Have you ever wondered why all jet planes leave white trails behind them at high altitudes?  It is called contrails made of carbon dioxide that leaves a trail in freezing temperatures [1]. Many people would find this fascinating, but in reality, these white trails show how much damage jet planes can cause to the environment. 

Almost 60 percent of airline passenger numbers dropped in 2020 to levels last seen in 2003 [2]. Lockdown protocols in countries prevented people from traveling for months, and air-travel industries suffered plunging revenue due to COVID-19 restrictions. With 49.6 percent of U.S. residents vaccinated as of August 1, 2021, E.U. leaders agreed to open European borders to foreign tourists [3]. With that loosening of restrictions comes the serious environmental consequence: unprecedented air pollution. 

Ironically, during the COVID-19 pandemic, air quality improved drastically. To stop the spread of the virus, countries set strict restrictions including quarantining and travel bans which brought carbon emission from aviation to fall by a record of 48% in 2020. Before the COVID-19 pandemic, experts expected aviation's carbon emissions to be 3 times as great in 2050. With people staying at home without traveling, they now expect the carbon emissions to be 2.3 times, which is a significant improvement from the previous estimates [4]. Limiting air travel does seem like a viable plan for air pollution, but maintaining this restrictive policy cannot be a sustainable solution. Instead, we need to find a way to reduce carbon emissions while people can travel safely and freely. 

One major approach to this effort was made on May 13 by Congress. It introduced a new bill, the Sustainable Fuel Act to increase the production of sustainable aviation fuel [5]. Meeting all the technical and safety requirements as fossil-based jet fuel, sustainable fuel is based on hydrocarbons derived from nature. Also, on May 20, three U.S. Representatives, Brad Schneider (D, IL), Dan Kildee (D, MI), and Julia Brownley (D, Calif), introduced a bill to “create a tax credit for the use of sustainable aviation fuel, which can cut emissions by up to 80% [6].” The efforts made by the U.S. Representatives have the potential to motivate the airline industry to use sustainable fuel for all commercial jets. 

With these policy supports, the airline industry’s biggest bet for its future sustainability is to move on from highly polluting kerosene to much cleaner biofuels. Made from cooking oil or waste vegetation, biofuels can cut carbon pollution from airplanes by up to 60%. Before the pandemic, biofuels only made up 0.01%, about 13 million gallons, of the aviation industry’s total fuel usage [7]. The EU is expected to set all carriers a target of 5% by 2030. In the U.S., the U.S. government and the airline industry globally have set a goal to reduce carbon dioxide emissions by “50% of 2005 levels by 2050” with the use of biofuels [8]. 

The ultimate goal for airliners is zero-carbon flight. Many airlines, such as KLM, use biofuels based on cooking oil. While cooking oil is cheap and easy to refine, the quantity is very limited to operate a large plane [9].  Instead of achieving a near-impossible task of using only cooking oil for fuel, United Airlines announced that it would use one-half conventional fuel and one-half fuel made of household trash, making planes hybrid. Like hybrid vehicles using two or more distinct types of power ‒ they use electricity during cruising or idling and diesel fuel to accelerate ‒ hybrid planes mix sustainable fuel with conventional fuel to utilize both at the same time. Although not completely clean, this method will reduce almost half of carbon emissions compared to using 100 percent conventional fuel. 

Another alternative aviation fuel is e-fuel that powers planes with electricity from batteries. How would e-fueled airplanes operate? In a propeller airplane, for example, a chemical reaction causes lithium to release electrons, causing lithium ions. The electrons then run from the anode to the cathode. Along the way, these electrons power the propeller. Once these electrons are used, they now help with the charging process. The electrons are replaced and returned to the anode to repeat the whole cycle [10]. Batteries in electric planes can charge themselves during the flight, which can give extra energy for the planes to operate. Electric motors use lithium ions to operate, so engineers can plug in a charger to charge planes instead of draining and replacing jet fuel every time. With no carbon emissions released into the atmosphere, e-fuel in the airline industry is the key to saving the environment. 

However, e-fuel has some drawbacks that could make airliners rethink their future in using it as the main power source: the sensitivity to extreme temperature and heavy weight of batteries. Most commercial jet planes fly between 33,000 and 42,000 feet. At that altitude, temperatures can range from anywhere between -40 to -70F. The concern is that batteries from e-fuel tend to drop in capacity in low temperatures. For example, batteries in electric cars can drop in capacity by about 20 percent in freezing weather and down to about 50 percent in temperatures that reach about -22F [11]. At -70F, batteries would lose all their capacity to the point where there won’t be enough e-fuel to power planes. 

Also, the weight of batteries to power planes would be heavy, slowing down e-fueled airplanes. As a comparison, a Tesla's 85 kWh battery weighs about 1,200 pounds. A jumbo-jet would need 440 times more batteries for the plane to even reach take-off speed. Currently, the biggest commercial jet, the Airbus a-380, weighs about 1,265,000 pounds with fully loaded passengers and luggage. A battery that can power this jet will be about 528,000 pounds [12]. Weight limits are important for planes to operate safely, and to match the weight limit on an e-fueled jet, airliners would have to limit the number of passengers, cutting profit. 

Both biofuel and e-fuel have their advantages and disadvantages. To save the environment, however, airliners need to tackle those disadvantages. Biofuel would still produce some carbon emissions, but it would be minimal compared to conventional kerosene. E-fuel, a cleaner option, comes with temperature sensitivity and weight problems. The solution then can be the combination of the two. airliners can use a hybrid engine that would include both biofuel and e-fuel. To utilize this hybrid engine, the world needs to find a way to produce enough biofuel from cooking oil and insulate batteries in extreme temperatures. The process of using sustainable fuel will be time-consuming and costly, especially after airliners suffered plunging revenue due to COVID-19 restrictions, but for the sake of the environment, actions need to be taken. 

Next time when you hear a loud jet engine, look up and see if you can spot a contrail left behind by the plane. If you see one, hold your breath. It’s toxic carbon emission from kerosene. If you don’t see anything but the blue sky, breathe in the fresh air. You’re looking at a clean carbon-free plane soaring through the sky!

Reference

[1]scientificamerican.com/article/why-do-jets-leave-a-white/ 

[2]news.un.org/en/story/2021/01/1082302 

[3]cnn.com/2021/08/01/health/covid-19-vaccine-doses-administered/index.html 

[4]digital.emagazines.com/time/20210528/index.html?t=4f967ae1-402d-468b-a03f-ba46b7cb11f9&utm_content=library&utm_campaign=time_web_front_issues#p12 

[5]congress.gov/bill/117th-congress/house-bill/741 

[6]digital.emagazines.com/time/20210528/index.html?t=4f967ae1-402d-468b-a03f-ba46b7cb11f9&utm_content=library&utm_campaign=time_web_front_issues#p12 

[7]bbc.com/future/article/20210525-how-aviation-is-reducing-its-climate-emissions 

[8]travelweekly.com/Travel-News/Airline-News/US-airlines-to-go-net-zero-carbon-emissions-2050 

[9]nbcnews.com/mach/science/eco-friendly-biofuels-made-food-waste-could-cut-flight-shame-ncna1030801 

[10]mashable.com/feature/electric-airplanes-future-flight 

[11]electrochem.org/ecs-blog/batteries-going-dead-in-cold-weather-explained/#:~:text=According%20to%20Lifewire%2C%20lead%2Dacid,on%20any%20given%20winter%20morning

[12]enrg.io/tesla-battery-weight-overview-all-models/