A Solar Powered Plane is an Engineering Marvel but it won’t Fly you Anywhere

Posted by PITHOCRATES - March 9th, 2013

Week in Review

The Boeing 747-8 is the latest derivative of the 747 family.  It can seat up to 465 people.  And has a gross takeoff weight of 975,000 pounds.  It’s cruising speed is 570 mph.  And has a range of 9,210 miles at maximum take-off weight.  Which means it could fly between California and New York in about 4 and a half hours.  The Boeing 747-8 is truly a remarkable aircraft.  But how does it measure up to other aircraft?  Well, here’s one with a similar wingspan (see Solar-Powered Plane To Make Cross-U.S. Flight by Jesse Emspak posted 3/4/2013 on Discovery News).

A plane that can fly on solar power, day or night, will make its way across the United States this summer — the first time the plane has attempted a cross-continental flight.

Wow.  Can it be the environmentalist were right all along?  That we can replace fossil fuels with solar power?  Well, this appears to be the proof.  A plane that can fly cross-continental.  Day or night.  Why, this can revolutionize air travel.  And put a serious crimp in global warming.  For as great as the 747-8 is it still burns a heck of a lot of jet fuel.  Putting a lot of emissions into the air.  Perhaps this is the future of aviation.  Clean solar power.  Perhaps with some minor adjustments required in our travel plans.  But if it saves the planet perhaps those minor adjustments will be worth it.

The Solar Impulse — built as a project of the Swiss Federal Institute of Technology, the brainchild of Bertrand Piccard and André Borschberg — has the wingspan of a 747 but only weighs as much a Honda Prius. It flies thanks to four turboprop engines powered entirely by batteries and solar panels.

Borschberg told Discovery News that the although the plane could make the whole trip from California to New York in one go, the pilot cannot. The plane travels at 40 to 50 miles per hour, so a cross-country flight would take days. And since there’s only room for a single person in the cockpit, in part to save weight, and no autopilot, the trip will have to be broke up into five legs…

The solar panels are conventional silicon, with an efficiency of about 25 percent. While there are more efficient solar panels such as those used in the satellite industry, those designs are often too heavy, Borschberg said, as they tend to be encased in glass. And although the power is stored in batteries, the engines can run directly from the energy collected by the solar panels. In fact, the plane could be flown on an empty battery.

A 747-8 at maximum take-off weight weighs the same as about 321 Honda Prius hybrids.  And it includes galleys.  And toilets.  So it can stay in the air and fly almost anywhere in the world nonstop.  While the Solar Impulse currently can’t carry any passengers, has no galley and no toilets.  Which may allow about three flights of 4-5 hours a day.  Allowing it to arrive in New York after leaving California some 6 days earlier.

So solar power is not a viable alternative to fossil fuel if we want to fly anywhere.  As remarkable as the Solar Impulse is, and it is truly remarkable, it is only an engineering marvel.  For there is no way that solar power can provide sufficient thrust to carry great weights into the air.  Solar power can work in weightless space for they only have to power electric loads.  They don’t have to provide any thrust to move a heavy mass.

This is a large-scale example showing the limitations of electric-powered transportation.  For transportation to be useful it must be able to move heavy weights.  But the more useful the transport vehicle (the greater the weight it can move) the more battery charge is used for motive power.  Drawing down the battery charge faster (which is drawn down even faster if lights, heat, radio and other electric accessories are used).  Reducing range.  And usefulness.  Leaving the fossil fuel-powered vehicle the only viable vehicle in the foreseeable future.

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