Underground water could be a great source of renewable energy

A new study published in the journal Applied Energy suggests that the use of underground water, through Aquifer Thermal Energy Storage (ATES), could reduce natural gas and electricity consumption by 40 percent in the heating and cooling sector of the U.S. This approach also has the potential to prevent blackouts caused by high power demand during extreme weather events.

“We need storage to absorb the fluctuating energy from solar and wind, and most people are interested in batteries and other kinds of electrical storage. But we were wondering whether there’s any opportunity to use geothermal energy storage because heating and cooling is such a predominant part of the energy demand for buildings,” said study first author A.T.D Perera. “We found that, with ATES, a huge amount of energy can be stored, and it can be stored for a long period of time.”

ATES works by pumping water up from existing underground reservoirs and heating it at the surface in the summer with environmental heat or excess energy from solar, or any time of the year with wind. The water is then pumped back down underground where it can be used to heat buildings during the winter or to cool them during hot summer months.

Study co-author Peter Nico is the deputy director of the Energy Geosciences Division at Berkeley Lab and lead of the Resilient Energy, Water and Infrastructure Domain. He explained that the Earth acts as a good insulator, allowing the water to maintain its temperature even after months of storage. “It’s a way of storing energy as temperature underground,” said Nico.

The experts found that ATES is a promising option for energy storage in the heating and cooling sector, which, when combined with other technologies such as batteries, could help reduce reliance on fossil fuel-derived backup power and enable a fully renewable grid.

ATES systems are not yet widely used in the U.S. but are gaining recognition internationally, particularly in the Netherlands. One significant advantage of these systems is that they obtain “free” thermal energy from seasonal temperature changes, which can be supplemented by artificial heating and cooling generated by electricity.

Moreover, ATES systems are designed to avoid using critical drinking water resources, as the water used often comes from deeper aquifers than the drinking water supply, and they do not introduce any chemicals into the water.

The research team developed a case study to estimate the potential savings in energy consumption and cost of using ATES. The experts designed a computational model of a neighborhood in Chicago composed of 58 two-story, single-family residence buildings to evaluate the feasibility and climate resilience of the ATES system.

ATES involves pumping water into a porous rock formation or an underground aquifer, where it is stored until needed to either heat or cool a building. This system is more efficient and takes up less space than traditional geothermal heat pump systems, which rely on heat transfer with the underground earth soil.

To test the effectiveness of ATES, the team used future climate projections to understand how much of the neighborhood’s total energy budget is currently taken up by heating and cooling demands, and how these demands might change in the future. They then designed a microgrid simulation for the neighborhood that included renewable energy technologies and ATES. The results showed that adding ATES to the grid could reduce consumption of petroleum products by up to 40 percent.

While ATES would cost 15 to 20 percent more than existing energy storage technologies, the researchers believe that after a few years of developing ATES, the costs would be offset by its efficiency. “But, on the other hand, energy storage technologies are having sharp cost reductions, and after just a few years of developing ATES, we could easily break even. That’s why it’s quite important that we start to invest in this research and start building real-world prototype systems,” said study co-author Professor Rizwan-uddin Perera.

One major advantage of ATES is that it doesn’t take up as much space as above-ground tank-based water or ice storage systems. ATES is also more efficient and can scale up for large community cooling or heating.

Another benefit of ATES is its ability to become more efficient as weather becomes more extreme in the coming years due to climate change. The hotter summers and harsher winters predicted by the world’s leading climate models will have many downsides, but one upside is that they could supercharge the amount of free thermal energy that can be stored with ATES. 

“It’s making lemonade, right? If you’re going to have these extreme heat events, you might as well store some of that heat for when you have the extreme cold event,” said study co-author Ashlynn S. Stillwell.

Furthermore, ATES could make the future grid more resilient to outages caused by high power demands during heat waves – which happen quite often these days in many high-population U.S. areas, including Chicago – because ATES-driven cooling uses far less electricity than air conditioners. It only needs enough power to pump the water around.

“It’s very much a realistic thing to do and this work was really about showing its value and how the costs can be offset. This technology is ready to go, so to speak. We just need to do it,” said study co-author Anna G. Stefanopoulou.

Overall, ATES has the potential to revolutionize the way we store and use energy for heating and cooling in homes and businesses, potentially reducing energy consumption and contributing to the goal of a fully renewable grid.

This research was funded by the Department of Energy’s Geothermal Technologies Office. The team’s diverse expertise across the energy geosciences, climate science, and building science fields allowed them to put all the factors together into one model, making this research possible.

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