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Clean, renewable energy is becoming more and more common in our everyday lives. But, as our cars and buildings become more green, tens of thousands of airplanes fly every day using petroleum-based fuel, and there's seemingly no end in sight. Why?

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Using clean, renewable sources of energy is really important for keeping our planet safe and healthy. That’s not that surprising.

It’s why so many buildings are switching to solar or wind power, and why electric cars are becoming more mainstream. But then there are airplanes. Every day, tens of thousands of commercial flights take off around the world, and just about all of them are loaded up with thousands of liters of petroleum-based kerosene.

So where are all the electric planes? Well, aircraft designers, including AirBus and NASA, are working on it. But don’t get your hopes up that you’ll be flying to your next vacation on a zero-emission electric airliner.

For now, jet fuel is just too good an energy source to get rid of. The challenge with designing any airliner -- electric or otherwise -- is figuring out how to store enough energy to power a flight. Since weight and space are kind of at a premium, your plane will ideally have some kind of power source that’s super effective but also pretty light.

This is something designers think about a lot -- specifically, in terms of something called fuel energy density. Also called specific energy, this is the amount of energy per unit volume or unit mass of fuel. Jet A and Jet A-1, the most common commercial jet fuels, are about as energy dense as fuels get.

They have about 43 million joules of energy in every kilogram of fuel -- or about 1 liter of the stuff. For comparison, that’s about the same amount of energy as if you crammed 43 sticks of dynamite into a soda bottle. Meanwhile, even the best commercially available batteries only have around 900,000 joules of energy per kilogram, more than 48 times less than Jet A and A-1.

The difference comes from /how/ jet fuel and batteries store their energy. Jet fuel stores energy directly within the structure of its chemical bonds. So all you have to do is burn it.

Batteries, on the other hand, store energy in charged molecules, or ions. They release their energy by transferring electrons between a negative ion, called an anode, to a positive ion, called a cathode. But to do that, batteries need a bunch of parts: an anode, a cathode, an electrolyte for charged ions to transfer through, and a container to hold it all together.

And that all just takes up a lot of space. If you tried to power a Boeing 737 -- your classic passenger jet -- with electricity, you would need a pile of batteries that weighed about 1 million kilograms. Which is about 13 times the max takeoff weight of the aircraft.

So yeah, batteries are about as good for air travel as a pile of bricks... Today, some companies are working on developing all-electric aircraft, but don’t expect them to be popular any time soon. For now, the major next steps involve building hybrid-electric systems -- which will be kind of like training wheels while we figure out the “electric” part -- and, maybe more importantly, making lighter and smaller batteries.

Researchers are working on it, though, and scientists at Cambridge have already developed special lithium-air batteries with about 10 times the energy density of current technology. But there’s still a long way to go. So for now, just sit back with your seat-back and tray-table in their upright and locked position and enjoy your kerosene-powered flight.

Thanks to Cheddar for supporting this episode of SciShow. Cheddar recently launched their YouTube channel where they cover future focused topics like electric cars, space travel, and even what we need to teach AI to keep it from enthusiastically killing all of humanity. I mean, what will keep the robots from taking us out?

To learn more, check out their video Can Empathy Prevent a Robot Uprising? And all their other videos at