Ameer Randolph
As fossil fuel reserves are expected to deplete, the aviation industry is actively exploring sustainable alternatives to power aircraft. While electric airplanes are almost here, they are only suitable for short-haul flights due to the weight of the batteries required. Other options include hydrogen, biofuels, electrofuels, solar, and ammonia. Hydrogen has a high energy density compared to its weight, but it is low compared to its volume, meaning more space is required to store it. Biofuels are a viable option, but they tend to get viscous or even solidify at low temperatures, and there may not be enough biomass to replace fossil fuels. Nevertheless, recent test flights powered by hydrogen or biofuels have been successful, and researchers remain committed to finding solutions to make aviation carbon-neutral.
| Characteristics | Values |
|---|---|
| Electric planes | Batteries need to be lighter and more energy-efficient |
| Hydrogen | Requires significant investment and changes to infrastructure |
| Solar | Current technology does not provide enough energy |
| Biofuels | Need to be blended with fossil fuels due to issues with viscosity and sustainability |
| Electrofuels | Liquid electrofuels can be made by combining hydrogen with captured carbon |
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What You'll Learn
Hydrogen-fuelled aircraft
The aviation industry is exploring several alternatives to power aircraft without fossil fuels. Hydrogen-fuelled aircraft are one of the most promising candidates for the future of aviation. Hydrogen has been used in the automobile and space industries for decades, and the aviation industry is now exploring ways to adapt its use for commercial flights.
Hydrogen has a high energy-to-mass ratio, making it appealing for aerospace applications and offering the potential for significant emissions reductions. Hydrogen can be burned in a jet engine or used to power a fuel cell to generate electricity for an electric propulsor. Hydrogen fuel cells are a scalable technology, allowing multiple cells to be combined into fuel cell stacks to increase power output.
However, hydrogen also presents challenges for aviation. Hydrogen fuel requires advanced storage technologies as it needs to be stored at extremely low temperatures of -253°C. Hydrogen-fuelled aircraft would also require significant investment, and the price of green liquid hydrogen relative to fossil fuels will be a critical factor in determining its uptake.
Airbus has been at the forefront of hydrogen-powered aircraft development with its ZEROe project, which aims to bring a hydrogen-powered aircraft to the skies. The project explores hydrogen combustion and hydrogen fuel cell technology, with fuel cells ultimately being selected as the preferred option. Airbus has also founded Aerostack, a joint venture with ElringKlinger, to research, develop and assemble fuel cell stacks for the ZEROe aircraft.
Airbus plans to launch a commercial hydrogen-powered aircraft by 2040-2045, with the first aircraft expected to enter service by 2035. The ZEROe aircraft will feature electric propeller propulsion powered by hydrogen fuel cells, which will convert hydrogen into electricity.
Other hydrogen-fuelled aircraft concepts have also been proposed, ranging from 7 to 90 seats, exploring the use of hydrogen with fuel cells and gas turbines. The UK expects hydrogen-powered aircraft to be commercially viable for short-haul and regional flights by the second half of the 2020s, with the potential to replace the entire UK regional fleet by 2040.
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Sustainable aviation fuels (SAF)
SAF technology is already fairly mature and has received significant funding for development and research. It is the most likely and suitable option for the short term, as it would require minimal changes to transition from traditional jet fuel to SAF. SAFs have already proven successful, with the first test flight of an A350 (a long-range large passenger airliner) fuelled with 100% SAF occurring in March 2021.
The ICAO (International Civil Aviation Organization) is working to facilitate SAF development and deployment through the four building blocks of the ICAO Global Framework for SAF, LCAF, and other Aviation Cleaner Energies. The ICAO ACT-SAF Series of events is providing comprehensive training to ACT-SAF Partners on a range of important SAF-related topics, such as sustainability, policy, economics, and financing certification.
The use of SAFs can actively contribute to sustainability goals within the aviation industry. SAFs can cut carbon emissions by up to 85% and reduce other harmful emissions like particulates and sulfur by 90% and 100% respectively. As biofuels become more economically competitive with kerosene, they will pave the way for commercial flights without fossil fuels.
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Electric airplanes
Electric aircraft have been in development for some time, with the first solar-powered flight occurring in 2005. This flight, by AC Propulsion, used an unmanned airplane named "SoLong" that was propelled entirely by solar energy and flew for 48 hours non-stop. Since then, there have been numerous other solar-powered aircraft, such as the Solar Impulse 2, which completed the first round-the-world solar-powered flight in 2016. Other notable electric aircraft include the X-57 "Maxwell", which is NASA's first all-electric aircraft, and the Taurus Electro, which was built by Pipistrel and is the world's first fully electric 2-seat aircraft.
While electric aircraft have come a long way, there are still some challenges to their widespread adoption. One of the main challenges is battery technology, as batteries need to be light enough to allow the aircraft to fly while still providing enough energy to power the plane. Improvements in battery technology will likely enable longer flights and increase the feasibility of electric aircraft for longer routes.
Another challenge is the energy density provided by fossil fuels compared to renewable alternatives. Large, long-range passenger planes consume huge amounts of energy, and finding a renewable fuel source that can match the energy density of fossil fuels is crucial for the aviation industry's transition to sustainable energy sources. Biofuels, such as hydro-processed renewable jet fuels, are currently the most likely short-term option as they require minimal changes to existing infrastructure.
Despite these challenges, there is ongoing research and development in electric aircraft, with projects funded by organizations such as the European Commission and the UK government. Hybrid-electric aircraft, which combine electric power with conventional piston or jet engines, offer a potential solution for long-haul flights while reducing the carbon footprint. Additionally, hydrogen fuel cells are being explored as a potential power source for electric aircraft, with projects such as the Boeing-led FCD (fuel cell demonstrator) making successful flights in 2008.
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Biofuels
However, there are also some challenges and drawbacks associated with biofuels. One issue is the limited availability of sustainable biomass to replace fossil fuels. Additionally, biofuels tend to become highly viscous or even solidify at low temperatures, which could be problematic at the typical air temperatures at which planes fly. Another challenge is the potential impact on food security and land use if food crops are used for biofuel production.
To address these concerns, sustainable biofuels that do not use food crops, prime agricultural land, or freshwater have been developed. Feedstocks like straw or used cooking oil, which do not compete with food production, are considered the most sustainable choices. Additionally, synthetic rubber substitutes that are not affected by biofuels, such as Viton, have been introduced to address the issue of rubber seals and hoses shrinking in the presence of pure biofuels.
While biofuels may not be the ultimate long-term solution for an emissions-free aviation industry, they certainly have a role to play in the transition to a low-emissions future. They can be used in conjunction with other alternatives, such as electrofuels, to reduce emissions and pave the way for more sustainable aviation.
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Electrofuels
As fossil fuel reserves are being depleted, jet fuel prices are expected to rise in the coming decades. This makes it crucial to explore alternative energy sources to power the aviation industry. Sustainable Aviation Fuel (SAF) is a low-carbon alternative to conventional jet fuel that is compatible with current aircraft engines and airport infrastructure. SAF can be produced from biomass, such as mustard seeds, used French fry oil, or other renewable materials.
The production of e-fuels requires a large amount of renewable electricity, especially for the production of green hydrogen, which is a critical component of e-fuels. The process involves using electricity to split water into hydrogen and oxygen through electrolysis, with the captured carbon or nitrogen forming fuels such as kerosene, methane, methanol, hydrogen, ammonia, and n-octane. While burning synfuel still emits CO2, it is considered "carbon neutral" because the carbon used to make the fuel comes from the atmosphere rather than the ground.
However, the high electricity requirements for producing e-fuels may have an adverse climate impact, especially when compared to using DAC to compensate for emissions from kerosene. The benefit of e-fuels in reducing emissions may be limited, and further research is needed to evaluate their overall lifecycle climate benefit. Nevertheless, with continued investment in technology and supportive policies, it is expected that we will move closer to flying on 100% sustainable fuels in the future.
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Frequently asked questions
Some alternatives to fossil fuels for airplanes are hydrogen, biofuels, electrofuels, solar power, and electricity.
Hydrogen-powered airplanes are being tested and are expected to be in use by 2026. Hydrogen has a high energy density compared to its weight, but it is low compared to its volume, meaning it takes up more space than other fuels.
Biofuels have been used in a blend with regular jet fuel, as well as in some 100% biofuel flights. However, the tendency for biofuels to become viscous or solidify at low temperatures is a significant barrier to their use in airplanes.
Electric airplanes are possible, but the weight of the batteries is a challenge. As battery technology improves, electric airplanes will become more viable for longer flights.
