August 11, 2017, ©. Leeham Co: In this Corner, we will design the hybrid propulsion system for our 50-seat regional turboprop. We could see in previous Corners that we can’t use batteries as a backup for our gas turbine core and main generator.
The battery gets too heavy as the specific power weight of a battery is simply too low. We will now design a hybrid power chain with a different redundancy concept.
We will use the ATR42 data to deduce the power requirements for our regional turboprop. It might be that a new design could reach improved values by designing a higher aspect ratio wing for example.
But it’s wise to use an established design to do the first sizing. It has passed certification and can serve as our base case for our hybrid propulsion chain. Once through this example we can discuss how the different performance values could be improved with a more aggressive design approach.
Through the specifications for the ATR42-600 (Figure 2), we can deduce the power we need for take-off, single engine take-off and maximum continuous cruise.
The take-off and maximum continuous cruise is 2,160shp or 1,600kW. For One Engine Inoperative (OEI), it’s 2,400shp or 1,800kW. For normal cruise, the power will be 2,000shp or 1,500kW per engine, giving 4,000shp or 3.000kW total for our aircraft.
Before we start sizing the hybrid chain, we need to design a viable redundancy concept. To get propulsion redundancy, we need to go a different route than for the regional jet. A battery as a redundant power source gives to little energy per mass unit, as we saw in Part 5.
One-way would be to double all components in the hybrid chain (Figure 3). This is not to smart. We decrease the propulsion efficiency by 6% (see Part 3) and increase the installed weight.
Our plan with one larger core driving the chain for normal use is to increase the dimensions of the core and by it, its efficiency (Part 3). The aircraft then needs an additional, redundant, power source, to not have a single point of failure in the propulsion chain.
At the same time, we need a power source on the ground for cooling/heating the aircraft, for producing electricity and to start the core gas turbine with. The normal source for ground power is an APU (Auxiliary Power Unit).
An APU is a small gas turbine, Figure 4. It’s designed to be small, light and reliable.
The sacrifice to get there is the fuel efficiency. Because of a low-pressure ratio (at or below 10) the thermal efficiency for APUs is not great. But this is not a problem; they run during ground stops and as a redundant power and air source during take-off or landing.
By choosing a large APU, the one for the Airbus A350 for example (HGT1700, figure 4), we get an APU that can function as our backup power source for our core turbine.
The HGT1700 delivers 1,300kW shaft power at a mass of 335kg. We then get 1274kW electrical power through our superconducting generator (98% efficiency). To get to our OEI 1,800kW propeller power through the electrical motor, we need to inject 1,826kW (we have a 98% efficient motor).
The missing 552kW we draw from our battery during the five minute OEI take-off time. Once on the downwind, the 1274 kW electrical power from the APU is enough for cruise and landing.
The battery now weighs 155kg instead of 5.8 tonnes. We use the superior power specific weight of fossil fuel to form the backbone of our redundancy concept while still utilizing a battery for the peak power part of the cycle.
In the net Corner, we will size the whole chain for weight and discuss some finer points about the use of the battery.