As is happening with cars, alternatives to fossil fuel based propulsion are also being developed for aircraft. But aviation is at a disadvantage compared to cars. Weight is a much bigger consideration for aircraft than it is for cars, airplane design options are constrained by the physics of flight and commercial operations in particular will want quick turn-around times so the system needs to allow for that as well. On top of that, certification of aircraft is a complex matter, even more so when it involves new and untested technology, which further complicates things.
But with all that being said, moving away from fossil fuels isn’t proving to be impossible and progress is being made. Let’s have a look at the various technologies being used or proposed and the pros and cons of each. When exploring these solutions, it’s good to be aware of the fact that much of the research and information comes from the industry itself which has an obvious interest in portraying things in the most positive light for them.
Electric aircraft
As with cars, a lot of attention is going towards electric aircraft. There’s three dominant approaches here, all-electric using battery power, all-electric using hydrogen fuel-cells and hybrid designs. These power sources can be coupled to a variety of different electric propulsion types as well.
All-electric using battery power
At a glance, this is the most basic system. Batteries power electric motors. And since it sounds simple, it’s no surprise that this isn’t a new idea. In 1884 the French Tissandier brothers proved that their airship “La France” could be navigated back to its original point of origin using a propellor powered by an electric motor. Interesting is that that motor, which produced 1.5hp, was manufactured by Siemens which remains involved in developing electric motors for aviation use in recent years but sold that part of their business to Rolls-Royce in 2019. But, despite the electric motor working as intended, the system had the same challenge that battery powered systems still have today: the batteries were too heavy.
Obviously today’s batteries are able to pack more energy into less weight than in 1884. The energy density of the system has improved. But even with today’s battery technology the endurance of electric aircraft does not come close to that of similar aircraft using combustion engines. For example, the Pipistrel Velis is an electric version of the Pipistrel Virus SW 121. The electric version has an endurance of 50 minutes while the conventional combustion variant has an endurance of about 5.5 hours.
This does not mean that all is lost. The energy density of batteries has been increasing at a rate of about 5% a year and engineers also find ways to make the aircraft themselves make more efficient use of that power. Just as electric cars are seeing an increase in their range over time, so should electric aircraft. But, there are limits to how much this can improve, and battery systems will remain heavy. But relatively short endurance figures may be fine for use cases like regional transportation and flight training. Not all flights cross great distances.
Improvements are also being made to charging times. Joby Aviation indicates that for their S4 air taxi when used on short hops the aircraft can be recharged in five to seven minutes, which isn’t a problematic amount of time since embarking new passengers takes a few minutes anyway. But for longer flights the recharge time starts to become a bit more problematic, with a 150 mile flight requiring 45 minutes of charging. This is a lot of down time for a commercial operation. Some airlines today can turn around an aircraft like a Boeing 737 in under half an hour so requiring a 45 minute recharge for a short flight with only a few passengers seems like a problem. But as with cars, technology is improving, and faster charging times should be expected in the future.
When we look at cost, since electricity does not cost as much as fuel, be it Jet A1 or AVGAS, operation will be cheaper from that perspective. For example, AVGAS (in 2020) typically cost between $4.06 and $5.51 per gallon in the lower 48 of the United States. Again, Pipistrel gives us an interesting real-world comparison by having two similar aircraft where one uses a combustion engine, and one uses battery power. The combustion aircraft uses about 4.9 gallons of fuel per hour, giving us a fuel cost of about $19.89 to $27.00 per hour. Electricity cost in the lower 48 of the United States ranges from $0.08 to $0.19 (per most recent data available calculated for the year 2020). The dual battery system offers 49.6 kWh of power for a total endurance of 1.5 hours. So, a one-hour flight would equal about 33.1kWh used or a cost of about $2.65 to $6.29 per hour in electricity. In short, the cost of the electricity used will only be a fraction of the cost of the fuel a similar aircraft would have consumed. 1 2
But electricity cost isn’t the entire picture. Maintenance in theory should be cheaper as there are fewer moving parts to worry about, however, for now this is specialized technology so not all repair shops will be able to perform the necessary maintenance, so the true cost here may be harder to determine. Another problem is that batteries do not last forever and will be expensive to replace. How often would they need to be replaced and what would that cost? It’s probably too soon to tell. Heart Aerospace is working with the assumption that their batteries will last 1,000 charging cycles. Joby claims to be able to get up to 10,000 charge cycles out of their system and Tesla reported in 2020 having achieved battery technology also capable of 10,000 cycles which makes that claim seem at the very least plausible. We also know that the cost of batteries is dropping and we’re not far off from batteries costing about $100 per kWh. 3 For the Pipistrel Velis, currently, 800 cycles are expected out of their batteries. If we assume that it is operated one hour at a time then charged, that means it can operate around 800 hours before the batteries need to be replaced. If by the time these battery replacements become necessary, the cost of the batteries would probably be around $5,000. Obviously, the total cost will be higher as this does not include the cost of labor. But, we should also consider that piston engines may require expensive overhauls, though the Rotax 912 S3 that the piston counterpart uses can do 2,000 hours between overhauls. The cost of the overhaul will vary depending on the work needed but estimates range from between $12,000 to $14,000 for this type of engine. So, the overhaul cost of a piston engine may end up being roughly equal to the replacement cost of batteries. Electric motors are expected to have relatively minimal maintenance cost. This cost picture may change further when comparing electric motors to turbine engines which require more costly maintenance.
So, what can we conclude from all this? In short, electric aircraft are a lot heavier and as a result offer reduced performance and range as a result. To move an equal amount of people or cargo you need more weight in batteries and supporting systems. Not only that, batteries are a fixed weight while fuel burns reducing the weight of the aircraft during the flight. This means that right now electric aviation only really works for carrying relatively small amounts of people over short distances, as larger loads or distances would simply make the aircraft too heavy. And no developments today suggest that we might come up with some magic battery technology that would bring energy density anywhere near that of fossil fuels. So, while there is a lot of buzz around battery powered electric aircraft, they only work in certain niche use cases and certainly will not replace medium-to-long range airliners.
Hybrid electric designs
Various different hybrid electric approaches exist. One approach is for a conventional piston or gas turbine engine to generate power for the aircraft’s battery system, which in turn can power electric motors and aircraft electrical systems. This approach is known as a series hybrid design. Aircraft using this system could operate short distances entirely on electric power but can rely on conventional fuels to extend their range.
In a parallel hybrid design, both electric and piston or gas turbine engines can deliver power directly for propulsion. In such designs, power is delivered either from one or more electric motors, the piston or gas turbine engine, or a combination of both. Under heavy loads such as during take-off, the power of both may be used, while during times of lighter load such as cruise or descent, the system can run entirely on its electric motor(s) which operate more efficiently than the piston or gas turbine engine.
A third option is to have two separate propulsion systems, for example the proposed Desaer ATL-100H, which has two conventional turboprop engines and two MagniX electric motors. This setup is also frequently used in testing of new electrical propulsion systems, such as the Cessna 337 used by Ampaire or the Yak-40 used by SuperOx.
Since there are many different possible configuration options performance will vary, but, generally speaking hybrid systems should offer considerable fuel savings when compared to conventional combustion alternatives, while at the same time offering more range than pure electric aircraft since it doesn’t require as much weight for batteries. The difference in range between pure electric solutions and hybrid ones is clear when we compare the all-electric Heart Aerospace ES-19 for which a range of 215nm is expected to a similar-sized hybrid aircraft, the Faradair BEHA for which a range of 1000nm is expected. 4
Hydrogen fuel cells
While there are different types of fuel cell designs, the most commonly used ones today leverage hydrogen. While the technology sounds complex, hydrogen fuel cells are not a recent development and the first published work about hydrogen fuel cells stems from 1838. Today, Proton-Exchange Membrane Fuel Cells (PEMFCs) are the most popular choice. To understand how it functions, it’s important to understand that a single hydrogen atom (H) consists of a single proton and a single electron. Hydrogen is typically found as a gas consisting of two hydrogen atoms (H2). In a PEMFC, H2 enters what is called the anode side of the system where it encounters a catalyst (commonly platinum). The catalyst encourages a reaction where the H2 is split into the two components that make up an individual hydrogen atom: an electron and a proton. On the other side of the system, oxygen (O2) enters what is called the cathode. Here the oxygen also encounters a catalyst (again commonly platinum) which splits up the oxygen into individual atoms.
In between of the anode and the cathode sits a membrane. The membrane is a special substance like Nafion that will only allow protons to pass through (hence the name “Proton-Exchange Membrane Fuel Cell”). As the protons pass through the membrane to the cathode side where they can recombine with the now split oxygen molecules (thus: oxygen atoms), the electrons will also want to travel to that side however they cannot permeate the membrane. They can travel through an electric circuit however, which is established separately, and which results in current passing through the connected electrical system. When both protons and electrons have reached the cathode side, they can recombine with the individual oxygen atoms to form H2O (water). Besides water, the process also generates considerable heat, which means cooling of the system is required. It’s also worth noting that the water produced as part of the process could lead to contrail formation, which in itself could contribute to the greenhouse effect.
Understanding the basics brings us to the obvious core reason why hydrogen fuel cells are interesting: running them doesn’t produce harmful waste, just water. However, it clearly is a more complex system which creates a lot of challenges. First of all, Hydrogen is a tricky fuel to work with compared to more conventional fuels. It carries a lot of energy by weight but not by volume, which means that hydrogen needs to be compressed for storage so that you can bring enough of it. However, storing compressed hydrogen means you cannot just use the typical “wet wing” design found in many aircraft. To further increase the storage capacity for hydrogen it is possible to store it in liquid rather than gaseous form, however, this requires reducing the temperature of below -252,87 °C and maintaining this temperature requires extremely good insulation. If the hydrogen warms up over time, the pressure inside the system will build and hydrogen must be vented to avoid the pressure reaching dangerous levels.
And with hydrogen being highly flammable, the storage must also be able to meet safety requirements. Further, with hydrogen being such a small molecule and stored under high pressure, it means systems are also very prone to leakage. Refilling hydrogen will also require specialized airport infrastructure and sufficient production of it to make it economically viable. And the production of hydrogen itself can cause pollution if this is done using power generated by conventional power plants using coal or gas. The system itself also contains some rather expensive components like the platinum catalyst (which is why catalytic converters are frequently stolen from cars since they also contain platinum).
In summary, hydrogen fuel cell technology, much like hybrid systems, allow for electric propulsion with a range far greater than batteries alone can provide. However, the challenges around working with this fuel generally mean that with the current technology, range and payload have to be sacrificed in order to accommodate the system. It’s also worth pointing out that hydrogen fuel cells have the potential to play a secondary role in larger aircraft, which is to replace the electric power commonly generated by the Auxiliary Power Unit (APU) on the ground with a hydrogen fuel cell system.
Alternative fuels
An alternative to switching to electric propulsion is to stick with existing propulsion methods but change the fuel used. The key paths today are Sustainable Aviation Fuels (SAF), eFuels and burning hydrogen (rather than using hydrogen in a fuel cell). Burning liquid ammonia is also being explored.
Sustainable Aviation Fuels
The important thing to understand about Sustainable Aviation Fuels is that they’re still hydrocarbons but they’re sourced in a different way. While fossil fuels will eventually run out (which doesn’t make it a sustainable source), alternative paths exist to produce the same jet fuel in different ways. A fuel that can be used with existing engines without any modifications is considered a drop-in fuel. However, we shouldn’t be too quick to think that sustainable always equals better. For example, if biomass crops are used as a source for fuel but the biomass is generated on land that used to be tropical rainforest that was cut down to make the biomass, then that cannot really be seen as a win for the environment.
Some will argue that the use of purpose-grown biomass can make SAF a near carbon-neutral fuel. Afterall, any carbon dioxide (CO2) emissions that may result from burning the fuel produced from this biomass was initially captured from the atmosphere during the growing process of the plants or trees used as biofuel, so this could be seen as a net-neutral process. And the plants or trees removed can be re-planted to continue to capture carbon. All this makes SAF similar to a carbon offsetting scheme. However, energy is required in production and transportation, which isn’t likely to be without CO2 emissions. Also, there are other emissions that contribute to global warming beyond just CO2 which are not eliminated by this process.
Not all options offer better overall performance as far as net greenhouse gas emissions are concerned when compared to conventional jet fuel either as reported by the International Council on Clean Transporation (ICCT) in 2021. For example, using palm oil as a source is actually worse. Further, a lot of the waste products used to produce fuel via the HEFA-SPK process also have other potential uses, and by diverting this waste to fuel production, those other use cases may now need to be addressed via other means that may cause additional greenhouse gas emissions.
There are many ways to produce these types of fuels. First, there are various different sources that can be used to produce the fuel. The source is often called the feedstock. Different types of feedstock require different processes to be converted into a useful product. The end result is then blended with conventional fuel. Currently, standards organization ASTM International defines seven different certified paths for producing Sustainable Aviation Fuel. Today, the various different paths all need to be mixed with conventional jet fuel to come up with an acceptable blend for drop-in use. This is because the various processes used to create SAFs today don’t yield a product entirely identical to the composition of jet fuel and thus cannot be used yet at 100% without requiring modifications to the aircraft. That does not mean that using 100% SAF is not possible, either by changing aircraft to allow for 100% SAF or to modify the process to create a fuel that can be drop-in. The existing certified processes and their mixture limits are described below.
Besides potential gains coming from how the fuel is produced, SAF also has the potential to burn cleaner. For example, measurements performed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt) have demonstrated that an A350 operated on 100% SAF fuel emitted fewer particulates that lead to pollution and contrail formation (which is also a contributing factor to climate change).
FT-SPK
The first type of fuel is Fischer-Tropsch Hydroprocessed Synthesized Paraffinic Kerosene (FT-SPK). While it could also be done using coal or natural gas, for the process to be considered sustainable biomass is used as the feedstock. Biomass can be organic household, industrial or agricultural waste products but could also be crops or trees grown with the intention of them being used as biomass.
The biomass is then converted into a gas. Different processes for this exist. Pyrolysis is a process where carbon-rich materials (like plant waste) degrade at high temperatures without oxygen. Since there is no oxygen, the material cannot catch fire. Instead, it breaks apart into components like carbon monoxide (CO), methane (CH4), cardon dioxide (CO2), hydrogen (H2) and nitrogen (N2). Together they form what is called syngas. This syngas can also be produced by breaking down CO2 from the atmosphere or processes that produce CO2 as waste and combining this with H2, however this process does not yet work at an industrial scale.
The syngas contains the right components (CO and H2) to produce various types of fuel products using the Fischer-Tropsch process, including aviation fuel. The primary difference between the various fuels is the number of carbon atoms in the hydrocarbon chain. Current standards allow for FT-SPK fuels to be blended with regular jet fuel for up to 50% of the mixture.
FT-SPK/A
The FT-SPK/A process is a variation of the FT-SPK process where the end product also contains aromatics (hence the /A). Normally, jet fuel already contains such aromatics and their presence is actually detrimental as far as emissions go. However, they also have a beneficial effect which is that they cause swelling used in the seals of aircraft fuel systems. Without the presence of aromatics, the opposite can occur which is that the seal is slowly eroded away by the fuel, thus causing unacceptable leakage. In this variant of the process the presence of aromatics is assured to avoid this problem. Though this is a good step towards making this process result in a drop-in fuel, as of right now the end product is still limited to a maximum of 50% of the fuel mix.
HEFA-SPK
A different approved path is Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosine (HEFA-SPK). This path uses used cooking oil, animal fats and algae and vegetable oils as its feedstock. These substances are either waste (cooking oil and animal fats) or purpose grown (vegetable oils).
Different paths are taken to convert these oils and fats to various types of fuels, including diesel and jet fuel. Various different processes are used to achieve this that all fall under the broader term hydroprocessing, which requires hydrogen. In a very non-technical nutshell, the oils or fats need to be cleaned up to remove any unwanted substances, then broken down into smaller hydrocarbon molecules that have the right number of carbon atoms for various types of fuel. The molecules with the right size for jet fuel can then be distilled out of the mixture. This can be blended with regular jet fuel for up to 50% of the mixture.
Today, the HEFA-SPK approach is the most common and most economical approach to producing SAF. But even with this being the most economical approach, it still easily costs twice as much as conventional jet fuel, though the cost varies considerably depending on the method used.
ATJ-SPK
The Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) route doesn’t mean that cheap beer nobody likes is getting turned into jet fuel. Instead, the focus is on particular alcohol molecules, primarily isobutanol, n-butanol and ethanol (the most basic alcohol). There are various ways to produce such alcohols, from simply fermenting sugars like you would do to produce drinking alcohols, to more complex chemical processes that allow other substances to be converted to alcohol as well. Further processing allows for these alcohols to then be converted to hydrocarbons suitable for use as jet fuel.
Various producers have started to ramp up the production of jet fuel through this process in both the United States and Europe. With regards to emissions, the ATJ-SPK story is similar to FT-SPK and HEFA-SPK. The fuel produces less harmful emissions and depending on the feedstock used, net carbon emissions can be reduced but with similar caveats around how this is measured. Fuel produced via the ATJ-SPK process is also limited to a maximum of 50% of the total fuel mixture.
HFS-SIP
The Hydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP) process goes down a somewhat different path. In this process, sugars are fermented by microbes which are known to produce hydrocarbons that can be blended in with conventional jet fuel. This differs from the ATJ-SPK process where fermentation is also used but with that process the fermentation results in different types of alcohol as an intermediate product. Currently, HFS-SIP fuel is only allowed to make up 10% of the total fuel mixture and the technique is not used at an industrial scale.
CHJ
Catalytic Hydrothermolysis Jet (CHJ) fuel is another process that takes oils as a feedstock, for example soybean or carinata oil or waste oil products from food production. It aims to improve on the HEFA-SPK process by requiring less hydrogen and creating a product that is more similar to existing jet fuel composition making it a potential drop-in fuel. However, as of the time of writing it cannot yet be used as a drop-in fuel and is limited to 50% of the fuel mixture.
HC-HEFA
Hydroprocessed Esters and Fatty Acids Synthetic with Hydrocarbons (HC-HEFA) is a variation of the HEFA-SPK process. The difference is that microalgae are used, at this time specifically Botryococcus braunii, which produce high amounts of hydrocarbons, most of which is deposited outside of the organism and thus relatively easily retrieved. These hydrocarbons can then be processed further. Currently, the limit for this process is 10% of the total fuel mixture.
FT / MTG eFuel
Since CO2 is generally seen as the primary greenhouse gas, and jet fuel needs the carbon atoms that CO2 happens to have, would there be some way to take this CO2 directly from the atmosphere (rather than through biomass) and turn it into fuel that way? This is basically the idea behind eFuel, which is mostly a nice marketing term for this process (the proper term is Electrofuel). CO2 is taken from the atmosphere and combined with hydrogen can be made to form jet fuel. It’s also possible to capture CO2 waste output from industrial processes.
This can be achieved using Fischer-Tropsch (FT) process (as with the previously discussed FT-SPK process) and falls under the same certification, allowing for blending of up to 50%. It is also possible to first produce methanol which with further processing can also be converted to more usable fuel, known as the Methanol-to-Gasoline (MTG) process. While producing hydrogen from seawater and then producing the fuel with it both take energy, if this is done entirely using zero emission sources (eg. wind, water, solar or nuclear power) this process could be nearly net neutral as far as carbon emissions go, as any resulting carbon emissions were previously captured from the atmosphere first. eFuel is considered a Power-to-X type process. The amount of power required by this process is a challenge however as this will need to be produced by power sources that themselves do not contribute to climate change and it is already difficult enough to build enough climate friendly forms of power generation to supply our existing energy needs.
So far, no project is producing aviation fuel this way at an industrial scale. The Haru Oni pilot project in Chile however is aiming to produce methanol using this process and potential further processing to aviation fuel is specifically mentioned by project participant Siemens. The Haru Oni pilot project will use wind energy as a zero-emission power source for generating hydrogen and other processes, direct air capture of CO2 and the MTG process to produce fuel.
Arcadia eFuels from Denmark is planning the construction of a facility as well in Vordingborg which will use the FT process instead and has a more specific focus on aviation. They expect their first facility to start operations by 2024 and be able to produce about 55,000 metric tonnes of aviation fuel per year. This sounds like a lot but that means a year’s worth of production would be used up in a matter of hours by the European aviation industry. Many thousands of plants of this scale would be needed to adequate supply the global need.
Hydrogen as fuel
Besides using hydrogen in fuel cells or as part of various SAF or eFuel production processes, hydrogen by itself can also be a fuel. It can be burned and with some modifications more-or-less conventional engines can be made to run on hydrogen. This technology isn’t new either with one of the earliest jet engine concepts, the Heinkel HeS 1, actually running on hydrogen rather than conventional fuel. The first aircraft to actually fly on hydrogen was a Martin B-57 Canberra in 1957. This wasn’t because of environmental concerns but because it was believed that aircraft using hydrogen would be able to fly at higher altitudes than those using conventional jet fuel with the US aiming for an altitude as high as 30.5km or about 100.000ft. This made the technology interesting for the development of surveillance aircraft, for example the proposed but never completed Lockheed CL-400 Suntan spy plane.
The Soviet Union revived the idea and in 1988 first flew a modified Tupolev Tu-154, designated the Tu-155, with one engine running on hydrogen. This test aircraft was also used to test flying on natural gas. Again, environmental concerns were not the driving forces behind testing hydrogen at this time, rather, there was a concern that oil could become scarce and that alternative fuels would need to be considered. It used an experimental Kuznetsov NK-88 engine which had been in development since 1974.
As with the hydrogen fuel cell technology, here too storage of the hydrogen proved to be a challenge. For the Tu-155, the hydrogen was stored at extremely low temperature in a pressure tank placed in the rear of the cabin, using up space that would normally be available for seating passengers.
With the fall of the Soviet Union, the Tu-155 project also came to an end. However, the idea didn’t. Further research was conducted in partnership between companies from various different countries, including European aerospace giant Airbus, Tupolev from Russia, Honeywell from the United States and more. The low temperature required for storing sufficient hydrogen resulted in the nickname for this project being the Cryoplane, which was to be an Airbus A310 modified do demonstrate the technology. However, the project never reached this prototype stage.
But the interest in using hydrogen as a direct fuel remains. Airbus is considering as a fuel source as part of their ZEROe plans. The FlyZero study lead by the British Aerospace Technology Institute also focuses on hydrogen as a fuel. And the Flying V concept being developed by TU Delft in the Netherlands is also considering hydrogen as a fuel.
However, due to the complexity of storing hydrogen and the space this takes up, no designs proposed today would be equivalent in performance to conventional jet powered aircraft. Either passenger and / or cargo space needs to be compromised to store enough fuel to get a range similar to that of other aircraft of equal size, or range is compromised to seat more people and / or bring more cargo, or the aircraft is made bigger. However, there are obviously limits to how much bigger an aircraft can be considering the space available at airports.
Burning hydrogen does not result in any direct CO2 emissions. However, it does still result in the production of another type of greenhouse gas, nitrogen oxides (NOx). Further, it also produces more contrails than conventional jet aircraft which also contribute to the greenhouse effect. Research suggests contrails actually contribute more to the global warming effect than CO2 so this is certainly a cause for concern.5 And compared to hydrogen fuel cells, when burning hydrogen there is less opportunity to control when and how the water vapor is released. It should be noted though that contrails produced by burning hydrogen would likely differ from those produced by conventional engines due to a lack of particulates in the exhaust stream, so the impact may not be equal.
Hydrogen isn’t necessarily doomed by this. Research in Japan has shown that about 80% of the impact from contrails is produced by only about 2% of flights that happen to cross areas that are most conducive to it. Much of the problem could potentially be solved by simply avoiding such areas.6
Liquid ammonia
A final and less frequently discussed option is to burn liquid ammonia (NH3). There already is a huge industry that produces ammonia, primarily for agricultural use. However, when liquified it can also serve as a fuel for conventional combustion engines. Burning liquid ammonia produces primarily water (H2O) and nitrogen (N2), and some nitrogen oxides (NOx). While it also needs to be stored under pressure and low temperature like hydrogen, it does not require nearly as much pressure to achieve a liquified state and the temperature only needs to be about -33 degrees Celsius. This means that while the storage isn’t quite as straightforward as that of conventional fuel, it is less complex than storing liquid hydrogen.
However, ammonia is energy intensive to produce and generates CO2 as a by-product. For the CO2 emissions alone, estimates range from it contributing from 1% up to 3% of all CO2 emissions worldwide, which is a huge number either way for one single industrial process. So, ensuring ammonia is produced without outputting a substantial amount of greenhouse gasses of its own would be key to ensure this is a worthwhile path to follow. And while it has no CO2 emissions when burned, it does still produce contrails (due to the water vapor emissions) and some amount of NOx meaning it does still produce greenhouse gas emissions, though exactly how much in comparison to conventional jet fuels is not known as at this point as no aircraft has flown using liquid ammonia as fuel. To prove the concept, Aviation H2, located in Australia, is working on making modifications to a Dassault Falcon 50 to allow one of its three engines to run on liquid ammonia.
Conclusion
So, what conclusions can we draw from all this? First of all, that no solution is going to make a significant impact to total greenhouse emissions from aviation in the next few years as pretty much all the technology and / or processes here are still in their relative infancy. Further, no option being developed today outperforms conventional fuels from both a performance and cost perspective. That being said, options to seriously reduce greenhouse gas emissions in aviation do exist and further development should certainly be encouraged. We can also see that different solutions may be good options for different scenarios. Let’s recap:
| Solution | Use case | Greenhouse Gas Emissions (most ideal implementation) | Potential impact | Availability |
| Battery power | Short range air taxis and other light aircraft | True zero | Minimal as the bulk of emissions are not in the segment of aviation this can serve. | Today at limited scale |
| Hybrid-electric | Smaller regional aircraft needing more range than battery power can provide | Reduced (up to 50% for total emissions)7 | Moderate if it can be used by regional airliners but the impact remains limited as emissions are not fully eliminated. | Only used in technology demonstrators |
| Hydrogen fuel cell | Regional aircraft | True zero | Could be significant if this can be adopted by narrowbody aircraft which represent the bulk of aviation emissions.8 There is a risk of contrail formation if not managed properly. | Only used in technology demonstrators |
| SAF | Jet fuel powered aircraft | Reduced (up to 80% for CO2)9 | While not fully eliminating greenhouse gas emissions it could still have moderate impact since it can impact the most important segments of aviation. However, the production method of SAF will determine how significant these savings will be and some processes could actually be worse. | Today at limited scale |
| eFuel | Jet fuel powered aircraft | Reduced (near net-zero for CO2) | Offers an even lower net-carbon emission path than other SAF however production volume in the near term will remain limited and is constrained by needing a climate friendly power supply. Still produces other greenhouse emissions. | Not yet available (est. 2024) |
| Hydrogen as fuel | Larger aircraft | Zero CO2 | Could be very significant as this can be implemented in the broadest range of commercial aircraft. However, increased contrail formation may result which could make this counterproductive if not properly managed. | Only used in technology demonstrators |
| Liquid ammonia | Jet fuel powered aircraft | Zero CO2 | Similar to using hydrogen as fuel, however with somewhat reduced transportation and storage complexity. Energy intensive to produce however, which means that the industry must move away from the conventional ways of producing ammonia to make a real impact. | Not yet used |
In a nutshell, fully electric aircraft have the potential to be truly without any emissions if the energy produced also comes from an emission free source. However, they lack the range needed for most applications. Further, many applications that are purely electric fall in the air taxi category, and these flights would mostly replace cars. And even with tens of thousands of active electric air taxis, on a global scale that impact would still be negligible. Hybrid systems may offer more range, but don’t eliminate emissions entirely, just reduce them. Hydrogen fuel cells also increase range and do offer a potential path for zero emissions, though right now hydrogen fuel cell technology in aviation is in an early stage and industrial scale “green” production of hydrogen is also not in place. SAF may offer some reduction in net carbon emissions and while not the most ideal solution, it probably offers the fastest path to meaningful reduction in emissions. However, if not carefully regulated, SAF could also do more harm than good. eFuel has the potential to reduce net carbon emissions even further, however, scaling this up to a meaningful level would be challenging. Finally, burning hydrogen offers an alternative to conventional fuels but even if sufficient hydrogen can be produced, airlines would have to be willing to accept the performance compromises likely to come with hydrogen technology, at least as it stands today.
It should also be noted that CO2 is not the only greenhouse emission produced, and NOx emissions and contrails also contribute to global warming, so a zero CO2 solution doesn’t fully solve the problem. Infact, CO2 may only represent a third or so of all aviation emissions that result in a warming effect and burning hydrogen in particular has the specific challenge of creating more contrails which could increase rather than reduce the global warming effect. So, it’s fair to say that all paths have their challenges. But that doesn’t mean they’re not worthwhile to pursue. The problem won’t be solved overnight, but we should still support technology that moves us in the right direction.
Footnotes and references
1 Note that at the time of writing (March 2022) the war in the Ukraine and resulting sanctions on Russia have increased the cost of both fuel and energy and it is not clear how the cost of either will develop over the coming months, hence using a snapshot for 2020.
2 While the Pipistrel Velis and Virus are similar, the performance of both aircraft isn’t entirely identical so it cannot be said this is a true apples-to-apples comparison.
3 The article referenced does not specifically talk about batteries for aviation use which may cost more.
4 These are unvalidated, pre-production claims from the respective manufacturers, with no details around the operational conditions under which that range might be achieved.
5 The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, 2020.
6 Mitigating the Climate Forcing of Aircraft Contrails by Small-Scale Diversions and Technology Adoption, 2020.
7 Report by Roland Berger, 2020.
8 Report by International Council on Clean Transportation, 2018.
9 IATA fact sheet, 2018.