ABesides earthworms, rocks and the odd skeleton, there is a giant battery right under your feet. Unlike a flammable lithium-ion battery, however, this one is completely stable, freely usable and ripe for sustainable use: the earth itself.
While temperatures above ground fluctuate throughout the year, the temperature below ground remains stable. That means there is geothermal energy bubbling away there that engineers can use. “Every building sits on a thermal reservoir,” says Cameron Best, director of business development at Brightcore Energy in New York, which uses geothermal systems. “I really don’t think there’s a more efficient or better way to heat and cool our homes.”
And now the big utilities are starting to take a closer look at this system. A few months ago, Eversource Energy in Framingham, Massachusetts, launched the nation’s first utility-operated interconnected geothermal district.
The pipes run into boreholes 180 to 215 meters deep, where the temperature of the rock is a constant 13 degrees Celsius. A mixture of water and propylene glycol (a food additive that acts as an antifreeze here) is pumped through the pipes, absorbing the geothermal energy and then flowing to 31 residential and five commercial buildings, where all-electric heat pumps use the fluid to either heat or cool a space.
If these geothermal systems were deployed across the country, they could make a major contribution to decarbonizing buildings, which account for about a third of total greenhouse gas emissions in the United States.
Once a system is installed, buildings can get heat from water pumped out from under their foundations, rather than burning natural gas coming from far away. Utilities use the same equipment for networked geothermal energy as they do for gas pipelines, and even the same type of pipes – they just circulate liquid instead of gas. The networks don’t need special geology to work, so they can be built almost anywhere. So the Framingham project could be the start of something big.
To increase scale, a geothermal loop like Framingham’s could connect one adjacent neighborhood, and that neighborhood could connect another. “Ultimately, we want the gas utilities to become heat utilities,” says Audrey Schulman, executive director of the nonprofit climate solutions incubator HEETlabs (a spinoff of the climate nonprofit HEET, which pitched the idea to Eversource and other utilities in 2017). “Each individual, shared loop can be connected together like Lego bricks to grow larger and larger.”
That goal may not be far off, as utilities face increasing regulatory pressure to phase out gas. Eversource Energy and two dozen other utilities representing 47% of U.S. natural gas customers have now joined an information-sharing coalition called the Utility Networked Geothermal Collaborative.
“We set out to think about: Are we really a gas company or are we a thermal energy company?” says Holly Braun, business development and innovation manager at Oregon-based utility NW Natural, who co-founded the coalition.
A heat pump in a geothermal system works in the same way as an air pump, except that the device extracts heat from the water flowing underground rather than from the air. In the summer, the heat pump cools a room by feeding the heat from the interior into the water, which is then pumped back into the earth. This helps to warm the ground and recharge the underground battery so there is enough energy available in the winter.
A networked geothermal system is extremely efficient, achieving a “coefficient of performance” (COP) of 6, meaning that for every unit of energy supplied, six units of heat are given off. In contrast, gas furnaces have a COP of less than 1.
An air source heat pump in the same area might need to run at 10 degrees outside, meaning it has to work harder to produce the same amount of heat. Accordingly, its COP of 2 or 3 would still be well above that of a gas furnace, but not close to the COP of 6 of geothermal.
“This means that you achieve greater efficiency with a geothermal system, which of course has a positive impact on operating costs,” says Jan Rosenow, who researches heat pumps at the Regulatory Assistance Project, a global energy NGO.
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This kind of efficiency will be critical if the U.S. is to wean itself off fossil fuels. The more gas furnaces are replaced with electric heat pumps, the more strain will be placed on the electric grid. But the more efficiently engineers can build heating and cooling systems, the less capacity utilities will have to add to the grid.
But why should a utility company invest in a new type of geothermal infrastructure when it already has a perfectly good underground infrastructure for delivering gas and is making good money from it?
The reality is that much of this gas infrastructure is not great and is downright dangerous if there is an explosive gas leak. A utility could use networked geothermal to simply replace water with gas. “If you’re in a situation where you’re going to have to upgrade or replace your line anyway, you might think about: Do I replace it instead with a line that doesn’t require fuel and draws energy naturally from the ground?” Braun said.
At the same time, utilities are under increasing pressure to phase out the use of natural gas: last year, New York became the first state to ban natural gas in most new buildings. In states such as California, Vermont and Colorado, utilities must also drastically reduce their overall carbon emissions. But they cannot do this if they continue to supply the same amount of natural gas.
Although networked geothermal energy is much more efficient than burning gas in a furnace, it is still unclear what impact this would have on the customer’s energy bill.
Because utilities are still experimenting with these systems, they have not yet agreed on a rate structure. One possibility would be a flat monthly rate for using the geothermal network, based on how much water a particular building needs to adequately heat and cool it.
Because it’s a relatively new technology, installation costs are still high: Eversource says its budget for the Framingham project for these 36 residential and commercial buildings was around $18 million (about £14 million). But as with any technology, costs will come down as the technology becomes more advanced.
If the U.S. really wants to reduce its carbon emissions, the homes of the future could forego natural gas and instead use a heat pump to use the air or the earth itself as a natural battery. The energy is there—it’s always been there—now it’s just a matter of harnessing its full potential.
