The combination is irresistible: photovoltaic cells covering the long, wide wings of an airplane, making the electricity to drive the propellor. If the solar cells could also charge batteries to run the plane at night, it would be possible to keep the plane in the air as close to perpetually as the weather allows. But heavyweight batteries and inefficient solar cells made such an accomplishment difficult.
Difficult -- but not impossible. Last week, AC Propulsion, an organization specializing in electric vehicle engineering, demonstrated a solar-battery unmanned air vehicle (PDF) that flew for just over 48 hours straight.
Remaining aloft for two nights is the milestone for sustainable flight. One night is possible just by discharging the batteries, but two or more nights means that the plane has to fully recoup and store the energy used at night while flying in the sunlight the following day. Once that is achieved, the cycle can repeat continually, and keep the plane airborne indefinitely.
The 76 photovoltaic cells used were only 20% efficient -- decent, but by no means cutting-edge -- but they produced enough power (around 225 watts) to charge the Lithium-Ion batteries and power the communication and control system. The UAV was controlled entirely from the ground, and never strayed more than 8km from the launch area; at 30-50 miles per hour, that restriction wasn't particularly difficult to stick to. But what makes the SoLong notable is the size: as the picture above shows, the wingspan is only about 4.75m, and the entire aircraft weighs about 12kg. It's not quite a microflyer, but it's definitely down in the hobbyist/NGO camera plane range. (More images can be found in this PDF document.)
Further up the size scale will be the Mercator solar UAV. Built by QinetiQ for a project run by the Flemish Institute of Technology, the aircraft is designated a "High Altitude Long Endurance" vehicle (HALE), and is intended to fly for months at a time at a height of 60,000 feet. The size difference is notable: the Mercator will have a wingspan of 16m -- nearly four times that of the SoLong -- and weigh 27kg. The difference is wingspan is related, in part, to the height at which it will fly; at so high up, the air is much thinner, and the wings have to be larger to keep the vehicle aloft.
Compare this to the Solar Impulse, a planned round-the-world manned solar aircraft. We posted about the Solar Impulse project in February; this week, at the Le Bourget Air Show, the Solar Impulse team revealed some of the specifications for the aircraft's design.
A team of 60 specialists has been working for 15 months to reach the present stage of this project, reflected in the model unveiled at Le Bourget: an impressive ultra-light structure with a wing span of 80 metres – similar in size to the brand new Airbus A380 – and a weight of just two tons. This airplane has the capability to fly day and night powered entirely by solar energy and to fly completely around the globe in fiveday stages, with the goal of promoting sustainable development and renewable forms of energy.
The "fiveday stages" aspect of the trip is almost certainly related to passenger comfort and health; as the SoLong excerpt notes, if the craft can make it for 48 hours, it can stay up indefinitely. Regardless, an 80m wingspan, 2000kg weight solar aircraft will be impressive, if it gets built.
Without substantially more efficient photovoltaic cells and batteries, solar powered flight will remain in the realm of robots and adventurers. But even if Boeing and Airbus never put solar planes on the runway, the research and development that goes into making these the frames, skin, solar and battery systems for these aircraft as light and efficient as possible will have real-world applications. If your gas-optional hybrid car of 2012 has a photovoltaic layer to help keep the batteries topped up, part of its genesis will have come from the air.
A carbon neutral aircraft is a vital technology if we want to keep our quality of life AND reduce carbon emissions. The huge amount of fuel used by planes, and the increase in warming caused by altitude (X2, according to the Independent newspaper) makes finding a replacement imperative. But such huge fuel costs also make such a craft more viable financially. I would like to see a Gravity Plane combined with photovoltaic fabric such as that made by Konarka. This would seem like a graceful, potentially viable alternative.
Carbon neutrality is relatively easy to achieve; all you have to do is run the plane on liquid hydrogen. (Making carbon-neutral hydrogen, building the fuel infrastructure, converting the aircraft... that's work.)
Gravity planes and the like suffer from the same flaw as trains: they are very slow over continental (not to mention trans-oceanic) distances and the labor costs of running them, let alone the time/opportunity costs of riding them, become very high.
Maybe we'll need to call them "cruise planes," and charge people much more for a luxury trip.
That said, the first applications for such planes will likely be in communication and remote sensing. There's all sorts of niches where UAVs would be helpful, including smuggling drugs and military operations. This technology could be quite disruptive even before it "gets big."
AC propulsion is an amazing company. It started out as a garage company selling hobby kits for electric vehicle enthusiasts (basically motors). And now this. Very nice.
This reminds me of the fact that GlobeTel / Sanswire's first prototype "Stratellite" has finally been built. Pictures of this solar-blimp are at their website:
Oh, yes, and check out: http://www.solar-flight.com
This is so cool. Someone should commercialize this. Excellent for eco-tourism excursions (make a two-seater). Quiet and clean!
Really check it out! If I had the money I'd buy one instantly.
That's cool , Lorenzo. Cheers!
"Gravity planes and the like suffer from the same flaw as trains: They are very slow over continental (not to mention trans-oceanic) distances and the labor costs of running them, let alone the time/opportunity costs of riding them, become very high."
What if they are automated? And can't they do the same thing that trains do and just ship huge amounts of materials so time doesn't matter as much? Trains and ships persist just by getting larger, longer and occasionally (like in Europe or Asia.) faster.
I doubt very much that someone going to a business meeting across the continent, or who wants important documents to sign or fresh fish for sushi, is going to be happy with five times the shipping volume with five times the delay. Sometimes time is of the essence.
This thread finally prompted me to put some thoughts down and crunch a few numbers; I blogged the subject of airliners here.
Its most likely that air travel will be powered by fuels provided by all the coal to liquids plants being planned. Coal is cheap. And cheap coal is very very cheap and alot cleaner to use if your converting it first.
By the time that runs out well lets just say I doubt power will be a problem by then.
As for cost the main cost of an airplane is its lease payments.
As i recall there was a Solar powered plane was built in the eighties or early nineties that crossed the English channel with a pilot. While it did not have the planned endurance of the discussion here it did provide a practical demonstration that the technology was ready to be pushed. It seems that not a great deal has been done since that time. It is good to see that the cudgel has been taken up again.