July 28, 2017, ©. Leeham Co: In the last Corner, we looked at the mass of the propulsion components for a regional airliner.
Now we complete the mass estimations for the complete hybrid electric system.
We do that for our aircraft project, which is a regional jet airliner with 50 seats. It’s using a hybrid propulsion system with four electric propulsors fed by a Gas turbine driven generator. The system uses a battery as a redundant energy source.
Last week we developed the masses for the propulsor units (fan+motor) shown in Figure 2. Now we continue with the other parts of the system. We start with the battery for redundancy during take-off and finish with the dimensioning of the generator and core.
By exploring the size of battery which is the minimum needed for redundancy, we also know what potential energy reserves we have for take-off and climb.
We use the battery as our redundant energy source, when the generator+core is not available. It shall only support finishing the take-off, leveling off and do a downwind with a direct landing. It shall support two such attempts to land, both in visual- and non-visual conditions.
We need to supply 7000kW during five minutes to finish the takeoff, 2,000kW during our leveling off and downwind for another five minutes and 1,500kW during our landing for an additional five minutes.
For two attempts, with a go-around after the first, we need 2* (7,000+2,000+1,500)* (5/60)=1,750kWh.
The problem is that at 170Wh per kg battery, this weigh us 10.3 tonnes of battery in the aircraft. For a jet aircraft category which has an empty weight of 14 tonnes, this is unacceptable. We will have to find another redundancy scheme or change our application for the hybrid concept.
Before we look at another solution to our jet regional airliner, let’s finish the generator and core sizing. We can see from the battery redundancy example that a battery as power boost during take-off is not practical. At least, not if we need to conserve battery energy to use for redundancy purposes.
After takeoff, during climb, we still need the battery as a backup for our generator+core. Should the generator or core fail, we need to be able to land the aircraft. If the 1,750kWh is sufficient we leave for now.
The generator and core then need to support our climb power, which is 9,332kW. The generator then weighs 1,170kg. The core would weigh 830kg. Together they weigh 2 tonnes.
A regional jet turbofan propulsion weighs 2*1,050kg=2,100kg. Our four propulsors weigh 2,800kg. To this we shall add generator with core of 2,000kg and any battery for redundancy. We are at 4.8 tonnes before we add the battery.
We can see that a hybrid electric propulsion system is not suited to jet speed applications. The required energy levels are too high and we cannot save any weight by using a battery as a redundancy component.
The energy requirements for a regional jet aircraft are simply too high. We need to find feasible alternative concepts, where an electric hybrid solution fits better.
There are better candidates for an electric propulsion system. Turboprops for 50 passengers use a quarter of the power during a single engine take-off.
The ATR42-600 has a single engine take-off power at V2 of 2400shp or 1800kW. This is 26% of the regional jet for the same number of transported passengers.
We will talk about why the power is so much lower in the next Corner. We will also discuss a hybrid configuration for such an aircraft based on the experiences we have gathered so far.