Bus passengers as far apart as Beijing, Barcelona and Aberdeen may be unaware they have been participating for years in trials to test the feasibility of hydrogen fuel cells in public transport.

The technology, which offers a cleaner and more efficient source of power, is now spreading to shipping as ports, just like congested cities, apply pressure to cut vessel emissions.

More distant but also feasible is upscaling fuel cells from the current focus on auxiliaries to larger MW systems for the actual propulsion of seagoing tonnage.