In the beginning, there were bacteria.
And among those were purple bacteria, whose regal hue comes from natural pigments that the microorganisms use to convert sunlight into energy.
Billions of years later, the U.S. Air Force evolved and began experimenting with ways to use the same natural purple dye to convert sunlight into electricity.
In purple bacteria, which thrive in shallow lakes, the pigment uses the energy from photons to convert carbon dioxide into carbohydrates, which the bacteria consumes as food.
In the Air Force, scientists are using a synthetic version of the dye to convert sunlight into electrons - electricity that it hopes can power an unmanned aerial vehicle on extended-duration flights.
The dye is the key ingredient in very lightweight dye-sensitized solar cells that are being developed at the University of Washington.
Essentially, these solar cells imitate the way nature harnesses sunlight.
The cells are made in layers: The top is a transparent electrode that is coated with a thin layer of titanium dioxide, which is then infused with molecules of photosensitive dye. The bottom is a counter electrode. And the two are separated by an electrolyte.
When sunlight shines through the electrode and strikes the photosensitive dye, the dye sheds electrons. The electrons travel from the electrode through a circuit to the counter electrode, creating electricity.
Meanwhile, the dye returns to its normal state by stripping an electron from the electrolyte. And the electrolyte, in turn, recovers its lost electron from the counter electrode, completing the electrical circuit.
Dye-sensitized solar cells are not new. They've been around as experimental energy sources for almost two decades and have been produced commercially for a few years for such tasks as recharging cell phones.
They are about 10 percent efficient, said Minoru Taya, a professor and researcher at the University of Washington. That means 10 percent of the light falling on the solar cells gets turned into electricity.
By contrast, the more common silicon solar cells are 10 percent to 20 percent efficient, Taya said.
But the silicon cells are expensive. Dye-sensitized solar cells could be produced for a quarter of the cost of silicon cells, solar cell experts say.
For Air Force purposes, the newest types of dye-sensitized solar cells have another compelling attribute - they're thin films.
Earlier versions of dye-sensitized solar cells were made with glass and metal plates, making them too heavy to be a practical power source for UAVs, according to the Air Force Office of Scientific Research.
But lightweight thin-film cells spread out on the wings of a good-sized UAV should be able to produce enough electricity to keep the UAV flying and its sensors, communications equipment and computers operating, Taya said.
A UAV with wings that are one meter wide with a span of 10 meters would accommodate solar cells that could produce about 22 kilowatt hours of electricity per day, he said.
By comparison, an Air Force Predator UAV has a wingspan of about 50 feet.
How large a solar-powered UAV would be remains to be seen, said Gregg Abate, a senior engineer at the Air Force Research Laboratory at Eglin Air Force Base, Fla.
Size depends on the efficiency and weight of the solar panels and the battery that stores the electricity they generate, he said. "Regardless, the larger the UAV - i.e., the wing surface area - the better power efficiency will become."
The Air Force envisions a solar-powered UAV for surveillance missions. Taya said it would fly at about 50,000 feet - above any clouds - converting sunlight into electricity as it goes.
The solar cell would charge a battery, which would power the UAV's electric-driven propeller, cameras, communications equipment, an onboard computer and flight controls.
"All those gadgets easily consume a lot of energy," Taya said, so considerable effort is going into keeping them turned off as much as possible. Activity would be limited to short bursts, he said.
Engineers also are working to limit the aircraft's weight by limiting sensors, eliminating as much heavy copper wiring as possible and finding an ideal battery. Because of their weight, batteries "are a bottleneck" in any airborne application, Taya said.
The Air Force is considering thin-film batteries. These are relatively new, and for now, they are very small and provide low power. Some are as small as a postage stamp, thin as two human hairs, and provide electricity in milliwatts - enough to power tiny sensors, smart cards and radio frequency tags.
An important advantage is that they can be recharged thousands of times.
Abate, who is the research lab's chief engineer for micro air vehicles, weapon dynamics and controls sciences, said that powering a UAV with dye-sensitized solar cells remains basic research at this point, and it is expected to take three to five years to complete.
Work on mating dye-sensitized solar cells with UAVs began in 2006, Abate said. So far, the Air Force has spent $450,000 on the project.
The Air Force isn't the only military service interested in thin-film solar cells. The U.S. Army is sponsoring research into thin-film cells - both dye-sensitized and silicon - that can be used as a "power shade" that fits over Army tents to generate up to a kilowatt of power for operating electronics and recharging batteries.
If the use of such solar cells becomes widespread, the Army can reduce the number of liquid-fuel-powered generators it needs, which would reduce heat signatures, eliminate noise and diminish the burden of keeping generators supplied with fuel, according to the U.S. Army Soldier Systems Center at Natick, Mass. ■
E-mail: bmatthews@defensenews.com.
The Army's Future Combat Systems (FCS) program is ready to test a few components that soldiers may have in their hands by 2010.
Help
