Given the weight of a fully-loaded passenger-jet, it seems highly improbable that an apu is going to deliver a sufficient supply of electrons to spark-up those little 'leccy motors.
You can't beat the laws of physics and some VERY substantial cabling and control-gear would be required,,,,as other posters have stated, that's a lot of extra work for the retraction-hydraulics,-though i suppose a dirty great counterbalance-spring could be devised
So, we have to find space for all this weighty tackle which has to be flexible-enough to go upand down with the gear, without fatigueing or breaking it's insulation.
no-one considered hydraulic motors? most tracked excavators use this propulsion-method,'cos just like an aircraft, they have a lot of hydraulic rams to power, so the pipework is already close-by and the motor-weight penalty is a lot less.
I fail to see any advantage in using a hybrid tug....conversion and efficiency losses would be large,-we're back to direct mechanical or hydraulic drive.
A good idea to drive the wheels direct,-using a jet pushing against air is extremely inefficient.....the layout of a tug could possibly be re-thought......it could be much lighter if the nosewheel was "carried" thus using the aircraft's own weight to give the tug traction.
Lithium-iron and li-poly batteries are many times more energy-dense than lead-acid, but their number of operating-cycles is quite limited and,though they can be rapidly charged,this carries a spontaneous-combustion risk.
I'm very sceptical of these professed savings.-sounds like a smart-alec sales pitch.it's also worth considering Cattletruck's observation.......the donks need running-up and t's & p's stabilising before flight anyway, so it does make sense to use at least part of it to manoeuver the aeroplane towards the T/O position.
Very simplistic original pitch made, those are just a few thoughts which have surfaced so far..