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Electric propulsion – the future of sustainable aviation?


The Paris Agreement on climate change came into force in November 2016, with a goal of limiting global warming to well below 2ºC, but preferably to 1.5ºC, compared to pre-industrial levels.

According to the International Energy Agency (IEA), CO2 emissions from aviation in 2019 equated to around 2.8% of global CO2 emissions from fossil fuel combustion. So, it is probably no great surprise that there has been much research in recent years on ways to reduce the carbon footprint of the aviation sector.

An IEA report from November 2021 highlighted a reduction in 2020 of CO2 emissions from aviation of one third from levels seen in 2019. Clearly this reduction was a consequence of travel restrictions resulting from COVID-19. However, as the world returns to a version of normality, airline travel and related emissions will increase. So, the need and desire for innovations to help reduce aviation’s carbon footprint will remain.

Making aviation more sustainable

At the COP 26 climate change conference, participant countries issued a declaration recognising international aviation’s contribution to climate change through its CO2 emissions and committing to work together to advance actions to reduce aviation CO2 emissions consistent with meeting the 1.5ºC goal.

To reduce emissions, aircraft can use clean fuels to reduce the number of pollutants released and change flight paths to fly at lower altitude, where contrail formation is avoided.

Indeed, work has been done on so-called “drop-in” fuels, being alternative fuels which can be used in place of kerosene without requiring extensive redesign of existing engines and related aircraft systems.

Innovations in electric propulsion

However, another area that has seen innovation in recent years is that of electric propulsion systems for aircraft.

An example is E-Fan X, which was active between 2017 to April 2020. E-Fan X was a hybrid electric aircraft technology demonstrator programme. The programme used a Bae 146 aircraft as a platform and replaced the core of one of the aircraft’s four turbofan engines with a 2MW electric motor.

Power for the electric motor was provided by a turboshaft engine and generator located in the rear of the aircraft fuselage, with an air intake provided in the fuselage to feed the turboshaft engine. Additionally, a 2MW battery was also provided in the fuselage to provide energy storage.

However, aircraft have also been developed solely employing electric power, without the need for conventionally powered engines to provide propulsion or to serve as means for generating electric power.

Much of the progress to date in this field has been in the light aircraft sector. Aircraft are being developed which use a large number of electrically-powered engines arranged on the aircraft – known as distributed electric propulsion.

One such example is the Lilium jet, with technology demonstrators built and tested for a version intended to transport four passengers and a pilot over a short range, and with a 60-minute flying time. Work is ongoing on a larger version which can carry 6 passengers and a pilot.

The aircraft employs a configuration of 36 electrically powered ducted fans arranged in different groups to provide propulsion, and able to tilt to provide vertical take-off and landing (VTOL) capability. The integration of the fans into the structure of the wings means that the housings/nacelles for the fans can also contribute to the generation of lift.

A collaborative effort led by Rolls-Royce resulted in the development of a single seat aircraft powered using solely electric propulsion – named the ‘Spirit of Innovation’. It laid claim to three different world records after flight runs on 16 November 2021, including speed records over 3km and 15km, plus the fastest time to climb to 3000m altitude.

A maximum speed of 387.4 mph was also claimed. The aircraft employs three 72kWh battery packs. The project involved collaboration between firms both large and small.

ICAO provides a list of various other projects relating to the development of aircraft employing electric propulsion systems. As you might imagine, research and development in electric propulsion has been accompanied by an increase in patent filings in this sector.

EP3048042A1, is one such filing, relating to a hybrid gas-electric propulsion system for an aircraft. For this hybrid system, power generated by two conventional jet engines is used to drive an electric motor provided in a boundary layer ingestion fan provided at the rear of the aircraft’s fuselage.

In-flight, the fan will ingest a boundary layer of air flowing over the fuselage of the aircraft, thereby reducing aircraft drag and enabling an increase in propulsive efficiency.

Another example, US2017/203839A1, relates to an aircraft having a propulsion system in which electrically powered ducted fans are integrated into the wings of the aircraft. This again is a hybrid design, employing a turboshaft engine for driving a generator, with the generator providing AC power to the ducted fans.

Future Challenges

Both patent filings mentioned above disclose the use of propulsion systems which rely upon a combination of conventional and electric power sources. Although powered flight using solely electric propulsion has been achieved, the upscaling of electric-only propulsion systems to larger commercial aircraft with longer range presents significant challenges.

One of the principal challenges is developing more mass-efficient ways of storing electricity for use in powering an all-electric propulsion system of an aircraft. This problem is illustrated by the fact that the specific energy – a measure of the amount of energy available per unit mass – of conventional hydrocarbon fuels, such as kerosene, is greater than what is available from current battery technologies.

In simple terms, a given mass of kerosene would provide a greater amount of energy than the same mass of batteries.

Closing this specific energy or energy density gap will be an important step in developing a commercial airliner powered solely by electric propulsion, which is capable of carrying several 100 passengers between continents. However, addressing this problem will provide opportunities for innovation and the development of valuable new intellectual property – both for large companies and small.

In the race to innovate in this sector, those firms and individuals who take steps to protect their intellectual property have the potential to secure their commercial position, as well as to benefit the wider global community.

About the author
Andy Williams is a senior associate of Reddie & Grose LLP, a firm of patent, trade mark and design attorneys.

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