There’s more to life than kerosene: between electricity and sustainable fuel, there are multiple energy sources to power an aircraft. But not everything is as efficient or as feasible in the near future.

One day there will be no more oil, and in any case, environmental issues are driving us to find other sources of energy, even if oil resources were infinite.

Without claiming to be exhaustive or to have in-depth technical expertise in the field, let’s try to give an overview of the possible options and make the subject more accessible to a wider public.

In this article:

• What makes today’s airplanes fly? Kerosene and gasoline
• Biofuels or sustainable fuels (SAF)
• E-fuel or synthetic fuel
• Hydrogen
• Electricity
• Plasma
• What’s the best option for the future of commercial aviation?
• Bottom line

What makes today’s airplanes fly? Kerosene and gasoline

Until now, you could put two types of fuel on a plane: kerosene and gasoline.

We’ll quickly skip the subject of gasoline, or Avgas for aviation gasoline: it only concerns small aircraft with piston engines, and has no weight in commercial aviation.

Just one point: even if the words sound alike, gasoline is not gasoil but a variety of petrol.

So until today, aircraft have used kerosene, also known as jetfuel . It is derived from the refining of crude oil, and its main advantage is its high energy content, which means greater autonomy for the same payload. Another characteristic is its very low freezing point (-47°), due to the temperatures encountered at altitude (-65°).

Its main drawback, apart from the fact that oil resources are not unlimited, is its CO2 emissions.

Biofuels or sustainable fuels (SAF)

These are alternatives to kerosene produced from biomass, which can be incorporated into fossil kerosene without having to modify engines, infrastructure, logistics, etc.

It’s also known as Sustainable Aviation Fuel (SAF).

Biokerosene can come from a variety of sources: plants, algae, flax, used oils, agricultural waste and, more often, a mixture of these components.

On the other hand, its environmental efficiency varies greatly depending on what is used to produce it. According to IATA, a biofuel made from household waste produces 94% less CO2 than fossil kerosene. However, if you use palm oil, the production of which has a high environmental impact, you increase CO2 production by 11%. If jatropha (a plant) is used, the gain is 75%. On average, the reduction in emissions is estimated at 80%.

But a flight using 100% SAF is not for tomorrow. While it is technically possible today to fly an aircraft on a 50/50 blend of SAF and fossil kerosene (Airbus has even flown an A380 on 100% biofuel in 2022), this is not realistic in commercial operation, for one main reason: production costs four times higher than with conventional fuel.

A price that explains why there’s little demand, a low demand that explains why there’s little production, and therefore high prices. The snake bites its own tail. Hence the willingness of many governments around the world to support the development of the SAF sector, and it is likely that the profitability of the transition to biofuels for airlines will depend on the willingness of local governments to subsidize their development, at least initially.

In any case, this transition will happen, hence the urgent need to make it economically bearable. Today, European regulations require 1% SAF, 2% in 2025 and a target of 70% in 2050, with a stepwise increase every 5 years. There’s nothing of the sort on the U.S. side, but there are objectives in terms of supporting production, perhaps with the idea that affordable supply will stimulate demand outside of any quota.

In any case, the planes are ready: according to Airbus, its planes are already certified to operate with 50% SAF, and will be certified for 100% SAF by the end of the decade.

E-fuel or synthetic fuel

According to France’s Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA), e-fuel is manufactured after “water electrolysis to produce low-carbon hydrogen, which is then combined with CO2 (…) to produce a fuel”.

This CO2 could be collected from the industries that generate the most, thus helping to decarbonize them.

The main advantage of e-fuel is thatit is totally carbon-neutral, but its production requires a technology that is still in its infancy and is much more expensive than biofuels and is very energy-intensive, even though experiments are under way to produce it using only air and sunlight.

While the use of this zero-carbon fuel may well be possible in the future, we’ll be flying on cooking oil for a very long time to come…

Hydrogen

Hydrogen is another option. It can be used in two ways:

– A fuel cell that powers an electric engine. Only suitable for small aircraft.

– A hydrogen-powered combustion engine, the only viable option for large aircraft

And hydrogen has almost everything going for it: it emits only water and no CO2, and is three times more fuel-efficient than kerosene at the same weight.

But its storage is problematic: much lighter than air, it has to be pressurized to be stored in liquefied form at -253°, and even then it takes up four times more space than kerosene.

And while we know how to produce hydrogen from fossil fuels (grey hydrogen), green hydrogen is still a field not yet mastered.

Once again, this is a technology to be developed and a sector to be developed from virtually scratch.

It may be a technology of the future, but a very distant one.

Electricity

The electric airplane is already a reality, or close to it, with the first commercial flights scheduled for the end of this decade. But if this is the solution most familiar to the general public, it’s by no means certain that it will be the most widely adopted, and for good reason.

The problem of the electric motor in an airplane is the same as in a car, but on a larger scale: given its weight, an airplane needs a lot of energy, which is stored in batteries, and these batteries are heavy and cumbersome.

Today, the electric aircraft suffers from two limitations: it can only be used on small aircraft, and its range is limited (400km by 2030). So it can’t be a global solution, at least not in the medium term and given the current state of technology.

But hybrid solutions do exist:

– hybrid engines, like those used in cars, to extend range.

– hybrid approaches using a liquid fuel (kerosene, SAF, e-fuel, etc.) for the engines and batteries to power all or part of the aircraft’s non-engine functions in flight.

Plasma

We talked about it in 2020: the plasma reactor could, in the future, power aircraft. At the time, researchers had succeeded in operating such a reactor in the laboratory, using only ambient air and electricity, producing thrust equivalent to that of kerosene engines and with zero CO2 emissions.

Plasma is the so-called 4th state of matter, and is found in the sun, lightning and neon lamps. It is obtained by heating ambient air to very high temperatures: over 20,000°, compared with 1,700° for a normal engine. This heat is transformed into energy, enough to propel an airplane without emitting CO2.

At the time, this technology, already used in space to move satellites, was still in the early stages of experimentation for operation in the earth’s atmosphere.

And since then? Numerous businesses are working on this subject, with roughly the same timeframe: a prototype in 2030.

What’s the best option for the future of commercial aviation?

Each of the options presented above has its advantages and disadvantages, and the state of the art in technology means that while some are very real and feasible in the medium to short term (or even mandatory for SAF), others are only at the research stage, and there’s no telling if or when they’ll one day become exploitable on a large scale by commercial aviation.

Even if all this works out one day, the question is not to choose today what will replace fossil fuels, but rather to define a realistic decarbonization trajectory.

Firstly, by using what we know works to reduce or even eliminate CO2 emissions in a known and controlled technological context. Secondly, by totally changing the rules of the game, engine types and even the way aircraft are designed (shapes, engines), which will have a gigantic impact on the entire industry, which will have to reinvent itself.

First play by the rules of the current game, then change the rules of the game when you have the means to do so.

The first stage is well known: first FAS, then perhaps e-fuel.

The second phase may be hydrogen, or plasma, or neither and a technology that is as yet unknown, it’s too early to say.

Bottom line

There are many alternatives to fossil fuels for powering aircraft, but not all are at the same stage of maturity, or impose constraints that make them unrealistic on a large scale.

Two things are certain. The first is that we already have the means to cut emissions by almost 80% over the next 20 years, but we’ll still be using at least some fossil kerosene until then. Less and less, but still a little.

Then there are highly promising breakthrough technologies beyond this horizon, but no one knows if or when they will be ready for commercial aviation.

And all this on condition that the public authorities give the necessary boost to the development of these new sectors.

Image : electric aircraft by Surasak_Photo via Shutterstock