January 24, 2020, ©. Leeham News: In the last Corners we tested some of the ideas why electric/hybrid propulsion would be more efficient and found the ideas could be easier and better implemented with existing technology, yet they are not. One wonders why. Perhaps the ideas are not that brilliant after all.
Before we look at alternative technologies that can help us lower air transports CO2 footprint we shall scope the problem and look at what part air transport plays. To what extent is air transport at the center of the problem?
First, does the world have a CO2 problem? Figure 1 shows our concentration of CO2 in the atmosphere is galloping since the 1960s. The effects of this change are becoming more and more visible. Of the contributors, air transport contributes around 2% at present.
The consumption statistics, which are hard to come by, are from the US Department of Energy and is their Worldwide energy consumption by sector from 2012. But as Figure 2 shows, fossil fuels dominate our energy consumption. Figure 1’s right-hand side, therefore, shows the main culprits in our CO2 emission problem.
One source of the phenomenal increase of CO2 concentration is shown in Figure 3. China has in the last 30 years gone from an emission level of Japan to double the level of the US. The major reason for the growth in CO2 emissions in China is the industrial growth, where steel and cement production and the generation of electricity is driven by fossil fuels.
As shown in Figure 1, air transport is a small part of the global CO2 problem. But it is highly visible and by it, an easy target. The airliners are very visible and the emission of exhausts from their engines is easy to spot. Further, air traffic grows by 5% per year, so the emissions will increase unless the fuel consumption of the worldwide aircraft fleet is decreasing with 5% per year.
We have experienced a 15-20% decrease in consumed fuel per passenger-mile for the last generation of aircraft but this is over 15 to 20 years. It means our decrease in fuel consumption per passenger-mile is more like 1% per year. And this is only for the new aircraft added to the existing 20,000 aircraft flying daily.
The above shows the importance of reducing fossil fuel consumption further for our airliners and our efforts to change as much of the consumed fuel from fossil to non-fossil fuel.
But as I have shown in this series the way to drastically changing our propulsion technology for our airliners is difficult. The reason is the efficiency of our modern airliner’s turbofans is very high. Road transport has engines with thermal efficiencies for the latest engine generations of below 30%. The thermal efficiency of our latest turbofans is 55%, double the efficiency of our cars.
On top of the differences in engine core efficiencies, road traffic is inefficient as a system as built-up potential energy is wasted at the next stoplight through our thermal waste brakes. Electric/hybrid cars recover part of this potential energy and can, therefore, compete with combustion cars. But the principle has a hard time to compete with airliner gas turbines. The airline mission has no segment that wastes energy, hence the energy recovery potential is not present.
It’s clear Air transport shall do its part to cut global green gas emissions. But the importance of these emissions must be put in its context, compared with other industries like energy production where non-fossil fuel alternatives are well established and can have a major impact on CO2 emissions.
The messaging on what is reasonable to expect from air transport must improve. It helps no-one if the world’s climate change is blamed on an easy target such as our airliners when it’s nigh impossible to replace hydrocarbons as fuel for anything but extreme short haul over the next decades.
It’s better to communicate what is done already, like the superior efficiency of the airline engine, the non-wasteful flight cycle compared with our road transport, the increased use of renewable fuels and the inefficiency of efforts to change long haul air transport to other propulsion technologies when the same efforts can achieve more elsewhere.
We will not contribute efficiently to solving the world’s CO2 problem if we let the public believe we will fix it with electric airliners tomorrow when the reality is we can’t. Instead, the focus shall be on what part of the problem does air transport amount to and how can it contribute to a fix.
With this said, we will focus the next Corners on what technologies can help us to a sustainable air transport system, including sensible applications of electric/hybrid technologies.
In 2018, global commercial operations emitted 918 million tonnes (Mt) of CO₂, 2.4% of all CO₂ emissions: 747 Mt for passenger transport and 171 Mt for freight operations.
Well written article. The current debate in Sweden is about bio based fuel, produced mainly from rests from the Tarja wood landscape inustry. But other sources can be used. To validate 100% biofuel, with likelly different mixes of standard fuels will take som time. But if this type of fuel or perhaps some new type of more enviroment friendly fuel based on e.g. fracking gas can be prioritized for the important aviation, this can be a promissing way forward as well.
Bio fuels primary purpose is as a gimmick to assuage critics There is room for it to take advantage of surplus food or crop production in my view. The problem is land use. One hectare of crop land will provide enough fuel to fly an A320 about 400km per year So to fly 4000km per day would every day of the year need about 4000 hectares. I find the idea of that amount of cropland hideous and would prefer to leave it to the birds, moose, wolves and bears. Subsidised European biodiesel already contributed to the near extinction of the organ outang since it encourage the clearing of forest to make way for palm trees. By contrast PtL (Power to Liquids) using wind power to generate synthetic fuel would required 1/10th to 1/20th as much land.
I think we can reason a bit differently.
We must realize our fossil fuels, that were produced millions of years ago, is a treasure. They have special characteristics, essential for some challenging applications where alternatives are hard to find (air transport is one). We shouldn’t squander these earth assets when producing energy-hungry essentials were perfectly viable carbon-neutral alternatives exits.
It’s not about going from burning our fossil fuels like if there was no tomorrow as we do today, to where they are not available for any use. It’s about preserving them for where we don’t have suitable alternatives. It’s about using energy with care and substituting with non-carbon generating energy sources where sensible, instead of today’s careless burning of oil, coal, and gas got sny purpose.
It’s about thinking that extra bit about tomorrow and the difficulties we create for our children because we prefer to live carelessly today.
Bjorn, excellent point & article. It’s about best & balanced use of resources. In the 70’s, peak oil was predicted to be near or already present, and there was talk of creating reserves for future aviation. There was recognition then of the lack of aviation alternatives.
With today’s climate concerns, we should be actively and voluntarily pursuing peak oil, rather than worrying about when it might happen. But the basic exception for aviation still exists, it’s still the best use of fossil fuels.
I would wish that we had a common sense approach like you counsel but I suspect many interests will sabotage this.
Public relations will be critical to ensure the industry is not senselessly attacked. IATA Industry body) has some interesting facts. Aviation has reduced per seat fuel burn by more than 50% in the period from 1990 to now mainly due to fuel efficiency gains in aircraft. IATA members have pledged a cap in emissions from 2020 and a reduction to half of 2005 levels by 2050. The 5% growth in aviation you mention compounds to a 4.2 times increase by 2050 though I expect the actual increase to be less.
IATA (industry body) has been using the ICAO (UN body) program called ( CORSIA) Carbon Offsetting Scheme for International Aviation as well as improving aircraft performance. I see no problem with this program which allows airlines to pay for offsets that compensate for the emissions of flights. I assume it includes good works such as reafforestation some biofuels and building wind turbines.
It’s worth noting that Iron and Steal emits 5% of global emissions but that production of steal by hydrogen is a much easier way to eliminate and equivalent amount emissions than attacking aviation. Aviation can help iron and steal make this transition.
The reason I feel PtL fuels are important is that from the public relations point of view being able to say 5%-10% of an aircrafts fuel burn is carbon neutral can not easily be undermined by critics. Offset accounting is esoteric and our media lacks the integrity anymore.
CORISA is a very good program. Taxes are not an effective solution
Experience shows that the effectiveness of taxes as incentives for cleaner/quieter aircraft is doubtful and does not contribute to decarbonizing aviation. No government that introduced a ticket tax has been able to demonstrate that such tax reduced CO2 emissions and rarely (if ever) used the revenues to support investment in technologies that would help mitigate aviation’s emissions. CORISA can account for its offsets.
You must have analysed this from a technical point of view what reduction in fuel burn is possible?
For instance ultra high aspect ratio braced wing aircraft with advanced engines like Boeings SUGAR concept? 50%? Such reductions make carbon neutral fuels more affordable and reduce offsetting requirements.
Here in the USA, we are already using biofuel for about 10% of our transportation fuel. As cars switch to electric and gasoline is phased out, the production resources for that biofuel could become available for aviation. The land use to replace all of our oil consumption with biofuel is unacceptable, but replacing the 5% or or so of oil used for aircraft is likely manageable.
In the very long run (think past the end of this century) we will probably, eventually, figure out a way to store electrical energy as densely as fossil fuel. After all, the theoretical energy density of lithium-air batteries is about thrice that of gasoline. So if we’re looking 100 or 200 years from now, I do expect all aircraft to become electric.
My understanding of US corn derived biofuel is that it has a efficiency factor of 1.6 (ie 1 unit of energy input produces 1.6 units of output). The more thorough analysis seems to be only 1.1. This includes allowances for tillage, tillage machinery manufacturing, harvesting and energy for distillation. Brazilian sugar cane has an efficiency factor of between 3.7 to 8 because the left over stalks from crushing out the cane juice, bagasse, is used to drive distillation and generate electricity. Carbon nano tube membranes may one day separate out the ethanol with great efficiency for US corn ethanol. Solar stills could also be used and if tillage were provided by John Deere’s electric tractor it may produce a good energy balance. Here in Australia grain is used for bioethanol despite our large sugar cane crop. I regard it as a good way of subsidising farmers to stay on the land and keep food production high rather than a viable fuel system at this pint. Mobil famously developed MTG “methanol to gasoline” which uses a zeolite lattice catalyst to constrain conversion of methanol to gasoline molecules with 90% efficiency. The process also works as ETG “ethanol to gasoline” and Mobil has demonstrated jet fuel production from methanol. Ethanol to Jet fuel would also work. Methanol and methane is easily produced with CO2 and hydrogen and one can imagine concentrated sources of CO2 such as cement and aluminium being used to make methanol when excess emissions free electricity is available. Ethanol and methanol can then both be converted to jet fuel. In Germany biomass has been burned to generate electricity and the C02 collected and combined with wind power generated hydrogen to make synthetic natural gas. Audi ran an Carbon offset scheme to power it’s compressed natural gas cars. This may be a model for aviation.
There has been some success to grow algea in saltwater dams in deserts for food and fuel. The other benefit is evaporation from these dams producing rain in other places.
Forests around the world planted for timber , if planted at all, is on average very poorly managed and could in most places grow at over twice the speed and thus double the conversion of CO2 to O2 and cellulosa per ha.
In the majorities there are seldom quick optimal replantation after harvest slowing the average growth over time.
you can actually grow algae from IC engine exhaust injected into water, irradiated by the sun. Skim of the bio mass growth. ( Works like algae bloom )
Yes algea can be great but hard to control as you easily can get the wronge algea mix growing, still I agree it is worth lots of scientific and engineering work as the benefits can be huge.
One can also set the ruling on net emissions letting airlines grow trees in well managed forests producing timber for construction and algea in desert salt water ponds to suck up more CO2 than they emit hence the airliners net emissions will be negative within a few years.
Similar net emission rules can then be applied to other industries operating less advanced equipment like power plants, concrete factories and steel mills hopefully globally including the US and China.
I agree with you.. and also with Bjorns below.
What is the optimal design of an airplane to save energy?
-save weight drastically (flying wing)
-aerodynamics (all surfaces add lift, relaxed stability….)
-new flight path (higher altitude, parabolic, longer free falling at low drag?)
Out of the box thinking…
“some new type of more enviroment friendly fuel based on e.g. fracking gas”
that is an Oxymoron. afaics.
Fracking gas is about as dirty to claim as possible.
oil from shale stuff too.
LNG loses 15% versus CNG in transport. liquifying is essentially lost effort. compression work is partly reclaimable.
“Further, air traffic grows by 5% per year, so the emissions will increase unless the fuel efficiency of the worldwide aircraft fleet is decreasing with 5% per year.”
The fuel efficiency rather needs to *increase*, doesn’t it?
Right! Changed. Bjorn
The Eviation , so called prototype electric 8 seater plane caught fire , before
even doing any taxi tests let alone fly…how sad
Nice to dig in the proble Bjorn. Few comments:
-You could have mentioned the ghg impact of nox and clouds which is as bad if not worse as co2 emissions alone…
-i agree that overhyping electric flight as an excuse not to make efforts in other areas is bad. But just pointing fingers at worse emitters to do nothing is even worse.
-Note that the international maritime organization announced a 5bn usd development programme for carbon neutral fuels (hydrogen and ammonia) funded a by voluntary tax per ton of fossil fuel. Aviation could do the same.
Looking forward the next corner!
I think that is Bjorn’s point. Technologies like these are much easier to implement on the ground and the sea, where weight and size and safety are not such an overwhelming concern. So those things should go forward while aviation waits for something that competes more favorably with fossil fuels. For aviation, advances can still be pursued with regard to efficiency, to minimize consumption and impact.
The NOX problem is not coming from aircraft or diesels. Fertiliser use is to blame. https://news.berkeley.edu/2012/04/02/fertilizer-use-responsible-for-increase-in-nitrous-oxide-in-atmosphere/
Similarly the sulphur based acid rain from coal and which was blamed for pine tree forrest die back but was also due to land use.
Aviation must make sure it does not become the visible scape goat for the mob.
I realise water vapour is a strong infrared absorber, the main one but it also forms clouds that blocks infrared and causes rain which leads to plant growth. The effects are complex and not generally negative yet uncertainty allows worst case fear since hundreds of climate models run backwards show that they don’t work. The billionaire bankers that want “cap and trade” are very keen to see that we and our children are frightened into taking their solution. The same applies to carbon taxes. The buerocrat and politician gets money and the student gets a free degree in international relations. Yet whereas the ICAO CARISMA program can accurately show where reductions have been obtained for offsetting aviation’s emissions carbon taxes can not.
NOX must clearly be minimised and the solution I feel is more efficiency. Current fuel burn is about 2L/100km/passenger but it seems 0.6L/100km/passenger is possible. The Boeing ultra high aspect ratio braced wing SUGAR concept should get a 56% reduction ie 0.9L/100km/passenger over a medium haul route. The technologies are not available now. Even the fuel cell to power the tail boundary layer suction electric fan system is available. I suppose a near complete elimination of NOX should be possible with fuel cells or perhaps closed cycle Brayton gas turbine.
Thanks for the balanced article. There are many industries that have a larger handle to avoid the massive increase in CO2 and other greenhouse gases. Would you mind, to show an actual graph of the Keeling curve. It ends in 2000 with a value of about 370ppm. We reached 415ppm last year. These 45ppm increase in these last 20 years alone respresents about 35% (!) of the increase since 1800 (280ppm). Solutions must be found to stop this increase.
CO2 is shooting up rapidly, no doubt about it. Fortunately infrared absorption shows a strong non linear logarithmic effect versus concentration which means we would need to double the concentration again to get the same effect, weve had most of the climate effects since about 1940. Its not saturated in upper atmosphere and some will reradiate down but not so much. With any luck planet will tolerate 1000ppm with little negative effect. Makes me a little uneasy of course. Where is nuclear when you need it.