<P> Another fuel is formic acid . The fuel is used by converting it first to hydrogen, and using that in a fuel cell . Formic acid is much more easy to store than hydrogen . </P> <P> HCNG (or H2CNG) is a mixture of compressed natural gas and 4 - 9 percent hydrogen by energy . </P> <P> Liquid nitrogen is another type of emissionless and efficient fuel . </P> <P> The air engine is an emission - free piston engine using compressed air as fuel . Unlike hydrogen, compressed air is about one - tenth as expensive as fossil fuel, making it an economically attractive alternative fuel . </P>

Describe one alternative energy source that would reduce