They compress a pellet of a type of hydrogen found in seawater by a factor of about 40, 000, like squashing a basketball to the size of a pea.

FORBES: A Nuclear Hybrid

Also a type of electric vehicle, a fuel-cell car takes hydrogen gas and converts it into electricity, while emitting only water vapor.

CNN: Hydrogen fuel-cell cars look to overtake electric autos

Finally, Declan looked at the BMW Hydrogen 7 - its only emission is water vapour but the technology for this type of green car is still being developed and currently, there are no models on sale.

BBC: NEWS | Programmes | Breakfast | Low carbon family: "Green" cars