Why don't we have electric planes?

The chronicles of energy density.

Jun 16, 2021

We have electric cars, electric toasters, and electric spaceships. Electric heating, electric cooling, and even electric books. With all these electrons to go around, why don’t we have electric planes? Aviation in the US accounts for 12% of our transportation emissions and 3% of total emissions. [1] While that’s not as high as other industries, it’s still a big enough number to beg the question: can we eliminate this through electrification? Ignoring all of the emissions that go into generating the electricity to charge our airplane’s batteries, the answer is, surprisingly, yes, if you’re okay with flying 50 miles at a time.

Picture this: you’re building a plane, and you want it to be electric. Great start. You don’t want to go too crazy, so you pick your baseline route as New York City to Charlotte. That’s about 540 miles. [2] Remember that number, we have to hit it, or else we run out of fuel before we land—not great. The best place to start in the aircraft design process is with what you know. Assuming we have engines, powered by electricity, that can produce similar thrust to current jet engines, all we would really need to do is replace the fuel with batteries and plug them in. To do this we need to understand energy density. Energy density is simply how much energy is contained per unit volume. Jet fuel has an energy density of 9.6 kWh/L. If we were to replace that same volume with batteries, which have a best-case energy density of 265 Wh/L, our plane would have (265 Wh/L)/(9,600 Wh/L)=2.8% as much energy. Already our range is 2.8% what it was before.

Bringing these numbers back to reality, the most common plane to fly this route (Embraer 175), has a range of 2,500 miles. [3] At 2.8%, our electrified version would have a range of just 68 miles. That’s about 8 minutes of flight time. There are, however, a few more strings we can pull. If we were to decrease the capacity of the aircraft to, say, 20 people, we would have about 7,500 kg more to play with. This brings us to the concept of specific energy. This is very similar to energy density—specific energy is the amount of energy per unit mass rather than volume. Batteries have a best-case specific energy of 250 Wh/kg, giving our reduced passenger configuration an additional (7,500 kg)*(250 Wh/kg)=1,900 kWh of energy.

This aircraft has a fuel capacity of 11,610 liters. From our battery’s energy density of 265 Wh/L, we can find that, before changing the passenger layout, we had (265 Wh/L)*(11,610 L)=3,076 kWh of energy on board. Changing the passenger layout gave us an additional 1,900 kWh, increasing our total available energy by almost 62%. Nice! We now have 4.5% of the original aircraft’s range, as opposed to 2.8%. That’s about 113 miles, for a total of 14 minutes of flight time.

Now we could continue this trend if we wanted to, add more and more batteries to the plane to increase range. But now we run into size limitations. Here’s a quick example: Imagine we go to the extreme with the largest commercial airliner in service today, the Airbus A380. If we filled the entire volume of that plane with batteries, we would have 1.134 million liters. [4] Using that volume to find our batteries’ total energy, we would have (1,134,000 L)*(265 Wh/L)=300,000 kWh on board. That’s a lot, and it corresponds to a range of 6,700 miles—or almost 12 hours of flight time. Woah! Looks like we did it! We could fly two whole people from New York all the way to Tokyo.

Just for yucks, how much would those batteries weigh? Assuming a specific energy of 250 Wh/kg, those batteries would weigh approximately (300,000,000 Wh)/(250 Wh/kg) =1.2 million kg. [4] Ah, crap, that’s just about twice as heavy as the plane is without batteries, and with all those people on board. This means we need wings that are twice as large, which means we’ll have a significant increase in drag, which means we need to use more energy to overcome it, which means we need more batteries, which means we’re going to be even heavier, which means we need larger wings, which means… This vicious cycle continues on and on, and it doesn’t even take into account the weight and space needed for all of the parts of the battery that don’t store energy: frames, cables, etc.

As if this wasn’t enough of a deal-breaker, we haven’t talked about taking off, where engines require a lot more power. These four engines usually require the equivalent of 93,000 kW during takeoff. [4] With all of our batteries on board, they would need a whopping 200,000 kW. That means just taking off and climbing to your cruising altitude, which for a plane this size takes about an hour, you’d use 2/3 of your energy. Giving you only a few hundred miles of range left to get where you need to go. So in summary, to realistically get you from New York to Charlotte, you’d need a plane two times larger than the world’s largest commercial aircraft. You’d also need to make an electric propulsion system 1,000 times more powerful than anything humans have ever put on an airplane.

All of these examples have seemed pretty grim for electric planes. And this more-weight-needs-more-batteries-needs-more-weight cycle means that we can’t just go bigger. Therefore, in the next 10 to 15 years, we will likely only see them as small, 4 to 6 person recreational planes. Some of these, like Pipistrel’s AlphaElectro, are FAA certified already but only have ranges between 50 and 100 nautical miles. [5]

Notice that this is where the electric car conversation began, but weight for an aircraft is a much larger roadblock than it is for cars. As such, we will most likely see electrification in the form of hybridization. Where jet engines will use hybrid motors to improve efficiencies of already-in-service engine components. This won’t eliminate emissions, but it could significantly help in reducing them. If you’re out on emissions from the aviation industry in general, check out the work that is being done on 100% sustainable aviation fuel (SAF).

Thank you for reading this week’s edition of It’s Not Rocket Science, see you all next week!

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