August 18, 2017, ©. Leeham Co: In this Corner, we will finish the design of the hybrid propulsion system for our 50-seat regional turboprop. We use the ATR42-600 as a reference, as before (Figure 1).
We found an acceptable redundancy concept in the previous Corner, with an APU+generator+battery as backup power source. Now we will finish the design of the hybrid propulsion chain and compare with the original turboprop propulsion.
The power chain that we envisage for our aircraft is shown in the lower part of Figure 2. We want a large gas turbine to drive our main generator (a large gas turbine is more efficient than a small).
As backup power source, we have a smaller, less efficient gas turbine (an APU) driving our backup generator. During peak power periods, with one engine missing (take-off, climb), the battery chips in. The APU is also our source for ground power and cabin air conditioning during passenger turn-around. We also use it to start our main gas turbine after the stop.
The battery system is now a 46kWh battery. With a specific energy of 0.3kWh/kg, we get the mass as 155kg. To this, we need an inverter to convert the DC current to AC for the propulsion motors. The mass of the inverter is negligible compared to the other components. We will discuss the lower mass components we need to add in a complete system in the next Corner.
The dual propulsion motors, of 1,800kW each, drive the propellers. For normal use, we need maximum 1,600kW (take-off, climb) but at One Engine Inoperative (OEI) situations, we can use 1,800kW as described before. At cruise the motors run at 1,500kW each.
We will now calculate the masses of the different components. From last Corner:
To this we add:
In total, we now have a system mass of 2400kg. This shall be compared with the ATR42’s turboprop engines (PW127M) of 480kg. We shall also count the ATR’s batteries of 40kg. In total, 1,000kg for the turboprop propulsion chain. Had the reference aircraft had an APU, we should also add its weight.
But the ATR doesn’t have an APU as standard. It uses the right-hand turboprop engine in “hotel mode” (prop brake engaged) for ground power and cabin cooling.
The hybrid chain for our 50-seat turboprop weighs 2.4 times as much as the original turboprop propulsion.
In the next Corner, we will discuss what implications this has on aircraft performance. We will also discuss different optimizations that could improve the picture.