Solar Airplanes: A Flight of Fancy

This article was originally posted on RealClearScience. It was also co-authored with Tom Hartsfield, a Ph.D. candidate in physics at the University of Texas.

Solar Impulse 2, a solar powered airplane that has enough room for exactly one person (the pilot), has made international headlines as it makes its historic trek around the planet. Media outlets from BBC News to Live Science have described the flight as a “revolution.”

Quirky, it is; a revolution, it is not.

Consider the longest leg of its global voyage, when Solar Impulse 2 flies from Japan to Hawaii. In a commercial aircraft, the journey takes about seven and a half hours; Solar Impulse 2, on the other hand, will arrive in Hawaii after four or five days. There are two reasons for this sluggish pace: (1) Jet engines require fossil fuels, so solar powered airplanes must revert to electric engines and propellers, which are slower; and (2) The laws of physics.

That second problem is particularly tricky because there is nothing we can do about it. Put simply, there is not enough energy in sunlight to fly a commercial aircraft.

To illustrate why, we will use as an example the Douglas DC-7, a propeller-driven airplane whose cruise speed was 359 mph and could seat 105 passengers. The total horsepower of its engines was 13,600, which is equivalent to 10,141,520 watts. This is the amount of power the aircraft needs to take off and fly. Any source of power must meet that threshold. If a DC-7 was plastered with solar panels, how much power would it get from sunlight?

The surface area of a DC-7’s wings is 1,463 square feet. To be generous, we will double that number to 2,926 square feet in order to account for all upward-facing surfaces that could potentially host solar panels. At its cruising altitude at noon on a cloudless day, a DC-7 would be exposed to sunlight that provides a power of 1,200 watts per square meter, which is equivalent to 111.48 watts per square foot. Now, a calculation:

(2,926 square feet of solar panels) x (111.48 watts/square foot) = 326,190 watts

Keep in mind, that figure is assuming 100% solar cell efficiency, which is an absurdly optimistic assumption since the best solar cells are only 30-40% efficient. But even assuming a miraculous 100% efficiency, the solar panels come up well short of the 10 million watts needed to operate the aircraft. In fact, it provides a measly 3.2% of the necessary power. And again, that’s in perfect conditions. An 8 AM flight would be lucky to capture even 1% of the needed power due to the low angle of the sun.

So, does that mean solar power is useless for flight? No, not at all. Extremely light aircraft, such as drones, could easily use solar power. But, for the foreseeable future, commercial aircraft will continue to be fuel guzzlers.



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