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Transportation is essential to our way of life in all but the most primitive of societies. For millennia much of it was powered by the renewable energies of horses on land and sails on water, and when motor vehicles became available they were celebrated as a much cleaner alternative that did not leave excrement on the roads. Today however a quarter of the world’s energy-related carbon dioxide (CO₂) emissions are from the transport sector and 95% of it is from oil[1].

There is an urgent need to decarbonise the transport sector, but in many ways the challenges in doing so are even greater than for the electricity grid. The energy must be provided in a way that can be accessible to a moving vehicle – in some limited cases such as trains this can be through electrical wires, but in most cases the energy is stored on board in some way. This adds weight and volume to the vehicle, requiring more energy to carry the energy supply, and creates the need for frequent and convenient refuelling.

In the early days of motor vehicles, various energy carriers were tried including batteries, eventually settling on petrol and Diesel to be burned in internal combustion engines. These fuels have clear advantages in the amount of energy they contain per volume and per weight, the relative simplicity of the tank needed to carry the liquid, and the speed of refuelling. Drivers have become accustomed to these advantages and have an expectation that switching to a different fuel will not cause them any inconveniences they do not currently experience.

The main technologies in development for carbon-free transportation are biofuels, batteries and hydrogen. Biofuels were the first of the three to be widely used, usually blended with regular petrol or Diesel. They include a range of products such as ethanol for spark ignition engines, biodiesel for compression ignition engines and renewable fuels that are chemically similar to petroleum based fuels without the unwanted components such as sulphur and aromatic compounds. In principle biofuels can be carbon neutral, but their agriculture often has negative environmental impacts and may compete with food production. Considerable work is ongoing to develop crops that minimise these negative externalities. Biofuels are handled essentially the same as petroleum based fuels, and since they are burnt in an internal combustion engine they produce similar tailpipe emissions such as oxides of nitrogen, ozone, particulates and unburnt hydrocarbons. Much of this can be removed with exhaust treatment systems – to the extent that today’s cars have cleaner exhaust than intake air in many cities in developing countries – but the removal is never 100%.

Battery electric vehicles (BEVs) have been growing their market share considerably in recent years as advances in all the associated technologies have enabled them to become more convenient for drivers. Whilst early BEVs had barely 100km range, there are models now on the market that have ranges similar to petrol and Diesel vehicles. The long time required to charge them is starting to be mitigated by faster charging batteries and the installation of infrastructure to provide fast charging on the road. If the electricity used to recharge the batteries comes from renewable sources, there are no carbon emissions from their operation. BEVs also have the potential to reduce the total energy consumption due to the higher efficiency of converting electricity into motion. Figure 1 compares the distance an average car can travel using ethanol or biodiesel in an internal combustion engine versus batteries with an electric motor, with respect to the quantity of stored energy equivalent to a litre of gasoline (Lge). The higher efficiency translates to less energy consumed, but the total range of BEVs is limited because petrol and Diesel have on the order of 10 times the energy per kilogram and 30 times the energy per litre compared to lithium ion batteries.