Could aquifers store renewable thermal energy?

Decarbonizing the energy sector requires ramping up power generation from renewable sources. However, increasing renewable energy generation poses some challenges, like mismatches between production and demand. Output from renewables varies seasonally and annually due to insolation differences and trends in weather, which means there may be periods of over- and undergeneration.

Seasonal heating and cooling—usually the largest energy expenses in households—don’t align often with renewable energy generation patterns, says Amarasinghage T. Perera, an associate research scholar in the Andlinger Center for Energy and Environment at Princeton University. For instance, there is higher heating demand in the winter, but more renewable energy generation during the summer. In such cases, it’s important to store additional energy in the summer to cater to the winter heating demand, he adds. This explains why long-term energy storage is needed to support renewable technologies.

According to a recent study published in Applied Energy, underground water has the potential for storing much-needed renewable energy. This approach, called aquifer thermal energy storage (ATES), uses naturally occurring groundwater or aquifers for long-term storage of thermal energy that can be used to assist the heating and cooling of buildings, says Perera, who was involved in the study.

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In an ATES system, there are two wells connected to the same groundwater reservoir. During the summer, cold groundwater is pumped up to provide cooling, warmed at the surface, and then stored. During the winter, the opposite happens—the warm groundwater is pumped up to provide heating, cooled at the surface, and then stored. The cycle repeats seasonally.

Energy storage is often discussed in relation to decarbonizing the transportation sector by replacing internal combustion engine vehicles with those supported by battery and hydrogen storage. However, for grid storage, the materials required to store electric charge in batteries have a high energy cost, while hydrogen storage results in significant energy losses. Perera says more research funding can help identify the broader potential of thermal energy storage technologies.

“Compared to conventional groundwater heat pumps, the extraction of heated or cooled groundwater which was previously injected into the subsurface enables a more efficient operation,” says Ruben Stemmle, a researcher from the Karlsruhe Institute of Technology (KIT)’s Institute of Applied Geosciences in Germany who was not involved in the study. ATES systems can also store excess heat from industrial processes, combined heat and power plants, or solar thermal energy. Overall, it helps bridge the seasonal mismatch between the demand and availability of thermal energy, he adds.

Long-term seasonal storage and demand-driven utilization of previously unused heat sources, like waste heat or excess solar thermal energy, can promote the decarbonization of the heating and cooling sector, as well as reduce primary energy consumption, says Stemmle.

According to the study, ATES can improve the flexibility of the energy system, allowing it to withstand fluctuations in renewable energy demand and generation from future climate variations. It could make urban energy infrastructure more resilient by preventing additional burdens on the grid during hot or cold months.

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ATES has very high storage capacities due to large volumes of groundwater available in many areas like major groundwater basins and complex hydrological structures. This enables ATES application for district heating and cooling or large building complexes with high energy demands, says Stemmle. It can significantly reduce the use of fossil fuels compared to conventional types of heating and cooling, he adds, like gas boilers and compression chillers.

Currently, there are over 3,000 ATES systems in the Netherlands alone. Some are also found in Sweden, Denmark, and Belgium. They aren’t as widely used in the US yet, but adding ATES to the grid could reduce the consumption of petroleum products by up to 40 percent.

To increase ATES deployment, policymakers can support funding programs for ATES systems and related technologies, like heat pumps and heating grids, says Stemmle. He emphasizes the importance of decreasing market barriers as well, which can be achieved by establishing a simple and rapid permitting procedure and a uniform regulatory framework governing ATES operations. The deployment of such thermal energy storage systems could help achieve a more climate change-resilient grid in the future.