Exploring Sustainable Aviation Fuels: The Future of Flight
The aviation industry is a significant contributor to global greenhouse gas emissions, and finding ways to make air travel more sustainable is crucial. One promising solution is the development and use of sustainable aviation fuels (SAFs). Let’s dive into what SAFs are, how they work, and why they are essential for the future of aviation.
What are Sustainable Aviation Fuels (SAFs)?
SAFs are a type of biofuel specifically designed for use in aircraft. Unlike traditional jet fuel, which is derived from fossil fuels, SAFs are made from renewable resources such as plant oils, waste oils, and agricultural residues. The goal of SAFs is to reduce the carbon footprint of air travel by offering a more environmentally friendly alternative to conventional jet fuel.
Current State of SAFs
Despite their potential, SAFs currently account for a tiny fraction of the fuel used in aviation. In 2022, the total SAF used was only enough to power global aviation for around 8 hours [1]. A common SAF today is Hydroprocessed Esters and Fatty Acids (HEFA), made from waste oils or fats. However, feedstock limitations mean that HEFA can only replace around 1% of jet fuel at its maximum potential.
The Challenge of Decarbonizing Aviation
Aviation contributes around 3.5% of the total radiative forcing emissions from human activity. This number is expected to increase as other energy markets, such as those for electric vehicles in ground transportation, decarbonize. Aviation is considered one of the most challenging sectors to decarbonize due to the energetic demands of flight, stringent safety standards, current airline operations and market dynamics, the long service life of aircraft, and the extended time required to design, build, and certify new hardware technologies.
Biomass-Derived SAFs
Biomass-derived SAFs are an alternative, made from a variety of feedstocks, including agricultural residues and forestry by-products. However, the volume of biomass required is immense – comparable to the world’s entire timber supply. This poses significant challenges for scaling up production to meet the aviation industry’s needs.
Synthetic SAFs
Synthetic SAFs are produced using renewable electricity and direct air capture (DAC) of CO2. This method would compete less with agricultural resources but would require around 40% of the world’s current electricity production to meet today’s jet fuel demand. This highlights the need for a balanced approach to SAF production.
Hybrid Approach: Power and Biomass to Liquid
A promising way to scale SAF production is through a hybrid approach that combines power and biomass to liquid technologies. This method uses all the carbon from the biomass, upgrading it with green hydrogen. By utilizing all the carbon, this approach requires one-third of the biomass of purely biomass-derived fuels and up to 50% less electricity than synthetic fuels.
The Role of Hydrogen in Aviation
Hydrogen is another potential game-changer for aviation, offering significant advantages in direct emissions reduction and weight efficiency. However, it must be stored as a liquid at cryogenic temperatures (-253°C), necessitating the use of insulated tanks. This storage requirement reduces the net weight advantage of hydrogen. Here are the key reasons to store in liquid state,
- Hydrogen has a very high specific energy (energy per unit mass) but a very low energy density (energy per unit volume) in its gaseous state. This means that, while it packs a lot of energy per kilogram, it takes up a lot of space when stored as a gas. Liquid hydrogen, on the other hand, is much denser than gaseous hydrogen, allowing more fuel to be stored in the same volume.
- For aviation, where space and weight are critical, storing hydrogen in its liquid state maximizes the amount of fuel that can be carried, thus making it more efficient for long flights.
Challenges of Liquid Hydrogen
Liquid hydrogen has about four times the volume of kerosene, meaning fuel tanks must be placed in the fuselage. A clean-sheet hydrogen replacement for a Boeing 737 could match the energy requirements per passenger to within 20%. Moreover, new types of jet engines designed to exploit hydrogen could reduce the energy requirement of flight by more than 20%.
However, realizing hydrogen's potential requires significant investments in technology development, liquefaction, distribution, and refueling infrastructure, as well as addressing safety and certification challenges.
Advantages Over Synthetic SAFs
Hydrogen requires 30-50% less renewable electricity compared to synthetic SAFs. However, the major challenge lies in developing the necessary infrastructure for its production, storage, and distribution.
Addressing Contrails
Contrails are the streaks of clouds left behind by aircraft. They form when aircraft emit water vapor and soot into sufficiently cold and humid air, occurring about 10% of the time during flights in ice supersaturated (ISS) regions. Contrails trap outgoing heat and reflect incoming sunlight, but overall, they have a warming effect comparable to the CO2 emissions from aviation.
Mitigating Contrail Formation
Since ISS regions are vertically thin and horizontally wide, contrail formation could potentially be averted with short-term altitude changes, typically flying lower in warmer air. This method may incur a 1-2% fuel penalty but offers a significant reduction in contrail-induced warming. Demonstrating this strategy at scale in operational conditions could be a powerful way to mitigate aviation’s climate impact in the short to medium term.
Advantages of SAFs
SAFs offer a relatively straightforward path for minimizing radiative forcing emissions from aviation, now and in the future. All currently approved SAFs are ‘drop-in’ fuels, meaning they can be used interchangeably with conventional jet fuel. This makes them highly practical as they require no modifications to existing aircraft infrastructure, engines, or fueling systems. SAFs meet stringent specifications for handling, fluidity, and altitude properties, ensuring they are as safe or safer than conventional fuels.
Economic and Logistical Challenges
The predominant issue with SAF scale-up is their current cost, which is higher than conventional jet fuel. This is due to increased production costs and the availability of cheap renewable carbon. SAF feedstocks may be partially or entirely oxidized, resulting in lower yields or requiring more energy to upgrade the feedstock to kerosene free of contaminants.
Additionally, SAF carbon tends to be more diffuse compared to conventional crude oil, which is sourced more centrally. This means SAF production faces logistical challenges in collecting and converting widespread feedstocks such as crop residues or atmospheric CO2.
Future Prospects and Technological Development
The cost of SAF will decrease with scale and technological advancement. Incremental gains will be made on existing pathways, and new SAF production methods will be developed. SAF can be produced from a variety of feedstocks, including municipal solid waste, corn ethanol, woody biomass, waste oils, and algae. For example, five companies are currently working with ASTM to qualify a new methanol-to-jet process for producing SAF [1].
Continued public and private investment in SAF technologies will drive down costs and increase production. This virtuous feedback cycle of investment and technological improvement will make SAFs more economically viable and widely available.
Conclusion
SAFs, hydrogen, and contrail mitigation represent significant opportunities to make aviation more sustainable. By leveraging a combination of these approaches, the aviation industry can reduce its carbon footprint and move towards a more sustainable future. Investments in technology, infrastructure, and research are essential to scale these solutions and realize their full potential, ensuring that air travel remains viable and environmentally responsible for generations to come.
Definitions:
- Sustainable Aviation Fuels (SAFs): Biofuels specifically designed for use in aircraft, made from renewable resources to reduce the carbon footprint of air travel.
- Hydroprocessed Esters and Fatty Acids (HEFA): A type of SAF produced from waste oils or fats through hydrogenation.
- Biomass-Derived SAFs: SAFs made from a variety of biomass feedstocks, including agricultural residues and forestry by-products.
- Synthetic SAFs: SAFs produced using renewable electricity and direct air capture of CO2.
- Power and Biomass to Liquid: A hybrid approach combining power and biomass to produce liquid fuels, using green hydrogen to upgrade the biomass.
- Contrails: Cloud-like trails left by aircraft, formed when water vapor and soot are emitted into cold and humid air, contributing to radiative forcing.
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