[HN Gopher] Ultralight lithium-sulfur batteries for electric air...
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Ultralight lithium-sulfur batteries for electric airplanes
Author : pross356
Score : 111 points
Date : 2020-08-19 18:14 UTC (4 hours ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| zzedd wrote:
| For speculative (near future) fiction, that uses a lightweight
| electrically-powered aircraft as a linking device, "News From
| Gardenia" by Robert Llewellyn, is a good read.
| ncmncm wrote:
| Can I just say this was an extremely well-written article? Each
| time a question came to mind, it was answered within the next two
| sentences.
| [deleted]
| rolleiflex wrote:
| > Oxis recently developed a prototype lithium-sulfur pouch cell
| that proved capable of 470 Wh/kg, and we expect to reach 500
| Wh/kg within a year.
|
| Meanwhile, gasoline / petrol / benzin (wherever you are in the
| world) has an energy density of 12200 Wh/kg.
|
| In other words, even in a world where all petroleum is perfectly
| depleted, we would still be producing synthetic gasoline for
| high-demand applications and capturing 100% of the emissions for
| recycling -- essentially using gasoline as a battery. It's just
| too good an energy storage to ignore.
|
| I've been looking at cars and geeking out on internal combustion
| engines for the past few weeks since I had to buy a car, and for
| the usual Silicon Valley guy such as yours truly whose standpoint
| on cars hadn't been much more than 'I want a Tesla', it was an
| outright revelation.
|
| ICEs are real technology -- and for a software guy it is easy to
| understand because the complexity is bounded by physical
| dimensions of parts (i.e they don't work past a certain small
| size) so you literally see human-size machinery with human size
| movements. It's been a refreshing change from potentially
| unbounded complexity of software.
| outworlder wrote:
| > Meanwhile, gasoline / petrol / benzin (wherever your are in
| the world) has an energy density of 12200 Wh/kg.
|
| Yeah, and yet, if you are talking about cars, you are bound by
| the Carnot efficiency, which is at most 35% no matter what(in
| reality more like 20% or so). Now gasoline goes to 4270 Wh/kg
| even assuming the best possible engine. That's without
| accounting for extraction, refining, transportation losses.
|
| Yes, still 10x more. However, you don't really need 10x, as
| electric motors are way more efficient (85 - 90%).
|
| The long range Tesla Model 3 only requires a 75kWh (72.5
| usable) battery for a 450km (~279 miles). In other words, it
| uses 161 Wh/km .
|
| > ICEs are real technology
|
| So are EVs. For all the tech they tend to have, it boils down
| to: Battery and electric motor. There are some pretty reliable
| solid state electronics to monitor and deliver power, but
| that's all you really need(even the 'charger' is optional - and
| for quick charging, it's located outside the vehicle).
|
| You could theoretically use decades old technology to control
| the amount of power delivered to the motors - so you could see
| with your naked eye. And, in fact, we have done that! The first
| cars ever designed were electric, battery tech just wasn't
| there. It doesn't make sense to do that in this day and age,
| electronics driven by software work better. We would replace
| ICE engines with solid state components if it was possible.
|
| Now, for an ICE car, you have valves. You have a crankshaft,
| controlling said valves (and tied to pistons) - hundreds of
| moving parts right there. You have spark plugs (and a coil to
| generate the voltage). Air filters. You have an alternator
| (usually driven by a belt). You have water pumps (optional on
| EVs - and for the battery only, when they are used). You need
| fuel pumps. Fuel filters. Radiators (because of all the engine
| inefficiency). You need oil - and replace said oil, as well as
| the oil filter. You have an exhaust with a catalytic converter.
|
| Each one of those things may break and some are even
| consumables. So much crap EVs don't have.
|
| And you still have software and the tiny electronics you can't
| see with your eyes ! Unless, of course, you still use
| carburetors (which add a few hundred more parts each).
|
| EVs are much, much simpler. And more reliable, less parts and
| most of them don't move. The only thing that degrades is the
| battery. For now. I drive a Leaf, which is notorious for having
| no battery thermal management, and degradation is minimal.
| zkms wrote:
| > you are bound by the Carnot efficiency, which is at most
| 35% no matter what(in reality more like 20% or so)
|
| Internal-combustion engines are not limited by the Carnot
| efficiency; see page 4 of these slides. In fact, latest-
| generation Priuses gets almost 40% thermal efficiency on the
| gasoline engine, and large diesel engines -- thanks to lean
| combustion (favorable ratio of specific heats for the gases
| that push on the piston), no throttling losses, higher
| compression ratio -- do even better.
|
| https://www.energy.gov/sites/prod/files/2014/03/f8/deer11_ed.
| ..
| petre wrote:
| True but gas turbines (not ICEs, those are mainly used on small
| aircraft) can reach a theoretical efficiency of 30% while
| electric motor efficiency of 92...93% is quite common. So you
| can multiply that number by 1/3 since 2/3 becomes heat.
| microcolonel wrote:
| With turbofans and turbojets it gets a little harder as well,
| since unlike stationary turbine generators, not all "waste
| heat" is actually wasted, when considering thrust-specific
| fuel consumption.
| pixelbash wrote:
| > ICEs are real technology
|
| They are real technology, in the sense that this is where over
| 100 years of optimising every aspect of ICE has got us to.
| Fascinatingly complex, extremely well engineered, and depending
| on the brand still relatively likely to break down within the
| first few years.
|
| The engines are also longer designed to be maintained without
| special processes, and are increasingly designed around
| emission regulations. To the point where a lot of the
| complexity is in emission systems and the cars are choked by
| their own extremely lean fuel maps.
|
| Electric motors on the other hand are still relatively
| unoptimised and the potential in things like torque vectoring
| is amazing. I'll miss driving manual, but it's getting just
| about impossible to buy those now anyway.
| DarmokJalad1701 wrote:
| Sure. The average car gets 25 mpg, consuming ~1400 Wh/mi. A
| Tesla Model 3 uses ~240 Wh/mi. Thats almost a factor of 6
| difference. You don't need quite as much energy when you are
| significantly more efficient and it is only getting more
| efficient as time goes on. Also add in the factor of recovering
| energy using regenerative braking which is impossible with ICE.
|
| With a car, unless you have a specific use-case where you are
| driving 200+ miles a day, an EV is a no-brainer when it comes
| to efficiency in operating cost as well as emissions and
| overall energy use.
| asdfadsfgfdda wrote:
| A Model 3 is smaller than the average car, it should be
| compared to something like a Honda Accord hybrid. Both have a
| similar cabin volume of ~100 ft^3. The fuel economy for a
| hybrid Accord is 48 mpg, or 760 Wh/mile.
|
| The Accord is also $10k cheaper, presumably that is because
| the Model 3 requires more materials and more embodied energy.
| mulmen wrote:
| > The Accord is also $10k cheaper, presumably that is
| because the Model 3 requires more materials and more
| embodied energy.
|
| Do economies of scale and manufacturing expertise factor in
| to "embodied energy"?
| jmercouris wrote:
| While energy density as an important measure when talking about
| a fuel source, we can not forget about the other part of the
| equation. Energy efficiency, of those units of energy in jet
| fuel, how much of it can we effectively utilize? In other
| words, what is the distance travelled for a given weight of jet
| fuel vs the distance travelled for a given weight of batteries.
| tenuousemphasis wrote:
| >Meanwhile, gasoline / petrol / benzin (wherever your are in
| the world) has an energy density of 12200 Wh/kg.
|
| Electric motors are 90% efficient, ICE are less than 20%. So in
| reality gasoline is 2-3x as energy dense, when you look at how
| much of the fuel's energy can be used to do useful work.
| throwaway189262 wrote:
| Electric vehicles have range problems in cold climates
| because you don't have waste heat.
|
| On a very cold day 60% of that waste heat might be used to
| warm the cabin. This translates to up to 50% range reduction
| in electric vehicles.
| DarmokJalad1701 wrote:
| This is a myth. It is not nearly that bad. I own a Model 3
| in a place where it is frequently below 30F in the winter.
| My last road trip when it was 17F throughout I consistently
| got around ~250-270 miles out of 310 rated range. The new
| hybrid heat-pump based system in the newer models makes it
| even more efficient.
| throwaway189262 wrote:
| Isn't the range penalty a lot worse on short trips? When
| I was researching Leaf I noped out of buying one because
| range penalty in the cold for daily commutes was almost
| 50%
| tengbretson wrote:
| You also have to factor in the weight reduction as petroleum
| fuel is consumed vs the fixed weight of a battery.
| Tuna-Fish wrote:
| True, but the effect of this is less than it sounds like,
| as while the fuel is consumed, in an ICE car the engine is
| the heavy part and that isn't consumed.
| klyrs wrote:
| Don't discount this so easily. You can't land a jumbo jet
| immediately after takeoff because its maximum allowed
| landing weight is much less than its maximum allowed
| takeoff weight. So you'll need a stronger fuselage and
| landing gear for electric plans, which means even more
| weight
| tenuousemphasis wrote:
| Yes, that's true. And the cost of electricity vs. fuel.
| Safety is another factor. The point is there's a lot more
| to it than Wh/kg of the energy source.
| outworlder wrote:
| This is a huge deal for aircraft. Not much for cars, as EVs
| are not carrying a lot of extra baggage that ICE cars
| require.
|
| When was the last time you saw an ICE with both a trunk and
| a frunk? :)
| throwaway189262 wrote:
| Modern gasoline engines are not as heavy as you may
| think. I once transported 2 liter car engine in the back
| seat of my civic. Only ~150 lb and quite manageable.
|
| Even monster SUV motors are only ~350 lbs. That doesn't
| include cooling, oil, accessories, or transmission but
| EV's have those too.
| ScottBurson wrote:
| > When was the last time you saw an ICE with both a trunk
| and a frunk?
|
| Seconds ago -- there's one in my carport :-) (A 1993
| Toyota MR2; it's mid-engine. The frunk is small, and
| completely filled by the spare tire and aftermarket
| stereo amp.)
| oh_sigh wrote:
| My SUV is 4500 lbs and has a 17 gallon tank. Gasoline
| weighs ~6lbs/gallon, so it only factors for about 2% of
| total mass between a full and empty tank. Seems safe to
| mostly ignore it.
| gibolt wrote:
| I'd love to see a future where part of a plane's battery
| pack is a detachable drone that could fly back right after
| takeoff (most taxing on energy use) and thus decrease the
| weight of the plane.
|
| You could also have battery swaps during flight as they
| pass over drone battery depots.
|
| Sounds crazy. Might not be worth the gains for the
| complexity, but could be worth it across a whole fleet.
|
| Microwave laser groundstations (or solar satellites) could
| come first, removing much of the battery requirement.
| mikeyouse wrote:
| Seems like you'd be better off with a ground-based
| catapult system to get the plane up to speed and in the
| air where the on-board motors could take over.
| klyrs wrote:
| Catapult really paints the wrong picture... but if you
| say ballista and actually mean a maglev rail accelerator
| then I'm down
| gibolt wrote:
| If these are passenger aircraft, that probably wouldn't
| fly
| microcolonel wrote:
| > _If these are passenger aircraft, that probably wouldn
| 't fly_
|
| Well, it doesn't need to have the same rate of
| acceleration as a carrier's, but honestly it just doesn't
| make sense in general.
| mikeyouse wrote:
| Right -- you could use the full length of an existing
| runway to have a gentle acceleration , bt I'd argue this
| makes a lot more sense than some sort of drone delivered
| jettisoned/retrieved battery if your goal is to reduce
| on-board energy expended for takeoff. Much less complex
| and the wear parts would be fixed at the airport rather
| than on every single airplane.
| tengbretson wrote:
| Hah, now you've got me imagining a gentle rain of 18650
| batteries following a plane as it travels across the sky.
| gibolt wrote:
| Yes, gentle :D
| woodandsteel wrote:
| Top-level ev's like the Model 3 are already superior for most
| drivers. The only problem is cost, and that is being fixed in
| the coming years through steadily falling battery prices.
|
| Other use cases like aviation and ocean boats are more
| difficult. It may well be that synfuels made with renewable
| energy will be the solution there.
| speedgoose wrote:
| I drive both ICE cars and electric cars and there is something
| about ICE engines in cars : they are very unresponsive compared
| to electric engines. They are fine at high RPMs, but you don't
| want to and shouldn't drive at high RPM. They have things such
| as turbo lag or really shitty torque curves. The gearboxes do
| not help as well. It's weird to wait half a second to get full
| power when you are used to the instant torque.
|
| You should test drive a random eletric car and a random ICE
| car.
| ramses0 wrote:
| If you have the opportunity, try comparing your ICE
| experience with a manual transmission ICE.
|
| Many modern ICE cars are effectively fly-by-wire with eco-
| junk-softwware in between the accelerator and engine.
|
| Manual transmissions would likely not have the lag that
| you're experiencing, but I definitely recognize what you're
| saying w.r.t. torque curves.
|
| I'm not challenging ICE v. Electric, but instead attempting
| to clarify that there is quite a wide variation in ICE-
| behavior that is less present in a manual transmission ICE
| car.
| jfindley wrote:
| Well engineered ICE cars are the opposite of unresponsive.
| Gear shifts are in the order of 100ms. Turbocharged engines
| require some revs to get moving, but good engines rev _so_
| fast that unless you have no idea what you 're doing this
| isn't a big problem.
|
| Electric cars are like synthetic computer benchmarks -
| amazing on paper, but to actually drive? On a real road, with
| corners and a competent driver? So, so much worse. We'll get
| there, in time - some hybrids are really great, but we need
| to work a lot on battery weight before a pure electric car
| can match an ICE/hybrid car for real world performance.
| Weight always has been and always will be the enemy, and
| right now a tesla is closer to a truck than a sports car in
| terms of weight. The day will hopefully come, but today is
| not that day.
| phonon wrote:
| Electric motors are simpler, more reliable, smaller, lighter,
| almost perfectly efficient, and have better torque
| characteristics than ICE.
|
| You can't compare the energy storage density in isolation.
| Engines are heavy...Model S's motor generates 362 horsepower
| (according to the official specs), and only weighs 70
| pounds...the equivalent ICE would be 500+ :-)
|
| (Yes, the inverter weighs something, but the transmission is
| much simpler as well for electric...overall you save a few
| hundred pounds easily...A Model 3 battery pack is between 600
| and 1000 pounds--so pretty close to the crossover point.)
| riffraff wrote:
| But the two aren't at odds, o e could use an electric engine
| and still burn gasoline to produce the electricity, sort of
| like ships do.
| outworlder wrote:
| Forgot about that on my longer reply. EV engines are _tiny_
| and incredibly efficient.
| BoorishBears wrote:
| You're making the parent comment's point, ICE != ICE
|
| Mercedes is selling cars with an engine that weighs 354 lbs,
| with just 2 liters of volume. Now 354 lbs is still pretty
| heavy, but it's 354 lbs _with fluids and accessories_.
|
| Some (many even) of those accessories have equivalents on a
| Tesla that aren't included in saying it weighs 75lbs, like
| pumps for coolant and the AC compressor
| throwaway189262 wrote:
| We're still not at the point where weight savings from motor
| offsets battery. Tesla cars are all extremely heavy for their
| size
| froh wrote:
| The mass is a problem of you don't t recuperate braking
| energy. Tesla's do recuperate braking energy. A heavy ICE
| driven vehicle in contrast just loses braking energy as
| heat.
|
| Edit: typo
| throwaway189262 wrote:
| Energy recovery from regen is minimal unless you're stuck
| in stop and go traffic. Usually less than 5%.
| baybal2 wrote:
| It highly, highly depends on motor setup.
|
| Tesla low gearing induction motor is by far not the best
| here.
|
| Synchronous motors have much better regen capability.
| throwaway189262 wrote:
| There's just not that much power available for regen.
| I've done a lot of dicking around with E-scooters. Even
| at their low speeds with lots of stop and go, regen is
| less than 10%.
|
| It's only used because dumping excess energy back to
| batteries is cheaper than including brake hardware. The
| math may work out the same for EV's. Regen just to
| decrease the cost of brakes rather than increase range
| significantly. In the e-scooter world, the cheap ones use
| regen and more expensive models have traditional disc
| brakes.
|
| Air resistance burns a ton of energy at any speeds over
| 20mph.
| baybal2 wrote:
| How much RPM were you getting at your scooter, and what
| was the vehicle mass?
|
| You say 5% at most... The difference in between, say, 5%
| and 10%, a bad and good regen is huge.
| throwaway189262 wrote:
| The RPM's are huge. Like 16k, because smaller motors are
| lighter for same power. Mass is generally 40lb + rider.
|
| The RPM doesn't matter much though. Regen efficiency is
| around 80% from wheel to battery. With cars you get much
| less regen because you lose tons of energy to air at the
| speeds they travel.
| usrusr wrote:
| The argument made wasn't so much about getting more range
| from regen as about regen lessening the impact of added
| mass: with perfect regenerative braking, a ten ton
| vehicle wouldn't use much more energy than a one ton
| vehicle if they shared the same outer hull.
|
| Real life doesn't have perfect regen, but on the other
| side of the equation real life ICE cars actually lose
| more efficiency to added weight than just what is
| converted to heat while breaking because they tend to
| compensate worth a bigger engine to get comparable (or
| better even) acceleration than a lighter counterpart and
| that means that during cruise where the mass is
| irrelevant the engine is running at all an even worse
| load point in terms of efficiency. ICE are terribly
| inefficient at partial load and when your engine is sized
| to get decent acceleration despite high total mass you
| simply can't gear long enough to get the engine to a
| reasonable load point in a moderate speed cruise.
| Electric motors don't have this problem (or a much, much
| smaller version of it), so they wouldn't suffer quite as
| hard from added mass as ICE even worth no regen at all.
| BoorishBears wrote:
| First off, 48 volt systems doing regen is already taking
| off, and there are already _many_ new cars doing it (not
| just ones that have a hybrid sticker on them either:
| https://en.wikipedia.org/wiki/Mild_hybrid#Examples)
|
| Second, as other comment says, the idea that braking
| regen is going to make up for an extra half a ton of
| batteries is pretty laughable
| jashmatthews wrote:
| Not by much. The Model 3 is approximately 1 person heavier
| than a comparable BMW 3 series. The Model 3 Long Range is
| about one person heavier than a 3 series wagon.
| throwaway189262 wrote:
| The BMW 330i is about 660lbs lighter according to some
| questionable internet sources for curb weight
| jashmatthews wrote:
| Base Model 3 is 1611kg vs 1545kg for a 330i. I'm looking
| at both a 330e and a Model 3 at the moment or maybe an
| M340i xdrive touring.
| Shivetya wrote:
| However you are ignoring that while you can spin the blades
| more efficiently, however scaling an electric motor to that
| size may end up with even more weight, you also have to
| replace the turbojet portion of these motors. So you will
| need even more engine. That turbo jet is more efficient at
| speed while the fan is better at lower speed.
|
| In the end we would have slower planes but we will eventually
| reach a point where we can do it.
|
| Someone can probably explain it better and my understanding
| is rough and not current; jets were cool when I was kid
| hwillis wrote:
| Your speculation about motors is incorrect; motors are
| actually more efficient the larger you build them. It's a
| property of the goodness factor:
| https://en.wikipedia.org/wiki/Goodness_factor
|
| It is profoundly foolish to compare motors and heat engines
| on first principles like this, obviously. Still, here's a
| turbine that handles 30x the power of a jet engine, running
| a generator that is about the same size as a jet engine: ht
| tps://www.ge.com/news/sites/default/files/Reports/uploads/.
| ..
|
| generator is in the upper left. Obviously not at all
| optimized for size or weight- there aren't even any magnets
| in that thing.
| rootusrootus wrote:
| > the equivalent ICE would be 500+
|
| It's not quite that heavy. An LT1 makes 500 horsepower and
| weighs less than 500 pounds.
| samatman wrote:
| Are you including the transmission?
|
| Teslas have a transmission, but it's not in the same league
| of complexity or weight.
| msaroff wrote:
| Two things: * The next great battery technology always seems 6
| months away.
|
| * What, if any, are the combustibility issues with this tech.
|
| Higher energy density typically means greater potential for a
| thermal runaway.
| elihu wrote:
| Higher energy density makes electric planes a little bit more
| reasonable, but they'd still be quite range-limited compared to
| gas. For some use cases, that might be okay.
|
| I'm more interested in how this affects cars. Getting four or
| five hundred miles out of a battery pack that's lighter than
| what's in a typical Tesla would be a great thing, especially if
| it's cheap.
|
| I'm currently working on an electric conversion of a Mazda RX-8.
| I just bought about 450 pounds of lithium iron phosphate
| batteries. They're the most expensive component, and provide
| about 27kwh; maybe enough for 100 miles if I'm lucky. I sort of
| assumed that in about ten years or so I'll probably replace the
| whole pack with whatever great new technology can provide more
| range with less weight, and probably cost less too. It would be
| wonderful if we had awesome batteries now.
|
| (I considered used tesla modules; they have much better energy
| density, but they're more dangerous and they wouldn't have fit
| well in the odd-shaped places I wanted to put them.)
| inamberclad wrote:
| Everyone is talking energy density but nobody is talking about
| Urban Air Mobility.
|
| Electric is the name of the game for a VTOL plane that will take
| you from SFO to downtown or Santa Cruz. They don't have to have
| all the performance in the world, they just need to have enough
| performance to do their job.
|
| Also, pilots will appreciate the operating costs and simplicity
| of these aircraft. Student pilots will love a plane that costs
| $10/hr instead of $100/hr in the Bay Area. 90 minutes of flight
| time (1 hour lesson + 30 minute VFR 'fuel' reserve) is all it
| needs.
|
| Neighbors will appreciate higher torque motors that turn modern
| props at 1500 RPM instead of 2200 for the noise reduction.
| the8472 wrote:
| > nobody is talking about Urban Air Mobility.
|
| Because flying cars have always been just behind fusion.
| Antipode wrote:
| How much of a prop plane's noise is from its propeller vs its
| engine?
| nickff wrote:
| This varies extremely widely depending on engine selection
| and propeller design. Urban air mobility vehicles are also
| very likely to be loudest during vertical take-off and
| landing, which is not comparable to a conventional small
| aircraft.
| dougmwne wrote:
| Here's a fun bit: in the article they say that lithium-sulfur is
| hard to measure charge level for due to the voltage properties of
| charging and discharging.
|
| "The upshot is that voltage is not a good proxy for the state of
| charge and, to make things even more complicated, the voltage
| curve is asymmetrical for charge and for discharge."
|
| Since it would be bad if your battery suddenly died and you
| dropped out of the sky, they had to develop complex statistical
| and neural network algorithms to accurately determine state of
| charge to within a few percent. One black box for staying in the
| sky and another in case you end up on the ground!
| mulmen wrote:
| This makes me wonder how accurate float bulb based fuel level
| sensors really are. This _sounds_ like a major problem and the
| solutions are interesting but potentially even better than what
| we are used to.
|
| My 1990 Toyota never read full. The buffer on the fuel sender
| was so extreme that by the time the needle made it to the "F" I
| had already burned 1/8 of a tank! My current car warns me when
| I have about 50 miles of fuel left, I wonder how much
| historical data it uses in that calculation.
|
| None of my motorcycles even have fuel gauges. I just keep an
| eye on the odometer and when I stop to stretch my legs I give
| the tank a shake or a peek.
| rlpb wrote:
| > This makes me wonder how accurate float bulb based fuel
| level sensors really are.
|
| They aren't. On light aircraft, the only thing they're good
| for is as a double check on a manual fuel reading (using a
| dipstick) or a time-based calculation, and to confirm during
| flight that the fuel cap wasn't left off. Beyond that, the
| needles bounce around so much during flight the only thing
| you can really verify is "yes there's some liquid there;
| somewhere between empty and full".
|
| Many light aircraft owners have since retrofitted "fuel
| totalizers" which measure fuel consumption, and are manually
| reset by the pilot to a dipstick value when fuel is added. My
| group aircraft's fuel totalizer seems accurate to within at
| least about 10%. It can be calibrated better, but manual
| dipstick readings are only accurate to a couple of gallons
| anyway.
|
| However, one key difference is that I do know, to within
| about half an hour, my fuel endurance before departure. I'd
| want the same from a battery.
| nippoo wrote:
| They're pretty inaccurate in light aircraft where you're
| often flying at an angle / slightly asymmetrically; in
| commercial airliners, under most flight conditions, they're
| generally within a couple percent. Aircraft will generally
| use fuel flow sensors and use those to calculate remaining
| fuel (by integrating the fuel flow over time) and
| float/capacitance sensors in tanks are used as verification /
| a sanity check (which can sometimes only end up being noticed
| inflight, eg https://www.flightglobal.com/safety/boeing-
| modifying-777-fue...)
|
| For more than you'd ever want to know about the fuel systems
| in a modern airliner, see http://www.b737.org.uk/fuel.htm
| TheRealSteel wrote:
| > float based fuel level sensors
|
| I initially read this comment thinking you meant floating-
| point based...
| cogman10 wrote:
| Dang floats ruin all calculations!
| mulmen wrote:
| Hah, didn't even think of that! I made a small edit to
| clarify.
| mrfusion wrote:
| Can you just measure the current coming out of the battery and
| keep track of it?
| hwillis wrote:
| AKA joule counting- for normal li-ion it gets you to ~5% most
| of the time, up to 20% off at the start and end. That's
| assuming the voltage stays constant the entire time- in
| reality the first bit is at 4.2+ volts, and the last bit is
| down to ~3 volts. That's a 40% energy difference per electron
| that leaves the battery.
|
| It's also one of the reasons you get electronics that die
| suddenly at 5%- current gas gauges usually account for it,
| but older stuff wasn't always good at knowing when the
| voltage would drop off. Nowadays (and always, for the most
| part) the sudden shutoff is because electronics often pull
| very brief power spikes that drop the battery voltage below
| the minimum voltage temporarily. The chemistry takes a moment
| to recover after that.
|
| The problem with Li-S batteries isn't just that they have a
| goofy curve- that can be charted and saved, even as the
| battery degrades (Note- I'm mostly up on conventional
| chemistry. Don't know much about Li-S). It's more have a
| couple phases they go through during discharge. Impedance and
| other properties of the battery change, which changes the
| discharge characteristics of the battery, which changes the
| voltage. Proportionally, the swing in voltage is also larger
| (although this kind of thing is always changing, so I may be
| out of date).
|
| There's also a small amount of self-discharge and parasitic
| reactions that will consume electrons, but that number is
| necessarily fairly small and predictable. The main thing is
| that 50% of the energy variance is in the voltage, and you
| need to know a lot about the current chemistry inside the
| battery (as well as the future load profile) to be able to
| predict the voltage that all the remaining electrons will
| have as they leave the battery.
| hn_acc_2 wrote:
| You could use this technique to estimate the remaining
| capacity __starting from a full charge / known charge __, but
| not to arbitrarily measure the remaining capacity of the
| battery
| dougmwne wrote:
| It sounds like the voltage curve is all over the place as
| the battery phases through its chain of different chemical
| reactions, unlike a normal battery. I take that to mean
| there's no way to just measure the voltage at a given point
| of time to estimate capacity, hence why the statistical
| method was required.
| rbanffy wrote:
| It's a good thing to measure the instant capacity, but
| the voltage and current (and, hopefully, thermal output)
| are being measured during the whole flight.
| ChuckMcM wrote:
| Many battery monitoring circuits do this, sampling both
| voltage and current to compute power over time. With a
| suitable inductor to limit the rate of current change to be
| within the nyquist sampling interval of the monitor, you can
| pretty accurately measure charge going in or coming out of a
| battery. Combined with a model for the battery and you've got
| a modern battery monitor circuit.
| cameldrv wrote:
| Or you could do what they did on Apollo (forget if it was the
| CM or LM). They had the problem of measuring how much was in a
| tank, but the tank was in 0G, so a float is no good. The
| proposed solution was some sophisticated radiation based thing
| where they measured the attenuation of some radioactive source
| through the tank. This wound up being highly complex, and the
| solution was simply to have a reserve tank. When the main tank
| ran out, you knew you had exactly the amount in the reserve
| tank.
| rbanffy wrote:
| Another possible solution is to measure how much fuel (or
| energy) got in and how much is getting out. You know the
| nominal capacity and you know the flow rate.
| cogman10 wrote:
| You'd have to account for charge losses (Some energy ends
| up just being heat while charging) but that could probably
| be simply guestimated on battery temp while charging and
| some constant factor.
|
| For example, assume a 90% charge efficiency for a battery
| at 20C... or whatever makes sense.
| ddoice wrote:
| Still orders of magnitude less energy-dense than jet fuel.
| henearkr wrote:
| To be fair, you need to compare a complete workflow including
| the renewable production of the jet fuel.
|
| If the overhead of heavy batteries does not annihilate the
| benefit of the carbon-neutral production rendered possible by
| using electricity (and associated carbon-neutral sources like
| photovoltaic etc... heck, even nuclear fission), then batteries
| are still the path to go.
|
| If there are carbon-neutral ways to produce the jet-fuel, and
| to have a completely carbon-neutral(or even negative) cycle
| production+consumption, then why not. If it could be done
| without turning the Earth into a giant bio-fuel crop, that
| would be nice.
| DesiLurker wrote:
| IIRC real-engineering on youtube did a detailed analysis of the
| current operating limitations with electric planes:
| https://www.youtube.com/watch?v=VNvzZfsC13o
|
| He also identified a couple of sweet spots where electric
| flight would make sense factoring in engine efficiency & cost
| of fuel etc. bottom line is that the picture is more complex
| than just comparing energy density of jet-fuel & batteries.
| with batteries becoming much lighter IMO it should open up many
| more use cases for short haul frequent flights without the need
| of big central hub airports. which is good. an more importantly
| give the trajectory of battery energy density it should provide
| enough justification for heavy investment into research into
| electric planes so i wouldn't dismiss it out of hand.
| umvi wrote:
| Yes but presumably electricity is order(s) of magnitude cheaper
| than jet fuel. And also order(s) of magnitude more available
| than jet fuel. And also order(s) of magnitude cleaner than jet
| fuel (depending on the source).
| asdfadsfgfdda wrote:
| Jet fuel is ~36 kWh/gallon raw energy density (13 kWh/gallon
| mechanical power assuming 35% engine efficiency). The pre-
| covid jet fuel price was $2/gallon, or $.15/kWh. The average
| price of commercial electricity is $.06/kWh in America, or
| $.08/kWh including charging/motor efficiency. This cost will
| definitely be higher if you only buy clean electricity, and
| this ignores battery wear out.
|
| But where the economics break down is aircraft utilization.
| If charge time is greater than ~1 hour typical turn time, all
| of your costs will grow. Capital cost, crew costs, and
| airport infrastructure cost will increase. To charge in <1 hr
| is a challenge, you need a huge power source (tens of
| megawatts per plane) and serious cooling.
| ramses0 wrote:
| Or swap the component, although that introduces its own
| design challenges and provenance risks.
|
| Aircraft refueling generally runs in-ground (at the largest
| airports), then 5-10k gallon trucks (~20-40k liters), then
| ~500-2000 gallon smaller trucks (2k-10k liters) for smaller
| aircraft or smaller airports.
|
| If you reimagined refueling trucks as "forklifts carrying
| batteries" instead of "tubes of gasoline on wheels" then
| you'd likely end up with similar delivery practices
| (central charging, swap/refuel, discard/recharge batteries
| instead of refilling the fuel tank on a fuel truck).
|
| Effectively it would be standardizing on some way to slot-
| in pre-charged batteries, and treat them similar to a
| propane tank rental company, where each removed battery is
| considered suspect and tested/refurbished/recharged after
| each use.
|
| Otherwise, yeah, putting a bunch of 220v outlets in the
| ground around an airport... you're going to be sitting
| there a while to recharge the ten planes that landed that
| day. It'd effectively be untenable for smaller airports to
| be able to provide "quick-turn" refueling services, and
| potentially risky to be able to guarantee overnight
| refueling.
|
| This is all nudging towards personal / corporate aircraft,
| not commercial aircraft operations, which would "never"
| want the plane in more than one spot for more than one
| hour, which would require something similar to battery-
| swaps that they control, OR some very fancy electrical and
| heat management associated with the airport/jetbridge that
| the plane pulls up at.
| bronco21016 wrote:
| I'd debate the 'more available' statement. It is more widely
| available overall but not in the places you'd want it. Of
| course that can be fixed but someone will have to build out
| that infrastructure to make electric planes viable.
|
| It's a bit like Tesla. Prior to them building out their
| charging network, electric cars had a bit of a chicken and
| the egg problem. You might buy a car but have no where to
| charge it, but nobody wanted to build places to charge them
| because nobody has an electric car.
| harg wrote:
| That's not the case at all. Everyone has electricity at
| their home. You can charge basically any electric car from
| a domestic wal socket and higher power chargers are easily
| available. Commercial charge points only really need to be
| used for long distance travel. Most EV owners can do the
| majority of charging at home.
| lacksconfidence wrote:
| One confounding factor is parking arrangements. If you
| have a garage then a wall socket is reasonable, but for
| many years the only place i could park a car was
| somewhere on the street, hopefully within 100m of my
| address. In that situation commercial charge points
| (hopefully near my employer, if lucky) would be the only
| reasonable charge point.
| bdamm wrote:
| Yeah basically all airports that have commercial service
| also have access to power. There are exceptions like
| seaplane bases and small country strips, but there are more
| than enough airports with access to commercial or even
| industrial grade power to make electric airplanes that
| require charging viable. The final step of linking up the
| airport power supply to the airplane charger is peanuts in
| the world of aviation. Almost all airports have a fleet of
| fuel trucks, therefore, the cost of buying a fuel truck is
| the low end of the acceptable cost for ground
| infrastructure investment to open a new route for an
| airline.
| bronco21016 wrote:
| I just reread what I wrote and I think maybe I wasn't
| very clear.
|
| There's electricity at every single gas station in the
| US. Why can't we pull into any gas station in the US and
| charge an electric car? Even now that electric cars are
| gaining market share and becoming more common.
|
| Someone has to build, supply, and hook up high power
| charging systems. You can't just fly your $5 million eJet
| into any airport in the US and run a 100' extension cord
| into the FBO. If that's the plan, you certainly can't
| hope to leave the same day. It will take at least 3 days
| for your 1 MWh eJet to finish charging.
|
| We're in the pre-Tesla days of electric aircraft. There's
| a few players working on the aircraft and they're getting
| close. However, until a 'Tesla' comes along where they
| also install charging infrastructure at the airports
| their customers are planning on using, we're not going to
| see a commercially viable electric aircraft.
| ramses0 wrote:
| The best way to do it would be start in the corners and
| cross in the middle. Seattle/SF/LA => { colorado? vegas?
| texas? chicago? } => NY/DC/Miami
|
| If you can link up some sort of route(s) to deal with
| range-anxiety / weather, and can criss-cross the country,
| you're in business.
|
| Once your route is built, it's straightforward to manage
| capacity/flight-plans (reservations / networks /
| routing), and then you move directly to demand-
| generation, but you'll have a real tough time competing
| directly with coast-to-coast direct flights.
| aphextron wrote:
| >Yes but presumably electricity is order(s) of magnitude
| cheaper than jet fuel.
|
| It's really not even about the cost of fuel. With aviation
| it's all about maintenance costs. Electric aircraft will be
| orders of magnitude simpler and cheaper to maintain than jet
| turbines. This is what will unlock cost effective small scale
| commuter routes, allowing you to just hop on a small 10
| passenger plane at a neighborhood airport and take a 300 mile
| flight with no need for security.
| blisterpeanuts wrote:
| >> Still orders of magnitude less energy-dense than jet fuel.
|
| Does that matter, if other aspects of the system compensate
| with lighter weight? For example, lighter weight electric
| engines versus heavier fuel-burning engines along with exhaust
| and cooling systems.
| threeseed wrote:
| If it's anything like ships/yachts then the lighter weight
| doesn't come anywhere close to making up for the loss of
| energy.
|
| Could still be very useful for flights between nearby cities
| e.g. LAX to SF.
| asdfadsfgfdda wrote:
| A PW150 turboshaft engine is ~5 kW/kg. Some electric motors
| are up to ~10 kW/kg. But, as an example, engines on a Q400
| regional airliner are only ~5% of the total weight. Fuel is
| up to 15% of total weight. So the savings are not
| significant.
|
| Also, the batteries will likely require a cooling solution.
| This can be challenging (heavy) for high altitudes (where air
| is cold but very low density). Jet fuel requires no cooling.
|
| https://en.wikipedia.org/wiki/Power-to-weight_ratio
| adammunich wrote:
| With every lithium-sulfur battery I've come across you need to
| have a lot of steel clamping plates together because they expand
| so much when charging, and will otherwise delaminate. So
| ultimately they become the same mass.
|
| This includes the oxis energy battery mentioned here.
| mulmen wrote:
| Is there some way to embrace this in the design of the plane?
| Could the wing structure be built in such a way that an
| expanding battery actually adds strength?
| nippoo wrote:
| The idea of making a battery a structural part of an aircraft
| wing has come up before, e.g. https://newatlas.com/axial-
| stack-battery-supersonic-electric.... I'm not sure if any
| progress has been made on this front recently, or whether
| they've come up against a materials-science roadblock!
| cmrdporcupine wrote:
| This might sound crazy but is it possible to do that in some
| way such that the clamping plates etc. are only there during
| the charging process? Prevent the delamination during charging
| and then remove the prevention mechanism?
|
| Or just charge really really slowly? Airports etc. could just
| have terminals full of trickle-charged batteries to swap in and
| out.
| ngold wrote:
| That was my first thought as well. But I don't have a clue if
| it's practical.
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