Thin Displays as Wristbands

The U.S. Army is evaluating full-color flexible displays that can be worn on the wrist.

Wrist flex: This prototype made for the U.S. Army is worn on the wrist and incorporates a thin, lightweight flexible OLED display.
Credit: Universal Display




The U.S. Army is testing a prototype "watch" that's lightweight and thin and has a full-color display. This display is built on flexible materials encased in a rugged plastic case and can be worn on a wristband to display streaming video and other information. It uses newly developed phosphorescent materials that are efficient at converting electricity into red, blue, and green light, which means the display needs less power to work.
Most phones, laptops, and TVs today use liquid-crystal displays (LCDs) controlled by electronics built on glass. To make more energy-efficient displays that are controlled by flexible electronics, which are lightweight and won't shatter like glass, many companies are turning to organic light-emitting diodes (OLEDs). The pixels in OLED displays replace the layers of electronics and filters in LCDs with organic dye molecules that emit light in response to electrical current.
For consumers, flexible OLEDs promise portable electronics with beautiful screens that don't drain battery life and won't shatter when dropped. But so far, no companies have developed economically viable manufacturing methods for producing flexible OLEDs with long enough lifetimes and consistent quality. The U.S. military has been funding development with the aim of providing soldiers with rugged, thin communications devices that can display maps and video without adding too much weight to their load.
The new display prototypes use efficient OLED materials developed by Universal Display of Ewing, New Jersey, and are built on foil-backed electronic controls developed by LG Display, headquartered in Seoul, South Korea. The devices were designed by L-3 Display Systems of Alpharetta, Georgia. The display is 4.3 inches. As part of military demonstration tests, the device has been used to stream real-time video from unmanned air vehicles.
"These prototypes represent not so much one major advance but continued progress on many fronts," says Janice Mahon, vice president of technology development at Universal Display. Those fronts include the OLED materials themselves, the electronics that control them, and the integration and packaging of the device.
The first generation of OLED materials, used today in glass-backed cell-phone displays and some small TVs, can convert only 25 percent of electrical current into light; the rest is lost as heat. Universal Display is designing and developing materials that work by a different mechanism and that have a theoretical efficiency of 100 percent. The prototypes for the Army use a full set of phosphorescent materials; the companies have not released specifications about power consumption, but Mahon says displays made with these materials use one-fourth the power of a conventional OLED.
Samsung Mobile Display, the biggest maker of OLED displays, currently uses Universal Display's red phosphorescent materials in its products; Samsung and other companies are currently evaluating green materials. Phosphorescent materials that work with higher energy light such as blue tend to be less stable over time and have been slower in coming. The companies have not disclosed information on the expected lifetime of the all-phosphorescent displays.
Universal Display applied the light-emitting layer to electronic controls made by LG Displays. The electronics are an array of amorphous-silicon transistors built on stainless steel foil instead of glass. Other companies, including Hewlett-Packard and Samsung, are developing flexible amorphous-silicon transistor arrays, mostly on sheets of plastic. Working with metal poses some challenges because the surface is rough, which can disrupt the structure of the transistors, but metal can withstand higher processing temperatures than plastic can. That's an important trait when it comes to laying down the silicon. High-temperature processing results in silicon crystal that's not only higher quality but also more stable over time.
"The broader story is that we're starting to see some good-looking demos of flexible OLED displays," says Nicholas Colaneri, who heads the Flexible Display Center at Arizona State University. Sony and Samsung Mobile Display have both demonstrated flexible displays built on sheets of plastic; both companies have been tight-lipped about these technologies. But, Colaneri notes, "just because you can do it doesn't mean you can afford to do it."
A major hurdle remains before displays like the prototype made for the Army will arrive on store shelves. Amorphous-silicon transistor arrays can be made at temperatures suitable for flexible electronics, and the LCD industry has created a lot of infrastructure for making them. But over time, they're not the best electronics for controlling OLEDs. The electrical currents required to switch OLED pixels burn out these transistors; the pixels that are on most frequently start to malfunction.
Canadian startup Ignis Innovation is developing software and other controls to extend the lifetime of the transistor arrays by ensuring that no single pixel is on too often. Colaneri says its initial prototypes are promising. In the meantime, Colaneri and other researchers are developing alternative transistor materials such as metal oxides to make OLED electronics that won't burn out.
The companies that made the Army prototype are not disclosing the metal-silicon electronics used to run it, but say they have met the Army's specifications.

Turbines Could Tap the Mississippi's Power Underwater turbines could harness a massive amount of energy—but could cause problems for boat navigation.

Power Up: Free Flow Power hopes to deploy tens of thousands of underwater turbines in the Mississippi River. That many turbines could generate enough power for a quarter million homes.
Credit: Free Flow Power

Tens of thousands of turbines anchored to the bottom of the Mississippi River could someday provide more than a gigawatt of renewable energy, enough to power a quarter of a million homes. That's the vision of Free Flow Power, a startup based in Gloucester, Massachusetts, that recently received preliminary permits from the U.S. Federal Energy Regulatory Commission (FERC) granting it the right to explore energy production at dozens of sites along the lower Mississippi over the next three years.
The proposed development is one of a number of "hydrokinetic" projects in the works. Such projects seek to generate electricity from wave movement, tidal flows, or river currents, without the use of dams.
The ambitious Mississippi project, however, relies on relatively unproven technology. The only commercial hydrokinetic river-power system operating in the U.S. is a single turbine deployed by Hydro Green Energy close to a conventional hydropower dam on the Mississippi River in Hastings, Minnesota.
Free Flow hopes to deploy hydrokinetic power on an unprecedented scale: hundreds of 40-kilowatt turbines, each the size and shape of a large jet engine and attached to pylons jutting out from the riverbed at 88 locations along the Mississippi.

Although most companies developing hydrokinetic technology have focused on tidal or wave energy, Free Flow's chief financial officer, Henry Dormitzer, argues that river power has distinct advantages. "The water flows in one direction, it doesn't have salt in it, and, in the case of the Mississippi, people have spent 100 years tracking water flows and velocities," he says.
But the Mississippi is also one of the world's busiest waterways, and the company will have to demonstrate that its turbines will not interfere with commercial shipping, and that it will have no negative impact on the river's wildlife.
In July 2009, Free Flow began a six-month test of a pilot turbine (a third the size of the planned commercial ones) in the Mississippi, and the company is now testing a commercial-scale prototype in the lab. Free Flow has also received $7.4 million in funding from investors and from the U.S. Department of Energy that will allow it to test its most recent prototype in the Mississippi next year. Free Flow Power is seeking additional funding to test four turbines, each attached to a separate pylon, over a 12-month period, as required by FERC as part of the licensing process.
Free Flow uses a "shrouded turbine" design that channels water through the turbine's blades. Water passes through a rotor with seven blades that are designed for a slow spin rate to minimize fish strikes. The turbines will be sited 10 or more feet off the riverbed. At this depth, water moves, on average, at one to three meters per second.
A 2007 study by the Electric Power Research Institute in Palo Alto, California, predicted that the U.S. could develop three gigawatts of hydrokinetic power from rivers by 2025. That's the equivalent of roughly two new nuclear power plants. "There is no question the potential for hydrokinetic river power is huge, but this industry is so young, it's very hard to say how economically viable it will be," says Andrea Copping, a senior program manager at Pacific Northwest National Laboratory in Sequim, Washington.
Copping says hydrokinetic power needs a strong commitment from commercial and government interests if it's to take off. "Unless there are public funds to help get this industry off the ground, we are not going to have an industry," she says. "Right now the early developers are being hit with really expensive studies, because the FERC doesn't know what the problems are, so they want the individual companies to look at all potential problems."
Michael Bahleda, an energy consultant with U.K.-based Halcrow Group, says securing the funding needed to carry out the necessary studies may prove difficult. "Until you get through the licensing process, investors aren't going to commit a lot of money," he says. "As it stands now, the permitting and licensing is very time-consuming. It's hard to attract capital until you are further along in that process."
Bahleda also questions whether some of the chosen sites along the Mississippi will prove viable, either because of insufficient water-flow rates or because of regulatory issues related to shipping and the environment.
The biggest environmental challenge will be preventing direct strikes to fish and other organisms. Even if individual turbines cause only a small number of strikes, the sheer size of Free Flow's proposed project raises significant concerns, says U.S. Fish and Wildlife Service biologist Joyce Collins, who is working with Free Flow to study strike issues. Collins says the company will have to pay particular attention to an endangered species that lives in the Mississippi called the pallid sturgeon.
Free Flow Power will also have to convince barge operators that their turbines won't interfere with commercial traffic. "There are times where you can have a low-water period where there is only 10 to 20 feet from the bottom to the top of the water; if you have pylons installed in certain areas, a vessel could run into them," says Mark Wright, vice president of the American Waterways Operators, a trade group representing the tugboat, towboat, and barge industry.
Edward Lovelace, Free Flow Power's vice president of engineering, says all of the sites selected by Free Flow Power will have sufficient clearance above them even during periods of low water. Drawing on historical flow data from the Mississippi, the company selected sites that maintained a depth of at least 40 feet during approximately 100-year lows. Such sites would allow for a minimum of 20 feet of water above the tops of the turbines for barges that draw no more than 14 feet of water.