Storing Energy Cryogenically

Researchers at the University of Leeds in the UK and the Chinese Academy of Sciences say they may have found a solution to dealing with short-lived power demand spikes—and it could be more environmentally friendly and halve the fuel needed when compared with gas-fired generation. Noting that gas-fired generators typically used to feed peaking demand are notoriously inefficient, expensive to run, and sit idle for long periods of time, the scientists have instead proposed use of an integrated energy storage system using natural gas and liquid nitrogen and oxygen—or “cryogen.”

“This integrated system is truly novel,” said University of Leeds Professor of Engineering Yulong Ding, who led the research and co-wrote an article on the system that appeared in the July issue of International Journal of Energy Research. The key idea is to use excess electricity to run a unit producing cryogen. At times of peak demand, the nitrogen would be boiled using heat from the environment and waste heat from the power plant. The hot nitrogen gas would then be used to drive a turbine or engine, generating “top-up” electricity.

Meanwhile, the oxygen would be fed to the combustor to mix with the natural gas before it is burned. “Burning natural gas in pure oxygen, rather than air, makes the combustion process more efficient and produces less nitrogen oxide,” the researchers said in a statement this August. “Instead, this ‘oxy-fuel’ combustion method produces a concentrated stream of carbon dioxide that can be removed easily in solid form as dry ice.”

A thermodynamic analysis under baseline conditions of 1 kg/s natural gas, combustor operating pressure of 8 bar, and cryogen topping pressure of 100 bar found that the integrated system’s exergy efficiency is as high as 64% and its corresponding electricity storage efficiency is about 54%. The results also showed that it could reduce the amount of fuel needed to cater to peak demand by as much as 50%, the researchers claim. But, that’s just “on paper,” admits Professor Ding. “We now need to test the system in practice,” he said.

Still, commercialization won’t be too far away, he said. “The components for the system are already mature technologies so we can have them and I think the key is integrating. So if there are power companies interested in going forward, it could be a five- to eight-year time scheme.”

—Sonal Patel is POWER’s senior writer.