No, solar powered flight isn’t coming in 30 years

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“We have to realise that we are between the Wright Brothers and Charles Lindbergh periods in the 20s of last century. So it will take, as it took in the past 25, 30, 35 years to fly clean. New technologies will have to be developed, this will take time.”

Once again solar powered flight is hitting the headlines, and misguided journalists are taking seriously the possibility of commercial solar flight.

If you read the Guardian you might even be convinced there is some kind of debate over the possibility of running a Boeing on solar panels. Someone quoted in the post gets it more or less entirely correct:

“Currently, an aircraft entirely covered in solar cells would not be able to generate enough power to enable it to fly”.

True, and replace the word “Currently” with the words “So long as the laws of physics apply” and that would do the job.

The Guardian manages to provide us with a comparison of a Boeing 747 and the Solar Impulse 2. Have a look:

Unfortunately for the Guardian this comparison gets one basic fact wrong. (And let’s not start with the idea that this solar powered plane has no CO2 emissions. How did they make the solar panels? Rub sticks together?)

A Boeing 747 does not weigh 154 tonnes. Or at least it doesn’t weight that when you include the fuel. And the fuel is the problem. A Boeing 747 can include something like 200 tonnes of jet fuel.

So, let’s do a back of the envelope calculation. Jet fuel has an energy density of around 40 GJ/tonne. So, 200 tonnes will have 8000 GJ of energy. Let’s assume it takes 20 hours to burn off this fuel – a rough estimate, but it will do.

This means that a Boeing consumes energy at an average rate of around 100 MW. At take off it is even higher, at around 280 MW.

Now, what about solar flight? A Boeing 747 is 70 by 60 metres, so it has a surface area of less than 4,200 square metres. But, let’s go with 4,200 square metres for now. (Yes, the actual surface area is much lower, but I’m being generous.)

Again, I’ll assume an average rate of energy consumption of 100 MW. This would equate to a rate of energy consumption per square metre of the plane of 24,000 watts.

24,000 W/m2. How does this compare with the solar energy striking the plane? Not very well. In the middle of a desert the average solar radiation striking the land is less than 300 W/m2. We are looking at differences of orders of magnitude, differences that cannot be bridged no matter how much “innovation” you chuck at it.

So, even if you can create a 100% efficient solar panel and connect them on to  a 100% efficient engine, a Boeing 747 will never be able to be powered by solar panels.

Solar panels can probably do little more than power your in-flight movie. (And, let’s not discuss battery powered flight.)

Note

Half of the arguments above featured in a previous piece I wrote. I am merely venting in annoyance at the perpetual nonsense about solar flight.

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