I was initially sceptical about the potential of using compressed air as an energy
storage medium for cars. The reason becomes obvious when the energy density is compared
to other energy sources. I have compiled a spreadsheet here which calculates the
tank volume required for 1 hours use of compressed air at 300 MPa for any variable
For a typical car travelling at highway speeds using 20 kW of power it would require
0.8 m3 of compressed air at 300 Mpa for one 1 hours travel, assuming 50% efficiency
of converting air pressure to mechanical drive. However, as stressed in the report
Environmental Transport systems. Cars are typically of the order of 10 m3 in volume,
so why should an extra 8% of volume be so restrictive? Even if space is a problem,
can’t the length of the vehicle simply be lengthened?
“There are some advantages to the air powered vehicle, as compared to an electric,
particularly that an expensive (lithium) or potentially polluting (lead, Cadmium)
large battery pack is not required. I suspect that air vehicles may have a future.”
In addition, there are resource issues with these metals, especially if a considerable
proportion of the worlds vehicle fleet used batteries for energy storage.
So let’s consider the fundamental thermodynamic and design considerations for compressed
air powered cars.
Compressing air heats it and expanding it cools it. To retain this energy some means
of heat storage medium might be required. ‘Trickle charging’ by slowly compressing
the air under near constant temperature would minimise the energy storage losses.
However, under practical conditions, rapid expansion occurs when work is extracted
for propulsion purposes during driving. In practice compressed air engines need
heat exchangers in order to avoid excessively low temperatures and maintain pressures
during the expansion process. To gain the maximum work from the expansion the process
may have to be carried out in various stages with the air passing through high pressure
cylinders or turbines before passing to low ones.
One possibility which occurs to me here is the use of a latent thermal store between
the compressed air tank and engine. The heat generated during compression could then
be stored then released to the air during expansion when the work in needed. This
could also act as an additional energy store, in addition to the compressed air through
the use of electrical heaters.
In theory an compressed air engine should be relatively efficient since propulsion
is obtained through direct pressure rather than indirectly through the combustion
of fuel and air as in a conventional engine, which results in significant loss of
heat energy. I envisage a situation where a compressed air car could additionally
inject Biofuel into the compressed airstream as a range extender, thereby making
the vehicle into a hybrid. This might only be used for the few long journeys or
emergencies when enough compressed air hasn’t been pumped into the tank for the full
journey. The Tata air car uses a similar principle although this seems to require
an external heat (or Stirling) engine using gasoline to achieve significant range
or speed. However, I suspect this relies on petrol for all but the shortest and
low speed journeys and the compressed air is being emphasised for public relations
No doubt there are significant technical challenges before compressed air can be
used routinely as the main source of energy for powering vehicles; however, just
like in the case of battery cars, their range could be further enhanced by a ferrying
concept such as the ELECAT. Therefore, I don’t really see any fundamental problem
which prevents compressed air being used as an energy store just like batteries.
With regard to primary energy use and carbon emissions all same assumptions would
apply to these as to batteries assuming the overall efficiency of the process was
equivalent. Unfortunately, there isn’t enough information available on this technology
at present to make an informed judgement on this issue.