A greener future for the aviation sector

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It may come as a surprise to some that, despite attracting attention in action against climate change, according to the International Energy Agency (IEA) the aviation industry accounts for only 2% of global energy related CO2 emissions. However the sector is growing fast. In recent decades this growth has outpaced rail, road and shipping, with the IEA anticipating that the efficiency gains afforded by updated aircraft will be unlikely to keep pace with the expansion of the sector. Clearly then, decarbonising the aviation industry will require more creative solutions than simply more efficient aircraft.

Electric and hybrid propulsion technologies

As outlined in a previous article, the fundamental issue with developing greener forms of aircraft propulsion is energy density. Most areas of ground-based transport are exhibiting a trend towards electric power. However, creating a fully electric commercial passenger aircraft of a reasonable size would require a battery weighing around an order of magnitude more than the aircraft itself. Battery power also eliminates the benefit of the aircraft becoming lighter as fuel is burned, improving efficiency as the flight progresses. Therefore, massive improvements need to made in battery technology before fully electric commercial aircraft are even remotely possible.

There have been continued developments in the hybrid-electric propulsion space in recent years. Airbus has invested heavily in hybrid technology, partnering with Renault and STMicroelectronics to advance its progress. Airbus’ ‘aircraft hybridisation principle’ centres on resource allocation. The aircraft dynamically changes between using different energy sources, either in combination or individually, during the course of a flight. Airbus claim that its energy management system will reduce fuel consumption by up to 5% compared to conventional technology.

This is a promising step forward, but advances of this scale clearly cannot be relied upon to solve the aviation sector’s emissions problems.

Airbus has also been involved in the development of ‘EcoPulse’, a distributed propulsion hybrid-electric aircraft developed in collaboration with Daher and Safran. The concept was exhibited at the 2023 Paris Air Show and aims to highlight the advantages of distributed propulsion on aircraft performance. Distributed propulsion involves dividing the thrust requirements between a number of smaller engines. According to Airbus, this allows for: “improved cruise, and take-off and landing performance; reduced cabin noise due to better synchronisation between several propellers; and increased energy savings thanks to reduced tail surface and better aircraft control”. The technology is very much still in its infancy, and since the prototype has only accumulated 27 hours of flight time no concrete data on efficiency has been released. However, according to Pascal Laguerre, CTO at Daher: “from this demonstration program, we plan to develop our future product roadmap and basically spec the hybrid aircraft we intend to produce by the end of our five-year plan. We expect by the end of 2027 to be able to offer our first hybrid aircraft to the market”.

Rolls-Royce has also been developing its own hybrid-electric propulsion system. They have produced a turbogenerator system which comprises a compact, power dense turbine that is intended for use in the Advanced Air Mobility (AAM) market (for example, short distance commuter aircraft with up to 19 seats). The turbogenerator system is capable of producing between 500 kW and 1200 kW of power, which is perfect for the short distance applications it is intended for. However, this is still several orders of magnitude too small for long haul commercial aircraft flights.

Gas-to-liquid (GTL) technology

As mentioned previously, gas-to-liquids (GTL) technology may prove to be the most viable solution for larger commercial aircraft. The gas-to-liquids process has been developed by Shell over a number of decades and involves three steps. First, a ‘synthesis gas’ is created which is a mixture of primarily hydrogen and carbon monoxide, formed via partial oxidation of natural gas. This synthesis gas then undergoes a catalytic conversion process, transforming the synthesis gas into liquid hydrocarbons. Finally, conventional cracking and isomerisation techniques can be used to create the desired hydrocarbon chain.

The gas to liquid market has expanded in recent years, with a valuation of 14.8 billion dollars in 2022 and growth predictions estimating the industry to reach a value of 24.4 billion dollars by 2030. The increasing needs of the commercial aviation industry was cited as a key factor for these figures. As governments and regulatory bodies increase the pressure on companies in the aviation sector to reduce emissions, GTL technology is becoming a far more feasible solution. Technological developments in catalyst manufacturing and reactor designs have resulted in higher conversion rates and reduced capital costs, increasing the potential reward for investors. For example, Shell has developed a process to 3D print the catalyst required for GTL conversion. 3D printed catalysts have the potential to improve production speeds, reduce raw material wastage and improve the efficiency of the catalyst by allowing greater control of the macroscopic and microscopic structure of the substance.

Powering the GTL process

Despite these developments, the most significant problem with GTL remains finding a sustainable energy source to power the process. Although nuclear power is contentious in nature, there is no denying that its extremely high energy density far surpasses its less controversial competitors such as wind or solar. The NIA recently issued a report stating that the UK has a “has a ‘golden opportunity’ to become a world leader in the production of emissions-free synthetic fuels using nuclear energy”. The report argues that current theoretical models for future carbon emissions downplay the potential benefits afforded by increasing nuclear energy usage. In the US, the Department of Energy is funding a $20 million project managed by Oxeon Energy to create synthetic fuels via nuclear energy.

Nuclear power has been consistently demonstrated to work and is not weather dependent, allowing for the consistent power output that would be required to meet the demands of the fast-paced aviation industry. Furthermore, both excess heat from the reactor and electricity generated by nuclear energy can be utilised in the GTL process, significantly improving efficiency. The development of small modular nuclear reactors (SMRs) by Rolls Royce promises to further improve the viability of nuclear based GTL production, as it will allow multiple synthetic fuel production plants to be installed across the country. SMR reactors could be implemented at every major airport, providing both synthetic fuel for aircraft as well as electrical power and heat for the airport and its surrounding population.

As the aviation sector continues to grow, more pressure will be put on major players in the industry to adopt more sustainable solutions. Energy density constraints mean that commercial aircraft in their current form are likely here to stay. However, the scalability and relatively low cost of SMR reactors means they have the potential make an important contribution towards a greener future for the sector.

Barker Brettell has a dedicated aerospace group. If you would like to discuss options for protecting your invention, please get in touch with the author or your usual Barker Brettell patent attorney.