Let’s go back to the example of the 747. Most variants of this plane have a fuel capacity of somewhere around 200,000 liters, which is really a lot of fuel. With a density of about 0.8 kilogram per liter, this gives it a fuel mass of 160,000 kilograms. The specific energy of jet fuel is around 12,600 watt-hours per kilogram. This means that with 1 kilogram of fuel, you could get a power of 12,600 watts for one hour—assuming you can use all of the energy, which you can’t.
Let’s say that the overall efficiency of the plane is 35 percent (which is the same as saying each jet engine is 35 percent efficient). That means that 1 kilogram of fuel will actually only give you 4,410 watts for one hour. But you see where this is going, right? I know the amount of fuel in the 747 and the required power. With that, I can calculate the flight time (and also the flight distance). Cranking the numbers gives me a flight time of 13.5 hours and a distance of around 10,000 kilometers, or 6,200 miles. That’s just a rough calculation, but it seems legit.
Now suppose I take all that jet fuel and replace it with batteries. Assume that I can replace the jet engines with equivalent electric-powered turbofan engines or something. So, that’s a 160,000-kilogram battery. Electric cars use a lithium-ion battery, and the best specific energy you can get is about 250 watt-hours per kilogram. Now you can already see the problem. If I assume an electric motor is 50 percent efficient, our electric-powered 747 could fly for 22.7 minutes with a range of 304 kilometers. Forget about that trip to Hawaii.
Actually, it’s even worse than that. I ignored the extra energy you need to get the aircraft up to cruising altitude at its cruising speed. It wouldn’t even make it that far.
Would it help to have a smaller aircraft like the Cessna 172? Of course, it uses less power, but it also carries less fuel—just about 170 kilograms. If we replace that fuel with a lithium-ion battery, it could fly for about 30 minutes. That’s still not great. If you reduce the speed from 220 km/hr to 150 km/hr, you can get a flight time of about 42 minutes, but you won’t really be able to get a better distance, since you are flying slower.
So, maybe lithium-ion batteries aren’t the best option. What about some other energy sources? Let’s just try some stuff for fun.
How about a nuclear-powered airplane? If you take uranium-235 and break it into parts (like in a reactor), you can get 79 million megajoules per kilogram. That’s 7.9 x 1013 joules for one kilogram of fuel. Still, you can’t just drop some uranium in a plane and expect to get power. A nuclear reactor doesn’t just contain fuel, it’s got all sorts of other stuff to turn that nuclear reaction into energy. The most important thing you would need is some heavy shielding to protect the humans on board from radiation. That adds a lot more mass. But still, it’s possible. Just 1 kilogram of fuel would be enough for a 747 to fly for over 200 hours.
