Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Electric cars and fossil fuels.

with 3 comments

I’ve recently heard a bit of debate centred around the question of electric cars – in a society such as Australia, where essentially all electricity generation comes from extremely polluting, carbon dioxide emissions intensive coal-fired plants, is there any environmental benefit to using electric vehicles?

First and foremost, of course, petroleum is one of our most finite and limited energy resources – concerns about the limits to conventional petroleum extraction are oft discussed, and the geopolitical insecurity associated with dependence on foreign oil also needs to be taken into account.

Gasoline/petrol has an energy content (calorific heating value) of 44.4 MJ/kg, and a mass density of about 737 g/l, and therefore a heating value of 9.1 kWh per litre.

A typical small car uses about 7 litres/100 km at an average of 90 km/h in highway use, and about 8.5 litres/100 km in city use.

In highway use, the engine uses about 1 litre of fuel – 9.1 kWh – to generate 1.7 kWh at the wheels – 18.7% efficiency. In city use, due to frequent braking, this overall efficiency in energy consumption is greatly reduced – let’s say 5-10%.

Averaging out highway and city use, let’s take – just rough approximations, remember – an overall efficiency of 12%, giving 1.48 kWh generated at the wheels per kilogram of fuel.

With an electric car, assuming 90% efficiency in the charger, and 90% efficiency in the control electronics, combined with regenerative braking, then we need to input 1.83 kWh of electricity from the grid, to get 1.48 kWh at the wheels. For 1 kg CO2 equivalent per kWh of electricity generated – this seemingly very high figure is typical for Australia – we have 1.83 kg of CO2 generation, for the equivalent of the use of 1 kilogram of fuel in a gasoline engine.

If we assume that the fuel is a mixture of only octane and its isomers (a rough assumption, but it will do), then the complete combustion of 1 kg must generate 3.08 kg of carbon dioxide – clearly the electric car is significantly ahead in this regard.

Electric cars are a technology we should at least give full, rational scientific thought to. Whilst the burning of coal to generate 80% of our  electricity needs is clearly unacceptable and cannot continue, as we replace the coal-fired base of the grid with  cleaner technology, maybe electric vehicles are potentially a technology where, in theory at least, benefits are to be had, especially if combined with a scalable, high-capacity, strong power grid, powered by clean nuclear electricity.

Written by Luke Weston

December 17, 2007 at 5:33 am

3 Responses

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