I don't know how really this works but according to engadget.com

Were this technology ramped up for commercial or municipal use, it could provide a viable alternative for utility-scale batteries, like the one's Tesla sells. That is, in times of excess energy production from renewable resources, rather than store that electricity in a giant battery, we could instead convert it to ethanol and use that to power generators when renewable sources aren't producing. Plus it would be carbon neutral since the carbon dioxide generated from burning the ethanol would be reclaimed by the catalytic process.