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.\u00a0 Even Eric and Irena Raymond\u2019s SunSeeker Duo, a more practical machine, performs only day-long flights so far.<\/p>\n
Part of this is the nature of sunlight and solar cells, both of which are limited in small areas.\u00a0 Despite the fact that every hour, each square meter of the upper atmosphere receives 1.367kWh of solar energy, and National Geographic claims that \u201cEvery hour the sun beams onto Earth more than enough energy to satisfy global energy needs for an entire year,\u201d it\u2019s hard to harness that energy on a relatively small surface area.<\/p>\n
Solar cells at their best convert only a small percentage of the energy beamed onto them into usable current.\u00a0 On Solar Impulse 1, this was about 22.7 percent, according to Sunpower, the cell\u2019s maker<\/a>.\u00a0 Newer cells are reported to be as efficient as 24.1 percent.\u00a0 A square meter of such solar cells would generate 329 Watt-hours per hour.<\/p>\n That would depend, though, on the solar cells being perpendicular to the sun.\u00a0 Any \u201coff-angle\u201d between the cells and the light source would reduce the amount of current flowing<\/a> from the cells.\u00a0 Finding ways to align more of the airplane\u2019s surface to be at or close to perpendicular to the sun has prompted some clever approaches, including that of Aurora Flight Sciences eight years ago<\/a>.<\/p>\n