The Chemistry Of A Solar Airplane

Sitting in a vast hangar in Payerne, Switzerland, nestled between the Alps and Lake Neuchâtel, is Solar Impulse HB-SIA, its four motors powered only by lithium-ion-polymer batteries charged by solar cells. The airplane’s 63-meter wingspan rivals that of a Boeing 747. But unlike a 747, the upper surfaces of its wings are coated entirely in solar cells. Built with lightweight materials and an innovative design, it weighs about the same as a family car.

Solar Impulse’s Swiss founders and pilots are Bertrand Piccard, 54, a psychiatrist and explorer, and André Borschberg, 60, a former fighter pilot, engineer, and founder of semiconductor technology start-ups. They came up with the notion to build a solar-powered plane in 2003 and since then have convinced corporate sponsors and partners to provide the project with $130 million, including materials and manpower.

In the process of developing and testing the plane, they have inspired many, including the research teams of their chemical company partners, Solvay and Bayer Material­Science, to think more creatively. Bayer and Solvay have reaped a significant public relations benefit from involvement with the project. But more important, company executives say, is the chance to apply what they have learned in other areas, including projects with car companies.

Solar Impulse isn’t an aircraft that you will be able to fly on anytime soon. “It’s not designed to carry people or even freight but as a message that sustainable energy is a viable option for mankind,” Borschberg says.

Flying at an average speed of 44 mph, it has already completed a series of flights across Europe, including one from Payerne to Morocco, and has even flown overnight.

In May the Solar Impulse team plans to freight the one-seat plane to the West Coast of the U.S. and fly it, in three or four legs, to the East Coast. The team also is building Solar Impulse HB-SIB, a stronger version of the plane, in which “everything is upgraded” so that in 2015 it will be capable of flying around the world, Borschberg tells C&EN.

It was a motivational lecture by Piccard that in 2003 led Solvay to join the project as its founding partner. “This type of project is unique in the history of Solvay. It’s a project with an idea to make a better world,” says Claude Michel, who heads up Solvay’s Solar Impulse team of about 10 staffers. “We recognized the value Piccard has in innovation, his pioneering spirit and respect for people and the planet, and we found we had the same set of values.”

Solvay’s contribution to the construction of the plane includes 11 materials used in 25 different applications and more than 6,000 parts. Among its activities, the firm has provided lightweight plastics to replace metals, techniques for improving lithium ion-polymer batteries, and a broad body of materials research and know-how.

Solvay’s Halar brand fluorine copolymer, for example, is being used to encapsulate the plane’s thin photo­voltaic cells. Halar is resistant to ultraviolet radiation, is waterproof, and forms a lightweight film less than 20 µm thick. Before the Solar Impulse project, Solvay used Halar only for coating materials such as metals, but the firm is now looking at using it across a range of applications, Michel says.

Solvay will have invested $16 million in the project, including a cash contribution and the value of its time and parts, by the time Solar Impulse HB-SIB makes its global flight in 2015. “It’s a good investment,” Michel says without hesitation.

The benefits to Solvay, he explains, have been multiple: Participation has driven the development of specialty plastics and chemicals across the company, enhanced the image of the firm as a solutions provider, and proven to be a powerful tool to motivate staff. R&D staffers typically don’t see a real-world outcome from their labors. With Solar Impulse, though, they quickly appreciate their role in preparing Solvay materials for the plane, Michel says.

Solar Impulse has had other influences on Solvay’s scientists, not least that they have adapted to the project’s tough time and performance requirements.

Meanwhile, working on Solar Impulse has led researchers at Bayer to be more creative in their approach to projects, says Martin Kreuter, a senior marketing manager in the firm’s materials science division.

“The removal of the expectation for commercial success has allowed people to work differently and to get into an open innovation mind-set,” Kreuter says. “Working with Piccard and his team is inspirational.” About 30 staffers from a range of Bayer departments have contributed to the Solar Impulse project.

Bayer joined Solar Impulse as a financial sponsor and materials partner in 2010. The German company’s contributions include polyurethane foam for the wingtips, motor casings, and cockpit; polycarbonate film for the cockpit window; and adhesive and coating materials used in the cabin and wings.

Bayer has used carbon nanotubes in combination with epoxy to make the spars—the backbones of the wings—and other structural components lighter and stronger.

For the project, Bayer has drawn on its experience in the automotive sector, where weight and performance are also key parameters, explains Kreuter, whose role at the firm involves partnering with car companies. And the materials and techniques Bayer has developed for Solar Impulse, such as lightweight and rigid insulating foam, could be used in cars.

“There are many things that we are developing with Solar Impulse that you might see in an electric vehicle 20 years from now,” says Kreuter, whose office in Leverkusen, Germany, has one wall covered in pictures of futuristic-looking cars. “Everything we are doing with Solar Impulse has high relevance to our most important sectors including automotive, electronics, and construction.”

In addition to helping reduce the weight of the solar plane, Solvay and Bayer are providing materials that can buffer the extreme temperatures of flight, which without safeguards could range from –40 to 30 °C .

To insulate the cockpit and other temperature-sensitive components of the plane, the chemical companies have codeveloped a strong and lightweight insulating foam based on Bayer’s Baytherm Microcell polyurethane and Solvay’s 365mfc fluorinated blowing agent. Owing to pores that are smaller than those in standard foam, the new product provides rigidity and structural strength but remains lightweight. The foam is designed to ensure that the temperature does not drop below 15 °C for the batteries and below freezing in the cockpit, Michel says.

Still, the conditions pilots experience in the Solar Impulse are extreme enough that they have had to resort to meditation and even self-hypnosis during flights. “It’s a case of knowing ourselves,” Borschberg says.

To try to make the pilots more comfortable, Solvay has provided a nylon 6,6 fiber for their undergarments. The material incorporates a special filler that helps keep the pilots cool in the heat and warm when it gets cold. The nylon recycles infrared heat back to the surface of the skin when it is cold but also prevents sweating during periods of intense heat. “We have had to be very clever, open, and curious,” Michel says.

The Solar Impulse team of about 80 staffers, excluding headcount from partners and sponsors, has engineering expertise from backgrounds as diverse as Formula 1 racing cars and aeronautics, but it had little experience building airplanes. “So we were extremely open and entrepreneurial and flexible in our thinking,” Borschberg says. This also meant that the staffers developed an approach that was unrestrained by protocol. Solar Impulse’s designers and engineers cross-fertilized their ideas with those of materials scientists and chemists from Solvay and Bayer, he adds.

Borschberg has been “extremely impressed” by the way researchers from Solvay and Bayer have engaged in the project, the way they have made resources available, and their culture of supporting the project’s goals. “The motivation of our partners and the public has helped us keep our energy levels high,” he says.

The project has hit pockets of turbulence, however. In the summer of 2012 development of Solar Impulse HB-SIB was set back when the main spar of the wing failed a load test. “We had pushed a little bit too hard to reduce weight. We were just on the other side of the limit,” Borschberg says. The Solar Impulse team has since modified the design, but the glitch set back the attempt to circumnavigate the world by more than a year.

Although someone with Piccard’s background in psychiatry and exploring may be an unusual partner for a chemical company, it is not the first time that the Piccard family and Solvay have worked together.

In 1911 Solvay founded the Conseil de Physique Solvay, a regular gathering of Europe’s finest scientific minds to develop solutions to the scientific problems of the day. A regular attendee was Auguste Piccard, Bertrand’s grandfather, a professor of physics at the Free University of Brussels and a balloonist who became the first man to view the curvature of Earth. Other participants included Marie Curie and Albert Einstein.

The goal of the Conseil de Physique was to advance the scientific thinking of the day. And the Solar Impulse project has already influenced Solvay and Bayer to think differently. Piccard and Borschberg hope to have shared their message about the possibilities for innovation and renewable energy with an even wider audience by the time they circumnavigate the world in 2015.