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A borderline case for carbon saving

Electric cars are undoubtedly beneficial for the local environment, especially in towns and cities, but whether they are beneficial, environmentally, for an economy as a whole depends on the source of their energy. The electricity used to charge the batteries may be relatively carbon free, as in mainly nuclear-powered France, or it may be generated from power stations that are predominantly coal-fired, as in some economies in eastern Europe.

For the most part, however, electricity is generated from a mixture of coal, gas, nuclear and renewables. The carbon dioxide "attached" to this mix in Britain and Denmark is low enough for the replacement of fuel-burning cars with electric vehicles to make environmental sense, but only just. In Germany and the United States it is a borderline case.

Medium-sized family cars with a conventional petrol (gasoline) engine emit around 16 kilograms of carbon dioxide every 100 kilometres. That is the target that electric cars must beat to become C02 neutral, or C02 positive. The energy that is needed to propel a car for that distance is about 20 kWh (main article).

That energy, however, needs to be fed to the vehicle batteries, with associated conversion losses both in charging the battery and in drawing energy from the battery, via the electric motor, to the wheels. The overall efficiency of these processes is about 80%, so 25 kWh must be drawn from the mains supply. Allowing for losses in transmission and distribution, 25 kWh at a consumer outlet to propel an electric car 100 kilometres demands around 28 kWh of generation.

If electric cars are to contribute fewer greenhouse gas emissions overall than petrol driven cars, then those 28 kWh must be responsible for less than 16 kg of carbon dioxide. In other words, the overall emissions per unit of electricity must be less than about 570g/kWh. France beats this target easily, with emissions around 56g/kWh, the UK also gets under the hurdle (about 450 g/kWh), as does Denmark (around 550 g/kWh). Germany and the United States, however, with emissions around 600g/kWh, do not meet the CO2 target.

Electric cars, however, would prevent the emission of various other noxious gases from petrol driven cars, such as carbon monoxide and oxides of nitrogen. On the other hand, a further problem with electric vehicles is the energy-intensive nature of battery production and this should be taken into account to establish whether there is an overall environmental benefit to massive investment in developing them and the associated battery-charging infrastructure.

Provided the extra demand for electricity created by electrical vehicles is provided by renewable energy, overall emissions per unit of generation will come down as the proportion of renewables in the supply mix increases. With a higher electricity demand, the need for constraints on wind plant (when output exceeds system demand) is likely to be reduced. Potential conflicts between wind and nuclear -- with both competing to meet base load demand (Windpower Monthly, June 2005) -- will also be deferred.

Comparisons of running costs between petrol and electric vehicles tend to be distorted by the high taxes on petrol. The 25 kWh needed to move 100 km would, at domestic electricity rates in Europe, cost around EUR 4. The cost of petrol for the same distance would be over double in Britain, whereas the much lower rates of taxation in the United States mean that the cost would be about the same.

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