Denver has a plan to heat and cool buildings without fossil fuels. It involves … sewage?

NPR is dedicating a week to stories and conversations about how communities are moving forward on climate solutions despite significant political headwinds. As the federal government halts plans to address climate change, states, cities, regions, and even neighborhoods are trying to fill the gap by cutting climate pollution and adapting to extreme weather. 

DENVER — Like in many American cities, Denver’s largest source of climate pollution is its buildings. Powering, heating and cooling the city’s skyscrapers takes a lot of fossil fuels.

Now, the city is trying a greener solution. It plans to heat and cool a cluster of large downtown buildings using a combination of water, the heat of the Earth — and sewage.

The Cherokee Boiler House, near downtown Denver, sits at the center of this plan. Despite the mothballed plant’s handsome brick exterior, inside it’s filled with rattling pipes, hazard signs and cockroach carcasses.

«It looks like a good place for a rave or potentially a horror movie,» says Denver Mayor Mike Johnston.

But the city sees potential in this relic. City officials think it could play a starring role in Denver’s goal to reduce greenhouse gas emissions to zero by 2040 — and save taxpayer dollars in the process.

 »We think we are standing in what can be the future of energy in Denver, which is both pollution free and affordable,» Johnston says.

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Denver will pilot what’s called a thermal energy network. Similar networks already exist on campuses and in some cities around the world. If it works here, it could set an example for how to decarbonize a dense, downtown core in the United States.

From steam system to an «ambient loop»

More than a hundred buildings in downtown Denver are currently heated by the world’s oldest continuously operating commercial steam system, which requires burning natural gas, a fossil fuel.

When the steam network was first built in the late 1800s, newspapers heralded it as a marvel. But today, it’s leaky and inefficient.

Customers’ steam bills have more than doubled in the past decade, according to the city’s climate office, because of increased maintenance costs, fossil fuel prices and a steady drip of customers quitting the system.

A 2021 city ordinance requires large buildings in Denver to cut their greenhouse gas emissions or potentially face penalties in a few years. But meeting those targets may be impossible for customers that are stuck on the aging steam system, according to the city.

So over the next decade, the city plans to repurpose parts of its old systems to create a new heating and cooling network for 11 city-owned buildings, which it calls an «ambient loop.»

The network will heat and cool buildings using underground pipes filled with water. That water circulates among buildings like a lazy river, linking them together on a loop (it’s «ambient» because of the relatively tepid water temperature).

Each building is then outfitted with water-source heat pumps. These are superefficient appliances that can transfer energy from the circulating water to either heat or cool the building.

«Basically, heat pumps can move heat wherever you need it,» says Elizabeth Babcock, the head of Denver’s Office of Climate Action, Sustainability and Resiliency.

When a building is too hot, heat pumps suck heat from the interior air and dump it into the circulating water. When a building is too cold, the pumps can suck heat from the water to raise the temperature inside.

Crucially, because buildings are linked together on a loop, they can share energy. If the art museum is overheated, for instance, the heat pump will dump its excess heat into the water. That water then flows to a nearby municipal building, where another heat pump can draw on that extra heat to warm up.

Eventually, the Cherokee Boiler House will be a central hub to manage the loop — the «brains and brawn» of the system, according to Drew Halpern, with the city’s climate office.

The city estimates it will cost roughly $280 million to $320 million to build out the network over the next decade, though it says those costs may fall. The pilot is being funded by a combination of city dollars and a state grant. Eventually, the city may have to issue bonds or seek private investment for more funding.

Even with the high up-front cost, the loop is up to 75% cheaper than other ways of decarbonizing those buildings, according to a a 2025 feasibility report, and will be cheaper and greener than staying on steam.

Tapping the heat beneath Denver’s feet

The city plans to start with just a handful of buildings. As more buildings join, the loop will need more energy to keep the water at the right temperature. So the city thinks it can tap into a nearly limitless source of clean energy — the heat of the Earth.

For more on communities pioneering geothermal energy, check out the Hotspots series from CPR News. 

Beneath downtown parking lots, the city plans to drill hundreds of geothermal boreholes, which will tap energy from more than 1,000 feet underground.

These will act as a battery of sorts for the network. Pipes filled with water will dive down into the holes, where they will exchange energy with the Earth. Then, those pipes will continue to buildings on the loop.

Geothermal heat is basically free once the system is constructed, though digging the boreholes can be a considerable expense.

But the city also hopes to tap into another unexpected source of «clean» energy: sewage.

Denver’s secret sauce: sewage

Most people don’t think of sewage as a source of energy, says Dan Freedman, director of technology and innovation at Metro Water Recovery, the city’s wastewater utility.

But taking showers, doing laundry and, yes, going to the bathroom generate warm wastewater brimming with thermal energy — which becomes heat.

Freedman admits it’s not the system’s most exciting selling point.

«If we’re being honest, geothermal just sounds sexier than wastewater thermal,» Freedman said during a tour of Metro’s treatment facility in Denver.

Currently, Denver’s wastewater is treated and dumped into the South Platte River while it’s still warm. That isn’t great for the river’s health, and in several years, to comply with state environmental regulations, Metro will have to cool it down.

It’s a lot of heat. In certain weather, the wastewater can contain about four times the heat used by buildings on the current steam system during the dead of winter, according to Freedman.

The city hopes to siphon off some of that heat for the loop, using a technology called a heat exchanger placed directly inside a major sewage line.

That would save the utility from paying more to chill its wastewater and burning more energy in the process. It could also open up a new source of revenue.

America’s largest «sewer heat recovery» system is just down the road, at a massive complex in Denver. Implementing the technology at city scale, though, could usher in much more widespread adoption.

«If successful, I’m incredibly confident that it’s just gonna take off,» Freedman says.

A model for other downtowns?

Denver is starting small — in about two years, just two buildings and a sidewalk snowmelt system will pilot a micro version of the loop. By 2030, the city plans for nine buildings to be connected.

Mayor Johnston is bullish that if the pilot works, it can be adapted to thousands of natural gas customers near downtown, accelerating the city’s push to eliminate its emissions.

«If you can come to one of the most bustling, vibrant downtowns in the world and discover any one of those buildings is heated and cooled by water,» he says, «that is a breakthrough for the city and, I think, a breakthrough for the country.»

Edited by Rachel Waldholz