Audi is building a plant in Germany that will use surplus power produced from renewable sources, such as wind energy generated when demand is low, to produce methane from water and carbon dioxide. The plant, which will use technology developed by Stuttgart-based SolarFuel, reportedly will produce enough methane to run 1,500 of the new natural-gas vehicles Audi is planning to start selling this year. To produce the methane, the company will utilize a combination of technologies: electrolysis, in which water is split into its hydrogen and oxygen components, and methanation, in which the hydrogen is combined with carbon from carbon dioxide to produce methane. While the combined process would normally be considered impractical because of inefficiencies, the availability of excess energy from renewable sources in Germany, which has increased from 150 gigawatt-hours per year to 1,000 in two years, makes the process economically feasible, according to a report in MIT’s Technology Review. “That’s electricity that we could use for nothing,” said SolarFuel’s Stephan Rieke.