Last Thursday, the Solar Impulse 2 touched down in Seville, completing its 70-hour jaunt across the Atlantic in its quest to fly around the world.

The transatlantic journey, helmed by one of the project’s pilots and co-founders, Bertrand Piccard, marked another important milestone for the solar-powered aircraft, having completed a Pacific Ocean crossing last year.

It’s had to overcome battery malfunctions that left the plane inactive for several months while inclement weather conditions hampered its flight plans but now the Solar Impulse 2 is back on track to complete its global flight.

The Swiss-built plane is powered by 17,000 photovoltaic cells, weighs 2.3 tonnes, and has an average speed of 70 km/hr. It plans to land in Abu Dhabi in August or September, the site of its launch in March 2015.

Lengthy jaunts over oceans are one of the biggest challenges for the Solar Impulse as it’s faced various postponements and delays since last year. To help guide it, the Monaco Control Center (MCC) acts as the Solar Impulse’s guardian angel as it crosses the world, over cities and oceans. The control centre in Monaco is supported by Prince Albert II as part of his pro-renewable energy efforts. Tech.eu paid it a visit and took some photos, as you can see below.

Teams of engineers, air traffic controllers, and mathematicians monitor the solar plane’s performance as it negotiates turbulence and ever-changing conditions.

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The MCC provides the pilot with an updated weather plan every six hours while he must also adjust his altitude at night much lower compared to day. It’s a testing experience for any person.

Engineering consultancy Altran, headquartered in Paris, has been one of Si2’s strategic partners, powering software and simulation technology that helps in training pilots and calculating flying conditions.

Solar Impulse CEO, co-founder and pilot André Borschberg was in the cockpit for five days last year over the Pacific and explained there’s only so much preparation you can do in advance with simulations.

“Last year was still full of question marks because of course we simulated that the airplane can do the mission but we never flew it. You have to make sure the reality fits the simulation,” he said.

Borschberg also had to develop a regimented sleeping schedule that allowed him to rest while the plane’s self-stabilisation system could keep the plane on track provided the weather forecast was calm.

Calculating the path

Each leg of the round-the-world trip presents different challenges and Altran’s data computations must adapt with that, explained Christophe Béesau, director of simulations at Altran.

Flights are coordinated to avoid certain seasons over different regions. For example, the plane first took off from Abu Dhabi and moved across Asia in spring and early summer of 2015 to avoid monsoon season in India.

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“The Atlantic crossing has some difficulties that we didn’t have for the Pacific,” he said, namely the instability of the weather.

“We worked a lot to improve the accuracy of the results and in particular the ability of the simulation to go deeper into details.

“We have 16 times more data to process now than last year. On the other hand we have better models to interpret and process this amount of data.”

This includes models on cloudiness, humidity in the air, and computing the effects the surround have on solar cells.

Unruly weather

The erratic weather conditions mean that pilots and engineers need to strike when the iron is hot. After landing in Japan last year, the next stage of the flight was delayed due to bad weather, leaving the team waiting for the right window to come.

When finally taking off for the Pacific flight, after weeks of delays, the Solar Impulse experienced another hiccup that could have scuppered Borschberg’s long awaited trek across the Pacific. Part of the alarm tool in the cockpit that alerts the pilot of any issues was malfunctioning early into the voyage. “There was interference between the virtual co-pilot and the alarm system,” said Borschberg. “It’s a question of software because there’s a lot of software. The alarm system was triggering an alarm that should not have been triggering.”

“I decided to continue because for the first time the weather was improving,” he said. “The weather I can’t control and if the weather is good and on my side… if I’m missing equipment, I can find ways to deal with the airplane. I had the impression I could handle the missing equipment. I was confident that as the weather was improving the overall risk was acceptable.”

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It was a move that caused tension in the team and some engineers even threatened to resign. “They thought I was crazy,” Borschberg said. Ultimately, the matter was smoothed out over a couple of months to rebuild trust, he further explained.

A series of firsts

The Solar Impulse project is carrying out the world’s first round-the-world flight for a solar plane and the project has made a habit out of being the first. Its predecessor, the Solar Impulse 1, was the first plane of its kind to fly over night and to fly internationally.

The project is also the first aircraft of its kind to fly over populated areas, according to Borschberg. It’s one thing to fly an experimental aircraft over the ocean but another when there are people below.

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“To fly this plane was new for everybody,” he said. “We had a test pilot from NASA, he was unable to land the airplane in the simulator, just to give you an idea that we’re so far away from normal type of aviation.”

Compared to regular or traditional aircraft, everything about the Solar Impulse 2 moves slowly, not just in the speed it travels but how it operates for the pilot. The learning curve differs from traditional aircraft.

Whether or not the Solar Impulse will inspire a wider uptake in solar power in the aviation industry remains to be seen, though.

The round-the-world voyage serves more as a proof of concept for solar energy in general rather than a pitch for aircraft manufacturers, but it’s not out of the question either, as Béesau envisions a future with small aircraft carrying five to 10 people on short one or two-hour journeys.

“I believe sets of small solar shuttles carrying people, this could be the future of aeronautics,” he said. But for now, that future will have to built one flight leg at a time.

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