The newest version of the solar-powered aircraft Solar Impulse features a wingspan of 236 feet, a weight of 2.3 tons (similar to a Jeep), and horsepower in the range of a motorbike. Power is generated by 17,200 solar cells on the wings.
The energy collected by the solar cells is stored in lithium polymer batteries, whose energy density has been optimized to 260 Wh/kg. The batteries are mounted in the four engine nacelles, with a system to control charging thresholds and temperature.
Weight of the batteries amounts to just under 1,400 pounds, or just over a quarter of the aircraft’s in-flight weight. In order to save energy, the aircraft climbs to almost 28,000 feet during the day and descends to 5,000 feet at night. It is proportionately 10 times lighter than the best glider.
Early next year, it is expected to traverse the globe without making a single stop for fuel. That’s a trip of five days and five nights for each leg flying at a speed similar to that of a car on the ground. Stops are planned to allow changes of pilots, who will have on board six oxygen bottles, a parachute, a life raft, and food and water for a week Total flight time is estimated at 500 hours.
Two global plastics companies, Solvay and Bayer, have contributed significant new technology to make the aircraft light enough to operate on solar power with relatively small batteries.
- The cockpit’s fairing is made of an ultra-light weight, insulating polyurethane with a special foaming agent, Solkane 365 MFC.
- The wing spar contains a refined honeycomb structure, made of paper impregnated with Torlon PAI polymer. This provides strength, torsion, flexion, vibration.
- Mechanical parts, such as fasteners and screws, are made of specialty polymers Ketaspire PEEK and PrimoSpire SRP.
- Complex mechanical parts, such as lighting clips or the housings for the cockpit equipment, are produced from polyamide 6 Sinterline via the 3D Selective Laser Sintering process.
- Baytherm Microcell is used for the aircraft door, while the rest of the cockpit shell is made of a different type of rigid polyurethane foam from Bayer MaterialScience.
- A polyurethane/carbon fiber composite is used for the door locks, and thin sheets of transparent, high-performance polycarbonate for the window. Although the cockpit is larger overall than in the first prototype, it is only minimally heavier. The size of the cockpit is 3.8 cubic meters.
- Rigid polyurethane foam from Bayer MaterialScience is used to insulate the batteries. The company also provides the raw materials for the silvery coating covering large portions of the aircraft and the adhesives that hold the textile fabric in place underneath the wings.
The aircraft was unveiled today to a large audience in Payerne, Switzerland invited by Bertrand Piccard and André Borschberg, who founded the Solar Impulse project more than ten years ago. They want the project to demonstrate the potential of existing or neart-term technologies for contributing to energy efficiency, renewable energies and new transportation solutions.
The actual commercial potential for a solar aircraft for anything beyond drones is hard to imagine, however, considering that the primary purpose of aircraft is to carry payloads: people, packages or industrial shipments.
One thing I have always liked about the Solar Impulse is that it is a collaborative technical development. At the heart of the matter, Piccard and Borschberg are really idealists and adventurers. They laid out a vision and a challenge and asked for funding and technical cooperation from a number of companies, even some who are at times competitors, such as Solvay and Bayer.
There are 80 technological partners on the project and more than 100 advisers and suppliers. Solar Impulse employs just 50 engineers and technicians. The technology for the first prototype was pretty much off the shelf. The technology required for the next prototype is next stage. For example the carbon fiber composites in SI2 set new standards for light weight. Some 140 carbon-fiber ribs spaced at almost 20-inch intervals give the wing its aerodynamic cross-section, and also maintain its rigidity.
May the wind be with them.