February 17, 2023, ©. Leeham News: Last week, we summarized that SAF is the short-term solution for GreenHouse Gas (GHG) reduction for Air Transport, and hydrogen is the longer-term solution for up to medium-haul flights.
What about battery and hybrid aircraft? It’s the go-to solution for ground transport (except for long-haul trucks, which are going hydrogen, Figure 1)?
Let’s first be clear about why battery and hybrid vehicles work for ground transport. Our standard cars are wonders in energy inefficiency! It comes from their engines being too large for efficient operation (they are sized for our egos, not for optimal efficiency), and they are forced to work 90% of their time in inefficient stop-go traffic between stoplights, junctions, and traffic jams.
The effect is the standard car has an energy efficiency of around 5%. It‘s not that difficult to improve on a propulsion system that wastes 95% of the energy in the fuel! As ground transport is also weight-insensitive, batteries work (a battery car weighs 50% more than a gas car).
Aircraft, on the contrary, are wonders of energy efficiency and are super sensitive to weight!
A turboprop commuter uses 25% of the energy in Jet fuel or 400% more than our cars. Single aisles jets better that. The engines for the A320neo or 737 MAX are at 40% average energy efficiency in the typical domestic flight. That’s 700% more efficient than the car!
I have met too many industry people who opine that electric aircraft must work. When asked, they have no clue about efficiency fundamentals. Most don’t even understand how energy efficiency is defined. But they are firm in their belief: “It works for cars, so it must work for aircraft. You are underestimating the pace of electric developments!”
Let’s agree: When the bar is 400% to 700% higher, you should stop assuming you can transpose technology from one industry to another!
When you understand these fundamentals, you start looking for the kingpin problem for electric aircraft. Electric motors are 95% efficient versus the gas turbines’ 25%-55% efficiency.
There is ONE fundamental problem that kills electric aircraft, the battery!
An aircraft-certifiable battery system is 70 times heavier than the equivalent Jet fuel energy today. It will improve to 40 times by 2030.
All experts on aircraft batteries agree on this, and it’s not changing because we have the typical yearly news about a new fantastic battery chemistry that works in the lab. For a battery to qualify for a car or aircraft application, it must be industry-grade and fully mature. The next step in batteries for the transport industry is solid-electrolyte batteries. The car industry is investing billions in making these mature and mass-producible by the end of this decade. With these, by 2030, we will get to a 40 times difference in energy efficiency.
That the battery spoils the day for battery-based aircraft is easy to understand. But why do they also destroy the story for hybrids?
Because the only high-efficiency energy the hybrid has is the energy tanked from the grid. And this can only be stored in batteries. Any energy tanked to the battery generated by the onboard gas turbine loses 10% each way, from the generator to the battery and then from the battery to the motor that drives the propeller or fan. It means you charge the battery with energy which is 20% down in efficiency from the gas turbine efficiencies we discussed.
If you drive the aircraft propeller/fan with the gas turbine, you are using 25% to 40% of the energy in the fuel for domestic flights. Drive it through a hybrid cycle with energy charged in the air, and you have a further 20% loss of efficiency.
Projects give up on serial hybrids early (their efficiency problem is evident). It takes a bit longer to realize what the deal breaker is for the parallel hybrid. It’s once again the battery. When you’ve done the math, including the hazard and failure mode analysis, you conclude that a parallel hybrid saves no fuel with today’s and tomorrow’s batteries. It just adds complexity and weight.
We can conclude the series in three elevator pitches: