A system that circulates underground water to heat and cool buildings could save 40% of the electricity and natural gas used for this purpose in the United States, according to a new study. The findings suggest that the method, known as aquifer thermal energy storage (ATES), could help decarbonize the energy system—and increase the resilience of the electrical grid to extreme weather.
A major challenge in decarbonizing the electrical grid lies in figuring out how to overcome the intermittent nature of solar and wind power. Most research has focused on electricity storage technologies such as batteries. But it turns out that energy for heating and cooling buildings can be stored not as electricity but as heat or cold itself, in underground aquifers.
That’s important because “heating and cooling demand plays a vital role when decarbonizing the building and industrial sectors which account more than 12% of total global emissions of CO2,” says study team member A.T.D. Perera, an energy systems researcher at Princeton University in New Jersey.
The basic idea behind ATES is this: during the summer, water is pumped up from underground aquifers, heated by the sun or with electricity from solar panels, then pumped back underground. In winter, the warm water is pumped back to the surface and used to help heat buildings, then the cooled water is stored back underground to help cool buildings in the summer. (The water can also be heated or cooled at any time of year with electricity from wind energy.)
The Earth basically functions like a giant insulated tumbler, keeping the water’s temperature pretty stable for months at a time. “ATES has the capability to operate as an energy storage for heating and cooling for a long duration, which is extremely challenging for most of the electricity storage technologies,” Perera says.
ATES is rare in the U.S. but is coming into increasing use elsewhere, such as in the Netherlands. The new study is one of the first to look at the potential role of the technology in the U.S. energy system.
Researchers constructed a computer simulation of a Chicago neighborhood composed of 58 single-family homes to explore the effect of adding ATES to the energy system. The simulation included a variety of energy sources (wind turbines, solar panels, a diesel generator, and a fossil fuel boiler) and storage options (battery banks as well as ATES).
ATES operates as a district heating system, Perera explains—one pair of hot and cold storage wells can serve a whole neighborhood.
The technology could aid integration of renewable power sources into the grid, the researchers report in the journal Applied Energy. It could also help prevent blackouts during extreme weather events by reducing electricity demand. (Pumping water up and down in the storage wells uses a lot less electricity than, say, air conditioning.)
The researchers also ran an analysis of how their simulated neighborhood’s energy demands for heating and cooling would shift with climate change. Climate change is likely to bring more extremes of both heat and cold, which could make ATES more necessary and also more efficient, with the climate providing additional “free” heating and cooling energy to be stored underground until it is needed.
ATES is an especially good technology for areas with big seasonal temperature fluctuations, but can work in most climates as long as there is a solar or wind power hookup. It doesn’t take up any space above ground, and is more efficient than geothermal heat pump systems. As a rule, ATES wells are too deep to interfere with drinking water supplies, but too shallow to raise concerns about earthquakes.
“The seismic concerns that you hear about with other subsurface applications are associated with processes taking place at much higher pressures and much deeper,” says study team member Peter Nico, a geoscientist at Lawrence Berkeley National Laboratory in Berkeley, California. “By definition you don’t really want to do ATES in a place where you have to push really hard to move the fluids around. You’d waste too much energy.”
The downside: right now ATES is about 15-20% more expensive than other energy storage approaches. But as with other green energy technologies, the researchers expect that the cost will come down fast as its use increases.
The researchers plan to study the technology’s potential elsewhere in the U.S., and analyze how it might integrate with other energy storage strategies, Perera says.
Source: Perera A.T.D. et al. “Enhancing flexibility for climate change using seasonal energy storage (aquifer thermal energy storage) in distributed energy systems.” Applied Energy 2023.