Solar Plane Soars Above Expectations

In early April, a Swiss research team finally test flew their solar plane prototype. The light craft has a wingspan of a Boeing 747 (approximately 65 meters[2]) and over 12000 solar cells. Its average flight speed is only 45 miles an hour, but that is not stopping the inventors from dreaming big. Their plan for 2010 is to circumnavigate the globe. And all without using a drop of fuel to power the plane [3].

Here, the plane begins to rise with the Alps looking on [3].

This little story shows that, perhaps, solar power has a future in aerospace applications. While current solar panels are not powerful or efficient enough to be a main method of propulsion, perhaps they could be used in hybrid applications. This would be similar to the first generation of hybrid vehicles, where the internal combustion engine was the primary energy source and the electric motor was secondary energy source.

Taking off is the most fuel intensive portion of flight. In fact, if the plane reaches a high enough speed, jet engines can shut off their compressors in their engines, allowing only the combustion reaction to propel them and maintain speed.  A lot of this is due to the lower drag at high altitudes due to lower air density. Is this a niche in which a solar powered plane could significantly lower fuel costs?

Unfortunately, the prognosis is not overly optimistic. The current Toyota Prius Permanent magnet AC synchronous motor is only 80hp[4], while each Boeing-747 jet engine is approximately 116000hp[2]. The weight and passenger size of these vehicles are absolutely massive, but scaling the Boeing engine power down to a 2-4 seat plane’s power doesn’t help much: 976hp to 80hp. A lot of this power isn’t necessary for steady flight, but the difference in magnitude is remarkable. It would be interesting to see if there are smaller, 2-4 seat planes for which the motor power is more comparable.

The other reason hybrid planes currently aren’t on the horizon is the two main restrictions of flight: weight and cross sectional area. Solar panels, electric motors, and large batteries would have to be added to make the hybrid plane function. For the scale of a plane, it’s hard to estimate what weight these would add, but it would not be insignificant. This, however, would be at least partially offset by requiring less fuel in the tanks. Finally, though, you get into area concerns. The solar panel arrays would require large areas to function. While it wouldn’t directly increase drag much (since solar panels are installed on a plane perpendicular to the velocity of the aircraft), I doubt manufacturers are about to make their planes’ bodies increase in size greatly so they can install more solar panels. Thanks to drag and weight concerns, there’s only so much room on a plane.

There are many technical and practical difficulties with making a hybrid plane (such as inclement weather suddenly becoming much more of a problem).  On the bright side, though, there have always been technical and practical difficulties associated with aerospace vehicles. It comes with the territory. Hopefully, this is one area they can get hammered out.







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2 responses to “Solar Plane Soars Above Expectations

  1. byrnemd

    As I first read this article, I didn’t think much of it. I mean, there has been successful solar powered flight for several years now. First, there was NASA’s craft “Pathfinder” followed by “Helios”. Helios broke altitude records for non-rocket flight, and ran on a combination of both solar power and gliding. [1]

    The Zephyr broke unmanned flight records in 2007 by flying for 54 straight hours. Its design looks flimsy, but it is very well suited for long term flight, and with a very lightweight construction of only 66 pounds, it can be launched by hand. [2]

    Then, in 2008, DARPA announced a competition for industry to create a solar-powered craft that could stay aloft for 5 years straight. The first entry to the competition, “Odysseus”, looks like a flying ‘Z’ and would weigh considerably more, on the order of 7,000 pounds. However, as it is designed to be a surveillance platform, it would fly at great altitudes and would hardly be visible from the ground. The solar power generated would in fact be more than required for sustained flight, so the construction also plans to incorporate flywheels or fuel cells to store energy temporarily to be used at night. [3]

    On the other end of the line, there are designs for solar energy harvesting using balloons as solar receivers, which claim the potential to lower solar energy costs to a measly 29 cents/Watt (about 10% the current solar average). [4]

    All this to say that I quickly realized the error of my thoughts, when I noticed that this blog is about manned flight. Previous solar flight successes have depended on the lightweight nature of the craft itself, with all nonessential parts and weight removed. This Swiss team though shows huge potential and incredible engineering skill to develop such a design that can carry a significant cargo weight compared to the vehicle weight itself. I agree with many of the points outlined above, though I believe there are many applications for solar cell use on existing aircraft to provide auxiliary power for non-critical electrical needs, such as space heating or entertainment systems. However, thin film solar cells must be further developed for this to be a reality, since weight is the primary concern for passenger or cargo flight.


  2. Christopher

    I feel, as you alluded to, that solar power has great potential at higher altitudes. In addition to causing less drag on the plane, a thinner atmosphere would absorb a smaller amount of the extraterrestrial radiation. In fact, of the 1367 W/m^2 that is incident on the outer atmosphere, I believe only about 200-300 W/m^2 reaches our surface.

    An extra 1000 W/m^2 (roughly a 500% increase) would be very usable for a high-altitude aircraft. While the added weight of solar cells and the systems to manage the extra power source (for propulsion or general use) are certainly hurdles, I would say that solar technologies could be a viable option for future use in high-altitude and long-duration passenger/cargo flights. As for low-altitude stop-and-go flights, I would not expect solar to be adopted for some time.

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