I’ve already spoken about how HB-SIA collects and processes the energy from the sun and I’ve also linked it to the flight cycle, but I still haven't spoken about a much smaller but absolutely vital part for the functioning of the aircraft: the solar cells.

There are currently 12’000 cells on HB-SIA’s wings and horizontal stabilizer. The next generation aircraft, HB-SIB, will have 15’000 of them. This might not sound so impressive until you know that the panel building process is all handmade. Yes, you got it; SunPower Corp. (NASDAQ: SPWR) provides the cells which are then put together, one by one, meticulously by our engineers, known as the Dübendorf boys.

I had the chance to see the unabridged process with my own eyes during one of my visits. I was in admiration of the engineer’s patience and constancy. For example, when a new batch of  solar cells arrive, each one of them needs to go through a final check before being tested, one by one, for their voltage three times. Can you imagine testing 15’000 solar cells thrice, that is, doing the same job 45’000 times!

If I am to run you quickly through the process, I can say that, after the healthy cells are promoted to the function “wing cell”, they will initially be stringed together into 300 cells, with a (+) and (-) pole on each end of the string (see solar energy article). This is followed by a layering process, placing a plastic resin under a glass foil, and so forth, eventually laminating the strings. The “sandwich” is then cooked at 95° for 7 hours before being placed on a mold that bends the cells into the desired shape, slightly rounded for the wings. What is most important during this long and tedious procedure is that nothing can fall on the panels before the curing process. Any microscopic piece of hair, dust or insect could potentially cause a short circuit, making the panel unusable and starting the process all over again with a new set of cells.  It takes 10-15 hours to make a panel and 48 of them are needed for HB-SIB alone; and that’s just the work involved in the solar panels…

To produce an aircraft that will take off and fly autonomously round the clock, propelled only by solar energy, is a tremendous challenge that requires the best and most reliable technology. SunPower’s Maxeon™ solar cell technology was selected because of its industry-leading efficiency and thickness of its solar cell. Each measures 135 microns, which is important for the power to weight ratio of the aircraft, and produces an efficiency of approximately 22.7%.

SunPower shares with Solar Impulse the values that have sustained SunPower since it was founded 27 years ago: a pioneering spirit, innovation, the human dimension, environmental awareness, in a world where solar energy can contribute to overcome the dependency on fossil energy and demonstrate that, by sharing the same vision, we can change the way the world is powered.

But let’s not get ahead of ourselves. What has been achieved is already an innovation in itself. 

When I activated the solar cells of the HB-SIA during my second flight today, the aircraft not only started to produce electricity, but it was also able to recharge its batteries. At this precise moment, when the Solar Impulse produced more energy than it was currently consuming, the fulfilment of a dream to fly solely on solar power, day and night came one step closer to reality.It was like a first encounter with the sun. After I had turned on the solar panel I could see the energy reserves increasing although the engines were continuously consuming power. Never before in my 40 years as a pilot have I experienced anything like this.I am exceptionally grateful for everybody who has supported us during the past seven years to put Bertrand's and my dream into action. Finally we can say we are flying a solar plane.

These thanks of course also go to our fans and followers, especially the proud "owners" of one of the 12000 solar cells who are part of our Supporters Program. You truly electrified me!

The heavier the aircraft and the pilot, the more energy will be needed to keep them flying. So are hunting down the very last gram.

An aircraft's wing loading is calculating by dividing its total weight by the wing surface. With the amount of solar energy captured by our 200 m² of solar cells we cannot afford a wing loading of more than 8 kg/m². Which is precisely that of a seagull.

You could say we are looking to fly on board a seagull with the wingspan of a Boeing 747. And yes, we're in our right minds.

As you can see from the video gallery, the HB SIA's carbon composite structure is slowly coming together, like a finely engineered Swiss watch, piece by piece, test by test. The fuselage has passed muster, but a few days ago the landing gear failed to make it.

The next big moment everyone is waiting for is in a week's time, when we receive and can finally test the main wing beams. Let's hope they carry no hidden kilos! A seagull's life is a hard one.