The year is 2023 and electrification is the name of the game. But is it possible to apply it to design of aircraft of tomorrow?
The electric future
The year is 2023, and the problem of climate change is finally getting the attention it deserves – not only from scientists but the general public as well. At the same time, an ever-increasing number of customers consider the environmental impact of their purchases when making them. Moreover, this trend transcended its natural niche of home appliances and heating devices and can be seen in other areas, such as transportation. And where demand goes, supply usually follows, which is the case in the automobile industry. A decade ago, most car brands offered only fossil-fueled models, and the Diesel engine was considered the pinnacle of nature conservation. Now almost all major brands have at least one fully electric car in their portfolio.
Electrification of cars and other means of transportation seems like a natural step. After all, transportation was responsible for 21 percent of global CO2 emissions in 2018 [1]. Roadgoing vehicles were responsible for almost 75 percent of those, eclipsing the runner-up, aviation, which came shy of 12 percent. However, as the market for aircraft-related services is predicted to more than double by the year 2041 in comparison with today [2], one can start thinking if aviation can receive the electric treatment just as cars did.
Problem 1: battery weight
In theory, most commercial and light aircraft could be electrified. After all, it makes no difference for a propeller what type of engine spins it. At the same time, other types of propulsion systems, such as turbofans found in most airliners, can be replaced by ducted fans or propfans, both of which can be electric-powered. However, the trouble starts with the weight of the battery pack necessary to power such an endeavor. In general, batteries suffer from poor specific energy density, or energy per unit mass. Though several new battery types have been proposed, Li-ion batteries are still the most energy dense of all commercially available ones as of 2023 [3].
Li-ion batteries are usually below 1 MJ/kg, though some new developments show potential for doubling that number [4]. However, it wanes terribly in comparison with liquid fuels currently in use. For comparison, petrol boasts around 46 MJ/kg, while the A-1 jet fuel used in commercial aircraft is slightly less energetic at 43 MJ/kg [5]. As a consequence, a battery pack is much heavier than a fuel tank holding the same amount of energy. In the case of the Fiat 500, the difference between the hybrid (1.0, 70 PS) and electric (42 kWh, 118 PS) versions is a 310 kg weight penalty for the electric one. Moreover, in mixed-cycle driving conditions the electric version has a stated range of 320 km only — significantly less than the 740 km of the hybrid [6, 7].
Let us now consider an actual aircraft, an Airbus A321neo: the biggest and, coincidentally, the most often ordered one from the A320neo family. With its range just below 6000 km, it can service both intracontinental as well as transatlantic routes. Its fuel supply consists of a maximum of 32 940 litres of fuel [8], which translates to approx. 26 500 kg of A-1 jet fuel at 15 centigrade. This corresponds to 1139.5 GJ of chemical energy stored in its tanks. If the same amount of energy was to be stored in Li-ion batteries with a specific energy of 1 MJ/kg, the entire battery pack would weigh a whopping 1140 tonnes. This surpasses A321neo’s maximum take-off weight of 97.4 tonnes more than 11 times. In fact, it is more than the weigth of the Cisne Branco — a tall ship of the Brazilian Navy.
In the above calculation, we assumed silently that the energy requirements for electric and jet propulsion systems are the same. Of course, that might not be the case. Let’s then assume for a moment that there exists an electric system that requires a battery pack of the same mass as the A321neo’s fuel load. Such a battery pack would hold 43 times less energy than jet fuel. To compensate for that, it would need to have an efficiency 43 times higher. Given that a normal turboprop engine has an efficiency above 30 percent, this hypothetical electric system would need to severely violate the laws of physics as we currently know it. Indeed, given an efficiency much higher than 100 percent, it would constitute a perpetual motion machine.
A further complication stems from the fact that batteries exhibit so-called internal resistance. It means that not all energy stored in them can be converted to actual work. Instead, some of it is lost irreversibly as heat. This effect has to be taken into account when sizing the batteries, so our previous estimates for total weight would need to be increased even more.
Problem 2: charging
Theoretically, the battery weight problem could be solved by new, as-of-yet-undiscovered battery technologies. However, they would still require charging. Let’s focus on Airbus A321neo again. As per the Airbus manual, a typical airport operation involving this aircraft encompasses loading it with 20 000 litres of fuel in approximately 16 minutes [9]. This is equivalent to transferring chemical energy at the rate of 0.72 GJ per second. If the same loading rate was to be achieved with batteries, a charger capable of 0.72 GW continuous power operation would be required. To put this number into perspective, loading two such planes simultaneously would require the entire capacity of Germany’s largest nuclear power plant — Isar-2 — operating at full power. Even the biggest German power station — Neurath — again running at full power, would produce enough electricity for about 538 such operations a day. This is much less than what was required by the Frankfurt airport alone in 2019 [10].
Additional complexity comes from charging efficiency and electricity transfer losses. Not only does it raise the power required for charging, but also creates waste heat, which has to be disposed of. How much, exactly? That all depends on how quickly we want to charge. If we need the charging done as fast as possible, transfer losses are equal to the battery charging power — the aforementioned 0.72 GW. Dissipating that much power is certainly possible — power stations do it all the time — but not without the usage of cooling towers or a steady supply of cooling water. Neither solution is unacceptable at an airport. Neither solution is unacceptable at an airport. Oh, and the total power required would be more than what is required to power a flux capacitor.
Problem 3: operating conditions
Internal combustion engines can operate in a wide variety of conditions. The same engine will run in the tropical summer or well above the arctic circle, save perhaps for different oil or fuel additives. Regardless, no significant drop in performance or longevity should occur. However, it’s a completely different story with batteries.
Batteries do not store electric charge directly as capacitors do, relying instead on chemical reactions. At low temperatures, the dynamics of those reactions are severely hampered, leading to much worse performance. For instance, one study of Li-ion batteries observed a 95 percent decrease in both specific energy and specific power at -40 centigrade compared to 25 centigrade. Moreover, the battery state of charge decreases with decreasing temperature. How much charge is lost depends on factors such as electrolyte composition, but can fall below 80 percent at -15 centigrade, again compared with 25 centigrade. Other detrimental low-temperature effects include more difficult charging and the possibility of short-circuiting.
High temperatures are also best avoided. Operating Li-ion batteries at temperatures around 100 centigrade leads to significant capacity loss and accelerated aging. Raising the operating temperature even higher might induce the so-called thermal runaway. This is a very dangerous phenomenon during which various chemical compounds inside a battery react in an exothermic fashion, raising the temperature even higher and irreversibly destroying the battery.
Such a constrictive operating envelope is extremely cumbersome for aviation applications. Aircraft operate in a wide variety of conditions, including taking off from extremely cold or hot airfields. As power requirements are the highest during takeoff, the aforementioned reduction in battery power at low temperatures is unacceptable. At best it raises the required runway length and at worst grounds the plane. The possibility of a short circuit is also not the most pleasing one, given the difficulties in putting down battery fires and the toxicity of resulting gasses. Even worse, if it occurs during take-off after a plane has reached the decision speed, it will almost certainly lead to exceeding the runway limits, and possibly loss of life. Conversely, taking off in hot weather requires making sure that the batteries are cooled appropriately to prevent thermal runaway.
To further complicate temperature management, batteries need to stay sufficiently warm at cruise altitude, where temperatures can fall below -70 degrees. Though the wing temperature is somewhat higher, it is still around -30 degrees. We now have two possibilities — the batteries either generate heat at a rate higher than losses to the atmosphere or not. In the first case, they require additional cooling with the outside air, which increases aircraft drag. In the second one, they will require additional heating so that their temperature does not fall below a certain threshold. One way or another, it introduces additional complexity and increases the possibility of a failure.
Outlook
The list of problems presented here is by no means exhaustive. There are others, such as battery life, economic considerations, or recycling, to name just a few. One could argue, however, that with sufficient technological progress, we can overcome them all. New battery chemistries will address the weight issue, while charging can be replaced by swappable battery packs. However, I personally do not believe it will happen in the near future, if at all.
Progress in battery technologies has been painfully slow for years. Though several new battery types that are more energy-dense than Li-ion batteries are now on the horizon, they still fall terribly short of liquid fuels. Moreover, service life and high production costs are still a concern.
As for swappable battery packs, they have already been introduced in other vehicles, such as e-scooters or mopeds. However, for aircraft, the situation is much more complicated. The batteries would most likely be located in wings, from where they cannot be easily removed due to interfering components such as spars or ribs. Also, replacing them before every flight would risk damaging other components. Apart from that, aircraft differ immensely between models and manufacturers, and a one-size-fits-all solution is hard to find, if possible at all.
I therefore highly doubt that commercial aircraft — save perhaps for some ultralight machines — will ever be electric-powered. That does not mean, however, that aviation will always be bound to fossil fuels. Sustainable aviation fuels are on possibility, and hydrogen is another, so net-zero aviation in 2050 is still on the cards.
References
[1] IEA Energy Technology Perspectives 2020 webpage (accessed 4.3.2023).
[2] Airbus. Airbus Global Services Forecast 2022-2041. Airbus, 2022. (accessed 15.2.2023).
[3] Houache, M.S.E.; Yim, C.-H.; Karkar, Z.; Abu-Lebdeh, Y. On the Current and Future Outlook of Battery Chemistries for Electric Vehicles—Mini Review. Batteries 2022, 8 (7), 70.
[4] Wu, F.; Fang, S.; Kuenzel, M.; Mullaliu, A.; Kim, J.-K.; Gao, X.; Diemant, T.; Kim, G.-T.; Passerini, S.Dual-anion ionic liquid electrolyte enables stable Ni-rich cathodes in lithium-metal batteries. Joule 2021,5 (8), 2177-2194.
[5] Air BP. Handbook of Products. Air BP, 2000.
[6] Fiat 500 BEV brochure. Fiat Auto Poland, 2023. (accessed 15.2.2023).
[7] Fiat 500 brochure. Fiat Auto Poland, 2023. (accessed 15.2.2023).
[8] Airbus A321neo homepage (accessed 15.2.2023).
[9] Airbus, Customer Services, Technical Data Support and Services. Aircraft characteristics – airport and maintenance planning. Airbus, 1985, rev. 39, updated 1.12.2020. (accessed 15.2.2023).
[10] Fraport AG, Market Research and Trends. Frankfurt Airport Air Traffic Statistics 2019. Fraport AG, 2020.f (accessed 15.2.2023).
[11] Ma, S.; Jiang, M.; Tao, P.; Song, C.; Wu, J.; Wang, J.; Deng, T.; Shang, W. Temperature effect and thermal impact in lithium-ion batteries: A review. Prog. Nat. Sci. 2018, 28 (6), 653-666.
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