This week’s news saw several celebrities dragged through the media spotlight for their use of private jets and their effect on climate change. Several were accused of using the jet to take a flight for a matter of minutes.
It’s no secret that the global aviation industry is coming under heavy scrutiny for its impact on the environment, even though its contribution to overall global carbon emissions is between 2-5%.
That said, if patterns were to continue unchanged, this could increase to 10% or more, depending on how successful other industries are at decarbonising.
As a result, the industry is investing heavily to reduce those emissions by moving away from burning fossil-fuel-based jet fuel and changing to more sustainable methods of propulsion.
Presently there are three big players in the world of sustainable propulsion — Sustainable Aviation Fuel (SAF), electricity, and hydrogen.
Sustainable Aviation Fuel
The most prominent alternative fuel currently available is sustainable aviation fuel or SAF. It is a blanket term used to cover a variety of non-fossil fuel types which are being developed to move aviation into a greener and more environmentally friendly future.
How does it work?
A key part of developing SAF is ensuring that the chemical and physical properties are almost identical to current aviation fuel. This means that they can be mixed into conventional aviation fuel, can use the same infrastructure and most importantly — do not require the adaptation of aircraft or their engines. These are known as “drop-in” fuels as they can be easily incorporated into existing airport fueling systems.
According to IATA, SAFs are being produced and used every day on commercial flights. In December 2021, United Airlines flew the first passenger flight to use SAf to 100% power one of the Boeing 737 Max 8’s two engines.
On the ground, airports are also developing their infrastructure to enable the use of SAFs. Last year, Heathrow Airport tested its ability to add SAF to the fuel supply. Even though the test was only enough to supply SAF for five to 10 short-haul flights, it proved that “drop-in” fuels like this can work well on a larger scale.
Compared to fossil fuels, the use of SAFs can result in a huge reduction in carbon emissions across the fuel’s life cycle. In the case of biomass fuels, those created from plants, the carbon dioxide created from burning the fuel is roughly the same that is absorbed by the plants grown to make the fuel.
There is almost a net-zero creation of carbon dioxide with this type of fuel and an 80% reduction in carbon emissions compared to current fuel when elements such as transport and fuel refining are taken into account. In addition to this, SAFs contain fewer other impurities such as sulphur, reducing the emissions of sulphur dioxide.
For SAF produced from municipal waste, the benefits come from using matter that would ordinarily be left to decompose in landfill sites, creating environmentally damaging gases such as methane.
The use of SAF would also reduce the environmental impact of the production of aviation fuels. In its current format, the fuel must be originally sourced from one of few oil-producing countries. Before it can be converted to aviation fuel, it must be transported by ship, a process that creates huge carbon emissions in itself.
If a country is able to grow its own biomass to create SAF, it’s no longer reliant on importing oil to create aviation fuel. Depending on the local environment, a variety of SAF feedstocks can be grown around the world, wherever the aviation industry needs it.
For now, SAF remains pricey — around two to five times more expensive than conventional aviation fuel. With the costs so high, no airline will be able to afford to fly exclusively on SAF.
Part of this problem is scaling up production. At the moment, SAFs contribute just 0.1% of global fuel and even optimistic estimates only see this rising to 8% by 2035. The issue comes from a lack of funding to ramp up the production. Like with many new projects, the costs only start to reduce once large-scale production increases.
In addition, not all airlines believe that SAF is the way forward. According to EasyJet CEO Johan Lundgren, “SAFs and, in particular, power-to-liquid (PtL) will play a role for long-haul, but it is definitely not something we as a short-haul operator would look at as part of our end game [of zero-emissions flying across Europe] at all.”
He suggested that short-haul operators “better go for electric or hydrogen solutions, or a combination of those two,” because he was sure that aircraft similar in size to today’s short-haul aircraft using those fuel types will soon become available.
If many airlines take this approach to SAF, agencies may find it more difficult to raise the funding needed to ramp up SAF production and reduce the cost to airlines.
Long seen as the holy grail of sustainable power for aircraft, if many of us are now driving 100% electrically powered cars, why are we not flying in 100% electrically powered aircraft? Well, the truth is that we already are… but not necessarily in the way in which you’d imagine.
According to Roland Berger, there are currently around 215 electric aircraft in development, each one aiming to be the breakthrough aircraft that provides airlines and small aircraft operators with a safe and viable alternative to many of the current-day aircraft.
Indeed, United Airlines has agreed to buy 100 ES-19 electric aircraft from Swedish start-up, Heart Aerospace.
How does it work?
Like with any electrical propulsion system, there has to be a source of power and a motor which can generate the torque needed to turn the propellor. In electric aircraft, the power comes from a pre=charged battery and an electrically driven motor that is used to turn the propeller, generating thrust.
Within the electric motor are a number of coils of wire which are energised at different times with electricity from the battery. The electricity moving through these coils creates a magnetic force which interacts with magnets on rotating stator discs. As this energising occurs thousands of times a second, the magnetic interaction causes the stator discs to rotate. The more electrical power that is sent to the battery, the greater the speed of the stators.
It’s the spinning of these discs that creates the power to turn the propellor. The more electrical current that is fed into the motor, the faster the propellor spins.
The beauty of electrically powered motors is that there produce zero emissions from the aircraft. Once the battery is charged, like in an electric car, you can disappear off into the distance and not have to worry about emitting CO2 and other greenhouse gases.
They are also extremely efficient in generating thrust. On a conventional jet engine, when we want to increase the thrust an engine is providing, we move the thrust lever in the flight deck forwards. This in turn delivers more fuel to the combustion chamber of the engine, increasing the temperature of the exhaust which creates higher thrust.
The problem with this is that there is often quite a lag between the thrust lever moving forwards and the thrust being delivered out the back of the engine, particularly when the engine is operating at low power. If you think of a normal take-off on a conventional jet aircraft, there is often a few seconds between hearing the engine’s power-up and then feeling the acceleration. The reality is even longer as you won’t be aware of the time between the pilots moving the thrust levers and hearing the engines power up.
However, with electric motors, the power is almost instantaneous. Anyone who has driven an electric car will know the speed at which the power is delivered from putting their foot on the accelerator. It’s the same with electric motors in aircraft. Simply sending more electrical power to the motor provides an almost instant increase in thrust.
The major problem with electrically powered aircraft is that their range is limited by the battery capacity. Once airborne, there is nowhere to pull over to top up the charge. The only way to increase the battery capacity is to have a bigger battery but a bigger battery means greater weight and now we start to enter the age-old conundrum of aviation.
The heavier an aircraft is, the more lift is required to fly. The more lift that is required to fly, the more engine power is needed. The more engine power that is needed, the bigger the battery needs to be and we end up in an ever-worsening cycle. So, for now, electrically powered aircraft are really only viable for short flights, up to around 500 miles.
In addition, it’s all well and good having a motor that creates no emissions when it runs, but we need to think about the entire cycle of the flight. An electrically powered aircraft would keep the air cleaner around airports and reduce emissions at high altitudes but we need to consider how that electricity was generated in the first place.
If it was generated by a dirty coal-burning power station, there could actually be a net negative impact on the environment and climate change. We would simply be passing the CO2 problem onto another area without actually making a positive impact on a global scale.
Probably the most viable and exciting long-term option for aircraft propulsion is hydrogen. In order to capture the gas and package it for commercial use, electricity is used to split molecules of water into their component parts – oxygen and hydrogen. The oxygen is released into the atmosphere and the hydrogen is stored for use to power hydrogen propulsion units.
How does it work?
Hydrogen-powered aircraft work in two ways – Fuel cells that create electricity and direct combustion, very much like how aircraft engines work today.
Fuel cells work by reversing the process that created the hydrogen in the first place. Hydrogen enters a tank on one side of the fuel cell and oxygen from the atmosphere enters a tank on the other side of the cell, with fluid in between. Using a platinum catalyst, the cell takes the hydrogen and splits it down into its component parts – a proton and an electron.
The protons move across the fluid towards the oxygen, but due to their negative charge, the electrons are unable to do so. Instead, they move by a wire to meet up with the protons on the other side where they react with the oxygen to create water.
You may remember from your high school physics that moving electrons are also known as something else – electricity. This is then used to drive the motor.
Hydrogen-powered engines are very similar to engines of today, instead burning hydrogen in the combustion chamber instead of traditional aviation fuels.
As part of a fuel cell system, there are a few benefits of using hydrogen. Firstly, the only byproduct of the reaction is harmless water vapour. This can be vented to the atmosphere without any of the other harmful gases that are emitted from burning fossil fuels.
Secondly, the purpose of the hydrogen fuel cell is to create electricity to drive the motor. As a result, you get all the benefits of electric propulsion (immediate engine power, no harmful emissions etc.) without the negative of an excessively heavy battery.
Famous for being the lightest gas, hydrogen is 11 times lighter than the air that we breathe. As a result, 1kg of hydrogen gas fuel contains nearly 3x the amount of energy of traditional fuel.
This provides an incredible weight saving for aircraft, which in turn, further reduces the amount of fuel needed for a flight.
The major problem with hydrogen fuel is its availability. At the moment, there are not nearly enough hydrogen production facilities globally to provide a significant supply of fuel to the aviation sector. The only alternative is to transport it from the site of production to where it is needed. The problem with this is that, depending on that mode of transport, you may be creating more carbon emissions just to transport the fuel than you would save by using it on the aircraft.
How hydrogen fuel is made is also a major factor. As mentioned before, the fuel is created by splitting water molecules into hydrogen and oxygen using electricity. If the electricity is not generated in a sustainable way, once again the upstream emissions from creating the may outweigh the benefits of using it on the aircraft.
Whilst significant progress has been made in the drive to reduce carbon emissions from aviation, there is a considerable way to go until we reach the point of zero-emissions flights, particularly on long-haul.
Sustainable Aviation Fuel is our most readily available source of more emissions-friendly fuel, but due to issues in the supply chain, it is far from perfect. Electric aircraft are already in the sky but due to their range being limited by battery weights, their scope is limited. Hydrogen does seem to be the overall best solution but until supply issues can be sorted, it is not going to be the silver bullet that we’re all looking for.