The world's first hydrogen-powered boat has docked in London, the last stop of its European tour.

The 30-metre Energy Observer acts a 'floating lab' for testing green maritime technology.

It harnesses three renewable energies – solar, wind and hydro – to propel the boat and charge its batteries.

The vessel has sailed around 18,000 nautical miles since leaving Saint-Malo, France in 2017, visiting 25 countries, propelled solely by electricity.

"Millions of boats are transporting everything from what we're wearing, to what we eat, to what we put over our heads. Everything is transported via maritime transport," says crew member Amelie Conty. "And so it's really important that we find a solution to limit their impact on the planet."

The 30-metre Energy Observer doesn't emit CO2, fine particles or even make noise. It is covered with 168 square metres of solar panels and its 12-metre high "Oceanwings" harness wind power, while propellers harness wave power to generate electricity.

But, most notably, Energy Observer is claimed to be the first vessel in the world that's capable of producing its own hydrogen on board, via seawater electrolysis, when renewable energy isn't available.

The vessel harnesses hydrogen power by pumping sea water on board. After removing salt and minerals, it goes through an electrolyser, which breaks H2O into its simple elements – oxygen and hydrogen. The hydrogen is stored for when renewable energy is not available. An on board fuel cell uses the hydrogen to generate electric and thermal power.

The crew of the Energy Observer recently viewed the impact of climate change first hand, sailing up to the island of Spitsbergen in Norway's Svalbard archipelago.

"We were able to observe the problem of global warming through the melting glaciers," says captain Victorien Erussard. "Simple as that. We could actually see, from photos that had been taken of the glaciers, say, 100 years ago, and what's there today."

But at a price of around €6 million, the technology isn't yet affordable for a great deal of the maritime industry. That will eventually change, claims Erussard.

"It's like all new technologies. Televisions, at the beginning, were really expensive. But then, as more and more people bought televisions, they would become a lot cheaper with each passing year," he says.

"With this it's the same. Hydrogen technology at the moment is between two and four times more expensive than thermal propulsion, than thermal motors. But I think in a few years we'll reduce that price."

Energy Observer was created and is financed by a group of public and private stakeholders, including auto maker Toyota.