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heyNear Brooklyn’s waterfront, the 34-story Riviery looks like any other tall building 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 the Earth’s natural heat reserves. Instead of burning fossil fuels, Rivari relies on geothermal energy; It uses the constant temperature underground to heat apartments in winter and cool them in summer. The building is part of a small but growing wave of urban geothermal projects that are reinventing heating and cooling in dense cities.
Rivery has 320 boreholes beneath its construction site, and developer Lendlease says this makes it the largest geothermal residential building in New York State and the largest high-rise geothermal system in the country. Last month people started moving into the building.
Unlike deep geothermal power plants, which harness hot rocks miles below the Earth’s surface to generate electricity, urban systems rely on comparatively shallow energy exchange. They use ground-source heat pumps, taking advantage of stable temperatures tens or hundreds of feet below. In a city, the first 100 meters (328 feet) of the Earth acts as a thermal sponge, soaking up heat from human activity – what Philipp Blum, a professor of engineering geology at the Karlsruhe Institute of Technology in Germany, calls a subsurface urban heat island. “When you go below 100 meters, you turn into a geothermal gradient,” says Blum.
In Riveri, U-shaped pipes called ground loops are buried beneath 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 move that heat and then passes it through a compressor to concentrate it. The heat is then circulated through the indoor air of the building. In summer this process is reversed, causing excess heat to escape back into the earth.
Installing such a geothermal system to heat a building as large as Riveri – which houses 834 rental units plus offices and common spaces – is a feat of engineering. Most of the work was done before the building’s foundations were laid, with the geothermal team being the first to arrive at the construction site. Team members used a dual rotary drill rig to drill the borehole. (Unlike a regular drill that rotates only the inner drill pipe, a dual rotary rig rotates the outer steel casing and the inner drill pipe at the same time.) This prevented the loose, waterlogged rock and soil of the coast from collapsing into the hole, keeping it straight and stable.
“Boreholes were drilled 499 feet into the ground, deeper than the height of the building,” says Meg Spriggs, Lendlease’s managing director of development. “Each rig completes a little more than one borehole per day.” The holes are approximately 4.5 inches in diameter and spaced 15 feet apart.
Spriggs says the underground geology at the Riveri site is largely composed of bedrock, “an ideal medium for efficient thermal exchange.” However, due to the site’s proximity to the East River, the holes needed to be protected 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 reinforced holes.
Spriggs says the length of all the piping reaches about 65 miles, although the site is just 2.6 acres. Those pipes ultimately connect to 1,100 heat pumps throughout the building.
Almost as soon as it is completed, this complex system becomes invisible forever when the foundation and then the building itself is placed on top of 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.
Success is not just a matter of installing the geothermal system correctly; Managing it is equally important. “If a bore area is not being monitored and managed, it is a matter of when it will overheat,” says Weber. “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 the summer to heat the building’s water before it goes to the boiler.
Viewed from above in 2023, the 2.6 acres of land provided hard work for engineers, who had to fit nearly 65 miles of piping into the compact footprint.
Ismail Firdous/Bloomberg via Getty Images
Because it transfers heat rather than simply generating it, Reverie is expected to reduce annual carbon emissions from heating and cooling by 53 percent compared to traditional residential buildings. As states and cities implement stricter building emissions limits, gas restrictions, and net-zero goals, technologies that can provide this kind of reliable, low-carbon heat are becoming a necessity.
So what will it take to convert more cities into geothermal?
Despite operating costs decreasing over time, the biggest hurdle is the high initial cost of installing a geothermal system. For Reverie, Spriggs says there was “only a 6 percent upfront premium on total construction costs. Over 25 years, these costs are offset through lower annual operating expenses and protection from (emissions) penalties.”
To completely bypass that capital hurdle, 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. “This is energy infrastructure, and part of the business model is treating it that way.”
Permitting regulations and building codes can also be a barrier. Regions that want to enable widespread deployment of geothermal need to establish policies that are conducive to these systems. “Cities need urban management strategies for the subsurface,” says Blum. “You have this space underneath the city, and more people want to use it, so you need to optimize the space for these types 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 the technology and business solutions to respond,” he says, adding that in Toronto, the list of developers not considering geothermal is now smaller than those that did.
