The Solar Impulse is a revolutionary solar powered plane with “the wingspan of a 747, the weight of a mid-sized car, and the power of a scooter.” With more than 10,000 solar cells on board, the plane completed its journey across America on July 6th, 2013.
Flying both day and night, Swiss pioneers Bertrand Piccard and André Borschberg, co-founders and pilots of the Solar Impulse, completed the passage without a drop of fossil fuel on board — not even for back up.
"We’ve been dreaming about this, and it is happening now," Piccard exclaims. “In the past the protection of the environment was boring and expensive,” but with the Solar Impulse, he hopes to demonstrate the capabilities of clean technology, and to ultimately bring about a clean technology revolution.
“People are inspired by aviation. If you can do it in the air, we can do it in our daily life,” he adds. “Our airplane is not designed to carry passengers, but to carry a message.”
The U.S. Secretary of Energy, Dr. Ernest Moniz expands on these points, stating that, “the Solar Impulse serves as an inspiring demonstration of solar power. More importantly, it demonstrates the advancement of energy storage and efficiency, system integration, and advanced materials.”
The flight was made possible through the collaboration of various companies, many of which have no specific ties to the aerospace industry. One of the founding companies of the Solar Impulse consortium, Solvay Chemicals, provided 11 different products that were used in more than 20 different applications, and in 6,000 parts.
Claude Michel, head of the Solvay/Solar Impulse partnership explains, “It wasn’t a question of building a plane; it was about energy, and lightweight materials.” Many of the materials developed throughout the production of the plane now have practical applications in everyday products, including electric cars, high-performance yachts, portable electronics, refrigerators, fabrics, and even nutrition, with input from Nestlé Health Science to keep the pilots healthy during long flights. “We didn’t have the feeling that we were building a plane, it was working on a series of projects that needed solutions,” adds Michel.
For every eight kilograms of structure on the plane, an additional 30 watts of power was required, as well as an additional one square meter of photovoltaic cells. These numbers are what made the structure design the most difficult challenge, as the plane needed to be lightweight, but also hold a pilot, the photovoltaic system, and batteries. “How do you build a structure of this dimension that is lightweight? We had to be clever in design and material selection,” Michel explains.
The Solar Impulse is constructed around a honeycomb structure, allowing the wings to be comprised of mostly air. Any parts that have traditionally been metal were replaced with ultra-light polymer components. Solvay’s polyphenylsulfone (Radel) and polyamideimide (Torlon) are two of these materials that lent themselves to the light design. Compared to metal, with densities ranging from 2.7 to 7.9 kg/m3, the polymers from Solvay have densities between 1.4 and 1.8 kg/m3.
On top of the wings are the plane’s 11,628 solar cells, each as thin as an average human hair. “Solar cells convert light to energy, but they are delicate and need to be protected from the environment they operate in,” explains George Corbin, head of research, development, and technology at Solvay.
To protect the cells, and to reduce the impact of temperature variations and solar radiation that occur while flying, Solvay’s Polyvinylidene fluoride (Solef) and Ethylene chlorotrifluoroethylene (Halar) polymers were used to “encapsulate” the cells. This resin is UV-resistant, waterproof, and is extruded in a film that is 17 microns thin.
The other primary challenge was harnessing energy, transforming it to electricity, and storing it in batteries. The bottleneck, was the battery systems, not the solar cells, as the team needed to improve the batteries’ energy density, increase the speed of charging, and extend the life cycle.
Over the period of a day, the sun averages around 250 W/m² of energy. When the sun is at its highest, each square meter of land receives the equivalent of 1.3 horsepower (hp) of light, or 1,000 watts. The Solar Impulse has 200 square meters of photovoltaic cells, and with a 12% efficiency of the propulsion chain, the plane averages 8 HP or 6 kW of power. “It is the most fabulous way to fly, because the more you fly, the more energy you have on board,” says co-pilot Andre Borschberg.
This energy is then stored in high-efficiency lithium batteries, which contain PVDF (Solef) and Solvay’s electrolyte component monofluoroethylene carbonate (FIEC). These battery systems are what allow the plane to continue flying at night.
“Human progress in flight has been amazing. Behind every milestone are pioneers, inventors, visionaries, pilots, engineers, designers, mechanics, and many other professionals who make dreams become reality,” says General John Dailey, director of the Smithsonian’s National Air and Space Museum. The next milestone for the Solar Impulse is the flight around the world in 2015.
Moving forward, the project will encounter a new set of complexities and complications. “The flight across America was one country and one language,” Gregory Blatt, head of marketing and communications explains. In 2015, the plane will be making stops in countries across the world, and communication may be a potential barrier.
Apart from communication barriers, there are many other challenges the team must overcome for the flight around the world, like dealing with the effects of humidity on a fully electric plane. Where the first plane was not water-tight, a second plane is being designed to meet the challenges of flying longer distances, across the ocean, and at higher altitudes.
As the new airplane reaches its final stages of construction, the Solar Impulse consortium continues to work together, bringing together new ideas and innovations. The plane should be fully assembled in early 2014 and will begin flight testing in preparation for its journey around the world, to show once again, just how far solar energy has come.
Dr. Moniz adds, “People are going to be surprised at where solar is in ten years, and not only in standard applications, but in applications we don’t know yet.” The technologies and innovations used to power the Solar Impulse flight across America and the future flight around the world, all have practical applications.
“We look back ten years and think the world looks really different today … but when we look ten years ahead, we think the world is going to look the same – it’s not. These are technologies that will make ten years from now look just as different as it does when you look back ten years. We will see the fruits of all these technologies changing the world.”