Many believe green hydrogen could provide a miracle solution for countries around the world seeking to decarbonize their economies. But the technology is still in its infancy. Generating sufficient quantities of green hydrogen would require a lot more renewable energy than is currently available. Right now, almost all hydrogen is produced using natural gas in a process that generates large amounts of carbon dioxide. Green hydrogen, by contrast, is climate neutral. It's derived using renewable energy. The principle itself is not new but has, at yet, only found limited usage. Engineers at the German Aerospace Center are now working with the world's largest artificial sun to try to produce hydrogen without any electricity at all, using only light. If they can succeed, it would allow large-scale production of this valuable gas in countries that receive a lot of sunshine. Hydrogen is already being used as a fuel for buses, trains and cars, with hydrogen-powered planes due to follow shortly. Hydrogen is even the fuel of choice for space rockets, and German submarines glide along almost in silence thanks to hydrogen fuel cells. Manufacturers of airplanes, trucks, and even steel are investing millions in the technology, hoping that hydrogen will be the go-to fuel of a climate-neutral future. But critics warn of major challenges ahead, saying billions stand to be wasted.