July’s successful circumnavigation of the globe by Solar Impulse was a triumph for human endurance and ingenuity, and proved solar power’s efficacy as a reliable source of electricity. But the project would not have been possible without batteries… and the journey proved a voyage of discovery for storage’s technical possibilities.
When pv magazine visited Solar Impulse’s vast hangar in Switzerland on a bright fall day back in 2014, a veritable shower of facts and figures poured forth from the presentations given by the aircraft’s pilots, Andre Borschberg and Bertrand Piccard. But amid the eyebrow-raising, record-breaking stats, one seemingly throwaway comment stood out: if the Solar Impulse 2 (si2) aircraft was made totally out of paper, it would actually be heavier than it is. The thousands of new patents filed during the plane’s design and creation was proof if proof were needed: this flight, if it was going to be a success, would be supremely dependent on weight.
Cue a few lighthearted jokes about the pilot’s forthcoming diet regimes, followed by the most obvious question: which components are proving the weightiest? The answer was the four lithium batteries, two to each wing.
Capable of storing 164,580 watts of power, the batteries – with cells developed by South Korean storage specialists Kokam – were key to the success of the operation, storing the solar energy harvested by the 17,000 solar cells to enable the aircraft to fly at night. But with a combined weight of 633 kg, they represented one-quarter of the aircraft’s total bulk – an obvious area for improvement next time out, perhaps?
“Typical lithium-battery sizing is between 10 to 50 AMP hours for a single cell,” Ike Hong, Kokam’s VP of power solutions division, told pv magazine. “So what we wanted to do for Solar Impulse was to minimize the number of cells required. We developed a large format battery, so one single cell was 150 AMP hour. That was the single biggest lithium polymer battery ever at the time, back in 2008-9 when we first began working with Solar Impulse. The second approach we took was to increase energy density, meaning that within the small space available the goal was to put much more energy.