Is The Full-electric Commercial Airliner Actually Feasible?

The aviation industry has relied so far on two main combustion technologies: piston engines and gas turbines. While the past was polluting and noisy, the advocates of the electric aircraft promise a world of clean skies and almost silent aviation. How can that be possible?

There are two main clean technologies competing on the ground and in the air: batteries vs fuel cells. Both technologies come with pros and cons. Batteries provide all the power you need, but are limited by the low gravimetric energy density. Fuel cells can use hydrogen fuel, so they are not limited by the amount of energy. However, they are limited by the low gravimetric power density. In other words, while batteries can't carry the energy required by an airliner, fuel cells can't provide enough power to sustain flight.

Sizing The All-electric Aircraft

The energy requirement of an aircraft comes from the sum of the energy needed during all the different flight segments, basically: taxi, take off, climb, cruise, descent, loiter, and landing. They can be calculated from primary requirements such as maximum take-off weight, take-off distance, vehicle aerodynamics, climb rate, cruise altitude, speed, and range requirements. A typical single aisle commercial aircraft has a maximum take-off weight (MTOW) of 56.5 tons. It can fly for about 8.5 hours using a tank of about 20,000 liters of kerosene. The fuel plus tanks and pumps has a weight of about 22 tons. Taking into account a fuel efficiency of 30%, the energy requirement is about 60 MWh for the longest flight, using about 40 MWh for the cruise segment of the mission profile. The engines weigh about 3 tons each (2x), providing a combined power of 40 MW, working near peak power during take-off and when climbing. Nonetheless, only 7.5 MW are needed when cruising.

So, the total mass of the energy storage and propulsion system is about 28 tons. For this typical 200 pax. airliner, the total gravimetric energy density is 60 MWh / 28 tons = 2.14 kWh/kg. A similar calculation taking into account the gravimetric power density returns 40 MW / 28 tons = 1.42 kW/kg.

Battery Based Aircraft

Since batteries are limited by the energy density requirements, we will focus on that. The current state of the art batteries are rated at 400 Wh/kg, while our 200 pax aircraft requires 2,140 Wk/kg. It is far, isn't it?. The gap is even larger than that, for a battery-based aircraft we still need a propulsion system. The current state of the art in electric propulsion is rated at about 6 kW/ kg. So, for 40 MW we still need 6.7 tons for motors and blades. This leaves only 21.5 tons for the battery. So, the actual gravimetric energy density requirement for the battery would be 60 MWh / 21.5 tons = 2,800 Wh/kg. The best option are the Lithium-Sulfur batteries with a theoretical limit of 2,567 Wh/kg. This is the hard limit imposed by physics and, as you can see, it is still below the requirements. The current estimate is to reach 1 kWh/kg batteries by 2040, so the future of battery-based aviation is limited to short distance commercial trips, limited by the weight of the energy storage system. Urban Air Mobility is considering batteries as the trips are short and payload is a small percentage of the MTOW.

For our single aisle aircraft, using a 500 W/kg battery, the complete energy storage and propulsion system would weight 127 tons, more than twice the whole MTOW of the typical aircraft in this class.

Fuel Cell Based Aircraft

Since fuel cells are limited by the power density requirements, we will focus on that. The current state of the art in fuel cell technology is rated at about 2 kW/kg, cooling system included, and increasing at a very promising pace. At the current gravimetric power density our aircraft would require 20 tons to deliver the 40MW of peak power. It will require also motors and blades. Using the same electric system proposed for the battery-based aircraft, it would require 6.7 tons.

The system still requires fuel, the hydrogen tanks. Hydrogen is very energetic, with a gravimetric energy density of 33.3 kWh/kg. The problem is the mass of the tanks required to store the small H2 gas molecules. For a 700 bar Type IV tank the gravimetric percentage is 6%. This means that the effective density, including the gaseous H2 container, is 2 kWh/kg. So, to satisfy the energy requirement of 60 MWh, the mass required for the filled tanks would be 30 tons. The technology for compressed hydrogen tank is evolving fast with Type V prototypes able to store hydrogen at 1000 bar, so this could be reduced 30% in the near future.

This fuel cell configuration, using compressed hydrogen for energy storage, requires 56.7 tons using today's proven technology. It is around the MTOW for the whole aircraft but less than half than the mass of the equivalent battery-based aircraft. We are closer to our goal, but not there yet.

Is there a better option? Well there is still proven aerospace technology used in space applications that we could leverage in aviation.

Fuel Cell based Cryogenic Aircraft

This option uses the same fuel cell technology, requiring the same 20 tons for the 40 MW power generator. But, instead of using compressed hydrogen, it will use liquid hydrogen stored in cryogenic tanks. These tanks sport a gravimetric percentage of 63%, delivering a gravimetric energy density of 21 kWh/kg. This storage system would have a weight of only 2.9 tons at take off.

The motors can be improved too. The low temperature of the fuel enables the use of superconducting coils instead of heavy magnets. High Temperature Superconducting motors operating at 77K deliver 20 kW/kg so a propulsion system of 2 tons could deliver the peak power of 40 MW required by the aircraft.

This system could provide all the energy and the power required by the aircraft using only 24.9 tons. It offers more than 3 tons of additional payload, with zero emissions.

There is still a long development and certification road in order to bring this aircraft to reality. The big problem is the volume required by the bulky cryogenic tanks. The current aircraft structure is the typical tube+wings. This configuration offers an easy way to manufacture and assemble the airframe. The problem is the low volume of the current configuration. There are other configurations such as the Blended Wing Body (BWB) or the Flying-V offering not only better aerodynamics, but also a larger interior able to contain the volume for the payload, plus the additional volume for the tanks and fuel cells.

Conclusion

The current and proposed mass budgets for the propulsion system of the proposed 200 pax single aisle aircraft is summarized in this table:

The objective of this study was just providing a rough analysis of the gravimetric energy and power densities to check the feasibility of a single aisle aircraft able to transport 200 pax. Today, we have the technology required to replace the current fuel and propulsion systems with a zero-emission electric system. We have the cryogenic tanks, we have the fuel cells, we have the superconducting motors, and the airframes able to contain all that. Developing an aircraft able to replace the Boeing 737 or the Airbus A320 is just a matter of money to engineer a safe and reliable solution, capable of being certified for commercial transport in a reasonable time frame.

We don’t know whether it is going to be Boeing, Airbus, Embraer, Comac, or a SpaceX-like startup disrupting the aviation industry with a new innovative and bold development approach. The technology is here, the need is there, is the money ready to bet for a zero-emission commercial aviation?