Killing two birds with one stone - a massive underground battery stores CO2 as well as renewable energy

Solving carbon and renewable energy storage in one go

A massive underground system designed by American researchers could provide a solution for carbon capture and storage as well as intermittency of renewables – two major challenges for grid decarbonisation.

The system involves injecting liquid CO2 into underground reservoirs. As the reservoirs would be created in sedimentary rock, the pressure of the CO2 would push brine from the rocks up production wells towards the surface. There, the brine could be heated using surplus renewable energy and injected back into the reservoirs to store thermal energy.

As a result, the pressure of the CO2 would increase, enabling the system to be charged or discharged depending on energy demand. At times when there would not be enough wind or solar power generation due to less than optimal weather conditions, the pressure would be released and used to power energy generating turbines. This way, the system would essentially function as a giant battery, storing energy from renewables for later use, virtually killing two birds with one stone.

"If you want to store the large quantities of renewable energy necessary to balance seasonal supply-demand mismatches and store it efficiently, we believe the best way to do that is underground," said Thomas Buscheck from the University of Ohio, who led the team behind the study published in the latest issue of the journal Mechanical Engineering.

"We believe this is a cost-effective way to store the energy long enough so it can be used later," he added.

Based on computer modelling results, the researchers believe the system could store up to 4 million tonnes of CO2 every year over a 30-year period - equivalent to the CO2 impact of a 600 megawatt coal plant.

"Storing such vast quantities of CO2 creates so much pressure. This is the biggest challenge for keeping it permanently underground, but it is manageable," Buscheck said. "To make sure we don't have too much pressure, we can divert some of the produced brine to generate water through desalination. Then, if we tap into the remaining pressure, we can recharge the system selectively and put energy into our storage system when there's excess and deliver it when it's needed."

The system, seven years in development, was unveiled shortly after global leaders had agreed on a new binding action plan to stop the progressing global warming and limit the average global temperature increase to less than 2 °C compared to pre-industrial levels.

To achieve the goal, the world would have to gradually abandon the use of fossil fuels. However, based on available data, even that would not be enough without also removing some of the climate warming carbon dioxide from the Earth’s atmosphere. 

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