(Bloomberg) -- The key to heating an 834-unit apartment tower under construction on the Brooklyn waterfront will be a hole in the ground.

Actually, it will be 322 holes, each about 4 inches (10 centimeters) across and exactly 499 feet (152 meters) deep — any deeper and New York state would consider it a mining project. These holes comprise the heart of a geothermal heat-pump system that is expected to reduce carbon emissions by 53% over a comparable building using conventional heating and cooling systems. When complete in 2025, 1 Java Street will be one of the biggest US residential buildings using the technology.

Buildings account for about 8% of global carbon emissions, mostly from burning fossil fuels for heating, and heat pumps are widely seen as an important tool to make homes cleaner. Spending on residential heat-pump systems climbed 9.6% to $64.3 billion worldwide last year amid a growing push to electrify more of the international economy. The Brooklyn project will be an important test for Lendlease Corp., the Australian developer that’s building 1 Java and seeks to eventually eliminate emissions entirely from its global operations.

“We don’t want to use natural gas anymore, in any of our buildings,” said Sara Neff, Lendlease’s head of sustainability for the Americas. If the system at 1 Java works out, the company plans to use more of them in future US projects.

Lendlease said its geothermal system will increase construction costs by about 6%, though it wouldn’t provide figures for the total budget. Part of that expense will be covered by a $4 million state grant. 

Heat pumps work by shifting warmth between the inside and outside of a building, and can be used for both heating and cooling. The more common direct-air systems use an electric compressor that applies pressure to a liquid refrigerant within closed coils of piping, and has valves to release the pressure. In the summer, warm indoor air is cooled as heat energy is absorbed by the liquid, which evaporates into a gas. The compressor puts pressure on the gas, making it even hotter as it flows into outdoor coils. Air from outside then absorbs some of that heat energy as the refrigerant cools and condenses back into a liquid. Valves reduce the pressure on the  fluid, which cools even more as it flows back inside to repeat the cycle. In winter, the process runs in reverse; the key is that adjusting the pressure can make the fluid hotter than exterior air in the summer or cooler than the outdoors in the winter.

Geothermal systems use the same basic principles, but they rely on the fact that underground temperatures stay around 55F (13C), no matter how hot or cold the air is above. In the summer, water mixed with an additive that inhibits freezing and corrosion is pumped down pipes into the boreholes, where it gets colder and is then brought up to cool the building. In frigid winter weather, the stable underground temperatures can bring the water temperature to 55F and then an electric heater makes it even warmer as it’s pumped through the building to provide heating.

The geothermal system is key to limiting energy use at 1 Java, which will be fully electric (including its stoves). For water delivered to the apartments, the constant underground temperature will be used to bring it to 55F and an electric boiler will then make it hot enough for showers and dishwashing. The 55-degree water in the closed-loop system will be used to regulate temperatures instead of electric air conditioners or gas heating.

Even though 1 Java will be more dependent on electricity than a conventional building, it will need less of it, said Scott Walsh, the project director. That’s because electric heat pumps are more energy-efficient than the heating and cooling systems widely used now. He expects to see more buildings using the technology amid a global push to shift away from fossil fuels.

“In a cold climate, an all-electric building isn’t financially feasible without a system like this,” Walsh said.

Geothermal systems tend to be significantly more expensive than other heat pumps to install, and the excavation required makes them difficult to add in existing buildings. Geothermal heat pumps are more common in large commercial buildings than single-family homes, and are becoming more popular with developers because the systems need less energy to operate than the direct-air version and are more cost-effective in the long run, said Lewis Williams, an analyst with BloombergNEF.

But the global fight to curb climate change is making all heat pumps more popular because they are all-electric and eliminate the need for gas heating. Spending on heat pumps has doubled since 2015, and government incentives, especially in Europe and the US, mean demand will continue to grow.

“Heat pumps are becoming a much bigger part of the solution for building decarbonization,” said Williams.

Utilities are also finding ways to use geothermal heat pumps on their grids. National Grid announced one such project in September in Massachusetts, and Vermont Gas Systems Inc. is planning one as well. Late last year, New York regulators ordered the state’s biggest utilities to come up with their own plans to implement the technology.

In a suburb outside of Boston, Eversource Energy is installing a geothermal heat-pump system that uses a networked design and will provide heating and cooling to 39 buildings in a single neighborhood. It’s the first US utility to try this approach, which is expected to reduce both customers’ bills and carbon emissions by not relying on natural gas or heating oil.

That project will link residential and commercial buildings, including some low-income apartments, to more than 100 boreholes that go down as far as 600 feet. Each building will have its own heat-pump device and the system is expected to go into service before the start of next winter’s heating season, said Nikki Bruno, vice president of clean technologies at Eversource. The effort is expected to cost $10 million to $12 million.

The system will use much less electricity for cooling than conventional air conditioners, lowering customers bills in the summer. In the winter, people who use gas heat will probably see some savings as well. About 24% of Massachusetts homes are heated by some type of fuel oil, and those customers will see more significant cost reductions, Bruno said. Eversource considers this a test and says it can’t yet provide more specific savings projections, but expects the average residential customer’s emissions to decline by about 60%. 

“This is an energy-efficiency play,” said Bruno. “You’re reducing your overall demand.”

(Company corrects details about stoves at 1 Java Street in eighth paragraph.)

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