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COP28 Series: Aviation

By Christopher King
COP28 Series: Aviation

For many of us, the ability to travel by air plays an important role in our lives, allowing us the freedom to explore new places and to stay connected with the people and things we love. Air transportation also plays a key role in international trade, with some studies estimating that, by value, 35% of global trade is carried by air. However, it is also well-known that aviation comes at a large cost to the environment. Almost all of today’s commercial aircraft are powered by gas turbine engines, which run on jet fuel. Jet fuel is a mixture of hydrocarbons derived from crude oil, which, when combusted, release carbon dioxide (CO2). It is estimated that the aviation sector is responsible for 2-3% of global CO2 emissions, and the total contribution of the aviation sector to global warming is higher than this as a result of other pollutants (e.g. NOx) and water vapour released during flight.

With the benefits provided by the widespread availability of aviation for air travel and trade, it is clearly something that society as a whole would be reluctant to give up. However, in the efforts to reach net zero by 2050 or earlier, the aviation sector’s contribution to global CO2 emissions cannot be ignored. This article looks into whether the adoption of new technologies could allow aviation to continue in a net zero future.

Industry commitment

In October 2021, the global body which represents the majority of the world’s airlines (the International Air Transport Association, IATA) passed a resolution committing to achieving net zero CO2 emissions from all aviation operations by 2050. This might suggest that the aviation industry itself considers a transition to net zero to be an attainable goal - at least in theory. However, the commitment is widely regarded as being somewhat ambitious. While many sectors are already starting to decrease their contribution to global CO2 emissions, the total amount of CO2 emissions connected with the aviation sector is still trending upward (disregarding the impact of the COVID-19 pandemic). This is because incremental improvements made to aircraft efficiency through the refinement of airframe design and the development of engine technology are outweighed by steadily increasing passenger demand.

For the aviation sector to bring about any significant reduction in CO2 emissions, there will need to be a move away from the reliance on conventional jet fuel. There are several technologies which may allow for conventional jet fuel to be replaced, each one having benefits, but also drawbacks and challenges that will need to be addressed before full commercialisation is feasible.

Electricity and batteries

With the rapid growth in the number of battery electric vehicles (BEVs) for road transport, an obvious question is whether batteries could also replace jet fuel as the energy storage means used in aircraft. Batteries onboard an aircraft could be used to power electric motors, which drive propellers. If the batteries are charged by electricity from renewable sources, the overall CO2 emitted by a battery-powered electric aircraft flight could be close to zero.

However, the concept of “battery electric aircraft” faces a fundamental problem: batteries have a much lower energy density (the amount of energy stored per unit mass) than jet fuel. The best commercially-available Lithium-ion batteries offer an energy density of around 1 to 1.5 MJ/kg, which is dwarfed by the 43 to 48 MJ/kg offered by jet fuel. Consequently, for a battery electric aircraft to have a range equivalent to that of a corresponding jet fuel-powered aircraft, it would need to carry batteries with a mass far in excess of the mass of fuel carried by the equivalent jet fuel-powered aircraft. An increased mass brings about myriad problems, including an increase in the total energy required for flight, increased structural requirements for the airframe and landing gear, and reduced payload capacity. This means that, barring extraordinary improvements in the energy density offered by commercially-available batteries, it is unlikely that batteries will ever be viable for use as the energy storage means on large planes or for long-haul flights.

Despite the disadvantages of using batteries to power aircraft, battery electric aircraft are in development, with start-ups such as Eviation successfully testing fully electric prototypes on short flights, and attracting large investments from high-profile organisations. Thus, while battery electric aircraft may not be suitable for all aviation applications, they may have some role to play in the phasing out of conventional jet fuel-powered aircraft, for example in general aviation (i.e. private, non-commercial aviation) for short-haul commercial flights, and for shorter-distance transport of cargo (which could be done using drones).

Hydrogen

One alternative to jet fuel which does not suffer from poor energy density is hydrogen. In fact, hydrogen has an energy density that is approximately three times greater than that of conventional jet fuel. Hydrogen could be implemented in an aircraft in two main ways: (i) in a fuel cell which generates electricity to power electric motors; and (ii) in modified gas turbine engines, where the hydrogen is combusted in much the same manner as jet fuel in conventional gas turbine engines today.

The development of aircraft with hydrogen-based propulsion systems is already well underway. Industry giant Airbus is aiming to bring a hydrogen-powered commercial aircraft to market by 2035 through its “ZEROe” project. There are also numerous start-ups engaged in the development of hydrogen-powered aircraft such as the Anglo-American start-up, ZeroAvia, who have already conducted successful tests with prototype aircraft powered by a combination of conventional jet fuel- and hydrogen-based propulsion systems.

Further, a recent report released by the EPO and the International Energy Agency (IEA) indicates that innovation (as measured by patenting activity) relating to the use of hydrogen in the aviation sector is growing rapidly – though this comes with the caveat that a large proportion of this innovation is thought to be directed mainly to the use of hydrogen in unmanned aerial vehicles (i.e. drones) as opposed to large commercial aircraft. Interestingly, the patenting activity seems to indicate that hydrogen fuel cells are currently of greater interest to the aviation sector than hydrogen gas turbine engines.

While hydrogen is certainly a promising prospect, it should be recognised that the technology is still in its infancy, and faces several challenges which must be addressed before it is a genuinely commercially-viable option. To store the volume of hydrogen required by an aircraft during flight, hydrogen needs either to be compressed to extremely high pressures (e.g. 450 to 700 bar), or turned into a liquid by cooling to approximately -253 °C. Storing hydrogen in such conditions requires complex and bulky storage tanks, which are difficult to integrate into standard airframe architectures. Other challenges arise in relation to the efficiency of hydrogen production, and the logistics of hydrogen transportation and storage. These challenges are discussed in more detail in one of our previous insights: What is the Hydrogen Economy and Can it Help Save the Planet? Of course, the hydrogen itself will also need to be produced using clean energy, which will place a further demand on that production, as discussed in the previous insight in this series.

Sustainable Aviation Fuel (SAF)

Another alternative to conventional jet fuel is so-called sustainable aviation fuel (SAF). Sustainable aviation fuel is a collective term used for jet fuel alternatives which are derived from sustainable sources. Relative to conventional jet fuel, it is estimated that use of SAF could reduce end-to-end CO2 emissions from aviation by up to 80%.

SAFs include biofuels (e.g. biodiesel and bioethanol), which can be obtained from feedstocks such as non-food crops and organic residues from agriculture and forestry, as well as waste fats, oils and greases, and municipal solid waste. SAFs also include synthetic “e-fuels”. Such e-fuels can be produced by harvesting CO2 (e.g. through capture from the environment or biogas plants), reacting the harvested CO2 with hydrogen to produce syngas (a mixture of carbon monoxide and hydrogen), and then, from the syngas, synthesising the e-fuel using the Fischer-Tropsch process. Biofuels are already commercially available, whereas e-fuels are still some way from commercial viability.

One advantage of SAF is that there are relatively few technical barriers to its implementation. In fact, many SAFs can be mixed with conventional jet fuel and used in conventional gas turbine engines today. All aircraft currently being manufactured by Boeing are certified to run on a 50/50 blend of SAFs and conventional jet fuel, and a Boeing 787 equipped with Rolls-Royce Trent 1000 engines recently completed a flight from London Heathrow to New York JFK using 100% SAF. Boeing has pledged that all of the aircraft it delivers will be certified to run on 100% SAF by 2030.

However, even though SAF could be used in some aircraft today, it currently accounts for less than 1% of global jet fuel consumption. This is largely because the production of SAF is both energy intensive and inefficient, so SAF typically costs three to four times more than conventional jet fuel.

Further, there are questions over whether it would be possible to scale-up the production of SAF to satisfy any significant proportion of the global demand for jet fuel without wider consequences. In 2019, the amount of jet fuel used globally was over three times greater than the total amount of liquid biofuel production. Increasing biofuel production to the point where it is able to satisfy any significant proportion of global demand for jet fuel would require a large amount of land to be dedicated to growing feedstocks. This could mean less land available for agriculture, and could actually encourage deforestation. Further, in a world which is attempting to build up sufficient renewable generation capacity to meet current electricity demand, ramping up production of SAF – and thereby considerably increasing global electricity demand – is undesirable. Thus, continued improvements in the efficiency of SAF production are necessary if its adoption is to become more widespread in the coming years.

What will it be?

IATA estimates that replacing crude oil-derived jet fuel with SAF could contribute 65% of the reduction in emissions required to reach net zero by 2050, with the adoption of electric- and hydrogen-based propulsion systems providing a smaller contribution (13%). This seems to be sensible considering that the infancy of electric- and hydrogen-based propulsion systems for use in aviation means that their adoption is likely to be gradual and relatively unpredictable. However, IATA’s estimates place a huge burden on the scaling of SAF production, which presents several challenges as discussed above.

Thus, while the adoption of new technologies may, in theory, allow aviation to continue in its current form and scale into a net zero future, it will be far from easy in practice and will require significant further advances. Realistically, therefore, the adoption of new technologies may need to be accompanied by a reduction in demand if the objectives set out in the Paris Agreement are to be met.

What is clear is that the conclusions of COP28 regarding the importance of technology and innovation to the transition to net zero apply unequivocally to the aviation sector. That is, for the aviation sector to make any significant progress in reducing the CO2 emissions for which it is responsible, we will need to see “rapid and scaled-up deployment and adoption of existing clean technologies and accelerated innovation, … demonstration and dissemination of new and emerging technologies, as well as increased access to those technologies”. Such innovation will require absolute commitment from all stakeholders, both in industry and across governing bodies.