Wednesday, February 6, 2008

Flexible, Nanowire Solar Cells

Exotic materials and cheaper substrates could lead to better photovoltaics.

Nano solar: A side profile of gallium arsenide nanowires growing on a silicon substrate. The nanowires grow upward from the substrate, creating a surface that's able to absorb more sunlight than a flat surface is.
Credit: McMaster University
See how the technology works.

Researchers at McMaster University, in Ontario, say that they have grown light-absorbing nanowires made of high-performance photovoltaic materials on thin but highly durable carbon-nanotube fabric. They've also harvested similar nanowires from reusable substrates and embedded the tiny particles in flexible polyester film. Both approaches, they argue, could lead to solar cells that are both flexible and cheaper than today's photovoltaics.

Now the researchers' challenge is to improve the efficiency of the cells without increasing cost. The research team, led by Ray LaPierre, a professor in the university's engineering physics department, has been given three years to achieve its goals--backed by about $600,000 from the Ontario government and private-sector research partner Cleanfield Energy, a Toronto-area developer of wind and solar technologies.

LaPierre says that the aim is to produce flexible, affordable solar cells composed of Group III-V nanowires that, within five years, will achieve a conversion efficiency of 20 percent. Longer term, he says, it's theoretically possible to achieve 40 percent efficiency, given the superior ability of such materials to absorb energy from sunlight and the light-trapping nature of nanowire structures. By comparison, current thin-film technologies offer efficiencies of between 6 and 9 percent.

"Most of the nanowire work to date has focused on silicon nanowires," says LaPierre, explaining that McMaster's approach relies on nanowires containing multiple layers of exotic Group III-V materials, such as gallium arsenide, indium gallium phosphide, aluminum gallium arsenide, and gallium arsenide phosphide. "It creates tandem or multi-junction solar cells that can absorb a greater range of the [light] spectrum, compared to what you could achieve with silicon. That's one of the major unique aspects of our work."

When used in conventional crystalline solar cells, Group III-V materials are known to have much higher efficiencies than silicon, but the great cost of these materials has limited their use. LaPierre says that cost becomes less of an issue with nanowires because so little material is needed. This is in part because the structure of the nanowires provides a more efficient way to absorb light and extract electrons freed by the light. In conventional solar cells, which are made of slabs of crystalline material, greater thickness means better light absorption, but it also means that it's more difficult for electrons to escape. This forced trade-off is overcome with nanowires. Each nanowire is 10 to 100 nanometers wide and up to five microns long. Their length maximizes absorption, but their nanoscale width permits a much freer movement and collection of electrons. "The direction in which you absorb the light is essentially perpendicular to how you collect electricity," explains LaPierre. "The dilemma is overcome."

In addition to reducing costs by using less active material, LaPierre's team can also cut the cost of the substrate that the nanowires are grown on. LaPierre's team doesn't require an expensive Group III-V substrate. It has successfully grown its nanowires on substrates made of more plentiful and relatively cheaper silicon. It's also working on using even lower cost substrates made of glass, which would be ideal for building-integrated PV applications. Flexible substrates such as polymer films and carbon nanotube fabric could be useful for many applications, and could be manufactured with inexpensive roll-to-roll processes.

To further drive down costs, the focus on cheaper substrates will be complemented by an attempt to replace the gold catalysts used to grow the nanowires with aluminum, although more work in this area is needed to achieve the necessary nanowire densities. "We have grown nanowires from aluminum, but gold works much better," says LaPierre.

Charles Lieber, a professor of chemistry at Harvard University who has created single light-harvesting nanowires made of silicon, says that his team is also pursuing the use of other materials for making nanowires. "But there are many challenges in going from nanowire to photovoltaic," says Lieber. He adds that comparison of approaches is difficult without data on the energy-conversion properties of each material.

Nathan Lewis, a professor of chemistry at the California Institute of Technology and an expert on nanowire structures, says that it's too early to say which approach and materials are best. "We know nanowires work in bulk form, but we don't know if you can make high-purity, high-quality nanowires and control all their electrical properties," says Lewis. "There's no theory that one works better than the other. It's just a question of getting any of them to work."

It's still early days for McMaster, which in prototypes has only achieved low efficiencies--"where silicon PV was in the 1950s," says LaPierre. But he's optimistic that the higher-efficiency materials and the approach chosen will get results.

Electromagnetic Railgun Blasts Off

A supersonic bullet is fired with a record-breaking 10 megajoules of muzzle energy.

Show of force: A shockwave is produced (above) as a supersonic bullet emerges from the navy's electromagnetic railgun, which was tested late last month.
Credit: U.S. Navy
Watch the railgun fire a seven-pound bullet.
See the flames produced by the bullet.

Last week at the Naval Surface Warfare Center, in Dahlgren, VA, a seven-pound bullet emerged from a truck-sized contraption at seven times the speed of sound and sent a visible shockwave through the air before crashing into a metal bunker filled with sand. With 10.6 megajoules of kinetic energy, this aluminum slug was propelled not by explosives but by an electric field, making this the most powerful electromagnetic railgun ever fired. The device is part of the navy's railgun development program.

While propellant-driven shells have been mainstays of naval warships for the past hundred years, the cost and safety issues related to storing explosive materials have driven engineers to seek alternatives like the electromagnetic railgun. "There are physical limits to what you can do with gunpowder," says Charles Garnett, the manager at Dahlgren, referring to the maximum velocities that explosions can produce. A railgun could eventually send a 40-pound slug 200 miles in six minutes--10 times the range of the navy's primary surface support gun, the MK 45--and it could be used to support Marine troops engaged in land-based operations.

"A lot of people think a railgun is not going to make a lot of noise," Garnett says. "It's electrically fired, and they expect a whoosh and no sound." In reality, when the bullet emerges, it lets out a crack as electricity arcs through the air like lightning.

The railgun gets its name from two highly conductive rails, which form a complete electric circuit once the metal projectile and a sliding armature are put in place. When current starts flowing through the device, it creates a powerful electromagnetic field that accelerates the projectile down the barrel at 40,000 gs, launching it in a matter of milliseconds. Aerodynamic drag along with a million amps of current heats the bullet to 1,000 °C, igniting aluminum particles and leaving a trail of flame in its wake. The researchers estimate the muzzle energy based on the mass and velocity of the bullet in the barrel and from precisely timed x-ray snapshots during flight.

"What's important," says Garnett, "is that this is the first step on the way to building a tactically viable system with 64 megajoules of energy."

The previous experimental railgun record of 9 megajoules had been set 15 years ago by a team at the University of Texas at Austin funded by the U.S. Army. But the Texas railgun was operating at the upper end of its capacity, while Garnett says that the new gun has been designed to handle up to 32 megajoules, and the ultimate goal of the project is to build a 64-megajoule model.

Jon Kitzmiller, an expert in electromechanical systems at the University of Texas, who worked on an earlier railgun project, says that the navy team is "going to have considerable difficulty getting [to 64 megajoules], but it's certainly achievable." He says that the navy's budget of $40 million a year secured through 2011 proves that it is serious about making the gun a reality in the next 15 to 20 years. The previous effort was derailed by funding constraints.

One of the biggest challenges, says Kitzmiller, will be in designing a power supply that can handle multiple shots. "In order to store multiple 64-megajoule shots on a capacitor bank, you would need an aircraft carrier full of capacitor banks," he says. One solution, Kitzmiller and Garnett agree, is a system of rotating pulsed alternators, called compulsators, rather than traditional capacitors.

Other challenges include developing a projectile guidance system that can withstand 40,000 gs--twice the acceleration of current systems--and building a gun barrel that can withstand the force and heat produced by repeated firings. The same force that drives the bullet out of the barrel also tears the rails apart. The Dahlgren prototype looks nothing like a typical gun, and parts will frequently have to be replaced.

"Firing a gun once or twice [makes it] a novelty," says Garnett. "Firing it a thousand times [makes it] a weapon."

A New Perspective on Search Results

Google is experimenting with different ways to serve up search results. But will any of them stick?

Results may vary: Google has announced an experiment in which users can sign up to receive alternate views of their search results. Instead of simply seeing the results in a standard list view, participants can view the results on a map, on a timeline (above), or by using filters that narrow down the results based on information such as dates, measurements, locations, and images.
Credit: Google

Amid the flurry of news over Microsoft's bid for Yahoo and Google's rebuttal, a research announcement by Google went largely unnoticed. Last week, the search giant began a public experiment in which users can make their search results look a little different from the rest of the world's. Those who sign up are able to switch between different views, so instead of simply getting a list of links (and sometimes pictures and YouTube videos, a relatively recent addition to the Google results), they can choose to see their results mapped, put on a timeline, or narrowed down by informational filters. Dan Crow, product manager at Google, says that the results of the experiment could eventually help the company improve everyone's search experience.

Google's experiment highlights the slow but steady push of engineers and designers to improve the Web search experience for the masses. While search algorithms are constantly improving, the interface has remained static for more than a decade: people submit keyword queries, and the engine spits back a list of 10 hyperlinked results. "If you compare Google search-result listings today to the Infoseek results in 1997, they're almost indistinguishable [in terms of presentation], except for the ads," says Marti Hearst, a professor in the School of Information at the University of California, Berkeley.

Hearst says that there continue to be attempts from non-Google engines to offer alternatives to the standard search interface., for instance, lets a user see a thumbnail view of each Web page before she clicks through to the link. And extracts words that are found on the search-results pages, letting a user drill down to a more specific search. For instance, a search for "MIT" can be specified to include references to "laboratory," "Massachusetts Institute of Technology," "project," and other words or word combinations.

But these slight alterations in search have been slow to catch on, as is evident from Google's dominance in the field and its relatively conservative approach to its user interface. Hearst thinks that many people tend to use Google and other simple interfaces for a couple of reasons. One is that search engines must accommodate a wide range of users, from the novice to the savvy. Less experienced users tend to get distracted when more information is presented on the screen, she says: people don't respond well to being overloaded with information, especially when they want a simple answer to a query. But perhaps more important, she says, is the fact that people are familiar with a decade-old interface and, as creatures of habit, they are reluctant to try something new.

Google's Crow says that people are generally happy with the interface as it exists today. "The basic format hasn't changed much because it's been successful ... It works well for most of the users most of the time," he says. "But that doesn't mean we couldn't do something beyond search today."

When a person signs up for the "alternate views" experiment on the Google Labs page, he essentially adds three search filters to his results page: "Info view," "Timeline view," and "Map view." (See below image). By default, a search for "Grateful Dead" serves up results in the "List view," which is essentially the standard results page. If a user selects Map view, he could see a map indicating where the group originated (San Francisco), and where it performed its last show (New York City). Clicking on Timeline view provides a bar graph of dates associated with the group--important concerts, for instance--over the years. And Info view lets the user filter the search by dates, measurements (in this case, Google offers units of years and, oddly, tons), locations, and images.

Crow says that when a person signs up for the experiment, Google collects the same information about his searches that the company would otherwise. This includes noting the search terms and result links that are selected, as well as logging the amount of time a user stays on the page of the selected link. Crow notes that all of the information collected is stripped of any identifying information. This data, in addition to market research collected from participants who visit Google's offices and participants who allow Google to come into their homes to track their search habits, will be used to determine the most helpful features of the experiment, and how best to sprinkle those features into search results without upsetting users, he says.

Improving the search interface isn't easy, but it's a crucial part of the technology, says Oren Etzioni, a professor of computer science and engineering at the University of Washington. There are billions of Web pages, and the results page only reveals 10 pages at a time. "Search is the process of drinking from the fire hose," Etzioni says. "This means that getting the user interface right ... is incredibly important." He doesn't see anything revolutionary about Google's experimental views in particular, but "throwing things out there and letting people react is very smart." He believes that in the next couple of years, search will evolve to provide more interface options for people, and not everyone will be using the same interface.

Search will change, but it will be a gradual process, since there's a fine line between providing helpful information and overwhelming the user with text and links. "One thing to remember is that it's still the early days," Google's Crow says. "People think that search is a solved problem. I think we're still in the early days of making search work on a universal global scale. We know we can do better."