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Oct 29

Net Zero Aviation: A Review of Alternative Fuels


Green aircraft flying over biofuel tank

Authors: Dilan Singh, Tiffany Hong, and Kyle Chiu


Mentor: Jack Proudfoot. Jack is a doctoral candidate in the Department of Engineering at the University of Oxford specializing in Active Flow Control with joint funding from Rolls-Royce and EPSCRC.

 

Abstract

Aviation is a carbon-intensive form of transport, critical to the global economy. Currently, aviation is responsible for ~3% of global CO2 emissions. This share of emissions is predicted to rise as the global civil aviation market is expected to grow at a compound annual growth rate of 8.62% from 2023 to 2030. There is no single solution to decarbonizing this industry with intense research ongoing in sustainable aviation fuels, zero carbon (hydrogen) aircraft, and increased electrification. Transitioning to a net zero framework will involve complex technical developments and changes to the current economic, political, and behavioral status quo. We review technologies in development focusing on the alternative pathways to powering flight through a transition to sustainable aviation fuels, battery electric, or hydrogen.


Introduction

 

Undoubtedly aviation brings together people, businesses and communities. In 2018, the global aviation industry supported 87.7 million jobs, contributing an estimated $3.5 trillion to global GDP, amounting to 4.1% of total economic activity [1]. Aviation infrastructure contributes to economic growth by enabling more efficient transportation of goods and people [2]. Flying is a highly carbon-intensive form of transportation, accounting for approximately 3.5-4% of global warming potential emissions (when factoring in non-carbon dioxide effects).  Between 1960-2018 there has been a 6.8-fold increase in CO2 emissions primarily driven by increased demand [3]. Meanwhile, technological advances in aviation have reduced this carbon intensity, with a 54% reduction of carbon dioxide (CO2) per revenue passenger kilometer (RPK) relative to 1990 levels [4]. Such advances have complemented other industries too including; materials science [5], safety standards [6], computer simulation [7], GPS [8], and tourism [9].


Graph of CO2 per passenger kilometer
Fig 1. Aviation global operational efficiency since 1990 (reproduced from [4]).

According to the US Environmental Protection Agency [10], aviation greenhouse gases (GHG) comprise 9–12% of total U.S. transportation GHG emissions. Similar estimates exist for aviation's contribution to transportation emissions in the EU, standing at 13.9% in 2022 [11].


Meanwhile, the International Air Transport Association (IATA) reports an 11.6% growth in total RPK and an increase of 10.4% in the number of global passengers in 2024 compared to the previous year [12]. It is estimated that by 2050, over 10 billion passengers will fly around 22 trillion kilometers each year and without any changes in technology, fuels, practices or operations, aviation-related activities will generate close to 2,000 megatons (Mt) of CO2 and require over 620 Mt of Jet A-1 fuel [4]. An encouraging trend reported in the US aviation industry is a 9.7% decrease in GHG emissions since 1990 [10]. Considering future growth predictions, the current position of the aviation sector is unsustainable and untenable [13].

 

Recently, the International Civil Aviation Organization (ICAO) codified an international agreement on carbon reductions from aviation to achieve net zero emissions by 2050 [14]. This strengthened existing commitments agreed upon in the; 2015 Paris Agreement [15], the 2016 CORSIA scheme [16], and the 2018 International Maritime Organization Initial Greenhouse Gas Strategy [17]. Therefore, the aviation industry is at a turning point. Action must be taken now to meet the above targets despite passenger growth predicted to rise by 3.1% compounded annually [4, 12].

 

Sustainable aviation refers to the entire supply and consumer chains of the aviation industry to minimize environmental impacts while adhering to sustainable goals. Several pathways have been identified to achieve a sustainable aviation industry [4, 14, 15, 16, 17, 18, 19].

 

The pathways can be summarized as follows:

  1. Lower Greenhouse Gas Emissions: Developing and deploying technologies and best practices that lower the carbon footprint of air travel. The footprint includes combustion emissions; CO2, nitrogen oxides (NOx), soot, sulphate aerosols, and persistent contrail formation due to supersaturated water vapor.

  2. Improve Design Efficiency: Enhancing the energy efficiency of aircraft through aerodynamic design improvements.

  3. Reduce Acoustics: Developing new materials, engine technologies and operational strategies that minimize noise pollution around airports and during flight to reduce disruption to communities near airports.

  4. Unlock Socio-Economic Benefits: Aligning with global sustainability goals that not only address environmental issues but also promote social and economic benefits, ensuring that aviation contributes positively to the communities it serves.

  5. Increase Systems Efficiency: Improving air traffic management, more efficient flight routes, and operational procedures that reduce fuel burn and emissions.

  6. Minimize Waste: Implementing practices that reduce, reuse, and recycle waste generated from airline operations and passenger services.

  7. Increase Market Drivers: Implementing market forces to drive change such as legislation, standards, compliance targets, carbon capture and trading schemes, etc.


These steps establish a holistic approach to sustainable aviation, inclusive of environmental impacts, economic viability, and societal well-being. The work will involve collaboration among the commercial aerospace sector, governments, research institutes, fuel producers, and other stakeholders to support, introduce, and co-fund the deployment of new technologies, policies, and regulations.


Whilst efficiency improvements in both engine and aircraft aerodynamics are expected to reduce emissions by 17-28% by 2050 [19, 21], this is not enough to offset the predicted rise in overall emissions due to increasing demand. Therefore, our strategy to power flight will fundamentally have to shift away from fossil fuels (Jet A-1). To the authors' knowledge, there are three alternative pathways to powered flight which have viable pathways to offset a significant portion of aviation emissions. These are; sustainable aviation fuels, battery electric, and hydrogen-powered flight. A combination of all three pathways will likely play a role in achieving net zero in aviation by 2050.

 

In this paper, we review potential pathways to achieve net zero by 2050 through the introduction of these alternate power sources. We discuss each source in detail, highlighting technical and economic challenges to implementation. We outline critical pathways and deadlines that must be met to reach the aspirational goals. We consider each of the key goals and attempt to plot the roadmap on a temporal vs. technological framework to link series and/or parallel pathways to achieving the goal.


Literature Review

 

Technology Roadmaps

Significant global collaborative efforts have united researchers, industry leaders, and policymakers to formulate common language and measurement systems, problem definitions and solution strategies that can achieve net zero emissions by 2050.  Several key organizations have published reports outlining strategies to achieve industry emissions goals. These include:

 

●        “Waypoint 2050” strategy [4]

●        World Economic Forum: “Target True Zero” strategy [13]

●        IATA: “Fly New Zero” strategy [19]

●        European Council: “Fit-for-55” strategy [20]

●        EUROCONTROL: “Aviation Outlook 2050” strategy [21]

●        Aviation Impact Accelerator: “Five Years to Chart a New Future for Aviation” plan [22]

●        Airlines for Australia and NZ: “The Australian Roadmap for Sustainable Flying – Net Zero by 2050” strategy [23]

 

Each report emphasizes the importance of a holistic approach to meet the aviation sector goals for 2050, rather than focusing on any single technology or new fuel source. The introduction of Sustainable Aviation Fuels (SAFs) has been proposed as a relatively short-term replacement with significant potential GHG reductions. SAFs are hydrocarbon-based fuels made from renewable resources and non-fossil feedstocks, thus offering lower net lifetime CO2 compared to conventional aviation fuels. Alternative power sources include the development of battery/electric and hydrogen-powered aircraft. The figure below represents one of the proposed pathways to meet net-zero targets by 2050, highlighting the importance of multiple technology unlocks required to achieve our goals.


Net zero emissions pathways by 2050
Fig 2. Impact estimates to achieve net zero emissions by 2050 (reproduced from [23]).

Various studies have proposed to reach net zero targets via a combination of so-called “technology enablers”. Table 1 summarizes each enabler with predicted percentage reductions of GHGs towards reaching net zero by 2050.



Table 1. Summary of key global organizations that have charted roadmaps and estimated impacts to net zero emissions by 2050.

Technology Enabler

Solution Strategies

Impact to Achieve Net Zero by 2050


●      Reform SAF policy [22].

●      Source from feedstocks that do not degrade the environment or compete with food or water [19].

●      Develop synthetic production techniques to meet future demands without exceeding global biomass limits [22].

●      Continue the trend of incremental increase in the use of SAFs [21].

●      In 2022, over 300 million liters of SAF had been produced and used with a goal of over 5 billion liters in 2025 [19].

41% to 71% [20, 22, 3, 18]

Market Enablers

●      Introduce effective market-based measures, e.g., the European Emissions Trading Scheme [19].

●      Use approved offsets including carbon capture and storage technology [19].

●     Implement a legal obligation, via European climate law, to reduce EU emissions by 55% by 2030 [20].

6% to 32% [3, 22, 18, 20]

Design Enablers

●      Improve current aircraft design technology [19].

●      Radical new aerodynamic solutions [19].

●      More efficient engines and aircraft [21].

17% to 28% [20, 3, 22]

Systems Enablers

●  Continued improvement in infrastructure and operational efficiency, with a particular focus on improved air traffic management [19].

●      Halve aircraft fleet age to 15 years.

●      Fly slower.

●      Match aircraft to the designed range.

3% to 14% [21, 3, 20, 18]

Battery (Electric) Propulsion Enablers

●      Introducing revolutionary aircraft powered by fuel cells EUC [19].

●      Develop batteries with higher energy densities.

●      Enhance battery longevity and efficiency.

●      Ensure that batteries for electric aircraft are charged using renewable or low-carbon sources.

2% to 13% [13]

Hydrogen Enablers

●      Introduce revolutionary aircraft powered by hydrogen.

●      Promote the production of hydrogen through electrolysis using renewable electricity.

●      Reduce the weight of fuel cell systems.

●      Innovate in the design and materials of hydrogen storage tanks to decrease their weight.

2% to 13% [13, 19, 21]

Contrail Avoidance Enablers

●      Develop a global contrail avoidance system by 2030.

●      Develop strategies to manage or eliminate the formation of contrails from hydrogen and electric flights.

●      Teoh [24] emphasized that mitigation efforts for contrails are crucial, given their potential warming effect which could be on the same order of magnitude as CO2 emissions from aviation.

●      Potential reduction uncertainty is very large due to a lack of research.

-18% to 81% [13] [22]

Other Enablers

●      Launch high-reward (“Moonshot”) technology demonstrations to develop scalable, transformative technologies by 2030. Examples include; cryogenic hydrogen, methane fuels, hydrogen-electric propulsion, or synthetic biology to dramatically lower the energy demands of fuel production [22].

12%-34% [4]


An important note is that each of the identified technology enablers carry various degrees of uncertainty in their respective likelihoods to reach market viability in a suitable time. For example, design enablers may be easier to research than to implement due to long product life cycles. Contrail research is in its infancy, therefore there is a large degree of uncertainty on their impact on overall Earth’s reflectivity. While it may still be too early, it is also important to note at this stage of our review, none of the technology enablers have yet factored any social impacts from the changes that these will create. There is uncertainty about whether society will trust new designs and fuels. Future innovations will certainly drive up the cost of flying [25], this is likely to be passed on to customers who may be unwilling to absorb increased ticket prices.


When positioned into a global roadmap for the aviation sector, the pathway to 2050 is more complex than a single-axis timeline. It is important to understand where each technology enabler lies in terms of their respective time frames for readiness. Each of these have barriers to implementation ranging from; technological, social, environmental, economic and political. Therefore, it would be useful to attempt to represent them in a 2D approach including factors for impact and readiness. This is done in Figure 3. With this representation, it would then be possible to construct a pathway that includes various combinations of technology enablers or a critical pathway that only links a few of the enablers. The goal is to recommend an optimum mix of global resources to be allocated to pathways with the highest probability of success in meeting 2050 goals.

 

The underlying assumptions for each technology enabler are detailed in the Appendix. The factors for readiness are broadly defined as:

 

A. Deployment speed: this factor ranges from 0-50 years and measures the likely time frame that the factor will be in production or implemented into a product or practice.

B. P(Research success): this is a qualitative measure of the probable delivery of the required science, given the level of collaborative university and industry research, government or organizational support, and is intended to be an overarching estimate of this parameter, rather than a quantitative assessment of any current or planned global research program.

C. P(Deployment success): this is a qualitative measure of the probable uptake of the science or technology by industry, and is intended to be an overarching estimate of this parameter, rather than a quantitative forecast of any current or planned uptake program (e.g. new fleet investments, new energy plants etc).

 

The factors for impact are:


D. Impact on achieving the 2050 target: this is a more qualitative factor, based on the cited literature already summarized in the above table.

E. Investment: this is a semi-quantitative factor, based on various investment scales (national and global) discussed in the cited literature already summarized in the above table.

F. Ripple effects: this is a semi-quantitative factor which indicates any flow-on (ripple) effects into other national or global sectors outside aviation (e.g. automotive, energy, communications, retail, etc).

 

The details behind figure 3’s metrics are included in the Appendix with relevant citations for each assertion.


Graph of the impact versus readiness for technological enablers of net zero
Fig 3. Roadmap showing impact vs. readiness factors for technological enablers in achieving net zero goals.

Ultimately, it is not the exact positioning that matters in our attempt to plot a 2D roadmap to 2050, but rather, the relative positioning. SAFs are positioned as the technology which is most ready with high impact potential. However, there are two distinct groupings:

●     HRLI -  high readiness, low impact enablers (i.e. Design, Market, Systems and Contrails) and

●        LRHI -  low readiness, high impact enablers (i.e. Battery, Hydrogen and Other)

 

The cumulative impact of HRLI has significant potential to match SAFs and this can assist research, industry and government organizations with prioritization of investments and risks. This assessment may be updated regularly as new technological innovations or international collaborating programs emerge.


Each factor is given an equal weighting, this is likely overly simplified, and could certainly be adjusted. Similarly, each factor has an uncertainty range that will need to be accounted for when deciding upon optimal investment pathways. Future work could include a deeper investigation into each factor to understand their relative influences on their overall positions and potentials to reach net zero by 2050.

 

The scope of this report herein focuses only on SAF, Electric and Hydrogen propulsion fuels, due to these being positioned as having the highest impacts on achieving 2050 targets in Figure 3.

 

Sustainable Aviation Fuels

The combustion of aviation fuels emits GHGs such as nitrous oxides (NOx), CO2, carbon oxides, sulphur oxides, and soot [26]. A key technology to reduce emissions is the introduction of SAFs replacing conventional fossil/petroleum-derived aviation kerosene (Jet A-1) and aviation gasoline.[1]. SAFs are advantageous over Jet A-1 because they are a non-fossil-derived aviation fuel. SAFs may use the same supply chain and refuelling infrastructure offering significant economic benefits over alternative fuel sources [27]. SAFs have a gravimetric energy density around 10 times higher than lithium-ion batteries and six times higher than compressed hydrogen, both of which currently require significant additional space in the aircraft [28].


Furthermore, SAFs are certified by the American Society for Testing Materials, ASTM D7566, which can be safely mixed with Jet A-1 to varying degrees [29]. These standards ensure that biofuels meet the necessary criteria for use in existing aircraft engines, including fuel stability, low freezing points, and compatibility with conventional jet fuels​.


ICAO [30] has established sustainability criteria that SAFs must meet including;  an overall reduction in lifecycle carbon emissions, limited use of land and freshwater, no competition with required food production, no damage to the environment, and no impacts on deforestation.


SAFs have the potential to reduce 80% of commercial aviation CO2 [19], offering significant potential to reduce the aviation industry's carbon footprint [31]. Unlike fossil fuels, which release carbon that has been sequestered for long periods, SAFs utilize carbon that has been recently captured by biomass, effectively recycling it and creating a closed loop. In 2022, over 300 million liters of SAF had been produced and used with a goal of over 5 billion liters in 2025 [19].


SAFs take two distinct forms. The first are biofuels which are not uncommon, with the first test flight with biofuels operated by Virgin Atlantic in 2008, using a 20% blend [32]. A timeline showing the gradual introduction of SAFs in the aviation industry is shown in Figure 4. In 2016, United became the first airline to introduce SAF into normal business operations by commencing daily flights [19]. The first commercial demonstration flight using 100% biofuels flew in December 2021 [33]. Rolls-Royce recently became the first jet engine manufacturer to publicly confirm all in-production engines for long-haul aircraft and business jets are compatible with 100% SAF blend and this was demonstrated in a transatlantic Virgin Atlantic flight [34]. To date, 787,159 commercial flights have operated using SAF since 2011 [35].


Timeline of first uses of sustainable aviation fuel flights
Fig 4. Timeline of first uses of SAF flights [32, 33].

Currently, SAFs offer the greatest readiness to substitute fossil-based jet fuel thanks to similar chemical and physical properties to Jet A-1. The supply and production of SAFs is limited by the ability to scale production plants. SAF blendstock is currently transported by truck, rail, or barge from stand-alone biorefineries. At present, 100% SAF blend stock (without ASTM D1655 approval) is not approved for transportation via petroleum pipelines [36].


Still in its infancy, SAFs amount to 0.05% of total EU aviation fuel use in 2022 [37]. Major implementation challenges include scaling up production whilst reducing costs to an economically viable level for widespread use in the industry.

 

A SAF grand challenge [38] has been launched by DOE to meet the following production levels:


●        A minimum of a 50% reduction in life cycle greenhouse gas emissions compared to conventional fuel.

●        3 billion gallons (11.3 billion liters) per year of domestic SAF by 2030.

●        35 billion gallons (132 billion liters) of SAF to satisfy 100% of domestic demand by 2050.


It is estimated that aviation will need between 330-445 million tons of SAF per annum by 2050 [4]. An EU initiative, ReFuelEU, has mandated a gradual ramp-up of SAF blending from 2% by volume in 2025 to 70% by 2050 [39].  EU legislation on renewable energy is aiming to increase the share of renewables to 42.5% by 2030, with an aspirational target of 45% [40].


The second form SAFs may take are electrofuels (i.e. produced directly from CO2 and Hydrogen) which are different to biofuels (i.e. made from plant matter). To date, the most promising SAF conversion pathways [41] are:


●        Power to Liquids (PtL)

●        Hydroprocessed Esters and Fatty Acids (HEFA)

●        Advanced Biomass to Liquids (ABtL)

 

A summary of different SAF electrofuel conversion processes is provided in figure 5.



Figure of sustainable aviation fuel pathways
Fig 5. Overview of SAF conversion pathways [41]

Power to Liquid processes involve using renewable electricity to decompose water molecules into oxygen and hydrogen (via electrolysis [42]) and combine the latter with a carbon source (e.g. non-crop-based biomass or carbon captured from the atmosphere) [43]. This gas is then converted into a synthetic oil via the Fischer–Tropsch (F-T) process, before being refined into SAFs. The F-T process is a chemical reaction that turns a mixture of carbon monoxide and hydrogen gas into liquid hydrocarbons usable for jet fuel [44]. F-T fuels are known for their cleaner burning characteristics compared to conventional fossil fuels. While F-T fuels themselves are non-toxic and produce fewer pollutants, combustion still produces CO2 and NOx. They have a high cetane number (beneficial for rapid ignition after injection into a combustion chamber), have reduced particulate emission, and produce low sulphur and aromatics [26]. Recently, hydroformylated Fischer–Tropsch (HyFiT) fuels comprising alkane–alcohol blends have been presented as a sustainable option for heavy-duty transportation [45]. The study shows that HyFiT fuels can lower particulate matter and NOx emissions and life-cycle assessments demonstrate their potential to complement electrification in heavy-duty transportation sectors. The two most important factors affecting the production cost of all PtL fuels are the electricity price and capital cost of the electrolyzer (for the hydrogen production cost) [46].


PtL fuels are considered sustainable because their feedstocks do not strain or compete with food production or the ecosystem. Furthermore, they do not contribute to environmental issues like deforestation, soil productivity loss, or reduction in biodiversity. Green hydrogen feedstock (i.e. hydrogen produced from renewable energy sources) can be produced in large quantities from wind and solar energy, which is advantageous for countries already engaged in or have favorable conditions for renewable energy. However, due to the need for large amounts of green hydrogen feedstock, the renewable power required to produce SAF via the F-T process is significant. For example, in the UK, producing enough SAFs to satisfy expected demand through PtL would require 5 to 8 times the UK's renewable electricity generation from 2020 levels [47].


The F-T process is already certified for use up to a 50% blending ratio [29]. PtL has the potential to reduce 89%-94% GHG emissions compared with Jet A-1 fuel [41]. It is understood that PtL is dependent upon the availability of low/zero greenhouse gas (GHG) electricity to produce SAFs with a low carbon footprint.


Shahriar and Khanal [27] describe Hydroprocessed Esters and Fatty Acids (HEFA) are currently the most economically viable pathway for producing SAFs, whilst the F-T is proven in lab-scale production, it is yet to be scaled to economically viable levels. HEFA is attractive because of the relative simplicity of converting feedstocks into fuel and is an established technology, it is constrained by limited feedstock availability.


​Waste products are derived from animal fats, recycled greases, plant oils, and agricultural residues [48]. These biofuels are characterized by their biogenic hydrocarbon base. HEFA is distinguished by its lack of sulphur and aromatics, along with a high cetane number, qualifying it as a “drop-in” fuel. HEFA fuels offer a 74%-84% GHG emissions reduction compared with Jet A-1 fuel [41].

 

However, some challenges in HEFA production are securing a consistent supply of appropriate and sustainable feedstocks, impacts on edible oil prices, land use, and possible environmental stress [49]. Tenenbaum [50] emphasizes the use of second and third-generation feedstocks, which do not compete with food crops and re-triggers the “fuel vs. food” debate, as crucial for the sustainability of SAF production. The report also suggests co-processing and keeping supply chains close to crude oil lines to share facilities and infrastructure as a practical means to ultimately keep SAF capital and operational costs low.

 

HEFA is expected to contribute to 6%-8% of the total SAF capacity required by 2050 due to limitations in the availability of suitable feedstocks [51]. HEFA was certified in 2011 by ASTM D7566 and is currently approved for a 50% blending ratio [29]. Usage is expected to grow until feedstock limitations become evident in several years [41]. Therefore, future predictions require large investments in F-T and ABtL processes to satisfy predicted demand in SAFs, as shown in Fig 6.

 

ABtL fuels are derived from a pathway that converts biomass and municipal solid waste into biofuels. Biomass includes organic materials including; crops, agricultural waste, and forest residues. It is considered renewable because plants and organic waste can be replenished over reasonable timeframes, unlike fossil fuels. ABtL produced via F-T is certified to be blended with up to 50% conventional jet fuel [29].


An advantage of ABtL is the variety of biomass inputs that can be used. An alternative to the F-T processing route is the alcohol-to-jet process (AtJ), where sugar-rich or lignocellulosic biomass feedstocks are converted into alcohols [52]. The AtJ process is also certified for a 50% blending ratio [29]. Ethanol to Jet (ETJ) and Sugar to Jet (STJ) processes were reviewed and it was found that STJ could generate well-to-wake (WTW) GHG emissions 59% below those of Jet A-1, while ETJ results were 73% below those of Jet A-1 [53]. ABtL promotes between 66%-94% GHG emissions reductions compared with Jet A-1 fuel [41].

 

Waypoint 2050 [4] reports there is enough feedstock available to meet the future demand for SAF, but the challenge lies in scaling up production and ensuring sustainability. However, SAF production needs to increase significantly—from around 0.05 million tons in 2021 to 330–445 million tons by 2050—to meet the aviation industry's net-zero goals. This increase is feasible if supported by substantial investment, policy changes, and technological advancements. Advanced and waste feedstock alone (for HEFA, ABtL, F-T) could supply almost 500 Mt of SAF per year, while 2030 demand for Jet A-1 is projected to be 410 Mt [54].

 

To date, over 42 million tons of SAF offtake agreements have been signed, with United Airlines consisting of a quarter of all such agreements [55]. The largest agreement to date was implemented by DHL [56]. Forecasts for the EU alone in 2050 reveal HEFA will supply 15% (7 million tons), ABtL will supply 30% (14.5 million tons) and PtL will initially supply 1 million tons by 2030. With production ramping up to between 35-50 million tons annually [41]. Figure 6 shows a scenario by the International Energy Agency Net Zero Pathway [41, 58].


Graph of forecast of sustainable aviation fuel usage
Fig 6. Forecast combined share of fuels [41, 57].

Biofuel supply can be categorized into five generations depending on the feedstock used for their production [58]:


●        First-generation biofuels are made from edible crops such as sugarcane, corn, soybeans, and other food crops. While they provide a renewable energy source, they compete with food crops, raising concerns about food security and land use.

●        In contrast, second-generation biofuels are produced from non-food sources like agricultural waste, algae, and switchgrass. These biofuels aim to reduce competition with food crops and are generally considered more sustainable.

●        The third-generation biofuels are produced from sewage sludge, municipal solid wastes, algae, and other microorganisms that can be grown on non-arable land. These biofuels are still in the developmental phase but offer high efficiency and sustainability potential.

●        Fourth-generation biofuels use genetically modified organisms to convert sunlight and CO2 directly into biofuels. This generation focuses on more advanced biotechnological methods and is highly sustainable.

●     Fifth-generation biofuels are currently in research and development, which aim to use synthetic biology to produce completely synthetic biofuels, potentially offering even more efficient and sustainable alternatives to fossil fuels.


Each generation represents a step forward in sustainability and a shift away from food-based sources to advanced methods that aim to reduce greenhouse gas emissions and dependence on fossil fuels. Feedstocks such as camilina, jatropha, algae, wastes and halophytes do not create biodiversity risks as they grow in underutilized and non-arable regions [26]. It is important to ensure that the combustion of biofuels does not contribute to higher net CO2 output. Rapid replanting, regrowth and harvesting can ensure this as shown by Abdudeen et al. [59]. According to the U.S. Department of Agriculture, the potential biomass annual supply could be as much as 1.37 billion dry tons per year, enough to produce biofuels to meet more than one-third of the current US demand for transportation fuels [60]. An optimal SAF production route remains a challenge as many aspects including production cost, GHG footprint and sustainable feedstock supply have not yet been resolved globally. We have shown that any viable pathway will likely require a blend of all sources discussed above due to a range of economic viability and scalability challenges.


Figure 7 shows that bio-feedstock availability worldwide is substantial but constrained by several factors. The total bio-feedstock available for aviation is projected to be around 20 ExaJoules (EJ) per year, with additional energy from waste gases and power-to-liquid (PtL) pathways augmenting this. When comparing bio-availability with the estimated feedstock requirements, aviation alone will not be able to access all feedstock supply, with other industries also demanding a share of the total supply. Figure 6 shows that biogenic SAF will be utilized in the short-medium term while PtL will need to ramp up to meet long-term demand. It is estimated that all SAF approaches could require up to 9% of global renewable electricity and up to 30% of sustainably available biomass in 2050 [61]. The availability of feedstocks like bio-waste, agricultural residues, and renewable electricity for PtL processes can meet aviation demand, but only if prioritized and supported by strong government policy.


Fuel feedstock availability by region
Fig 7. Feedstock availability by region (ExaJoules) (reproduced from [4])

Scaling demand and production levels will be a significant challenge. Up to USD 1.45 trillion worth of investment over the next 30 years will be required to develop a fully vertical SAF and energy system [4]. This investment is aimed at building around 5,000 to 7,000 facilities globally to produce enough SAF to meet the aviation industry's climate goals by 2050. With support from governments and the energy sector, this could be achievable, it is the equivalent of around 6% of typical oil and gas capital expenditure [4].

 

The production of SAF is currently significantly more costly than the production of Jet A-1 fuel. Taking Jet A-1 as a baseline at USD $1000 per ton, HEFA is the most cost-effective pathway to creating SAF to date at around USD $1500 per ton, followed by ABtL which comes with a 25% premium compared to HEFA due to limited uptake and associated economies of scale [41]. The global production cost for PtL is estimated at USD 3000 per ton by 2030, decreasing to around USD $1500 per ton by 2050 [41, 55]. The cost of producing green hydrogen is the biggest contributor to the PtL pathway, with renewable power generation accounting for 60-80% of the total cost.

 

Martinez-Valencia et al. [62] reported the state of the SAF supply chain and proposed methodologies for incorporating broader economic and environmental benefits into supply chain strategies to help mitigate the supply, capital investment and operating cost barriers to entry. Nevertheless, PtL is still projected to be 2-4 times more expensive than historical Jet A-1 prices [63]. Efforts by airlines to decarbonize are predicted to increase operating costs by up to 18% in 2050, this is likely to be passed on to the customer [63, 64]. Evidence of this approach can already be seen with Lufthansa introducing a levy for SAF on ticket prices from January 1, 2025 [65]. An ICAO report [25] estimates the potential impacts on airline ticket prices to be between 1.5 to 3.3 times by 2050 due to net zero efforts (including the introduction of hydrogen and other fuel sources). This will inevitably lead to a reduction in passenger demand from current predictions. A 2023 report by the Sustainable Aviation Alliance predicts a 14% reduction in emissions due to lower demand driven by the rising cost to fly by 2050 [66]. Figure 8 breaks down the multitude of factors which contribute to the costs of producing SAFS.


Figure of factors affecting the economics of sustainable aviation fuel
Fig 8. Factors that affect the economics of SAF (reproduced from [62]).

Battery Electric

 Battery electric aircraft, recognized by its availability to run aircraft without combustion, has been gradually influencing the sustainable aviation market/industry. Due to current limitations of battery technology in terms of gravimetric energy density, cost efficiency, and cycle life, electric aircraft may only operate short-range flights with fewer passenger seats available, primarily targeting regional markets. Current state-of-the-art Lithium-ion (Li-ion) battery packs are capable of achieving a gravimetric energy density of 200-300 Wh/kg, which would be capable of powering electric air taxis with 1–4 passengers over a 100 km range flight [67]. If an aircraft already in service is to be converted to electric flight, further modifications to reduce weight are needed to make a design economically and operationally viable. Therefore, it is optimal to start with a clean sheet design, with an airframe optimized for electric flight. This inevitably comes with increased cost to the manufacturer.

 

The Breguet range equations (Eqs. 1 & 2) are used to predict an aircraft's maximum range. Depending on if the aircraft is battery-electric or utilizes combustion, the equations differ. When using combustion, the aircraft fuel is burnt off reducing the flying mass over time. Batteries do not gain this benefit, reducing the overall range.

The Breguet range equations

The same is seen when comparing the energy required per revenue passenger kilometer for jet engine aircraft (JEA) and all-electric aircraft (AEA) [67]. Despite roughly double the total efficiency from well to wake of electric aircraft, the higher weight factor and lack of weight reduction throughout the flight results in an approximate increase of 50-100% energy intensity per RPK for AEA[2].

enEquations for energy required per revenue passenger kilometer for jet engine aircraft and all-electric aircraft

Comparing characteristics between conceptual energy density and “real-life” energy density will help in recognizing places for improvement. Figure 9 visualizes several factors, such as industrial features and base savings, which need consideration for flights to operate safely. The following requirements show that it is essential to develop batteries with higher gravimetric energy density if electric flight is to become viable.


Conceptual representation and “real life” representation of battery efficiency
Fig 9. Conceptual representation and “in real life” representation of battery efficiency (reproduced from [68])

Li-ion batteries are a rechargeable power source widely used across industries. Based on both theoretical value and commercially obtained value, it's assumed that a typical Li-ion battery has a gravimetric energy density ranging between 200-300 Wh/kg and an approximate volumetric energy density of 650 Wh/L; these corresponding values are expected to increase around 500 Wh/kg and 1000 Wh/L in the upcoming future [69]. Compared to Jet A-1 this is over an order of magnitude smaller for both gravimetric and volumetric energy density. Other battery technologies, such as lithium-sulphur (Li-S) and solid-state (SS) batteries, offer the potential for higher gravimetric energy density; lithium-ion remains highly relevant due to it being a proven technology.

 

To date, only Li-ion technology has been demonstrated in any form of electric aviation, whilst Li-S and SS are still in research and development at lab demonstrator levels. As shown in Table 2, Li-ion batteries can be used for a maximum of 2,000 cycles, which is significantly more compared to proposed high-energy-density batteries. Cycles are another important characteristic of batteries which determines their potential/efficiency for real-life usage. If low, then packs would require constant replacement which is both environmentally damaging and economically inefficient. At the same time, there are several challenges to the implementation of Li-ion batteries due to safety concerns, including thermal runaway of a damaged pack [70]. Li-ion batteries perform poorly at low temperatures [71, 72, 73] which is a challenge when operating at high altitudes. Even if lithium-ion batteries become less ideal options for aircraft operation in the future, they provide the industry with a temporary solution until other viable battery technologies emerge.


Table 2. Battery technology outlook for aviation (reproduced from [86])

 

Lithium-ion (Li-ion)

Advanced Li-ion

Solid-state battery

Lithium-sulphur (Li-S)

Cathode (+)

Lithium metal oxide such as

LFP, LMO, Li-NMC

Li-NCA [74]

Lithium metal oxide

with Ni-rich fraction

(e.g. Li-NMC811) [75, 76, 77]

Lithium metal oxide With Ni-rich fraction (e.g. Li-NMC811) [75, 76, 77]

Sulphur

Graphene

Acetylene black [77, 78, 79]

 

Anode (-)

Graphite (with silicon)

Lithium metal or Silicon [76, 80]

Graphite Silicon [74]

Lithium metal Graphite [76, 81]

Electrolyte

Organic liquid (e.g. lithium salt-LiPF6) [74]

Organic liquid

(e.g. lithium salt-LiPF6) [74]

Inorganic solid

(e.g. Li10SnP2S12)

Organic solid

(e.g. polycarbonate) [76]

Organic liquid

(e.g. LiN(SO2CF3)2) [74, 76]

Gravimetric (Wh kg-1) (MJ kg-1)

300 [82]

(1.08)

450 [82]

(1.62)

400-500 [83]

(1.44-1.80)

300-400 [82]

(1.08-1.44)

Volumetric (Wh L-1) (MJ L-1)

700 [82]

(2.52)

1,200 [82]

(4.32)

Not specified

400 [82]

(1.44)

Lifetime (cycle)

1,000-2,000 [84]

1,000

500 [83]

-1,000 [82]

100 [82]

Development status

Commercial scale (TRL 9) [82]

Demonstration scale (TRL 7) [82]

Technology validation Small-scale prototype

(TRL 3-4) [82]

Small-scale prototype

(TRL 4) [82]

Lithium-sulphur (Li-S) batteries, having a relatively high theoretical energy density of  400-500 Wh/kg [86, 82], could meet requirements to operate commercial routes with electric aircraft with adequate range for intra-continental flights (300-400 km). However, further advancements are needed to make them practical for commercial aviation. Li-S batteries face internal chemical challenges, such as the shuttle effect, where dissolved polysulfides migrate between the anode and cathode. This movement causes the loss of active sulphur material, leading to capacity decline, self-discharge, and ultimately poor cycling stability, which reduces the battery lifespan to approximately 100 cycles [82].

 

In contrast, table 3 shows that future urban air transport eVTOL requires batteries capable of at least 500 cycles. This gap is significant, as long-range aircraft require batteries with higher cycle life. Longer-lasting batteries would ensure consistent performance and reliability over extended periods, as longer flights put more strain on battery capacity and cycle life. Rapid battery replacement also reduces the sustainability of electric aircraft. Without resolving these issues, Li-S batteries, despite their high energy density, would remain unsuitable for aircraft. If technical issues are addressed, Li-S technology could enable longer-range electric flights, supporting both urban and regional air mobility in sustainable aviation.


Table 3. Illustrative schematic demonstrating main markets suitable for Li-S technology now and in the future (reproduced from [88])

Future commercial vehicles [88]

Maritime

Aviation

Heavy EVs

Future urban air transport

Examples

AUVs2

Electric aircraft

eBuses, eTrucks 4

eVTOL

Required Egrav

>400 Wh kg−1

>300 Wh kg−1

>400 Wh kg−1

>400 Wh kg-1

Cycle life

60–200 cycles

500–1000 cycles

1000 cycles

500 cycles

Environmental requirements

Low temperature (4 °C)

 

High pressure (45 MPa)

−10–60 °C

−10–60 °C

−10–60 °C

Main remaining challenges

-

Cycle life (>500)

 

Safety regulations

Cycle life (>1000)

 

Safety regulations

Fast discharge rate

 

Cycle life (>500)

 

Enhanced thermal management.

Solid State Batteries (SSB) are in the early stage of research, having shown potential characteristics which encourage further development. Gravimetric energy density is potentially very high at 400-500 Wh/kg. Lab tests have shown SS batteries are capable of achieving a 500-1000 cycle long life span. SS batteries offer an attractive set of capabilities to be used in short-range flight applications [82, 83].


In terms of safety characteristics, SSBs have a strong safety profile due to the inherent properties of solid electrolytes and enhance safety by lowering risks of flammability (thermal stability over 100 ºC), reduced likelihood of thermal runaway, and leakage. The absence of flammable liquid reduces the risk of fire and thermal runaway, making SSBs significantly safer in environments where heat and physical impact are concerned, such as aviation. However, challenges remain in lowering costs, poor environmental stability, and managing dendrite growth, which is the formation of needle-like lithium structures that can penetrate the electrolyte over repeated charge cycles which leads to potential internal short circuits [69]. Additionally, solid electrolyte durability is an area of ongoing research, as these materials can degrade with use, affecting battery performance and safety.


For future applications, achieving higher durability in solid electrolytes, lowering manufacturing costs, and improving resistance to dendrite formation will be critical. With advancements, SSBs could offer a sustainable, safer, and more efficient power source for short-range flight, aligning with the growing demand for sustainable aviation technologies.


Despite limitations in low lifespan, Li-ion batteries and Li-S batteries currently have the highest potential for sustained usage due to their reliable performance over repeated use. They are expected to continue replacing regional flights and to support growth in urban aviation. Although energy density is still lower than some sustainable aviation alternatives, lithium-ion batteries' stable cycle efficiency and cycle life offer promise for gradually replacing fuel-powered aircraft on longer routes and carrying more passengers if current technological challenges are overcome. Meanwhile, Li-S batteries have more promising performance to replace jet aircraft which operate in further range, this will happen if cycle limitations are overcome.


If commercially usable battery packs can advance up to an energy density of 800 Wh/kg, the electric aircraft will be able to operate within a range of 600 nautical miles (1,111 km), which can replace approximately 15% of commercial aircraft fuel use and eliminate around 40% of global landing-and-take-off-related NOx emission [89]. Figure 10 demonstrates this by breaking down global commercial aviation by distance of flights and proportion of departures, NOx, RPK, and fuel usage. Small changes can be accumulated to eventually contribute a noticeable amount towards sustainable aviation.


Break down of global commercial aviation by distance of flights and proportion of departures
Fig 10. Cumulative distributions of key operational variables by the global commercial aircraft fleet in 2015 [67]

In terms of cost of production, all battery types discussed above are expected to reduce. With ongoing investigation towards scaling up the production of batteries, growth of demand in battery-powered electric aircraft, advances in materials and manufacturing, and development of recycling and resource recovery, an increasing trend of battery costs is very unlikely [69]. For instance, a Li-ion battery pack currently has an approximate cost of 137 U.S. dollars per kilowatt hour in 2020 and is expected to decrease further due to mass production [90]. This downward trend in costs is likely to encourage the continuous use of battery-powered electric aircraft, making sustainable technologies more affordable and practical. Despite weaker capabilities, batteries have the potential to have an important impact on global energy/aviation markets and environmental policies.


Hydrogen

While there are various methods of producing hydrogen, blue hydrogen and green hydrogen are the two most common. Blue hydrogen is created using natural gas through steam reforming [13]. Water electrolysis is the primary process that produces green hydrogen [13], with biomass gasification serving as an alternative [91]. Although both eliminate in-flight carbon emissions when powering aircraft, blue hydrogen has higher net carbon emissions due to release of CO2 and methane during production. Hydrogen conversion efficiencies for fossil fuels and biomass are estimated to be around 50% and 40% respectively [91].


Electrolysis is used to extract hydrogen from water using electricity. Anodes and cathodes are submerged in electrolyte solutions, such as NaOH and KOH, which split the water into hydrogen and oxygen. Hydrogen is generated at the cathode while oxygen is created at the anode [94]. However, for electrolysis to be carbon-free, renewable energy sources such as wind and solar must be used to generate electricity. Mature water electrolysis technologies include alkaline water electrolysis, which consumes 3.8 to 5.4 kWh of energy per cubic meter of hydrogen gas, and proton exchange membrane (PEM) electrolysis, which uses between 4.3 and 5.2 kWh of energy per cubic meter of hydrogen gas. Currently, around 4% of hydrogen is produced by electrolysis [95]. A key issue slowing the further expansion of electrolysis technology is cost. Alkaline electrolysis costs USD $500-1000 per kW and PEM costs USD $700-1400 per kW in upfront costs. Moreover, alkaline electrolysis has a low current density and PEMs have inefficient membranes, both requiring further development [95].

 

In addition, biomass gasification is another method for generating green hydrogen. Biomass is converted into synthetic gases through thermochemical conversion [94]. This process has three main steps: gasification, gas cleaning, and hydrogen separation. One current gasification technology is the dual fluidized bed (DFB) system. This system separates the gasification process into two parts, a steam gasification steam reactor and a combustion reactor that provides the necessary heat for gasification. Typically, 35-45% of gas produced by a DFB system is hydrogen. Absorption-enhanced reforming is the most efficient biomass gasification process, with a hydrogen output of 75%. AERs are similar to a dual fluidized bed system but produce higher amounts of hydrogen by removing carbon dioxide from the gasification reactor. [91]

 

When stored at ambient temperature and pressure, hydrogen has poor volumetric (120 MJ/kg) but excellent gravimetric energy (0.01 MJ/L) densities [96].  An ideal hydrogen storage system for aviation should maximize volumetric energy density. A high volumetric density allows more fuel to be stored within the limited space of aircraft without expanding the fuselage or removing passenger space. This is made challenging as hydrogen may not be stored in the wings of aircraft (like current designs do for Jet A-1), due to the shape requirements for pressurized hydrogen tanks.

 

Compressed gas storage is a common hydrogen storage technology. Hydrogen is compressed to pressures of 350 - 700 bar. The compressed gas density rises, reducing the required storage space. Figure 11 shows the density of hydrogen stored at different temperatures and pressures. The goal is to achieve the highest possible density whilst being economically and technically viable. This would seem to indicate that liquid storage would be most suitable unless the technical challenges involved in cryo-compressed storage are overcome.


Hydrogen density as a function of pressure and temperature by storage method
Fig 11. Hydrogen density as a function of pressure and temperature for different storage methods (reproduced from: [97])

Moreover, strong materials, such as steel, aluminum, or select composites, must be utilized in the tank to withstand the high pressure [94]. Currently, Aluminum 6061 or 7061 and stainless steel are used for metal parts (due to the embrittlement risk of other metal alloys) and carbon fiber-reinforced polymers are commonly used composites. Four types of compressed hydrogen tanks are in use. The most used type I design is a full steel cylinder. Type II tanks reinforce the steel cylinder with composite-covered steel or aluminum liners. In Type III tanks, composites are used for the cylinder, with liners covering the entire tank. Type IV tanks are made of carbon fibers and polymers [98]. As shown in table 4, each design has varying gravimetric and volumetric densities as well as storage pressure:


Table 4. Storage pressure, gravimetric, and volumetric densities of four types of hydrogen tanks (reproduced from [98])

Type

Gravimetric Capacity

Max Storage Pressure (bar)

Volumetric Density (MJ/L)

I

1.1

200

1.4

II

2.1

300

2.9

III

4.21

700

-

IV

5.7

700

4.9

When in contact with hydrogen, certain metal alloys can corrode or embrittle. The Type IV avoids this issue but faces the problem of permeation, a process where hydrogen penetrates the fibers of the tank. Although the Type IV is the most efficient storage system in terms of volume and weight, it is also the most expensive, costing 633 USD/kg of hydrogen. Carbon fibers make up 50-70% of the cost, so decreasing the amount of fiber used is necessary to reduce cost [98].

 

Liquid hydrogen storage requires decreasing the tank's temperature to -253°C. Tanks are designed to hold low-temperature hydrogen but are usually unable to sustain large amounts of pressure. Thus, the tanks must be properly insulated to reduce heat transfer and hydrogen must be allowed to vent through a relief valve, both elements of maintaining low pressure [96]. Venting causes the tank to lose storage efficiency as hydrogen is lost before energy is extracted. Generally, larger hydrogen tanks correspond with lower operating costs due to their higher ratio of hydrogen volume to insulation volume. A high volume-to-surface area ratio is also necessary to reduce heat transfer and hydrogen venting.

 

Hydrogen can also be stored in other materials through chemical reactions. Materials such as ammonia and metal hydrides are suitable for this purpose [99]. Chemical storage offers higher energy density and can be reused, but requires a catalyst to speed up the time-consuming process of releasing hydrogen. (role of hydrogen). Metal hydrides are chemical compounds capable of absorbing and releasing hydrogen. However, metal hydrides need temperatures of 120-200 degrees Celsius to release stored hydrogen. NaAlH, AlH3, LiBH4, Mg(BH4)2, Li2NH, Li3NH, LiAlH4, MgH2, and NaBH4 are some hydrides able to reach 9 wt % gravimetric density [99].


Conclusion - Net Zero Aviation

 

Our review has confirmed and consolidated eight key technology-enabling pathways that can currently achieve ICAO’s target for zero emissions by 2050. While the ultimate solution will likely require a combination of pathways, depending on impact and technology readiness levels, SAFs have the most significant role to play in achieving this (up to 71%). SAFs also have the potential to reduce CO2 emissions by up to 80% compared with current Jet A-1 fuel. The key reason is from a life cycle viewpoint where SAF processes rotate the CO2 cycle using biofuel or electrofuel feedstock already present and being produced on Earth, rather than releasing sequestered CO2 that has been stored in the Earth for millennia.


SAFs are already in use today by airlines and supply logistics are improving at most major airports. Governments are successfully encouraging more growth via new policies, regulations, and investments. Many challenges still exist including implementation and scaling in an economically viable way for SAF to be used as predicted to keep up with the future demand for flying. SAFs not only have the highest impact on GHG emissions but they can also be manufactured through several processes, including F-T. Food sources and waste are not a long-term feedstock, so PtL will be the inevitable method for producing SAFs.


Currently, SAFs are expensive to produce, and until the costs come down, the added cost will likely be passed onto consumers. SAF research, development and deployment outlooks are promising, albeit still in its infancy stages. However, with ongoing commitments from governments and other organizations to reduce the initial high-cost barrier into an economically viable one, SAFs are likely to achieve its impact on maturity by 2050.


Within minor industries/markets, batteries are gradually becoming a more viable option in terms of both short-range flights and small passenger aircraft operations. Fuel-powered aircraft for regional flights are the main replacement targets. Few start-up companies are aiming to add a new transportation market involving urban aircraft as well. Due to the battery’s undeniable limitations caused by constraints on energy density and weight, battery-powered aircraft are prevented from meeting the criteria for long-range flight-range aircraft. It is predicted that batteries will contribute to an expected  2%-3% reduction in GHG emissions by 2050.


Hydrogen-powered flight has promise. Zero carbon combustion emissions are highly enticing. However, several technical and economic challenges stand in the way of hydrogen-powered flight becoming commonplace. Significant among these are production and storage limitations. To produce enough hydrogen to satisfy the expected demand for the commercial aviation industry, global renewable energy production will have to increase by at least an order of magnitude. For long-range commercial aviation to viably fly with hydrogen, storage tanks will need to use liquified hydrogen with a far greater storage efficiency than has currently been achieved. This will require significant investment from government and industry but is by no means an impossibility.


All of this is neatly summarized in Figure 12, indicating the predicted entry to the market, range limitations, and emissions reductions predicted for 2050. By then the world will look very different, the aviation industry is not immune to change and drastic action is needed now to achieve the goals we have all set out to achieve.


Technology potential of low-carbon fuels
Fig 11. Technology potential of low-carbon fuels [100]

References

 

[6] T. B. Spence, R. O. Fanjoy, C. Lu, and S. W. Schreckengast, “International Standardization Compliance in aviation,” J. Air Transp. Manag., vol. 49, pp. 1–8, 2015, doi: https://doi.org/10.1016/j.jairtraman.2015.06.015.

[11] “Reducing emissions from aviation,” EU Energy, Climate change, Environment, Climate Action Report, June 2023, Accessed: Oct. 16, 2024.  [Online]. Available: https://climate.ec.europa.eu/eu-action/transport/reducing-emissions-aviation_en

[13] Barker, B., Hodgson, P., Hyde, D., Miller, R., Rapeanu, M., “Target True Zero: Unlocking Sustainable Battery and Hydrogen Powered Flight,” World Economic Forum, 2022.

[15] “Paris Agreement,” United Nations, 2015. Accessed: Oct. 20, 2024. [Online]. Available: http://unfccc.int/files/essential_background/convention/application/pdf/english_paris_agreement.pdf

[37] “European Aviation Environmental Report 2022,” EASA - European Union Aviation Safety Agency, 2022. Accessed: Oct. 17, 2024. [Online]. Available: https://www.easa.europa.eu/eco/sites/default/files/2023-02/230217_EASA%20EAER%202022.pdf

[40] “Directive (EU) 2023/2413 of the European Parliament and of the Council,” European Union, Oct. 2023. Accessed: Oct. 22, 2024. [Online]. Available: https://eur-lex.europa.eu/eli/dir/2023/2413/oj

[54] “Clean Skies for Tomorrow Sustainable Aviation Fuels as a Pathway to Net-Zero Aviation,” World Economic Forum, Insight Report, Nov. 2020. Accessed: Oct. 22, 2024. [Online]. Available: https://www3.weforum.org/docs/WEF_Clean_Skies_Tomorrow_SAF_Analytics_2020.pdf

[82] “High-energy battery technologies,” The Farady Institute, Jan. 2020. Accessed: Oct. 27, 2024. [Online]. Available: https://faraday.ac.uk/wp-content/uploads/2020/01/High-Energy-battery-technologies-FINAL.pdf

[89] Dray, Lynnette M., “AIM 2015: Documentation,” Dec. 19, 2020. Accessed: Oct. 27, 2024. [Online]. Available: https://www.atslab.org/wp-content/uploads/2019/12/AIM-2015-Documentation-v9-122019.pdf

[91] M. Binder, M. Kraussler, M. Kuba, and M. Luisser, “Hydrogen from biomass gasification,” IEA Bioenergy, 2018. Available: https://www.ieabioenergy.com/wp-content/uploads/2019/01/Wasserstoffstudie_IEA-final.pdf. [Accessed: Oct. 10, 2024]

[100] “Making Net-Zero Aviation Possible,” Mission Possible Partnership, Jul. 2022. Accessed: Oct. 16, 2024. Available: https://www.missionpossiblepartnership.org/action-sectors/aviation/. 



End Notes

 

[1] In 2022, fuel consumption by volume by the US aviation sector was 98.8% Jet A-1 and 1.2% aviation gasoline [10].

[2] Assuming a typical short commercial airline design (B-737 900 baseline).


Appendix

 

Table A1. Compiled scores used in modelling and presenting the current state of impact vs. readiness for key technology enablers.







 

Technology Enabler

X – Readiness Factors

 

Y – Impact Factors

Source

 

 

A

B

C

Total

 

D

E

F

Total

 

 

Systems Efficiency Enablers

7

7

7

21

 

2

1

1

4

[4, 19, 21, 22]

 

Contrail Avoidance Enablers

7

7

4

18

 

2

1

1

4

[13, 22, 24]

 

Design Efficiency Enablers

5

7

6

18

 

3

4

1

8

[4, 19, 21, 23]

 

Market Enablers

7

7

7

21

 

3

3

4

10

[4, 19, 20, 21, 23]

 

Other Enablers

1

3

3

7

 

6

7

4

17

[4, 19, 22]

 

SAF Enablers

7

7

7

21

 

6

7

7

20

[4, 19, 21, 22, 23]

 

Electric Propulsion Enablers

4

3

4

12

 

3

6

7

16

[13, 19]

 

Hydrogen Propulsion Enablers

4

4

4

12

 

3

7

4

14

[13, 19, 21]



Table A2. Raw scorecard and factors used in modelling and presenting the current state of impact vs. readiness for key technology enablers.

X - Critical Readiness Factors









 

 

Weight

 

 

Weight

 

 

Weight

A

Deployment Speed

100%

B

P(Research Success)

100%

C

P(Deployment Success)

100%

1

50 years

 

1

0%

 

1

0%

 

2

40 years

 

2

17%

 

2

17%

 

3

30 years

 

3

33%

 

3

33%

 

4

20 years

 

4

50%

 

4

50%

 

5

10 years

 

5

67%

 

5

67%

 

6

5 years

 

6

83%

 

6

83%

 

7

0 years

 

7

100%

 

7

100%

 

 

 

 

 

 

 

 

 

 

Y - Critical Impact Factors









 

 

Weight

 

 

Weight

 

 

Weight

D

Impact on 2050 target

100%

E

Investment (CAPEX + OPEX)

100%

F

Ripples into other sectors

100%

1

0%

 

1

USD$0.1b

 

1

Minimal

 

2

10%

 

2

 

 

2

 

 

3

20%

 

3

 

 

3

 

 

4

30%

 

4

$1b

 

4

National impact on sectors and economy

 

5

40%

 

5

 

 

5

 

 

6

50%

 

6

 

 

6

 

 

7

60%

 

7

$10b

 

7

Global impact on sectors and economy

 


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