Wrong.
There is now an exception to that rule, and it's name is Solar Impulse. It's a solar-powered airplane that can fly without a drop of fuel. And, in spite of the adjective "solar-powered", it doesn't require sunlight at all times. It can even fly at night.
The plane shows significant promise for clean energy proponents. Back in 2012, it was flown from Switzerland to Morocco. Eventually, the plane's creators plan to circumnavigate the globe with it. But that will have to wait until it finishes its flight across the United States, which began on May 3. For some reason, that flight did not originate in Kitty Hawk, North Carolina.
Solar Impulse started its transcontinental journey in San Francisco, setting course for Sky Harbor airport in Phoenix, Arizona. From there it will travel to Dallas, Texas sometime soon. At the end of this month, the plane will travel to St. Louis, Missouri. Then it will travel to the nation's capitol, and finally to New York City.
So why doesn't a plane which is described as "solar-powered" need perpetual sunlight for power? It's got lithium polymer batteries housed in nacelles under its wings. The actual solar-powered cells (photovoltaic cells for geek-speakers) are situated on top of the wings.
These cells absorb the sunlight, and convert that energy into electricity. This electricity is used to propel the plane and charge the batteries. When the light isn't present, the energy from the batteries can still propel the plane. Lithium polymer batteries are also rechargeable, so once the energy in the batteries is spent, they can be recharged again via the solar-powered cells. The plane generates about as much energy as a scooter.
How fast does the plane travel? About 45 miles per hour, according to the founders. That's not nearly as fast as the commercial airliners, but it's still in early stages of development.
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