Green Cars Part 4- Compressed Air

Well the answer is yes and no. The cars certainly have their limitations. Range and power are obvious hurdles since the air-tank stores the equivalent of about a gallon of gasoline, the advertised range for an MDI air car is 125km (78 miles, possibly optimistic?), though the addition of conventional fuels may change this dramatically to over nearly 1000 miles (I'd be very surprised if that were true but time will tell) and apparently boosting the top speed from 35mph (55km/h) to 100mph (160km/h). According to this interview it uses the liquid fuel to heat the air 'increasing the volume' to extend the range. Having air stored at 300 Bar (4350psi) under your seat is a little disconcerting but the experts assure us that carbon fibre will do the job, air isn't flammable on its own so there is no fire risk. Compressed air cars have the same chicken and egg infrastructure problem as hydrogen cars, since standard compressors at service stations don't go anywhere near the required psi and the on-board compressor takes approximately 4 hours to charge. When the air is compressed at the compressing station the process also heats the air as a byproduct. This can effect range since when the air cools in the tank the pressure will also drop. More energy can either be used to cool the air before it goes into the car or the heat can be recycled eg. for domestic hot water. The cooling effect of the engine can cause ice formation especially at lower ambient temperatures and requires addition fuel for heating the engine. But efficiency wise compressed air cars perform better than any of the alternatives we have seen in this series. According to the efcf, the cars can have a theoretical compression efficiency of 46% and a tank to wheel efficiency of up to 84%, the report concludes that given 100MJ of energy a hydrogen FCEV could travel 42km, an air car 46 and a Li-ion BEV an impressive 133km.

 

Compared to the other alternatives we have looked at, the compressed air car certainly has some impressive advantages. It is obviously superior to liquid fossil fuels since it can be cheaper to build and has the potential to be renewable and emission free. They don't have the wider environmental and humanitarian consequences of biofuel vehicles. Provided the range and power issues are resolved, it betters hydrogen on both efficiency and production cost. Its only rival in the alternative fuel game is the battery electric car, which trumps it on efficiency but not on production costs and battery life. The widespread adoption of compressed air cars I think will depend a great deal  on public preference rather than the technology itself (unless it manages to exceed battery electric efficiency with clever thermodynamic tricks, or battery cost and life can be drastically reduced). Will consumers choose lower vehicle production and maintenance costs despite the increase in infrastructure and energy usage? Or perhaps will they decide that the higher vehicle costs of battery electrics are worth the price for the higher efficiency? Personally I think there is probably room for both as the technologies develop, but we will perhaps be able to make a more informed decision after we look at battery electric cars next week.