Solar Impulse 2, a solar-assisted battery-powered airplane, finished its world tour on July 26, 2016, returning to Abu Dhabi more than one year and 40,000 km (25,000 mi) after its departure. With seventeen individual flights accumulating nearly 600 hours in the air, the sun-powered plane broke nineteen world records on its historic journey. Along the way, its solar panels provided more than eleven megawatt-hours of electrical energy to the plane - running the four 13 kW motors during the day, and storing the excess for night flying.

Solar Impulse 2

Here are Solar Impulse 2's specifications and flight path:

The Batteries

The plane's world tour had one complication: the potential overheating of its Kokam 154 kWh battery bank during the trip's 118 hour Japan-to-Hawaii leg. The batteries feature Kokam's patented Ultra High Power Nickel Manganese Cobalt (NMC) technology, a variation of Li-ion chemistry. They can survive up to 10,000 charging cycles, have a low internal resistance, and dissipate heat 60% better than standard NMC batteries. Their combination of energy density and power density makes Ultra High Power NMC batteries ideal for electric vehicles.

On its flight to Japan, Solar Impulse 2 encountered some rough weather, so the ground crew decided to run an unscheduled test flight prior to the trans-Pacific crossing. Everything checked out okay and the plane was quickly cleared for take-off. However, they didn't allow the batteries to cool after the test flight. That, combined with the tropical weather and the extra long flight to Honolulu, caused the battery temperature to approach 50C during the flight, exceeding the batteries' specified limits. The team decided to err on the side of caution and delay the remainder of the trip while engineers determined the cause and devised a solution. (I suppose the fact that they were "stranded" in Hawaii made that decision a little more palatable.)

Upon landing in Honolulu, technicians removed the batteries and shipped them back to their assembly plant in Germany, where they underwent extensive testing. New batteries were installed for the rest of the journey. As it turned out, the original batteries were undamaged by the heat and the engineers determined that Solar Impulse 2 would have been safe to fly without replacing the batteries. Better safe than sorry.

Chill Out, Batteries

The delay proved to be worthwhile from a design standpoint. While the plane was grounded, engineers tweaked the cooling system by adding a pilot-controlled air vent (shown in the image below), which allows outside air to cool the batteries when needed. The vent can be closed in colder temperatures to prevent the batteries from freezing.

Learning to Fly (Electric)

While a completely solar-powered plane isn't practical, hybrid and fully-electric powertrains - solar-assisted or not - can reduce the carbon footprint of medium-to-small aircraft. The research and development that the Solar Impulse team has conducted and the lessons that they've learned will provide lift to the aviation and automotive industries as they reach new heights in energy efficiency and electric propulsion. I like the outlook!

Here's a video about Solar Impulse: