When the rainwater falls gets soaked in a aquifer, a layer of porous rock, or loose materials such as sand or gravel. For thousands of years, humans have been digging into these bands of liquid to get drinkable water. But interest is growing in another smart use for these underground pools: aquifer thermal energy storage, or ATES.
A battery contains energy to be used later. Aquifers can be harnessed to do something similar: they can exploit the insulating properties of the Earth to conserve heat energy and transfer it to and from buildings above ground. The temperature of the water in an aquifer tends to remain fairly stable. This provides a way to heat and cool nearby structures with energy stored in water, rather than burning natural gas in furnaces or harnessing fossil fuel-derived electricity to run air conditioners.
ATES systems consist of two separate wells, one warm and one cold, running between the surface and the aquifer below. In the winter, it pumps groundwater from a warm well that’s about 60 degrees Fahrenheit and runs it through a heat exchanger. Combined with a heat pump, this process extracts heat from groundwater to keep the interiors of structures warm.
You then pump that now cooler groundwater into the second well. This gives you a pool of cold water, around 45 degrees F, to pump outside summer to cool the buildings. “You heat the groundwater by extracting the heat from the building and directly injecting it into the other well,” says hydrogeologist Martin Bloemendal, who studies ATES at Delft University of Technology in the Netherlands. “Then in winter you draw from your warm well.” This process alternates indefinitely as the seasons progress because the groundwater is reused, not consumed. The system could even take advantage of brackish or contaminated aquifers that cannot be tapped for drinking water.
Because water pumps and other equipment run on renewable energy, such as solar or wind, this hyper-efficient energy storage would reduce demand for fossil fuels and prevent a large amount of carbon from entering the atmosphere. Heating and cooling are responsible for a third of US energy consumption, and half of the energy consumption in Europe. In fact, a new paper In the diary Applied Energy found that ATES could reduce the use of natural gas and electricity for heating and cooling in US homes and businesses by 40 percent.
It’s a way of storing massive amounts of energy for long periods of time: a kind of underground battery, always ready to explode. “In a local city, you can store heat and cold, and now you don’t have to pay for it later,” says Erick Burns, United States Geological Survey leader for the Geothermal Resources Research Project. (The USGS is part of a new international consortium that’s investigating geothermal energy on a city scale). “The good thing about this is that you don’t need critical minerals, like batteries.”
The technique is ideal for large buildings, such as hospitals, or a group of buildings, such as a university campus, because they can share a dedicated facility for the well and other equipment. It would be particularly effective during times of high demand on the network. In the US, demand increases on late summer evenings when people turn on your energy-hungry air conditioning units. ATES uses much less power, which would lighten the load on the network and help prevent accidents. If these systems could not only run on solar or wind power, but were also backed by a distributed grid of lithium ion batteriesThey can be resistant to power outages total.