RAT is only used when sh*t hits the fan. Even then, it can help you power some hydraulics / electrical, not “store” energy for further flight.
The OP asked - in a low fuel situation, can the energy spent on a climb get effectively recovered - and the answer is not really. We convert as much as we can into unpowered (low-powered) descent. But once you are at a spot where you make a final decision to land or not, you are by design low and slow - and all that energy you had 15m ago is gone.
If you need to keep flying, those engines need to spool back up. And that takes fuel.
Isn't it when air hits the fan, technically?
(Sorry.)
Such aircraft do exist. For example, the Pipistrel Velis Electro trainer. And more recently, the Rhyxeon RX4E became the first electric aircraft to be type-certified for commercial passenger operations.
It's likely that we'll see many more electric fan aircraft in the coming years/decades, whether powered by batteries and/or hydrogen fuel cells, or hybrids with both conventional turbofan and electric propulsion in order to improve efficiency and environmental performance.
While it’s a real plane, I think we can both agree this is more of a prototype than what the general public would consider to be a passenger plane.
Yes, energy recovery is possible. It’s not happening currently in commercial aviation.
An electric fan aircraft absolutely can recharge it's batteries on descent. The fans simply act as turbines, creating drag to slow the aircraft and electricity to charge the batteries. Large commercial airliners already have a small turbine that works this way, the Ram Air Turbine (RAT) which is used to generate electrical power in emergencies.