Let's be optimistic

In 'Air-brained scheme' (

) Chris Finn is unduly pessimistic about the possibilities of compressed air (CA) energy as the motive power for cars.

His point, regarding the energy available from a 13A power socket, is arithmetically correct — but why limit ourselves? A previous owner of my house installed a 45A power point, connected to a timer to use cheap overnight power for heating. If many people started charging their cars in this way the National Grid would have to be upgraded — but it isn't a physical barrier.

My objection is that CA is dismissed with assertions and no numbers to support them. My work has shown that, at a pressure of 30MPa/300bar, a 250litre carbon fibre pressure vessel would hold 38MJ. By using compact heat exchangers to reheat the cold air stream, before it enters the motor, this can be increased to 45MJ. Excluding the motor, the fully charged weight of the system would be about 130kg.

That would give a specific energy density of 90Wh/kg. Better than NiMh batteries at 80Wh/kg. This is miniscule by the standards of petrol at 40MJ/kg but an SI engined car averages 25 per cent efficiency while any CA motor can do 50 per cent and, I believe, can be optimised to 90 per cent. Good enough for a daily commute in a small car and, at 250 litres, it will fill the boot but not the whole car.

CA will not compete with petrol, but I am exploring a CA/SI engine hybrid that should yield efficiency benefits for both motive power systems.