In the last couple years an old technology, flywheels, has been used in motor sports. In Formula 1 and 24h of Le mans. The energy density with KERS is much higher than it is with batteries, allowing it to be used. The flywheel spins in a vacuum meaning there is minimal loss of energy.
So is there an application in aviation? Airplane engines most of the time spin at a relatively constant speed. But they are designed for the much higher thrust needed during take-off and subsequent climb.
The energy used to obtain a given height and speed is then burned off without it being re-used.
I can see 3 potential uses, but not convinced they all make sense yet:
A) During take-off, add energy from KERS to the engine, so the engine does not have to work as hard, or so we may even be able to use smaller turbines.
B) Allow engines to spool up quicker when power is re-applied or needed . This could allow idling to being more fuel efficient as the engine does not have to spin as fast during idle.
C) Power other airplane systems rather than using bleed air, or electricity generators(787).
The KERS system could be powered by the equivalent of braking in aviation This is now using flaps only, but it is possible to come up with a way to reuse that energy (like a RAT turbine). Or it could take surplus rotational energy from the engine at idle.
So does this make sense? Or is there a reason why it wouldn't work?