Biofuels, electricity, plain old gasoline: there are many different ways of fueling cars, buses and airplanes.
As technology develops at a rapid pace and the transport industry also changes, hydrogen could soon play a significant role.
Here, Sustainable Energy takes a look at its properties, applications and the role it might have in the years ahead.
A chemical element — the most abundant in the universe — hydrogen's symbol in the periodic table is H. Colorless and odorless, it is an energy carrier and able, as the U.S. Department of Energy's (DOE) office of Energy Efficiency and Renewable Energy says, to both "store and deliver usable energy."
The DOE adds that hydrogen does not usually "exist by itself in nature" and needs to be produced from compounds that contain it.
A number of sources — from fossil fuels and solar to geothermal — can produce hydrogen using a number of methods. These include biological processes, thermochemical processes and electrolytic processes, the DOE says.
The European Commission has described hydrogen as an energy carrier with "great potential for clean, efficient power in stationary, portable and transport applications."
The DOE says that a fuel cell — which turns the chemical energy in hydrogen into electricity — combined with an Electric Motor is "two to three times more efficient" than an internal combustion engine that runs on gasoline.
Hydrogen is already being used in vehicles around the world. To give just one example, a fleet of hydrogen fuel cell buses is currently in operation in the Scottish city of Aberdeen.
In 2016, European airline easyJet revealed plans for a zero emissions hydrogen fuel system. The concept was based around the idea of stowing a hydrogen fuel cell in the hold of the aircraft.
Like the DOE, the European Commission says that when combined with fuel cells, hydrogen can boost energy efficiency in transport. In this way, it can help contribute to mitigating climate change, "especially when produced by renewable primary energy sources."
This notion of "green hydrogen" is an intriguing one, but what exactly is it?
"It's hydrogen stemming from green sources," Jorgo Chatzimarkakis, secretary general of Hydrogen Europe, told CNBC's Sustainable Energy.
"So if you produce energy from wind or solar, turn it into electricity and then turn it into hydrogen, this is green hydrogen," he said.
In Switzerland, H2 Energy and IBAarau have been working on the installation of a hydrogen generation plant at a hydropower facility in Aarau, in the north of the country.
With air pollution in urban areas an increasingly serious concern, there is one clear benefit of using hydrogen. The DOE's Alternative Fuels Data Center says that the only emissions from hydrogen-powered fuel cell electric vehicles are water vapor and warm air.
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