December 20, 2019, ©. Leeham News: We continue our series why e in ePlane shall stand for environment and not electric.
If our target is to lower the environmental footprint from air transport we must have a target that focuses just that, lowering the CO2 load from our airliners. Electric or Hybrid-electric aircraft are not the most efficient way to achieve this. There are better ways to this target.
We increase the CO2 content in our atmosphere when we burn fossil fuel. With present technology fully electric aircraft other than UAMs, light sport aircraft or short-haul air taxis are not feasible.
Anyone who says it is have not done even the most basic checks with realistic system assumptions and an aircraft performance model. And it won’t be feasible for at least another decade (if not two).
And hybrids won’t be much better. The initial hybrid-electric aircraft will have a shorter range and burn more CO2 generating fuel than today’s airliners. I realized this when I started modeling the possible hybrid concepts two years ago, in preparation for my first article series on the subject.
Since then I have talked and listened to every expert on the subject, and my view hasn’t changed. In anything, I see a longer route to a worthwhile hybrid today than I did then.
You can do a hybrid aircraft that can fly airliner routes. Different to full electric aircraft it’s possible. It’s just not a smart way to do it.
After spending a lot of money you will get it to fly with some payload and range. After spending even more money and time you will get it certified for carrying passengers. But it will be a long route there.
And no-one has been able to explain the real point in a hybrid propulsion system for aircraft. You have the gas turbine already, why not bolt the propeller or fan to it and be done?
And don’t come with a story the gas turbine can get more efficient if we have a hybrid concept. Marginally yes, but this gain is consumed before we are past the generator. We will analyze the other claimed gains in later Corners. To summarize, they don’t change anything.
The complexity of a hybrid brings us problems but little else. You now shall convince the FAA/EASA/etc. it’s safe to have 3kV electrics routed through the aircraft pumping 15,000 Amps of current to generate the 50 MegaWatts of power the propulsor fans need to get an A320 sized aircraft into the air.
Nothing bad will happen. Not in normal operation and not if there is a major upset/runway excursion/crash or system/isolation failure. It will work with total safety at up to 41,000ft and with all possible cosmic effects on its control electronics, creating no hazard for at least the next 2,000 flights until major maintenance can have a look if something has happened. And so on.
Realize we have NO experience of the tensions and currents involved, including flying with these types of Volts and Amps, at the altitudes and atmospheres (wet, humid) involved. The military are in this bracket when they do experimental airborne death lasers. But our safety-oriented civil air transport has no experience of such challenges.
And you have the same gas turbine generating the propulsive power to drive these 50 MegaWatts to get the aircraft to take off and later 15 MegaWatts to cruise. Only now through a heavy, complicated and risky hybrid chain instead of directly to the fan. Your gains from the hybrid chain will not compensate for the added weight, complexity and risk.
No-one who has real expert knowledge of all parts of the chain has so far been able to explain to me how we will gain in fuel consumption and by it CO2 emissions with this technology.
The route described above spends our invest money and burns the time we don’t have. Instead, we should focus on more productive options to get the job done. I will give examples of such options below and in the coming Corners.
The options are more straight forward and attainable than the electric/hybrid investments and will achieve several times more over the next decade than all these projects combined.
We start with the one which needs no new technology and no big investments, just the will for change:
With a focus on reducing the fuel consumed per transported passenger-km the most obvious advancement would be an improved air traffic system. We are today flying inefficient routes rooted in historical incomplete knowledge of where our airliners are in the air. And we fly a lot of these routes every day, Figure 1.
The present air traffic routing and separation technology necessitate separating principles that fly the aircraft on routes that are up to 10% longer than necessary. With every aircraft in US-controlled airspace having ADS-B from January 1st, 2020 (which tells every aircraft and air traffic control where everyone is all the time) flights can be completed more efficiently once this technology is embraced and used. And an ADS-B mandate like the US can be applied worldwide today. It’s just to decide.
For this global change, the technology is there. The production lines for the equipment are up to speed and all aircraft OEMs have developed the mod kits (they need it for the US market). And the software changes for the ATC systems have been trialed over the last five years. Yet progress is tepid. This is shameful.
The state in the US won’t give the FAA the money to get the NEXTGEN system ready soon and the air traffic controller unions in Europe block SESAR, the European effort, as they want to stay with the old. It’s more inefficient and keeps more controllers on the state’s payroll.
It’s a low hanging fruit that can reduce our emissions the same amount as upgrading all our 20,000 airliners flying our airspaces every day to a newer generation aircraft. It would with one feasible, realistic and badly needed change significantly lower our CO2 emissions from air transport. Yet it’s not done.
NexGen is debated ad nauseam in the US and the European equivalent, SESAR, is going nowhere. Our politicians pay lip service by allocating research funds to environmental projects with electric and other buzz words, yet they don’t do what they have control over. Order our airspace providers (they are all state organizations) to settle with the air traffic controllers on how we can get the new more efficient airspace done. Any delay is blocking a real chance to reduce CO2 emissions and do it now.
Instead, the policymakers prefer to believe in the dreams of e as electric to fix the problem. Then they don’t have to do anything or spend any money with their airspace providers. New technology will save them from having to do what can be done now.
In the next Corner, we will list more nearby options to reduce our CO2 emissions.
This is a good series of articles on a subject that should be discussed.The aircraft industry is becoming an easy ( tax) target as it has to run on high energy density fossil fuels.
Clearly neither the industry nor the consuming public are taking the matter seriously,frankly it’s all ‘hot air’ ( sorry).
Why? Because the solution to a 20% reduction on CO2 emissions is available now but no ones interested because ‘the public wouldn’t want it’ -they say,and who knows they might be right.
The answer of course is high speed tubo props.An up to second engine such as the TP 400 already exists.Along with the latest in prop technology.It powers the A400 which ( although a heavy lift veichle) can fly at close on jet speeds.A ‘thinner wing’ would no doubt further improve this.
Keejse did some photoshop examples of a 150 seater.But the industry is bringing out two new/heavily revised aircaft in this category that are conventional jet planes.
Clearly no one is really serious about CO2 emissions if they don’t take the 20% that’s sitting on the table right now.
Thanks, Phil. You are bringing up the subject of one of my future Corners. Agree fully. And there is an even better implementation of this principle than the A400M. Stay tuned.
Sadly I have more bad news.Last week the ICS ( global shipping companies).The key subject was ‘slow steaming’ I think dropping from 24 Its to 20kts.Drrag ( as you know far better than me) dies down ( or up)x4 with speed.So the lowering of fuel consumption is instantaneous (+20% I believe).Did they do it -of course nor ‘too difficult’.
Instead they agreed a fund to study zero carbon ships.Pie in the sky long term stuff.
Note that Mersk did manage to do it voluntarily when oil was $100 a barrel -strange that ( not).
IMHO this climate change experiment is going to play out.We just have to hope all the experts are wrong, and the Greenland I’ve sheet really isn’t melting 9x faster than predicted….Ships-Planes-Power stations etc no one’s really doing anything material.
The TP400, as it is, is a low hours usage military specific engine. Not the best for high cycle short routes 2 hrs and under.
We already have a engine produced in high volumes designed for short haul, Pratt and Whitney GTF , redeveloped so that could use its core with propellors instead of a front fan. Luckily P&W has its own turboprop division.
This Book :Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes (Aerospace Series), 2013 by Egbert Torenbeek is available at Amazon Kindle ( but pricey)
The PW800 ( the version without a GTF but based on same core as the A220 PW1500G) was supposed to be the basis of the Pratt offer, PW180 for a new TP for the A400M.
Yup that engine ( appropriately modified) would certainly do the job.
“”But the industry is bringing out two new/heavily revised aircaft in this category that are conventional jet planes.””
Turboprops would have put Irkut and COMAC over the top, that was a missed chance. Maybe Bjorn can tell us why they didn’t do it.
I suspect the advantages of some level of Hybridisation is that it may provide a more efficient lighter way of providing emergency reserves for diversion, hold or headwinds than batteries that are used only on say say 5% of occasions. The effect of the “APU” will be to reduce the battery capacity noticeably.
There was considerable experimentation some time ago with electric taxiing systems. That itself would save considerable amounts of jet fuel. Perhaps the APU could be replaced with a large battery charged on the ground. This is the Safran Honeywell EGTS
My own view is that a continuing drive to fuel efficiency with a gradual substation of carbon neutral synthetic hydrocarbons made out of atmospherically captured CO2 is the best way forward. One Dutch company is already producing 1 ton/day of such fuel but the idea and experimentation has been around for maybe 50 years.
For the design of an environment friendly airliner, the most difficult task seems to eliminate the NOx exhaust of the engines. Compared to this challenge, it even seems easy, to eliminate the CO2 exhaust (by using liquid H2 instead of kerosine).
Therefore I expect, that the future aircraft architecture will be based on the design target of zero NOx exhaust to atmosphere, and everything else will be derived from this.
Agreed totally. Even if something is claimed to be CO2 neutral, one must consider the associated particulates that result from non-complete combustion. Liquid methane is for instance much cleaner and has a higher volumetric energy density than traditional or bio-kerosine. It can also be produced via a renewably-powered hydrolysis process that then combines CO2 and H2 in an exothermic reaction. Still, even at very high purities, some remaining particulates such as (NOx) are formed if the methane is combusted with air. The best would be to react it with liquid oxygen but at that point you are essentially powering a rocket due to the high temperatures and resulting pressure. Perhaps airplanes should therefore have rockets as engines in the future? The difficulty there is in control of thrust but perhaps having many individual motors that can be nominally modulated by 50% could allow for a finer degree of control. The remaining problem relates to the pressure vessels that would be needed for storing the substances. Clearly, this isn’t impossible from a feasibility stand point as very strong-yet light carbon-reinforced fiber vessels could make this a possibility. The engineering challenge would be to get this configuration approved, certified and publicly accepted but hey, maybe SpaceX will be the group to make this happen as they already have/are developing the technology for methalox engines and are building the infrastructure behind it. Perhaps a further ‘sky’ Roadster plan would be the answer here? First, develop a supersonic mini-rocket-powered bizjet. Then, as the price falls and the technology matures, bring out the commercial designs that regular airlines can afford to buy. Perhaps this is the way. Or perhaps H2-burning planes are the way. Clearly, innovation needs to move ahead and existing savings opportunities such as in airspace-control need to be valorized.
The name of the process for production of methane from CO2 and H2 is the Sabatier reaction CO2+4H2->CH4 = 2H2O done over Rhenium catalysts but others will work as well. Exothermic but not so much as to be inefficient and much of this energy can be recovered via a steam cycle. The German national grid uses excess wind power to make synthetic methane which is simply pumped and stored into the national grid.
Using Cobalt catalysts produces long chain like hydrocarbon jet fuel, diesel fuel, kerosene and is now a Fischer-Tropsch Reaction. Normally syngas (Carbone monoxide CO and 2H2) would be used but CO2 works as well. This was one of the the processes used by German coal to fuel industry in the 30s and 40s.
Using Iron catalysts will produce gasoline directly but its more likely a copper Zink catalyst would be used to produce methanol which would be converted to gasoline via MTG (Methanol to Gasoline) catalyst in a zeolite structure to constrain molecular length to get gasoline instead of diesel/jet.
The efficient Extraction of CO2 from the air is not as difficult to imagine. Its is being done for producing greenhouse atmosphere and people have hoped they can use it to sequester CO2 underground, in minerals or to push out more oil from oil fields. The efficiency is marginal for these purposes however when the CO2 is used as a carbon neutral fuel it makes much sense.
There are many processes. One primitive but quite workable one is to absorb CO2 from air running up a stack of glass beads with a downward flow of weak Sodium Hydroxide solution. The resulting Sadium carbonate is converted to NaCl and CO2 by reaction with Hydrochloric Acid and the salt converted back into NaOH (sodium hydroxide). This system, I saw in a patent by Northrop Grumman in the early 60s, was about 34% efficient to produce gasoline. The German ZSW in 1990 did 38.4% using a mini photovoltaic plant and ran a VW Golf Car of it as a demonstration. They reckoned they could do 60% on a large scale. The absorption of CO2 from the atmosphere is now about 5 times more efficient (much more) down from about 1000WHr/KG to less than 250WHr//Kg. The alternative is cryogenic hydrogen but it has of course handling difficulties that add expense and the efficiency is likely to also not better than 60% due to liquefaction losses.
The thing to take from this is that while this process is possible and could produce a carbon neutral fuel (I think it should be done as a 1% token) it is best to focus on ground based applications first.
Slight correction: The German national grid uses excess wind power to make synthetic methane which is simply pumped and stored into the national gas pipeline system.
A few of the (potential) benefits for electric aircraft are:
1) More propulsor redundancy
2) Downsizing of the gas turbine
3) Easier implementation of BLI
4) Higher propulsive efficiency
5) Reduced L/TO emissions
Agreed that these (potential) benefits are outweighed by the added weight/complexity/risk currently. There might be something there in the long term, though.
Hi Albert. Thanks for the list. I will address all these in subsequent corners. The majority of these could be easier implemented with existing technology, yet it’s not been done. Tells one something about the realistic gains of the claimed benefits. I’ll cover this all in detail in the next Corners.
it would be nice, if you could also discuss the concept of a liquid hydrogen powered fuel cell aircraft, since this seems to be the only concept for an emmision free (except water vapor) longe range aircraft.
The penaltys of this concept are:
-Energy ineffecient production of liquid H2 on ground (can technically be solved by invest in enough renewable enegie sources)
-High OEW of the aircraft, due to the weight of the fuel cells and the electric motors
-No NOx exhaust to atmosphere
-No CO2 exhaust to atmosphere
-Liquid H2 has a three times higer specific energy (120MJ/kg) than kerosene (43MJ/kg)
-Cooling fluid for the supra conductors inside electric aircrafts motors is always on board of the aircraft
-Silent propulsion by many distributed electric fans
Hindenburg is a strong public perception to overcome
There is a lot of hydrogen in methanol. No need for special tanks. Methanol even works with fuel cells.
The Airbus Cryoplane study (LH2 plane with conventional jet engines) came to the conclusion that safety issues of LH2 planes are a “Psycological problem primarily”:
Methanol weights 6x more than LH2 – therefore it’s not suitable as aircraft fuel.
Hydrogen is liquide at -252 C (21 K). Maybe LH is light but insulation won’t be easy, light and cheap.
Also how much hydrogen can be stored per volume?
According to energy density methanol is just 50 % of jet fuel but LH is at just 30 %.
LH is about 0.07 gr/cm³
Methanol at 0,79 gr/cm³ just about Jet-A.
So energy density per weight is about 4 times better for LH than for Jet-A but how heavy ist the LH tank?
In case LH+tank is lighter than kerosine than it could be an option for long range flights with huge amount of fuels.
Apologies for a trivial question. What is the difference between the yellow aircraft and the orange aircraft in the FR24 image at the start of the article ?
According to FlightRadar users:
“The yellow icons mean the data is processed in real time via MLAT and ADS-B. The orange icons mean that due to the use of FAA data (located in the USA) there is a 5 minute delay (FAA regulations)”
So 5 minute delay in reporting for the FAA data (aircraft shown in orange).
US Public perceives props to be inferior.
How much noise you can remove is also a factor.
Ramp issues with hits to the props as well as people would go up.
Not that we did not live it at one time but that is past.
Maybe a composite Constellation would change minds!
TW, in future designs that use unducted fans, the fan is in the back as a pusher configuration. This allows for better sweep-back and noise control, especially if mounted at the back of the airframe.
Also can make use of boundary layer effects for better efficiency with rear mounting. And the engine nacelles become quite narrow again if the fan is outside. But I agree noise is still a problem, and rear mounting has its own set of issues.
Ultimately the dominant factor in efficiency is the size of the fan. So either the ducted engines get larger nacelles or the fans become unducted, in the limit becoming a large free swept propeller.
It’s an interesting problem, will be fun to see how future designs develop.
Having engines off wings makes for clean and efficint wings like on the latest Gulfstreams still I think UHB engines or UDF engines will be wing mounted on high wings due to complex to have 2ea huge engines mounted in the rear compare to the VC-10
The weight of an engine ultimately has to be carried by the wing, so placing engines on the fuselage means a strong and therefore heavy structure must carry the weight to the wings. Its a trade off. Plus flaps can get some downward thrust etc.
Executive jets almost universally have engines in the rear fuselage for a few reasons, they are high up clear of foreign object damage, put the noise behind the cabin and allow a low ( clean) wing /undercarriage combination so baggage and passengers can load easily. There was a McDonnell prototype executive jet/military trainer for the USAF which used 4 engines under the wing much like a 707/DC8 of the time. The competition was won by the 4 (rear ) engined Lockheed Jetstar, but the USAF trainer requirement for 4 engined jet was dropped but the Jetstar persisted and was high end large cabin jet before the Gulfstream came along and took its market.
Executive jets would make ideal test platforms for alternative fuels, maybe a 3 engined Dassault 900/7X, but the interest so far is small propeller driven planes for their higher efficiency.
What are typical values for grams/passenger-km for the latest regional airliners, narrowbodies, and widebodies?
This varies with a very large number of factors, but here is a white paper focused on transatlantic routes of various airlines and aircraft. The average was 33 pax-km per liter, which converts to 24 grams per pax-km for Jet-A (note that the result varies fuel temperature).
Values for specific aircraft are given in the report.
Should point out that the model used in the paper overpredicts fuel consumption by an average of 5 pax-km per liter. So the adjusted average would be 29 pax-km/l or 27 gram per pax-km.
Thanks, Rob. I asked about g/pax-km because I’ve seen this metric commonly listed for Russian/Ukrainian aircraft but not for Western aircraft. BTW – did you mean “underpredicts fuel consumption” or “overpredicts fuel efficiency”, instead of what you wrote?
The conversion formula is (Jet-A fuel density, in grams per liter) divided by (airplane or airline mileage, in transported passenger-kilometers per liter). Wikipedia lists Jet-A density as 0.804 kg/L, and Figure 2 in your link has the A350-900 with the highest mileage (kilometrage?) at ~43 pax-km/L, so the value for the A359 is (804 g/L)/(43 pax-km/L) = 18.7 g/pax-km.
I looked around for values of fuel density and found different values. I used .79 as an average value, but noted it depends on temperature.
The model yields 5 pax-km per liter greater than observed. So it over-predicts fuel efficiency and under-predicts fuel consumption. The paper mainly uses the model for comparison so they weren’t that concerned with absolute values.
So the 43 modeled value you found for the A350 would be more like 38 or 39 in the real world.
The best next step in my opinion is a combined propulsion system with normal engines providing more electricity than actual to drive laminar flow control fans e.g. a ring of fans around the tail cone.
Worldwide, there is an average of 100.000 commercial flights, every day.
Air transport, passengers carried (ICAO)
In 1990 there were 1.025 billion passengers
In 2018 there were 4.233 billion passengers
Too bad policy makers and lobbyists seem to pit certain ways to reduce emissions over others, when all of the solutions at hand should be pursued especially when they can add up!
Low hanging fruits now and building the ladder for high hanging fruits.
Nox are an important topic, not just for local pollution but also for its indirect effect on climate too via the ozone production.
When the science is clearer on this impact, the aviation industry will have to brace for hard landing, the lack of fuel taxes will be no more tenable. And biofuels, which have been the easy reply of the industry to reduce emissions, will hardly be an answer to Nox (on top of its cost issues and scalability)
Bjorn, do you by ‘All Electric Aircraft’ mean ‘Battery Electric Aircraft’? Since Hydrogen Fuel Cell Electric Aircraft are already flying (as experimental and as drones, for now…) I think it would be better for your articles if you used a clearer terminology than the ‘All Electric’ term. Separating this would also make it easier to clearly understand where you see the problem areas: Electric Propulsion, or Batteries as Energy Carrier? Or both? And would, in your opinion, a Fuel Cell Electric Powertrain with either Compressed or Liquid H2 solve some of these problems you see? And for which segments of the market?
Check back on past articles – use the search box
“Former US Energy Secretary Dr. Steven Chu summarized the problem: “We need four miracles to happen (for fuel cells to become practical for transportation) and Saints only need three:”
Former US Energy Secretary and Nobel laureate Dr Steven Chu has had a change of mind. I guess ‘miracles’ do happen… 🙂
I remember reading the article and paragraph you linked, but it’s not exactly one of Bjorn’s finest moments, so I wasn’t going to rub that in by bringing it up. Essentially, much or what Bjorn wrote in September 2017 is not accurate.
I’ll leave it at that for now.
Bjorn , beeing a european ATC i really look forward to know in your next corners in which ways i’m blocking improvement in airspace management ? As a former military pilot you must know longer routes are often dictated by huge military segregated areas for one or two fighters….
happy new year holidays to all, even for lots of us working to get others safely home !
Single European Sky
“The European ANS system (¹) covers 37 air navigation service providers (ANSPs) participating in a cost-efficiency benchmarking report, which is a business of EUR 8.6 bn with some 57 000 staff (compared to Airbus worldwide employment of 52 000) and 16 900 are air traffic controllers (ATC) compared to 13 000 ATC in the USA.
In 2014, the European ATM system controlled 26 800 flights on an average daily basis.
On average each flight is 49 km longer than the direct flight.
European airspace: 10.8 million km², 60 control centres – fragmentation of airspace.
Estimated costs of fragmentation of airspace amounts to EUR 4 bn a year.
37 (now 38) Air Navigation Service Providers with 57,000 staff with just under 17,000 ATC.
Airbus has only 52,000 – and uses a lot of suppliers as well.
Not all the ANSPs are equal as the UK one , has just 2 centres at Swanwick and Prestwick, has a large area of the North Atlantic to control and had 4600 employees of which 1782 are ATC
If you want to reduce carbon dioxide emissions, then make a fuel using renewable energy (wind, solar, hydroelectric, nuclear) that contains no carbon!
Such a material is ammonia, NH3. We already make millions of tons of it, it contains more hydrogen atoms per unit volume than liquid hydrogen at a fraction of the pressure and it can either be burned direct in a gas turbine or put into a fuel cell.
I’d imagine an efficient way of using it would for it to intercool a gas turbine, the heat taken in that operation being used to catalytically separate the stuff into N2 and H2 molecules, and then either burn it or give it to a fuel cell or both.
Has there ever been an aircraft powered by ammonia? Yep, they called it the X15 and Neil Armstrong (among others) flew it at up to M6.7 at the edge of the atmosphere (with liquid oxygen mind you).
Using spare renewable capacity to manufacture ammonia is easy.
I’d still be a big fan of fixing air traffic control and mandating the use of turbo-props on short routes (initially, longer routes later).
The main selection criteria for an aircraft energy storage is the specific energy, the weight you need to store a given ammount of energy. Amonia is ‘heavier’ than Jet-Fuel and the weight of a fuel cell and electric motor comes on top, while liquid hydrogen is three times lighter than kerosene:
Some specific weights (source Wikipedia):
-Lithium-Ion-Batery: 0.36–0.875 MJ/kg
Use of H2 is great one way or the other but …
first get a large supply of H2 especially if you burn it (Yield limited by Carnot principle !!)
To get there use electricity to get H2 from water and elctrolyse
Get electricity trhu a very small amount of H2 using nuclear fusion
Problem solved in few decades (ITER in France and China already working on fusion technologies)
BE PATIENT !!!
E for environment.
Does anyone remember Mount Pinatubo?You know the big eruption a short while ago.Tge reason it’s releva is because on its own it caused a 1% cooling of the globe.That may not sound a lot but believe the scientists -its humungus!
How did it do this? By pouring vast amounts of sulphur dioxide and carbon soot into the stratosphere ( you know the place where jet aircraft fly -hint).The sulphur dioxide converted to aerosols And was carried by the soot particles.
Now we hear that the aircraft industry must desulphurise its kerosene and reduce the carbon soot particulates.Why? Because ( apparently putting this sulphur dioxide and soot into the upper atmosphere causes global warming! Confused??
Well you should be! Indeed not so long ago there was indeed a plan to put military planes up into the stratosphere to indeed try a geo engineering experiment.(vapour trails to be oversimlistic.Try cooling via exactly this process …But apparently that’s not they way to cool the planet ( nasty way) So the climate emergency is not that bad after all.
Perhaps they need to make their minds up!!