Special report: Partners play a weighting game

Sitting in its huge hangar in Switzerland, it may appear that the latest incarnation of the Solar Impulse plane has barely changed from its prototype form. Most of its slender frame is still covered in photovoltaic panels, the nose of the diminutive cockpit pokes out from beneath its wings and the plane retains its trademark silver finish. Look a little closer, however, and it becomes clear that there have been innumerable enhancements to assist the plane on its epic journey around the globe.

The original plane was a resounding success, proving the concept of flight powered only by the sun. However, an even more ambitious attempt to fly a solar plane around the world required the project’s engineers to head back to the drawing board and design a bigger, more powerful machine not only capable of crossing continents and oceans, but of comfortably carrying a pilot for up to five days at a time. And it had to still remain as light as possible.

To withstand the gruelling conditions and cope with the longer flight times of a round-the-world attempt, Solar Impulse 2 (SI2) had to be larger and more robust than its predecessor. Despite its size, however, the new plane weighs only around 2,300kg – more than the prototype’s 1,600kg but still little more than a standard family car. SI2 may boast a wingspan of 72 metres and be wider than a Boeing 747 commercial airliner but its engines only provide the power of a motorbike.

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During the challenge SI2 will soar to 8,500m (27,000ft) during the day to absorb the sun’s rays and descend to 1,500m at night to conserve energy. A 300m2 surface spread over the wings, fuselage and horizontal stabilizer is home to over 17,000 monocrystalline silicon photovoltaic cells to soak up the sun – almost 50% more than were used previously. The panels are protected by Solvay’s Halar ECTFE (ethylene chlorotrifluoroethylene), a thin fluorine copolymer film that at only 17 microns thick is far thinner than the 26 microns of the alternative film that was considered. The result is a weight saving of around 35% without compromising the electrical performance.

The cells themselves are among the best currently available and are only 135 microns thick, equivalent to a single human hair. They are UV resistant and waterproof, offering vital protection from the elements.

With weight such an important element of the project, the team often had to make tough decisions to reject materials that would otherwise have been suitable, says Bernd Rothe, Solar Impulse project lead, Bayer MaterialScience. “For Solar Impulse, the main priority was to be as light as possible. We developed a glue to attach a film to the top of the solar cells but the Solar Impulse team said no as our solution was 28g/m2. Even though it had excellent properties and transmission, and it was a faster process, the existing solution was 25g/m2. The weight is the main factor for them.”

More time in the air requires more energy storage and the prototype’s larger sibling now plays host to 630kg of lithium polymer batteries – up from 400kg in its predecessor. The batteries are hidden beneath the wings within four gondolas that also house 17.5hp engines. The team was able to wring out as much power as possible thanks to a fluorinated polymer from Solvay. As a result, the energy density is optimized to 260 watt hours/kg (Wh/kg) – far higher than the 180Wh/kg targeted for the original plane. Solef PVDF (polyvinylidene fluoride) is used as a binder for both electrodes and reduces the weight of the batteries, while a monofluoro-ethylene carbonate solvent helps to improve the ion flow and helps the batteries to carry more current.

“The main bottleneck was not really the energy capture,” says Claude Michel, head of the Solvay Solar Impulse Partnership, “the big challenge was the intermediate energy storage. You need to capture renewable energies and transform them into electricity but most of the time it’s produced when you don’t need it. We had to find the right solution. We contributed to their improvement through our electrolytes, additives, binders and separators and I’m convinced that the progress we’ve made with Solar impulse will help us make advances with electric cars.”

Bayer MaterialScience provided the insulation – a lightweight yet strong foam that ensures the batteries are protected from the extreme -40oC to +40oC temperatures experienced mid-flight. The polyurethane (PU) used for the insulation saves 70 times more energy than is used to make it.

Over 80% of the plane’s body is made from ultralight composite materials, providing an incredibly strong frame at a fraction of the weight a conventional fuselage. Rather than conventional epoxy-enhanced solid carbon, a lighter carbon sandwich structure was specified at only 25g/m2.

Its distinguishing trademark silver finish is thanks to the Impranil coating from Bayer Materialscience – the same coating that has been used on the Adidas Brazuca, the official match ball of the 2014 FIFA World Cup. The company also provided the adhesives that hold the textile fabric in place underneath the wings.

Even the landing gear has undergone an overhaul, helping to shave a few pounds off the plane’s load, says Michel. The metals used in the pneumatic cylinders that raise and lower doors have been replaced by a polymer, Ixef PARA (polyarylamide) – marking the first time a pneumatic cylinder or actuator has ever been entirely made of plastics. This innovation proved 20% lighter than the original version.

“Not only have the materials been improved for the second plane but the techniques have evolved too,” says Michel. “For example, the fasteners, bolts and screws – all made from high performance plastics – were machined in the first plane but there was some brittleness because of this process. For the second plane we used a moulding process, which means better quality and a more consistent performance. Solvay also developed a new lubricant, which will reduce the amount of maintenance needed – very important for long duration flights.”

• Wings

Engineers placed 144 ribs at regular 50cm (20 inch) intervals along the wings, providing a rigid structure capable of withstanding the harsh conditions of flight. A flexible film was used to fill gaps between the solar cells and aid aerodynamics. At 72 metres, the wings are wider than those of a Boeing 747.

• Solar Cells

Over 17,000 ultra-thin photovoltaic cells cover 300m2 of sky-facing surfaces to absorb as much of the sun’s energy as possible and power the flight. A special film coating ensures that they are UV resistant and waterproof.

• Fuselage

Larger than the prototype, 83% of the new plane is made from light carbon fibre honeycomb structure, which strengthens the structure while significantly lowering the weight. This technique is already used in competitive yacht racing. The body’s silvery appearance uses the same coating that is used on the FIFA World Cup football.

• Gondolas

The four carbon fibre gondolas beneath the wings incorporate a 17.5hp motor, lithium polymer battery and propeller. Specially-formulated insulation foam protects the batteries from the fluctuating temperatures (-40oC to +40oC) in flight. The motors are up to 94% efficient.

PICTURE CREDIT: Bayer MaterialScience

PICTURE CAPTION: The cockpit has been given a thorough overhaul for the new record attempt