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 to be as efficient as 24.1 percent. A square meter of such solar cells would generate 329 Watt-hours per hour.
That would depend, though, on the solar cells being perpendicular to the sun. Any “off-angle” between the cells and the light source would reduce the amount of current flowing from the cells. Finding ways to align more of the airplane’s 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.
Boeing thinks, as others have with high-altitude, long-endurance (HALE) attempts, that capturing and storing energy wisely will enable an airplane to stay up for years at a time.
While Aerovironment’s long-wing, multi-motor flying wings posited using hydrogen generation to enable overnight flight, Boeing’s unpiloted aircraft is designed to stay in the air with sunlight alone.
Its long wings, with perpendicular winglets to capture sunlight even when solar energy is radiated from low on the horizon, should be able to have the “persistence” needed for communications systems using radio signals, mobile signals, microwave transmissions, broadcast television and even Internet. It can also be fitted with an atmospheric sensor system for calculating the wind, temperature, humidity, gases present, and sunlight, among others.
The company filed for a patent, the application noting several potential uses for a solar-powered craft that can maintain a fixed position:
The patent applications acts as something of a confessional, showing that a Boeing 747 equipped with perfectly efficient solar cells on its entire upper wing surface would receive at most approximately 600 kilowatts, or about 800 horsepower from the solar cells, far short of the 100,000 horsepower required for the 747 to maintain cruising speed and altitude.
Solar power can provide only 0.8-percent of the needed power to a conventional 747; even good cells of 0.3-percent efficiency would produce only 0.3% of the needed power to a 747. At least for the time being, big wings, slow speeds and light weights are a reasonable way to accommodate solar power.