Green promise

Tesla Model S is a perfect fit with intermittent renewables and offers the prospect of zero-emissions personal transport

Terence Rook asks important questions on the relative efficiencies of burning fuel in a power station to produce electricity for use in an electric car compared with just burning it in the car itself. 

The following are official figures for the Chevy Volt plug-in electric hybrid: Gasoline only - 37 MPG (US gallons which are 20% smaller than ours), Electric only - 98 MPG. The lower number is a modest improvement over a conventional car of similar size but the higher one is a huge improvement.

I understand that coal fired power stations operate at around 40% efficiency while ICEs in cars average around 14 to 20% rather than the suggested 25% - in any case electric wins out here. ICEs require gear boxes and differentials and these have similar efficiencies to battery/mechanical power conversion systems - both around 90% so not much difference here. Lugging heavy batteries around, in addition to the vehicle weight, does not hugely affect the efficiency of electric vehicles since regenerative breaking recovers around 90% of the kinetic energy rather than dumping it as heat - electric wins out big time on this issue.

The Tesla Model S (large saloon car, 89 MPG) with 85 Kw.hr battery has an official range of 265 miles but a father and son have recently achieved 423.5 miles on one charge. With this sort of range, you would need to plug-in only about once-a-week but would do so whenever cheap off-peak power was available. This is a perfect fit with intermittent renewables and thus offers the prospect of zero-emissions personal transport. This 'promise' is as green as it gets.

Stuart Kirby, Derby

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