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Electric aviation for aircraft larger than 4 passengers is extremely hard. Batteries don't have the energy density needed, and fuel cells are barely at the prot

Electric aviation for aircraft larger than 4 passengers is extremely hard. Batteries don't have the energy density needed, and fuel cells are barely at the prototype stage for aviation. ARPA-E has been looking at this for while, see slide 6 in [1] for an overview of payloads by energy storage medium. If you read a little more through the deck you'll see there are also issues with power output of electric motors in this application. I'm much more optimistic about low-carbon jet fuels. Biofuel options here have already been trialed successfully. And some new synthetic fuel companies [2] that use direct air capture CO2 are looking promising. [1] https://arpa-e.energy.gov/sites/default/files/Grigorii-Soloveichik-Fast-Pitch-2018.pdf https://arpa-e.energy.gov/sites/default/files/Grigorii-Solov... [2] https://en.wikipedia.org/wiki/Carbon_Engineering https://en.wikipedia.org/wiki/Carbon_Engineering

cjrp 7y ago
Even for aeroplanes with just two people, the most usable, all-electric endurance I've seen is about 1 hour[1]. Just about usable for training (mostly circuits around the airfield), but not much else. [1] https://www.pipistrel-aircraft.com/aircraft/electric-flight/alpha-electro/#tab-id-2 https://www.pipistrel-aircraft.com/aircraft/electric-flight/...
- VBprogrammer 7y ago
  Useable in the loosest possible sense of the word since charging it would then take a few hours. By which point lack of daylight or a change in the weather would probably mean no more flying for the day. And even if that could somehow be made to work (perhaps with swappable batteries), it ignores the fact that anyone trained in flying an electric aircraft would be poorly equipped to fly a conventional light aircraft with issues like carburettor icing, shock cooling, leaning with altitude etc.
  - iguy 7y ago
    I thought these were in use at flight schools already. With spare batteries being charged on the ground. Perhaps you can't practice quite everything, but as a cheaper way for people to get some of the hours it sounds great.
    - VBprogrammer 7y ago
      I had a bit of a look, I found one flight school that had committed to buying some electric aircraft and had apparently trialed them before. No real indication of how it works practically. Bearing in mind that a Tesla battery pack weighs 500kg (1200lb) or so. Changing them over wouldn't be a simple operation.
      - iguy 7y ago
        Looks like this is what I was remembering: https://www.pipistrel-aircraft.com/locations/#tab-id-2 https://www.pipistrel-aircraft.com/locations/#tab-id-2 https://www.dw.com/en/electric-flight-finds-its-wings-at-slovenian-startup-with-lightweight-alpha-electro/a-18610096 https://www.dw.com/en/electric-flight-finds-its-wings-at-slo... They have electric planes, and they have flight schools, but whether the two actually intersect or not isn't immediately clear.
  - cjrp 7y ago
    I imagine there'd have to be swappable batteries, but you're absolutely right that you'd be missing out on learning all the other aspects of flying. Although by the time electric aircraft become mainstream for training I'd hope we'd seen the end of carbs!
brohee 7y ago
Or you rethink completely the problem, and beam the energy to the plane. This is more on an engineering challenge, unlike much better batteries which will require a fundamental scientific breaktrough. https://en.wikipedia.org/wiki/Beam-powered_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion
- nradov 7y ago
  Beam propulsion is impractical for aviation due to weather effects and line of sight. It's more than an engineering challenge, it just won't work (except for maybe a few limited cases of very short range flight in clear conditions).
  - nkoren 7y ago
    I know some very smart people who are working on this. http://el-sky.com/ http://el-sky.com/
    - nradov 7y ago
      I wish them luck but I doubt they'll find a magic solution to beaming power through clouds.
  - brohee 7y ago
    You can beam from over the clouds (beaming antenna on a tethered balloon or even drone), and have a huge line of sight to boot. It's an absolutely huge engineering challenge, but at least it doesn't require a discovery worth of the Nobel prize in chemistry (which I guess much improved batteries would warrant).
    - nradov 7y ago
      Having a huge line of sight doesn't solve the problem. The receiver has to be oriented roughly perpendicular to the incoming beam in order to get any reasonable efficiency. And the receiver has to be large in order to account for beam dispersion and aiming inaccuracy. For aerodynamic reasons the receiver can only be on the top or bottom of the aircraft and can't be angled much. So therefore the power transmitter must be close to directly above or below the aircraft. Battery energy density is steadily improving every year. The 2019 Nobel Prize in Chemistry went to the inventors of the lithium ion battery. https://www.nobelprize.org/prizes/chemistry/2019/press-release/ https://www.nobelprize.org/prizes/chemistry/2019/press-relea...
mikepurvis 7y ago
It's also hard because batteries don't get lighter as they deplete, so you're landing a plane with the exact same weight as what you took off with, meaning you need much more rugged (and heavy) landing gear. This is extremely significant when you consider that a fully loaded 747-8 is 970,000 lb, of which up to 326,000 lb is fuel (per Wikipedia's numbers). It's a hard problem, and most proposed solutions are extremely novel and risky, like having an all-battery tugboat that tows the plane up to cruising speed/altitude before returning to the airport to charge or get a battery swap— obviously bad because risky, also adds 50% to runway capacity to land all the tow planes.
- chrisseaton 7y ago
  Apparently they do actually get lighter, but not usefully so.
  - w0utert 7y ago
    Why is that? I’m not a battery expert but it would seem all that charging a battery does is move electrons in the cells around, no?
    - chrisseaton 7y ago
      e=mc2 I think - there’s a Tesla forum thread about it, and Quora articles and things if you look. I’m not a physicist.
      - rubatuga 7y ago
        The mass depletion is less than a milligram...
        chrisseaton 7y ago
        Yeah I read the same threads - that’s what I was referring to.
    - 7y ago
swalsh 7y ago
"Electric aviation for aircraft larger than 4 passengers is extremely hard" To be honest, the worst parts of flying are usually all those other passengers anyways. If you could scale up a fleet on-demand small electric planes. Flying would improve. If you account for all the waiting time, it might be just as fast too.
- Mountain_Skies 7y ago
  That would also make point to point flights easier to coordinate. It might not make sense to have a direct commercial flight from Birmingham, Alabama to Fort Collins, Colorado with today's aircraft but if you only needed to fill four seats, such pairings might work. Not sure how much quieter an electric airplane would be but if it is significantly quieter, smaller airports spread out in greater numbers would also make point to point more viable.
  - notahacker 7y ago
    Four seater aircraft have been around since the first decade of flight, and I'm not sure batteries are going to solve the cost and logistics issues of using them as taxis, except in edge cases where there's a mountain in the way. Flying anywhere near as far as from Birmingham, Alabama to Fort Collins, Colorado is also an exceptionally hard problem for electric aircraft.
Robotbeat 7y ago
What doesn't make sense is to use number of passengers as the figure of merit, here. Technically, that's no limit at all. Aircraft scale up just fine from 2 people to 20 to 200 people.[0] "Power" is even less of a concern: fundamentally, electric motors are just as high performance as turbine engines. The appropriate figure of merit is range and specific energy (i.e. useful energy stored per unit mass), and that IS a limitation, but range of up to 500 miles is feasible for pure electric using existing batteries and structure and aerodynamic efficiency tech. Double or quadruple that is possible with cleverness and more exotic approaches. Even long-haul is possible with certain approaches to pure electric flight (either lithium-air, which has just as high theoretical useful energy as something like gasoline, or staging). The only way "large aircraft" or "high power" is a limitation is just the speed at which things can be certified. Large passenger aircraft cost billions to bring to market, so companies are super conservative about it. But this isn't a fundamental technical limit of electric flight. [0]Technically, there are structural scaling laws which detract from efficiency as you get bigger, but they don't become strong until you're very large... And they're countered by perhaps even more important positive Reynolds Number scaling effects, i.e. your lift to drag improves as your scale increases.
- p_l 7y ago
  Don't forget that most long-haul planes can't land at their MTOW - so the batteries need to be scaled down apropriately.
  - Robotbeat 7y ago
    Or more attention must be paid to engineering landing gear and structures to handle high landing weights. One must approach the problem with an eye to identifying solutions or we'll never get anywhere.
    - p_l 7y ago
      There's more to it than just structural integrity - the heavier plane is generally less safe in an emergency, and despite some ideas proposed around you can't "get rid" of takeoff-power engines unless you drop Go-Around capability.
simion314 7y ago
Could we use Hydrogen that is generated sing clean electricity or is too dangerous?
- asdfadsfgfdda 7y ago
  It's possible, but even liquid hydrogen is not very dense and requires a lot of insulation. https://en.wikipedia.org/wiki/Tupolev_Tu-155 https://en.wikipedia.org/wiki/Tupolev_Tu-155
  - __sy__ 7y ago
    Liquid hydrogen has about 4X the specific energy of Jet-A. But yes, the system as a whole isn't great once you consider what it takes to keep the H2 liquid...
    - sokoloff 7y ago
      Energy density per unit of mass is about 3x in favor of hydrogen. Energy density per unit of volume is about 4x in favor of Jet-A.
      - __sy__ 7y ago
        Sorry you are absolutely correct. Volume matters a lot, maybe almost as much as weight, for aircraft design.
olau 7y ago
> Batteries don't have the energy density needed ... today. There's a lot of money in batteries these days. But from what I gather, it's not just about the batteries, but also about coming up with a design that takes advantage of the possibilities offered by simple electric motors.
SECProto 7y ago
> Electric aviation for aircraft larger than 4 passengers is extremely hard. There is at least one airline transitioning to only electric aircraft already [1]. It's one that focuses on short range flights. And as energy density continues to increase, the range and applications of all-electric aircraft will too. [1] https://electrek.co/2019/03/26/harbour-seaplanes-electric-airline/ https://electrek.co/2019/03/26/harbour-seaplanes-electric-ai...
- AWildC182 7y ago
  Ignore the PR fodder and go run the actual numbers. The actual range/endurance/payload capability of these aircraft is a tiny tiny fraction of their Jet-A/100LL counterparts. It will eventually get better, and a lot of organizations are smart to keep that in mind, but we're not there yet and won't be for a decade at least.
  - SECProto 7y ago
    I specifically mentioned that the range and applications of this initial version is very limited. I was mostly just pointing out a company (with a very unique service) that is at the point where it's possible. It won't make its way to major carriers for a long time (i think a decade would be quite optimistic, given lead times in aviation)
miles_matthias 7y ago
This is the same for the train and shipping industries. I’ve always thought those vehicles had ample opportunities to capture solar rays and wind since they’re out in the open all the time, but we simply don’t have batteries and engines powerful enough yet.
- jimktrains2 7y ago
  I mean, basically every train engine you'll see that isn't steam is powered by electric motors. The diesel engine just makes electricity for them, and is not used to directly drive them via transmission like in a car or truck.
darksaints 7y ago
Solid Oxide Fuel Cells are reaching 2.5kw/kg power densities, Electric motors are reaching 15kw/kg power densities. Combined, that is already better than current jet engines. With cogeneration, SOFCs are reaching 80-85% efficiencies, double that of jet engines. Liquid hydrogen has an energy density of 39kwh/kg, compared to JP8 at 13kwh/kg, but SOFCs could run on pretty much any hydrocarbon. They don't even need cryogenic storage, because fuel consumption rates would be higher than evaporation rates. All the pieces exist for fuel cell powered jets. They've already got the performance levels needed. All they need is to put them together into a cohesive package, and put a shit ton of effort into reliability engineering.
- londons_explore 7y ago
  Cogeneration isn't much use on a plane... Granted a little heat for the cabin is handy, but the rest is largely useless.
  - darksaints 7y ago
    I meant in the context of energy conversion. Solid oxide fuel cells produce high grade heat (~800C) that is perfect for conversion via organic rankine cycles (which will convert ~30% of that heat to electricity) or supercritical co2 brayton cycles (~50% heat to electricity with extremely high power densities[0]). Even without heat capture, SOFCs run at 60-65% efficiency, which is probably good enough. [0] https://www.ge.com/reports/call-ecomagination-ge-building-co2-powered-turbine-generates-10-megawatts-fits-table/ https://www.ge.com/reports/call-ecomagination-ge-building-co...
- 08-15 7y ago
  > Combined, that is already better than current jet engines. Wikipedia lists the GE90-115B at 10kW/kg, four times better than your SOFC. Did you just combine motor and fuel cell to get 17.5kW/kg? That would be hilarious. > With cogeneration, SOFCs are reaching 80-85% efficiencies That's creative accounting. You're counting waste heat as if it was useful.
- kortilla 7y ago
  Energy density of jet fuel is 45MJ/kg. Fuel is a significant portion of the weight on the plane. Requiring a 10x+ increase in fuel weight is a non-starter no matter how light the engines.

cjrp 7y ago

Even for aeroplanes with just two people, the most usable, all-electric endurance I've seen is about 1 hour[1]. Just about usable for training (mostly circuits around the airfield), but not much else. [1] https://www.pipistrel-aircraft.com/aircraft/electric-flight/alpha-electro/#tab-id-2 https://www.pipistrel-aircraft.com/aircraft/electric-flight/...

VBprogrammer 7y ago
Useable in the loosest possible sense of the word since charging it would then take a few hours. By which point lack of daylight or a change in the weather would probably mean no more flying for the day. And even if that could somehow be made to work (perhaps with swappable batteries), it ignores the fact that anyone trained in flying an electric aircraft would be poorly equipped to fly a conventional light aircraft with issues like carburettor icing, shock cooling, leaning with altitude etc.
- iguy 7y ago
  I thought these were in use at flight schools already. With spare batteries being charged on the ground. Perhaps you can't practice quite everything, but as a cheaper way for people to get some of the hours it sounds great.
  - VBprogrammer 7y ago
    I had a bit of a look, I found one flight school that had committed to buying some electric aircraft and had apparently trialed them before. No real indication of how it works practically. Bearing in mind that a Tesla battery pack weighs 500kg (1200lb) or so. Changing them over wouldn't be a simple operation.
    - iguy 7y ago
      Looks like this is what I was remembering: https://www.pipistrel-aircraft.com/locations/#tab-id-2 https://www.pipistrel-aircraft.com/locations/#tab-id-2 https://www.dw.com/en/electric-flight-finds-its-wings-at-slovenian-startup-with-lightweight-alpha-electro/a-18610096 https://www.dw.com/en/electric-flight-finds-its-wings-at-slo... They have electric planes, and they have flight schools, but whether the two actually intersect or not isn't immediately clear.
- cjrp 7y ago
  I imagine there'd have to be swappable batteries, but you're absolutely right that you'd be missing out on learning all the other aspects of flying. Although by the time electric aircraft become mainstream for training I'd hope we'd seen the end of carbs!

brohee 7y ago

Or you rethink completely the problem, and beam the energy to the plane. This is more on an engineering challenge, unlike much better batteries which will require a fundamental scientific breaktrough. https://en.wikipedia.org/wiki/Beam-powered_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion

nradov 7y ago
Beam propulsion is impractical for aviation due to weather effects and line of sight. It's more than an engineering challenge, it just won't work (except for maybe a few limited cases of very short range flight in clear conditions).
- nkoren 7y ago
  I know some very smart people who are working on this. http://el-sky.com/ http://el-sky.com/
  - nradov 7y ago
    I wish them luck but I doubt they'll find a magic solution to beaming power through clouds.
- brohee 7y ago
  You can beam from over the clouds (beaming antenna on a tethered balloon or even drone), and have a huge line of sight to boot. It's an absolutely huge engineering challenge, but at least it doesn't require a discovery worth of the Nobel prize in chemistry (which I guess much improved batteries would warrant).
  - nradov 7y ago
    Having a huge line of sight doesn't solve the problem. The receiver has to be oriented roughly perpendicular to the incoming beam in order to get any reasonable efficiency. And the receiver has to be large in order to account for beam dispersion and aiming inaccuracy. For aerodynamic reasons the receiver can only be on the top or bottom of the aircraft and can't be angled much. So therefore the power transmitter must be close to directly above or below the aircraft. Battery energy density is steadily improving every year. The 2019 Nobel Prize in Chemistry went to the inventors of the lithium ion battery. https://www.nobelprize.org/prizes/chemistry/2019/press-release/ https://www.nobelprize.org/prizes/chemistry/2019/press-relea...

mikepurvis 7y ago

It's also hard because batteries don't get lighter as they deplete, so you're landing a plane with the exact same weight as what you took off with, meaning you need much more rugged (and heavy) landing gear. This is extremely significant when you consider that a fully loaded 747-8 is 970,000 lb, of which up to 326,000 lb is fuel (per Wikipedia's numbers). It's a hard problem, and most proposed solutions are extremely novel and risky, like having an all-battery tugboat that tows the plane up to cruising speed/altitude before returning to the airport to charge or get a battery swap— obviously bad because risky, also adds 50% to runway capacity to land all the tow planes.

chrisseaton 7y ago
Apparently they do actually get lighter, but not usefully so.
- w0utert 7y ago
  Why is that? I’m not a battery expert but it would seem all that charging a battery does is move electrons in the cells around, no?
  - chrisseaton 7y ago
    e=mc2 I think - there’s a Tesla forum thread about it, and Quora articles and things if you look. I’m not a physicist.
    - rubatuga 7y ago
      The mass depletion is less than a milligram...
      - chrisseaton 7y ago
        Yeah I read the same threads - that’s what I was referring to.
  - 7y ago

swalsh 7y ago

"Electric aviation for aircraft larger than 4 passengers is extremely hard" To be honest, the worst parts of flying are usually all those other passengers anyways. If you could scale up a fleet on-demand small electric planes. Flying would improve. If you account for all the waiting time, it might be just as fast too.

Mountain_Skies 7y ago
That would also make point to point flights easier to coordinate. It might not make sense to have a direct commercial flight from Birmingham, Alabama to Fort Collins, Colorado with today's aircraft but if you only needed to fill four seats, such pairings might work. Not sure how much quieter an electric airplane would be but if it is significantly quieter, smaller airports spread out in greater numbers would also make point to point more viable.
- notahacker 7y ago
  Four seater aircraft have been around since the first decade of flight, and I'm not sure batteries are going to solve the cost and logistics issues of using them as taxis, except in edge cases where there's a mountain in the way. Flying anywhere near as far as from Birmingham, Alabama to Fort Collins, Colorado is also an exceptionally hard problem for electric aircraft.

Robotbeat 7y ago

What doesn't make sense is to use number of passengers as the figure of merit, here. Technically, that's no limit at all. Aircraft scale up just fine from 2 people to 20 to 200 people.[0] "Power" is even less of a concern: fundamentally, electric motors are just as high performance as turbine engines. The appropriate figure of merit is range and specific energy (i.e. useful energy stored per unit mass), and that IS a limitation, but range of up to 500 miles is feasible for pure electric using existing batteries and structure and aerodynamic efficiency tech. Double or quadruple that is possible with cleverness and more exotic approaches. Even long-haul is possible with certain approaches to pure electric flight (either lithium-air, which has just as high theoretical useful energy as something like gasoline, or staging). The only way "large aircraft" or "high power" is a limitation is just the speed at which things can be certified. Large passenger aircraft cost billions to bring to market, so companies are super conservative about it. But this isn't a fundamental technical limit of electric flight. [0]Technically, there are structural scaling laws which detract from efficiency as you get bigger, but they don't become strong until you're very large... And they're countered by perhaps even more important positive Reynolds Number scaling effects, i.e. your lift to drag improves as your scale increases.

p_l 7y ago
Don't forget that most long-haul planes can't land at their MTOW - so the batteries need to be scaled down apropriately.
- Robotbeat 7y ago
  Or more attention must be paid to engineering landing gear and structures to handle high landing weights. One must approach the problem with an eye to identifying solutions or we'll never get anywhere.
  - p_l 7y ago
    There's more to it than just structural integrity - the heavier plane is generally less safe in an emergency, and despite some ideas proposed around you can't "get rid" of takeoff-power engines unless you drop Go-Around capability.

simion314 7y ago

Could we use Hydrogen that is generated sing clean electricity or is too dangerous?

asdfadsfgfdda 7y ago
It's possible, but even liquid hydrogen is not very dense and requires a lot of insulation. https://en.wikipedia.org/wiki/Tupolev_Tu-155 https://en.wikipedia.org/wiki/Tupolev_Tu-155
- __sy__ 7y ago
  Liquid hydrogen has about 4X the specific energy of Jet-A. But yes, the system as a whole isn't great once you consider what it takes to keep the H2 liquid...
  - sokoloff 7y ago
    Energy density per unit of mass is about 3x in favor of hydrogen. Energy density per unit of volume is about 4x in favor of Jet-A.
    - __sy__ 7y ago
      Sorry you are absolutely correct. Volume matters a lot, maybe almost as much as weight, for aircraft design.

olau 7y ago

> Batteries don't have the energy density needed ... today. There's a lot of money in batteries these days. But from what I gather, it's not just about the batteries, but also about coming up with a design that takes advantage of the possibilities offered by simple electric motors.

SECProto 7y ago

> Electric aviation for aircraft larger than 4 passengers is extremely hard. There is at least one airline transitioning to only electric aircraft already [1]. It's one that focuses on short range flights. And as energy density continues to increase, the range and applications of all-electric aircraft will too. [1] https://electrek.co/2019/03/26/harbour-seaplanes-electric-airline/ https://electrek.co/2019/03/26/harbour-seaplanes-electric-ai...

AWildC182 7y ago
Ignore the PR fodder and go run the actual numbers. The actual range/endurance/payload capability of these aircraft is a tiny tiny fraction of their Jet-A/100LL counterparts. It will eventually get better, and a lot of organizations are smart to keep that in mind, but we're not there yet and won't be for a decade at least.
- SECProto 7y ago
  I specifically mentioned that the range and applications of this initial version is very limited. I was mostly just pointing out a company (with a very unique service) that is at the point where it's possible. It won't make its way to major carriers for a long time (i think a decade would be quite optimistic, given lead times in aviation)

miles_matthias 7y ago

This is the same for the train and shipping industries. I’ve always thought those vehicles had ample opportunities to capture solar rays and wind since they’re out in the open all the time, but we simply don’t have batteries and engines powerful enough yet.

jimktrains2 7y ago
I mean, basically every train engine you'll see that isn't steam is powered by electric motors. The diesel engine just makes electricity for them, and is not used to directly drive them via transmission like in a car or truck.

darksaints 7y ago

Solid Oxide Fuel Cells are reaching 2.5kw/kg power densities, Electric motors are reaching 15kw/kg power densities. Combined, that is already better than current jet engines. With cogeneration, SOFCs are reaching 80-85% efficiencies, double that of jet engines. Liquid hydrogen has an energy density of 39kwh/kg, compared to JP8 at 13kwh/kg, but SOFCs could run on pretty much any hydrocarbon. They don't even need cryogenic storage, because fuel consumption rates would be higher than evaporation rates. All the pieces exist for fuel cell powered jets. They've already got the performance levels needed. All they need is to put them together into a cohesive package, and put a shit ton of effort into reliability engineering.

londons_explore 7y ago
Cogeneration isn't much use on a plane... Granted a little heat for the cabin is handy, but the rest is largely useless.
- darksaints 7y ago
  I meant in the context of energy conversion. Solid oxide fuel cells produce high grade heat (~800C) that is perfect for conversion via organic rankine cycles (which will convert ~30% of that heat to electricity) or supercritical co2 brayton cycles (~50% heat to electricity with extremely high power densities[0]). Even without heat capture, SOFCs run at 60-65% efficiency, which is probably good enough. [0] https://www.ge.com/reports/call-ecomagination-ge-building-co2-powered-turbine-generates-10-megawatts-fits-table/ https://www.ge.com/reports/call-ecomagination-ge-building-co...
08-15 7y ago
> Combined, that is already better than current jet engines. Wikipedia lists the GE90-115B at 10kW/kg, four times better than your SOFC. Did you just combine motor and fuel cell to get 17.5kW/kg? That would be hilarious. > With cogeneration, SOFCs are reaching 80-85% efficiencies That's creative accounting. You're counting waste heat as if it was useful.
kortilla 7y ago
Energy density of jet fuel is 45MJ/kg. Fuel is a significant portion of the weight on the plane. Requiring a 10x+ increase in fuel weight is a non-starter no matter how light the engines.