September 22, 2017, ©. Leeham Co: After 12 articles about electric aircraft, it’s time to wrap up. We will go through what we have learned and discuss future developments.
Our designs were aimed for the next decade and the result was sobering. Electric aircraft have important challenges to traverse. As had electric cars, and they have turned the corner.
We learned that batteries as energy stores leave a lot to be desired. Here a summary:
The inefficiencies make the battery virtually impossible as an energy store for longer range aircraft. In addition, the battery has four times higher maintenance costs than gas turbines; it needs replacement after 1,500 charge cycles.
But batteries will improve. The car industry is turning electric with force, and it needs efficient and lower cost batteries. This will result in batteries with improved characteristics.
The problem is that we are 40 times behind and batteries might improve two to three times in specific energy over the next decade. We need at least 10 times to make longer range battery driven aircraft practical.
Fuel cells hold the promise to solve the problem. But the promise has been there for the last 30 years and we are not closer to a breakthrough. Former US Energy Secretary Dr. Steven Chu summarized the problem: “We need four miracles to happen (for fuel cells to become practical for transportation) and Saints only need three:”
The problem with fuel cells is that they need a new eco system. Aviation cannot be the driving force to solve all these problems and the automotive world isn’t close to a fuel cell driven car.
Hybrids in cars are successful because the electric motor driving the wheels can be reversed into an energy-recovering generator when the car decelerates for the stoplight. The wasted energy in a car’s journey is recovered. Electric cars can therefore compete on efficiency, if not range.
The airliner flight cycle doesn’t have such waste parts in its trajectory. The descent is done with the engines at idle, transforming the aircraft’s height energy to forward motion. The stored height energy is compensating for aircraft drag on the way down.
Comparing a gas turbine-driven airliner with a hybrid, where the gas turbine drives a generator which in turn drives a motor-fan combination, we found the hybrid is not competitive.
This is no wonder. We add complexity and weight, and unlike the car, we have no energy gains.
The peak power take-off phase could be where the battery complements the gas turbine, so it could be sized for climb/cruise. But we find the battery can’t be the power surge back-up, it’s just too inefficient.
Instead, we go for a small, light (and rather inefficient) gas turbine, an APU, to complement at power surges and for redundancy. The inefficiency is no problem, it’s only active a fraction of the mission time.
So now we have a solution, but it’s not a good one. The main power chain of gas turbine-generator-inverter-motor-fan is more complex, heavier and more inefficient than today’s gas turbine-fan combination.
If the battery was an efficient energy store, we would be better-off. We could charge it from the power grid during our ground stops, then use it for take-off. The APU could then fill it up again when in the air. But the batteries are too inefficient, weighing in itself more than the whole hybrid chain.
Electric aircraft (and quad-copters) will make sense as urban commuters at first. They can be made quieter and more neighbor-friendly than gas turbine or piston aircraft (no noise, no fuel or exhaust odors).
Gradually, their practical range will increase. Battery swap systems will be developed to shorten turn times. It will also allow charging the batteries in an optimal way.
Once batteries can take part as an efficient energy source, hybrids can start to make sense for longer range flights.
A gas turbine that can spin at an ideal constant rpm can be made simpler (and by it, lighter/cheaper) and more efficient than one that has to modulate its power during the flight cycle.
The battery would cover power peaks and at the lower fan power need after climb, the turbine-generator can recharge the battery.
But it will take time until we are there. It might be that other technologies like fuel cells mature during this time and changes the picture.
There is potential for more optimal aircraft architectures when the motor-fan unit can be made smaller/simpler. Propulsors can be placed more freely. But the gains are counted in percent. Perhaps one can ultimately achieve a 20-30% efficiency gain.
The problem we create with going electric is a 4,000% efficiency loss (battery versus jet fuel specific energy). So, while one happily accepts the architecture gains, they don’t solve the problem. Only battery development will (or a miracle or two on the fuel cell side).
There’s much hype around the electric aircraft. It can be instructive to look at the well-known hype versus progress cycle, Figure 2. It has proven it selves in many industrial developments.
The hype is often a decade before the reality slump, then comes the time of real progress. We are somewhere at the first peak right now.
So, electrical aircraft will come, just not as fast as many thinks.