Along with earthworms, rocks, and the occasional skeleton, there’s a massive battery right under your feet. Unlike a flammable lithium ion battery, though, this one is perfectly stable, free to use, and ripe for sustainable exploitation: the Earth itself. 

While temperatures above-ground fluctuate throughout the year, the ground stays a stable temperature, meaning it’s humming with geothermal energy that engineers can exploit. “Every building sits on a thermal asset,” said Cameron Best, director of business development at Brightcore Energy in New York, which deploys geothermal systems. “I really don’t think there’s any more efficient or better way to heat and cool our homes.”

At the start of June, Eversource Energy commissioned the United States’ first networked geothermal neighborhood operated by a utility, in Framingham, Massachusetts. Pipes run down boreholes 600 to 700 feet deep, where the temperature of the rock is consistently 55 degrees Fahrenheit. A mixture of water and propylene glycol (a food additive that works here as an antifreeze) pumps through the piping, absorbing that geothermal energy, then flows to 31 residential and five commercial buildings, where fully electric heat pumps use the liquid to either heat or cool a space. If deployed across the country, these geothermal systems could go a long way in helping decarbonize buildings, which are responsible for about a third of total greenhouse gas emissions in the U.S. 

Once a system is in place, buildings can draw heat from water pumped from below their foundations, instead of burning natural gas piped in from afar. Utilities use the same equipment to deploy networked geothermal as they do for gas lines, and even the same kind of pipes — they’re just circulating fluid instead of gas. The networks don’t need special geology to operate, so they can be set up pretty much anywhere. The project in Framingham, then, could be the start of something big.

In Massachusetts, commercial buildings tend to be more cooling-heavy, meaning that they cool more than heat over the course of a year, whereas residential homes tend to be more heating-heavy. Lots of different structures, with different heating and cooling needs, share one loop of piping in a geothermal network. “When you combine them onto the same loop, you keep the ground temperature stable,” said Eric Bosworth, manager of clean technologies at Eversource Energy. “You’re not putting energy in or out of the ground when you add all of the loads up.” 

To scale up, a geothermal loop like Framingham’s might connect to an adjacent neighborhood, and that one to another. “In the end, what we would like is if the gas utilities become thermal utilities,” said Audrey Schulman, executive director of the nonprofit climate-solutions incubator HEETlabs. (A spinoff of the climate nonprofit HEET, which began pitching the idea to Eversource and other utilities in 2017.) “Each individual, shared loop can be interconnected, like Lego blocks, to grow bigger and bigger.”

That goal may not be far off as utilities face increasing regulatory pressure to phase out gas. So Eversource Energy and two dozen other utilities, representing 47 percent of the country’s natural gas customers, have joined into an information-sharing coalition called the Utility Networked Geothermal Collaborative. “We’ve made a point to think about: Are we really a gas company, or are we a thermal energy delivery company?” said Holly Braun, business development and innovation manager at the Oregon utility NW Natural, which co-founded the coalition. 

These geothermal systems hinge on the humble heat pump. For most homes, an “air source” heat pump is currently the best option: Using an outdoor unit, it extracts warmth from even chilly winter air and pumps it inside. It then reverses in the summer to act like an air conditioner. 

The heat pumps in a geothermal system work the same way, only instead of extracting heat from air, the appliance extracts it from the water that’s been coursing underground. In the summer, the heat pump cools a space by injecting indoor heat into the water, which is then pumped back into the earth. That helps warm up the ground, recharging the subterranean battery so there’s plenty of energy to extract in the winter. 

A networked geothermal system is extremely efficient. It scores a “coefficient of performance,” or COP, of 6, meaning for every one unit of energy going in, you get six units of heat out. By contrast, gas furnaces have a COP of less than 1.

These heat pumps are exploiting water moving through rock that’s consistently 55 degrees. An air-source heat pump in the same neighborhood might have to run when it’s 10 degrees out, meaning it’ll have to work harder to provide the same amount of heat. Accordingly, its COP of 2 or 3 would still far outpace a gas furnace, but not approach geothermal’s COP of 6. “That means you have a higher efficiency with a ground-source system, which, of course, helps then with running costs,” said Jan Rosenow, who studies heat pumps at the Regulatory Assistance Project, a global energy NGO.

That kind of efficiency will be critical if the U.S. is going to wean itself off fossil fuels. The more gas furnaces people replace with electric heat pumps, the more demand on the electrical grid. But the more efficient that engineers can make heating and cooling systems, the less capacity utilities will have to add to the grid. “Ground-source heat pumps, and particularly those community networked shallow geothermal, take the lowest electricity draw on that coldest day in winter,” said Tamsin Lishman, CEO of Kensa Group, which is pioneering networked geothermal in the United Kingdom. “It supports a substantial saving in the upgrade needed in the grid.”

But if a utility has perfectly good infrastructure already in the ground to deliver gas, and it’s making good money doing so, why would it invest in a new kind of geothermal infrastructure? The reality is that a lot of that gas infrastructure isn’t particularly good, and is downright dangerous if it’s leaking an explosive gas. A utility might use networked geothermal to just swap in water for gas. “If you’re in a situation where you’re going to need to upgrade your pipe anyway, or replace it, you maybe think about: Do I replace it instead with a pipe that doesn’t require fuel, and it’s naturally replenishing energy from the ground?” Braun said.

At the same time, utilities are under mounting pressure to phase out natural gas: Last year, New York became the first state to ban it in most new buildings. Utilities are also staring at mandates in states like California, Vermont, and Colorado to slash their overall carbon emissions, and they can’t do that if they keep delivering the same amount of natural gas. “If you’re in a jurisdiction that says ‘no new gas,’ well, you don’t put in new gas,” Braun said. “You got to have something else, or you just keep shrinking your business.”

For new housing developments in particular — especially where recent ordinances have limited the amount of new buildings that can be connected to gas — they can drill the boreholes and lay the piping for buildings, and the homes will be ready to go fully electric. “We could lose those customers — we could just take ourselves out of the game — or we could present them with a new, decarbonized option that utilizes our existing strengths,” said Morgan Hood, manager of innovative products and services at Vermont Gas Systems, which co-founded the Utility Networked Geothermal Collaborative. “That’s what geothermal does.”

Though networked geothermal is vastly more efficient than burning gas in a furnace, it’s still unclear how it would impact a customer’s energy bill. Because utilities are still experimenting with these systems, they haven’t settled on a rate structure. One option may be a flat monthly rate to tap into the geothermal network, depending on how much water a given structure needs to provide adequate heating and cooling. It’s a relatively new technology, so the costs to install are still high: Eversource says its budget for the Framingham project was around $18 million for those 36 residential and commercial buildings. But as with any technology, costs will come down as the technique matures.

If the United States is going to properly decarbonize, the home of tomorrow could ditch natural gas and instead use a heat pump to tap into the air or the earth itself as a natural battery. The energy’s there — it’s always been there — now it’s just a matter of realizing its full potential.