Question About Electric Aircraft Propulsion

[Z0dCHiY8]

Summary: Question About Electric Aircraft Propulsion

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

the very heart of any transportation & machinery is fuel == if you have one w/ sufficient specific energy, you could do anything. All-electric propulsion has many solutions. For instance, magnetic-plasma ramjet == such engine has no moving parts == it inlets air, heats it w/ microwaves, focuses plasma w/ magnetic fields & pushes it w/ electric field to outlet. needless to say, mp-ramjet can variate thrust & Isp. For vacuum, we can have powerful ion thrusters. But we need such a minor trifle == BATTERY to meet those requirements on specific energy.. such a trifle :)

 

[phinds]

But we need such a minor trifle == BATTERY to meet those requirements on specific energy.. such a trifle :)

HUH ? Do you have any idea what is the energy density of a battery vs the energy density of gasoline?

 

[russ_watters]

HUH ? Do you have any idea what is the energy density of a battery vs the energy density of gasoline?

I was detecting sarcasm in that statement.

 

[sophiecentaur]

I was detecting sarcasm in that statement.

Perhaps it was more Irony (a more sophisticated weapon). I can't imagine the word "trifle" used seriously in that way.

 

[mikelorrey]

Turbine engines do not necessarily avoid the efficiency losses of supersonic propeller tips. The specific impulse of a turbine engine is much lower than that of a propeller driven craft. Turboprops are in between, and turbofans are always more efficient than turbojets (A turbofan allows most of its first stage compressor air to bypass the combustion chamber, so it is pushing more air than just the combustion gasses).
If you were to somehow get batteries to have a similar energy density to fossil fuels, they still would not be as efficient, because with combustible fuel, you are burning it and tossing it overboard as you go, so you reduce the mass of the vehicle by burning fuel. With batteries, you remain fully loaded with the full mass of the batteries from beginning to end, so you have to haul the full load the whole way, requiring more energy and thus you still cannot fly as far or as long for the same Joule budget with batteries as you can with fossil fuels.
That being said, converting electricity to thrust could theoretically be more efficient. A turbine engine without thermal recovery is 35-40 % efficient. An electric battery powered propeller aircraft, from battery to propeller, has a system efficiency also of 35%. A battery powered turbine impeller would necessarily be less efficient than this simply because you are moving less mass over a shorter radius. So the propulsion side of the system is not where you are going to gain anything unless you come up with some super efficient electrostatic or MHD field effect to move air. Making batteries more energy dense has lots of room for improvement, but the theoretical maximum possible energy density for batteries is still far below fossil fuels.
The new "artificial leaf" technology might be a good hybrid: coat the top surface of the wings and hull with artificial leaf material. This material uses photosynthesis and ambient air (including CO2) to produce methanol, and at a conversion efficiency of 10%, which is on a par with current market thin film photovoltaic cells, but the methanol is far more energy dense than any batteries, so you should be able to reduce aircraft mass, depending on how much mass is required for this artificial leaf material.

 

[sophiecentaur]

With batteries, you remain fully loaded with the full mass of the batteries from beginning to end,

That's a good point and it suggests that, just as the load of fuel is tailored to the proposed journey, batteries would need to be modular and interchangeable from plane to plane in order to keep the dead weight to a minimum. That would entail some good organisation and cooperation.
I can't imagine that the 1kW/m² that even a long haul craft wings would produce would pay its way for the extra weight. The one good thing would be that the cells would be in direct sunlight for the whole of a daytime flight (no clouds).

 

[essenmein]

IMO the best way to use electricity for air travel is to use it to make H2 or NH3, and then feed combustion turbines with this. Since this electricity doesn't have to actually fly, you could park a 4th gen nuke plant next to your ammonia or H2 generator and have almost limitless carbon free fuel, bonus, you can use it in cars and trucks as well. Bam, global warming solved.

 

[skystare]

Even better; use electricity and waste heat from 4th gen nuke plant to make synthetic kerosene from aboveground (perhaps even atmospheric) carbon. Aviation related global warming (2% of total problem) solved, without poisonous or impractical fuels.
Of course the mere adoption of 4th gen nukes would solve another 50% to 60% of the problem.

 

[sophiecentaur]

Aviation related global warming (2% of total problem) solved,

Perhaps not as strong as that but it would be going in the right direction, perhaps.
I just watched the Chernobyl series on TV so my present view of the much vaunted 'Nukes' is a bit coloured. We can't blame the whole of that episode on the shortcomings of a bad regime.
Slower and fewer flights would produce the same advantages as all the untried high tech solutions, of course and it would require very little startup time.

 

[member 656954]

4th gen nuke plant

Given we don't have working Gen IV plants, are not likely to for a decade, and the economics of such are questionable given the price trend of utility scale wind and PV, it's not an immediate or arguably even a desirable solution.

without poisonous or impractical fuels

No, just tons of highly toxic, long-lived radioactive waste!

 

[essenmein]

Given we don't have working Gen IV plants, are not likely to for a decade, and the economics of such are questionable given the price trend of utility scale wind and PV, it's not an immediate or arguably even a desirable solution.

Gen 4 plants are being actively pursued, eg Terrestrial energy (I only pick this one because its closeish to home), I'd say we will see working 4th gen in less than 5 years, whether that's here or in China/Korea depends on the willingness of the population. Your economic argument would make more sense backwards, give then questionable price of utility scale wind and solar its not an immediate or even desirable solution. Look at Germany , expecting to spend somewhere to the tune of 1.4 to 2.1 trillion Euro to get 100GW of renewable electricity, that's 2-3 times the annual US defense budget to get a meager 100GW for 1% of the worlds population, renewables will bankrupt us. Thats $15-20/W, advanced nuclear (in Asia) is about 1/10th of that!
But this is off topic somewhat... lol

No, just tons of highly toxic, long-lived radioactive waste!

On topic through is the miss conception of waste streams... Our current nuclear waste stock pile is technically fuel for 4th gen, so adopting more nuclear power would hilariously reduce our total amount of nuke waste.

Battery electric on the other hand, given that we don't have a good way to recycle Li ion batteries (too many different chemistries, mechanical form factors etc etc), bat electric planes and cars will produce literal mountains of highly toxic waste and unless we find an economical way of getting that lithium back, its a one way trip for a quite limited resource, not quite as sustainable as people think.

But don't take my word on it:
https://techxplore.com/news/2019-11-uk-electric-vehicle-battery-mountain.html

 

[russ_watters]

Slower and fewer flights would produce the same advantages as all the untried high tech solutions, of course and it would require very little startup time.

Unless governments are going to become autocratic about this, solutions will need to have similar levels of functionality or economics to existing ways of doing things otherwise people won't want to do them.

 

[russ_watters]

No, just tons of highly toxic, long-lived radioactive waste!

Which is great compared to the alternative! I'm a big fan of nuclear waste!

No, seriously - more nuclear waste means more nuclear power (unless we start recycling the waste), which is great, and the waste itself is no big deal.

 

[member 656954]

and the waste itself is no big deal.

No idea if you're being serious or not with that closing line Because, so far, it's been a huge deal for pretty much every country planning a waste repository, apart from Finland's Onkalo site.

 

[russ_watters]

No idea if you're being serious or not with that closing line Because, so far, it's been a huge deal for pretty much every country planning a waste repository, apart from Finland's Onkalo site.

I'm being completely serious. The "big deal" is 100% political. From a technical standpoint it is essentially nothing. Heck, most nuclear plants store their spent fuel on site and people barely give it a first, much less second thought. It's only when we talk about permanent storage that people suddenly decide it's a big deal. There is no rational reason for it.

 

[hmmm27]

- away from population centres ; not because of accidents, but because of terrorists and the abysmally stupid.

- neutral territory : Antarctica
(Greenland (Denmark) and Canadian tundra would be suitable also, but governments change. Existing hot deserts are too accessible)

- above ground. No good reason to put it underground, unless you plan on pretending it isn't there.

- united nuclear-world organization, to administer, collect storage fees, oversee experiments.

 

[skystare]

I just watched the Chernobyl series on TV so my present view of the much vaunted 'Nukes' is a bit coloured. We can't blame the whole of that episode on the shortcomings of a bad regime.

No, not a regime thing; a technical thing. That plant was the equivalent of some 1920s car with no seatbelts, no safety glass, and cable operated brakes. Britain's Windscale plant was of similar design and came to a similar, if less spectacular end.

Modern designs can have walk-away levels of safety. Some, like Terrapower's plant, or thorium reactors, can even burn what we now store as waste, thus eliminating another environmental hazard.

The main drawback to nuclear power today, even more than entrenched coal interests, is widespread superstition.

 

[phinds]

The main drawback to nuclear power today, even more than entrenched coal interests, is widespread superstition.

NIMBYism plays a large part, although I think that's more of an issue for spent fuel.

What you call superstition I would just call ignorance. People who don't understand science (read "most people") cannot / will not take the time to understand the issues and are only aware of the most sensationalist media reports on things like Three Mile Island and Chernobyl.

 

[skystare]

Given we don't have working Gen IV plants, are not likely to for a decade, and the economics of such are questionable given the price trend of utility scale wind and PV, it's not an immediate or arguably even a desirable solution.
No, just tons of highly toxic, long-lived radioactive waste!

Wind or solar can just as easily be used to synthesize airplane fuel. In fact, synthesizing methanol (which makes a terrific ground transport fuel) has been proposed as a power storage method to take better advantage of the cyclic and irregular nature of wind and solar.

And, yes, Gen IV reactors are a decade or more away, but should still be pursued, even if only to use to burn the hot waste we've already created. Meanwhile, all coal plants should be converted posthaste to natural gas for an interim 30% to 40% reduction in CO2 emissions, on the way to whatever clean power tech wins the race.

 

[phinds]

No, not a regime thing; a technical thing.

Sure, but the EXTENT of the disaster was very much a "regime thing".

 

[russ_watters]

Wind or solar can just as easily be used to synthesize airplane fuel. In fact, synthesizing methanol (which makes a terrific ground transport fuel) has been proposed as a power storage method to take better advantage of the cyclic and irregular nature of wind and solar.

Totally agree. The intermittency of wind and solar is an intractable problem when it comes to grid power, but it isn't for other storage media. Why pay extra for the storage (for the grid) when you can generate the power at any time of day or night specifically to store it?

Caveat: I'm not clear on the chemistry of synthetic hydrocarbons; I'm just talking about the electricity. Whether you store it in batteries, hydrogen or synthetic hydrocarbons is a secondary issue.

 

[skystare]

Sure, but the EXTENT of the disaster was very much a "regime thing".

NIMBYism plays a large part, although I think that's more of an issue for spent fuel.

What you call superstition I would just call ignorance. People who don't understand science (read "most people") cannot / will not take the time to understand the issues and are only aware of the most sensationalist media reports on things like Three Mile Island and Chernobyl.

That's the root of it, indeed. Even electric lights are magic to at least a large minority of the population.

As far as my backyard goes, I would rather live next door to a nuke (not graphite moderated!) than a hundred kilometres downwind of a coal plant and all that lead, mercury, cadmium, and uranium going up the stack.

 

[Anand Sivaram]

We use around 70 Million Tonnes of Hydrogen per year, not as an energy storage, but for NH3, fertilizer and petrochemical processing. Out of which 1-2% hydrogen is from ChlorAlkali process (to produce NaOH and Cl2, byproduct is H2). All remaining Hydrogen is from Steam Methane Reformation, SMR of Natural Gas, basically fossil fuel based Hydrogen. There is already a Hydrogen storage, transportation infrastructure tailored for the above mentioned chemical processes. We should increase the use of Renewable electricity based Hydrogen for these chemical processes rather than H2 storage for Fuel Cell Vehicles and transportation.

 

[Tom.G]

Back to the thread title subject:
https://www.msn.com/en-us/news/world/nasa-unveils-its-first-electric-airplane-a-work-in-progress/ar-BBWugPN

 

[cjl]

That's interesting, and I'll be curious to see more details about the design. However, this statement from the article:

"Because electric motor systems are more compact with fewer moving parts than internal-combustion engines, they are simpler to maintain and weigh much less "

is simply false. As discussed earlier in this thread, one downside of electric motors is that they weigh substantially more than turbine engines for a given power output. They are (based on the numbers given by @essenmein earlier in this thread) a bit lighter than piston engines, but not much lighter, and certainly not enough lighter for it to be a large advantage.

 

[russ_watters]

.. is simply false. As discussed earlier in this thread, one downside of electric motors is that they weigh substantially more than turbine engines for a given power output. They are (based on the numbers given by @essenmein earlier in this thread) a bit lighter than piston engines, but not much lighter, and certainly not enough lighter for it to be a large advantage.

For cars it is probably true since you can connect them directly to the wheel hubs and eliminate a lot of drivetrain, but that's not even the main issue: The weight of the batteries swamps any improvement even for cars (a Tesla is like 40% heavier than a comparable car due to the battery) and it is much, much worse for a plane.

 

[anorlunda]

The NASA X-57 is not trying to research efficient electric flight per se. They are trying to find out how wing efficiency can be improved by fans that blow air over the top of the wing along the entire length.

It was because of the entire length requirement, that they added 6 small electric fans along each wing.

If that proves to be smart, someone could invent a jet plane where a portion of the exhaust gasses are piped to blow over the top of the wing rather then exiting straight back, that would replace the electric fans.

 

[cjl]

For cars it is probably true since you can connect them directly to the wheel hubs and eliminate a lot of drivetrain, but that's not even the main issue: The weight of the batteries swamps any improvement even for cars (a Tesla is like 40% heavier than a comparable car due to the battery) and it is much, much worse for a plane.

You don't even want to do that with cars though. The lightest solution in most cases will be to run a single motor per axle, with a conventional differential and driveshafts (though this does also eliminate the transmission, which as you state is a significant weight savings). Even running one motor per wheel, you want them inboard - hub motors are terrible for vehicle dynamics due to the very large amount of unsprung weight they add. I agree that the batteries are the biggest concern, I just wanted to point out that even if you ignore that, a 50-60MW electric motor is around 20 tons while a GeNX gas turbine engine (which makes in that same range of power and also includes the fan, nacelle, nozzle, etc) is only 6 tons, so electric motors are heavier than turbines by a factor of ~3 (and pretty much every commercially relevant airplane these days uses a turbine - only the very smallest light aircraft use pistons).

 

[cjl]

The NASA X-57 is not trying to research efficient electric flight per se. They are trying to find out how wing efficiency can be improved by fans that blow air over the top of the wing along the entire length.

It was because of the entire length requirement, that they added 6 small electric fans along each wing.

If that proves to be smart, someone could invent a jet plane where a portion of the exhaust gasses are piped to blow over the top of the wing rather then exiting straight back, that would replace the electric fans.

Sure, and that makes sense (though modern multi-engine cargo planes like the C130 already blow a significant portion of the wing with propwash, and it does improve lift at low speed). Also, the jet engine thing has been done - take a look at the Antonov AN-74 for example.

 

[anorlunda]

Sure, and that makes sense (though modern multi-engine cargo planes like the C130 already blow a significant portion of the wing with propwash, and it does improve lift at low speed). Also, the jet engine thing has been done - take a look at the Antonov AN-74 for example.

I'm sure that's all true. I think NASA is trying to push it further. They wouldn't make a X plane unless there was some advantage to research. Look at the wings on this picture of the X-57

The Wikipedia article says

The Leading Edge Asynchronous Propeller Technology (LEAPTech) project is a NASA project developing an experimental electric aircraft technology involving many small electric motors driving individual small propellers distributed along the edge of each aircraft wing.[7][8][9] To optimize performance, each motor can be operated independently at different speeds, decreasing reliance on fossil fuels, improving aircraft performance and ride quality, and reducing aircraft noise.

The same article also repeats the same wrong statement about electric motors being lighter. The statement probably misses some qualifications, that might make it true in restricted circumstances. I agree with @cjl that it is not true in general.

Distributed propulsion increases the number and decreases the size of airplane engines. Electric motors are substantially smaller and lighter than jet engines of equivalent power. This allows them to be placed in different, more favorable locations.
,,,
The wing features 12 1.89 ft (0.58 m) diameter cruise propellers that each require 14.4 kW (19.3 hp) of motor power at 55 kn (102 km/h) and turn at 4,548 rpm. The five-blade propellers fold in cruise to reduce drag. Each wingtip hosts two 3-blade 5 ft (1.5 m) diameter cruise propellers that each require 48.1 kW (64.5 hp) at 150 kn (280 km/h) and turn at 2,250 rpm.
...
The optimized wing has 40% of the baseline area, reducing friction drag, and a wing loading 2.6 times higher. The high-lift array of 12 propellers should maintain the 58 kn (107 km/h) stall speed.