Compressed-air technology is nothing new. During the late 19th and early 20th century compressed-air engines were used in trams in France and Switzerland, and to drive mining locomotives in Europe and the US. It is a simple technology where electricity is used to compress air into a tank. Expansion of the compressed air can be used to drive pistons and power a vehicle, doing away with the need for a tank of gasoline or an electric battery.

Compressed-air vehicles are clean; releasing no particulates or exhaust fumes, which could help to keep cities smog-free. If powered with renewable electricity, compressed-air cars could be considered carbon-free.

However, battery electric cars share the same advantages, so how do the two forms of technology square up? Felix Creutzig from the University of California, Berkeley, US, and colleagues have analysed the efficiency of compressed-air, battery-electric and conventional gasoline engines, as well as various hybrids.

They found that compared to battery electric cars, compressed-air vehicles are incredibly inefficient. Even when making optimistic assumptions about the development of the technology, Creutzig and co-workers found that compressed-air cars have a grid-to-wheel efficiency of less than 27%, compared with more than 75% for battery electric cars.

"Large amounts of energy are lost while compressing and expanding the air," explained Creutzig. What's more, vast storage tanks – double the size of batteries – are required for the compressed air and even then a compressed air car has a very small range. "In the near future distances beyond 100 km won't be seen, and even up to now, only a laughable range of less than 10 km has been demonstrated," Creutzig told environmentalresearchweb.

To make matters worse, the current electric-power sources in most countries mean that compressed-air cars would have a very high carbon footprint. Creutzig and his colleagues calculated that a compressed-air car using electricity produced from a coal-based power mix would have emissions of 330 g of carbon dioxide per kilometre, as compared with 120 g of carbon dioxide per kilometre for a similar-sized battery electric car or a conventional gasoline-fuelled car. When the electricity is sourced from greener forms of energy the emissions improve, but still can't compete with battery electric cars. Their findings are published in Environmental Research Letters.

However, compressed-air technology could come into its own when used in conjunction with gasoline. In a hybrid compressed-air car a small tank of compressed air, recharged via the car's braking mechanism, supports a conventional combustion engine. Creutzig and his colleagues explain that such a hybrid improves the overall car efficiency. "The advantages are that less power is lost when braking and a smaller motor size is required because the compressed air acts as a support – similar to a battery hybrid like the Toyota Prius," said Creutzig.

In hybrid form at least, compressed-air technology may yet come back into fashion again.