The Future of Aviation - True Zero Emissions?

The Future of Aviation: Transitioning Towards Sustainable Fuel Options

The aviation industry is experiencing a paradigm shift, as environmental concerns grow and the pressure to reduce carbon emissions intensifies. Airlines, manufacturers, and regulatory bodies are exploring various alternatives to conventional fossil fuels, with electric and hydrogen-based propulsion systems at the forefront of research and development. In this article, we will delve into the potential of these two emerging technologies, and discuss how they may shape the future of aviation.

Electric Aircraft

Technology Overview

Electric aircraft are powered by electric motors, which use energy stored in batteries. The technology is still in its nascent stages for commercial aviation, with numerous challenges that need to be addressed, including energy density, weight, and charging infrastructure. Nonetheless, electric aircraft are gaining traction for short-haul flights due to their lower operating costs, reduced noise, and zero-emission operations.

Market Projections and Adoption

Several companies are making significant strides in electric aviation. For instance, Israeli start-up Eviation has developed Alice, an all-electric nine-passenger aircraft, which is expected to enter service in 2023[1]. Pipistrel, a Slovenian manufacturer, is also making waves with its two-seater electric trainer, the Velis Electro[2].

Although these developments are promising, wide-scale adoption of electric aircraft is projected to be slow. According to UBS, electric aircraft are expected to represent around 3% of the total aircraft in service by 2040 [3].?

Challenges and Considerations

The main challenges facing electric aircraft are the weight and energy density of batteries. Presently, batteries cannot match the energy density of conventional jet fuel, which limits the range of electric aircraft. As the technology progresses, new battery chemistries and energy storage solutions are expected to emerge.

Hydrogen Fuel Cell Aircraft

Technology Overview

Hydrogen fuel cells produce electricity by combining hydrogen and oxygen in a chemical reaction. Hydrogen can be used as a fuel in two ways: in fuel cells to generate electricity or by direct combustion in a modified gas turbine engine. Both methods have the potential to significantly reduce greenhouse gas emissions.

Market Projections and Adoption

Airbus is one of the major players investing in hydrogen technology with its ZEROe concept aircraft. The company aims to put a hydrogen-powered commercial aircraft into service by 2035 [4]. Additionally, smaller players like ZeroAvia are working on retrofitting existing aircraft with hydrogen fuel cells for short-haul flights[5].

Challenges and Considerations

Hydrogen storage and distribution are the major challenges for hydrogen-powered aircraft. Liquid hydrogen, which requires cryogenic storage, and hydrogen gas under high pressure are the two main storage methods being considered. Additionally, the production of green hydrogen, which doesn't emit CO2, is crucial for making this technology truly sustainable.

An Insightful Study on Liquid Hydrogen (LH2) Powered Aircraft

A study by the International Council on Clean Transportation (ICCT) titled "Global evolutionary designs of liquid hydrogen-fueled aircraft" explores the potential performance characteristics, fuel-related costs, and emissions of LH2-powered aircraft anticipated to enter service in 2035 [6].

The study focuses on two LH2 combustion designs; a smaller turboprop aircraft aimed at the regional market, and a narrow-body turbofan aircraft for short and medium-haul flights. These designs were benchmarked against the ATR 72 and the Airbus A320neo, respectively. Both LH2-powered aircraft designs necessitate a longer fuselage to house LH2 storage behind the passenger cabin. The study looks into Gravimetric indices (GI) in the range of 0.2 to 0.35, which is the ratio of fuel mass to the mass of the complete fuel system including the cryogenic tank. Also, seating pitch (SP) values of 29 and 30 inches were considered to replicate the seating densities of low-cost and regular airliners.

While the study revealed that LH2-combustion aircraft would not perform as well as jet fuel counterparts, they still hold potential for contributing to the aviation industry's 2050 climate goals. The LH2-powered aircraft are expected to be heavier due to an increased maximum takeoff mass (MTOM) and less efficient, with higher energy requirements per revenue-passenger-kilometer (MJ/RPK). They would also have a shorter range compared to fossil-fuel aircraft. Nonetheless, the study estimates that LH2-powered narrow-body aircraft could carry 165 passengers up to 3,400 km, and LH2-powered turboprop aircraft could transport 70 passengers up to 1,400 km. Collectively, they could service about 31-38% of all passenger aviation traffic as measured by revenue passenger kilometers (RPKs).

In terms of fuel costs, using green hydrogen is expected to be more expensive than fossil jet fuel but less costly than blue hydrogen and e-kerosene. To make green hydrogen cost-competitive, carbon pricing would be essential, with breakeven expected to be between $102 and $277/tonne CO2e in 2050, depending on the region. Importantly, green hydrogen is predicted to be cheaper than e-kerosene on routes up to 3,400 kilometers.

Under optimistic assumptions, evolutionary LH2-powered aircraft could cap, but not significantly reduce, aviation CO2 compared to 2035 levels. This scenario would necessitate all replaceable missions in 2050 to be serviced by LH2-powered aircraft using green hydrogen and would result in the mitigation of 628 Mt-CO2e in 2050, accounting for 31% of passenger aviation’s CO2e emissions. The study's internal modeling suggests that a 20%-40% adoption rate is realistically attainable and would mitigate 126 to 251 Mt-CO2e in 2050, representing 6% to 12% of passenger aviation’s CO2e emissions.

LCA Analysis of Single Aisle Aircraft: SAFs, Hydrogen, and Fuel Cells with Superconducting Motors

It is also essential to address the non-CO2 emissions associated with Sustainable Aviation Fuels (SAFs) and combustion processes. SAFs are fuels derived from sustainable sources and can be used in existing aircraft engines. While they can substantially reduce CO2 emissions, it’s important to understand that SAFs when combusted, still produce non-CO2 emissions, such as NOx, which have climate impacts.

Under the most optimistic fuel and fleet turnover assumptions, evolutionary LH2-powered aircraft could cap, but not absolutely reduce, aviation CO2?compared to 2035 levels.?This would require all replaceable missions in 2050 to be serviced by LH2-powered aircraft using green hydrogen and would result in mitigation of 628 Mt-CO2e in 2050, representing 31% of passenger aviation’s CO2e emissions. Internal modeling suggests that a 20% to 40% adoption rate is realistically achievable and would mitigate 126 to 251 Mt-CO2e in 2050, representing 6% to 12% of passenger aviation’s CO2e emissions. (See Figure below) Other technologies, including more fuel-efficient aircraft and sustainable aviation fuels, along with measures to moderate traffic growth will be needed to meet airlines’ aggressive climate goals of net-zero emissions by 2050.

In a life-cycle analysis (LCA), integrating SAFs with hydrogen and superconducting motors can offer a more comprehensive solution. Superconducting motors, coupled with hydrogen fuel cells, can provide significant weight savings and higher efficiencies compared to conventional electric motors. This combination can significantly reduce both CO2 and non-CO2 emissions, representing a deeper shift towards sustainable aviation.

Conclusion

As the aviation industry aims for a more sustainable future, it is imperative to harness the strengths of various technologies. Electric aircraft offer benefits for short-haul flights, while hydrogen-based systems hold promise for longer routes. SAFs can be a bridging technology, helping to reduce emissions in the near term. Combining these with advanced propulsion systems, such as superconducting motors, can lead to holistic solutions that address both CO2 and non-CO2 emissions. As these technologies evolve, collaboration among manufacturers, policymakers, and other stakeholders will be crucial in achieving the aviation industry’s ambitious climate goals.