June 7, 2019, ©. Leeham News: In last week’s Corner I wrote: “The reason electric cars work and airliners don’t is the Sky lacks Stoplights“.
The discussion was part of my previous series on Electric aircraft, but it was in the comment section. Here is a more exhaustive run through of the main reasons.
The key reason electric cars with a battery energy source work and airliners don’t is that the normal car is an energy hog in our daily use and is less sensitive to weight.
Car: We burn energy when accelerating the car from a standstill after every stop, be it for stoplights or traffic jams. Then after a while, we brake the car to a slower speed or to a stop with our friction brakes, wasting the energy we used to get to speed. It repeats over and over again.
The electric car recovers the built-up motion energy when slowing down, by it making it less wasteful with energy than the normal car.
Airliner: The airliner accelerates once from standstill on the runway to takeoff speed and then gradually to cruise speed during the climb. When descending the investment in potential energy during the climb is recovered. It overcomes the drag during the descent.
A minimum of energy waste characterizes the airliner operation. All energy is used to overcome drag. The potential energy from cruise height is to a large part recovered during the descent before landing. So there is no energy waste to gain for an electric airliner compared with a combustion one.
Car: The car is less sensitive to the added weight of batteries or hybrid propulsion than an aircraft.
A normal 1,500kg car when driving at 100km/h (60mph) has a tire rolling resistance of 220N/50lbf and an aerodynamic drag of 220N/50lbf. If we substitute the fuel with the typical Tesla battery pack of 65kWh the car weighs 2,000kg. Drag stays the same with tire rolling resistance due to weight now at 294N/66lbf.
Aircraft: If we take a similar loaded weight aircraft, like the Cessna 206 with five passengers, drag due to weight (induced drag) is 440N at 100kts. If we substitute fuel for a Tesla 480kg battery pack the induced drag increases to 780N. The substituted fuel has an energy content of 2,600 kWh whereas the pack has 65kWh but we use the pack weight of 480kg to make the weight increase comparable.
Electric cars can recover the wasteful brake energy lost if many situations. They can, therefore, successfully compete with our energy-wasting combustion cars.
There is little energy waste in a modern airliner’s operation. Electric airliners, therefore, have no energy waste to recover.
The drag increase due to weight for the Car is 33% when going from combustion to electric. The airliner’s drag due to weight, induced drag, increases with 78% when we convert our aircraft to electric with the Tesla battery pack. As noted we would cut range to a fraction of the fuel based aircraft. We need a much larger battery pack to keep range, making the induced drag increase much larger (induced drag scales to weight squared).
Aircraft are very sensitive to weight, cars are not. Weight is the aircraft designer’s arch enemy. This is ignored/forgotten by those proposing battery-based or hybrid transport aircraft.
If you add fuel cell aircrafts into the comparison it would be interesting as they normally burn hydrogen and produce steam and electrical Power, hence the mass of the fuel reduces during the flight just like JET-A.
A hybrid with a fuel cell APU driving an aft prop energizing the boundary layer to increase efficiency. Toyota makes hydrogen composite fuel tanks for its Mirai hydrogen car.
Well said Claes.
I am particularly interested in the prospect of fuel cells running on green ammonia (which also per volume contains more hydrogen the LH2).
‘Green’ because it can be produced from renewables – solar, wind, hydro or nuclear power.
In combination with a small gas turbine to produce heat (which is used to help catalytically split the NH3 and heat the fuel cell to its operating temperature) and very intricate fuel cells (perhaps constructed through additive manufacturing) it ought to be possible to build a very efficient hybrid powerplant that produces little or no CO2.
Not for aircraft initially perhaps – trains and ships, even airships, first – but I think that would be the future, not batteries for the reasons given by Bjorn.
Hydrogen fuel cell aircraft is a different ball game. If the problems around the hydrogen distribution and fuel cell use in the aircraft can be solved it makes electric propulsors an interesting alternative.
Its all about storage and specifically storage density. There is more hydrogen in a litre of water than there is in a litre of liquid hydrogen.
There is some interesting research ongoing on storing H2 within carbon nano-structures, but its nowhere near the TRL needed for use on aircraft.
You could just directly burn the H2 within an existing gas-turbine (with some modifications) if it came to it. An example would be the Cryoplane DLR were going to make off a Do 328 in the late 90s.
**However**, there is a school of thought that dropping loads of water vapour into the upper atmosphere might be as harmful as dropping CO2 there.
Yes, the first gas turbine in Germany ran on hydrogen gas, but there is a difference in emissions between buring hydrogren in air v/s in a fuel cell. There are other fuels that work in fuel cells besides hydrogen. You can today subscribe to a street legal production hydrogen Toyota Mirai if you live in California or Hawaii (close to a hydrogen station). Toyota has the resources to improve a concept like hybrid cars into a commercial sucessful product. Unfortunately today the cheapest way to produce hydrogen is starting with natural gas, but with time other methods will be competetive.
Hi, might be a stupid question.
But if you insist on flying water (H2O) around – sounds like in your first sentence – and then burn it to H2O, you won’t free up any energy?
At least as far as i remember from Newton in Physics back the days in Universtiy.
So you have to use H2.
And this leads to the problem why electrical cars are way better then hydrogen fuel cell cars:
A electric engine takes eletricity storred in batteries and frees them.
Efficeny is very high,
you can assume 20% loss for loading and then have a
90% efficency from battery to kinnetic energy (electric engine, while combustion engine is about 30% ideal)
If you see a hydrogen fuel cell, you need H2 first.
That’s either produced from Oil – then you can use fuel also, or via electrolitiy split of water with a efficency of about 50%.
The fuel cell has about a effieceny of 60%.
If you compare the numbers,
you will have:
Electric car: 80% and 90%
fuel cell car: 50% and 60%
Yet, a fuel cell car has storrage and handling issues.
So overall, the fuel cell doesnt stand a chance, unless we find a ceap way to produce so much energy we can bear the loss of 50% to prodcue H2. Or we gain knowledge and are able to use plants to gain H2 or any other source.
If we get a better battery though, discussion is done.
For fuel cell power one of the issues is producing H2 and its storage either as compressed H2 to approx 400bars or as LH2, today the cheapest way to produc H2 is from Methane but work is intense to develop new catalysts to reduce electrical engergy consumption to separate water into O2 and H2 like at my Alma Mater Stanford U. Another problem to consider is hydrogen embrittlement in most high strength steels. France has a modified nuclear powerplant to produce LH2 shipped on a ship in a gigant thermos for the Arianne space program. Still certifying LH2 or compressed H2 into commerial aircraft require some certification rules and work, some will come from NASA/ESA but to commercaila aircraft safety levels.
Regarding dumping H2O vapour into the upper atmosphere – we’re already doing that. The products of combustion for jet fuel are CO2, H2O, + bits of soot, NOX, etc.
It’s what makes the contrails.
It would be a pity if that turns out to be as much of a problem as the CO2.
Think the soot particles from the jet engine helps alot for the H2O to create contrails, fuel cells running on H2 and O2 does not produce soot, so the amount of contrails with an airline burning JET-A v/s H2 in a fuel cell would be great to know.
burning Hydrogen in a turbine is limited by Carnot process metrics. ( theory ~55..60%, real: 40%
A fuel cell does allow much higher efficiencies. (theory:90, real:60)
Then, with burning hydrocarbons _you_already_are dropping massive amounts of water vapor into the atmosphere _beyond_ massive amounts of CO2 🙂
There is some uncertainty, but the warming effect of contrails is currently believed to be the same magnitude as the warming effect of CO2. This is one of the reasons to pursue all-electric aircraft. Though, range may be an insurmountable issue for larger aircraft.
Albert, there’s hardly any consensus about contrails causing as much damage as the CO2 for fossil powered airplanes. A man made cirrus cloud can dissipate in 30 minutes, while CO2 can stick around in the atmosphere for 10s of thousands of years. It’s tricky to get good data, as we’ve only had 3 days of US Airspace shut down after 9/11 when accurate ‘baseline’ measurements could be taken. So far most research I’ve read concludes that while contrails do have an effect, it’s short lived, mostly local, and cirrus clouds can also have a cooling effect under specific circumstances. Also, it’s possible to reroute aircraft so they don’t create as much contrails or the contrails don’t form cirrus clouds. The same can not be said about the aircraft’s CO2 emissions.
There have been multiple studies showing that the net effect of contrails is as important (on a radiative forcing basis) as CO2. However, exactly like you say, contrails are short lived and their effect would stop immediately if fossil fuel powered aviation were to stop. As far as I know, rerouting to avoid forming contrails is not currently done, as it is not financially advantageous.
Water vapor influences radiation transmission in the atmosphere.
Water vapor content is in a buffered balance.
With the gas phase gaining on increased temps.
( and this also increases energy levels drastically, the atmospheric activities gain significant energy,)
The “real” green house gases effect temp rises that are amplified by the changing water vapor balance.
The greenhouse paradigm is a nice one but rather unsuitable to describe the real atmospheric processes.
Björn, it would be interesting to see how high energy density for Hydrogen Fuel Cells need to go before they become competitive for long range flight. I posted a few links about a NASA sponsored Hydrogen Aircraft research project in the discussion after your last Electric Airplane article, but I erroneously linked to an article that didn’t include the institutions and companies supporting the NASA sponsored effort. Since those are familiar names to people here (Boeing, General Electric, etc), I figured it might be worth mentioning again:
“The project includes participation from eight additional institutions: the Air Force Research Laboratory, Boeing Research and Technology, General Electric Global Research, The Ohio State University, Massachusetts Institute of Technology, the University of Arkansas, the University of Dayton Research Institute, and Rensselaer Polytechnic Institute.”
And a separate interview on this very hydrogen hostile website:
“…the weight of the fuel cell systems on the aircraft are fixed and, as a result, the aircraft would likely be heavier for short-range missions. However, the cumulative weight of the liquid hydrogen and fuel cell system is anticipated to actually be lower than conventional Jet A systems for longer ranges, due to the aforementioned high specific energy of hydrogen.”
“15 years ago, a standard fuel cell had a specific power of around 0.3 kW/kg. Now fuel cells are commercially available with specific power up to 2 kW/kg at the stack level, with additional mid-term future projections featuring even larger increases up to as far as 8-10 kW/kg.”
And then the issue Björn didn’t touch on in this article: Superconductivity. Which is necessary for large, electric aircraft at jet speed:
“…the cryogenic hydrogen doubles as a way to maintain the ultra-low temperatures for this superconductive state. By removing ohmic (resistive) losses from the transmission system, the delivery of electrical power becomes incredibly efficient.”
Hope that helps the debate.
Excellent article – I’d had weight and regenerative braking in the back of my mind, but hadn’t made them front and central and quantifying the induced. I realise it isn’t aerospace per se, but how much is gained through regenerative braking?
A second thought is that one of the reasons why electric vehicles are so attractive (in Europe at least) is that electricity is not taxed nearly as much petrol – £10 to charge a Tesla in the UK is £8 electricity plus 20% VAT. In the UK, a litre of fuel is about £1.30, which is made up of 57p duty, 22p VAT and 51p fuel. Without the duty, it would be 61p, reducing the cost of filling up from £60-70 to £30-35 – still a lot more than the Tesla, but significantly reduces the difference.
Electric vehicles are politically “good”, so I do not expect a tax on their energy anytime soon – aviation remains politically “bad”, so electric aircraft may incur duty quicker (debate and discuss).
Regenerative braking, it past mny mind too. 400 kts at 40k ft to standing on the runway losses a lot of energy. I can image windmilling the engine to do the braking / extract the energy creates so much energy, batteries that can’t absorb/ stow that much energy that fast..
The new US Ford carrier uses some of the braking energy when the Aircraft catches the wire.
I agree with this – fuel energy density is the problem for aircraft.
Side note – heavier electric cars do have some engineering challenges, road wear, brakes, tyre wear … high levels of low speed torque and heavy vehicle puts a lot of load into the sidewall of the tyre.
The brakes aren’t used much – regnerative braking – so they really don’t wear out at all. They’re still needed for emergency stops and slow speed braking.
I’m wondering about a change to modern turbo props for longer range airliners. The earliest turbo props like Bristol Brittania had prodigous range for the time- late 50s.
efficiency for jets had moved on since then while large turbo props have atrophied.
The best comparison would be the US C17 compared to A400M. They aren’t the same size but likely could be scaled.
Airliners, even widebody have a tubular fuselage and tapered towards tail, so better aerodynamics than military cargo haulers.
I wonder if turboprops will satisfy those who want even more fuel economy, however there is a fetish for anything electric that goes beyond reason.
There is a maintenance cost problems with large turboprops as you have to pay for a regular Engine+ Power gearbox+prop and its components &controls. The only gain is a much smaller nacelle maintenance cost. Hence UDF’s without a Power gearbox and higher flight speeds might be the solution as its own Life/cost problems are getting solved, For now noice, fuel consumption and cruising speed has acceptable solutions but I think Life cycle cost and suitable airframes are still missing.
The smaller engine is because you have a smaller turbine. The maintenance costs of a CFM56 engine are much less than the much larger CF6. It should more than cover the propellors and gearbox which are relatively low maintenance anyway.
My suggestion is large turbo props are greater fuel efficiency and much more than say 20%, which gets back to the weight of fuel carried for the range required. It could be half that of a large modern turbofan.
Sounds good to me. GE did say it almost offered a UDF-style engine for the Boeing 787.
I’m agnostic as to gearbox vs. direct-drive, but if a gearbox open rotor is feasible, it would be more efficient than a direct-drive version.
If you ordered a turboprop to power an A320neo the engine shaft hp would be of similar order to thrust in lb. Hence you need 2ea 25000shp turboprops. My expericence tells me that would be an expensive power gearbox and prop system where you have 15-20 different CMM’s involved from beta tubes to electrical de-ice with slip rings. A NASA demo might be done using a GE LM2500 engine with a PW type of reduction gear and new Ham Sund /Collins prop with pitch control/anti-ice and the other controls. Most likely will you end up with a counter rorating system like the one on the Tu-95 /Tu-114 engine of original BMW Berlin design.
Not so . Propellors are more efficient especially for takeoff but not so much for high speed cruise. I think the numbers are much better below about 500mph.
So the peak engine requirements are takeoff and top of climb
The A400 crusies at 780km/hr at 31,000ft, the C17 at 830km/hr
Its difficult to compare the engines maximum output directly as they measure different things , Thrust and Power.
Here’s how I understand the thrust-to-power conversion.
Speed shouldn’t be a concern. The 7J7 was designed for a cruise speed of Mach 0.83, and the UDF/propfan version of the MD-91/92 would’ve cruised at the same M0.76 mark as the MD-80.
Bjorn had an earlier article contradicting what we both stated. He says 1.36 hp => 1 lb at takeoff, and 2 hp => 1 lb at cruise.
At the transonic speeds of long-range flight (e.g. M0.85), turboprops have low propulsive efficiency and are noisy. Last I heard, UDFs remedy the former, but worsen the latter.
The analysis tools have made progress, now the Safran Open Rotor design with LEAP-1 type of carbon blade design is certifiable to todays noice standards and can keep the same cruing speed – a few %.
Hello Bjorn, maybe an idea to specifically state the before and after weights for the Cessna too so that its really clear how comparable the car and aircraft weights are (but the consequences are not).
The Tesla battery pack has an energy content of 65kWh weighing 480kg. Specific energy is 0.135 kWh/kg.
It’s 65kWh is the equivalent of 5.4kg of Fuel. The Cessna starts at 1500kg with five pax on board, full tanks and lands on the nearest possible airfield or farmland with 1495,6kg landing weight when fuel driven and using 65kWh of energy. It’s a trip of 10nm at 55% power and at 1,000ft.
The Cessna starts and lands at 2,000kg when driven by the battery pack. The flight will be shorter than 10nm as the induced drag is now 78% higher. When it parks the aircraft the battery pack is at 0% charge. You better have good weather, a nearby destination airfield and a lot of farmland around without power/telephone poles/lines because your planning margins are 0%.
When using the 2,600kWh in its tanks the Cessna 206 can fly 500nm.
Bjorn, the last paragraph says it all ‘aircraft are sensitive to weight, cars are not’.
The main reason for the push towards electric cars is to make (crowded) cities cleaner. Cities that are getting more and more crowded, traffic is slow and cars are filling the air with CO2 NOX and associated particulates. Here electric is the good thing, even though the perfect public transport would have ‘done better’. Diesel fuel is being banned in some cities, gasoline not; but for the global warming perspective, diesel is better (less CO2 emissions). So charging (car) batteries from a coal operated plant can be justified, it moves the ‘pollution generator to a desolate location’.
So, is electric aircraft more of a gimmick, electricity from batteries being the future propellant of larger flying machines seems like mission impossible. The problem with batteries is its energy density, you have to carry the same high weight from takeoff to landing, the undercarriage must be designed for landing with takeoff weight, and your maximum altitude will remain the same throughout the flight. And will ‘waiting on batteries being charged’ be the next top delayed departure reason?
Aircraft must be as light as practicable, ships on the other hand needs a lot of weight in the bottom. So here batteries may do the job; a cleaner environment and effective ballast. Water is, after all, a thousand times heavier than air, an difference that becomes greater and greater, the higher you fly. Aircraft and ships have in common that they both ‘fly and float’ in a fluid.
Finally, let me stress that I am not negative to developing ‘cleaner aircraft’ propellants, but we should begin in the same lane as cars, – with small aircraft flying short distances. We develop better and new technology as we go, that’s the nature of science and the human mind ‘we can do this better’! It reminds me of the first GPS receiver I used; it was a large one-channel Trimble, I could barely lift it, and we had to wait for satellites to ‘come up’. Today, you get a 50+ channel receiver as an add-on to your wristwatch. Being born the same year as the transistor, and ‘worked with it since’, I say as a fellow countryman once said ‘the impossible takes just a bit longer’.
My thinking exactly, as I mention it to my students frequently. Recovering energy normally lost to heat by braking is a great plus for cars, which increases their efficiency significantly, especially in city driving. Without such recovery, electrical energy becomes just a replacement for energy in hydrocarbon fuel and little else. On top of that, at least an order of magnitude improvement in battery technology would be needed to address increased inert (empty) mass fraction issue. Aircraft manufacturers have gone to great lengths to reduce the empty mass fraction by using CFRC wings and fuselage, and many other mass-saving measures, and it is hard to imagine them embracing a much higher empty mass fraction, even if electrical propulsion is feasible.
Thanks for a great article.
In my humble opinion, flying an A320 on a 500nm route, at 60% capacity, isn’t exactly the epitome of weight optimization.
I believe there is a place for an hybrid-electric regional aircraft.
How about Hybrid airplanes? An S-Duct plane such as the Dassault Falcon 7X and the Dassault Falcon 900 can work by having one kerosene burning jet engine and two electric fans. The plane would need enough battery storage to get at cruising speed, which is at about 20 minutes of flying and then the jet engine would recharge the batteries.
See story about Cessna 337 hybrid. This is the scale that is possible at present.
Long range business jet? , I don’t think so.
Propellor business aircraft are far more efficient than jets anyway
A car, boat, or train is energy neutral at rest. An electric blimp might be closer from an energy perspective.
This analysis is over simplified. Conventional and electric propulsion systems are not equivalent. Though the battery may be a large penalty, there are benefits of electrification outside of accelerating/stopping. Also, it does not come down to weight, it comes down to cost (both fixed and operating). Cost is the aircraft designers’ arch enemy.
You quantify none of those and just claim with out justification there are other benefits. Plus you you are mixed up over cost. Weight is cost, the more kg of conventional structure the higher the cost to make it , and same for new carbon fibre , to save weight leads to higher build costs but the payoff is reduced fuel burn from lower weight. Which gets back again to the weight of fuel when transferred into a battery equivalent.
Read again about the 4 seat Cessna, its liquid fuel has energy equivalent of 2600kWh , the replacement battery pack is 65kWh
I knew when Bjorn spelt it all out, there still would be some that would say black is weight , night is day and electric is still best. All without any justification other than ‘it is’
That is exactly my point; it is not a simple better or worse answer. As requested: https://doi.org/10.1016/j.paerosci.2018.11.002
Weight is not cost. Cost is cost.
Clearly you have no manufacturing experience. In my business all the materials including Al extrusions were priced on weight, even the recycling bin was weighed and paid on that basis.
Your academic studies mean nothing , as you ‘juggle the parameters’
to get your results like this
“A factor of four increase in battery pack specific energy from current values of 200 Wh/kg to 800 Wh/kg enables 500 nmi flights”???
“based on the Airbus A320neo configuration are designed and evaluated at 200–1600 nmi design ranges with 2–10 propulsors and 400–2000 Wh/kg batteries ???
Magic wand thinking you would expect from a Graduate research assistant with MIT whos aiming for research citations.
Tesla battery backs are around 150Wh/kg
Airplanes are not priced per kg, unfortunately. Operating cost also depends on factors outside of weight as well. The linked paper is a conceptual study and there is reasoning behind the selected parameters. Though, admittedly anything beyond 800 Wh/kg is unlikely with current knowledge. You requested justification of other electrification benefits, and I am attempting to provide them, Dukeofurl.
To net it out, the linked paper is a feasibility that suggests a 180Pax airliner with a 500nm range might be possible with batteries that have a 4x higher specific energy density than todays best.
Now a 4x improvement is within the theoretical max energy density for Li chemistries (for example Li-Si has a theoretical capacity of 10x Li-Graphite). But.. going from theoretical energy capacity to practical batteries is a slow process. Historically battery capacity has doubled about every 10-12 years, either by moving to a new chemistry or by incrementally improving existing chemistries.
At that rate it will be 20 years before there is a battery that can power such an aircraft.
Agreed, and MW-scale superconducting motors for aviation are also far off. The paper mentions an entry into service of 2050. Small all-electric aircraft are not that far out though, especially if the operating cost is significantly lower.
oh so it turns out different changes that occur on the plane with a car? is this influenced by grapitation too?
Hydrogen isnt viable because the contrails emitted would eventually all not dissipate and blanket the earth in a ice crystal shield blocking out the sun. The tank isse is easily solved with proposed double bubble fuselages. Other issues are how to contain the small H molecule ( recall upper left of periodic chart) .
Hybrid airplanes are a viable concept – and turbojet is an energy equation – all non-787 aircrafts use thr0w away waste heat which comes off the engines. Bleed air is extracted from the engines at 600 degrees and cooled through precooler claptrap on the engine struts to still crazy hot but lower temperatures where it can be used for anti-ice and bleed air accessories like air driven hydraulic pumps. The other side of the precoolers are exhausted over board – very hot heat exchanged air – IE pure energy. There are many opportunitys to recover energy from the fuselage using pizeoelectric transducers on a vibrating and twisting body while from the flight controls in the form of back-EMFs to charge the batterys.
There is one more factor which needs to be considered. A combustion engine can be highly optimised for a narrow operating range while an electric drive is efficient over a wide operating range. An aircraft in cruise is operating under conditions where it is optimised, while a combustion engine vehicle (especially in the city) is hardly ever operating under the best operating conditions. A vehicle like the Nissan e-Note or BMW i3 with range extender solves that by having a small combustion engine running at a fixed (optimised) RPM driving a generator which charges the battery which in turns provides power to the electric drive. A normal petrol car solves this by having more speeds in the gearboxes like the latest 9-speed automatic gearboxes. For aircraft, this is not needed because the operating conditions vary far less.
So I see far less benefits of moving to electric drive for aircraft even if you do not consider energy storage.
Another issue is auxillary systems. You can refer to the 787 and claim that that is electrical, but it works because power is generated by the aircraft engines. The moment you need to create any cabin heating/cooling/pressure control, there is a large energy requirement on top of the energy needed for propulsion. It is ok for light aircraft but nothing above that. An electric vehicle in a hot and humid environment gets a big range hit due to air-conditioning. I have seen range reductions of European electric buses operating in tropical environments by up to 50% (it is particularly poor due to doors opening and closing frequently). I have heard statements that in Hong Kong double decker city buses can not go electric because there are no efficient electric AC systems available which enable operation in summer while single deck buses are possible.
787 and its engines do use bleed air, just they dont pipe the ‘hot compressed’ air to the fuselage to power other subsystems.
Bleed air is still required in the engine internals- where most bleed air is used anyway.
plus as Boeing says there still are some engine bleed powered systems
On the 787, bleed air is only used for engine cowl ice protection and pressurization of hydraulic reservoirs. The electrified functions are wing deicing protection, engine starting, driving the high-capacity hydraulic pumps, and powering the cabin environmental control system.
The reduced bleed air requirement is over egged for these massive high BPR turbofans as most air is passed around the engine after going through the front fan.
That is not what I am referring to. The 787 has totally 4 generators which are driven by the engines which in turn power most of the plane including cabin environmental system. Total power generation is 2MW max and more importantly, it is at 800Hz. Solid state converters then transform this in DC and/or 50/60Hz power of various voltages. This makes the generators and electrical system extremely compact and efficient.
For a fully electric plane, this becomes additional energy which needs to be stored before takeoff and therefore additional batteries. This is often forgotten but the amount extra energy storage / weight required is significant. I have never seen this listed in any calculation.
I think one has to get away from the all electric airplane with the only power source being batteries, at last in regards to today’s power density of batteries.
If one starts looking at electric drive as part of a system, the possibility for change is getting enormous.
The first point is a fuel cell. A hydrogen fuel cell, is about twice as efficient as burning hydrogen in a combustion engine. There has also been the dream of using different, even liquid fuels to run a fuel cell.
As it is the cost of fuel cells are prohibitive. If those cost would come down and perhaps new ideas about storage of hydrogen or new fuels being more than theoretically possible, the electric plane would have a future.
Second point could be hybrid drives, were the main power would still come from a combustion engine.
While we are talking pie-in-the-sky, has inflight refuelling for commercial aircraft ever been considered? What if there were refuelling “bases” in Iceland, Alaska, Hawaii and, Dubai. Assuming a 500nm diversion to the refuelling base in case of a missed tanker then with a single refuelling a 3500nm aircraft would become a 6500nm aircraft.
There would be some loss of efficiencies due to the refuelling operation and non-optimal routing. There is also the cost to lift the fuel up and out to the refuelling point. But total system fuel consumption should be lower because the fuel for the second 3000nm is only being carried from the base to the refuel point (say 500nm) not the full 3000nm from the departure point. I believe Bjorn had a graph showing how the cost of caring fuel for the long haul became dominant after about 3000nm.
It seems this approach, despite all the regulatory and technical hurdles may be a quicker way to improve efficiencies than completely changing out almost all systems for new ones.
What is the point you are making..air refueling isn’t feasible for passenger aircraft. The numbers of planes are just too large now. Planes can just land if you want a cheaper flight than non stop. Icelandic Air business model relies on it, same as many others say from aAustralia to US, stopover in Hawaii for a cheaper airfare
The industry is going to come under ever greater pressure to reduce carbon emissions. So my question is can inflight refuelling actually save on fuel and reduce emissions?
If not then that’s that. But if so then we are onto the engineering and logistics problems. Daunting though the may be it strikes me they are more tractable than powering a long-haul flight with batteries, or even a hybrid power system.
There are proponents for this idea of taking off pretty light weight, do a refuel and fly to your target. Like the SR-71 ops. Besides certification the issue is cost of manned tankers and their fuel consumption. It might be needed again for hypersonic interceptors and hypersonic airlines. Then cost is not the same issue..
Interesting ‘hybrid’ , a modified Cessna 337 , with one engine fueled and the other one electric
‘Los Angeles-based Ampaire publicly demonstrated its electric propulsion system with a 25-minute flight yesterday afternoon of its 337 Ampaire from Camarillo Airport (CMA), the company announced. The aircraft is a push-pull Cessna 337 Skymaster with a proprietary, battery-powered electric motor replacing one of its two combustion engines.’
There is another big difference between cars and commercial used aircraft: time of use per day. A commercial aircraft goes 16 hours a day a car maybe 1 or 2. A car does 2 legs while a single aisle does multiple legs. There is no time to reload the batteries. Only solution is refueling. Either kerosine to run the generators or hydrogen/methanol to run the fuel cells.
In theory you could swap battery packs at the Airport. Bolted onto the wing underside, a special vehicle should be able to do it quicker than todays refueling truck refuels a similar size aircraft. Aircraft batteriers needs mainenance quite often and it could be a new business for the companies running the Airport handling/refueling besides doing the recharge and certify the large batteri packs and attachment bolts.
You may remember a DC-10 crash due to a not properly locked cargo door. So now you want to have save bolts all over the aircraft? For real world applications these batteries must be replaced after each flight (in case your batteries are more than half full you should have left them at home in first place – dead weight). Therefore a lot of wear for the bolts.
Next thing is a very sturdy wing structure able to cope with holes in its structure. Therefor heavier than a normal sealed wing with just a few fuel pipes.
A stable liquid is the best way to store and move energy around.
In this therotical case the wing that normally carries fuel would carry battery packs whose large bottom frame would be structural and work as the wing aerodynamic underside. Industry knows well how to inspect bolts for reuse, normally with GO & NO-GO thread gauges and Magnetic Particle Inspection for crack inspection before reuse.
An interesting analysis, but I think incomplete.
Among other things – the arbitrary line drawn at energy in the car biases towards the electric vehicle.
Yes, the gasoline or diesel burning internal combustion engine has a lower theoretical efficiency than the electricity from battery electric motor.
However, the electricity that is stored in the battery has to first be generated somewhere, then transmitted to the battery. This process introduces 10% to 20% losses, on top of the losses from battery to motor.
Of course, theoretical full cycle analysis would look at overall energy efficiency from raw resource to final usage – and again, the electric battery has a very large amount of energy usage in its creation; it isn’t clear to me that the rare earths in battery and electric motor are any less energy intensive than the materials in the IC engine.
For all the conversion losses you can look it up at one of Bjorn’s article series about electric aircraft:
A jet engine can provide 30 % energy conversion from stored chemical energy. Even better than for Diesel engines at about 25 %. Now try a hydrogen fuel cell with a ratio of about 60 % with an electric motor at about 97 % (0.60 * 0.97 = 0.58). For more factors see articles by Bjorn.
Fuel is not only more energy-compacted ,but it is also consume d along the way, making the aircraft lighter, which reduces a lot of weight. However batteries don’t become any lighter. Deadweight all the way. Just imagine cruising at MTOW for hours.
And I seriously doubt the idea of storing batteries in the wings. Anyone who’ve played with Li-poly batteries know that it’s extremely sensitve to deformation. Wing flex will easily destroy the whole thing in a fiery explosion. You can put many small batteries on an elastic structure to cater this problem, but with hundreds, if not thousands of batteries it may only take one failed one to ignite . And that’s why there is accidents and safety concerns over TESLA.
It’s going to be a 787 battery snag on a larger scale.
The lack of weight reduction throughout the flight is also a concern for landing. Even with thrust reversers, the landing distance for an all-electric aircraft (still at MTOW) can be considerable.