Case Study 09

Solar Impulse 2

May 22, 2017

Solar impulse

Flying day and night using only stored solar energy, the Solar Impulse 2 (Si2) defied odds and made history when it circumnavigated the world without any fuel. Taking turns in the single-seater cockpit,  co-founders Bertrand Piccard and André Borschberg covered 43,041km in 17 legs, crossing Asia, the Pacific Ocean, the USA, the Atlantic Ocean, the Mediterranean Sea and the Middle East. 

When designing the plane, Piccard and Borschberg knew that they had to construct an ultra-light form that had a large wingspan to reduce drag and an ample surface to insert enough solar cells to reserve maximum energy to fly throughout the night on batteries. “The aircraft structure uses the most advanced technologies and has stimulated scientific research in the fields of composite structures, lightweight materials, electric propulsion and methods for managing and storing energy,” said André Borschberg.

Borschberg set a record for flying five consecutive days and nights over the Pacific Ocean from Japan to Hawaii. And Bertrand Piccard achieved the first crossing of the Atlantic Ocean in a solar airplane. A total of 19 world records were set or are still pending by the World Air Sports Federation (FAI).

“We had to make maximum use of every single watt supplied by the sun, storing any surplus in our batteries. We tracked down every possible source of energy efficiency,” said Borschberg.

Si2 used batteries based on Kokam Co., Ltd’s advanced Ultra High Energy Lithium Nickel Manganese Cobalt Oxide (NMC) battery technology. The battery was chosen for its high energy density and efficiency, along with its ability to operate over a wide range of temperature, humidity and pressure conditions.

With four 38.5kWh Ultra High Energy NMC battery packs with 150Ah cells, totaling 154kWh of energy storage, Si2’s 17,248 solar cells produced 11,000kWh of electricity, much of which was stored in its NMC batteries and then discharged to power the plane at night.

The Ultra High Energy NMC batteries feature an energy density of approximately 260 watts hours per kilogram (Wh/kg). This high energy density enables the Si2 to store more energy without increasing the plane’s weight or size. In addition the batteries have a 96% efficiency, meaning less energy is wasted when the batteries charge or discharge.



• 17,248 solar cells built into the wings that power the four batteries (38.5kWh per battery) that in turn power the four electric engines (13.5kW each) and the propellers

• Single-seater aircraft made of carbon fiber

• Unpressurised and unheated cockpit of 3.8m3

• Wing span: 72m (larger than a Boeing 747: 68m)

• Length: 25m

• Weight: 2,300kg (similar to a large family car)

• Cruising Speed: 45-55km/h
(25-30 KIAS (Knots-Indicated Air Speed) at sea level)

• Max. Altitude: 8,500m

Flight Level: 280 (28,000ft.)

• The propulsion system is 93% efficient  i.e. only 7% energy loss

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