One of the nation’s largest solar energy projects, being built near Springfield, will provide electricity to Chicago city properties

The largest solar arrays operating in the United States are located in the kinds of sunny states you would expect — California, Texas and Arizona. But a project that began construction last month near Springfield and that will provide electricity to Chicago is going to be about as large or larger than all of them.

The 593-megawatt Double Black Diamond Solar farm near Springfield is being built by Swift Current Energy of Boston.

Among the buyers of the power is the city of Chicago, which plans for the project to provide most of the electricity used by municipal properties, including some giant power users like O’Hare Airport and Midway Airport and two water-purification plants.

Chicago’s involvement with the project is tied to the city’s goal, adopted by former Mayor Rahm Emanuel and affirmed by Mayor Lori Lightfoot, to use 100% renewable electricity in city buildings by 2025.

But the city doesn’t have a contract with the project itself. Instead, City Hall has a deal with Constellation Energy to supply 100% renewable energy. The Baltimore company sells electricity and natural gas contracts in states — like Illinois — that allow customers to choose their supplier for those resources.

Constellation plans to get 70% of that from Double Black Diamond once construction is done, which it expects will be next year. The rest could be renewable energy from anywhere, though city officials have said they would like to have much of it come from projects in the Chicago area.

“I am as excited as anybody to see shovels in the ground, so to speak, this spring,” says Jared Policicchio, City Hall’s deputy chief sustainability officer. “We are hoping that we can demonstrate to other public agencies and frankly private organizations in the region that you can cost-effectively continue to spur the quick development of this critical industry.”

The city’s electricity contract is helping to support the solar project but still paying electricity rates that are competitive, according to Policicchio.

Double Black Diamond’s other customers include State Farm, the insurance giant, and PPG, the paint manufacturer, which have signed up to buy power from the project.

The largest operational U.S. solar project, based on capacity, is Topaz Solar Farm in California, with 586 megawatts, according to the Solar Energy Industries Association. Topaz is slightly smaller than Double Black Diamond by that measure but encompasses more space: about 4,700 acres for Topaz compared with about 4,100 acres for Double Black Diamond.

But Double Black Diamond won’t be the largest in the country because others are in development that will be bigger.

Wind and utility-scale solar projects in Illinois have faced challenges. One has been opposition from nearby residents who have criticized the developments as ugly. Another: a lack of transmission lines to deliver the electricity from the plants.

To get past those challenges, Double Black Diamond got approval in 2021 from the two county governments that host the project, over the opposition of some neighbors, and is being built near a relatively new transmission line with available capacity — the Illinois Rivers project built by the utility Ameren.

Double Black Diamond also got a boost from clean-energy policies, including the Chicago 2025 target and several state laws that encourage wind and solar development. A 2021 state law, the Climate and Equitable Jobs Act, is one of the reasons that the project is required to be built using workers being paid union wages, with the companies required to meet standards for diversity in hiring.