

Limitations of batteries

Lithium-ion batteries have the advantage of being able to dispatch electricity quickly, to stabilize the grid. They remain relatively expensive, though.

But even as prices of lithium-ion batteries further decline, they will have a major shortcoming. They can store electricity for only a few hours. Extended storage will be needed, as several utilities pointed out in February, when a deep cold was accompanied by windless days.

Tri-State Generation & Transmission, with three coal-burning units located a half-hour drive west of Hayden that will close by 2030, has also been studying its options.

Duane Highley, the chief executive of Tri-State, said at a February forum organized by the Sierra Club that no decisions had been made about its plans for the three coal-burning units at Craig.

“Today, people talk about battery storage a lot. However, the battery that a utility can buy today lasts somewhere from 2 to 4 hours,” he said. “A 6-hour battery is pretty much of a stretch.”

He talked about the Valentine’s Day cold snap that, in Texas, caused widespread power failures. Colorado had its problems. “We had about 3 days of gray skies and no wind. Those would be very difficult days for us if we didn’t have fossil fuels in the mix today.”

What Tri-State hopes to see, he continued, “is a 24- or 48-hour battery. And battery prices are linear. If you double the size of the battery, it costs twice as much. It’s very much a function of the size. We are looking for the storage technology that is better than the lithium-ion battery and has a scalability that would be suitable for a former coal plant such as at the Craig site. We think it is one of the best in the Western grid for mass storage at utility scale.”

Tri-State has been working with the Electric Power Research Institute, which does what its title indicates and has a $100 million research effort. Highley said the goal is to find dispatchable energy storage technology. The technology needs to take advantage of surplus renewable energy, both wind and solar. The leading contenders today, he said, are hydrogen and ammonium. For Tri-State, the technology needs to be available by the year 2030.

NREL research

A federal agency created in 2007 is also at work on the challenge. The Advanced Research Projects Agency-Energy, or ARPA-E, was tasked with advancing high-potential, high-impacted energy technologies that are too early for private-sector investment.

One of its current grants, for $2.8 million, went to the National Renewable Energy Laboratory in Golden. That project, has a goal of creating a modular and scalable thermal storage system that can operate at the scale of conventional thermal power stations.

In the case of natural gas plants, says a statement from ARPA-E, that’s usually about 100 megawatts. In the case of coal generation, it’s 300 megawatts.

Thermal energy storage, as the name implies, relies upon heat to store energy, and there are a great many media for heat, even rocks or aluminum. A thermal power station is one that creates electricity by producing heat, such as occurs in coal and gas-fired power plants.

The NREL project assumes that the electricity dispatched from a storage unit must cost no more than 5 cents to be useful to utilities. This formula assumes energy arriving into the storage medium can cost no more than 2.5 cents per kilowatt-hour. That’s likely, as Xcel has been getting wind for less than 2 cents and storage for not much more than that. That leaves the cost for the storage technology at a maximum of 2.5 cents per kilowatt-hour.

As for nitrate salts, Xcel’s front-runner in the report to the Routt County commissioners, it would be heated by the arriving electricity to Hayden to 1,000 degrees. Then the heat would be tapped to power the existing coal-fired turbines when needed to produce electricity. The technology is largely the same, but the coal will be gone from the equation.

Xcel – this is from the Steamboat Pilot report—is also thinking about solar electrolysis, which would convert sunlight into hydrogen that could then be sold to the local natural gas distributor.

Another idea would use biomass, presumably wood, to create a synthetic gas, also to be distributed locally.

Xcel does not sell natural gas in northwestern Colorado, but other gas utilities will be under the gun in the next few decades to eliminate emissions from their product.

Also in the mix is potential for a fish hatchery, to be built in partnership with the Colorado Parks and Wildlife, at site of the power plant in Hayden.

Salts and solar

Molten salt isn’t a new technology. A 2009 story in Scientific American examined the technology and the looming problem of how to buttress renewables, solar in particular. The story used as a case study was a plant in Spain.

At the time, NREL had found batteries wanting, because too much energy that goes in is not returned, along with the expense.

Melting salts do better with what NREL senior engineer, Greg Glatzmaier called “round-trip efficiency.” At temperatures above 435 degrees Fahrenheit (224 degrees Celsius), the molten salt can deliver back as much as 93% of the energy. Too, salts are ubiquitous, commonly used in fertilizers.

By 2017, molten salt technology represented three-quarters of the thermal energy storage deployed for electricity applications worldwide, reported, citing a report from the International Renewable Energy Agency. Salt by then had also edged ahead of batteries in the United States.

That same Greentechmedia story noted more and larger molten-salt storage projects associated with concentrated solar projects in the American Southwest.

A 2019 story by InsideClimateNews focused on a molten salt project near Tonopah, Nev.. The concentrated sunlight at the Crescent Dunes Solar Energy Facility—developers later declared bankruptcy—heats molten salt up to 1,050 degrees F in a shimmering tower. At that temperature, the salt looks and flows pretty much like water.

Many more salt towers

The molten salt runs through a heat exchanger to make steam to run a standard turbine generator, the story explained. The tank holds enough molten salt to run the generator for 10 hours. That represents 1,100 megawatt-hours of storage, or nearly 10 times more than the largest lithium-ion battery systems that had been installed at that time to store renewable power.

Mark Mehos, program manager for concentrated solar power research at NREL, predicted many more CSP molten salt towers. He said that such towers with 8 or 10 hours of molten salt storage were then much cheaper than a solar PV farm with an equivalent amount of lithium-ion batteries. But for that January 2018 story, Mehos also cautioned that molten-salt towers associated with concentrated solar power still needed to prove their reliability and promised prices.

Figuring out storage is absolutely huge, the holy grail of decarbonization. Renewables have their problems. There are always land-use impacts. At legislative hearings in Denver, Republican detractors sometimes wanted to know the carbon footprint of the infrastructure needed to produce renewable energy.

But renewable generation is now cheap. On its own, though, it can’t get any utility to 100% short of building massive amounts of infrastructure.

Colorado utilities are deep in the hunt of finding solutions. Look for another report from Big Pivots soon.

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