Hybrid-Electric Aircraft: 101

Hybrid-Electric Aircraft are receiving extensive attention at the moment and are viewed by many as the next great thing to reduce aviation kerosene fuel use and reduce aviation emissions. This article looks at the fundamentals of how this technology generates these benefits and where the challenges and opportunities are.

To summarise, Hybrid-Electric aircraft can generate global environmental benefits in 2 principal ways:

i) Reduced emissions by 'off-shoring' the energy generation from burning kerosene (or similar) to a 'cleaner' electrical power generating system. This must also take into account any additional inefficiencies and increases to aircraft system mass.

ii) Reduced aircraft total energy consumption that are a result of the Hybrid-Electric propulsion system opening up the airframe and engine design space to provide weight and efficiency benefits.

'Off -Shoring' power-generation

With no changes to airframe or engine efficiencies or aircraft weight, the total energy to complete a specified mission will be unchanged, irrespective of fuel or electrical energy sources.

Studies using my RawAvCon design suite with Hybrid-Electric propulsion system applied to a single-aisle aircraft resulted in significant fuel load reductions on shorter mission lengths where reasonable, but challenging specific energy and specific power assumptions were included.

One of the fundamental keys to reduced aviation related emissions through Hybrid-Electric technology is that the ground based power generation is 'cleaner' than burning fuel in flight. There are probably further Hybrid-Electric technology benefits for moving emissions from higher altitudes to the ground.

Thus, a global emissions improvement requires readily available and sufficient 'clean' renewable or nuclear power generation and transmission capacity. Using Coal fired power stations to generate the additional electrical energy is likely to generate increased global aviation related emissions without Carbon Capture technology.

An even broader perspective could be considered if the total energy to process and deliver the kerosene or electrical energy to the aircraft is considered. This would clearly favour renewable energies, although there are clearly challenges related to supply reliability (i.e. on a still, dark night) and also the required power levels. This raises another issue, the migration of societal energy usage from oil based fuels (e.g. cars, potential Hybrid-Electric aircraft, etc) will require increased power generation capacity as the existing capacity is sized to current needs.

Hybrid Car Comparison - There are similarities and differences to hybrid cars. Similar attributes are an 'off-shoring' of emissions from the car (in a congested city centre) to the power station although, again, the emissions are greatly reduced if nuclear of renewable power generation is used.

Although any aircraft emissions improvements around airports would be welcome (and possible), most of the aircraft's emissions are away from people.

A major difference with cars is the usage patterns. Hybrid car benefits are greatest in an urban environment where continuous cycles of braking and acceleration are ideal for energy recovery in the braking system that can then assist the ensuing acceleration. There is little wasted energy in flight: just spoiler deployment (a few minutes per flight) and braking on the runway (30 seconds per flight) If in-flight generation is used additional fuel must be burned and any associated benefits of 'off-shoring' emissions are reduced accordingly - see my previous note on this.

A Car can even turn off almost all power usage when stationary in traffic, an aircraft must still burn fuel in a holding pattern to maintain lift.

Hybrid-Electric Aircraft: Increased Total Energy Requirement

Increased Aircraft Weight - Electrical power systems, (batteries, power management, transmissions and motors) are much heavier than the liquid fuel they replace even with aggressive specific energy (Wh/kg) and specific power (W/kg) assumptions - increased 'Wh' and 'W' are beneficial - increased 'kg' is a penalty.

Increasing aircraft weight due to the electrical propulsion system means that a hybrid-electric concept without significant airframe or engine efficiency and weight improvements actually needs more energy for a given flight.

This is compounded by a Hybrid-Electric powered aircraft burning off less fuel weight through the mission making its average weight even higher with an associated increase in its average propulsive energy requirement.

This mitigates the fuel burn benefits of Hybrid-Electric technology but, as noted previously, my RAWAvCon modelling has forecast significant fuel burn benefits even with significant aircraft weight increases.

Hybrid Car Comparison - This is a similar issue to hybrid cars: the Hybrid VW Golf GTE is quoted at about ~200kg (~15%) heavier than the equivalent GTI model and will therefore require more total energy to complete a long motorway journey.

Increased weight is greater issues on aircraft as increased car weights do not increase the car drag - it simply requires more energy to accelerate but increases braking energy to be harvested. Increased aircraft weight requires increased lift and lift-induced drag resulting in increased energy consumption.

Aircraft TLARs

Hybrid-Electric Aircraft Landing Weights will be higher than non-hybrid due to reasons described above. There is little scope to increase approach speeds at the Maximum Landing Weights for the largest family members of existing single-aisle aircraft families. These approach speeds tend to be close to, or at the 140kt maximum to qualify for ICAO's approach category C definition (operational benefits). Increased approach speeds also lead to longer landing rolls as aircraft deceleration rates on the runway tend to be similar between aircraft.

To maintain approach speeds at the non-hybrid levels, a larger wing and/or more powerful high lift system are required, both of which increase weight and therefore even more energy would be required to complete a mission.

The larger wing may also require a larger engine to meet altitude and Take-Off performance requirements. Any increase in engine size would add further weight and total energy requirements.

Airframe and Engine Design Space

The above sections all describe issues that increase the total energy that is likely to be required by a Hybrid-Electric aircraft where airframe and engine weight and efficiencies are unchanged.

However, Hybrid-Electric propulsion system may offer new architectural features to the aircraft systems and these could provide opportunities to open the airframe and engine design space. If these opportunities result in improved airframe and/or engine efficiencies or lower weight, the total energy required by the aircraft to will reduce.

If these improvements also permit reductions in the critical sizing cases for the wing, engine and/or systems, even greater reductions in the total energy requirement can be achieved.

I have identified a number of opportunities that could reduce the total energy consumption of the aircraft (offsetting the increases described above) and consequently the global emissions benefits. These will be explored and quantified in the coming month. This will include considering whether consuming additional fuel in flight to generate electrical power for later use has any benefits.