Sustainable Energy

How compressing air in underground salt caverns could help the planet's shift to renewable energy

Key Points
  • Compressed air energy storage may not be that well known, but it has been around for decades. 
  • As the planet looks to transition to renewable sources of energy, compressed energy air storage could become increasingly important. 
Compressing air underground could help the planet shift to renewables

You may not have heard of it, but compressed air energy storage (CAES) has been around for decades. A CAES plant was built in Huntorf, Germany, in 1978, while another facility was developed in McIntosh, Alabama, in 1991. Both these developments use underground salt caverns to store air.

The Alabama project is operated by the PowerSouth Energy Cooperative. According to PowerSouth, the McIntosh facility can generate up to 110 megawatts of electrical power "within 14 minutes of startup during periods of peak demand."

The thinking behind CAES is relatively simple. U.K. based firm Storelectric, which is looking to develop CAES projects, breaks the process down into several parts.

Firstly, electricity from the grid or a renewable source is used to pressurize air. That compressed air is stored at a high-pressure underground.

When it's needed, the air is released, heated – in Alabama, natural gas is used to do this – and then expanded through a turbine to produce electricity. Storelectric says its process offers options that either negate or reduce the need for gas to re-heat air.

"We can store megawatts and gigawatts," Tallat Azad, the firm's CEO, told CNBC's Sustainable Energy.

"We can also… store long durations, meaning days, weeks, even possibly seasonal storage," he explained. "In addition, we have a proven concept: the first CAES plant in Germany has been running now for almost 40 years, it shows 99 percent reliability... with a CAES plant, the lights are just not going to go off."

For its part, Storelectric is aiming to make its technology energy efficient, cost-effective and flexible.

"You can't store the air underground at much above ambient temperature," Mark Howitt, CTO of the business, told CNBC. "But compressing the air heats it up to somewhere north of 650 centigrade," he added.

"Traditional compressed air energy storage puts that heat up a chimney, wastes it, and when you come to regenerate the electricity later they burn gas to put the heat back in so that you're not going to freeze your environment and equipment and destroy it all."

Storelectric's system, Howitt explained, extracts the heat of compression, stores it separately and then puts it back in during the expansion phase. This he said, "eliminates the gas burn and therefore eliminates the emissions."

With renewable sources of energy set to become increasingly important in the planet's energy transition away from fossil fuels, energy storage systems that are reliable and efficient will become prized assets.

Daniel Mansson is an associate professor at the KTH Royal Institute of Technology in Stockholm, Sweden. His research focuses on topics such as energy storage and smart grids.

Speaking to CNBC's Sustainable Energy, Mansson said that, given the right conditions, CAES could be "feasible both technologically and economically."

There were challenges, however. "Since it is a very site-specific technology then this is a problem – finding the right geological structures in the ground," Mansson said. Challenging perceptions was also an issue. "But also, I think it's a problem that people are a little bit afraid of it, they don't want it in their own backyard."