'Nanocoax' Solves Solar Cell 'Thick and Thin' Dilemma

A nano-scale solar cell inspired by the coaxial cable offers greater efficiency than any previously designed nanotech thin film solar cell by resolving the "thick & thin" challenge inherent to capturing light and extracting current for solar power, Boston College researchers report in the current online edition of the journal Physica Status Solidi.

The quest for high power conversion efficiency in most thin film solar cells has been hampered by competing optical and electronic constraints. A cell must be thick enough to collect a sufficient amount of light, yet it needs to be thin enough to extract current.

Physicists at Boston College found a way to resolve the "thick & thin" challenge through a nanoscale solar architecture based on the coaxial cable, a radio technology concept that dates back to the first trans-Atlantic communications lines laid in the mid 1800s.

"Many groups around the world are working on nanowire-type solar cells, most using crystalline semiconductors," said co-author Michael Naughton, a professor of physics at Boston College. "This nanocoax cell architecture, on the other hand, does not require crystalline materials, and therefore offers promise for lower-cost solar power with ultrathin absorbers. With continued optimization, efficiencies beyond anything achieved in conventional planar architectures may be possible, while using smaller quantities of less costly material."

Optically, the so-called nanocoax stands thick enough to capture light, yet its architecture makes it thin enough to allow a more efficient extraction of current, the researchers report in PSS's Rapid Research Letters. This makes the nanocoax, invented at Boston College in 2005 and patented last year, a new platform for low cost, high efficiency solar power.

Constructed with amorphous silicon, the nanocoax cells yielded power conversion efficiency in excess of 8 percent, which is higher than any nanostructured thin film solar cell to date, the team reported.

The ultra-thin nature of the cells reduces the Staebler-Wronski light-induced degradation effect, a major problem with conventional solar cells of this type, according to the team, which included Boston College Professors of Physics Krzysztof Kempa and Zhifeng Ren, as well as BC students and collaborators from Solasta Inc., of Newton, Mass., and École Polytechnique Fédérale de Lausanne, Institute of Microengineering in Switzerland. The research was funded in part by a Technology Incubator grant from the Department of Energy.

Email or share this story:

Genetic Secrets That Allow Tibetans to Thrive in Thin Air Discovered

Tibetans carry a special version of a gene called EPAS1 that enables them to live at high-altitude without getting sick. (Credit: Wikimedia Commons, creative commons license.)

A new study pinpoints the genetic changes that enable Tibetans to thrive at altitudes where others get sick.

In the online edition of Proceedings of the National Academy of Sciences, an international team has identified a gene that allows Tibetans to live and work more than two miles above sea level without getting altitude sickness.

A previous study published May 13 in Science reported that Tibetans are genetically adapted to high altitude. Now, less than a month later, a second study by scientists from China, England, Ireland, and the United States pinpoints a particular site within the human genome -- a genetic variant linked to low hemoglobin in the blood -- that helps explain how Tibetans cope with low-oxygen conditions.

The study sheds light on how Tibetans, who have lived at extreme elevation for more than 10,000 years, have evolved to differ from their low-altitude ancestors.

Lower air pressure at altitude means fewer oxygen molecules for every lungful of air. "Altitude affects your thinking, your breathing, and your ability to sleep. But high-altitude natives don't have these problems," said co-author Cynthia Beall of Case Western Reserve University. "They're able to live a healthy life, and they do it completely comfortably," she said.

People who live or travel at high altitude respond to the lack of oxygen by making more hemoglobin, the oxygen-carrying component of human blood. "That's why athletes like to train at altitude. They increase their oxygen-carrying capacity," said Beall.

But too much hemoglobin can be a bad thing. Excessive hemoglobin is the hallmark of chronic mountain sickness, an overreaction to altitude characterized by thick and viscous blood. Tibetans maintain relatively low hemoglobin at high altitude, a trait that makes them less susceptible to the disease than other populations.

"Tibetans can live as high as 13,000 feet without the elevated hemoglobin concentrations we see in other people," said Beall.

To pinpoint the genetic variants underlying Tibetans' relatively low hemoglobin levels, the researchers collected blood samples from nearly 200 Tibetan villagers living in three regions high in the Himalayas. When they compared the Tibetans' DNA with their lowland counterparts in China, their results pointed to the same culprit -- a gene on chromosome 2, called EPAS1, involved in red blood cell production and hemoglobin concentration in the blood.

Originally working separately, the authors of the study first put their findings together at a March 2009 meeting at the National Evolutionary Synthesis Center in Durham, NC. "Some of us had been working on the whole of Tibetan DNA. Others were looking at small groups of genes. When we shared our findings we suddenly realized that both sets of studies pointed to the same gene -- EPAS1," said Robbins, who co-organized the meeting with Beall.

While all humans have the EPAS1 gene, Tibetans carry a special version of the gene. Over evolutionary time individuals who inherited this variant were better able to survive and passed it on to their children, until eventually it became more common in the population as a whole.

"This is the first human gene locus for which there is hard evidence for genetic selection in Tibetans," said co-author Peter Robbins of Oxford University.

Researchers are still trying to understand how Tibetans get enough oxygen to their tissues despite low levels of oxygen in the air and bloodstream. Until then, the genetic clues uncovered so far are unlikely to be the end of the story. "There are probably many more signals to be characterized and described," said co-author Gianpiero Cavalleri of the Royal College of Surgeons in Ireland.

For those who live closer to sea level, the findings may one day help predict who is at greatest risk for altitude sickness. "Once we find these versions, tests can be developed to tell if an individual is sensitive to low-oxygen," said co-author Changqing Zeng of the Beijing Institute of Genomics.

"Many patients, young and old, are affected by low oxygen levels in their blood -- perhaps from lung disease, or heart problems. Some cope much better than others," said co-author Hugh Montgomery, of University College London. "Studies like this are the start in helping us to understand why, and to develop new treatments."

Email or share this story:

Florida Ridges' Mystery Marine Fossils Tied to Rising Land, Not Seas, Geologist Says

Sea level has not been as high as the distinctive ridges that run down the length of Florida for millions of years. Yet recently deposited marine fossils abound in the ridges' sands.

Now, a University of Florida geologist may have helped crack that mystery.

In a paper appearing June 1 in the June edition of the journal Geology,Peter Adams, a UF assistant professor of geological sciences, says his computer models of Florida's changing land mass support this theory: The land that forms the sandy Trail Ridge running north to south from North Florida through South Georgia, as well as lesser-known ridges, was undersea at the time the fossils were deposited -- but rose over time, reaching elevations that exceeded later sea level high stands.

"If you look at the best records, there's no evidence that global sea level has come close to occupying the elevation of these fossils since the time of their emplacement," Adams said, referring to Trail Ridge's elevation today, nearly 230 feet above modern sea level. "The only thing that explains this conundrum is that Trail Ridge was underwater, but later rose to an elevation higher than subsequent sea levels."

At the heart of the phenomenon are Florida's unique weather patterns and geology, Adams said.

The state's abundant rain contains a small amount of carbon dioxide, which forms carbonic acid in lake and river water. This slightly acidic water slowly eats away at Florida's limestone bedrock, forming the karst topography for which Florida is so well known, replete with pockmarks, underground springs and subterranean caverns. The surface water washes the dissolved limestone out to sea, over time significantly lightening the portion of the Earth's crust that covers Florida.

A mass of slow-moving mantle rock resides 6 to 18 miles below the crust. As the Florida land mass lightens, this mantle pushes upward to equilibrate the load, forcing Florida skyward, Adams said. The process is known as isostatic rebound, or isostatic uplift.

"It's just like what happens when you get out of bed in the morning. The mattress springs raise the surface of the bed back up," Adams said, adding that the uplift is similar to what takes place when glaciers retreat, with Maine and Norway, for example, also gaining elevation.

Glaciers melt off the land surface to drive isostatic uplift. But in Florida, varying rainfall rates during different periods have slowed or quickened the karstification just below the land. This has in turn slowed or quickened the mantle's push up from below. Additionally, sea level high stands do not always return to the same elevation, which creates a complex history of which beach ridges are preserved and which aren't, Adams said.

For instance, during periods when sea level rose quickly, some pre-existing ridges were overtaken and wiped out. During other periods, however, when sea level rose slowly or did not reach a certain ridge's elevation, a beach ridge was preserved. In effect, Trail Ridge, Lake Wales Ridge and other lesser-known ridges are the remains of isostatically uplifted land that was kept out of harm's way, Adams said. The ridges carry with them the marine fossils that are the evidence of their lowly early beginnings.

Today, the land surface of Florida is rising at a rate of about one-twentieth of a millimeter annually, far more slowly than sea level rise estimated at approximately 3 millimeters annually. Adams noted that Florida's rise is not nearly rapid enough to counteract sea level rise -- and that society should be mindful that low-lying coastal areas are threatened.

Neil Opdyke, a UF professor emeritus and a co-author of the recent paper, first proposed the uplift process in a 1984 paper. Adams tested it using computer models that matched known information about sea levels dating back 1.6 million years with historic rainfall patterns, karstification rates and mantle uplift. The models concluded that Trail Ridge is approximately 1.4 million years old -- and has been preserved because of uplift and the fact that sea levels have not reached the ridge's elevation since its formation. In addition, Florida's one-twentieth of a millimeter rise is twice as fast as previously thought.

"The neat thing about this paper is, it combines many different systems that people work on. There are people who work on uplift, people who work on erosion of karst, people who work on precipitation and paleoclimate," Adams said. "And I knew just enough about all these things to be dangerous. So I said. 'Let's take what we know from the literature and put it together in a simple mathematical model to see how the whole system responds.'"

Email or share this story:

enlarge

Topographic Map of Florida. Including state borders, rivers and longitude/latitude lines. Relief texture and satellite images courtesy of NASA (http://earthobservatory.nasa.gov). Further data source courtesy of CIA World Data Bank II database. (Credit: iStockphoto/Frank Ramspott)

AMD to Phase Out Several Phenom II X4 CPUs, Readies Six-Core Chip

According to AMD's updated CPU release schedule, the chip maker plans to put several Phenom II X4 900 series chips on the chopping block. The company has already stopped taking orders for the Phenom II X4 910 (2.6Ghz) and 945 (3.0GHz) and will stop shipments in the second quarter of 2010.

Starting in the first quarter of 2010, AMD will no longer take orders for its Phenom II X4 965 (3.4GHz) and 925 (2.8Ghz) processors, while orders for the Phenom II X4 955 (3.2GHz) are scheduled to end in the second quarter.

In addition to the above named parts, AMD will start phasing out its Phenom II X4 800, X3 700, and X2 500 series, and Athlon II X4 600 and X3 400 series sometime next year.

To replenish its CPU lineup, AMD plans to launch a 95W Phenom II X4 955 processor in Q2 2010, as well as its six-core desktop chip codenamed Thuban. The 2.8Ghz Thuban part will be built around a 45nm manufacturing process and come with 512KB of L2 cache and 6MB of L3 cache.

Image Credit: AMD