“Back in my day airplanes used to be a lot faster… plus we were allowed to smoke”

This is a quote from my Dad, who by the way does not smoke, and never has. He likes to complain about the good ole days, and although it can get a little annoying, he reminded me of an interesting conversation with a friend. It’s true that flight time has increased, in fact, airplanes are around 80km/h slower than the average speed in the 60s. A friend of mine, who I will call Barnie (not his name), told me this during his second year of studying Aerospace at TU Delft. He was talking about one of his projects that required him to limit the GHG emissions under a certain threshold. Barnie’s approach was the minimization of fuel consumption, which meant significantly lowering the speed. This is why nowadays planes fly at a slower speed, because its more fuel efficient. Less speed equals less drag so the engines don’t have to work as hard. The main reason airlines do this is because it saves them thousands in fuel costs, resulting in cheaper tickets. An important added benefit is that less fuel consumption also means less GHG emissions.

I mention this story because, in all honesty, I was quite surprised Barnie’s university undertook the daunting task of incorporating emission parameters in their bachelor program projects. Barnie says that TU Delft is a lot more focused on sustainability than some would expect, and that sustainability is an integral consideration taught early in the bachelor program. After all, Aviation makes up 3% of all global GHG emissions, and is widely regarded as one of the hardest industry to decarbonize.

“If you study Aerospace, you begin to learn that it’s all about weight, weight, and weight” – Barnie

The reason why it’s so hard to decarbonize aviation is because of weight. Believe it or not, planes are heavy, and unlike cars, planes do not have the luxury of simply using batteries. As you can see in Figure 1, batteries are not yet powerful enough in relation to their mass and volume to power a heavy aircraft. That said, smaller electrical airplanes, like Alice, do exist, although their air-time and distance covered is extremely small. For now, batteries are simply not fuel-efficient enough, and will likely not be technologically developed enough to be commercially viable in more than a decade. sources: _{1, 2, 3}

Figure 1 does more than highlight the inadequacy of batteries, however. Liquefied Hydrogen has immense potential because it’s energy-per-unit mass being three times higher than traditional jet fuel and, as you may already know, it emits no CO2 emissions. So why aren’t we using hydrogen if it can so efficiently power large airplanes over long distances? Although the foundational technology is there, hydrogen has a very low volumetric energy density, making it much bulkier than jet fuel. Because hydrogen storage needs more space, aircraft structures need to be changed significantly. This means revamping the entire aviation industry, which is both costly, but also faces many political barriers. That said, the potential is there and many leading aviation companies, like Airbus, are invested in the opportunity. Still, the implementation of hydrogen in aviation is still in the R&D phase. Like electrification, it is widely considered a long–term solution to reach net-zero.

Can we decarbonize in the short-term?

The short answer is: to some extent. Short-term solutions are ones that reduce emissions by optimizing the efficiency of, among other things, aerodynamics, fuel-efficiency, and engines. For the sake of not writing an entire book, I will name two personally interesting methods that have been especially popular in Barnie’s bachelor program. The first is the Flying V, a radical change in airplane structure that reduces emissions by 20%. Patented by TU Delft, it has been tested (at a model scale) and proven to be more weight-, and therefore fuel-, efficient. As you see in Figure 2, the seats are located inside the wings, and there is no need for a tail because of the aerodynamic center of mass between the wings. According to Barnie, his university won’t shut up about this.

The second method is Sustainable Aviation Fuel (SAF), which has gained increasing popularity for its high potential in reducing emissions and is already commercially used today. Unlike hydrogen or electrification, it does not require a structural revamp. It’s still usually around 50% jet fuel, but combined with other forms of fuel such as biomass and Hydrotreated Ester and Fatty Acids (HEFA). Additionally, techniques that use recycled carbon from Direct Air Carbon Capture (DACC) technology are also in place today. That said, it is important to mention that these fuels, like HEFA and DACC contain carbon and other GHGs, meaning the amount of flight-emissions roughly stays the same. The reduction in emissions comes from the fact that the fuel is largely recycled and repurposed, as opposed to more jet-fuel being extracted and processed.

Am I too Techno-Optimist?

Both Barnie and I understand the aviation industry from being far from net-zero. Full decarbonization will likely come from a combination of short- and long-term solutions. I personally think that these solutions make aviation, along with concrete perhaps, the hardest industry to decarbonize because of its dependency on breakthrough technologies. I believe it to be the biggest barrier to the industry, along with the eventual scaling-up implementation of these technologies. That said, perhaps my vision of decarbonization is too techno-reliant. I am curious to hear the opinions and criticism of my readers. Perhaps there are other non-technological barriers that deserve to be part of the discussion.