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Tackling climate change with technology
Hydrogen can be burned in combustion engines or used to drive fuel cells that combine it with oxygen to produce electricity.
It's clean - the only waste product is pure water - and it's the most abundant element in the universe.
Hydrogen production is energy-intensive, often using fossil fuels or biomass. Flammable nature raises storage and transport risks.
It's too early to give an accurate estimate of cost. The US National Research Council says $55bn needs to be spent on R&D.
Transport can run on electricity stored in batteries, or in next-generation storage devices called supercapacitors.
Mechanically simple, and newer electric motors very efficient. Existing power grid can be used as basis for charging infrastructure.
Much depends on how electricity is produced. From a carbon-intensive source, overall emissions may be higher than petrol.
Far cheaper than petrol per mile but cost of battery makes cars more expensive. Also requires entirely new infrastructure.
Road, Rail, Ships, Air
Fuels made from plant matter or organic waste. Bioethanol, from sugar-rich crops such as maize, used in place of petrol.
Biofuel blends can be used in existing cars. Second generation fuels will make use of waste biomass such as seeds or husks.
Growing and cropping biofuels burns carbon - maybe more than they save. Grown on arable land that could be used to grow food.
Cost comparable to petrol - sometimes cheaper, depending on oil price. Effect on food prices needs to be factored in.
Alternatives include the burning or pyrolysis (heating) of municipal waste. Pyrolysis results in a combustible gas or oil, and more heat.
Many alternative fuels' greatest advantage is that they utilise something that would otherwise go to landfill.
A dense waste product may result. Amount of CO2 saved varies, depending on method of combustion and type of fuel used.
Waste fuel technology is at an early stage of development, but experts say it could be competitive with other fuels in 10 years.
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