Sun planes are cool, but they are not the permanent source of the flight

Have you heard about the solar pulse? It is a Swiss plane, which is powered entirely by solar energy. The ambitious goal of this project is to fly around the world, using only solar energy. On May 1, they will start their journey from San Francisco to New York City, with many stops along the way. They have already made a 26-hour flight, as well as an intercontinental journey from Spain to Morocco, powered exclusively by sunlight (they use batteries to store energy reserves and power the aircraft at night).

When I first heard about it, I was surprised that it is possible at all. Are the solar panels really enough to power an aircraft? And when can I expect to fly in one?

To find out how they did it, let's do some numbers.

How much energy can you get from the Sun?
First of all, let's find out how much energy a plane gets from the sunlight. The Solar Pulse has about the same wingspan as the 747, and its wings are covered by almost 12,000 solar cells. This is about 200 square meters of solar cells.

Now, the amount of energy provided by sunlight is a well-known number. If we ignore the clouds, and the average day and night are about 250 watts delivered per square meter of earth. This number, 250 Watts per square meter, is the amount of energy that we, sitting here on earth, can draw directly from the Sun.

Combining these two numbers produces 250 watts/m² × 200 square meters = 50,000 watts. This is the maximum amount of power this aircraft can theoretically capture from the Sun, given its wingspan.

But we do not have the technology to use all this power. The best solar cells available on the market are about 20% efficient in capturing solar energy, and then there are more losses in batteries and electric motors, all of which waste some energy. In general, Solar Impulse staff tells us that 12% of the solar energy received is pumped by electric motors. That's 12% of 50,000 Watt, leaving us with 6,000 W of useful energy. Remember this number. We will come back to it.

How much power does it take to fly a plane?

So far, good. On average, we have 6,000 Watt pumped to fly this plane. But is that enough? To answer this question, we need to find out how much power it takes to fly the plane. In fact, there are two elements to answer this question.

Heavier planes need more power to fly them. This is because planes fly by throwing air down. They need to shed enough air to counteract their own weight, so heavier planes need to "work harder" by shedding air down. To stay on the water surface, a heavier plane must fly faster than a lighter one, so it can shed more air every second, counteracting its own tendency to fall from the sky. (If you want to learn more about how it works, see my post titled Can we build a more efficient plane? Not really, says physics).

So you need more power to fly a heavier plane. This is quite intuitive. (By the way, as we're talking about it, that's also why you can't say something like, "The plane flew anyway, so my flight on it was carbon neutral." No - it needs extra power to support its extra weight! Not to mention the fact that airlines would fly fewer planes if there were fewer people).

Some planes are simply better at standing up than others. If I throw a paper plane, it slides around the room. If I take the same piece of paper, crush it into a ball and throw it with the same force, it will not go that far. The difference is that a paper plane is more aerodynamic - it is more capable of throwing air down and staying on the surface. Therefore, if the 747 ran out of fuel, it would not fall from the sky like a rock but would slide as effectively as a wing.

This gliding ability is captured by a number called a glide rate. This is how it works. Imagine you turn off the Boeing 747's engines in the middle of the flight (do not try it at home). You will end up falling 1 foot for every 12 feet you move. This means that the slip rate is 12/1 = 12. The Albatross moves 20 feet forward for every foot that falls (slip rate 20), while the Sparrow moves 4 feet forward for every foot that falls (slip rate 4). Here is a table with some other examples.

The higher the slip rate, the more energy-efficient the plane is because it means more capacity and less resistance. Think of the Albatross and the Sparrow. The most direct way to increase the speed of gliding is to increase the wingspan because this way, you can throw much more air down.