Tony Webster / Creative Commons
The Minnesota city has made significant investments in its aging district energy system, adding natural gas boilers and replacing steam pipes with hot water loops.
The city of Duluth, Minnesota, continues to lower emissions from downtown buildings as it expands a redesigned district heating system.
Originally built in 1932, the system acts like a shared boiler for a network of buildings in the city’s core. Over the last decade, the city has mostly phased out coal, converting two of its four boilers to run on natural gas. More recently it began replacing old steam pipes with a more efficient closed-loop system that circulates hot water between customers’ buildings and its centralized plant.
The investments are a critical component of the city’s effort to cut carbon emissions by 80% by 2050, as the heating plant that powers the district energy system is by far the largest source from city government operations. The system serves more than 160 customers and covers 5.6 million square feet of building space.
“The biggest benefit of the new system is that it will generate less energy but heat the same amount of square feet,” said Terry Nanti, general manager of Duluth Energy Systems, an affiliate of St. Paul-based Ever-Green Energy that operates the system for the city of Duluth. “When we reduce the amount of energy we produce, we reduce our reliance on coal.”
In January, warmer than typical weather allowed the system to rely exclusively on the newer gas units, burning no coal at all for the first time in its 90-year history. Overall, annual coal use is down about 80% since 2012. Even gas emissions have declined by 20% as the system has grown more efficient.
Ever-Green Energy, which also operates district energy projects in St. Paul, Milwaukee, and Cincinnati, is nearing completion of a project that involves replacing pipes and equipment on 11 blocks of the city’s main drag, Superior Street, and converting 27 buildings to hot water heating.
This year Ever-Green will extend the hot water system to two hospital campuses beyond Superior Street and a handful of other buildings, extending the system’s heating load. Work will continue connecting buildings in summer and fall.
The company faced a few hurdles during the project. Converting century-old buildings “required some intricate work,” said senior program manager Sean McFarling. Not every structure on the network could transition from steam because of their age, he said. Making sure the heating plant’s floor could support new equipment and water load also added complications.
Contractors struggled with other issues — among them, tucking pipes underneath Interstate 35 to connect the plant, east of the freeway, with downtown, west of it. And COVID-19 slowed but did not stop progress, Nanti said.
City leaders are hopeful that adding customers and converting more buildings to hot water heat will continue to reduce emissions associated with the system.
“Our city and our utility are taking the hardest steps to tackle, to be more efficient and to convert to hot water,” said Mindy Granley, the city’s sustainability officer. “Improving the efficiency of the Duluth Energy Systems plant, moving away from coal, and adding hot water loops are critical elements to help the city meet its climate goals.”
The coal boilers remain in place for extremely cold days — such as those in recent weeks when wind chills dipped as low as 40 degrees below zero — but officials also imagine a day when the plant mostly runs on renewable energy, with natural gas as the backup fuel source.
“Eventually, we would like to get rid of [coal] with other fuel sources,” McFarling said. “I think we’d want to burn renewable fuel and natural gas for backup. That won’t happen tomorrow, but I think eventually that will happen.”
The Duluth project stands out in Ever-Green’s portfolio, said Nina Axelson, the company’s vice president of public relations. “This is one of the largest infrastructure transformations we’ve taken on, not just because of the size of the construction but because of what it achieves on the other side in terms of efficiency in the customer buildings improvements to the plant,” she said.
Other aging district energy systems now have an example in Duluth’s approach to modernizing and decarbonizing its infrastructure, Axelson said: “We think Duluth could be a model.”