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.)
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.
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!
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.
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.