Despite having a wing, horizontal tail, and fuselage top full of solar cells, Solar Impulse 2 needs long days and short nights to make it through the dark periods between recharging. Even Eric and Irena Raymond’s SunSeeker Duo, a more practical machine, performs only day-long flights so far. Part of this is the nature of sunlight and solar cells, both of which are limited in small areas. Despite the fact that every hour, each square meter of the upper atmosphere receives 1.367kWh of solar energy, and National Geographic claims that “Every hour the sun beams onto Earth more than enough energy to satisfy global energy needs for an entire year,” it’s hard to harness that energy on a relatively small surface area. Solar cells at their best convert only a small percentage of the energy beamed onto them into usable current. On Solar Impulse 1, this was about 22.7 percent, according to Sunpower, the cell’s maker. Newer cells are reported …
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