Bjorn Fehrm
January 24, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.
We have covered why the progress of battery-based aircraft is slow and also described what to expect at the end of this decade and the beginning of next.
Now, we look at hybrids, an inherently more complex design. Upstarts are changing to hybrids after realizing that battery-only aircraft will not have useful range this side of 2030.
Figure 1. The Heart Aerospace ES-30 has passed the phases in the article. Source: Heart Aerospace.
Hybrid eAircraft and eVTOLs
In last week’s Corner, we examined the operational utility of battery-only eAircraft and eVTOLs. The designs were restricted to VFR trainer missions near the airfield, nine-place commute hops of 100nm, and eVTOLs to the 10-minute Airport to City Center shuttles in fair-weather environments.
As upstarts realize there is no way around these problems this side of 2030, which usually takes about two to three years (we call it the battery-electric to hybrid incubation time), the next step is to look at adding a thermal engine + generator “Range Extender” to the design.
This is the path of almost all battery-eletric upstarts, so also Heart Aerospace, Figure 1. It takes a year or two for the upstart to realize that one range extender reduces the design to a single point of failure, so two are needed.
Dual Range Extenders
Once it has applied two range extenders and accommodated the doubled systems of a complete battery-electric propulsion system combined with a dual hydrocarbon-fueled gas turbine system, which includes dual generators with charging regulator electronics, the aircraft’s complexity, weight, and cost become problematic.
When the aircraft efficiency calculations then show the complex, heavy aircraft has higher sales and operating costs than what it replaces, despite a low electric energy cost (which is not that low once the airport charging infrastructure is amortized over the energy price), the search for a better alternative starts.
The reason is that the battery-electric eAircraft or eVTOL is a serial hybrid (Figure 2), which is inefficient by design.
Figure 2. The different battery and hybrid propulsion concepts. Source: Leeham Co.
The path from the Jet-fueled gas turbine over the generator and battery charger/inverter into the battery and then back through the propulsion inverter and motor to the propeller has a path loss of ~20%.
So, the range extender cannot really be used as a “range extender,” as the CO2 emission values and total efficiency tanks.
Why can’t the gas turbine generator be coupled directly to the electric motor? It can, but then it must have the same size, including the path losses as the cruise propulsion system, and you are better off coupling it directly to the propeller. This is what you do in the next concept.
Parallel hybrid
So, after a “Range Extender” hybrid incubation time of about two years, the design team looks at a parallel hybrid, Figure 2. A parallel hybrid’s design efficiency can be higher than that of a serial hybrid. But the complexity increases, forcing development and production costs above the aircraft it shall replace.
As long as the parallel hydrocarbon-fueled part is used to provide mission reserves, the aircraft has the low emissions of a battery-electric aircraft. However, the gains in this mode are marginal compared to a simpler battery-electric concept.
The reason is, once again, the batteries. Theoretically, you can now empty them on the mission part to 0%, as you don’t rely on them for reserves. But batteries shall not be emptied below 25% State Of Charge (SOC), or you need to replace them several times a year, to the tune of around $75,000 for a battery modules renewal for a 30-seater like in Figure 1.
So, regardless of how hard the upstarts calculate different mission profiles with the gas turbine part on or off, the operational economics in real-world use, where the gas turbines have to be on and to consume fuel (thus generating emissions), the complex and heavy hybrid aircraft fail to be the low emission deal. And its operating economics fails to convince.
The turn to hydrogen.
The above trip through the realities of the hybrid for eAircraft and eVTOLs has made many projects move on to hydrogen-fueled propulsion concepts after yet another incubation time. We will look at these in the next Corner.
Note: So why do hybrids work for cars and not eAircraft or eVTOLs? In the car case, you have energy recovery when braking for the corner or stop light. There is no energy recovery phase in a flight mission.
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