Could water be the energy source of tomorrow? It can be used to produce the wonder known as green hydrogen（[化学] 氢）, a fully climate neutral gas. But it's still time-consuming and expensive. That's why billions are being spent on research such as on this artificial sun. It has 149 lamps, each as powerful as a large cinema projector（[仪] 投影仪）. The idea is to produce fuel using only light and water and release no greenhouse gases.

  It's already being used in solar plants just like these. Will green hydrogen save the climate or is it just an environmental fad（时尚） as volatile（飞行的） as the gas itself? This submarine also relies on hydrogen. The U-35 is ready to head out. Six torpedo（鱼雷） tubes, 4,200 horsepower, maximum diving depth, deep, so deep, it's a secret. It's great that we can finally put out to sea.

  A crew of 29 women and men. Final orders have been given（做） and the crew goes aboard. Their home for the next few weeks is just 56 meters long and 7 meters wide. A tugboat pulls the U-35 away from the pier（码头） out to sea, where first a diesel（柴油机） engine is fired up. But when the boat goes underwater, a hydrogen fuel cell with a battery and electric motor kicks in. This technology was first used in Germany in 2005.

  The advantages are one. As we're moving, the craft is silent. We make（使） very little noise. In addition, we emit an extremely low level of heat and that makes us almost completely undetectable underwater（水下）. Ultra-quiet voyages with hydrogen. Secrets submerged missions of up to two weeks are now possible.

  Quieter, cooler（更冷的（cool的比较级）） and all without harmful exhaust gases. We mainly have water which is left over from the electrochemical reaction and we can use that as grey water if need be. The Navy now has six hydrogen-powered submarines. They were developed more for tactical（战术的） advantages than climate protection. But the German submarines prove hydrogen technology has potential and works. But can it succeed elsewhere?

  Ralf Ueswein from Grosszemann in Hesse loves technology. He collects Unimog's historic military vehicles. Although they still run on diesel, his SUV runs on hydrogen. Both fascinate him. For Ueswein, both represent progress. It's simple.

  The time has come. We are entering a different era. We grew up with this old technology. It got us this far and brought us the prosperity（繁荣） we have today. And now we have to take this old technology and develop new technology. Everything looks neat and tidy under the hood.

  Ueswein's car and the submarine have essentially the same technology and it works like this. Oxygen from outside air and hydrogen from the tank flow into the fuel cell where a chemical reaction occurs. This produces electricity for the motor that powers the car. Only pure, clean water vapour comes out of the exhaust. Even the air is filtered（滤过） in the process. It almost can't get any cleaner than this, at least while it's moving.

  The hydrogen in the tank is not yet climate neutral. Up until now, it's been made almost exclusively（排他地） from natural gas, which emits carbon dioxide during production. And Ueswein had to dig deep into his wallet for this car. Starting price, nearly 80,000 euros. As a luxury car, it's still just a niche（壁龛）. At the moment, only Hyundai and Toyota offer hydrogen vehicles and around 300 were registered for the first time in 2020.

  This makes Rauf Ueswein a bit of a trailblazer. So far, at least. Hydrogen is already used far more often than you might think. More and more cities are converting their bus fleets（舰队）. Fuel cells help the large vehicles last longer during the day than with just（ 公正的） batteries. And the first hydrogen trains have been rolling through Lower Saxony since 2018, replacing diesel locomotives（机车） in places where there are no overhead lines.

  And this is what the future could look like for trucks. A prototype（原型） from Daimler Trucks. Futuristic, but not at all science fiction. It could go into production as early as the mid-2020s with a range of up to 1,000 km. A current impossibility with just an electric motor and battery. Hydrogen is also taking to the skies.

  This may look like just any other plane, but it too is powered by a fuel cell. Manufacturer ZeroAvia developed the plane in just three years and tested it last fall in Cranfield, England. Hydrogen has arrived in aviation, in small aircraft for now, but the manufacturer is already thinking bigger. The objective of this program was to figure out and design all the components that would go into the larger powertrain. That is really our commercial target, about 600 kW in power, replacement（归还） for turbine engines of similar size that go into 10-20 seat aircraft. More power, more range.

  A solution for longer travel too. According to a concept from Airbus, yes, hydrogen powered jets could take off as early as 2035, but they would only be climate neutral if they used exclusively green hydrogen. These kinds of futuristic visions are not new. 150 years ago, also Jules（朱尔斯（男子名）） Verne predicted that water would be the energy of tomorrow. The concept of green hydrogen existed even back then. But first came the era of crude（天然的） oil, its cheap price giving it a clear advantage.

  Hydrogen was instead used to elevate（提升） the zeppelin, but in 1937 it became clear that hydrogen was not to be taken lightly. Leaks（漏洞） and a discharge likely triggered the catastrophe. To this day, hydrogen's image remains tarnished. Yet the element is present almost everywhere, never in its pure form here on Earth, but rather as a building block, for example in water. A renewable（能再生的） and inexhaustible（无穷无尽的） source. The problem?

  A lot of energy is needed to use it. Katarina Reiche is just one of the researchers dedicated to solving the problem. She is head of EON（永世） subsidiary（子公司） Vest energy and also chairwoman of the National Hydrogen（[化学] 氢） Council, a body of experts advising the German government on just how to implement its hydrogen strategy. Its goal? Using green hydrogen as one of the solutions to make Germany climate neutral by 2050, with federal backing（ 资助） of 9 billion euros. Green hydrogen is not yet produced in large quantities.

  It's simply too expensive to produce without large economies of scale. So to overcome this, what we need is both a robust（精力充沛的） market to sell hydrogen（[化学] 氢） and the ability to produce it in large enough quantities for such a market.