Colorless and odorless, hydrogen is an energy carrier. Indeed, hydrogen can both "store and deliver usable energy," according to the U.S. Department of Energy's (DOE) Office of Energy Efficiency and Renewable Energy.

The DOE adds that hydrogen does not usually "exist by itself in nature" and needs to be produced from compounds which contain it.



A number of sources — from fossil fuels and solar to geothermal — can produce hydrogen using a range of methods. These include biological processes, thermochemical processes and electrolytic processes, the DOE says.

When it comes to hydrogen, one of the big challenges is producing it from renewable sources. "Right now, most hydrogen is made from fossil fuels," Bruce Logan, from Pennsylvania State University, told CNBC's "Sustainable Energy".



"What we're doing is taking primarily natural gas, making that hydrogen and then using that hydrogen," Logan added. "So we have benefits from using the hydrogen but it still relies upon fossil fuels."



While it may rely on fossil fuels at the moment, hydrogen has a lot going for it from an environmental perspective. The European Commission, for example, has described it as an energy carrier with "great potential for clean, efficient power in stationary, portable and transport applications."



Penn State's Bruce Logan was equally optimistic. "If we start with water, and we electrolyse water and … use some renewable energy source to do that, then we can use the hydrogen and … end up with water when we're done," he said. "It's a water to water cycle and we don't need to burn all that air like we do when we burn a fossil fuel," he added.

The 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 "Sustainable Energy" earlier this year.

"So if you produce energy from wind or solar, turn it into electricity and then turn it into hydrogen, this is green hydrogen," he added.

The applications of hydrogen could be transformative. 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 which 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.



The project means that 10 hydrogen buses are ferrying residents around the city's streets as authorities look to reduce city center emissions and boost air quality.

"They're a very good fit for us because we have, like many other cities … air quality issues," Barney Crockett, the Lord Provost of Aberdeen, told CNBC's "Sustainable Energy".