and has the potential to provide a  clean, near-infinite source of energy.

It's already used in oil refining  and fertiliser manufacture.

But despite efforts, and a lot of  hype, it has never taken off as a fuel.

That could be about to change, as the need  to tackle climate change becomes more acute.

Analysts agree that much of the global  economy can be decarbonised in the near term  

by electrifying things like cars and heating,  and by using renewables to generate power.

But something else will be needed to clean  up other carbon-intensive sectors such as  

aviation, shipping, long-haul trucking and heavy  industry like concrete and steel manufacture.

Enter hydrogen.

Hydrogen gas does not emit carbon when  burnt and has been touted by some as  

the ideal replacement for fossil  fuels in these problem sectors.

Despite the abundance of hydrogen as an  element, its atoms do not exist by themselves.

They need to be separated from other elements  in order to be used to deliver energy.

This can be done in a number of different ways,  each colour coded according to emissions produced.

At the dirtier end of scale is  brown hydrogen, made from coal.

Grey hydrogen is made from natural gas, a  process that still creates a lot of carbon waste.

The cleaner blue variety is also made  from natural gas, but with carbon capture  

technology that stores the CO2, rather  than spilling it out into the atmosphere.

Green hydrogen tops the eco-friendly table.

It's produced by the electrolysis  of water using renewable energy.

An electric current produced from wind or solar  splits water molecules into hydrogen and oxygen.

Green hydrogen is attracting the  attention of investors and policymakers  

as the world scrambles to cut emissions.

But there is a catch.

The costs of green hydrogen remain  too high at around $5 per kg.

To be competitive with fossil fuels, this  figure needs to fall below $1 per kg.

Analysts say that could happen within  the next few decades, or even sooner,  

as agencies like the US Energy Department  get behind research and development.

If it does take off, it's  estimated that the hydrogen  

economy could be worth as much as $2.5tn by 2050.