The massive “batteries” hidden under your feet

by The Insights

When the rainwater falls, it seeps into an aquifer, a layer of porous rock or loose material like sand or gravel. For thousands of years, man has been digging into these strips of liquid to draw drinking water. But interest is growing for another clever use of 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 harness the Earth’s insulating properties to store heat energy and transfer it to and from buildings above ground. The water temperature in an aquifer tends to remain fairly stable. This allows nearby structures to be heated and cooled 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 hot, one cold – that extend between the surface and the aquifer below. In the winter, you pump groundwater from a hot well at about 60 degrees Fahrenheit and pass it through a heat exchanger. Combined with a heat pump, this process extracts heat from the water table to keep the interior of structures warm.

Then you pump this now cooler groundwater into the second well. This gives you a pool of cold water – around 45 degrees F – to pump into. out summer to cool the buildings. “You heat groundwater by extracting heat from the building and injecting it directly into the other well,” says hydrogeologist Martin Bloemendal, who studies ATES at Delft University of Technology in the Netherlands. . “Then in the winter you mine from your hot well.” This process alternates indefinitely with the seasons as groundwater is reused and not consumed. The system could even take advantage of brackish or contaminated aquifers that cannot be tapped for drinking water.

Since water pumps and other equipment run on renewable energy, such as solar or wind power, this hyper-efficient energy storage would reduce the demand for fossil fuels and prevent a large amount of carbon from entering the environment. ‘atmosphere. Heating and cooling are responsible for a third of energy consumption in the United States and half of energy consumption in Europe. In fact, a new article in the journal Energy applied found that ATES could reduce the use of natural gas and electricity in heating and cooling US homes and businesses by 40%.

It’s a way to store huge amounts of energy for long periods of time – a kind of underground battery, always ready to be exploited. “In a local town, you can store heat and cold, and now you don’t have to pay for it later,” says Erick Burns, United States Geological Survey manager for the resource survey project. geothermal. (The USGS is part of a new international consortium studying geothermal energy at the city scale.) “The cool thing is that it doesn’t need essential minerals like batteries.”

The technique is ideal for large buildings, such as hospitals, or a group of buildings, such as on a college campus, because they can share a dedicated facility for the well and other equipment. It would be particularly effective in times of high demand on the network. In the United States, demand increases in late summer when people turn on their energy-guzzling air conditioners. ATES uses much less energy, which would lighten the load on the network and help avoid accidents. If these systems could not only run on solar or wind power, but be backed up by a distributed network of lithium-ion batteries, they could well withstand power outages.

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