Bertrand Piccard had the idea for a solar-powered aircraft during a long-distance hot-air balloon expedition where he attempted to circumnavigate the globe in 20 days. During the journey, he watched his fuel supply get lower each day, and he wondered: “How could I imagine to fly forever with no fuel?”
Thus, the idea of the solar-powered aircraft took hold. Thirteen years after a 2003 feasibility study, co-founders of Solar Impulse Foundation Bertrand Piccard and André Borschberg completed the first circumnavigation of the globe with no fuel.
It was a 40,000 km journey for the solo-flying aircraft, and each of the co-founders flew for half of the journey. But instead of the plane having the speed of a 747 to chase the sun and be in perpetual daylight, the battery system on board stored the solar energy so they could fly during the day or night.
Each flight cycle made maximum usage of energy. Beginning with a full battery, the plane takes off from the runway using stored energy. Then during daytime flight (6am – 6pm) the plane runs at maximum power, steadily using solar energy while also storing energy to use at night. The goal is to continually gain altitude, reaching its maximum altitude of 9,000 meters by 6 pm.
Now as the sun’s rays fade, the motors can be throttled down and the plane can glide, reducing its altitude. It take about four hours to reach 1,500 meters, using almost no energy during that time. At about 10 pm, the motors are then throttled up using the stored energy from the batteries. It flies through the night on the stored battery power until the sun rises again.
The aircraft weighs about as much as a family car, and has the wingspan of a Boeing 747 Jumbo Jet. This makes the Solar Impulse 2 the largest aircraft ever built at such low weight.
It has to be very efficient with both space and energy in order to achieve the great distances, including 5 days of consecutive flight achieved by Borschberg, who crossed the Pacific from Japan to Hawaii.
Power: Over 24 hours, the average power is 15hp, or that of a small motorbike. However, the maximum power is 70hp.
Energy: The plane was equipped with 17,248 solar cells. “The solar cells, batteries and motors have record beating energy efficiency: 23% for the solar cells, and 97% for the motors, which only lose 3% of energy against 70% for standard thermal motors,” said the team.
Propulsion: With four propellers, the plane achieved an average airspeed of 75 km/h.
Weight: At 2.3 tons, the plane is 10x lighter than the best glider. One quarter of the plane’s mass, or 633 kg, is taken up by the batteries.
Dimensions: The wingspan is 72 meters. However, the cockpit is only 3.8 cubic meters, a very small space that is the size of about seven refrigerators. The pilots spent up to five days in this unheated and unpressurized cabin.
Body Structure: The body of the plane, cockpit, and spars are made from an innovative ultralight construction. Carbon fiber weighing 25 g/m2 — which is three times lighter than paper — creates an exterior shell sandwiching alveolate foam that is in a hexagonal pattern like honeycomb.
Wing Structure: While the upper wing is covered with a skin of encapsulated solar cells, the lower wing surface has 140 carbon fiber ribs at 50 cm intervals to give the wing rigidity and make it more aerodynamic.
Exterior Protection: Because of the harsh conditions of wind, rain, and temperature, an exterior barrier protects the plane from the elements and mold while still allowing for necessary flexing. The plane was protected using a combination of polyurethane foam, high performance polycarbonate sheets, and ultrathin polymer film.
A Look Inside
In the video below, you can get a look at the plane’s construction, including seeing smart details such as a pilot’s chair that reclines into a bed.
The core of the endeavor was a mission of clean energy. It sought to not only push the technological boundaries to make solar powered flight possible, but it also sought to capture the public’s attention with a Jules Verne-style sweeping adventure.
While it seems quite unrealistic to imagine a world where every person has their own gigantic hangar to keep their solar airplane, the technology is a great start. Just as computer storage evolved from floppy disks to 120GB smartphones, perhaps solar flight will also evolve to get smaller and faster.
Even beyond personal planes, solar flight technology could have applications for increasing battery capacity of flying drones. While research such as the SpiderMAV explores ways of reducing power usage while mid-air, another approach to drone batteries could be through harvesting solar energy during flight.
“Solar Impulse was not built to carry passengers, but to carry messages,” said Bertrand Piccard. “We want to demonstrate the importance of the pioneering spirit, to encourage people to question what they’ve always taken for granted.”