De-carbonization of Civil Aviation
Global warming will be the next and the most crucial challenge for humankind. In the very near future, civil aviation will be under even more scrutiny, as will other transportation mediums. From air to ground, water to railroad, all transportation solutions create a quarter of the total CO2 emissions. The motivation to transform engine systems from internal combustion to pure electricity is to decarbonize our methods of transportation.
Commercial aircraft, on the other hand, are responsible for 3% of carbon emissions. Even though this amount seems very low, the much needed revolution in alternative methods of producing power will also affect the big jets that have been using the 100 years of technology behind fossil fuels. High-tech gas turbine and jet engine manufacturers are aware of recent paradigm shifts and; they have already started developing alternative solutions for propulsion systems.
Some industry experts suggest that pure electrification would be the final solution. Whereas, engineering backed consultants insist on hybrid methods, combining both electricity (or hydrogen) and internal combustion, which seems more realistic due to power requirements of the ‘big-birds’. Contrary to the aforementioned solutions, World Economic Forum’s Clean Skies for Tomorrow (CST) Coalition believes engineering of new engine types, either electric or hybrid, could take 10 to 20 years. Thus, during the revolution of new types of propulsion systems, aviation fuels could be a catalyst for zero aviation shift1 at the beginning.
WEF CST was established in 2019 and is the collaboration between aircraft manufacturers (Airbus, Boeing etc.), airline operators (KLM, Spicejet etc.), airport authorities ( London Heathrow etc.), non-governmental organisations, academic institutions and energy giants (Shell etc.) focusing on decarbonization of civil air transportation via alternative fuel and propulsion systems. Initiatives are being planned to alter recent practices by the mid of 21st century via sustainable aviation fuels (SAF) and alternative clean propulsion technologies.
In this study, alternative solutions are ordered according to infrastructure change requirements. SAFs seem to be the fastest way that this can be applied. Hydrogen powered propulsion systems, on the other hand, require significant amounts of infrastructure and technological investment.
Source: Hydrogen-powered aviation, A fact-based study of hydrogen technology, economics, and climate impact by 2050
Scaling Sustainable Aviation Fuels (SAF)
Instead of researching or developing SAF, industry has focused on scaling the usage of new types of fuel since SAFs have already been developed and have been in use for a long time. As mentioned in the SAF Analytics report; municipal waste, agricultural residues and waste lipids are main ingredients for SAFs, and more than 250,000 commercial flights have already been fueled by SAF and tested in practical manner. That means existing fuelling and propulsion technology allow SAF usage without significant investment.
The only hurdle in front of scalability is the cost of refining household waste into jet fuel. Recent technologies, however, do not provide cheaper solutions when compared with fossil fuels. Only stimulation, therefore, that can be suggested by industry experts is mandatory regulations dictated by governments. CST suggests four pathways in order to attract industry attention: Hydroprocessed esters and fatty acids (HEFA), alcohol-to-jet (AtJ), gasification/Fischer-Tropsch (gas/FT); and power-to-liquid (PtL). In the below matrix taken from Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) by ICAO explains pathways depending on maturity, feedstock and contribution to green aviation.
Source: CORSIA, RED II; De Jong et al. 2017; GLOBIUM 2015; ICCT 2017; ICCT 2019; E4tech 2020; Hayward et al. 2014; ENERGINET renewables catalogue; Van Dyk et al., 2019; NRL 2010; Umweltbundesamt 2016
Although SAFs are far from general applicability, and are...significantly more expensive than fossil fuel, SAFs have enormous potential if scalability challenges are resolved.
Collecting sufficient amounts of feedstock is the first challenge. The other one is the demand for eco travel to reduce footprint. Therefore, regulations and passenger preferences could be a catalyst for alternative fuel investments. That would eventually result in highly dispersed SAFs as an economical choice.
In order to reduce carbon emissions, SAFs have a significant role during transition. Once the scalability is achieved, the cost of fossil fuel per gallon and some SAFs would be almost the same for airlines. Public support, government incentives and strict regulations will also motivate investors and industry players to be part of energy transition as seen on fully electric personal vehicles.
Pure Electric Propulsion
Today’s jet fuel consumption breakdown shows that almost %66 of consumption comes from short and medium range commercial flights. CO₂emission directly correlates with this burn rate; the shorter the distance, the more emission the atmosphere is exposed to. Therefore, analysts and industry professionals will initially focus on a shift in short-haul flights.
Improvement in battery technology is devastating if we look back at the last 20 years. However, for commercial airline use, batteries are not capable of carrying 200 passengers from point A to B. Low energy density of batteries and lack of fast charging techniques ( 10 to 15 mins of charging are not available on the market) are pioneering challenges of sole electric flight. Recent airport infrastructures also limit scalability of electric propulsion.
In recent market conditions, electrically driven vehicles are limited to personal use only. For commercial trucks and busses, hydrogen and electric powered hybrid solutions are suggested instead of full electric propulsion. The reason is simply to do with the excessive power requirements mentioned earlier. Similarly, for commuting purposes, personal use or low range very light flights could be converted to %100 electric very soon. Otherwise, the airline industry has to wait for battery and charging technology developments which require new advances and innovation in engineering materials. That means that planning a roadmap for pure electric transformation is almost impossible with current technologies. Therefore, commercial aircraft manufacturers do not waste time with pure electricity and they rely on hybrid solutions esentially.
Hydrogen Powered Aircraft
Technical complexities of using Hydrogen as an energy source have already been resolved. In the technical perspective, Hydrogen is burnt in a modified combustion chamber in order to rotate turbines, or it can be used in fuel cells to drive electric engines. In fact hydrogen carries more energy compared to fossil based jet fuel; kerosene. However, the only inconvenient condition in front of widespread usage of H₂ is storage. Since it has more volume, it requires bigger storage tanks in airplanes that change aircrafts’ design and configuration. For long-haul flights, tank capacity is a significant limitation that reduces its profitability.
Decarbonization of air travel via hydrogen is not a new trend. Urban or commercial usage of H₂ has been studied for a long time. However, these studies basically depend on technological research and development. Recent considerations, on the contrary, are highly related to scalability and widespread usage of Hydrogen propulsion.
For instance, the UK based aviation startup, ZeroAvia, focusing only on hydrogen powered aircraft, drafted a timeline that shows how transition from kerosene fuel powered jet engine to hydrogen propulsion is projected to occur.
ZeroAvia’s plan starts with changing general aviation that needs less power and offers less seats. In the timeline, seat capacity starting from 10 with limited mileage will increase to 200 seats with more than 5,000 NM. Within 10 years, mid-range short haul aircraft are expected to have hydrogen plants and operate in domestic networks at short distances. However, the final transformation will take as long as 20 years and not only will it change the aerodynamic structure of aircraft completely, but also the propulsion systems that will be based on hydrogen fuels.
ZeroAvia does not suggest only a propulsion solution but also a complex system that starts from energy generation via renewable sources. The company also explains how to convert water and energy into usable hydrogen fuel via hydrogen electrolyzer. This mid step has crucial importance since it requires significant amounts of energy. In order to have zero carbon aviation, this mid-step should be powered with renewable energy resources. Eventually, the final product will be stored in hydrogen tanks and used in fuel cells to propel either gas turbines or electric engines. The concept is very similar to electric car brand Tesla’s offerings, starting from Solar Roof to battery, followed by superfast charger to car.
On the commercial aviation front, Airbus is the leading aircraft manufacturing company that has been investing on reducing CO₂emissions and green air travel. The company offers three concepts which are expected to fly by 2035.
- A turbofan aircraft (120 to 200 passengers with more than 2000NM) having modified jet turbine running on hydrogen,
- A turboprop aircraft (up to 100 passengers with an approximate range of 1,000NM) having hydrogen combustion gas turbine engine,
- A blended-wing body aircraft (up to 200 passengers) merging new aerodynamic design and hydrogen turbofan efficiency, are pole stars for Airbus decarbonization action.
As expected, hydrogen fuel sources and powerplant integration requires significant technical research and development both in infrastructure, design and aircraft systems as well as decisive regulation change. Hydrogen power is only a fairy tale without financial support from industrial partners and authorities’ desire to change.
- Hydrogen-powered aviation, A fact-based study of hydrogen technology, economics, and climate impact by 2050
- CORSIA, RED II; De Jong et al. 2017; GLOBIUM 2015; ICCT 2017; ICCT 2019; E4tech 2020; Hayward et al. 2014; ENERGINET renewables catalogue; Van Dyk et al., 2019; NRL 2010; Umweltbundesamt 2016