Low-Energy Fridays: Why don’t we have electric airplanes?

A commonly proposed solution to the carbon emissions problem is to replace as much fossil fuel use as possible with electric alternatives. Since we already know how to produce carbon-free electricity cheaply, this comes up a lot among those of us working in climate policy. But some sectors are difficult to electrify no matter how hard we try—and aviation is a prominent example.

Greenhouse gas emissions from global aviation are expected to rise substantially, with international aviation emissions potentially tripling from 2019 levels by 2050. While many love to talk about the golden age of air travel when people could knock back a few cocktails with some live music in an airplane’s piano bar lounge, the dirty secret is how much more expensive and inaccessible air travel was back then. Before the industry was deregulated, airlines couldn’t establish new air routes to compete on cost, so they had to compete on service. Air travel may be less luxurious nowadays (for those of us in economy class, at least), but it is certainly cheaper. And because the rest of the world is finally enjoying its benefits, demand is rising alongside emissions.

So if we can electrify cars, then why not airplanes? The answer boils down to two issues: energy density and energy efficiency.

Although everything that consumes energy needs a source for that energy, not everything contains the same amount of energy. As an example, uranium has about 14,000 times as much energy as coal in the same amount of mass. Part of this is due to the extreme energy potential of uranium and other heavy elements; the rest relates to coal’s low energy density. Liquid fuels like gasoline—while nothing compared to uranium—tend to be quite energy dense. In fact, 1 kilowatt-hour (kWh) of electricity has the equivalent energy density of 0.03 gallons of gasoline.

Put more simply, a Tesla Model 3 with a 75.0 kWh battery holds the energy equivalent of 2.3 gallons of gasoline. Inversely, a car with a 13-gallon tank holds as much energy as a 433 kWh electric vehicle (EV) battery.

Obviously, a Tesla can get a lot further on that lower amount of energy than a conventional car can. But why is that? This gets into the issue of energy efficiency. While gasoline has a lot of energy, combustion engines are inherently energy inefficient. A combustion-engine car only converts about 20 percent of the energy from gasoline into driving motion, while an electric car gets 89 percent (and a jet engine about 40 percent). This is why batteries—a way worse form of energy storage than liquid fuel in a tank—enable an EV to go about as far as a conventional car on one tank of gas.

Also important when comparing fuel types is the weight involved in the energy storage. An electric car’s battery pack usually weighs over 1,000 pounds, while a full tank of gasoline weighs only about 100 pounds. This matters little for vehicles that sit on the ground; however, keeping mass aloft requires continuous energy consumption. It’s the real reason we don’t have electrified passenger aviation: It would to take more energy to keep a lithium-ion battery in the air than it’s worth.

Of course, whether one way of doing something is worth more than another depends on economic conditions. Fuel costs represent a major expense for airlines at about 29 percent. Profit margins for airlines are also narrow, at about 3 percent (sounds like those leg room cuts didn’t help much, huh?). Electrifying air travel is appealing because electricity is much cheaper than jet fuel. One company, Zunum Aero, attempted to crack this code by fulfilling a niche demand to make short air routes more convenient and cheaper but was ultimately unsuccessful.

The real reason we don’t have electrified air travel boils down to economics, as all things do. The economics of keeping a heavy battery aloft don’t favor electrification over liquid fuels. The worthwhile insight is that a policy attempting to force electrification of air travel would be economically costly because of the physical challenges associated with such an energy policy scenario.

While electrification of air travel is obviously too challenging to be a reasonable policy, it’s worth noting that policymakers meddle in energy consumption all the time—and not always efficiently. For example, although there’s currently a focus on facilitating a coal revival, coal’s low energy density compared to natural gas and the comparatively worse energy efficiency of coal power plants constrain the economics of such an approach. Similarly, biofuels have many defenders; however, growing corn as a fuel supply isn’t as efficient or scalable as petroleum fuels.

The main insight is that physics and economics interplay to make what is merely possible into what is practical. Policies that run counter to that wisdom cost more and offer fewer benefits. If economics should shift and the electrification of air travel become worthwhile, we want to ensure relevant industries are free to make those changes as necessary. However, policymakers ought not to force outcomes that can’t be achieved efficiently.