Aviation’s Path to Decarbonization

In early April, I had the opportunity to travel to Cambridge, MA to attend the 2022 MIT Energy Conference. While a 360-degree review of the world’s current energy situation was on display, one message resonated through most of the perspectives offered: Electrify everything possible and decarbonize what you cannot.

Certainly, there are several hard to abate industries, yet aviation stands out as one that is ripe for innovation and decarbonization. One of the keynote addresses was given by Glenn Llewellyn who is the Vice-President for Zero Emission Aircraft at Airbus. At present, air travel produces about 3% of global carbon dioxide emissions in addition to being one of the fastest growing sources of pollutants. Between the years of 1990 to 2017, sectors, on average, increased emissions by 60% whereas aviation emissions increased by 110%. As aviation returns to growth following the effects of the coronavirus pandemic, the industry will drive a growing share of emissions.

Llewellyn presented several mechanisms by which the aviation sector is looking to decarbonize over the coming decades. One of the most notable technologies is hydrogen propulsion. These aircrafts are considered hybrid-hydrogen as they are powered by hydrogen combustion through modified turbine engines utilizing liquid hydrogen as opposed to kerosene for combustion with oxygen. Additionally, hydrogen fuel cells can be utilized for electric systems to complement turbines resulting in a hybrid-propulsion system. These technologies are being explored in both turboprop and turbofan designs for the 50-200 seat market with ranges spanning up to 2,000 newton-meters.

However, there are several challenges that need to be overcome when looking at this hybrid propulsion system. Firstly, while hydrogen is an energy-dense fuel, it is roughly only 25% as energy dense as traditional aviation fuels like kerosene on a volumetric basis. Therefore, for every 1L of jet fuel that you are replacing, 4L of liquid hydrogen is necessary. Saying this, hydrogen is less mass dense and therefore lighter than kerosene; however, increased containment for fuel tanks and pressure regulating facilities will be necessary given the unique chemical state of liquid hydrogen. Therefore, a fully loaded tank of hydrogen fuel will weigh nearly the same as its traditional counterpart; but will take more space and will weigh more upon landing than kerosene given a greater amount of the weight is embodied in the tank and containment system itself.

Further challenges will include the sourcing of green hydrogen to ensure that there is a carbon reduction in the lifecycle of the fuel, as well as the introduction of new transportation and storage facilities for this new fuel type.

One potential alternative solution with lower barriers to adoption is the use of ‘drop-in’ sustainable aviation fuels (SAF). These fuels are made from biogenic feedstocks such as municipal waste, agricultural remnants, or even next-generation power-to-liquid processes which enable carbon recycling to produce SAF. Currently available SAF has the ability to reduce emissions by nearly 80% when compared to fossil fuels on a life-cycle basis. However, the supply remains limited and the cost remains high. In the EU, SAF represents just 0.05% of total aviation fuel consumption. Nevertheless, SAF will likely represent the largest piece of tomorrow’s decarbonizing aviation sector as several airlines note the fuel type as a crucial piece in their net-zero strategies.

Overall, aviation represents a sector that will continue to be a challenge to decarbonize. However, the scaling of sustainable aviation fuels represents a key piece of a future low-carbon aviation system along with more novel technologies like hydrogen combustion and hydrogen fuel cells.

William Deo

Undergraduate Seminar Fellow

William Deo is a graduate student studying mechanical engineering at the School of Engineering and Applied Sciences. Deo was previously a 2020 Undergraduate Student Fellow with the Kleinman Center.