MINNEAPOLIS — As states, cities and towns set goals for 100% renewable energy, critics question whether it can actually be done, and done cost-effectively.

In Colorado, Xcel Energy said its recently announced goal of 80% reduction in carbon emissions by 2030 can be achieved with existing technology. Costs of renewables have declined, weather forecasts have improved, and engineers have learned how to integrate higher and higher levels of clean power without sacrificing reliability.

But for that final 20% of emissions-free power, the company wants to see improved weather forecasts, and, most critically, improved storage technology. Market reforms will also be needed to better manage the grid.

Even these strides will not be enough. As the company noted in announcing a new study, it will need new carbon-free technologies not yet commercially available at the cost and scale required.

In the report, Dr. Brian O’Neill and Steve Hedden, climate modelers from the University of Denver, said Xcel’s goals are consistent with, and in most cases larger than, what will be needed from electrical utilities to minimize global warming as identified in the Paris climate agreement.

Xcel’s mid-century goal is believed to be the most ambitious goal of an investor-owned utility in the United States.

On Dec. 3, with the sun blazing in the windows of the Denver Museum of Nature and Science, Xcel chief executive Ben Fowke announced the ambitious new goals. Just three days before, shortly after sunset as wind picked up on the Great Plains, Xcel operations in Colorado had set a new record for renewable generation — 72% — for a full hour.

The 80% goal Fowke outlined, however, was not for just an hour, but overall. And not just Colorado, but throughout the company’s eight-state service area.

“We have the technology today to allow us to achieve that interim goal of 80% by 2030,” he had told reporters earlier.

Scores of municipalities across the country have adopted goals of 100% renewable electricity. Colorado Gov. Jared Polis, in campaigning for governor last year, declared a goal of 100% renewables for Colorado by 2040. In Minnesota, where Xcel is headquartered, Gov. Tim Walz announced a 100% by 2050 goal.

Larger utilities, tasked with the practical matter of delivering across broad geographic areas, have been more restrained. But Xcel believes that strides in new technology of recent years will continue.

“I have a lot of confidence they will be developed,” Fowke said. “All we have to do is look at how far we have come in the last decade. We have seen reduction in the pricing of large-scale wind and solar by over 70%, which means they now compete head-on with fossil alternatives. In fact, we are saving customers money.”

To reach that goal, expanded wind generation coupled with improved forecasting techniques top the list.

Xcel easily blew past the 2004 mandate imposed by Colorado voters of 1,800 megawatts of renewables, hitting 3,845 megawatts a year ahead of the original deadline of 2020. Wind delivers 82% of that generation, with another 18% coming from community, rooftop, and utility-scale solar.

More wind came online last fall with the 600 megawatts of capacity in Rush Creek, a wind farm that sprawls across six counties about 150 miles southeast of Denver. Next will come Cheyenne, a 600-megawatt wind farm farther east, near the Kansas border, to be completed by late 2020.

New wind was a large part of Xcel’s Colorado Energy Plan approved by state regulators last September. With the new renewables, Xcel plans to close two aging coal plants at Pueblo by 2025. With that, Xcel expects to be 55% powered by renewables.

Improved weather forecasting in the last decade has allowed Xcel to deepen penetration of wind without risking reliability to customers. The error rate in day-ahead forecasts has dropped from 18 to 20% a decade ago to 10 to 11% as of two years ago.

Global Weather Corporation, a Boulder company, projects wind — and hence electrical production — from turbines for 10 days ahead. The company updates its forecasts every 15 minutes. The technique uses statistical post-processing to compare production from Xcel’s 6,000 to 7,000 turbines to compare against forecast production and inform future forecasts.

Forecasts have become so good, said John T. Welch, director of power operations for Xcel in Colorado, that the utility uses 95 to 98% of the electricity generated by turbines. It would use all the electricity but for transmission constraints and other considerations.

Knowing when the wind will blow and how strongly has allowed Xcel to reduce the use of coal and natural gas plants. The trick, Welch explained, is to keep plants operating but at reduced volume.

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