Nature: In an industrial warehouse on the outskirts of Stuttgart, the sooty past is shaking hands with Germany's green-energy future. In one corner sits a relic destined for a museum: a cast-iron engine as big as a bus, which was used until the early 1970s to compress coal gas for lighting, cooking and heating. Nearby, a gleaming network of stainless-steel tanks uses electricity to create methane out of water and carbon dioxide.
This power-to-gas (P2G) pilot plant is the largest of its kind in the world,...