The totally solar-powered aircraft, Solar Impulse 2, was built to be used in an attempt to fly around the world powered by solar power alone.

I must say that this aircraft is a wonderful piece of technology and I marvel at the science and design which went into its construction.

The aircraft is very lightweight and has a massive wingspan of 72 m. Its takeoff speed is only 45 km/h. The whole design was a delicate balance between building an aircraft that could generate enough solar electricity to power it and building it light enough so that it could take off with the limited available power.
It also has rechargeable batteries on board to keep it flying during the night.

The team responsible for the Solar Impulse 2 aircraft had said that their intention was to demonstrate the possibilities of green energy.

Well, the aircraft certainly is succeeding in achieving that goal because, at the time of writing, the aircraft had been forced to make an unscheduled landing in the central city of Nagoya, in Japan. The craft was on its way from Nanjing, in China, to Hawaii, which it had been hoped it could achieve in one hop, lasting six days and six nights of nonstop flight. They failed to achieve this owing to bad weather and so had to come down at Nagoya.

When the aircraft landed at Nagoya, there were gusting winds, which were strong enough to blow the plane over, so support staff had to hold onto the aircraft with their hands for 24 hours while others desperately built a temporary hangar large enough to take the 72 m wingspan. The mission initiator, Bertrand Piccard, said: “Our team of a dozen people has been holding the plane on the ground since its arrival on the Monday night.” On Tuesday, the wind was still gusting, so they could not let go.

This was the seventh leg of the epic mission and probably the most dangerous, since pilot Andre Borschberg had to cross so much open ocean.

The mission was initiated to demonstrate the potential of solar power. Well, to my mind, it is doing just that. It is on the ground. I do not want to sound sarcastic – I really do think that it is an amazing piece of scientific design and engineering. However, what it shows is that you need a wingspan three-quarters the length of a football field to lift a really lightweight, which cannot fly in windy or overcast conditions.

Imagine, for a flight of fantasy, that you build a fast-food shop selling hamburgers and hotdogs. Imagine it is solar powered. Now, imagine that it employs six people but has to be in a lightweight building as large as a football field with the roof covered in solar panels. It has to close if there are two rainy days, or if the wind blows somewhat above mild. Also, if a strong, gusty wind blows, all employees have to spend 24 hours holding onto their stock by hand until special storage containers are built at emergency speed, at great expense. How profitable do you think the fast-food outlet would be?

My point is that the reality of solar power is that it is very dilute and intermittent. Solar energy strikes the ground at about 1 kW/m2 – and that is that. Contrary to popular belief, it cannot be concentrated with lenses.

So, an area of 1 000 m2 is hit by a 1 000 kW – end of story. Well, not quite. The reality is worse than that. The red part of white light is heat and the blue part is high energy, which can give you sunburn. It is the portion that gives you sunburn that also can produce electricity in solar photovoltaic, or PV, systems, such as on the aircraft. So, the amount of real extractable energy is, in reality, much less than the 1 kW/m2. You also only get the 1 kW when the sun is directly overhead, at midday. During midmorning and midafternoon, when the sun is at an angle, the power per square metre is much less. When a cloud passes over the sun, it drops even more.

The reality is that solar can be used for specific applications for which it is good, and supplying electricity into a national grid is not one of them.
Solar Impulse 2 is demonstrating beautifully what reliance on solar power means.