Air-brained scheme

David Cutter (Letters, 5 May) suggests the future might lie with vehicles driven by compressed air, which could be topped up 'at home or at the office using electricity, or for £2 at the filling station'. There are two main reasons why this is impracticable.

First, Lord Robens (one time chairman of the National Coal Board) once succinctly described electricity as 'coal by wire'. In other words, electricity is not a source of energy, but merely a means of transmitting it. Hence, any vehicle propelled by compressed air still has to use a primary source of energy to provide that air. (In general terms, this also applies to hydrogen — a point which is often conveniently overlooked by those proposing a 'hydrogen economy').

Second, the amount of energy capable of being taken from a 13A power socket overnight (say 35kWh) is roughly equal to that contained in one gallon of petrol. So a vehicle charged with compressed air all through the night could only have a range of about 30-40 miles, and the air receiver needed to contain this energy at practical safe pressures would be rather larger than the vehicle itself.

It also follows that a call at a filling station for a top-up of air would add to journey times. The alternative (exchanging an empty receiver for a full one) is in my view equally impracticable.

I remember buses being run on gas during the last war. They had a large balloon type holder on the roof as their 'fuel tank'. Perhaps there is a lesson there?

Chris Finn, by email