Across North America, hundreds of downtowns, college campuses and hospitals are heated by steam carried through networks of underground pipes. Electric companies installed many of these "steam loops" or district energy systems more than 100 years ago in older East Coast cities like New York, Boston, and Philadelphia.

Today, these systems, which often provide chilled water for cooling as well, are experiencing a renaissance as a potential solution to climate change.

In Philadelphia, the steam gets generated at a red brick plant built in 1915, one of the few remaining industrial sites that sits along the Schuylkill River. Steam travels through 41 miles of pipe to dozens of buildings, including the Philadelphia Museum of Art, the University of Pennsylvania and the newly built Comcast Towers.

"It's like a ring of steam where you have a bunch of people connected to that ring taking the steam and using it," said Mike Ancona, operations manager for Vicinity Energy, which owns and operates Philadelphia's steam loop system.

The plant is a study in contrasts, and illustrates the evolution of electric generation. Ancona points to the high-arched ceilings that are lined with tiles, and the building's detailed brickwork, depicting an era when electricity had begun to replace gas lighting in earnest. It now houses modern boilers alongside a highly efficient cogeneration facility that feeds electricity to the grid, while at the same time, uses excess heat to produce the steam that heats Philadelphia buildings. It also produces food grade steam that is used to sterilize equipment at nearby hospitals, and cook cold cuts.

From coal to gas to renewable energy

"So, when this place was built, like 100 years ago, these boilers ran on coal," Ancona said. "If we can go back in time, there would have been a 60,000-ton coal pile sitting here."

The plant originally burned coal that arrived on river barges. It switched to burning oil during World War II. Today it burns natural gas and some waste grease from nearby restaurants to produce the electricity, and uses the waste heat to generate the steam. Most people only see this underground network through the residual steam that rises through sidewalk grates.

Vicinity Energy says this highly efficient and flexible system could easily ditch natural gas and replace it with renewable energy or lower carbon fuel to generate that steam.

In fact the company has begun to do that with the steam loop system it owns in Boston.

"And the building owners don't have to do a thing," said Vicinity Energy CEO Bill DiCroce. "No major retrofits. No big capital expense on their part. No disruption in the operation of the building. So we become the easy way to decarbonize huge swaths of building space in urban cores."

And it's not just cities that can benefit. Dozens of colleges have district energy systems in place where the fuel source is getting switched in order to lower a university's carbon footprint.

"Instead of doing 150 individual buildings, if you can decarbonize the primary supply to a central plant, then you really achieve lower carbon operations at scale," said Rob Thornton, president and CEO of the International District Energy Association.

More than 900 of these systems exist in the U.S. and Canada, while there are thousands more worldwide. Thornton says switching those systems from fossil fuel to cleaner energy is a more cost-effective way to achieve electrification without overloading the grid.

"District energy is actually a very elegant solution, particularly for cities, campuses, communities, and clusters of buildings," Thornton said. "It enables buildings to get to net zero."

A clash with federal climate policy

But in Philadelphia, where city officials pledged to reach net zero by 2050, two federal agencies plan to switch from Vicinity Energy's district steam system to natural gas boilers. This despite President Biden's commitment to tackling climate change, and his executive order mandating federal agencies work to limit emissions.

Both Amtrak's 30th Street Station and Independence National Historical Park operated by the National Park Service, which includes the historic Independence Hall and the Liberty Bell Center, have inked deals with the city-owned Philadelphia Gas Works to remove the steam systems and install new natural gas infrastructure.

This angers environmental groups that support the steam loop system because of how easy it would be to use it to cut the city's overall carbon emissions.

"The Federal Government has pledged [about] $14 million dollars to install these natural gas boilers that can only burn natural gas for heat," said Joseph Ingrao, an attorney who worked with the Clean Air Council until recently.

The group is especially frustrated by the fact that, in the case of Amtrak and the National Park Service, there were no attempts to look at the impact on greenhouse gas emissions, no public hearings or comment opportunities, and no public bidding processes.

A spokesperson for Amtrak said switching from district steam to natural gas boilers would be both more efficient and save the transportation system money.

"Diversifying the sources offers more reliable heating for the building and provides significant energy cost efficiencies to Amtrak over the contract term," wrote Amtrak spokesperson Olivia Irvin in an email. "Amtrak shares the administration's commitment to a lower carbon future and we will be working to integrate carbon assessments into our projects. "

Amtrak would give no further details on the contract, or any environmental assessment it may have done for the gas boilers it's installing.

The National Park Service says plans to convert to natural gas began in 2015, under the Obama Administration, and the move is in line with President Biden's executive order. NPS says the newly installed system will consume less fuel and cost less, but that could not be independently verified.

"The energy required to generate high-pressure steam and transport it 2.5 miles to the park is substantially higher than the park will use to self-generate heating hot water and pump it through closed loop systems serving multiple buildings," wrote a spokesperson in an email. "The future park system, because it will not consume the amount of fuel needed to convert water to steam and will recirculate the heating medium, will use much less fuel than that required to supply steam from the existing provider."

Vicinity COO Kevin Hagerty says the Park Service is "either disingenuous or uninformed."

"How can any party claim that a process that will burn additional natural gas will save carbon emissions over a process that captures waste heat that powers 70% of the steam that heats Philadelphia [buildings]," said Hagerty.

DiCroce says that while natural gas may be cheaper, the environment should be taken into consideration.

"People solve for two greens; they solve for the dollars and carbon," DiCroce said. "Different institutions, different people decide which green to lean towards. So those who decide to lean toward the cheapest alternative will make different decisions than those who are trying to look forward to a less carbon-intensive future."

Supporters say district energy has also proven resilient in the face of extreme weather. After Superstorm Sandy hit the East Coast in 2012, the district energy heating and cooling systems remained online despite power failures. And when a deadly winter storm hit Texas last year, crashing the state's power grid, district steam systems that served downtown Houston, the Texas Medical Center's seven hospitals, Texas A&M University and the University of Texas in Austin continued to operate with no service interruptions.