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  • The aviation sector is on the brink of a crisisIts future is in limbo as the world moves towards  

  • decarbonisation. Planes are currently  only responsible for 2-3% of the world's  

  • carbon dioxide emissions, but that's  expected to rise to 25% by 2050. [1]

  • Most major polluters have clear technology  pathways to a cleaner future. The automotive  

  • industry has batteries and electric motorsThe shipping industry has a range of potential  

  • alternative clean fuels to choose from. Our  electrical grids are rapidly investing in solar  

  • and wind, and future nuclear energy projects  are being researched intensively. There is  

  • still plenty of work to do, but the path ahead  for these sectors has been surveyed and marked.

  • However, the aviation industry has no  clear way forward for replacing kerosene,  

  • and if the aviation sector can't find answers  to this problem, it's projected that with the  

  • continued growth of passenger numbers and the  expected decarbonisation across other industries,  

  • that it could represent as much as 25% of  total world wide emissions by 2050. [1]

  • To understand this problem, and the potential  technologies we could see in the future,  

  • we first need to understand the  current state of aviation fuel.

  • Today, nearly all jet engines use kerosene,  

  • but internal combustion turbine engines are not  actually that picky about the fuel they consume.  

  • Gas powered turbines power grids all over  the world [2] , and many of them are being  

  • converted to run on bioethanol [3].Early jet  engines were powered by mostly gasoline. If it  

  • burns hot and can be pumped into a combustion  chamber, chances are it can drive a turbine.

  • But, it's not quite so simple for a jet  engine that flies and carries humans.

  • There are two main types of jet  fuel used for commercial aviation.

  • Jet A and Jet A-1. Jet A is primarily  used in the United States and Jet A-1  

  • is used in the rest of the world. [4] So is this just another case of the United  

  • States insisting on being different because they  are too stubborn to admit the rest of the world  

  • may just have a better system? In this case, no.

  • The primary difference between  the two is their freezing point,  

  • with Jet A-1 having a lower freezing  point of -47 degrees versus Jet A at -40.

  • For domestic flights within the US, Jet  A's freezing point is just fine, but  

  • for colder climates, or colder international  routes like those that fly over the arctic,  

  • a lower freezing point is needed to  prevent the fuel from turning to wax. So,  

  • a lower freezing point is  desirable, but it comes at a price.

  • The United States uses Jet A because  it is cheaper. To understand why,  

  • we need to understand how crude oil is refined.

  • Crude oil is essentially justblend of many different hydrocarbons,  

  • all with different carbon chain lengths.  [5] We have short chain gas molecules like  

  • methane and butane, with 1-4 carbon atoms in each  chain. Then we have longer gasoline molecules,  

  • with chain lengths between 5 and 10. Whilekerosene molecules range from around 10 to 16.

  • We can separate each fuel type from crude oil  thanks to these chain lengths impacting the  

  • boiling point of each component, which allows us  to separate them with fractional distillation.

  • We simply heat the crude oil up and  pump it into a distillation tower.  

  • The longer chain hydrocarbons liquify  lower in the distillation tower,  

  • thanks to their lower boiling point, and  when they do so, they are tapped off.

  • The shorter chain molecules will remain  gaseous and continue rising through the tower,  

  • but the tower gets gradually colder as it risesSoon Kerosene will turn to liquid and be removed,  

  • then gasoline, and finally the lightest methane  and butane gases rise right to the very top.

  • So how does this explain Jet A-1's lower freezing  point? Freezing points and boiling points are  

  • generally linked, so Jet A-1 can lower its  freezing point by excluding hydrocarbons  

  • with longer chains, and therefore excludes  lower boiling point molecules from the mix.

  • Jet A, in comparison, is less  picky about the freezing point  

  • and can take a larger cut of this distillateMeaning, there is a broader percentage of  

  • the crude oil that can be included in  Jet A, making it cheaper than Jet A-1.

  • So, it makes perfect sense forcountry like the United States,  

  • that doesn't need to worry too  much about low temperatures,  

  • to manufacture a cheaper wider cut fuel  for their domestic airline industry.

  • So, these are our first two properties we need  to consider when choosing a future aviation fuel:  

  • freezing point and cost. The freezing point issue  rules out longer chain molecules like diesel.  

  • Diesel powered vehicles in Canada  and Alaska actually have to cut their  

  • fuel with kerosene to prevent the fuel  from freezing in the winter months. [6]

  • This is the same reason a different jet fuel, Jet  B, is used in parts of Canada and Alaska. It's  

  • also known as wide-cut fuel, which gets its name  because it takes a much larger cut of the crude  

  • oil distillate, with a mix of 30% kerosene and 70%  gasoline, giving it an even lower freezing point  

  • of -60. So if this wide-cut fuel can be used  in engines, why isn't it used in all engines?

  • Gasoline, thanks to it's  shorter carbon chain lengths,  

  • is too volatile for general use in aviationIt's flash point is much lower than kerosene.  

  • Flash point is the lowest temperature vapors can  form from a liquid to create an ignitable mixture  

  • in air. So low flash points make unintended  explosions and fires much more likely,  

  • not something airports and planes are particularly  fond of. The lower temperature of vaporization can  

  • also cause problems with vapor locks in plumbingWhere gas bubbles can form and cause blockages.  

  • This becomes an even larger issue for jet  engines, as boiling points lower as pressures  

  • decrease at altitude. So gasoline is notdesirable jet fuel for general applications.

  • The US Navy and US Airforce even use two  different Kerosene grades for a similar  

  • reason. The U.S. Air Force uses JP-8 [1], which  is similar to Jet A-1, but with the addition of  

  • corrosion inhibitors and anti-icing additives  that are not required for the Jet A-1 standard.

  • While the US Navy uses JP-5. The primary  difference between the NAVY and Air Force  

  • fuels is that the navy fuel has a higher  flash point. 60 degrees versus 38 degrees.  

  • This makes it much safer to handle during  refueling operations on aircraft carriers,  

  • and makes explosions much less likely in the event  of an attack. This was a constant worry during WW2  

  • with the predominantly gasoline powered  piston engines. Fuel fires were not a  

  • rare occurrence during the war. [7] This is the  third property we need to consider: flash points.

  • But we aren't done yet. We haven't even  mentioned the most obvious. Energy content.

  • The primary function of aviation turbine fuel  is to power the aircraft. This is achieved by  

  • igniting the fuel, which releases  heat, which raises the pressure,  

  • which causes air flow. To fulfill this role  most effectively we want high energy content.

  • We can measure the energy content of  a fuel pretty easily. It's simply the  

  • heat released when a known quantity of the  fuel is burned under specific conditions.

  • There are twoquantitymeasurements  however. Energy per unit mass,  

  • measured in megajoules per kilogram, and energy  per unit volume, measured in megajoules per liter.

  • In general a dense fuel with a high  volumetric energy content is desired,  

  • especially for military aircraft that always take  off with their fuel tanks filled to the brim,  

  • so volumetric energy density is a more important  metric. Commercial aircraft only fill their tanks  

  • with enough fuel to reach their destinationwith a little extra in case of emergency,  

  • but volumetric energy density is  still generally a better measurement.

  • Let's add this to our shopping list, and  start looking at potential alternative fuels.  

  • First, let's look at the numbers for ourmain identified properties with a typical  

  • kerosene jet fuel. Cost, freezing pointflash point and volumetric energy density.  

  • These will be our measuring  sticks for our alternative fuels.

  • The first stop on our proverbial  shopping trip is the biofuel aisle.  

  • We have a tonne of options to choose from here.

  • In terms of production volumesbioethanol and biodiesel  

  • are currently the most available biofuels.

  • Ethanol is a short chain alcohol. Similar to  the short chain hydrocarbons, it's freezing  

  • and flash point is quite low, minus 115 degrees  celsius and 13 degrees respectively. [8] The  

  • low freezing point is useful, but the low flash  point is a problem. This makes ethanol volatile,  

  • which makes it undesirable as a jet fuelIt's volumetric energy density is about 61%  

  • of kerosene, meaning range would be reduced  if fuel tanks remained the same size. [9]

  • Biodiesel suffers from the opposite problem to  bioethanol because it's carbon chain lengths  

  • are much longer. As a result it's flash point is  very high, between 98 and 150 degrees depending on  

  • the feedstock used, and as expected comes with  a very high freezing point of about 1 degrees.  

  • This fuel would turn to wax in  the fuel tanks. It's unusable.

  • However, we can further process these biofuels to  create fuels that are so similar to kerosene that  

  • they can even be used in current generation  planes with very little modification. [10]

  • Airbus began testing a fuel composed entirely  of biofuel this year in an A350 powered by Rolls  

  • Royce XWB engines. [11] Testing the plane's  performance and emissions using the fuel,  

  • which was manufactured be Neste. A company  that manufactures biofuels from palm oil  

  • and waste oils, like cooking oil. Results  of this test have not yet been published,  

  • but NASA has already published data from their  own tests with a 50-50 fuel blend or traditional  

  • jet fuel and a similar plant oil derived biofuel.  [12] Their tests showed, with only a 50-50 blend,  

  • that particulate emissions in the contrail  were reduced by up to 70%. That's important,  

  • because those particulates have a much  larger impact on earth's atmosphere  

  • than the carbon emissions. This is  positive news, but these biofuels  

  • are a long way from being cost effective or  even environmentally friendly to manufacture.

  • The main challenges facing biofuels are scaling  the feedstocks in an environmentally friendly  

  • way and cost. Waste oil products as feedstocks are  fantastic and every country should be working on  

  • ways to collect waste products to feed this  growing industry, but sourcing oil from the  

  • palm oil industry is obviously problematic, as  the palm oil industry is driving the destruction  

  • of the Borneo rainforest. Sourcing enough  feedstocks to completely replace fossil fuels  

  • in the aviation industry is going to be a massive  problem to solve, and right now we have no answer.

  • Cost is also a huge issue. Norway announced a 0.5%  

  • biofuel mandate for the  aviation sector in 2019. [13]

  • This is a tiny fraction of the total fuel usedbut Scandavian Airlines has said that this 0.5%  

  • mandate will add an additional 3.3 million dollars  in fuel costs a year. Making it 100%, assuming  

  • prices wouldn't rise with the extra demand, would  cost 660 million dollars extra a year. That would  

  • Completely wipe out Scandinavian Airlines'  2019 profit of 84 million dollars. [14] So,  

  • these biofuels currently fail the cost metricdespite being suitable alternatives to kerosene

  • Even if we ignore the questionable  environmental benefit of the feedstocks,  

  • the real issue here is the difficulty  in scaling up feedstocks to meet demand

  • So, are there any other alternatives

  • Hydrogen is also being explored  as a potential future fuel.

  • Airbus has published several concept  aircraft that could utilize hydrogen,  

  • because, unlike biofuels, hydrogen cannot be  used in existing planes. This would require a  

  • complete overhaul of airlines plane inventories  and would cost trillions over several years

  • Hydrogen's main advantage is that's feedstock  is just water, and we are surrounded by water

  • However, hydrogen currently needs very  pure fresh water to prevent corrosion  

  • to the electrodes that split the water  apart during electrolysis. Researchers are  

  • working on ways to extend the life of these  electrodes while preventing the salt ions,  

  • like chloride, that are found in seawaterfrom breaking down the electrodes. [15]

  • The alternative is simply pairing the system  with desalination process, but this would draw  

  • even more electricity for what is  already a very expensive process.  

  • Hydrogen, right now, does not  satisfy our cost requirement.  

  • But let's move forward with the expectation  that we will have massive amounts of excess  

  • renewable energy looking for a home in the  future and assume these costs will come down

  • Hydrogen has insanely good  gravimetric energy density,  

  • at 120 MJ/kg. [16] Completely blowing kerosene  out of the water at around 44 MJ/kg. However,  

  • hydrogen's volumetric energy density, the quantity  we actually care about, is complete dog trash

  • The only way to get it to a reasonable number  is by pressurizing it or making it cold,  

  • but even then it's volumetric energy density  is terrible. At 700 bar, that's 700 times  

  • atmospheric pressure, hydrogen still  only has a volumetric energy density  

  • of 5.6 MJ/L, compared jet fuels 38.3 MJ/L. [17] Pressurizing a fuel tank to 700 bar comes with its  

  • dangers, as repeated pressure cycles can lead to  rapid failure due to fatigue. This is made worse  

  • by hydrogen's habit of attacking and embrittling  materials, a phenomenon that is also accelerated  

  • by higher pressures. [18] So, most designs for  hydrogen fuel tanks instead call for cryogenic  

  • storage. Where the hydrogen is cooled to  achieve a higher volumetric energy density  

  • with much lower pressures. [16] This also  results in higher energy densities of 8 MJ/L,  

  • but still much lower than the  38 MJ/L of traditional fuels

  • This low volumetric energy density, and need  to pressurize, makes hydrogen fuel tanks a  

  • nightmare to integrate to an aircrafts airframe. Planes these days place a large amount of fuel  

  • inside the wings. [19] This is ideal for several  reasons. It takes up no useful space inside the  

  • cabin of the plane. Aircraft wings need to be  hollow to increase the strength of the wings.  

  • The weight of the fuel being located so close to  the center of lift means the plane does not need  

  • to adjust it's control surfaces during flight  to compensate for changes in center of gravity  

  • as the fuel gets used up, which reduces drag. Finally, when flying, the wings deflect upwards  

  • due to the upwards lift force they create. This  creates stress in the supporting structures  

  • of the plane. So, by putting the fuel in the  wings it actually helps the wings deflect less  

  • as the weight of the fuel pushes them  down, and as the fuel is used up,  

  • the lift the wings need to generate reduces, and  the upwards lift forcing the wings up reduces

  • Storing the heavy fuel in the wings is an  incredibly elegant solution, and it's not possible  

  • with hydrogen. There simply is not enough space  in the narrow hollow structure of wings to fit  

  • the equipment needed. This space is also getting  even smaller as newer generation composite planes  

  • enter the market [19], with their sleek elegant  wings being much thinner than older metal versions 

  • Because hydrogen needs to be pressurized and  cooled, it requires specialized fuel tanks that  

  • are too bulky to fit into these small spaces. The  matter is only made worse because of hydrogen's  

  • dismal volumetric energy density. Some designs for  hydrogen planes simply call for the massive fuel  

  • tanks to be placed inside the fuselage, replacing  valuable space that could be used for passengers  

  • or cargo. This just compounds the issue of cost  even more, as airlines will now be making less,  

  • while also having to pay more for fuel. While some have proposed a more drastic change in  

  • flight architecture, the blended wing. The blended  wing offers fantastic drag characteristics and  

  • leaves plenty of space within the wing to store  the large fuel tanks. There is a lot more to be  

  • said about this design, but we will explore this  kind of plane in more detail in a future video

  • Now we need to deal with the safety concernsHydrogen is a gas in normal conditions,  

  • so flash point is not a relevant quantityIt's gases are going to ignite at all ambient  

  • temperatures if exposed to an ignition source. It is a difficult fuel to handle for this reason

  • Hydrogen also has no odor and it's flame is nearly  invisible, so detection of leaks is difficult.  

  • It's also difficult to mix odorising agents, like  the sulfur odorants we add to natural gas, because  

  • the freezing temperatures of liquid hydrogen  would simply turn them solid in the tanks and they  

  • wouldn't exit with the gas when there was a leakThese odorants would also contaminate any fuels  

  • cells using hydrogen to generate electricity. [18] This is a problem because many future hydrogen  

  • powered jet engines, including all of  Airbus' concepts, call for hybrid engines,  

  • mixing electric motors powered by hydrogen  fuel cells with combustion turbines burning  

  • hydrogen. [20] Gas alarms will be essential  early warning systems and they will need  

  • to be located anywhere large quantities of  hydrogen are stored. In the case of a leak,  

  • modular tanks, with shut off valves between  each section will be essential to minimize risk

  • These storage and handling difficulties are likely  the largest barrier for hydrogen moving forward,  

  • and this is why some have proposed an  extra step, that will use hydrogen to  

  • generate a new type of hydrocarbon fuel. E-Fuels. This would be done by combining carbon dioxide,  

  • which will be drawn directly from the  atmosphere using direct air capture,  

  • with hydrogen to produce methanol. This methanol  would be liquid at ambient temperatures and  

  • could be further processed, like our ethanol from  earlier, to produce kerosene efuels. E-Fuels are  

  • fuels that are created entirely using sustainable  feedstocks and renewable electricity. This would  

  • solve the scalability issues of biofuels, but  more than likely cost a lot more due to the  

  • sheer amount of energy needed to both create  hydrogen and draw carbon dioxide from the air

  • It's hard to make predictions on the  future of the air travel industry.  

  • If I was placing bets, I think biofuel mandatesdespite their questionable environmental benefit,  

  • will continue to be introduced, and then, as  excess renewable electricity floods the market,  

  • energy intensive processes like efuels may  take over. Primarily because these fuels are  

  • compatible with current jet engines. Hydrogen  has a chance of succeeding, but it will require  

  • massive investments to completely  overhaul airport and plane architecture,  

  • which alone will cost trillions of dollars. This cost barrier is going to be something  

  • the aviation industry is going to have to accept  in the near term. It's more than likely that air  

  • travel will get vastly more expensive during  this transitional period. That cost inflation  

  • can be minimized by a gradual introduction of  biofuels and efuels that are compatible with  

  • current generation infrastructure. Howeveras we saw in Norway, even just a 0.5% biofuel  

  • mandate increased fuel costs significantly. And  this may just be a hard truth we as a society