Q&A: Electric Airplanes

Question: I was out on my deck yesterday and saw this small airplane approaching maybe 1000 feet up. It seemed really quiet, and when it went past me I could tell for sure it wasn’t using a regular airplane engine. All I can think is it must have been an electric airplane. We have electric scooters and bikes and cars, so why don’t we have more electric airplanes? They say electric power is cleaner than burning gasoline, and airplanes use a huge amount of fuel.  — BR, Dinuba, CA

Answer: You’re correct about the emissions from aircraft engines. In 2018, airlines around the world carried 4.4 billion passengers and accounted for 2% of the total 35 billion metric tonnes of CO2 from all human activities. So there is serious interest in electric aircraft, but it’s a tough engineering problem.

Nonetheless, there’s actually dozens of electric aircraft on the market today, even if you don’t count drones. If you follow that link you’ll see they’re all in the light aircraft or experimental aircraft categories. And all are optimized for the lowest possible weight.

But I suspect you were asking about larger aircraft, like long-haul cargo and passenger planes. The engineering hurdle in that case is what’s called energy density: How much energy can be crammed into a given amount of space and/or weight, as measured in J/kg (joules per kilogram) or J/m3 (joules per cubic meter). To put things in perspective, one joule is a measure of energy about equal to that released by striking a single match.

Let’s examine the energy needs of a typical Boeing 747 passenger liner on a transcontinental flight. The amount of fuel needed on a fully-loaded flight is around 183,000 liters (183 m3). When combusted, that fuel releases a total of 5.2 TJ (terajoules = 1012 joules) of energy. The red “FUEL” cube in the graphic shows how much space that fuel takes up compared to the size of the aircraft. Most of that fuel is stored in the main wings. This graphic is approximately to scale.

Electric batteries are getting better all the time thanks to research in the automotive and renewable energy sectors, but they still can’t compete with the energy density of liquid fuels. If we use the best battery technology currently available (cobalt lithium-ion), to get that same 5.2 TJ of energy, we’d need a battery bank with a size of 5000 m3 and a weight of 9800 tons. The green cube in the graphic shows the size of that battery to scale. Obviously, there’s no way it could fit into the aircraft.

Weight is also a problem. Even if we use the “heavy lift” Boeing Dreamlifter 747-400 the maximum payload is 125 tons. No way it could carry that battery, even if it did fit inside. At present, barring some breakthrough in battery energy density, the only truly “green” alternative for large aircraft is powering them with hydrogen — this combustible gas could be used in existing turbine engines with a few modifications, and the exhaust would be pure water vapor.

Energy cost is yet another issue. At the current average price of $0.50/liter, 183,000 liters of jet fuel runs around $91,500 per fill-up. At the current average electricity cost of $0.12/kwh, a 5.2 TJ full charge will run you $180,000. Granted, large users like electric airports would likely get a discount from the local utility. They also have the option of building their own solar energy farms to charge the aircraft with free sunlight — most airports have acres of unused real estate. Alas, the cost of the batteries themselves is still prohibitive. The going rate for Li-ion batteries is around $176 per kwh capacity, so a 5.2 TJ battery (uncharged) would set you back $264 million. The Boeing 747 sells for around $360 million.

Disclaimer: This was a fun analysis, but comparing batteries to jet fuel is like comparing the proverbial “apples and oranges”. Jet fuel is burned in turbines, the most efficient combustion engines available, converting over 70% of fuel energy into motion. An electric aircraft uses batteries to power electric motors to spin propellers. Propeller-driven aircraft are fairly efficient in their normal realm of operation, but performance drops at the speeds and altitudes accessible to jets. And of note, it is aerodynamically impossible for propeller-driven aircraft to achieve speeds greater than Mach 1.

The numbers used in this analysis are all rough approximations, but you can see the problems involved with transforming the entire aviation industry to electric propulsion. If you want the cruising range and speed of commercial airliners, or the performance of military fighters, electric batteries just aren’t up to the challenge. That may change in the future, but for now, highly refined jet fuel is about all we have.

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