For both land and air, total payload is another hard limit. Tesla achieves admirable results for a passenger vehicle by using high energy/volume and energy/mass batteries integrated into the vehicle structure itself. Battery EV trucks have had range limits of measured in single-digit kilometers. Their fuel-based counterparts exceed four digits (1,000 km). Accellerating, decellerating, and managing mass costs joules.
> Battery EV trucks have had range limits of measured in single-digit kilometers.
I don't believe you. Find me an EV truck that any company has fielded in the past 20 years that has less than 10km range.
Also you're comparing a prototype to a mature technology. One might as well note that early automobiles had less range, lower speed, worse reliability, and higher cost than horses. And unlike horses, automobiles require infrastructure such as roads and gasoline stations. All of those things were true at the time, but horses couldn't be improved upon and automobiles could. Now we're in the same position with respect to gas vehicles vs electric vehicles.
Battery technology is improving at an impressive rate. Energy density has doubled since 2010. Price per kilowatt-hour is 1/9th what it was a decade ago.[1][2] These advances have made EVs competitive on the ground. It's quite likely that improvements will continue and EVs will become competitive in the air. EVs have lower refueling costs, lower maintenance costs, and are simpler designs (making them cheaper and safer). We already have battery-powered commercial drones. Human flight is more conservative when it comes to adopting new technologies, so it'll probably take longer for EV planes to become widespread. But long term? I would bet on EV planes replacing regional jets.
> I would bet on EV planes replacing regional jets
Energy density of batteries are around 1/40th of that of kerosene (0.9 mj/kg vs 44mj/kg) so we are talking 44x the energy density we need to make up for to get perfect parity. From what I can find I’ve seen 35% thermal efficiency for modern airliners and I believe 95% for electric motors. Ok so this means we can reduce the density gap required for parity to just under ~ 44/2.5 so ~18/19 mj/kg? (I’m doing this in my head so it’s rough). Rough calls show we need batteries with 20x the capacity to replace what we have right now with electric alternatives. While the gains in battery densities have been impressive we need to keep these gains in mind that we need to be making much bigger gains for this to become a reality. Personally I don’t see it being feasible for a very long time given the progress and I believe we’re better off taking fewer unnecessary flights than hoping on some miracle battery breakthrough
BMW's Terberg YT202-EV electric tractor, tested~2015, comes to mind. Its route during trials was 2km, though the claimed range was 100 km. I strongly suspect the latter was slow and unloaded. Speed was restricted in any regard:
BMW and the SCHERM Group have put a massive 40-ton EV into service for a one-year pilot. The Terberg YT202-EV electric tractor will travel a 2 km route eight times a day between the SCHERM group logistics center and the BMW plant in Munich, transporting vehicle components such as shock absorbers, springs and steering systems.
The standard tractor has two batteries (112 kWh), and a third battery can be fitted for extended operations. It is equipped with a 138 kW, 720 N·m Siemens motor and an Allison 3000 transmission. Top speed is 25 mph.
The YT202-EV has a range of up to 100 kilometers (62 miles), theoretically enough for a full production day. BMW says it will be charged exclusively with renewable electricity, and will save 11.8 tons of CO2 annually compared to a legacy diesel truck.
The basic problem with electric aircraft vs. electric cars is that aircraft spend a much larger proportion of their journeys operating at high power levels.
A slow, economy cruise setting in a Cessna 172 is a 45% power setting - ~80 hp, and climbs will be at full power (180 hp).
On the other hand, you might only need ~20 hp to maintain 65mph on a level road in a sedan; and most people never use anything like the full power of their cars for more than seconds at a time.
>aircraft spend a much larger proportion of their journeys operating at high power levels.
Luckily the high power level is maintained at reliably high speed [0], ensuring airflow for cooling.
Even better, aircraft by its very long-range nature will have much larger battery:engine ratio. For example this 2018 article [1] discusses airplane with 9x the Tesla's battery capacity and 3x the Tesla's engine power, in other words, its batteries would need to handle only 1/3rd electrical load as much at peak load (per unit). The lower power density[2] of the battery pack again provides for easier cooling; perhaps even passive cooling or air cooling instead of forced liquid cooling. Whereas Tesla's sustained performance is limited by battery pack thermals.
Lastly, it's worth noting the ICE aircraft engines are "overbuilt" as compared to the roadgoing ones, mostly for sake of reliability.
--
[0] aside of the engine run-up and initial acceleration, but both of those are short anyway
That, and the fact that fuel-powered aircraft will shed half their takeoff weight during flight. Battery-powered aircraft either maintain constant mass or gain mass in the case of metal-air batteries, as energy is discharged.
Long-haul alt-energy aircraft utilise very low airspeed (low drag), extreme mass budgets (they're wonders of materials and structural engineering), and altitude profiles (potential energy0 to manage overall energy budgets and use.
I think this depends where you're driving. If you're at 130kmh in a 70hp subcompact, something perfectly common in Europe, you're pretty close to max power..
I mean, this plane is a sprinter. If you want range, use sailplane techniques and with similar chemistry as for terrestrial lithium ion you can get >1000km range with multiple passengers and speed just as high as high speed rail (or higher). Eviation Alice and other concepts show how this could be done. Just a matter of time and a LOT of effort.
Battery EV trucks (lots of prototypes and maybe even low volume production going on, from Tesla to Daimler) regularly get multiple hundred kilometers of range and 1000km isn’t at all unreasonable although 500 miles is probably the sweet spot in the near term to avoid reducing available payload (any greater range has diminishing returns due to regulatory requirements for rest stops). And that’s without advanced chemistries that you can get now at low volumes like metal anode or lithium-sulfur which double the specific energy and allow double the range (to say nothing of lithium-air, which is easily over a decade away from practical use but would eliminate the disparity in useful specific energy between battery-electric and hydrocarbon combustion in almost all cases—rocketry and munitions being the major exception).
A few years ago an electric bus hypermiled to get some ludicrous range. Turns out the trial stripped all spare mass from the rig (seats, other interior), pumped tyres to munition-rated pressures, hugely oversized the battery, and held a constant speed of about 24 kph (15 mph).
> Battery EV trucks have had range limits of measured in single-digit kilometers
Battery powered trucks that you can buy today from the likes of Scania/Daimler has a range of 200+ km. Not some prototype but currently in production for delivery next year.
Sure, it's not 1000 km, but that single digit km is off by two magnitudes.
So basically Elon has suggested doing all the things they already do today to maximize fuel savings in ICE planes...
Note that airplane fuel has significantly higher power density than an electric battery (~40x as much, see https://www.theverge.com/2018/8/14/17686706/electric-airplan...), so no EV will ever be able to achieve the same range as an ICE plane, ton-for-ton.
Is it possible to assist the plane during its take off phase? Maybe with a electric wire that supplies energy and detaches. I know it’s messy. Or maybe using something like balloon to gain altitude and then engines to cruise? Or a special battery bank that gets depleted during take off and detached + a recovery mechanism?
Electric motors in the wheels might help on the takeoff roll during initial acceleration to rotation speed (100-150 mph). Perhaps also regenerative braking on landing. And taxiing.
I was just htinkin the same thing. Having a huge battery pack attached and then parachuted off after takeoff in a designated spot could work really well. Or even like a "plane assister" that just stayed attached for takeoff and then dropped off and landed back at the airport to re-charge / help the next plane.
I think it's all moot tho. Airbus seems to be going hydrogen for their next planes to have 0 emission.
I find it interesting that Rolls-Royce Motor Cars is developing this, as opposed to Rolls-Royce plc - the company that works more closely with aviation.
-Even more interesting given that Rolls-Royce plc has a number of boffins at a subsidiary in Trondheim, Norway working on exotic PM motor designs, including for aviation. (The company is called SmartMotor, was purchased by what was then Rolls-Royce Marine, then was retained under the RR umbrella when the rest of the Marine division was sold to Kongsberg a couple of years ago.
Most diesel-electric trains are not hybrids, they have no batteries and therefore always get power from the single source, the diesel engine. There have been attempts to make them hybrid by putting a car in that contains a lot of batteries for say storing power down a mountain to use going back up the next mountain rather than converting the power to heat in the dynamic braking grid. My understanding is so far it is not worth the cost or complexity for the minimal savings.
Diesel-electric is actually less efficient than a mechanical coupling at steady speed due to double conversion losses which is about 80% efficient vs high 90's for gears through standard mechanical transmission. Trains use the an electric drivetrain because they need precise traction control with huge torque to get the train moving, a mechanical transmission with torque converters, clutches and gears would be difficult to route power and wear out too easily due to the low speed lugging. Once up to cruising speed however it would be more efficient.
This is why there are no hybrid semis or even diesel electric semi's, standard mechanical transmission are more efficient and hybrids only have significant gains in stop and go traffic where as for mostly highway travel the hybrid would normally disengage and simply be dead weight in a parallel hybrid or simply a efficiency drain in a series hybrid. This is also why most hybrid cars are not series, they mechanically couple the engine to wheel for efficiency once up to speed.
Planes again would have little gains from being a hybrid as they would spend most of their time at cruise running the combustion engine and a straight mechanical connection from engine to prop is much more efficient and lighter than any electric double conversion scheme.
Electric planes might make sense because there is no double conversion or engine in the plane itself, but you still have the issue of energy density with batteries.
Battery-electric trains (not hybrids) are certainly a promising emerging technology, however. Countries like the UK and Germany are looking to replace diesel trains with battery-electric on non-electrified branch lines:
Excellent and well-written answer. I hadn't considered the dead weight of the generator. It's more proof that questions with seemingly simple answers rarely exist in the real world. As a friend of mine (a data analyst) likes to say: "the real world is messy".
Yeah much higher bypass ratio. Current turbofans bypass ratio is limited by the fan diameter. With a series hybrid you can power multiple fans from one engine. Or more likely engines. So you could design a plane with two engines and four fans.
For a reaction engines thrust is proportional to mass flow X delta V. And power is proportional to mass_flow X delta V squared. You can see why high bypass turbofans are more efficient. More thrust/less power.
Couple of other benefits. Much safer in an engine out condition because you can power your fans off the remaining engine. With a battery hybrid you could power the fans for a limited time with all engines out.
Probably less throttle lag with an battery hybrid. Throttle lag is what kills people during take off and landing in bad weather. Wind shear causes the airspeed to drop, pilot needs more power but the engines take seconds to respond.
Take off and landing under battery power --> way less noise.
Yes. Or at least the concept/idea is being explored. From [1] "The hybrid version would generate electric power through a turbine within the plane. That power would be used to turn the fan blades of the single electric turbofan engine."
[2] Uses a slightly different approach "The propeller is powered by an electric motor with 65 kilowatts of continuous output. The electricity is supplied through a generator by a small Wankel engine that consumes little fuel"
The Wankel engine one is interesting. It seems the only way any of these make sense is with a new type of engine. Makes no sense to put a turbine in a plane just to run an electric motor, since it's more efficient to just use the turbine directly.
That Rolls Royce proposal seems to be based on the fact that airplanes draw power from the turbines and/or and auxiliary power unit.
They're driving one of the fans off of power parasitized from the other turbines. Technically that's a hybrid, but I was thinking more of a self-contained unit.
Well, Airbus had an event[0] last week about their hydrogen-hybrid passenger planes. And there the main power would come from adapted jet engines that combust hydrogen but the hybrid aspect would be fuel cell sourced electrical motor that would give some additional boost for some phases of the flight.
By singling our trains I’m thinking you are referring to diesel electric which is not the same as hybrid. On a train the generator to motor link it to act as essentially a clutch/gearbox allowing theoretic full torque at 0 rpm.
Electric propulsion is very cool in and of itself.
If the goal is to fly without adding CO2, there are other approaches as well, though.
One could synthesize fuel usable by nornal existing airplanes from atmospheric CO2. This would require a lot of CO2-free energy though.
There is an ongoing project [1] by US NAVY to turn seawater into jet fuel. This way we would not need to burn fossil fuels and would not need to throw away much of existing aviation technology.
Wouldn't converting seawater into jet fuel effectively just have us contribute to global warming even more aggressively by converting CO2 sequestered in the ocean to atmospheric CO2 (+ huge energy consumption for the intermediary step of jet fuel)? I'm sure I must be missing something there.
If source of energy is low CO2, such as nuclear, then losses dont contribute to CO2 increase.
As for taking CO2 from the ocean, it is done for this project as it is military for airplane carriers. CO2 can be had from atmosphere as well, with more energy cost.
From fuel efficiency and aerodynamics perspective, CELERA 500L is a great advancement that could possibly also be converted to electric in the future - https://www.ottoaviation.com/
Electric planes will replace car and bus trips before they replace gas planes. Short range, on demand, point to point, cheap travel could really be a game changer. Think what you used to do as a two hour drive- battery electric planes will be able to do that, a distance that today isn’t economical because of the high cost of jet and turboprop maintenance and fuel, and the need for runways. All those commenting- “but the energy density” are right, but don’t miss the forest for the trees. The sky is an underutilized asset.
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Range is quite another issue. A Cesna 172 has a 1,289 km range, four times the Rolls-Royce prototype, and carries more than just the piolot and fuel.
https://en.wikipedia.org/wiki/Cessna_172#Specifications_(172...
For both land and air, total payload is another hard limit. Tesla achieves admirable results for a passenger vehicle by using high energy/volume and energy/mass batteries integrated into the vehicle structure itself. Battery EV trucks have had range limits of measured in single-digit kilometers. Their fuel-based counterparts exceed four digits (1,000 km). Accellerating, decellerating, and managing mass costs joules.
I don't believe you. Find me an EV truck that any company has fielded in the past 20 years that has less than 10km range.
Also you're comparing a prototype to a mature technology. One might as well note that early automobiles had less range, lower speed, worse reliability, and higher cost than horses. And unlike horses, automobiles require infrastructure such as roads and gasoline stations. All of those things were true at the time, but horses couldn't be improved upon and automobiles could. Now we're in the same position with respect to gas vehicles vs electric vehicles.
Battery technology is improving at an impressive rate. Energy density has doubled since 2010. Price per kilowatt-hour is 1/9th what it was a decade ago.[1][2] These advances have made EVs competitive on the ground. It's quite likely that improvements will continue and EVs will become competitive in the air. EVs have lower refueling costs, lower maintenance costs, and are simpler designs (making them cheaper and safer). We already have battery-powered commercial drones. Human flight is more conservative when it comes to adopting new technologies, so it'll probably take longer for EV planes to become widespread. But long term? I would bet on EV planes replacing regional jets.
1. https://cleantechnica.com/2020/02/19/bloombergnef-lithium-io...
2. https://www.statista.com/statistics/883118/global-lithium-io...
Energy density of batteries are around 1/40th of that of kerosene (0.9 mj/kg vs 44mj/kg) so we are talking 44x the energy density we need to make up for to get perfect parity. From what I can find I’ve seen 35% thermal efficiency for modern airliners and I believe 95% for electric motors. Ok so this means we can reduce the density gap required for parity to just under ~ 44/2.5 so ~18/19 mj/kg? (I’m doing this in my head so it’s rough). Rough calls show we need batteries with 20x the capacity to replace what we have right now with electric alternatives. While the gains in battery densities have been impressive we need to keep these gains in mind that we need to be making much bigger gains for this to become a reality. Personally I don’t see it being feasible for a very long time given the progress and I believe we’re better off taking fewer unnecessary flights than hoping on some miracle battery breakthrough
(Edited for clarity)
BMW and the SCHERM Group have put a massive 40-ton EV into service for a one-year pilot. The Terberg YT202-EV electric tractor will travel a 2 km route eight times a day between the SCHERM group logistics center and the BMW plant in Munich, transporting vehicle components such as shock absorbers, springs and steering systems.
The standard tractor has two batteries (112 kWh), and a third battery can be fitted for extended operations. It is equipped with a 138 kW, 720 N·m Siemens motor and an Allison 3000 transmission. Top speed is 25 mph.
The YT202-EV has a range of up to 100 kilometers (62 miles), theoretically enough for a full production day. BMW says it will be charged exclusively with renewable electricity, and will save 11.8 tons of CO2 annually compared to a legacy diesel truck.
https://chargedevs.com/newswire/bmw-tests-40-ton-electric-tr...
> In 1937 they produced a ride-on four wheeled vehicle, suitable for a payload of 8–10 cwt (410–510 kg) and with a range of around 35 miles (56 km)
A slow, economy cruise setting in a Cessna 172 is a 45% power setting - ~80 hp, and climbs will be at full power (180 hp).
On the other hand, you might only need ~20 hp to maintain 65mph on a level road in a sedan; and most people never use anything like the full power of their cars for more than seconds at a time.
Luckily the high power level is maintained at reliably high speed [0], ensuring airflow for cooling.
Even better, aircraft by its very long-range nature will have much larger battery:engine ratio. For example this 2018 article [1] discusses airplane with 9x the Tesla's battery capacity and 3x the Tesla's engine power, in other words, its batteries would need to handle only 1/3rd electrical load as much at peak load (per unit). The lower power density[2] of the battery pack again provides for easier cooling; perhaps even passive cooling or air cooling instead of forced liquid cooling. Whereas Tesla's sustained performance is limited by battery pack thermals.
Lastly, it's worth noting the ICE aircraft engines are "overbuilt" as compared to the roadgoing ones, mostly for sake of reliability.
--
[0] aside of the engine run-up and initial acceleration, but both of those are short anyway
[1] https://news.ycombinator.com/item?id=18602771
[2] not to mix it up with energy density
Long-haul alt-energy aircraft utilise very low airspeed (low drag), extreme mass budgets (they're wonders of materials and structural engineering), and altitude profiles (potential energy0 to manage overall energy budgets and use.
https://youtu.be/MBItc_QAUUM?t=2422
Battery EV trucks (lots of prototypes and maybe even low volume production going on, from Tesla to Daimler) regularly get multiple hundred kilometers of range and 1000km isn’t at all unreasonable although 500 miles is probably the sweet spot in the near term to avoid reducing available payload (any greater range has diminishing returns due to regulatory requirements for rest stops). And that’s without advanced chemistries that you can get now at low volumes like metal anode or lithium-sulfur which double the specific energy and allow double the range (to say nothing of lithium-air, which is easily over a decade away from practical use but would eliminate the disparity in useful specific energy between battery-electric and hydrocarbon combustion in almost all cases—rocketry and munitions being the major exception).
EDIT: Daimler has delivered some 300-400km range electric Freightliner semis for customer testing already: https://electrek.co/2020/03/04/daimler-electric-freightliner...
Ideally, aside from parasitic losses, shouldn't decelerating pay joules?
A few years ago an electric bus hypermiled to get some ludicrous range. Turns out the trial stripped all spare mass from the rig (seats, other interior), pumped tyres to munition-rated pressures, hugely oversized the battery, and held a constant speed of about 24 kph (15 mph).
https://www.latimes.com/business/autos/la-fi-hy-proterra-ran...
(The article, as with most, reveals no test conditions though does at least note "a Proterra spokeswoman said they won’t reveal the test bus speed".)
In production, range claims seem to be about 1/3 this.
Typical city busses stop every block or two, accelerating to about 40 kph, then braking, every 60 seconds or so.
Both mileage and wear and tear suffer immensely.
Battery powered trucks that you can buy today from the likes of Scania/Daimler has a range of 200+ km. Not some prototype but currently in production for delivery next year.
Sure, it's not 1000 km, but that single digit km is off by two magnitudes.
Unless I'm mistaken? Can ducted fans match the speed of a jet engine?
https://youtu.be/MBItc_QAUUM?t=2422
- Going higher
- Using gravitational energy on the second half/landing part of the journey. Like a car going down hill.
- Making use of the higher power density of the engines
- (Doesn't mention this, but using batteries as structural elements)
I also think that this:
"Prandtl Wing Minimum Drag Update" - Al Bowers (Chief Scientist at NASA) would be a key technology
https://www.youtube.com/watch?v=bCwtcDNB15E
A new type of wing and maybe more importantly a new type if turbine fan.
Note that airplane fuel has significantly higher power density than an electric battery (~40x as much, see https://www.theverge.com/2018/8/14/17686706/electric-airplan...), so no EV will ever be able to achieve the same range as an ICE plane, ton-for-ton.
I think it's all moot tho. Airbus seems to be going hydrogen for their next planes to have 0 emission.
Deleted Comment
It seems like weight and displacement could be more of a concern. And turbines run at higher temperatures than diesel engines...
Diesel-electric is actually less efficient than a mechanical coupling at steady speed due to double conversion losses which is about 80% efficient vs high 90's for gears through standard mechanical transmission. Trains use the an electric drivetrain because they need precise traction control with huge torque to get the train moving, a mechanical transmission with torque converters, clutches and gears would be difficult to route power and wear out too easily due to the low speed lugging. Once up to cruising speed however it would be more efficient.
This is why there are no hybrid semis or even diesel electric semi's, standard mechanical transmission are more efficient and hybrids only have significant gains in stop and go traffic where as for mostly highway travel the hybrid would normally disengage and simply be dead weight in a parallel hybrid or simply a efficiency drain in a series hybrid. This is also why most hybrid cars are not series, they mechanically couple the engine to wheel for efficiency once up to speed.
Planes again would have little gains from being a hybrid as they would spend most of their time at cruise running the combustion engine and a straight mechanical connection from engine to prop is much more efficient and lighter than any electric double conversion scheme.
Electric planes might make sense because there is no double conversion or engine in the plane itself, but you still have the issue of energy density with batteries.
https://www.railwaygazette.com/battery-traction-agreement-si...
Battery-electric trams are also a thing:
https://www.railtech.com/infrastructure/2019/12/12/first-uk-...
For a reaction engines thrust is proportional to mass flow X delta V. And power is proportional to mass_flow X delta V squared. You can see why high bypass turbofans are more efficient. More thrust/less power.
Couple of other benefits. Much safer in an engine out condition because you can power your fans off the remaining engine. With a battery hybrid you could power the fans for a limited time with all engines out.
Probably less throttle lag with an battery hybrid. Throttle lag is what kills people during take off and landing in bad weather. Wind shear causes the airspeed to drop, pilot needs more power but the engines take seconds to respond.
Take off and landing under battery power --> way less noise.
[2] Uses a slightly different approach "The propeller is powered by an electric motor with 65 kilowatts of continuous output. The electricity is supplied through a generator by a small Wankel engine that consumes little fuel"
[1]https://phys.org/news/2017-11-airbus-rolls-royce-siemens-hyb... [2] https://phys.org/news/2013-07-electric-hybrid-aircraft.html
They're driving one of the fans off of power parasitized from the other turbines. Technically that's a hybrid, but I was thinking more of a self-contained unit.
[0] - https://www.airbus.com/innovation/zero-emission/hydrogen/zer...
Deleted Comment
If the goal is to fly without adding CO2, there are other approaches as well, though.
One could synthesize fuel usable by nornal existing airplanes from atmospheric CO2. This would require a lot of CO2-free energy though.
There is an ongoing project [1] by US NAVY to turn seawater into jet fuel. This way we would not need to burn fossil fuels and would not need to throw away much of existing aviation technology.
[1] https://www.eurekalert.org/pub_releases/2020-07/uor-lch07152...
As for taking CO2 from the ocean, it is done for this project as it is military for airplane carriers. CO2 can be had from atmosphere as well, with more energy cost.
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