While temperatures above ground fluctuate throughout the year, the ground temperature is stable. “Every building sits on a thermal asset,” said Cameron Best of 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.”
The United States’ first networked geothermal neighborhood operated by a utility is 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.
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. You’re not putting energy in or out of the ground when you add all of the loads up.
47 percent of the United States’ natural gas customers have joined into an information-sharing coalition called the Utility Networked Geothermal Collaborative.
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. A heat pump in a geothermal system works 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. 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.
Utilities are under mounting pressure to phase out natural gas and are also staring at mandates to slash their overall carbon emissions, and they can’t do that if they keep delivering the same amount of natural gas.
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.
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.
You can read the original article at grist.org