What do you think about constructing a hybrid car that consists of a hydraulic accumulator, connected to a pneumatic drive motor, with an energy battery electric motor keeping the accumulator charged?
I was thinking about that because apparently current electrical regenerative braking tech is only 30% efficient, but the pneumatic recapture is about 70% efficient.
The accumulator could act sort of like a cheap ultracapacitor, allowing the batteries to be optimized for energy storage rather than power burst AND storage. And this would not interfere with the eventual electrification of transportation, as other types of pneumatic hybrids might.
The hydraulic systems have high efficiency but a relatively low specific energy. This makes them very good for short start-stop cycles (e.g. garbage trucks) but not so good for typical passenger car cycles; you would need a very large and heavy accumulator (relative to the vehicle) to absorb all the braking energy.
I went searching for the 30% figure, because I was wondering about the methodology and the split with the friction brakes. The one hit I found was a pointer to a page which is now 404. Regardless, larger batteries (e.g. PHEV-class) will have a higher regeneration efficiency because they can absorb more power.
I wonder how much energy could be stored in a compressed air tank of reasonable size. Those tanks are pretty heavy, though as you point out. Perhaps a carbonfiber tank? Crash protection would also be a worry.
There have been several compressed-air cars, including one now being promoted in India. The problem with using air for regenerative braking is that it’s hard to package all the filtration, intercooling, drying and such gear that stationary high-pressure compressors have, and the tanks for low-pressure would be mighty big.