Question About Electric Aircraft Propulsion

[boneh3ad]

In the overall picture, this is one of the most relevant facts. It's only when all electric transport energy produces almost no climate effect that the quoted massive ratio can be ignored. An intermediate solution would be to store energy in the form of Hydrogen, which sits somewhere in between. Hydrogen is something that seems to vary in popularity over the years.

Hydrogen and manned aviation have a fraught history, as it turns out.

 

[sophiecentaur]

Hydrogen and manned aviation have a fraught history, as it turns out.

Yes. The shadow of Hindenburg doesn't fade.

 

[cjl]

It also has a very poor density, so even though it has the advantage of being very light, you need massive tanks (and the associated drag and weight penalty) to carry very much of it.

 

[sophiecentaur]

It also has a very poor density, so even though it has the advantage of being very light, you need massive tanks (and the associated drag and weight penalty) to carry very much of it.

The timescale is a bit different for rockets but why not store it cryogenically? That must have ben considered for planes.

 

[cjl]

Even cryogenically the density sucks. LH2 has a density of 71kg/m^3, while kerosene (or Jet A) is around 810 kg/m^3. Jet fuel has 42.8 MJ/kg, so the volumetric energy content of jet fuel is 34.7 GJ/m^3. Hydrogen has 130MJ/kg, but combine this with the low density and it only has 9.2 GJ/m^3, so you need nearly 4 times the tank volume to store identical energy compared to jet fuel.

 

[Klystron]

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.

My post stipulated an upper limit of one cargo container built from light weight materials similar to cargo space for a typical delivery truck that carry a few tons at most or equivalent 10 passengers with luggage. I fail to see the humor in citing a ridiculous 100,000 tons cargo criteria for a single aircraft. Unless you meant kilograms?

The largest civilian aircraft I have entered a C-5A Galaxy could possibly lift 10E5 Kg under stringent take-off and landing conditions, minimum fuel, minimum crew, perfect weather conditions but so what?

...but the maximum allowable payload was reduced from 220,000 to 190,000 lb (100,000 to 86,000 kg). At the time, a 90% probability was predicted that no more than 10% of the fleet of 79 airframes would reach their fatigue life of 19,000 hours without cracking of the wing.[15]

 

[cjl]

The point of the 100k tons is because that would make it similar to modern cargo ships in capacity, since the suggestion was to replace earthbound transportation with airships (not to replace existing cargo aircraft). You're right that that would be a pretty crazy amount to move by air though, since the largest existing cargo plane can only carry around 250,000 kg of payload.

 

[Klystron]

The point of the 100k tons is because that would make it similar to modern cargo ships in capacity, since the suggestion was to replace earthbound transportation with airships (not to replace existing cargo aircraft). You're right that that would be a pretty crazy amount to move by air though, since the largest existing cargo plane can only carry around 250,000 kg of payload.

Negative. I made no such suggestion. Read the post. While unclear, perhaps, the reference to marine freight was only to compare the requirement of time in transit. Ships are slow relative to jet aircraft. The counter-argument devoid of sarcasm would be that ships carry large tonnage as the weight is supported by water. This argument could lead to comparison of transit times versus payload. Can we state that generally the faster the transportation, the lighter the payload?

Earthbound transportation is unclear as the atmosphere is earthbound.

My reference to LTA and airships was a carry-over from a parallel thread on NASA airships for exploring solar system objects such as Titan and Mars. No 'giant gasbags' mentioned. Amazon, among others, is diverting cargo from trucks to aerial delivery systems, all electric powered AFAIK.

 

[russ_watters]

My post stipulated an upper limit of one cargo container built from light weight materials similar to cargo space for a typical delivery truck that carry a few tons at most or equivalent 10 passengers with luggage. I fail to see the humor in citing a ridiculous 100,000 tons cargo criteria for a single aircraft. Unless you meant kilograms?

The largest civilian aircraft I have entered a C-5A Galaxy could possibly lift 10E5 Kg under stringent take-off and landing conditions, minimum fuel, minimum crew, perfect weather conditions but so what?

It was 6 weeks ago, so I don't recall if I read the post correctly (whether you intended an exact container ship replacement), but I do know my post was way less than 50% sarcastic. Whether what you suggest would take a large number of obscenely enormous airships or an obscenely enormous number of large airships is just two sides of the same coin. So, some related numbers I'd like to see for the other side:

1. How many airplanes are in flight over the US at any given time?
2. How many single container trucks are on the road?
3. What is the global annual helium production volume?
4. What volume of helium would be required to lift that many containers simultaneously?

A quick google tells me that replacing one large container ship with single container airships would roughly double the number of aircraft in the air at one time, globally.

If you're going to propose an outside-the-box idea, you really should do a little work to check on feasibility.

Can we state that generally the faster the transportation, the lighter the payload?

No, I wouldn't say that. There are several constraints affecting payload and transit speed. The constraints tend to be more fundamental to the mode of transport than related to speed/capacity.

 

[Klystron]

OK. Leave it at 6 weeks, sleepless night, mixed threads on my part. The LTA concept was asked on a speculative forum. I must have merged airship with electric propulsion. An interesting concept but I never mentioned helium or gas bags. You are using a form of argument of going to extremes in order to ridicule. ALL ground vehicles. ALL marine tonnage. Largest bulk cargo aircraft ever built.

Other engineers on this thread got it right that current e-propelled aircraft frames are designed for minimum weight. You are stuck in Leviathan. Current electric aerial delivery systems mimic hummingbird.

 

[sophiecentaur]

Current electric aerial delivery systems mimic hummingbird.

Local deliveries by drone could work - except that there WILL be accidents. Get indoors if you ever hear one!

 

[russ_watters]

OK. Leave it at 6 weeks, sleepless night, mixed threads on my part...

Ok...yeah, looking back at the original discussion, you already acknowledged the original description was "inapt". So I'm not even sure why you are circling back to it (I was letting it drop at that).

[first post]
...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.

[second post]
The reference to water-borne cargo was meant as analogy, perhaps inapt. I have seen an articulated truck loading three standard containers. Most ground vehicles carry only one or two containers.

I didn't reply to the second, but my reply would have been the same as the one I gave today; a lot of one-container airships or fewer 10,000 container airships are two sides of the same coin/problem.

The LTA concept was asked on a speculative forum. I must have merged airship with electric propulsion. An interesting concept but I never mentioned helium or gas bags.

Now I don't follow: if you didn't mean helium or gas bags, what does "LTA" mean? As far as I know, that's the only way to achieve it.

You are using a form of argument of going to extremes in order to ridicule. ALL ground vehicles. ALL marine tonnage. Largest bulk cargo aircraft ever built.

I am not intending that, nor did I say most of those -- indeed, I think my examples substantially understate the scale problem in what you are suggesting. Most of my examples were for replacement of a tiny fraction of our shipping (one container ship). Asking how many trucks are on the road was more for scale -- I have no idea how many shipping containers are in transit at anyone time. And on the other end, saying how many planes were in the air wasn't to say I think you're talking about replacing all planes, but using that number to show how unworkable what you propose is, to replace even one container ship. But if you have a more specific idea in mind as to what "much ground transportation" means, please say so. 1%? 10%? 40%? If you don't say what you mean, what choice do I have but to guess what you mean?

 

[Klystron]

Thanks. I finally remember my point in returning to this thread. Not as important or profound as I thought but here is my reasoning.

Around the time this type of electric aircraft was reported in the media, my family noticed a grocery delivery truck with an enormous fairing stretching from the tractor cab over part of the top of the refrigerated cargo section high balling through the Mojave desert on a lower section of highway.

My artist granddaughter commented on the "decorative panels" and thick coils running under the fairing into the refrigeration unit. My engineer granddaughter recognized solar panels on the fairing and uncovered area of the roof of the cargo container from school projects and working with her dad. Were the panels providing electricity from solar to help power the refrigerators?

Discussion led to comparing the super-light solar powered aircraft with broad wing geometry carrying two passengers to a speculative electric delivery truck with batteries recharged by the sun between roadside charging stations.

What was the optimum "solar area" for an e-truck? An e-train? E-car?
Should the truck "follow the Sun" as the solar aircraft was reported to have done?
How efficient were the solar panels versus weight and size?
Instead of frozen groceries, should our hypothetical E-truck just carry solar panels to market...?

 

[sophiecentaur]

"much ground transportation"

much ground transport can be achieved on the ground on rails. Very efficient and nothing to drop out of the sky. Sea transport costs more to run and is slow but, as in the days of canal transport, a delay is not necessarily a problem if the arrival rate is high enough. The safety aspect makes sea transport attractive, too. Except when approaching land, engine failure just means you stop. Near land, you may need rescuing but that probably has a timescale measurable in hours.
Passenger transport is a different matter but, as I have remarked several times in the past, there are alternatives to many journeys by plane. Modifying lifestyles is not an unthinkable concept if the result is to reduce 'carbon' footprint.

 

[merriam]

In the overall picture, this is one of the most relevant facts. It's only when all electric transport energy produces almost no climate effect that the quoted massive ratio can be ignored. An intermediate solution would be to store energy in the form of Hydrogen, which sits somewhere in between. Hydrogen is something that seems to vary in popularity over the years.

Actually, Hydrogen has an energy density (per kg) of approximately 3 times that of jet fuel (varying slightly with temperature) and at least 100 times that of Lithium polymer batteries. However, it is tough to use. Among its drawbacks: Takes a lot of volume store as a gas, though hydrogen fuel cell cars (like Toyota Mirai) have a range comparable to that of gasoline vehicles 2) hydrogen tends to degrade any metal it comes in contact with (see hydrogen embrittlement) though this may be overcome with coatings and liners. 3) it is explosive at all concentrations between 5% and 95% (safety issue). The most serious however is that hydrogen gas contains some molecules traveling at escape velocity, so it might depart the Earth's atmosphere (as helium does). Normally it is attached to oxygen (or carbon) so this isn't a problem, but in a hydrogen economy it could be a problem.

 

[cjl]

The most serious however is that hydrogen gas contains some molecules traveling at escape velocity, so it might depart the Earth's atmosphere (as helium does). Normally it is attached to oxygen (or carbon) so this isn't a problem, but in a hydrogen economy it could be a problem.

Nah, that isn't an issue at all. We have so much hydrogen available that the small amount lost to atmospheric escape is completely irrelevant.

 

[256bits]

What was the optimum "solar area" for an e-truck? An e-train? E-car?
Should the truck "follow the Sun" as the solar aircraft was reported to have done?
How efficient were the solar panels versus weight and size?
Instead of frozen groceries, should our hypothetical E-truck just carry solar panels to market...?

If the top roof area of an 18 wheeler was covered with solar panels, expect about 5 kW production bright sun overhead, and then decreasing to 0 at night. Considering that a reefer ( the refrigeration container ) will produce about 10kW of refrigeration, and that depends on ambient / interior unit temperature difference, one comes up a bit short even at peak sunlight. One could thicken the insulation to reduce the cooling load, if one live and make a profit with reduced cargo carrying capacity compared to the competitors running on current technology. People are probably working on it. Perhaps you saw a test model.

Are you sure the fairing was not just there for reduced aerodynamic drag - very common these days.
I don't know what the hoses would be except the hookup from the tractor to trailer for lighting and braking for an 18 wheeler if that is what it was.

 

[Klystron]

If the top roof area of an 18 wheeler was covered with solar panels, expect about 5 kW production bright sun overhead, and then decreasing to 0 at night. Considering that a reefer ( the refrigeration container ) will produce about 10kW of refrigeration, and that depends on ambient / interior unit temperature difference, one comes up a bit short even at peak sunlight. One could thicken the insulation to reduce the cooling load, if one live and make a profit with reduced cargo carrying capacity compared to the competitors running on current technology. People are probably working on it. Perhaps you saw a test model.

Are you sure the fairing was not just there for reduced aerodynamic drag - very common these days.
I don't know what the hoses would be except the hookup from the tractor to trailer for lighting and braking for an 18 wheeler if that is what it was.

Right you are about each detail. +10.
As driver I only glanced at the grocery truck. The side I saw was encased in removable film announcing experimental delivery technology for regional stores (now owned by Kroger). Most likely a test model, as you say.

The front fairing looked enormous, certainly to reduce aerodynamic drag and protect the film. In retrospect the cable bundles and coils were probably part of the reefer-truck mechanism but also for radio communication. If memory serves, I saw a tall whip antenna for citizens band (CB), an AM/FM antenna and possibly (?) some kind of compact microwave rig -- like a Lewis antenna but smaller.

My (artist) granddaughter corrected my recollection. She liked the irredescent colors of the truck. My daughter and her daughter, who had some experience with home solar panels, noticed what they thought were solar cells on the trailer but not on the cab. Certainly that section of the Mojave high desert would be ideal for testing solar-augmented reefer trucks. The road alert signs, some lights and all emergency phones have solar panels that charge during the day. Thanks.

 

[zekise]

Pretty much all modern turbofans run with a blade tip speed at full throttle of mach 1.5 or so.
4MW isn't anywhere close to enough. Propulsive power at cruise for a modern jetliner the size of a 787 is more on the order of 40MW. At takeoff, each engine has to be making more like 50-60MW of shaft power just to run the front fan. Rerun the electric numbers knowing that and you'll see why running jetliners on electric power is a pipedream without a massive breakthrough in technology.

I think you need about 10 tons of thrust per engine during cruise for a 737. If the thrust to power ratio of an electric fan is 6 kg/kW, then you need 10,000 / 6 = 1,660 kW or 1.7 MW per engine. This assumes 95% motor efficiency and 90% propulser efficiency. Developers are building 0.75 MW motors. So four of these will do.

 

[cjl]

I think you need about 10 tons of thrust per engine during cruise for a 737. If the thrust to power ratio of an electric fan is 6 kg/kW, then you need 10,000 / 6 = 1,660 kW or 1.7 MW per engine. This assumes 95% motor efficiency and 90% propulser efficiency. Developers are building 0.75 MW motors. So four of these will do.

You actually don't need anywhere close to 40klb of thrust at cruise for a 737 - that's about a factor of 4 or 5 high. In reality, a 737 might have between 7000 and 10,000 pounds of drag (and thus thrust) during cruise, depending on gross weight, which 737 variant it is, etc.

Unfortunately, though, your thrust to power ratio is also way, way too high. At cruise, a 737 engine might be ingesting around 300kg/s of air at 250m/s. To generate 20kN (4500 lb) of thrust (since it has 2 engines), this air will need to be accelerated to 317m/s by the engine. This involves adding about 11MW of power to the air jet, so using your propulsor efficiency value of 90%, we need 12.5MW of shaft power per engine to generate 4500lb of thrust, for a thrust to power ratio of less than 2N/w.

The reason the power level needs to be so high is because of the high speed and the low density of air at altitude, resulting in a fairly low massflow. With a higher massflow (larger prop disk size, lower altitude) and lower airspeed, you probably could get a 737 to fly on only 3 or 4MW, but you'll never get anywhere close to the altitude or airspeed without 20-30MW.

 

[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.