Electrifying Airplanes

January 25, 2018


Having grown up in North Carolina, one thing Shrey is proud of (and reminded of every time he see’s an NC license plate) is the fact that the Wright Brothers conducted the first sustained flight of an aircraft in Kitty Hawk, North Carolina.  Fast forward over a hundred years later, we now see that there are companies like Airbus and Rolls Royce are working on building hybrid electric plane technology.  Furthermore, there a number of smaller companies who have built and tested smaller electric passenger aircrafts that can go for a few hours at a time on a single charge.  With investment dollars flowing into this new aviation technology, we decided to take a deeper look at the potential for electric aviation.

Benefits of an Electric Plane

The major benefits of electrifying planes are pretty much the same as electric cars- lowering our dependence on oil, and reducing emissions.  Furthermore, electric planes could potentially fly faster and at higher altitudes more efficiently than conventional planes.  Like cars, as planes reach their max speeds, the worse the fuel efficiency becomes.  In electric planes, the electric motors powering the aircrafts would be able to speed up and slow down more efficiently than their traditional counterparts.

Jet Engines and Electric Motors

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Jet Engine.jpg

Before getting into electric motors, lets take a look at two common engine types: Jet engines, and Prop Engines.  Jet Engines (pictured on top above) take in cold air and compress it, and push it out of the back.  As this happens, the aircraft is thrust forward. Jet engines are better suited for higher speed travel, are less fuel efficient then prop engines, and are better suited for higher altitude flights.

Prop engines are somewhat similar in its basic process.  However, the process happens from the back of the engine to the front, with energy produced from the process being used to power propellers facing the front of the aircraft.   When the propellers spin at a reasonable speed they are able to generate thrust by creating a pressure imbalance between the air in front of the propeller (lower pressure) and behind the propeller (higher pressure), which then pulls the airplane forward. Prop engines are better for lower speed travel, are more fuel efficient than jet engines, and are better suited for lower altitude flights.

Both of these engine types can potentially be electrified.  Looking at the prop engine, instead of going through the combustion process to power a propeller, an electric motor can be used to directly supply power to turn the propeller.  In the jet engine, the process is a bit more complicated since the exhaust itself is used to thrust the aircraft forward. However, one company, Lilium (who is working to build what is pretty much a flying car), replaced the gas turbine in the engine with an electric motor to turn the compressor fan.  Given that both engine types can potentially be replicated using electric motors, we can see that the true limiting factor for electric air travel is energy density, and the added weight of those giant battery packs.

Weight Considerations

As airplanes get heavier, they need to compensate with extra power.  The current issue with batteries is that they are relatively heavy without compensating for that weight with extra juice.  Thus, one of the primary issues of electrifying airplanes are weight related considerations.  

While EV manufacturers have figured it out, and it’s very easy to say that companies should just replicate electric car technology in aircrafts, it is not that simple.  Electric cars can mount the heavy batteries under the carriage, which will have the effect of lowering the center of gravity and providing more traction.  To compensate for the added weight, auto manufacturers can use lighter materials elsewhere.  However, the final product is still going to be heavier than a similar gas powered car with the currently available technology.  For example, an 85kwh tesla battery weighs close to 1,200 lbs, which can make the car as a whole weigh 20-30% more than a comparable gas-powered car!

Similarly, planes can attempt to offset the added weight of batteries with lighter parts, however aircrafts are a bit more complex than cars.  Two of the bigger considerations when comparing them are that aircrafts travel at much faster speeds, and they have different mechanics during takeoff and landing as they do when cruising, which is not a relevant consideration with cars.

So while both industries need batteries with high energy to weight ratios, the threshold is lower for cars (although one can argue that if Tesla were to shift their focus to airplanes, they might get there soon as well).  But given that there is a lot of brainpower and money being put into making higher density and more efficient batteries, it’s not crazy to think that electric airplanes will be a reality.

How Much Lithium is Needed?

Now back to what we care about- how much lithium would be consumed in constructing an all electric aircraft?  

In cruise mode, a typical Boeing 747 burns about 10 tonnes of jet fuel per hour.  If replaced with an electric power source, and assuming you can produce a battery dense enough to match the jet fuel energy yield pound for pound, you would need a 10 tonne battery for simply a one hour long flight.  Sticking with the Tesla battery as a guide (which has ~60kg of lithium in a battery pack weighing ~550 kg), this 10 tonne airplane battery would contain about 1 tonne of lithium.  

Now coming back to the crucial energy density issue.  According to an analysis by wired.com, right now, jet fuel yields about 14 times more energy than a battery with the same weight.  So in the example above, to get the same power as 10 tonnes of jet fuel, an electric plane would actually need a  ~140 tonne battery pack.  So there’s a long way to go in terms of battery technology before we can begin to think about large scale commercial aircrafts running on electric power.  According to Don Hillebrand, who was quoted in that wired.com article, if we can get to battery densities of 1000 watt-hours per kg, than it would be practical for small scale commercial aviation.  To put that into perspective, Tesla’s current battery density for the 100 kwh battery pack is about 250 watt-hours per kg.  So battery density would need to quadruple just for small scale use, something that isn’t expected to happen until 2045.  

Notable Projects

Given this massive gap in battery technology, it seems more likely that innovation surrounding other aspects of aviation will need to occur to help make electric aviation a reality.  Luckily, there are some bright minds working on doing just that, with companies and organizations like NASA, lilium, and Zunum Aero testing cool new products.

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NASA’s X-57 Electric plane (pictured above) consists of 14 separate electric motors turning propellers. According to NASA’s website- “Energy efficiency at cruise altitude using X-57 technology could benefit travelers by reducing flight times, fuel usage, as well as reducing overall operational costs for small aircraft by as much as 40 percent. Typically, to get the best fuel efficiency an airplane has to fly slower than it is able. Electric propulsion essentially eliminates the penalty for cruising at higher speeds.”

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Zunum Aero expects to launch 12-passenger planes (pictured above) for use on regional flights by 2022. They expect the planes to cruise at a max of 340 mph, and have a hybrid-electric range of 700 miles.  They claim that their flights will get you to your destination faster and at a lower cost, while reducing emissions and noise.

lilium plane.jpg

Lilium, a German startup backed by Tencent, is essentially working on a flying car (pictured above).  Their two seater car will be lifted vertically by 36 electric ducted fans. While the fan engines are directed downward, the car hovers.  However, once in the air, the engines are slowly directed backwards, which then allows for forward flight.  Finally, the company isn’t just aiming to sell these flying electric vehicles, they are hoping to start an air-taxi service to change the way we commute.  

From commercial aircrafts to flying Ubers, the electrification of the aviation industry is certainly on the radars of big and small companies alike.  With unique and creative projects like these and others already set in motion, we can’t wait to see what’s in store going forward.