8.14 - Comparing hydrogen and petrol as fuels
Compare hydrogen and petrol by considering emissions, how the fuel is made, storage and safety. You will use those comparisons to make a judgement for a particular driving situation.
Emissions at the car
Petrol is a mixture of hydrocarbons. Complete combustion forms carbon dioxide and water. Carbon dioxide is a greenhouse gas. Incomplete combustion can also produce toxic carbon monoxide and harmful particulates; high engine temperatures allow nitrogen oxides to form from air.
Hydrogen contains no carbon. Its reaction with oxygen forms water:
A hydrogen fuel cell uses this reaction to generate electricity for a motor. It emits water without the harmful exhaust pollutants from burning petrol. A hydrogen combustion engine is different: burning hydrogen in air at high temperature can still form nitrogen oxides. Neither system produces carbon dioxide from the hydrogen itself.
Emissions before the car is used
Petrol is obtained from crude oil, a finite fossil resource. Extraction, refining and transport also require energy.
Hydrogen is an energy carrier: energy is used to make it before it releases useful energy in the car. It can be produced by electrolysis of water or from natural gas. Electrolysis powered by renewable electricity can give low production emissions. Producing hydrogen from natural gas without capturing carbon dioxide releases carbon dioxide; methane can also leak from the gas supply chain.
A car's water-only exhaust therefore does not establish that its fuel has zero overall emissions. Include production, compression and transport when comparing the fuels. Hydrogen made using renewable energy can be replenished without consuming a fossil fuel feedstock; hydrogen made from natural gas still depends on a non-renewable source.
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