More On Compressed Air

Here's a followup to the earlier entry on compressed air cars.

It turns out lots of people are looking into this. Ford and UCLA have been looking at compressed air hybrid vehicles. Here, a conventional internal combustion engine is modified (through camless valves) so some or all of its cylinders can be made to act as a compressor. This enables engine braking to convert vehicle kinetic energy into energy of compressed air, which is stored in a tank. When extra power is needed, the valves are again tweaked, allowing stored air into the cylinder. This enables the engine to operate without having to (at that moment) expend energy in a compression stroke.

Prof. Tsao at UCLA has an abstract on their work here (go down to D.19). The relevant paragraph:

The latest development in camless engine technology makes compressed air hybrid vehicle feasible. By variable valve control, the engine is configured to work in 4 different modes. During deceleration, the engine absorbs the energy of vehicle motion by operating as a compressor, charging an air reservoir with compressed air. This is the compressed braking mode (CB). Air motor (AM) mode and air-power-assisted (APA) mode allow the engine to recover the stored energy during acceleration. The other times the engine works in conventional unthrottled (CU) mode. We conducted a EPA cycle simulation and showed a 52.3% fuel consumption reduction, compared to conventional engine.

See also this page.

One issue I see with this idea is adiabatic heating of the air. If it's too hot, that complicates the design of the storage tank and reduces its capacity; also, if the air must be cooled it will lose energy. You could get around that by interposing a thermal store between the engine and tank; air flowing through the store would be cooled on the way to the tank and reheated on the way back to the engine (an example of a countercurrent exchange system.)

Storing energy as sensible heat in a material can actually be very effective. Active Power, an Austin company, sells compressed air UPS systems. Their TACAS system extends the energy density by electrically heating a block of steel. This block is used to heat the stored air before it is run through a turboexpander to drive a generator.

I wonder how expensive a small diurnal compressed air storage system for homes or small businesses would be. Many utilities have time-sensitive electricity rates, with off-peak rates a factor of five or more cheaper than peak rates. Not only could such a system produce electricity at peak times, it could also be used directly for air conditioning -- the air coming out of the expander will be very cold. And, if the owner also had a compressed air vehicle, the stationary tank could be used to at least partially 'refuel' the vehicle in a few minutes.