In a normal year, the aviation industry burns 65 billion gallons of jet fuel producing 750 million tons of carbon dioxide. Although this carbon footprint is a quarter the size of the automotive industry, it is still quite substantial. Moreover, the automotive industry has taken large steps to reducing their footprint as the industry shifts to electric vehicles. However, the safety and weight constraints of aircraft make it difficult to implement similar changes. The Green New Deal goes so far as to state that large portions of the aviation industry needs to be displaced by high-speed rail service to stave off climate change.
Although it will be difficult for the aviation industry to cut their carbon footprint, the International Air Transport Association has committed to capping their carbon footprint at the 2020 level, while creating a substantial reduction by 2050. While these goals are ambitious, the aviation industry has rallied around sustainability and is currently developing the requisite technology for achieving these goals.
The key to sustainability is replacing petroleum-based jet fuel with a more sustainable alternative. The likely long-term candidate for replacing jet fuel is hydrogen. Hydrogen offers numerous benefits over traditional jet fuel, especially that the only byproduct of combustion is water. Additionally, a kilogram of hydrogen has three times the energy of a kilogram of jet fuel. With these benefits in mind, Airbus recently announced that it is developing a concept aircraft, termed the ZEROe airplane, which will be powered completely by hydrogen. This aircraft will have a prototype developed by 2035.
Unfortunately, significant challenges still exist for hydrogen. First, given the volatile nature of hydrogen, safety is a key issue. It must be stored such that it is as stable as jet fuel, which only combusts in the presence of flames and high pressures. In turn, this causes the hydrogen storage systems to be voluminous and heavy, with most storage tanks being ten times heavier than the hydrogen inside it. Moreover, hydrogen is currently produced from methane, resulting in a carbon footprint comparable to jet fuel. However, many countries, including the UK, Norway, Japan, and France, are building hydrogen production facilities that use renewable energy sources to electrolyze water to produce hydrogen. Perhaps the largest hurdle is that current aircrafts cannot readily use hydrogen. As such, hydrogen-powered aircraft would be phased in to replace conventional aircraft. This undertaking would span decades, given the 25,000 aircraft in the world coupled with their average 30-year lifespan.
With these challenges in mind, the International Air Transport Association is focusing on short term gains with the development of sustainable aviation fuels (SAF). Similar to biofuels used in the automotive sector, SAF is not oil-based. While biofuels are typically derived from corn or sugarcane, SAF is produced from used cooking oils, animal fat, and organic waste material. These materials allow SAF to not compete with agriculture for food production. Additionally, since SAF is derived from organic matter, much of the carbon dioxide released from combustion is offset by carbon dioxide absorbed by the organic material while alive. Several companies are working to mass produce SAF for the aviation industry, with BP and Shell each citing that their SAF can achieve an 80 percent smaller carbon footprint than traditional jet fuel.
Currently, commercial aircraft limit the amount of SAF in the fuel blend to 50 percent, with several airlines—including Japanese Airlines, Qantas, and United Airlines—flying with these blends. Recently, Rolls-Royce recently tested a new engine that can run on 100 percent SAF, helping pave the way to allowing engines to run completely on SAF. Meanwhile, Boeing committed to certifying that their jets can handle running completely on SAF by 2030.
Despite its benefits, SAF accounted for less than 1 percent of jet fuel used in 2020, with a key issue being cost. Currently, SAF is approximately five times the cost of traditional jet fuel. Given the financial issues in the aviation sector, this cost increase is difficult to absorb. One promising technique for driving down the cost of SAF is to produce it from algae. Although numerous technical challenges exist, the high crop yields of algae could allow it to be an economical source for producing SAF. Another option is termed solar jet fuel, which is produced by concentrating sunlight on carbon dioxide and water vapor to synthesize into a hydrocarbon, which can be refined into jet fuel. This technique was demonstrated in 2014 by the Zurich Institute of Technology and is expected to reach technical maturity by 2035.
Although the aviation industry is still recovering from the COVID-19 pandemic, it remains committed to decreasing its overall carbon footprint. Airlines are now adopting sustainable fuels that will potentially supplant jet fuel. Meanwhile, several longer-term initiatives, such as hydrogen powered planes, have the potential to revolutionize the industry.