Question About Electric Aircraft Propulsion

[Anand Sivaram]

TL;DR

Question About Electric Aircraft Propulsion

I have been thinking about this subject for some time now, based on the following considerations

1) Many companies are working on Electric Aircraft Propulsion, high energy density Li-Ion batteries are used to power Electric motors and they drive propellers. This is the approach every one is using.

2) We know the problems of Propellers, when the tip of the propeller starting to approach supersonic speed, then the efficiency goes down drastically.

3) That is one of the reasons of jet engine development, first turbojet and then turbofan. Using these engines planes could reach supersonic speed.

4) Hypothetical consideration: When the battery technology improves and the energy density starts approaching that of fossil fuels, it could store large amount of energy.

Now the question:
Could we reach supersonic speed with battery powered aircraft considering (4) and (2). Or, could we power gas turbine with electricity?

 

[Drakkith]

4) Hypothetical consideration: When the battery technology improves and the energy density starts approaching that of fossil fuels, it could store large amount of energy.

That's very unlikely in the near to moderate future in my opinion. Fossil fuels are extremely energy dense, which is one of the features that makes them useful over other energy sources, and it's very difficult for batteries to even approach their energy density. Jet fuel has an energy density of around 43 MJ/kg, while lithium ion batteries are about 0.8 at the top end, so we're talking roughly a fifty-fold difference between them.

Could we reach supersonic speed with battery powered aircraft considering (4) and (2). Or, could we power gas turbine with electricity?

The issue is that jet engines don't rely on a moving prop to generate their thrust (with the exception of turboprop engines), they rely on the heating and expansion of gas powered by the combustion of fuel. How would you do something similar using electric power?

 

[russ_watters]

The issue is that jet engines don't rely on a moving prop to generate their thrust (with the exception of turboprop engines), they rely on the heating and expansion of gas powered by the combustion of fuel. How would you do something similar using electric power?

Well, theoretically you could use a giant electric heater to heat the air in the "combustion chamber", but I'm not sure if that's really doable.

 

[anorlunda]

Well, theoretically you could use a giant electric heater to heat the air in the "combustion chamber", but I'm not sure if that's really doable.

Yeah that's true but you wouldn't get the volume expansion ratio of burning liquid fuel to produce combustion gasses. Perhaps you could use the electric power to boil water instead of heat air. I agree, not sure it is really doable.

 

[Anand Sivaram]

Thanks All. Got the points about electrical heating and the volume expansion of liquid fuels.
It could be one of the reasons for using After Burners in supersonic aircraft to get extra thrust at the expense of fuel consumption.

Just came to my mind, how about in situ electrolysis to get H2 and burn it in gas turbine?

 

[Drakkith]

Just came to my mind, how about in situ electrolysis to get H2 and burn it in gas turbine?

Why not just carry a tank of H2 instead? The cost, weight, and complexity of storing it is almost certainly better than carrying around all the equipment for electrolysis, especially batteries.

 

[Anand Sivaram]

True

 

[gmax137]

Just cut out the middleman, by using nuclear heat to drive the jet (rather than charging your batteries off the grid at the airport).
https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion

 

[hmmm27]

Does anybody know how to make ball lightning ? Just park one of those suckers in the erstwhile combustion chamber.

 

[Anand Sivaram]

Basically I was thinking about sustainable, Carbon neutral aircraft propulsion - propeller free, as all commercial jet airlines are faster than propellers.
This is the recap of what I understood, including from our discussions here.

- Currently, turbine fuels like Jet A-1 are selected based on Gravimetric and Volumetric energy density, flash point, safety etc. in mind.
- Battery based propulsion are not an option - they could power propellers only.
- Nuclear propulsion is discussed here - quite interesting concept - but, does not seem to have any traction after 1950s and 60s.
- Stored hydrogen could be used for the jet engine, but H2 has very bad volumetric density and safety also could be a concern.
- Other forms of heating could be used, but heat transfer to air and volume expansion of air many not be sufficient enough.

- With all of these, I think the only option could be renewable biofuel based aviation fuel.

 

[anorlunda]

With all of these, I think the only option could be renewable biofuel based aviation fuel.

Pretty good summary, but why dismiss propellers just because of speed?

At some point, being green must necessarily address behavior modification. Is it off limits to ask people to travel slower for the benefit of the environment?

 

[russ_watters]

Pretty good summary, but why dismiss propellers just because of speed?

At some point, being green must necessarily address behavior modification. Is it off limits to ask people to travel slower for the benefit of the environment?

Also, the actual sacrifice for many routes is not that large. There are a ton of short-hop flights that currently use regional jets that don't even reach typical cruising altitude before descending to land. These flights used to be flown in turbo-props. The fractional change in total trip duration from adding 15 minutes to a 45 minute flight (when you spent an hour waiting at the gate, 30 minutes loading and 20 minutes waiting to take off) is not a major sacrifice.

I also think for longer-haul flights they may need to sacrifice the current small plane, lots of flights business model and go back to fewer flights in larger airplanes. Even with conventional fuel this increases efficiency, but it also opens up the door for alternate fuels with lower volumetric energy density (e.g.; hydrogen).

 

[hmmm27]

Basically I was thinking about sustainable, Carbon neutral aircraft propulsion

You do know that "sustainable, carbon neutral" means denying the planet of a portion of its carbon-sequestration abilities.

- propeller free, as all commercial jet airlines are faster than propellers.

Why would speed be an issue ?

This is the recap of what I understood, including from our discussions here.

- Currently, turbine fuels like Jet A-1 are selected based on Gravimetric and Volumetric energy density, flash point, safety etc. in mind.

cost, availability, lubricity...

- Battery based propulsion are not an option - they could power propellers only.

motorjets.

- Nuclear propulsion is discussed here - quite
interesting concept - but, does not seem to have any traction after 1950s and 60s.

Science experiments, paid for by the military, with promises of reconnaisance aircraft and ready bombers that could stay up for weeks at a time. More modern reactors, and you could probably stuff one into an A380... okay, maybe not an A380.

- Stored hydrogen could be used for the jet engine, but H2 has very bad volumetric density and safety also could be a concern.

BWB's and similar designs could take care of the volumetric issues. What safety issues ? (apart from those of any gaseous fuel)

- Other forms of heating could be used, but heat transfer to air and volume expansion of air many not be sufficient enough.

Take a closer look at the nuclear engines.

- With all of these, I think the only option could be renewable biofuel based aviation fuel.

Feel free to explain the advantages of constantly burning the same area of land over and over, compared to simply letting existing vegetation soak up CO2.

I'm not claiming there are none but - mostly for the sake of debate - I posit that it's a zero sum game which has the side effect of destroying natural habitats. Secondary effects, unrelated to mitigating global warming - redistributing income from countries that may be perceived as having too much, for instance - might be beneficial overall, or might be disastrous : I can't tell.

 

[russ_watters]

motorjets.

That isn't battery powered.

 

[Baluncore]

Modern jet engines employ a high-bypass design. The outer part of the engine is effectively a ducted fan, which is a propeller. Efficient propulsion will involve propellers turning slowly, with n=3, or more blades. n is proportional to 1/RPM. High tip velocity will not occur as it is inefficient.

High efficiency requires low speed because drag is proportional to the square of the airspeed. Lift is also a square law function of airspeed. A heavier-than-air craft must be supported by lift for the duration of the flight. The rate of energy flow from the batteries determines the maximum airspeed possible. Propellers can be optimised for that situation.

High efficiency requires long wings, like a glider, to reduce induced drag. The top surface of the wings can be covered with a solar cell film, which becomes more efficient at high altitudes, and can charge the batteries to increase the range.

 

[hmmm27]

motorjets

That isn't battery powered.

It is if the motor driving the impeller is electric (and battery powered). I don't recall how much of the thrust going through the turbine (the spinny bit at the back of the combustion chamber) is required to drive the compressor (the spinny bit in front of the combustion chamber), but it's not trivial.

If the compressor is driven by an electric motor, the jet doesn't need to drive it, itself.

 

[Anand Sivaram]

Thanks all for the wonderful answers. I get the advantages of propeller especially in short-haul regional where the extra time taken is minimal.

 

[cjl]

If you're just talking about replacing commercial airplanes, you don't need the heating or expansion effects mentioned above. You could just as well simply have electric ducted fans that would look much like the existing turbofan engines except with the core replaced with an electric motor. Modern turbofans gain almost all of their thrust from the front fan, rather than from thermal expansion of the core flow through a supersonic nozzle (the way turbojets do). As a result, you'd gain just about the same performance without the core and just driving the fan with an electric motor.

The problem with this is the amount of power required. A GE90 used on the Boeing 777 required something on the order of a hundred thousand horsepower to drive the front fan at full power. An electric motor of this power level is both too large and too heavy to replace the jet core, so we'd need some way of substantially improving the power density of electric motors in order to do this. In addition, there's the above-mentioned issue with storing enough energy. A long range jetliner like a 777 might be 30-40% fuel by weight at takeoff, and batteries are more than an order of magnitude worse in energy density than jet fuel. In addition, long range jetliners take advantage of the fact that they get lighter as they use fuel, improving their efficiency late in the cruise and decreasing the mass and requirements on parts like landing gear and brakes, since their maximum landing weight is significantly lower than their maximum takeoff weight. Batteries have none of these advantages, so it would have to be designed to be at full weight for the entire duration of the flight.

 

[Anand Sivaram]

@cjl Got your points about ducted fan.
Good point about fuel getting consumed. Now I remember one post somewhere by a commercial pilot going from Buenos Aires to Paris. How he takes the cruising altitude in steps from FL250 to FL390 as and when the aircraft gets lighter, to optimize the fuel consumption.

 

[russ_watters]

It is if the motor driving the impeller is electric (and battery powered). I don't recall how much of the thrust going through the turbine (the spinny bit at the back of the combustion chamber) is required to drive the compressor (the spinny bit in front of the combustion chamber), but it's not trivial.

If the compressor is driven by an electric motor, the jet doesn't need to drive it, itself.

It still needs fuel to be a jet. I think something on the order of 20% of the power is for turning the compressor and the rest for thrust. So you could only save 20% of the fuel at the expense of increasing the weight.

[edit: or I suppose you could just forgo the "jet" part of the jet as per cjl.]

 

[russ_watters]

If you're just talking about replacing commercial airplanes, you don't need the heating or expansion effects mentioned above. You could just as well simply have electric ducted fans that would look much like the existing turbofan engines except with the core replaced with an electric motor. Modern turbofans gain almost all of their thrust from the front fan, rather than from thermal expansion of the core flow through a supersonic nozzle (the way turbojets do). As a result, you'd gain just about the same performance without the core and just driving the fan with an electric motor.

I thought that was only true at low speed whereas at high speed the "jet" part provides most of the thrust?

[edit: Hmm -- I'm seeing someone's off the cuff answer in another forum saying about 75% at takeoff and 60% at cruise is coming from the fan/bypass. That's a lot more than I realized at cruise.

 

[Anand Sivaram]

>> I thought that was only true at low speed whereas at high speed the "jet" part provides most of the thrust?

I just had a look at bypass ratio of turbofan engines.
It is the ratio of airflow through the outside ducted fan to the air flowing through the engine core. It is selected considering the application and the speed in mind.
0 = pure turbojet, 0.3 to 0.6 for Mach 2 supersonic fighters, for modern A380 and B777 engines the bypass ratio is between 8.0 to 10.0, for turboprops >50 ratio.

https://en.wikipedia.org/wiki/Bypass_ratio

 

[sophiecentaur]

Well, theoretically you could use a giant electric heater to heat the air in the "combustion chamber", but I'm not sure if that's really doable.

Energy carrying capacity is crucial with electrical propulsion. Using the stored electricity to heat air in a simulated gas turbine would mean a serious drop in efficiency. Regular jet engines have little better than 40% thermodynamic efficiency and an electrically powered heat engine would not be worth while. It would, by implication, give a range of only 40% of what could be achieved by turning a turbine / fan to move the air through it. Best to stick with the 'high grade' energy that's stored in the batteries.

I believe that the maximum altitude for prop craft is lower than for jet craft and that could be a significant efficiency factor, compared with jets, of you want to travel fast.

 

[hmmm27]

A GE90 used on the Boeing 777 required something on the order of a hundred thousand horsepower to drive the front fan at full power.

Could you post rough calcs on that, please ? The GE90 series produces about 100k lbf thrust, and I always thought that hp was generally less than lbf (at speed).

 

[skystare]

Theoretically, an electric ducted fan (EDF) could be used for supersonic flight. No gas turbine today operates in a supersonic airflow; variable inlet geometry is used to slow the airflow to subsonic speed at the first compressor or fan. The same would apply to an EDF. For the practical objections, see cjl's post. The actual turbine part of a jet engine has an astonishing power to weight ratio, unlikely to ever be matched by an electric motor.
As aircraft contribute only a percent or two of human CO2 output, IF we could go all nuclear/solar/whatever powered, including electric surface transport, aircraft as they are would be entirely sustainable. As well, just to be extra green, it would be quite practical to synthesize all av-fuel from aboveground carbon.

 

[Klystron]

While the thread centers on electric aircraft propulsion, consider what type of airships benefit most from improved props driven by efficient e-motors. @Baluncore 's post hints at modified air frame designs. Dropping supersonic flight and high speed as requirements opens the door for re-evaluating lighter-than-air (LTA) designs.

Neutral/positive buoyant air frames could provide lift for heavy batteries, solar cells and electric motors plus the air frame, crew and cargo. The props only need to provide thrust for movement and to maintain position in wind. Given improved materials, weather forecasting, and other factors absent in the 'golden age of airships'; canny aero engineers should reconsider LTA ships for practical transport including cargo. Time is not a prohibitive factor for tons of cargo and passengers on the water. LTA ships could replace much ground transportation dependent on fossil fuels, even using similar modular containers.

A school was recently constructed in a nearby cul-de-sac. A modest dirigible carried in much of the material and acted as a sky hook crane during construction; an increasingly common sight in our valley. The engines and props were very quiet compared to helicopters and jet engines.

 

[cjl]

Could you post rough calcs on that, please ? The GE90 series produces about 100k lbf thrust, and I always thought that hp was generally less than lbf (at speed).

It's been a while since I ran the numbers (so I'm going from memory here), but I'll do some digging. There's no real direct relationship though. It's also worth noting that the front fan horsepower per pound of thrust is going up as higher and higher bypass ratios are becoming common, since obviously a turbojet requires exactly 0 horsepower to drive the front fan, while a large fraction of the engine's power output on a modern high-bypass design is going towards the front fan.

 

[skystare]

Force through distance in time equals power. Without moving anything, no power is expended; thus if you push on a wall with 20 pounds force you use no power because nothing is being moved.
In the case of an aircraft in flight, a jet engine produces one hp per pound of thrust at 325 mph. On the ground with brakes set the engine will still produce power as it is moving air, but not as easily calculated as the in-flight formula.

 

[sophiecentaur]

As aircraft contribute only a percent or two of human CO2 output,

That could be used as an argument against many possible ways to reduce CO2. I can see the logic behind it but an awful lot of that CO2 is due to unnecessary travel. Why have foreign holidays, for instance?

Edit: Unnecessary trade is also a factor. Items are transported from A to B whilst identical items go from B to A; the only reason is the trade deals and the politics involved. As with many such issues, 'Freedom' to trade does not mean that it's justified. This is not Physics, of course but it's certainly a point worth noting.

 

[russ_watters]

While the thread centers on electric aircraft propulsion, consider what type of airships benefit most from improved props driven by efficient e-motors. @Baluncore 's post hints at modified air frame designs. Dropping supersonic flight and high speed as requirements opens the door for re-evaluating lighter-than-air (LTA) designs.

Neutral/positive buoyant air frames could provide lift for heavy batteries, solar cells and electric motors plus the air frame, crew and cargo. The props only need to provide thrust for movement and to maintain position in wind. Given improved materials, weather forecasting, and other factors absent in the 'golden age of airships'; canny aero engineers should reconsider LTA ships for practical transport including cargo. Time is not a prohibitive factor for tons of cargo and passengers on the water. LTA ships could replace much ground transportation dependent on fossil fuels, even using similar modular containers.

I'd really like to see a calculation on the volume of lifting gas required to carry 100,000 tons, to replace a cargo ship.

Then I'd like to see the kinetic energy of impact if one breaks apart at 10,000 ft.