The Future Of Aviation: Sustainable Fuel Alternatives

The aviation industry is exploring several alternatives to jet fuel to reduce its carbon footprint and achieve net-zero carbon emissions by 2050. Sustainable Aviation Fuel (SAF) is one of the most promising options, with the potential to cut carbon emissions by up to 100%. SAF can be produced from various feedstocks, including household waste, industrial waste gases, agricultural and forestry waste, and vegetable oils. Hydrogen is also being explored as a potential alternative, but it faces challenges due to its low density and high flammability. Additionally, biofuels, ethanol, and synthetic fuels created from atmospheric carbon capture are being considered, but they are currently more expensive than conventional jet fuel.

Hydrogen-powered aircraft

Hydrogen has a specific energy of 119.9 MJ/kg, compared to ~43.5 MJ/kg for usual liquid fuels, which is 2.8 times higher. However, it has a much lower energy density than liquid fuels, even when pressurised or cooled. This poses challenges when designing an aircraft, where weight and exposed surface area are critical. Hydrogen tanks have to be housed in the fuselage or be supported by the wing.

Gaseous hydrogen may be used for short-haul aircraft, while liquid hydrogen might be needed for long-haul aircraft. Hydrogen's high specific energy means aircraft would need less fuel weight for the same range, despite the added volume and tank weight.

Fuel cells are more efficient than modern 7 to 90-passenger turboprop airliners, but their engine efficiency is less than large gas turbines. They make sense for general aviation and regional aircraft. Hydrogen is suited for short-range airliners, while longer-range aircraft need new aircraft designs.

A study in the UK, NAPKIN (New Aviation, Propulsion Knowledge and Innovation Network), investigated the potential of new hydrogen-powered aircraft designs to reduce the environmental impact of aviation. The findings suggest that in the UK, hydrogen-powered aircraft could be commercially viable for short-haul and regional flights by the second half of the 2020s. Airlines could potentially replace the entire UK regional fleet with hydrogen aircraft by 2040.

Airbus and Boeing, the world's top two aircraft makers, are both developing hydrogen technologies. Airbus aims to bring the world's first hydrogen-powered commercial aircraft to market by 2035. Its ZEROe project is exploring a variety of configurations and technologies, as well as preparing the ecosystem that will produce and supply the hydrogen.

In 2023, aviation startups ZeroAvia and Universal Hydrogen claimed their novel aircraft would be ready to start flying commercially as early as 2025. Both companies are using hydrogen in its gaseous form to power fuel cells during flight testing, but they plan to use liquid hydrogen eventually.

While hydrogen-powered aircraft are a promising solution to carbon-free flying, there are some challenges and limitations to consider. Hydrogen has a low volumetric energy density, and it needs to be stored under very high pressure or cryogenically, which can cause problems. It also causes metal embrittlement and leaks out of storage containers more easily than most fuels. Additionally, the infrastructure to support hydrogen-powered aircraft, such as the production, transportation, and storage of hydrogen, needs to be further developed.

Sustainable Aviation Fuel (SAF)

SAF presents a near-term opportunity to reduce emissions from aviation, which accounts for 2% of global carbon dioxide (CO2) emissions and 12% of transportation CO2 emissions. The aviation industry has set a goal of achieving net-zero carbon emissions by 2050, and SAF is estimated to contribute around 65% of the required emissions reduction. To meet the growing demand for SAF and achieve emissions goals, a massive increase in production is necessary, with policy support and incentives playing a crucial role.

One proposed method for producing SAF involves using waste-to-fuel refineries built near major travel hubs, utilising waste produced by modern society, such as household garbage, food scraps, and sludge from water treatment plants. This approach could potentially replace a significant portion of the nation's jet fuel supply while reducing carbon intensity. Additionally, producing SAF from wet waste, such as manure and sewage sludge, offers environmental benefits by reducing pollution in watersheds and keeping methane out of the atmosphere.

Another method for producing SAF is through synthetic means, capturing carbon directly from the air. SAF is considered 'sustainable' because the raw feedstock does not compete with food crops or water supplies and does not contribute to forest degradation. SAF recycles the CO2 absorbed by the biomass used in the feedstock, in contrast to fossil fuels, which release previously locked-away carbon.

Biofuels

• Plant sources such as Jatropha, algae, tallows, waste oils, palm oil, Babassu, and Camelina.
• Animal sources.
• Solid biomass using pyrolysis processed with a Fischer-Tropsch process (FT-SPK).
• Wet wastes (manures, wastewater treatment sludge).
• Municipal solid waste streams.
• Dedicated energy crops.

There are some challenges associated with biofuels. For example, the production, processing, and transport of biofuels can emit greenhouse gases, reducing the overall emissions savings. Additionally, the availability of certain feedstocks, such as oils and fats crucial for Sustainable Aviation Fuel (SAF) production, may be limited as demand increases. Nevertheless, biofuels present a promising solution for reducing carbon emissions in the aviation industry.

Ethanol

However, there are challenges to using ethanol as an aviation fuel. One issue is the higher energy density of jet fuel compared to ethanol, which means that more ethanol is required to produce the same amount of SAF. Additionally, ethanol production has been associated with increased groundwater use, which could negatively impact one of the nation's most important resources. There are also concerns about the emissions associated with ethanol production, particularly the use of nitrogen fertilizers in corn production, which can volatize into nitrous oxide.

To address these challenges, researchers and policymakers are exploring ways to reduce the emissions and environmental impact of ethanol production. For example, the Biden Administration has proposed adopting a different model, called GREET, to calculate ethanol's emissions for SAF, which could reduce the emissions gap between ethanol SAF and the required threshold for tax incentives. Additionally, ethanol plants may need to invest in emissions reduction technologies, such as carbon capture and sequestration, to meet sustainability goals and qualify for tax credits.

Overall, ethanol has the potential to play a significant role in the future of aviation fuel, but further developments and investments are needed to ensure its long-term viability and sustainability.

Electric planes

The most common method of supplying electricity to electric planes is through batteries. However, other methods include solar cells, microwave energy beamed from a remote transmitter, and power cables connected to a ground-based electrical supply.

The first crewed free flight by an electrically powered plane, the MB-E1, was made in 1973. Since then, several companies have been working on developing electric aircraft. For example, in 2018, EasyJet announced it was developing an electric 180-seater plane for 2027 with Wright Electric. In 2024, Aura Aero rolled out its first prototype of an electric aircraft, the Integral E.

However, electric planes also face some challenges. The energy density of batteries is recognised as a bottleneck for zero-emission electric powertrains. Additionally, the weight of batteries can be a hindrance, especially for larger aircraft. Nevertheless, electric motors weigh less than their piston-engine counterparts, and electric motors do not lose power with altitude, unlike internal combustion engines.

Overall, electric planes show promise as a more sustainable alternative to traditional aviation fuel, but further advancements in technology are needed to address the current limitations.

Frequently asked questions