July 07, 2017, ©. Leeham Co: In last Corner we could see that battery driven electric aircraft will be for the extreme short-haul.
The stored energy per kg battery is 70 times lower than for fuel. In addition the weight is constant. Fuel gets burned off during the flight.
A more useful configuration is the hybrid, which adds a combustion driven generator as energy source.
An hybrid electric aircraft adds an extra energy source on top of the battery we discussed in the last Corner. A generator driven by a combustion engine (running on fuel) is added. The components in a normal hybrid drive chain compared to a Turbofan engine is shown in Figure 2.
There are electric hybrids for cars which mixes the combustion engines shaft output (core in the figure) to the drive of the vehicle. We will only discuss hybrids where the combustion engine is driving a generator. This in turn feeds energy to the aircraft’s electrical motor (driving a propeller/fan) and battery.
To convert the batteries DC output to the AC waveform, needed by the electrical motor, we need an Inverter. The generator creates energy as AC current, but the frequency of the waveform needs adaptation to control the speed of the AC motor. Therefore, we also need the inverter between the generator and the motor.
Today’s airliners use Turboprops or Turbofans to generate propulsive power. In both cases the main propulsive power comes from the air mass being accelerated by the propeller or fan. The jet power from the engine’s core exhaust is 10% or less in modern aircraft engines.
From Figure 2 it’s obvious we introduce more steps between the core and the fan in a hybrid case. Each step involves energy conversions and conversions means efficiency losses.
The challenge for a hybrid electric aircraft is to minimize such losses. Otherwise an electric propulsion systems cannot compete with Turboprop or Turbofan driven aircraft.
The hybrid will use a propeller or fan accelerating the air mass, the same as today’s solutions. So we can skip over the efficiencies of these at first. The assumption is, these are unchanged for the two cases.
The Cores that will be used when electric airliners can be realized will have efficiencies around 55%. Comments in last week’s Corners said present cores have lower efficiencies, more like 40%. This is correct. But we are here examining power chains which will be realized after 2025.
The projected core for the Rolls-Royce Ultrafan Turbofan has a cruise efficiency of around 55%. Its pressure ratio is over 50 at cruise and it uses advanced turbine cooling techniques to reach such efficiency levels. Other engine OEMs are working on similar cores. We must therefore assume hybrid cores with efficiencies in the 55% bracket.
The next step is the Generator. Superconducting generators suitable for aircraft use are planned which will reach 98% efficiency. As the electrical Motor is in principle the inverse of a generator, we can assume an efficiency of 98% there as well. Siemens and Airbus joint venture for electrical aircraft is working on such designs, as are others.
Batteries we discussed in last Corner. This leaves the Inverter.
We assumed inverters of 90% efficiency last time. These use present technology switching semiconductors to shape the AC current that suit the motor, from the battery or generator power. They also feed the battery when the generator produces excess power that can fill the battery. And the inverter reverses this process to feed the motor from battery DC power when needed.
New switching semiconductors are developed (based on Silicon Carbide technology) which can raise the inverter efficiency to 97-98%. We will assume the availability of such inverters in our discussions.
We have now described the efficiencies of the extra components in a hybrid chain. As these are put in serial we would have a chain efficiency of 0.98*0.98*0.98=94%.
If we introduce a generator, an inverter and an electrical motor between the core and the fan we loose 6% efficiency compared to the direct drive Turbofan.
This assumes everything else being the same. In the next Corner we discuss why this might not be the case.