...AND CHEAPER...:

A battery holds energy to be used later. Aquifers can be leveraged to do something similar: They can exploit the insulating properties of the Earth to conserve thermal energy and transfer it to and from buildings above ground. The temperature of water in an aquifer tends to stay fairly stable. This provides a way to heat and cool nearby structures with energy stored in water, instead of burning natural gas in furnaces or tapping into fossil-fuel-derived electricity to run air conditioners.

ATES systems consist of two separate wells--one warm, one cold--that run between the surface and the aquifer below. In the winter, you pump groundwater up from a warm well that's around 60 degrees Fahrenheit, and run it through a heat exchanger. Combined with a heat pump, this process extracts heat from the groundwater to keep the structures' interiors warm. 

Then you pump that now-cooler groundwater down into the second well. This gives you a cold pool of water--around 45 degrees F--to pump out of in the summer to chill buildings. "You heat up the groundwater by taking out the heat from the building and directly inject it into the other well," says hydrogeologist Martin Bloemendal, who studies ATES at Delft University of Technology in the Netherlands. "Then in winter, you extract from your warm well." This process alternates indefinitely as the seasons roll on because the groundwater is reused, not consumed. The system could even take advantage of brackish or contaminated aquifers that can't be tapped for drinking water. 

Because the water pumps and other equipment are run on renewable power, like solar or wind, this hyper-efficient energy storage would lower fossil fuel demand and keep a lot of carbon from entering the atmosphere.

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