Date: Sep 07, 2013 Source: Economist Magazine (
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FLYING using only the power of the sun is an enticing prospect. But manned solar-powered aircraft are fragile and slow, like the ungainly craft which a Swiss team called Solar Impulse plan to fly around the world. Take the weight of the pilot out, however, and things change. A solar-powered unmanned aerial system (a UAS, more commonly called a drone) could fly long, lonely missions that conventional aircraft would not be capable of. Advances with the technology mean many such craft could be swarming into the sky.
Small, battery-powered UASs are already common. The Raven, made by AeroVironment, a Californian company, is a hand-launched reconnaissance aircraft with a wingspan of about 1.4 metres that is widely used by the American armed forces. Thanks to advances in electronics a small UAS like this is now capable of beaming back images from high-resolution video cameras and other sensors.
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Eventually, though, the civilian use of small drones may well exceed that of military ones. They can survey pipelines and power cables, perform aerial filming for anyone from television news stations to estate agents, monitor fires, assist in search-and-rescue operations and help carry out research. Aviation regulators are now forming rules that would allow far greater civilian use of such systems.
For many of these missions long flight times would be important, and solar power can help with that. It also means not having to transport fuel or carry recharging equipment. With electrically powered small drones the limitation is the battery. This means the flight time of some small craft like the Raven is limited to around 90 minutes. So the race is on to find ways to fly for a lot longer.
Kevin Jones, of the US Naval Postgraduate School in Monterey, California, is developing a hand-launched drone called TaLEUAS (for Tactical Long Endurance Unmanned Aerial System). It combines solar power with an ability to soar on thermals, created by rising hot air. Most solar cells are based on crystalline forms of silicon. To increase the power being generated Dr Jones is using cells with a highly efficient, but more expensive "monocrystalline" structure. These cells are being produced for him by SunPower, another Californian company, which is supplying the cells for the manned Solar Impulse.
Sunshine come lightly
But it takes a lot more than just sticking better solar cells onto the wing. The cells have to be bent over the curved aerofoil of the upper wing without disrupting the airflow, which would increase drag and thus reduce lift. And then the cells still have to work efficiently and be able to flex in flight as wings do. Getting all this right, says Dr Jones, was a big part of the process. Computer modelling provided an indication of how to do it but the only reliable way to find out if a design works well is to fly it.
The latest set of solar cells will give TaLEUAS some 60 watts of power, which should extend the flight time significantly compared with other drones. The first solar-augmented flights are scheduled for November. The lessons gained will then be used to build another craft with more solar cells, enough to recharge the batteries in flight. Combined with an ability to rise up on thermals to gain altitude, Dr Jones hopes TaLEUAS will be capable of flights lasting for days or even weeks.
New types of solar cell are coming and prices are expected to fall. MicroLink Devices of Niles, Illinois, has developed a solar sheeting based on gallium arsenide, which has greater efficiency than silicon cells. Although brittle, the photovoltaic material is flexible enough to be wrapped around the curves of a UAS wing.
MicroLink has worked with the American air force to retrofit its solar cells to AeroVironment's small drones. The first Raven fitted with these cells showed a 60% improvement in performance. But this is dwarfed by the capability of a small UAS called Eturnas which MicroLink is collaborating on with Design Intelligence, a company based in Oklahoma.
By treating solar power as an add on technology to an existing UAS design it is possible to achieve a 200% to 300% improvement in flight times over battery power alone, says James Grimsley, Design Intelligence's boss. But a craft specifically built to use solar power can do better. It would be able to fly as long as there is good sunlight, and then use its battery for an hour or two once the sun goes down.
Two versions of Eturnas are under development: a 4kg version and a 2.7kg one with a "loitering speed" of around 64kph (40mph). Both can be taken apart and packed into convenient carrying cases. Test flights are planned later this year.
Meanwhile, Alta Devices of Sunnyvale, California, has developed a new technique for manufacturing solar cells from extremely thin layers of gallium arsenide. According to Rich Kapusta, the firm's marketing chief, the layers are just a few microns thick, which makes the solar cells very lightweight indeed. The cells are being produced in 5cm-by-2cm units which can be "stitched" together to cover the surface of a wing of any size and shape.
Last month AeroVironment and Alta Devices revealed a new version of their Puma UAS, a 6kg craft with a 2.8-metre wingspan (pictured). This solar-powered Puma flew for over nine hours, compared with three hours for the battery-only version.
Mr Kapusta says his firm's solar cells are robust enough to withstand the impacts that can be associated with hard landings and are encapsulated to protect them from wind and rain. Similar types of cell are used on satellites, he adds. Indeed, solar power has had a transformational effect on satellites, giving them an almost unlimited life. The new sentinels in the sky may end up with not just long endurance but also a long operating life.
