This series of white papers concerning the sustainability of air transportation is published in the midst of the COVID-19 period, which has had a tremendous effect on the worldwide aviation sector. Some voices are rising to push governments and institutions to tackle, in the recovery efforts to address the economic crisis, the sustainability issue of this transport mode at the source, and redefine its global rules. We see these debates as a great opportunity to condense and review the main challenges of the aviation sector regarding sustainability. This paper gives an interesting overview of the current development and challenges of electric aviation, which has long been called a potential solution for reducing the carbon-intensity of the sector.

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Part 3: Electric aviation, a flight into the future?

This white paper is the 3rd segment of our ICARUS series, “Sustainability in air transport”, which is divided into 4 parts. If you missed the previous posts, you can find them here and here.

In recent years, the aviation sector has received a lot of criticism because of its environmental impact. As recalled in the ICARUS 1 paper, it was responsible for 859 million tonnes of CO2 emissions in 2018. This was approximately 2.5% of the global total and proportionally 3.6% of EU28’s total [1]. While the global fleet of aircraft has become 3.2% more efficient per year between 2000 and 2014, this increase in efficiency has slowed down to 1% in recent years. However, this increase was not enough to offset the growth of the aviation sector. Solutions must be found to cope with the challenge.

One of the principal candidate innovations designed to help reduce the environmental impact of the sector is the electrification of aircraft. It could significantly reduce emissions and eventually eliminate CO2 and other GHG emissions from flight operations, on the condition that the electricity would come from renewable energy sources. This piece delves into the different methods and technologies available that could electrify aviation. It will focus on long-haul flights, which represent a significant portion of the impact of the sector, and for which no viable alternatives exist yet. Electrification by batteries will be explored in this paper, and fuel cells or hydrogen will not be examined.

The paper is divided into two parts. First, the technical improvements that are required to electrify an aircraft are discussed; and second, the main techno-economic challenges of the concepts in regards to feasibility and sustainability are touched upon.

From electrification of conventional aircraft to electric propulsion

The introduction of the More Electric Aircraft concept with the Boeing 787 and Airbus A380 has sparked a new wave of research and development of new actuators and power electronics, for instance, but also system architectures. This direction can create more efficient engines, ensure cleaner air in the cabin and, most importantly, create lighter aircraft. However, this is still a developmental step. An aircraft is essentially the same as it was 70 years ago; a tube with wings and engines underneath.

The More Electric Aircraft (MEA) Concept

In aircrafts, all onboard power is generated by the engines. However, for different systems, different types of power are needed. Hydraulics are needed for the aerodynamic control surfaces, pneumatics are used to keep the cabin pressurized, and electricity is needed for computers and entertainment systems. All pumps and generators are connected to the engine through gear boxes. For the pneumatic system, high pressure ‘bleed’ air is taken from the compressor, directly reducing the engine efficiency.

Electricity is easier to transport around the aircraft. The goal for MEA is therefore to replace these subsystems with electric versions and, for instance, manage the control surfaces with electro-hydraulic actuators. This increases engine efficiency and could save weight on the complex subsystems.

Figure  1 – Schematic of the More Electric Aircraft with conventional systems architecture (left) and the MEA (right) [2]

Recent news disclosed the latest ambitious achievements of engineers regarding electric aviation. In May 2020, MagniX performed a test flight with a retrofitted Cessna Caravan, a small aircraft for 9 passengers [3]. While this is still far from the size and range of a commercial airliner, it shows the first steps towards electric flight.

Electric propulsion

At the 2019 Paris Airshow, the world was surprised by the introduction of the Eviation Alice: the first full-scale prototype of a fully electric commercial aircraft [5b]. Powered by Li-ion batteries, the business jet should have a range of 1000 km with up to nine passengers onboard. The aircraft is expected to go into service in 2022.

Figure 2: Eviation Alice at the Le Bourget 2019 [5] www.aironline.com

Figure 3: The Lilium jet (www.lilium.com) [4]

 

 

 

 

 

 

 

The Alice already shows one great potential benefit of (hybrid-)electric propulsion: distributed propulsion. The pusher propeller at the back of the aircraft fills the wake of slow-moving air behind the fuselage. The propellers at the wingtips counteract one of the biggest contributors to aircraft drag, which are the vortices at the wingtips, created by the pressure difference between the top and bottom surfaces of the wing. This is only one example of how propulsion could be used to our benefit. A different, more extreme example would be the Lilium Jet [4]. This is a ‘flying taxi’ in which the electric motors are spread over the entire top surface of both the canard (front wing) and the main wing.

Figure 4: Parallel (upper) vs. serial (lower) hybrid electric propulsion. From [1].

For the use of distributed propulsion, an aircraft does not necessarily have to be fully electric. An intermediate option would be a hybrid-electric flight. This seems like a decent step, especially for large airliners, and it has already shown significant potential for emissions reduction. Airbus, together with Rolls Royce, presented such a concept in 2013 [6]. It uses one big jet engine at the back of the fuselage that is used mainly for power generation, and 6 electric fans distributed over the wings that provide propulsion. The aircraft has a battery pack on board to help with power-intensive flight phases like take-off and landing. During cruise, the batteries recharge while the rest of the power is used to provide propulsion. This concept is called parallel propulsion, where electric and conventional jet engines operate side by side. The counterpart of parallel electrification, presented in Figure 4, is serial electrification. Here, the electric motor is placed on the same shaft as the fan of the jet engine. Again, the electric motor is only engaged during take-off and climb, but now helps the main engine directly. This reduces the required size of the main engines. In addition to this, they can continually operate at their most efficient revolutions per minute.

 

Challenges of electrification

One of the biggest obstacles that the implementation of electrification with batteries faces has to do with their energy density (both per mass and per volume). Current Li-ion batteries can store 240 Wh/kg [5a]. In order for larger aircraft to make a significant impact, researcher M. Voskuijl suggests a minimum specific energy of 600 Wh/kg [8]. The study was for a short-haul (1500 km) airliner based on the ATR-72 with around 70 passengers. With 1000 Wh/kg specific energy and with 34% of the power being provided electrically, the hybrid aircraft could save 28% in fuel required for a flight.  An increase in specific energy of the battery can have great snowball effects on the overall aircraft: a smaller battery leads to less mass, which leads to smaller wings, which leads to a smaller engine, less energy required, etc…

The Eviation Alice is already at the forefront of current battery technology. As described in the Greenfish whitepaper on Li-ion batteries, there are several battery-chemistry candidates, including lithium-sulphur, lithium-silicon, and lithium-air. All of them currently have one important issue, which is cathode volume increase. Lithium-based batteries use graphite cathodes in which the potential lithium content is not so high (this determines the specific energy of the battery). The other chemistries can hold a lot more lithium compared to their cathode volume, but this also means that the cathode increases in size when in use. In turn, this expansion damages the cell, reducing the number of charge cycles.

Another issue is charging. Fast charging shortens the battery lifetime, while the typical turnaround time (time between touchdown and take-off), is between 45 minutes and one and a half hours, depending on the type of aircraft. In this period, you have to unload passengers and cargo, fuel, clean and reload with passengers, catering, and cargo. During this time the airline essentially does not make money. Therefore, it is of paramount importance for airlines to keep this as short as possible to maximise airplane usage. One could avoid this issue by replacing entire battery packs, but that brings up several new challenges like hauling around tons of batteries and requiring more than one battery pack per aircraft. Many start-ups are trying to solve these challenges and develop better batteries. Some of them, such as Sila Nanotechnologies or QuantumScape (solid-state batteries) [9], have collected more than a hundred million dollars in funding for the development of these technologies.

Besides the current shortcomings of batteries, there are other technological challenges to overcome. These challenges include superconducting motors and generators with their accompanying cryogenic cooling requirements and low-weight power transmission lines that can handle high currents. However, once the battery issues are solved, which could still take several decades, solutions to these challenges will probably quickly follow suit.

Figure 5: Airbus E-Fan X, a hybrid testbed based on a BAE 146 regional airliner

One last important question that casts doubt on electrification being a fast-enough solution to the decarbonisation challenge of the industry is the airliner’s development timeline. Big aircraft programs, depending on the starting point, typically take around a decade to complete. This is further compromised because the aircraft market is dominated by four major players: Boeing, Airbus, Embraer, and Bombardier. Last year, extensive collaboration examples between Boeing and Embraer (joint venture/takeover), but also between Airbus and Bombardier (takeover of CRJ program), were established, which further strengthens a duopoly that is detrimental to fast innovation. Each new aircraft program is only slightly better than the competition and the previous versions of it. The drive for radical design changes is associated with risks that companies are not yet willing to take. Furthermore, these changes need to be seen through the lenses of the current economic downturn that our economies are experiencing. As we speak, the aviation sector is already taking large hits from the Coronavirus lockdown. For instance, Airbus’ E-Fan X program got canceled in April 2020 together with the Boeing and Embraer takeover. If the sector takes the decarbonisation question seriously, will it give these electrification programs the chance to further accelerate their development or on the contrary, completely stop them?

In conclusion

There is no doubt that from a technical standpoint, electric flights could be the future. With the presentation of the Eviation Alice, this concept has become much more tangible. The More Electric Aircraft is already developing subsystems that are essential for hybrid or fully electric planes. However, batteries and their density, motors, and power electronics still need to make big steps for large aircraft to become viable. Once these technologies have matured, the fact remains that new aircraft programs typically take close to a decade to go from an investment decision to the market. In fact, with major companies like Airbus, Rolls Royce, Siemens, and DLR developing serious testbeds, it is more a question of “when” and not “if.”

Nonetheless, in reaching the decarbonisation target of the sector by 2050, this “when” question is paramount. It comes down to two main questions:

  • Will the technological developments of electrification programs shift our current air transport fuel usage to electric consumption fast enough?
  • If yes, will the large upcoming electricity demand for aircraft be decarbonized by then?

Nothing could be less certain! Therefore, looking at reducing the demand is also imperative. Stay tuned for ICARUS 4 which will explore the feasibility of such an idea.

Roger Coenen – Consultant at Greenfish
Quentin Lancrenon – Knowledge & Content Specialist at Greenfish
Nassim Daoudi – Chief Executive Officer at Greenfish

[1] ATAG 2018, Air Transport Action Group – Facts and figures
[2] Wheeler, et al., 2016. Technology for the more and all electric aircraft of the future. Proceedings of IEEE International Conference on Automatica (ICA-ACCA), pp. 1-5.
[3] Flight Global News, 2020, Harbour Air to resume electric-powered Beaver flights as certification work begins
[4] Lilium, 2019. Lilium
[5a] Flight Global, 2019 Cape Air named as launch customer for the Alice electric aircraft
[5b] Eviation, 2019 https://www.eviation.co/
[6] International Aviation News, 2014. Youtube.
[7] Anderson, Aaron D., et al. “System weight comparison of electric machine topologies for electric aircraft propulsion.” 2018 AIAA/IEEE Electric Aircraft Technologies Symposium (EATS). 2018
[8] Mark Voskuijl et al. , 2018. Analysis and design of hybrid electric regional turboprop aircraft. CEAS Aeronautical Journal, 03, 9(1), pp. 15-25.
[9] Zaleski, et al., 2019. Battery start-ups are raising millions in the battle to crush Tesla
[10] Airbus, E-Fan X, https://www.airbus.com/innovation/future-technology/electric-flight/e-fan-x.html