2022 was a significant year for the segment of the aviation industry that is looking at electricity as its main source of propulsion. Many key milestones were achieved, these include test flights of full scale prototypes. Contrary to popular belief, the main challenge this developing industry faces is not the development of more energy dense batteries but the hurdle of the certification process. Even with existing batteries, aircraft can be and have been developed that can complete a variety of missions.
The progress and maturity of most emerging technologies follows a well-known curve called the “Gartner hype cycle.” For electric aviation the preceding years can be associated with the period of “peak of inflated expectation” on the curve. Riding these expectations, there were hundreds of start-ups that mushroomed all around the world with the aim to venture into the Urban Air Mobility market alongside established aviation companies like Boeing, Airbus and Embraer. The current period on the Gartner chart for electric powered aircraft can be said to be the “trough of disillusionment”. It is the phase on the hype curve, where things don’t look as promising as they did earlier. At present, many of the startups that sprouted almost at the same time a few years ago are now defunct. A high profile casualty of this was Kitty-Hawk, a company that had developed one of most energy efficient eVTOL aircraft called the Heaviside, but unfortunately was not able to scale it up to a product that would be viable for the urban air mobility market.
Looking back, one will notice that the key driver for the electric aviation industry was the Li-ion battery. Within two decades of its release, the Li–ion battery with its 5 times higher charge carrying capacity compared to lead acid battery, found itself in a variety of electronic and electrical products. Initially it was used to power digital cameras & smart phones. It then made its way into bigger electrical gadgets like mobility scooters and electric cars.
It was because of the Li-ion battery that many new technologies spawned. The Lithium-Ion battery was the primary reason behind the development of quadcopter drones. These drones were able to demonstrate the versatility of electric propulsion over conventional propulsion. Through a small, solid state control system (i.e. the motor controller) the motors could be easily sped up or sped down which allowed the control of lift, yaw and roll. The need for a complex gear system was eliminated as required in a conventional fuel based propulsion system, be it a jet engine or an IC engine system. With the added benefit of reduced maintenance of electric motors compared to engines, it was a natural course of inspiration to pursue scaled up drones that would act as personal flying machines. But there was an issue, and it was the weight. Batteries with their heavy weight don’t provide as big a challenge in electric cars as they do in an electric aircraft. Physics also dictates that as the scale is increased, the weight of the object grows cubically.
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Even the best batteries today are nowhere close to the energy capacity of fossil fuel. If we just compare the specific energy of AVgas, it is 50 times more than the best performing Lithium Ion battery i.e 44 MJ/kg compared to 0.9 MJ/kg. On the face of it, this looks a huge difference however, electric propulsion does provide some advantages that reduces this gap significantly. For example, electric motors can be spun with efficiencies of over 90% whereas most of the internal combustion engines have efficiencies of around 20%. This disparity alone brings down the gap of 50 times to less than 12 times. Another leverage electric motors provide is that they can be scaled up or down and maintain the same level of energy efficiency and they are lighter compared with the IC engines of the same power. Several smaller motor propulsors can therefore be installed instead of a few large ones, which allow for distributed propulsion i.e. a more efficient way of powering an aircraft. The energy consumption through higher aero propulsive efficiency (distributed propulsion) as suggested by NASA through their Maxwell X-57 can be decreased by 3 times. Therefore, theoretically the gap in performance between battery electric powered and AVgas powered aircraft can be reduced to just 4 times. Note that this comparison applies to small fixed wing aircraft, such as the Cessna 172, that run on internal combustion engines.
A good example of this is the Pipestrel Velis electro, which is the first type certified electric aircraft. The Velis electro has shown that the difference in endurance is certainly not 50 times but around 6 times to that of a comparable conventionally propelled aircraft. This gap will be further reduced as the battery technology improves and aircraft are produced ground up with electric propulsion technology at their heart.
Cost is another factor that comes into play and favors electric propulsion. For example, the Pipestrel Velis electro with a battery pack capacity of 24.8 kWh can be charged for about 8 dollars. The fully charged pack provides an hour-long flight. The Cessna 174 on the other hand would burn around 8.5 gallons per hour. This means it would consume about 54$ of fuel alone in an hour long flight (Fuel price $6.4/gallon).
There is also the impetus for reducing carbon emissions that is driving the technology forward. With the goal of net zero by 2050 in sight, there is a pressing need to find ways to curb CO2 in the aviation industry and electric propulsion in this regard is a way forward.
Despite all of the listed advantages of the electrified propulsion system, it is apparent that battery technology at present is not sufficient to replace our existing air transport infrastructure. To reap the benefits of electric propulsion for longer range missions, one has to look towards hydrogen. The high specific energy of hydrogen (120 MJ/kg) that is 2.7 times higher than traditional jet fuel (44 MJ/kg) lends itself well to be considered for aviation. However, if this fact is taken only on its face value then it can be misleading. As hydrogen is the lightest gas, to have a substantial mass of it onboard, it either has to be compressed at very high pressures or be liquified.
If hydrogen is stored as gas, then it cannot be stored in wings like normal aviation fuel. Hydrogen in gaseous form is stored in specialized pressure vessels. On the other hand if hydrogen is stored as liquid then it requires a cryogenic fuel tank. In either case, the volumetric energy density is lower than that of Avgas. Hydrogen compressed to 700 bars has volumetric energy density 7.1 times lower than jet fuel. In liquid form at 20 K (−253 °C), the volumetric energy density is 4.1 times lower than jet fuel (8.5 MJ/L compared to 35 MJ/L).
So hydrogen storage has its own set of problems. It costs much higher to install a fuel cell system compared to a battery. The running costs are also higher nonetheless it can provide 5 times more energy than a battery. And therefore purely on the amount of energy that can be carried Hydrogen-Fuel Cell system has a much better starting point compared to Lithium Ion batteries. The higher gains of electric propulsion coupled with the hydrogen-fuel cell system can certainly push the performance to surpass the AVgas fuelled aircraft particularly if an aircraft is designed from ground up to support this new powertrain.
There are new hydrogen storage technologies also on the horizon. This includes Metal Hydride storage that is more compact than gas hydrogen storage and is being pursued by several commercial concerns. Another solution is the Powerpaste that was developed by the Fraunhofer Institute. Both the metal hydride and the magnesium based powerpaste trap hydrogen for storage within their structural lattice and release it at high temperature.
The term “electric aircraft” often brings to mind 4 seater eVTOL aircrafts like the Joby S4 from Joby Aviation, the VA-X4 from Vertical Aerospace and Eve by Eve Air Mobility. It can be said with certainty that due to the challenges of the certification process, these aircraft will not be the ones that will take to the skies in the near future. Certification is most likely to be achieved first by the fixed wing conventional and short take-off and landing electric aircraft, like Eviation Alice. These aircraft will be used for middle mile logistics for cargo transport.
While the battery technology may have not progressed as many of the electric aviation companies would have hoped but there is still plenty to be excited about in 2023. The release of solid state batteries will provide a boost and so will the availability of cheap green hydrogen. Electric aviation is certainly here to stay.
This article was written by Adnan Khan. Adnan currently works within the Low Carbon Fuels business for Shell Plc and is located in Houston, Texas. He joined Shell in 2002, where he has held various roles in Downstream, Upstream and Projects & Technology businesses and brings a strong passion for energy transition.