Applying Tesla technology to aviation - a 100 seat aircraft with propellers driven by electric motor

[Peter Egan]

GE 93 turboprop has the air/gas flow from the the rear to the front of the engine. Air is sucked in rather than rammed in. Rather than the air compressor powered by a shaft from the exhaust gas turbines, it could be powered by an electric motor. Air and fuel could then be supplied solely on the basis of the exhaust gas flow required. An electric motor could be fitted on the turbine side of the planetary gears to provide power to the electric compressor and the aircraft. In an electric aircraft, the planetary gears could go and two electric motors provided. A large motor to drive the propeller to produce the desired short term power, and a smaller motor attached to the exhaust turbine to provide power to aircraft systems and to batteries to power the aircraft. The engine will be greatly simplified and its costs will fall. In a standard turbine engine, there is a fixed ratio. generally 1:1, shaft connection between the exhaust and air turbine to power the air turbine. The fixed ratio means some inefficiency in the engine. A simpler, cheaper, more efficient engine will justify an engine that is somewhat heavier in a plug-in hybrid aircraft.

 

[Saghost]

The question is whether the inefficiency from not matching the airflow is more or less than the efficiency loss involved in your double conversion (mechanical to electric back to mechanical.)

Even if it is, is it enough more to justify the weight and cost of the electric half of the system?

I'm pretty sure that there isn't that much inefficiency, but I haven't tried to run numbers.

The bigger difference in my experience comes from running the turbine at part throttle much of the time - turbines are generally very peaky on efficiency maps.

If there's an opportunity for a hybrid turboprop, that's where I think it would be - a turbine sized for cruise operation and a battery pack for takeoff and climb.

 

[Peter Egan]

The question is whether the inefficiency from not matching the airflow is more or less than the efficiency loss involved in your double conversion (mechanical to electric back to mechanical.)

Even if it is, is it enough more to justify the weight and cost of the electric half of the system?

I'm pretty sure that there isn't that much inefficiency, but I haven't tried to run numbers.

The bigger difference in my experience comes from running the turbine at part throttle much of the time - turbines are generally very peaky on efficiency maps.

If there's an opportunity for a hybrid turboprop, that's where I think it would be - a turbine sized for cruise operation and a battery pack for takeoff and climb.

Good to know

 

[Peter Egan]

While the aviation industry is starting with 1 to 4 seat electric aircraft to mature technology, an indicative specification for a plug-in hybrid version of an industry standard 100 seat regional propeller powered aircraft could look as follows below. Telsa technology would be used in the battery system and in the ground 'drone' used to power the aircraft while taxiing to the runway.

In the attached image of a modified CRJ1000, I have left the turbofans in place to compare with the new electric drive layout.

The aircraft has a 100 km electric range. However, the batteries are mainly used on take-off with the turboshaft-generator APU contributing power on take-off and fully powering cruise flight, aircraft services and battery recharging.

The 100 km electric range, along with aircraft glide capacity, is kept in reserve in the event of turboshaft engine failure. The aircraft has one jet fuel burning turbine compared to three for conventional 100 seat aircraft - a large cost saving and a big start on the journey to fully electric aircraft. 800 kWh of usable battery storage for the 100 km range is expected to weigh 4 tonne. An electric range of 500 km for a 100 seat aircraft is likely some decades away.

This aircraft would compete with the 78 seat ATR72-600 and 86 seat Q400 turboprops and 100 seat CRJ100 on short hall routes.

100 seat plug-in hybrid electric aircraft – indicative specification
Cabin – 26 seat rows with two 2 toilets, two front doors, 4 emergency exits over wings
Cabin – 24 m long, 2.65 m wide, aisle headroom 1.90 m
Cargo hold – 20 cu.m, 4 m long, behind cabin
Cockpit – 4 m long
Tail – 4m long – houses propulsion equipment
– elevators/horizontal stabilisers mounted on propeller motors frame
Fuselage – 2.8 m wide, 3.0 m high, 32 m long
Wing area – 70 sq.m
Wing span – 27 m
Payload – 10.0 tonne including 3 tonne Cargo
Fuel – 4.0 tonne, 5000 litres
Typical operating empty weight – 20 tonne including 4 tonne flight power batteries
Maximum take-off weight (MTOW) – 32 tonne (2 tonne less aircraft with max. payload and max. fuel)
Flight battery – 800 kWh (2880 MJ), 4.0 T, with 40 x 100 kg modules each with 1152 x 70200 NCA Tesla cells
– module – 1152 cells in series, 1 cell parallel, each cell 18.36 W.hr, 3.6 V, 5.1 Amp.hr
– module – 4147 V (nominal)
– module – 1152 x 18.36 W.h = 21.15 kW.hour (nominal)
– battery – 21.15 kW.h x 40 = 846 kW.h (3046 MJ, 3GJ) – 5.75% above nominal capacity
Turboshaft APU – 1 tonne (including 483 kg PW127TS):
– Pratt & Witney PW127TS – 1864 kW class with 2386 kW take-off power ((813 H, 686 W, 1626 L, mm))
– 2 x 1000 kW generators, each with 1200 kW take-off capability
2 x motor/counter-rotating propeller units, each with:
– 2 x permanent magnet electric motors, 1200 kW continuous (1400 kWx4 = 5600kW for take-off),
weight 4x280 kg, 1120 kg, 4147 Volt (nominal), 337.6 Amp (likely rated to 400 amp)
All wheel drive (in-wheel motors) for taxiing and regenerative braking
Ceiling – 25,000 ft, 7620 m
Range at MTOW – 2500 km at cruise 301 kt, 558 kmh, 155 m/s
Range - battery (APU fails) – 100 km plus 10 km reserve plus altitude glide reserve plus battery reserve
Flight time without APU – 12 minutes at cruise plus reserves
Fuel economy Jet A-1 – 1.6kg/km average, 1.5kg/km cruise – 37% energy conversion to thrust
Fuel economy - Battery – 7.27 kWh/km (26.2 MJ/km) – 90% energy conversion to thrust
Take-off run – 1400 m
Price – US$30 m

At large airports with long taxi paths, a ground drone (cart) with Tesla driverless technology, and 200 kWh battery capacity (for numerous taxi runs before recharge), will power aircraft out to runway, disconnect and return to terminal. Planes will generally land with enough electricity stored to reach terminal. If needed, the plane could restart its APU.

Propellers set high on tail to be well out of the way in the event of tail strike on take-off.

Aircraft will recharge its batteries by plugging-in while being serviced at terminal gate – 1000 kW charger needed.

Aircraft accelerates faster down the runway as electric motors reach full power much faster than jet engine.

APU started at push-back, or at ground drone detachment where they are available. APU has sufficient power to recharge battery while at cruise.

Aerodynamic drag, rather than aerodynamic lift to carry aircraft weight, is the prime user of power at cruise. The weight of the batteries is mainly a concern at take-off.

25% improvement in fuel economy expected compared to 86 seat Bombardier Q400 turboprop.

Aircraft construction has ‘armoured cockpit’ and ‘propulsion unit tail’ constructed separately and bolted to hull. Fuel lines and power cables run in separate ducts on underside of hull for ease of construction and maintenance. Fuel stored in wings, flight batteries under floor ahead of the wings.

Plug-in hybrid systems could be trialled on a Bombardier Q400 with the electric motors/propellers mounted on the wing in place of the PW150A engines, with the PW127TS turboshaft engine and generators replacing the UTC Aerospace Systems APS 1000 in the APU bay.

View attachment 184773

On further reflection, having seen the Tesla battery module deconstructed, its suggested there be two 400 kWh batteries, each comprised of 40 modules with each module packed as shown in the image.

The 21-70 cells are envisaged to be rated at 3.6 V and 5.85 Ah for 21.06 Wh of which 20 Wh (95%) is usable. Each 400 kWh battery thus requires 20,000 cells. Each battery is nominally rated at 421.2 kWh (1516 MJ). Each cell weighs 75 grams. Each battery has 1500 kg of cells and weighs 2000 kg with pack, BMS, coolant tubing, radiators and ducts, plane access doors, and air-frame vibration and landing shock fastenings included.

The cells have a nominal energy density of 281 Wh/kg, 869 Wh/L, and the pack 211 Wh/kg, 281 Wh/L

Each module has 500 cells - 20 rows of 25 cells arranged as in the image with the liquid coolant tube between every second row. Each module has a coolant circuit. Each group of 50 cells is connected in parallel with a Tesla style fuse wire connection to the collector plate. Each module is nominally 36 Volts and can be handled as a low voltage item. Each module has 37.5 kg of cells and weighs 40 kg.

The battery is thus 36x40=1440 V, with a charging high of 1680 V and a low of 1200V. The propeller motors (4 motors in total) are thus sized for this voltage and need to be able to sustain 972 Amps (~1,000 Amps) for the required 1400 kW. The wiring needs to be uprated for this higher current.

At 25 cells wide and 20 deep, the battery pack is 600 mm wide and 500 mm deep. 40 modules make it 4 metres long with extra space required for the battery management system, coolant radiators and their ducting to the outside of the aircraft.

 

[Peter Egan]

On further reflection, having seen the Tesla battery module deconstructed, its suggested there be two 400 kWh batteries, each comprised of 40 modules with each module packed as shown in the image.

The 21-70 cells are envisaged to be rated at 3.6 V and 5.85 Ah for 21.06 Wh of which 20 Wh (95%) is usable. Each 400 kWh battery thus requires 20,000 cells. Each battery is nominally rated at 421.2 kWh (1516 MJ). Each cell weighs 75 grams. Each battery has 1500 kg of cells and weighs 2000 kg with pack, BMS, coolant tubing, radiators and ducts, plane access doors, and air-frame vibration and landing shock fastenings included.

The cells have a nominal energy density of 281 Wh/kg, 869 Wh/L, and the pack 211 Wh/kg, 281 Wh/L

Each module has 500 cells - 20 rows of 25 cells arranged as in the image with the liquid coolant tube between every second row. Each module has a coolant circuit. Each group of 50 cells is connected in parallel with a Tesla style fuse wire connection to the collector plate. Each module is nominally 36 Volts and can be handled as a low voltage item. Each module has 37.5 kg of cells and weighs 40 kg.

The battery is thus 36x40=1440 V, with a charging high of 1680 V and a low of 1200V. The propeller motors (4 motors in total) are thus sized for this voltage and need to be able to sustain 972 Amps (~1,000 Amps) for the required 1400 kW. The wiring needs to be uprated for this higher current.

At 25 cells wide and 20 deep, the battery pack is 600 mm wide and 500 mm deep. 40 modules make it 4 metres long with extra space required for the battery management system, coolant radiators and their ducting to the outside of the aircraft.

The battery would need to be split into 8 (200 kg) or so parts so it can be inserted through a hatch between the frames of the aircraft. A special hoist would be needed to lift the sections into aircraft and they and the coolant tubing would need to be secured.

The US national Fire Protection Association conducted tests of a 100 kWh Tesla energy storage Powerpack with 18650 cells and found a cell runaway does not affect neighouring cells, and a fire next to a Powerpack takes a long while to initiate a fire in a pack. Essentially, Tesla batteries are safe for airline use.

I note with the Tesla car battery packs, the cell direction is reversed for each sub-module so the one metal sheet can attach to two sub-modules and reduce the wiring quantity. This no doubt improves the rigidity of the pack. The electricity flowpath is very much like water. The use of the thin fuse wire connections to the plates ensures each battery gets a full flow of electrons.