Towards low-carbon aviation

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ecarbonizing aviation poses a number of difficulties. Aviation's emissions, of which CO2 is just one, have a significant warming effect when released at high altitudes. The industry is characterized by long innovation cycles and the costs of developing and implementing new technologies can be substantial. New technologies must meet stringent safety and reliability standards, delaying integration into service. Existing infrastructure and aircraft are designed for conventional fuels, forcing major investments to transition to other technological alternatives.

 

These factors collectively make aviation among the hard sectors to abate. Significant advancements in aircraft efficiency, driven by technological innovations, larger average aircraft sizes, and better passenger load factors, have dramatically improved aviation transport efficiency. In the 1950s, emissions exceeded 2,000 grams of CO2 per Revenue Passenger Kilometer (RPK)—a measure of the environmental impact per paying passenger transported over a kilometer. By 2018, this figure had dropped to 125 grams of CO2 per RPK. However, this efficiency improvement must be considered alongside the rapid growth of the aviation industry, fueled by greater affordability, economic expansion, and globalization. This growth has led to a continual increase in overall emissions. Indeed, the aviation industry, like other hard-to-abate sectors, must strike a delicate balance. On one hand, it is crucial to focus on reducing emissions within aviation. On the other hand, there is the challenge of achieving these reductions while ensuring that key stakeholders can transition in a fair and sustainable manner, avoiding major disruptions to operations and, by extension, to passengers. Therefore, the industry must pursue a well-informed and carefully considered strategy to support a sustainable transition. This article will explore the leading methods currently available to the aviation sector for achieving carbon neutrality.

 

Three primary strategies

Recent literature highlights three primary strategies for aviation decarbonization: enhancing efficiency, reducing demand, and innovating fuel and propulsion technologies. To meet the 2050 target, residual CO2 emissions might be offset using carbon credits, which play a more significant role in efficiency and demand reduction strategies than in technological innovations.

Efficiency improvements, such as better aerodynamic designs and the use of advanced materials, can help reduce emissions from current aircraft. Sufficiency measures involve optimizing flight routes and reducing air travel demand. Innovations in fuel and propulsion technology offer a direct solution to cutting tailpipe emissions. While these approaches can work together to reduce emissions more quickly, they may not be sufficient on their own.

 

 

 

 

The aviation industry tends to favor efficiency improvements and technological innovations over sufficiency measures, as the former align with the industry's growth objectives and focus on technological progress. In contrast, sufficiency measures, which might involve reducing the number of flights or optimizing operations to lower emissions, could be seen as counterproductive to the industry’s growth goals. However, significant decarbonization, meaning a substantial reduction in actual tailpipe emissions, largely depends on advancements in propulsion and fuel technologies, as tailpipe emissions are currently the largest source of aviation emissions. Therefore, this article will focus on the third strategy: fuel and propulsion technology innovation, examining the primary technological approaches to reducing aircraft tailpipe emissions.

 

 

The key role of technological innovation

 

Fuel and propulsion technology innovation efforts have been focused on two primary areas. The first involves advancing existing propulsion technologies and their associated fuels. The second concentrates on developing entirely new propulsion technologies. In terms of propulsion technologies, the High-Bypass Ratio (HBR) turbofan engine is the primary technology used in today’s civil aviation.

The most used technology is the high bypass ratio (HBR) turbofan engine

However, the potential for further efficiency gains with HBR turbofan engines is nearing its theoretical limits. In recent decades, significant strides have been made in enhancing the fuel efficiency of modern jet engines. Efforts have focused on improving propulsion efficiency, increasing thermal efficiency, reducing noise, and lowering Nitrogen Oxides (NOX) emissions. From the 1970s to the early 2000s, these advancements led to a remarkable 35 percent reduction in fuel consumption and nearly eliminated smoke emissions.

Theoretically, a further 30 percent improvement in fuel consumption is achievable. However, practical limitations suggest that without groundbreaking low-NOX technologies, engines may realistically achieve only an additional 20-25 percent reduction while meeting future NOX emissions standards. Even with maximum efficiency improvements, some aircraft produced in the coming years might still be operational by 2050. Therefore, without a technological breakthrough in engine efficiency, we must address a fundamental question regarding the root cause of emissions: the fuel itself.

To advance fuel technology, the International Civil Aviation Organization (ICAO) identifies two primary approaches under the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA): Lower Carbon Aviation Fuels (LCAFs) and Sustainable Aviation Fuels (SAFs).

LCAF amounts to ensuring conventional fossil-based aviation fuel complies with the CORSIA Sustainability Criteria. To be classified as LCAF, the fuel must achieve a 10 percent reduction in lifecycle emissions compared to the standard aviation fuel baseline of 89 gCO2e/MJ. Achieving this will hing on transport and upstream activities needed to make the fuel. Although LCAF efforts to bring emissions down to 80 gCO2e/MJ may not appear to make a substantial impact on GHG emissions, its effect would be comparable to the peak efficiency improvements achieved in the 1970s with the introduction of the Boeing 747, which featured a wide-body design and HBR turbofan engines. SAF, on the other hand, refers to aviation fuel derived from renewable or waste sources and is subject to specific sustainability criteria. Currently, SAF is certified according to ASTM D7566 Annex 1 to 8 and D1655 Annex A1 (Figure 3).

After SAF is produced, it must be blended with conventional fossil-based jet fuel and certified under existing standards. SAF can be managed like traditional aviation fuel and seamlessly integrated into existing infrastructure. Blending SAF with conventional jet fuel ensures compatibility with the majority of commercial aircraft in operation. The standards set forth limitations on specific compounds (e.g., aromatics, cycloparaffins, or trace compounds) that a fuel must meet to be certified as aviation fuel, fulfilling secondary functions such as lubrication and sealing. However, as older aircraft fleets retire and new aircraft engines impose fewer constraints, this issue is expected to diminish.

 

 

The new technologies include hydrogen propulsion

When considering solutions poised for long-term commercial viability, novel propulsion technologies take center stage. These include hydrogen propulsion and chemical battery electric propulsion. Hydrogen can power aircraft through two primary methods: hydrogen combustion and fuel cells. The first method uses hydrogen similarly to traditional jet fuel in hydrogen-compatible jet engines, completely eliminating carbon emissions. The primary exhaust from these engines would be water vapor (H2O), NOX, and residual heat. The second method is a novel development in aviation, using electricity generated from hydrogen-fed fuel cells instead of combustion for takeoff. This approach, along with electric battery-powered flight, marks a significant shift from conventional aviation propulsion. In fuel cell-powered aircraft, hydrogen is converted into electricity, which drives an electric motor and a fan or propeller to generate thrust. The main emissions from fuel cells are primarily H2O and residual heat.

 

 

 

 

When it comes to chemical battery propulsion, battery-electric systems have revolutionized ground transportation and various other markets, leading to significant advancements in operational capabilities and energy storage properties. Battery-based systems for aviation present clear benefits as they do not emit direct emissions during operation. Additionally, battery-electric systems offer higher end-to-end drivetrain efficiencies compared to traditional jet engines and hydrogen-based propulsion. For instance, a study calculated that for every 1 MJ at an aircraft’s fan, it would require approximately 1.3 MJ from a battery-electric system, 4.1 MJ for a hydrogen fuel-cell system, 5.2 MJ from a hydrogen combustion system, and 5.7 MJ from an SAF-based system.

However, battery-electric systems, despite their higher efficiencies, face numerous challenges in commercial aviation

The main issue is their relatively low specific energy, making them suitable primarily for light-payload and short-range aircraft in the foreseeable future. Nonetheless, the aviation industry has adopted battery-electric systems by integrating more-electric aircraft systems, which replace engine-bleed and hydraulic actuation systems with electrical alternatives to improve overall efficiency and reduce weight. This shift towards electrification has also led to numerous studies exploring fully electric and hybrid-electric aircraft configurations. These studies indicate that fully electric battery energy storage systems for commercial transport aircraft are not feasible in the near to mid-term, with lightly hybridized configurations offering only modest fuel efficiency improvements.

Aviation must overcome unique challenges to achieve a timely and orderly transition to decarbonization. Yet, with each challenge, the industry is given valuable opportunities to innovate and reach its decarbonization targets.