Their idea is to use microbes to convert excess renewable energy into methane, which could be burned as needed. In nature, the microorganism Methanococcus maripaludis consumes hydrogen and carbon dioxide and exudes methane. So, the researchers are using renewable energy-powered electrodes to split water and free its hydrogen atoms. Those hydrogen atoms are fed to the microbes, which then pull carbon dioxide from the air and release methane. The gas doesn't dissolve in water, so it can be captured and stored.
Then, at times of peak demand or when renewables aren't producing, the methane can be burned much like fossil fuel sources. It might seem backwards to turn renewable energy into methane, which releases carbon dioxide when it burns. But, this methane is produced by pulling carbon dioxide from the atmosphere, so the process is carbon-neutral. One significant advantage over battery storage systems, like Tesla's Powerpacks, is that the methane can be converted into electricity using existing infrastructure.
The researchers are still working on the technology, but they believe this can be cost effective at scale. The Department of Energy has already committed funding, and the Lawrence Livermore National Laboratory and Southern California Gas are helping to develop more efficient designs. It's hard to say if this can solve renewable energy's biggest flaw -- its storage problem -- but it's certainly a novel approach.