Aviation: Part 2 #35
Solutions to sustainable aviation
It’s good to be back! I had covid for the past two weeks, and it turned out one of the worst flu that I remember having… Feeling so lucky that I had three vaccines!
Today, we are continuing with the deep dive series on aviation. In Part 1, we learned about the terrible climate impact that aviation is making on our planet. (If you haven’t yet read Part 1, I highly recommend giving it a read.)
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The solution set of sustainable aviation
There are three key ways to slash aviation’s climate impact:
Sustainable Aviation Fuels (SAFs)
In battery-electric airplanes, the energy required is stored in batteries instead of jet fuel.
Batteries are an amazing option for decarbonizing aviation. If the electricity is produced renewably, battery-electric airplanes can decrease the in-flight emissions by 100%!
The downside of batteries is that they weigh a lot and take a lot of space relative to the amount of energy they can store. In other words, batteries have low specific energy and low volumetric energy density.
Specific energy and volumetric energy density are key measures in aviation.
Firstly, airplanes have a restricted space to store the energy required for the flight. Jet fuel (kerosene) is currently stored conveniently in the airplane’s wings. Secondly, we want to build as light airplanes as possible. Every gram adds to the fuel consumption of the airplane.
Below you can a variety of energy storage methods or fuels mapped out based on their specific energy (here MJ/kg) and volumetric energy density (here MJ/L).
As you can see lithium-ion batteries are located at the bottom left corner meaning that they have both low specific energy and low volumetric energy density. What I’ve learned is that kerosene (and other hydrocarbons in general) are annoyingly great…
It is expected that battery-electric airplanes will decarbonize regional and short-term flights up to 500-1000km.
Pioneers of the battery-electric aviation
2. Sustainable Aviation Fuels (SAFs)
Sustainable Aviation Fuels (SAFs) have similar characteristics to the conventional jet fuel. The key difference is that they are made from alternative, sustainable feedstock material.
SAFs can be divided into two types of fuels: 1) Biofuels and 2) Synthetic fuels.
Biofuels are made from biomass. The biomass can be waste biomass like food and forestry waste or it can be grown and harvested.
Synthetic fuels are made by combining atmospheric CO2 and green hydrogen.
A clear advantage of SAFs is that they provide a drop-in solution. No major fuel infrastructure or jet engine changes need to be done, and we can start decreasing air travel’s impact right away.
We are in a climate crisis and should be acting accordingly right away.
The main disadvantage of SAFs is that their reduction of climate impact is limited to 30-60%.
With biofuels, it is crucial to pay attention to where the feedstock has been produced. There is a risk that the production of energy crops could negatively affect food prices and water scarcity if the land use is not properly managed.
Pioneers of SAFs
Hydrogen is one of the abundant elements in our universe and an energy carrier.
There are two ways to release the energy carried by hydrogen: via 1) Fuel cell and 2) Combustion.
A clear advantage of hydrogen is its high potential to reduce climate impact. Hydrogen combustion could decrease the in-flight emissions by 50-75%, and hydrogen fuel cell engines would decrease those by 75-90%.
Another advantage of hydrogen is its high specific energy. If you look back at Figure 1, you can see that all forms of hydrogen (gas, compressed, and liquid) are located in the bottom-right corner of the chart.
In fact, hydrogen can store more than 3x the amount of energy per mass unit!
A major limitation of hydrogen is its low volumetric energy density. Even liquified hydrogen requires 4x more storage space than jet fuel does. This limits the economic adoption of hydrogen for short-haul flights. We would need to develop a new airframe design (e.g. Blended Wing Body) so that the adoption of hydrogen to makes economic sense also on medium- and long-haul flights.
Hydrogen infrastructure needs to be built. Firstly, we need to ramp up the production of green hydrogen (=production via electrolysis). Secondly, the pipes and storage methods at airports need to be built as well. When building the infrastructure, hydrogen leakage across the value chain must be limited in order to avoid adverse climate impact.
Pioneers of hydrogen-powered aviation:
Below you can find two charts that summarize well the key information that we have covered in this deep dive.
There is no silver bullet to more sustainable aviation. We need to work hard on all of these solutions.
It is likely that battery-electric propulsion will decarbonize regional and short-haul flights up to 1000 km (600 miles).
Hydrogen offers the largest reduction of climate impact for medium- and long-haul flights, but ramping up the hydrogen infrastructure and building hydrogen-powered airplanes is going to take some years.
Luckily, sustainable aviation fuels are able to kick-start aviation’s transition already today.
If you are interested in chatting about sustainable aviation, I’d love to hear from you (firstname.lastname@example.org)!
Clean Skies for Tomorrow: Sustainable Aviation Fuels as a Pathway to Net-Zero Aviation by World Economic Forum and McKinsey
FlyZero reports by Aerospace Technology Institution
Hydrogen-powered aviation by Fuel Cells and Hydrogen 2 Joint Undertaking
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Fuel Cells and Hydrogen 2 Joint Undertaking (2020). Hydrogen-powered aviation: a fact-based study of hydrogen technology, economics, and climate impact by 2050. Source.
IATA. What is SAF? Source.
Lee et al. (2021). The contribution of global aviation to anthropogenic climate forcing for 200 to 2018. Atmospheric Environment, 244. Source
Mukhopadhaya, J. & Rutherford, D. (2022). Performance analysis of evolutionary hydrogen-powered aircraft. ICCT. Source.
Ocko, I. B. & Hamburg S. P. (2022). Climate consequences of hydrogen leakage. Preprint. Atmospheric Chemistry and Physics Discussions. Source.
Popp, J., Lakner, Z., Harangi-Rákos, M., and Fári, M. (2014). The effect of bioenergy expansion: Food, energy, and environment. Renewable and Sustainable Energy Reviews, 32, 559-578. Source.
van Renssen, S. (2020). The hydrogen solution? Nature Climate Change, 10, 799–801. Source.