Why Big Electric Planes Won't Fly, According to Vaclav Smil

The batteries just don't have the energy density to get a plane across an ocean.

Rolls-Royce's all-electric Spirit of Innovation takes to the skies for the first time
Rolls-Royce's all-electric Spirit of Innovation takes to the skies for the first time.


Treehugger loves the idea of electric planes and has shown quite a few of the new ones. But even Treehugger's Sami Grover, desperate for a decent beer back in the United Kingdom, and admiring this little Rolls-Royce electric marvel, is worried that they won't scale in size or distance traveled.

"The trouble is, of course, that the biggest climate-related challenge in terms of aviation is long-distance commercial travel," writes Grover. "It’s hard to see how offering an electric and low carbon option for a new and inherently inefficient application like flying taxis gets us nearer to that goal. And while electrifying and decarbonizing an existing segment of the market like commuter planes may serve as a technological stepping stone, it also runs the danger of distracting us from policy-level efforts at a demand-side reduction."

Smil book


Vaclav Smil is known to Treehugger for his books: "Growth, from Microorganisms to Megacities," "Energy and Civilization: A History," and most recently, "Numbers Don't Lie." Now, in an article for IEEE Spectrum, he munches on the numbers for electric planes and concludes that all these little electric planes won't make that much of a difference. "The problem is much more fundamental," he writes. Aviation is a giant business, and most of it is in much bigger, heavier planes.

In his book on energy, Smil explained how improvements in energy density, going from wood to coal to gasoline and natural gas, built the world we live in:

"By turning to these rich stores we have created societies that transform unprecedented amounts of energy. This transformation brought enormous advances in agricultural productivity and crop yields; it has resulted first in rapid industrialization and urbanization, in the expansion and acceleration of transportation, and in an even more impressive growth of our information and communication capabilities; and all of these developments have combined to produce long periods of high rates of economic growth that have created a great deal of real affluence, raised the average quality of life for most of the world’s population, and eventually produced new, high-energy service economies."

Back in IEEE Spectrum, Smil is back to talking about energy density and says batteries don't have enough of it.

"Large  turbofan engines powering these planes are fueled by aviation kerosene that provides nearly 12,000 watt-hours per kilogram. In contrast, today's best commercial Li-ion batteries deliver less than 300 Wh/kg, or 1/40th the energy density of kerosene. Even when taking into account the higher efficiency of electric motors, the effective energy densities go down to about 1/20th. That's more than better batteries can bridge within the next decade or two."

He notes that even if maximum energy density tripled, it still wouldn't be enough to get a plane from New York to Tokyo, and that is before even taking into account the fact that planes running on liquid fuel get lighter as they go and electric planes don't. Read enough Smil and you learn energy density is everything—it's what makes our world.

Comments are dismissive, suggesting that "just like the auto industry, the aviation market will start with smaller aircraft, as that’s where the technology is at, it will scale to larger aircraft over time." But Smil has been writing about the nature of technological progress as far back as when people found out that burning wood to cook meat delivered more food energy density. He could respond to the commenters by quoting his conclusion from his book on energy:

"Techno-optimists see a future of unlimited energy, whether from superefficient PV cells or from nuclear fusion, and of humanity colonizing other planets suitably terraformed to the Earth’s image. For the foreseeable future (two-four generations, 50–100 years) I see such expansive visions as nothing but fairy tales."

Looking at our much shorter window to cut our carbon emissions to slow and then stop the increase in temperature, it is likely that the proponents of electric planes flying across oceans are like those pushing hydrogen planes: It all seems like a way of maintaining the status quo by promising that someday, somehow, it will all be green and wonderful. But when you look at the real numbers, it just doesn't fly.