Reaching Jet-Zero: We need a revolution, not an evolution, in aircraft propulsion systems.

We need a revolution, not an evolution, in aircraft propulsion. Aviation is?currently?projected to account for over 22% of all transport CO2 emissions by 2050?(International Energy Agency, 2020).?Moreover, to negate aviation’s impact on our climate?by 2050,?we need to achieve at least?a?50%?reduction in the next 10-15 years.         

In 2019 the aviation industry accounted for?12%?(Air Transport Action Group, 2021)?of all transport CO2 emissions.?49%?of?those?emissions came from?international flights?(Graver, Zhang, & Rutherford, 2019). This?latter fact?has made?decarbonising?aviation?particularly challenging. When a large fraction of an aircraft’s?emissions?occurs?over international waters, whose carbon budget are they added to??

Aviation’s impact on the climate goes beyond CO2. The?high-altitude?emission of?sulphur?and NOx also has?serious consequences. In fact, together with contrails, the actual?total?climate impact from aviation is?three?times?larger than?(Lee et al., 2021)?that of CO2 alone.?

Whilst today’s figures indicate that aviation has a relatively minor carbon footprint,?the real challenge is?yet to come.?There has been little?progress developing?the technologies?necessary?to?decarbonise?the industry. This?contrasts with?other areas of transportation, where great strides have already been made.?As a result, aviation is?currently?projected to account for over 22% of all transport CO2 emissions by 2050?(International Energy Agency, 2020).?

Whilst the COVID pandemic has undoubtedly?reduced air traffic in the short term, the long-term trend?is?one of?continued growth.?Experts project than global passenger numbers will double?by?2040?(Boeing, 2021). In fact,?bookings have?already?returned to within 20% of?pre-pandemic figures?in?both the US and Europe?(IATA, 2021).?

At first glance, the simplest means of de-carbonising?the aerospace industry?appears?to be a?switch to?sustainable fuels. However, sustainable aviation fuels (SAFs)?are far too expensive and not widely available today or in the future.?Even?in?2050?sustainable fuels?are projected to be 2-3 times more expensive than?jet fuel?(World Economic Forum, 2020). With fuel accounting for around?22%?of?an?airline’s?operating costs?(IATA, 2019), this?cost increase?is?not?financially?sustainable, even if passengers are willing to absorb some of the cost.?2)?Scalability:?It is not certain that the world could ever produce enough biofuel?to meet the?aviation?industry’s needs. The long-term impact on agricultural land?use?is also unclear.??

If we are to use?SAFs?then we must?radically?reduce the amount of?fuel aircraft use?and we must do it quickly. To put this in context, it has taken the aviation industry?46?years to reduce jet engine fuel consumption by 45%?(Kharina & Rutherford, 2015).?To negate aviation’s impact on our climate?by 2050,?we need to achieve at least?a?50%?reduction in the next 10-15 years. This will require a revolution, not evolution, in aircraft propulsion.???

How can we achieve such radical improvements? One opportunity is?in?the?use of fuel cells instead of gas turbines or ICE engines. Fuel cells?can be?20-30%?more efficient at converting fuel into power?than combustion processes?(US Department of Energy, 2015).?However, they are often?heavy, which means their power densities are low.?As?the propulsion system in an aircraft needs to be sized to meet the peak power requirements?during take-off, which are often at least double?power requirements once the plane is cruising, this low power density?is a challenge.?Hybridisation?offers a potential solution by?combining?a fuel cell sized for cruise power?with a battery system that provides additional power during take-off.?By?further?optimising?the fuel cell for cruise,?we can reduce fuel usage by an additional?25%.???

At?Qdot?we are working on such hybrid propulsion systems to bring?about a clean future for aviation.???

References?

Air Transport Action Group. (2021, Nov).?Facts & Figures. Retrieved from https://www.atag.org/facts-figures.html?

Boeing. (2021).?Commercial market outlook: 2021-2040.??

Graver, B., Zhang, K., & Rutherford, D. (2019).?CO2 emissions from commercial aviation, 2018.?The International Council of Clean Transportation.?

IATA. (2019).?Economic Performance of the Airline Industry - December 2019.??

IATA. (2021).?Update on the latest air traffic data.??

International Energy Agency. (2020).?Energy Technology Perspectives 2020.??

Kharina, A., & Rutherford, D. (2015).?Fuel efficiency trends for new commercial jet aircraft: 1960 to 2014.?The International Council on Clean Transportation.?

Lee et al., D. (2021). The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018.?Atmospheric Environment, 244.?

US Department of Energy. (2015).?Fuel Cells.?Fuel Cell Technologies Office.?

World Economic Forum. (2020).?Clean Skies for Tomorrow: Sustainable Aviation Fuels as a Pathway to Net-Zero Aviation.??