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On a stretch of Brooklyn’s waterfront, the 34-story Riverie looks like any other high-rise in New York City’s crowded skyline. But beneath its foundation, an invisible network of boreholes extends hundreds of feet into the ground to tap Earth’s natural reservoir of warmth. Instead of burning fossil fuels, the Riverie relies on geothermal energy; it uses the steady temperature underground to warm apartments in winter and cool them in summer. The building is part of a small but growing wave of urban geothermal projects revamping how heating and cooling work in dense cities.
There are 320 boreholes underneath the Riverie’s building site, and developer Lendlease says this makes it the largest geothermal residential building in New York State and the largest high-rise geoexchange system in the country. People started moving into the building last month.
Unlike deep geothermal power plants that tap hot rock miles below Earth’s surface to generate electricity, urban systems rely on comparatively shallow energy exchanges. They use ground-source heat pumps, which leverage the stable temperatures just tens or hundreds of feet underground. In a city, the first 100 meters (328 feet) of earth act as a thermal sponge, soaking up heat from human activity—what Philipp Blum, a professor of engineering geology at Germany’s Karlsruhe Institute of Technology, calls the subsurface urban heat island. “When you go below 100 meters,” Blum says, “you change to the geothermal gradient.”
At the Riverie, U-shaped pipes called ground loops are buried under the building to circulate a mixture of water and propylene glycol, a food-grade chemical that is similar to antifreeze and absorbs heat from the ground. A pump inside the building uses a refrigerant to transfer that heat and then passes it through a compressor to concentrate it. The heat is then circulated through the building’s indoor air. In summer the process is reversed, dumping excess heat back into the earth.
Installing such a geothermal system to heat a building as big as the Riverie—which has 834 rental units plus offices and common spaces—is a feat of engineering. The bulk of the work was done before the building’s foundation was laid, with the geothermal team being first on the construction site. The team members used a dual rotary drill rig to create the boreholes. (Unlike a regular drill that only spins the inner drill pipe, a dual rotary rig rotates an outer steel casing and the inner drill pipe at the same time.) This prevented the loose, waterlogged rock and soil of the waterfront from collapsing into the hole, keeping it straight and stable.
“The boreholes were drilled 499 feet into the ground, deeper than the building is tall,” says Meg Spriggs, managing director of development at Lendlease. “Each rig completed slightly more than one borehole per day.” The holes are about 4.5 inches in diameter and spaced 15 feet apart.
Spriggs says the underground geology at the Riverie site is largely composed of bedrock, “an ideal medium for efficient thermal exchange.” Because of the site’s proximity to the East River, however, the holes needed to be shielded from groundwater in the soil above the bedrock. Engineers used steel casing for this, and pipes made of high-density polyethylene—a corrosion-resistant material designed for long-term use—were installed in the reinforced holes.
The length of all the piping adds up to around 65 miles, Spriggs says, though the site is just 2.6 acres. Those pipes ultimately connect to 1,100 heat pumps throughout the building.
Almost as soon as it’s completed, this intricate system becomes invisible forever when the foundation and then the building itself are placed over it, leaving no room for errors. “Your entire heating and cooling system is dependent on [the geothermal system] and it’s buried under the building, so failure is not an option,” says Tim Weber, co-founder and CEO of Diverso Energy, which has built multiple projects similar to the Riverie in Toronto.
Success isn’t just a matter of installing the geothermal system correctly; managing it is just as critical. “If a bore field is not being monitored and managed, it’s a matter of when it will overheat,” Weber says. “You also need the opportunity to reject excess heat to help balance the building.” One way to balance the temperature is to use excess heat in summer to preheat the building’s water before it goes to the boiler.
Seen from above in 2023, the 2.6-acre lot presented a tight squeeze for engineers, who had to pack roughly 65 miles of piping into the compact footprint.
Ismail Ferdous/Bloomberg via Getty Images
Because it simply moves heat rather than generating it, the Riverie is expected to reduce annual carbon emissions from heating and cooling by 53 percent compared with traditional residential buildings. As states and cities roll out strict building emissions caps, gas bans and net-zero targets, technologies that can deliver this kind of reliable, low-carbon heat are becoming more of a necessity.
So what will it take for more cities to go geothermal?
Despite operating costs being lower over time, the high up-front cost of installing a geothermal system is the biggest barrier. For the Riverie, Spriggs says that “there was just under a 6 percent up-front premium on the total construction costs. Over 25 years, these costs are offset through lower annual operating expenses and protection from [emissions] fines.”
To bypass that capital barrier entirely, Diverso has implemented an “energy as a service” business model: the company not only drills and maintains geothermal systems, but also becomes the HVAC (heating, ventilation, and air-conditioning) provider for the building’s residents, who pay the company a fixed rate year-round. “Geothermal should not be considered a piece of equipment or an extension of the building,” Weber says. “It’s energy infrastructure, and part of the business model is treating it as such.”
Permitting rules and building codes can also be a barrier. Areas that want to enable wider deployment of geothermal need to establish policies that are conducive to these systems. “Cities need an urban management strategy for the subsurface,” Blum says. “You have this space below the city, and more people want to use it, so you need to optimize the space for these kinds of systems.”
Weber is optimistic that cities and companies will rise to the challenge. “If a city is bold enough to create aggressive climate and decarbonization mandates, the private sector will come up with technology and business solutions to respond,” he says, adding that in Toronto, the list of developers that aren’t considering geothermal is now smaller than those that are.