Flying on Sunshine

What’s the real deal with electric aircraft?

Solar-powered airplanes are very cool, but there aren’t that many electric airplanes flying in 2023. At first glance, it seems like electric commercial aviation is totally impractical and won’t happen for many decades, if ever.

As Bill Gates wrote in 2021:

“The best all-electric plane on the market can carry two passengers, reach a top speed of 210 miles per hour, and fly for three hours before recharging. Meanwhile, a mid-capacity Boeing 787 can carry 296 passengers, reach up to 650 miles an hour, and fly for nearly 20 hours before stopping for fuel.”

On the other hand, United Airlines and Archer have announced that they will launch an electric air taxi service in Chicago beginning in 2025. Michigan Medical has announced that electric drone company Zipline will begin prescription delivery by air in 2024. The Federal Aviation Administration is planning for our skies to be filled with electric and autonomous flying machines.

So, when will you be able to take a trip in an electric aircraft using batteries charged by solar electricity?

Why People Think Electric Aviation Is Impractical

Let’s review why we haven’t had electric aircraft until very recently. Tad McGeer in his article, “The total impracticality of electric aviation,” does a good job explaining the challenge: per unit of energy delivered, batteries have been more than tenfold heavier than avgas or jet fuel. Batteries that heavy in an airplane don’t leave enough room for cargo or passengers.

The obvious question is, how much lighter can batteries get?

Back in 2021, Mr. Gates opined:

“Batteries are getting better […]. If we’re lucky, they may become up to three times [emphasis added] as energy dense as they are now.” But that wouldn’t be enough to make battery-powered airplanes practical.

This prediction is now hard to explain. Why would anyone think that batteries couldn’t quickly become much more than three times as energy dense as they were in 2021?

What Has Changed

The development of electric aviation can’t follow the same path as the electrification of the automobile industry. While car manufactures could rush electric vehicles to market by replacing the fuel tanks and engines with lithium-ion batteries and electric motors, that path isn’t practical for aircraft manufacturers. Lithium-ion batteries are just too heavy to adapt historical aircraft designs to become battery powered (although Wright Electric is trying).

Instead, advanced software algorithms are allowing smaller unmanned electric aircraft to take flight. Experience flying smaller aircraft will inform the development of larger aircraft once batteries are light enough. Anyone who is thinking about the future of electric aviation without anticipating the impact of technological breakthroughs in basic battery science will miss their flight.

Just two years after Mr. Gates published his thoughts about battery-powered airplanes, researchers at the Illinois Institute of Technology and U.S. Department of Energy’s Argonne National Laboratory announced that they have developed a lithium-air battery that can be almost four times as energy dense as the best lithium-ion batteries on the market in 2023.

The February 2023 Science article describing the lithium-air battery research includes this striking sentence:

“A lithium-air battery based on lithium oxide formation can theoretically deliver an energy density that is comparable to that of gasoline.”

Knowing that lithium-air batteries can at least theoretically become competitive with gasoline in terms of energy density gives investors confidence to provide start-up companies enough funding to bring their designs to market.

Despite the bulk of lithium-ion batteries, some companies are already using them to deliver packages in flying electric drones. These pioneers are getting operational experience while better batteries are being invented. If aircraft are able to fly with the heavyweight ballast of today’s lithium-ion packs, imagine how they’ll perform when they can swap in featherweight lithium-air batteries!

Based on peer-reviewed science and working prototypes, merely tripling the energy density of lithium-ion batteries is not the best we can hope for. Lithium air is one plausible design for batteries that can compete with gasoline. Here’s how it ranks in a comparison of the gravimetric energy density of fuel and other battery technologies:

• Aviation gas and jet fuel: ~12,000 Wh/kg
• Lithium-air batteries: 11,000 Wh/kg (theoretical), 1,200 Wh/kg (expected)
• Lithium-sulfur batteries: 2,600 Wh/kg (theoretical), 900 Wh/kg (achieved in 2017)
• Lithium-ion batteries with nickel-based positive electrode materials, liquid electrolytes, and carbon or silicon-based negative electrode materials: 1,191 Wh/kg (theoretical)
• Lithium-ion batteries available from Amprius and CATL in 2023: 500 Wh/kg
• Lithium-ion batteries from 2021: 250 Wh/kg

The prediction that we’d be “lucky” if batteries reached 750 Wh/kg, triple the gravimetric energy density of the best lithium-ion batteries available in 2021, seems laughable now. In two years, companies have already doubled the specific energy of the industry’s best batteries and peer-reviewed science has described a clear path to more than quadrupling it.

The very limited basic science we’ve done so far already suggests that it should be possible to improve battery specific energy by an order of magnitude. If lithium-air batteries reach their theoretical potential, their energy density will be equivalent to jet fuel. And there’s little reason to believe we have exhausted the search space for all possible materials that could be used to build better batteries. We’ll probably find many more materials than can rival lithium-air batteries. It’s just a question of looking.

The theoretical limit to the specific energy of a battery is given by the amount of charge (i.e., number of electrons) a material can deliver per gram (the “specific capacity”) multiplied by the electrochemical series voltage. If a material can hold more electrons more tightly it can store more energy per mass.

It will be very interesting to see what comes out of the $209 million in battery research and development the US Department of Energy invested in 2021 and the $2.8 billion invested in 2022 for the American Battery Materials Initiative. Some of that new funding could be used for basic science to fill in our gaps in knowledge about battery materials that can provide an energy density similar to jet fuel, and some of that new funding can be used to refine manufacturing processes for older generations of battery materials (like lithium cobalt oxide and lithium iron phosphate and Prussian White) that we know don’t have the potential to electrify commercial aviation but can be used for ground-based vehicles and stationary electricity storage in buildings.

If you’re betting against electric commercial aviation, you have to assume that none of the new funding being invested after 2021 will be used to make batteries significantly lighter. In other words, you’d be betting that despite the huge amount of money we pour into improving battery manufacturing this decade, we won’t be able to make the switch from lithium cobalt oxide to lithium oxide despite knowing that lithium oxide provides four times the gravimetric energy density, and we won’t discover any valuable insights into improving specific capacity or electrochemical series voltage in battery materials that can be commercialized.

Who Is Flying Electric Now

We don’t have to wait for better batteries to come out of the labs before electric aviation takes off. While new battery technologies are being perfected and commercialized, a whole fleet of electric aircraft are getting off the ground lugging heavy lithium-ion batteries along for the ride.

In some ways, the weight penalty of lithium-ion batteries is forcing faster innovation than we might see if we had invented lithium-air batteries first. Since it’s physically impossible to build a lithium cobalt oxide (or any other nickel-based lithium-ion chemistry) battery aircraft with similar performance to a gasoline or jet fuel aircraft due to the difference in energy density between lithium-ion batteries and fossil fuel, designers are forced to be more creative.

An obvious way to lighten the load of any flying craft is to eliminate the pilot on board. That’s what these flying drone delivery companies do:

• Zipline has been delivering packages using autonomous flying electric drones in Africa since 2016, in North America since 2020, and in Asia since 2022.
• Matternet announced in 2022 that its electric M2 drone was the first to non-military unmanned aircraft to receive Type Certification by the Federal Aviation Administration in the United States.
• Wing Aviation has made more than 300,000 commercial deliveries across three continents as of 2023 using its autonomous electric drones.
• Flytrex is delivering food by flying electric drones in one city in Texas and four in North Carolina as of 2023.
• Zing Drone Solutions at a site in Tierra Verde, Florida, is testing ways to enhance electric drones built by other companies to make deliveries, so that one day companies could create their own airborne delivery fleets using drones from different manufacturers the way they can create ground-based delivery fleets using vans and trucks from different manufacturers.
• Wingcopter is carving out a niche in the crowded drone delivery market with its Wingcopter 198 model that can carry heavier loads and fly further than competing drones.

Even though the FAA and other regulators are comfortable allowing flights over densely populated areas for package delivery without requiring pilots in the aircraft, they (and passengers) might be more hesitant about electric air taxis without pilots. The nascent electric aircraft industry is hoping regulators will split the difference between requiring two pilots (a pilot and a co-pilot) and no pilots on board. A couple companies have announced plans to reserve a seat for one human pilot when their electric air passenger service starts:

• Archer plans to be flying 6,000 electric aircraft by 2030. In each one, 20% of the passenger capacity will be occupied by a pilot. Each aircraft will have one pilot and four passengers.
• Joby likewise imagines flying air taxis with one pilot and four passengers with an “aerial ridesharing” business model comparable to how Uber operates with one human driver and several paid passengers.

Another player in the scramble to launch an air taxi service is more ambitious:

• Volocopter has a vision for its electric two-seat VoloCity air taxi be initially piloted, with one pilot one passenger, but eventually autonomous so that both seats can be sold to paying passengers. A larger VoloRegion electric air taxi will seat four passengers.

If you have your pilot’s license (or are willing to get one) and want to fly yourself, you already have a choice of electric airplanes:

• The Velis Electro is a two-seater approved for pilot training.
• The e-SWAN electric foldable airplane is a single seat ultralight aircraft.
• A Corsair e-motion ultralight is for sale, pre-assembled or as a kit.

Making their way through the regulatory process are a few more electric airframes:

• The Eviation Alice nine-passenger, two-pilot commuter-class airplane is billed as “the world’s first clean-sheet, all-electric design.”
• Heart Aerospace is hedging its bets by designing its ES-30 to fly 30 passengers 200 km in all-electric mode, or 400 km in hybrid mode.
• Wright Electric claims that “all short flights can be zero-emissions starting in 2026” thanks to its Wright Spirit 100-passenger design (although the company doesn’t explain how by then we’d replace the over nine thousand fuel-burning commuter aircraft that are currently providing short flights — perhaps they meant “any short flight” not “all short flights”).

If you just want to go have electric fun in the sky, pretty soon you might be able to hop in your own human-size drone, no pilot license required:

• Jetson is taking orders for its $98,000 Jetson One, which the company notes requires no pilot’s license in the United States.
• Opener has been showing off its BlackFly, which it describes as “a safe, easy-to-operate, electric vertical take-off and landing vehicle.”

Batteries to Power Electric Aviation

How quickly electric commercial aviation takes off depends on how quickly better batteries are developed. Any battery technology that doesn’t have at least a theoretical chance of reaching 10,000 Wh/kg is probably not that helpful for electric flight. That includes all lithium-ion batteries that have a nickel-based positive electrode and a liquid electrolyte. Even if lithium cobalt oxide batteries reach their theoretical maximum, their energy density is abysmal compared to any type of fossil fuel.

Newer battery technologies, like lithium-air batteries, are more promising. If you have any influence over how research and development funds are allocated, you can help accelerate the advent of electric aviation by encouraging some investment to develop batteries that can compete with avgas and jet fuel.

In the meantime, we can expect to soon see flocks of flying drones that wring the maximum performance from the heavy batteries available now.