Hybrid-Electric Aircraft In-Flight Battery Charging: Where does the energy come from?

In some Hybrid-Electric aircraft studies, an energy recovery system is included to recharge the battery and further improve the overall aircraft efficiency and mitigate charging requirements on the ground. Given the principle of energy conservation, any recovered energy must come from some other energy source, but where?

The only potentially feasible flight phase for significant energy recovery is descent where the aircraft is reducing both potential and kinetic energy. In both take-off and climb, potential and kinetic energy are being added to the aircraft, so any harvesting will detract from field and climb performance. At cruise, the aircraft is operating at close to peak aerodynamic and engine efficiency - there is no 'waste' energy to harvest.

However, any energy 'recovered' in descent cannot be potential or kinetic energy: a hybrid aircraft at a given weight descending from 39,000ft/0.8Mn to 1500ft/0.4Mn will lose the same amount of kinetic and potential energy whether the recovery system is active or inactive, even though the energy recovery system will undoubtedly do its job and generates some electrical energy.

Hence, any energy recovery must come from the fuel energy, either directly or indirectly, as it is the only other significant energy source on board the aircraft. Where else can it realistically come from?

To confirm and understand the mechanisms of this, additional energy transfers must be considered and how these interact with each other as well as potential and kinetic energy. Chemical energy, from either kerosene or batteries, converted to thrust adds energy to the aircraft at all times when the aircraft is moving and the engines are in operation (excluding reverse thrust operation and some high altitude high speed idle cases).

The atmosphere is the energy sink: energy is imparted into the air to generate both lift and drag whenever the aircraft moves relative to the atmosphere. This 'Aero energy' is transferred the atmosphere in the form of downwash, friction, vortices and shockwaves.

Consider constant altitude and speed cruising flight where potential and kinetic energy levels are fixed. An energy flow is still required from the fuel or battery to replace the energy loss due to the lift and drag.

In descent, the fuel energy flow is greatly reduced while 'aero energy' remains. Hence, a transfer of the aircraft potential and kinetic energy to the 'aero energy' is necessary to maintain a desired descent speed profile.

Hybrid electric energy recovery systems are often described with some form of 'windmill' energy recovery system in descent. Fundamentally, a bladed shaft is immersed into the airflow such that the drag on the angled blades rotate a shaft that in turn is mechanically linked to an electrical generator to convert the mechanical energy to electrical energy.

A good example of this is the Ram Air Turbine that is standard on many conventional transport aircraft for emergency use when the engine generators become inactive. However, the powers involved with hybrid electric system require a much larger bladed shaft or multiple bladed shafts, e.g. the propellers or the fan systems.

Consequently, any windmill energy recovery will increase to the drag term in the 'aero energy' that will in turn will cause faster reductions in potential and kinetic energy, i.e. faster descent rates.

Given a fixed potential and kinetic energy reduction in descent, the increased descent rates will shorten the descent time and more importantly the descent distance (with a fixed descent speed schedule). However, the distance between the origin and destination of the flight is fixed, so any reduction in descent distance must be compensated by a similar increase in the cruise distance. The distance transferred from descent to cruise will be operated at substantially increased fuel flows (a factor of 4-5).

The environmental impact of this extra cruise fuel, burnt at Stratospheric altitudes, must be offset against the benefits of any energy harvested in descent to determine the net benefit of energy recovery, whether it is emissions, economic or reduced time between flights.

To conclude, to achieve any energy harvesting in descent, additional fuel must first be consumed in a longer cruise that I suspect that it is more beneficial to eliminate any in-flight charging capability and increase the ground charging accordingly.

Thank you for reading this note, I would very much welcome any comments or debate on the issue or flaws in my analysis.