Scientists Develop 3-D Model of the Ionosphere F-Region

Developed by Dr. Joseph Huba and Dr. Glenn Joyce at the NRL Plasma Physics Division, SAMI3 is a fully three-dimensional model of the low- to mid-latitude ionosphere. SAMI3 has been modified recently to use a sun-fixed coordinate system to eliminate rotation of the dawn-dusk line and a high-resolution longitudinal grid to capture the evolution of equatorial plasma bubbles in the pre- to post-sunset sector.

The new modeling capability with SAMI3 has found that ESF can be triggered by pre-sunset ionospheric density perturbations and that an existing ESF plasma bubble can trigger a new bubble.

"Understanding and modeling ESF is important because of its impact on space weather," said Dr. Joseph Huba, head of the Space Plasma Physics Section of the Beam Physics Branch."ESF anomalies can cause radio wave scintillation that degrades communication and navigation systems and serves as the primary focus of the Air Force Communications/ Navigation Outage Forecast System.

Post-sunset ionospheric irregularities in the equatorialF-region were first observed in 1938 by terrestrial magnetism researchers, H.G. Booker and H.W. Wells at the Carnegie Institution of Washington. During that time, analysis of the scattering of radio waves by theF-region of the ionosphere at an equatorial location (Huancayo, Peru) revealed ESF is fundamentally a nighttime event, with greatest frequency of occurrence in the period from four hours before midnight to four hours after midnight.

"The ionosphere builds up after sunrise and reaches a maximum electron density in mid-afternoon, said Huba."Subsequently, the ionosphere can be lifted to higher altitudes just after sunset because of the pre-reversal enhancement of the eastward electric field. During this time the ionosphere can become unstable."

TheF-region of the ionosphere is home to theF-layer, or Appleton layer, and is the densest part of the ionosphere as it extends from about 200 km to more than 500 km above the surface of Earth. Beyond this layer is the topside ionosphere. Here extreme ultraviolet solar radiation ionizes atomic oxygen. TheF-layer consists of one layer at night, but during the day, a deformation often forms creating layers labeledF₁andF₂. TheF-region is the region of the ionosphere that is very important for high-frequency (HF) radio wave propagation facilitating HF radio communications over long distances.

The upgraded version of SAMI3 represents a unique resource to investigate the physics of equatorial spreadF, particularly the processes that control the day-to-day variability of ESFs. Future improvements to the current model include: modification to the geomagnetic field to have a tilt allowing the inclusion of longitudinal effects; coupling SAMI3 with a physics-based model of the thermosphere; and replacement of the full donor cell algorithm, currently being used for crossfield transport, with a high-order flux transport algorithm allowing for the capture of complex bubble evolution involving bifurcation.

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Better Turbine Spacing for Large Wind Farms

To help steer wind farm owners in the right direction, Charles Meneveau, a Johns Hopkins fluid mechanics and turbulence expert, working with a colleague in Belgium, has devised a new formula through which the optimal spacing for a large array of turbines can be obtained.

"I believe our results are quite robust," said Meneveau, who is the Louis Sardella Professor of Mechanical Engineering in the university's Whiting School of Engineering."They indicate that large wind farm operators are going to have to space their turbines farther apart."

The newest wind farms, which can be located on land or offshore, typically use turbines with rotor diameters of about 300 feet. Currently, turbines on these large wind farms are spaced about seven rotor diameters apart. The new spacing model developed by Meneveau and Johan Meyers, an assistant professor at Katholieke Universiteit Leuven in Belgium, suggests that placing the wind turbines 15 rotor diameters apart -- more than twice as far apart as in the current layouts -- results in more cost-efficient power generation.

Meneveau presented the study results recently at a meeting of the American Physical Society Division of Fluid Dynamics. Meyers, co-author of the study, was unable to attend.

The research is important because large wind farms -- consisting of hundreds or even thousands of turbines -- are planned or already operating in the western United States, Europe and China."The early experience is that they are producing less power than expected," Meneveau said."Some of these projects are underperforming."

Earlier computational models for large wind farm layouts were based on simply adding up what happens in the wakes of single wind turbines, Meneveau said. The new spacing model, he said, takes into account interaction of arrays of turbines with the entire atmospheric wind flow.

Meneveau and Meyers argue that the energy generated in a large wind farm has less to do with horizontal winds and is more dependent on the strong winds that the turbulence created by the tall turbines pulls down from higher up in the atmosphere. Using insights gleaned from high-performance computer simulations as well as from wind tunnel experiments, they determined that in the correct spacing, the turbines alter the landscape in a way that creates turbulence, which stirs the air and helps draw more powerful kinetic energy from higher altitudes.

The experiments were conducted in the Johns Hopkins wind tunnel, which uses a large fan to generate a stream of air. Before it enters the testing area, the air passes through an"active grid," a curtain of perforated plates that rotate randomly and create turbulence so that the air moving through the tunnel more closely resembles real-life wind conditions.

Air currents in the tunnel pass through a series of small three-bladed model wind turbines mounted atop posts, mimicking an array of full-size wind turbines. Data concerning the interaction of the air currents and the model turbines is collected by using a measurement procedure called stereo particle-image-velocimetry, which requires a pair of high-resolution digital cameras, smoke and laser pulses.

Further research is needed, Meneveau said, to learn how varying temperatures can affect the generation of power on large wind farms. The Johns Hopkins professor has applied for continued funding to conduct such studies.

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New Device May Revolutionize Computer Memory

Traditionally, there are two types of computer memory devices. Slow memory devices are used in persistent data storage technologies such as flash drives. They allow us to save information for extended periods of time, and are therefore called nonvolatile devices. Fast memory devices allow our computers to operate quickly, but aren't able to save data when the computers are turned off. The necessity for a constant source of power makes them volatile devices.

But now a research team from NC State has developed a single"unified" device that can perform both volatile and nonvolatile memory operation and may be used in the main memory.

"We've invented a new device that may revolutionize computer memory," says Dr. Paul Franzon, a professor of electrical and computer engineering at NC State and co-author of a paper describing the research."Our device is called a double floating-gate field effect transistor (FET). Existing nonvolatile memory used in data storage devices utilizes a single floating gate, which stores charge in the floating gate to signify a 1 or 0 in the device -- or one 'bit' of information. By using two floating gates, the device can store a bit in a nonvolatile mode, and/or it can store a bit in a fast, volatile mode -- like the normal main memory on your computer."

The double floating-gate FET could have a significant impact on a number of computer problems. For example, it would allow computers to start immediately, because the computer wouldn't have to retrieve start-up data from its hard drive -- the data could be stored in its main memory.

The new device would also allow"power proportional computing." For example, Web server farms, such as those used by Google, consume an enormous amount of power -- even when there are low levels of user activity -- in part because the server farms can't turn off the power without affecting their main memory.

"The double floating-gate FET would help solve this problem," Franzon says,"because data could be stored quickly in nonvolatile memory -- and retrieved just as quickly. This would allow portions of the server memory to be turned off during periods of low use without affecting performance."

Franzon also notes that the research team has investigated questions about this technology's reliability, and that they think the device"can have a very long lifetime, when it comes to storing data in the volatile mode."

The paper,"Computing with Novel Floating-Gate Devices," will be published Feb. 10 in IEEE'sComputer. The paper was authored by Franzon; former NC State Ph.D. student Daniel Schinke; former NC State master's student Mihir Shiveshwarkar; and Dr. Neil Di Spigna, a research assistant professor at NC State. The research was funded by the National Science Foundation.

NC State's Department of Electrical and Computer Engineering is part of the university's College of Engineering.

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New Reactor Paves the Way for Efficiently Producing Fuel from Sunlight

Solar energy has long been touted as the solution to our energy woes, but while it is plentiful and free, it can't be bottled up and transported from sunny locations to the drearier -- but more energy-hungry -- parts of the world. The process developed by Haile -- a professor of materials science and chemical engineering at the California Institute of Technology (Caltech) -- and her colleagues could make that possible.

The researchers designed and built a two-foot-tall prototype reactor that has a quartz window and a cavity that absorbs concentrated sunlight. The concentrator works"like the magnifying glass you used as a kid" to focus the sun's rays, says Haile.

At the heart of the reactor is a cylindrical lining of ceria. Ceria -- a metal oxide that is commonly embedded in the walls of self-cleaning ovens, where it catalyzes reactions that decompose food and other stuck-on gunk -- propels the solar-driven reactions. The reactor takes advantage of ceria's ability to"exhale" oxygen from its crystalline framework at very high temperatures and then"inhale" oxygen back in at lower temperatures.

"What is special about the material is that it doesn't release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves," Haile explains."When we cool it back down, the material's thermodynamically preferred state is to pull oxygen back into the structure."

Specifically, the inhaled oxygen is stripped off of carbon dioxide (CO₂) and/or water (H₂O) gas molecules that are pumped into the reactor, producing carbon monoxide (CO) and/or hydrogen gas (H₂). H₂can be used to fuel hydrogen fuel cells; CO, combined with H₂, can be used to create synthetic gas, or"syngas," which is the precursor to liquid hydrocarbon fuels. Adding other catalysts to the gas mixture, meanwhile, produces methane. And once the ceria is oxygenated to full capacity, it can be heated back up again, and the cycle can begin anew.

For all of this to work, the temperatures in the reactor have to be very high -- nearly 3,000 degrees Fahrenheit. At Caltech, Haile and her students achieved such temperatures using electrical furnaces. But for a real-world test, she says,"we needed to use photons, so we went to Switzerland." At the Paul Scherrer Institute's High-Flux Solar Simulator, the researchers and their collaborators -- led by Aldo Steinfeld of the institute's Solar Technology Laboratory -- installed the reactor on a large solar simulator capable of delivering the heat of 1,500 suns.

In experiments conducted last spring, Haile and her colleagues achieved the best rates for CO₂dissociation ever achieved,"by orders of magnitude," she says. The efficiency of the reactor was uncommonly high for CO₂splitting, in part, she says,"because we're using the whole solar spectrum, and not just particular wavelengths." And unlike in electrolysis, the rate is not limited by the low solubility of CO₂in water. Furthermore, Haile says, the high operating temperatures of the reactor mean that fast catalysis is possible, without the need for expensive and rare metal catalysts (cerium, in fact, is the most common of the rare earth metals -- about as abundant as copper).

In the short term, Haile and her colleagues plan to tinker with the ceria formulation so that the reaction temperature can be lowered, and to re-engineer the reactor, to improve its efficiency. Currently, the system harnesses less than 1% of the solar energy it receives, with most of the energy lost as heat through the reactor's walls or by re-radiation through the quartz window."When we designed the reactor, we didn't do much to control these losses," says Haile. Thermodynamic modeling by lead author and former Caltech graduate student William Chueh suggests that efficiencies of 15% or higher are possible.

Ultimately, Haile says, the process could be adopted in large-scale energy plants, allowing solar-derived power to be reliably available during the day and night. The CO₂emitted by vehicles could be collected and converted to fuel,"but that is difficult," she says. A more realistic scenario might be to take the CO₂emissions from coal-powered electric plants and convert them to transportation fuels."You'd effectively be using the carbon twice," Haile explains. Alternatively, she says, the reactor could be used in a"zero CO₂emissions" cycle: H₂O and CO₂would be converted to methane, would fuel electricity-producing power plants that generate more CO₂and H₂O, to keep the process going.

The work was funded by the National Science Foundation, the State of Minnesota Initiative for Renewable Energy and the Environment, and the Swiss National Science Foundation.

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Breakthrough in Converting Heat Waste to Electricity: Automotive, Chemical, Brick and Glass Industries Could Benefit from Discovery

The material exhibits a high thermoelectric figure of merit that is expected to enable 14 percent of heat waste to electricity, a scientific first. Chemists, physicists and material scientists at Northwestern collaborated to develop the material. The results of the study are published by the journalNature Chemistry.

"It has been known for 100 years that semiconductors have this property that can harness electricity," said Mercouri Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in The Weinberg College of Arts and Sciences."To make this an efficient process, all you need is the right material, and we have found a recipe or system to make this material."

Kanatzidis, co-author of the study, and his team dispersed nanocrystals of rock salt (SrTe) into the material lead telluride (PbTe). Past attempts at this kind of nanoscale inclusion in bulk material have improved the energy conversion efficiency of lead telluride, but the nano inclusions also increased the scattering of electrons, which reduced overall conductivity. In this study, the Northwestern team offers the first example of using nanostructures in lead telluride to reduce electron scattering and increase the energy conversion efficiency of the material.

"We can put this material inside of an inexpensive device with a few electrical wires and attach it to something like a light bulb," said Vinayak Dravid, professor of materials science and engineering at Northwestern's McCormick School of Engineering and Applied Science and co-author of the paper."The device can make the light bulb more efficient by taking the heat it generates and converting part of the heat, 10 to 15 percent, into a more useful energy like electricity."

The automotive, chemical, brick, glass and any industry that uses heat to make products could make their system more efficient with the use of this scientific breakthrough, said Kanatzidis, who also has a joint appointment at the Argonne National Laboratory.

"The energy crisis and the environment are two major reasons to be excited about this discovery, but this could just be the beginning," Dravid said."These types of structures may have other implications in the scientific community that we haven't thought of yet, in areas such as mechanical behavior and improving strength or toughness. Hopefully others will pick up this system and use it."

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Coiled Nanowires May Hold Key to Stretchable Electronics

"In order to create stretchable electronics, you need to put electronics on a stretchable substrate, but electronic materials themselves tend to be rigid and fragile," says Dr. Yong Zhu, one of the researchers who created the new nanowire coils and an assistant professor of mechanical and aerospace engineering at NC State."Our idea was to create electronic materials that can be tailored into coils to improve their stretchability without harming the electric functionality of the materials."

Other researchers have experimented with"buckling" electronic materials into wavy shapes, which can stretch much like the bellows of an accordion. However, Zhu says, the maximum strains for wavy structures occur at localized positions -- the peaks and valleys -- on the waves. As soon as the failure strain is reached at one of the localized positions, the entire structure fails.

"An ideal shape to accommodate large deformation would lead to a uniform strain distribution along the entire length of the structure -- a coil spring is one such ideal shape," Zhu says."As a result, the wavy materials cannot come close to the coils' degree of stretchability." Zhu notes that the coil shape is energetically favorable only for one-dimensional structures, such as wires.

Zhu's team put a rubber substrate under strain and used very specific levels of ultraviolet radiation and ozone to change its mechanical properties, and then placed silicon nanowires on top of the substrate. The nanowires formed coils upon release of the strain. Other researchers have been able to create coils using freestanding nanowires, but have so far been unable to directly integrate those coils on a stretchable substrate.

While the new coils' mechanical properties allow them to be stretched an additional 104 percent beyond their original length, their electric performance cannot hold reliably to such a large range, possibly due to factors like contact resistance change or electrode failure, Zhu says."We are working to improve the reliability of the electrical performance when the coils are stretched to the limit of their mechanical stretchability, which is likely well beyond 100 percent, according to our analysis."

A paper describing the research was published online Dec. 28 byACS Nano. The paper is co-authored by Zhu, NC State Ph.D. student Feng Xu and Wei Lu, an assistant professor at the University of Michigan. The research was funded by the National Science Foundation.

NC State's Department of Mechanical and Aerospace Engineering is part of the university's College of Engineering.

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Researchers Helping Electric-Wheelchair Users Move More Easily

To address this problem, researchers at the Florida A&M University-Florida State University College of Engineering are working on technology that will enable electric-powered wheelchairs to detect hazardous terrain and automatically adjust their control settings to maneuver more safely.

Emmanuel Collins is the John H. Seely Professor of Mechanical Engineering at the college and director of Florida State's Center for Intelligent Systems, Control and Robotics (http://www.eng.fsu.edu/ciscor/) (CISCOR). He said that a device known as a laser line striper, originally developed for military use, has been adapted to classify terrain conditions so the wheelchair control system can self-adjust.

"I'm inspired by the idea of applying technology originally meant for the battlefield to improve the quality of everyday life for injured soldiers and others," Collins said.

Engineers had previously developed automatic terrain-sensing controls for military robotic vehicles, and several four-wheel-drive automobiles now on the market include such controls for improved safety. So, Collins wondered, why not integrate this type of system into electric-powered wheelchairs to provide more mobility and independence for their operators?

Collins' team, working with colleagues from the University of Pittsburgh, began experiments this year to add instrumentation based on current driving control systems. The new technology is designed to enable an electric-powered wheelchair to detect hazardous terrain and implement safe driving strategies while avoiding wheel slip, sinkage or vehicle tipping.

Collins said that, to his knowledge, no one else is working on this type of application.

The U.S. Army Medical Research and Materiel Command's Telemedicine and Advanced Technology Research Center saw the promise in this collaboration and has provided funding and guidance for the researchers to pursue their ideas together. The partnership joins CISCOR, which has worked extensively with control and guidance of autonomous vehicles, with the University of Pittsburgh's Human Engineering Research Laboratories. The latter group has developed several assistive technologies already in use by wheelchair manufacturers and rehabilitation hospitals nationwide.

The partnership began when Collins heard a presentation by Professor Rory Cooper, director of the Human Engineering Research Laboratories and chairman of Pitt's rehabilitation science and technology department. Cooper has used a wheelchair since receiving a spinal cord injury in 1980 during his service in the Army. He won a bronze medal in the 1988 Paralympic Games in Seoul and has been recognized nationally for his research and leadership efforts to aid veterans and others with spinal cord injuries.

In his presentation, Cooper mentioned the need for terrain-dependent, electric-powered wheelchair assistance. Collins approached him about working together, and the two of them began developing ideas with other collaborators at the National Science Foundation-sponsored Quality of Life Technology Center, an engineering research center affiliated with the Human Engineering Research Laboratories that Cooper co-directs.

Cooper also is the founding director and a senior research scientist of the VA Rehabilitation Research and Development Center of Excellence in Pittsburgh. His laboratory has been collaborating with the Veterans Administration for 15 years, and with the military since 2004, to develop robotic and other advanced assistive technologies. Cooper noted that the lab has a very good relationship with the orthopedic and rehabilitation departments of Walter Reed Army Medical Center and the National Naval Medical Center.

Army Maj. Kevin Fitzpatrick, director of Walter Reed's wheelchair clinic, said,"This technology will provide electric-powered wheelchair users with an increased degree of independence that may significantly increase their ability to participate in recreational and functional activities."

The project is part of the Rehabilitation Engineering and Assistive Technology sub-portfolio, recently managed by Craig Carignan, within the Telemedicine and Advanced Technology Research Center's Advanced Prosthetics and Human Performance research portfolio.

"The Human Engineering Research Laboratories and the Pittsburgh VA center are considered among the top wheelchair testers in the United States, and are playing critical roles in developing international wheelchair standards," Carignan said."The researchers on this project are excellent investigators, and we are looking forward to the solution they develop."

Collins estimated that if the team develops a strong commercial partner, the technology could be assisting electric wheelchair users in approximately five years.

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'Nanoscoops' Could Spark New Generation of Electric Automobile Batteries

The new material, dubbed a"nanoscoop" because its shape resembles a cone with a scoop of ice cream on top, can withstand extremely high rates of charge and discharge that would cause conventional electrodes used in today's Li-ion batteries to rapidly deteriorate and fail. The nanoscoop's success lies in its unique material composition, structure, and size.

The Rensselaer research team, led by Professor Nikhil Koratkar, demonstrated how a nanoscoop electrode could be charged and discharged at a rate 40 to 60 times faster than conventional battery anodes, while maintaining a comparable energy density. This stellar performance, which was achieved over 100 continuous charge/discharge cycles, has the team confident that their new technology holds significant potential for the design and realization of high-power, high-capacity Li-ion rechargeable batteries.

"Charging my laptop or cell phone in a few minutes, rather than an hour, sounds pretty good to me," said Koratkar, a professor in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer."By using our nanoscoops as the anode architecture for Li-ion rechargeable batteries, this is a very real prospect. Moreover, this technology could potentially be ramped up to suit the demanding needs of batteries for electric automobiles."

Batteries for all-electric vehicles must deliver high power densities in addition to high energy densities, Koatkar said. These vehicles today use supercapacitors to perform power-intensive functions, such as starting the vehicle and rapid acceleration, in conjunction with conventional batteries that deliver high energy density for normal cruise driving and other operations. Koratkar said the invention of nanoscoops may enable these two separate systems to be combined into a single, more efficient battery unit.

Results of the study were detailed in the paper"Functionally Strain-Graded Nanoscoops for High Power Li-Ion Battery Anodes," published Thursday by the journalNano Letters.

The anode structure of a Li-ion battery physically grows and shrinks as the battery charges or discharges. When charging, the addition of Li ions increases the volume of the anode, while discharging has the opposite effect. These volume changes result in a buildup of stress in the anode. Too great a stress that builds up too quickly, as in the case of a battery charging or discharging at high speeds, can cause the battery to fail prematurely. This is why most batteries in today's portable electronic devices like cell phones and laptops charge very slowly -- the slow charge rate is intentional and designed to protect the battery from stress-induced damage.

The Rensselaer team's nanoscoop, however, was engineered to withstand this buildup of stress. Made from a carbon (C) nanorod base topped with a thin layer of nanoscale aluminum (Al) and a"scoop" of nanoscale silicon (Si), the structures are flexible and able to quickly accept and discharge Li ions at extremely fast rates without sustaining significant damage. The segmented structure of the nanoscoop allows the strain to be gradually transferred from the C base to the Al layer, and finally to the Si scoop. This natural strain gradation provides for a less abrupt transition in stress across the material interfaces, leading to improved structural integrity of the electrode.

The nanoscale size of the scoop is also vital since nanostructures are less prone to cracking than bulk materials, according to Koratkar.

"Due to their nanoscale size, our nanoscoops can soak and release Li at high rates far more effectively than the macroscale anodes used in today's Li-ion batteries," he said."This means our nanoscoop may be the solution to a critical problem facing auto companies and other battery manufacturers -- how can you increase the power density of a battery while still keeping the energy density high?"

A limitation of the nanoscoop architecture is the relatively low total mass of the electrode, Koratkar said. To solve this, the team's next steps are to try growing longer scoops with greater mass, or develop a method for stacking layers of nanoscoops on top of each other. Another possibility the team is exploring includes growing the nanoscoops on large flexible substrates that can be rolled or shaped to fit along the contours or chassis of the automobile.

Along with Koratkar, authors on the paper are Toh-Ming Lu, the R.P. Baker Distinguished Professor of Physics and associate director of the Center for Integrated Electronics at Rensselaer; and Rahul Krishnan, a graduate student in the Department of Materials Science and Engineering at Rensselaer.

This study was supported by the National Science Foundation (NSF) and the New York State Energy Research and Development Authority (NYSERDA).

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Is the Hornet Our Key to Renewable Energy? Physicist Discovers That Hornet's Outer Shell Can Harvest Solar Power

"The interesting thing here is that a living biological creature does a thing like that," says physicist Prof. David Bergman of Tel Aviv University's School of Physics and Astronomy, who was part of the team that made discovery."The hornet may have discovered things we do not yet know." In partnership with the late Prof. Jacob Ishay of the university's Sackler Faculty of Medicine, Prof. Bergman and his doctoral candidate Marian Plotkin engaged in a truly interdisciplinary research project to explain the biological processes that turn a hornet's abdomen into solar cells.

The research team made the discovery several years ago, and recently tried to mimic it. The results show that the hornet's body shell, or exoskeleton, is able to harvest solar energy. They were recently published in the German journalNaturwissenschaften.

Discovering a new system for renewable energy?

Previously, entomologists noted that Oriental wasps, unlike other wasps and bees, are active in the afternoon rather than the morning when the sun is just rising. They also noticed that the hornet digs more intensely as the sun's intensity increases.

Taking this information to the lab, the Tel Aviv University team studied weather conditions like temperature, humidity and solar radiation to determine if and how these factors also affected the hornet's behavior, but found that UVB radiation alone dictated the change.

In the course of their research, the Tel Aviv University team also found that the yellow and brown stripes on the hornet abdomen enable a photo-voltaic effect: the brown and yellow stripes on the hornet abdomen can absorb solar radiation, and the yellow pigment transforms that into electric power.

The team determined that the brown shell of the hornet was made from grooves that split light into diverging beams. The yellow stripe on the abdomen is made from pinhole depressions, and contains a pigment called xanthopterin. Together, the light diverging grooves, pinhole depressions and xanthopterin change light into electrical energy. The shell traps the light and the pigment does the conversion.

A biological heat pump

The researchers also found a number of energy processes unique to the insect. Like air conditioners and refrigerators, the hornet has a well-developed heat pump system in its body which keeps it cooler than the outside temperature while it forages in the sun. This is something that's not easy to do, says Prof. Bergman.

To see if the solar collecting prowess of the hornet could be duplicated, the team imitated the structure of the hornet's body but had poor results in achieving the same high efficiency rates of energy collection. In the future, they plan to refine the model to see if this"bio-mimicry" can give clues to novel renewable energy solutions.

The research team also discovered that hornets use finely honed acoustic signals to guide them so they can build their combs with extraordinary precision in total darkness. Bees can at least see what they are doing, explains Prof. Bergman, but hornets cannot -- it's totally dark inside a hornet nest.

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New Solar Cell Self-Repairs Like Natural Plant Systems

"We've created artificial photosystems using optical nanomaterials to harvest solar energy that is converted to electrical power,"said Jong Hyun Choi, an assistant professor of mechanical engineering at Purdue University.

The design exploits the unusual electrical properties of structures called single-wall carbon nanotubes, using them as"molecular wires in light harvesting cells," said Choi, whose research group is based at the Birck Nanotechnology and Bindley Bioscience centers at Purdue's Discovery Park.

"I think our approach offers promise for industrialization, but we're still in the basic research stage," he said.

Photoelectrochemical cells convert sunlight into electricity and use an electrolyte -- a liquid that conducts electricity -- to transport electrons and create the current. The cells contain light-absorbing dyes called chromophores, chlorophyll-like molecules that degrade due to exposure to sunlight.

"The critical disadvantage of conventional photoelectrochemical cells is this degradation," Choi said.

The new technology overcomes this problem just as nature does: by continuously replacing the photo-damaged dyes with new ones.

"This sort of self-regeneration is done in plants every hour," Choi said.

The new concept could make possible an innovative type of photoelectrochemical cell that continues operating at full capacity indefinitely, as long as new chromophores are added.

Findings were detailed in a November presentation during the International Mechanical Engineering Congress and Exhibition in Vancouver. The concept also was unveiled in an online article (http://spie.org/x41475.xml?ArticleID=x41475) featured on the Web site for SPIE, an international society for optics and photonics.

The talk and article were written by Choi, doctoral students Benjamin A. Baker and Tae-Gon Cha, and undergraduate students M. Dane Sauffer and Yujun Wu.

The carbon nanotubes work as a platform to anchor strands of DNA. The DNA is engineered to have specific sequences of building blocks called nucleotides, enabling them to recognize and attach to the chromophores.

"The DNA recognizes the dye molecules, and then the system spontaneously self-assembles," Choi said

When the chromophores are ready to be replaced, they might be removed by using chemical processes or by adding new DNA strands with different nucleotide sequences, kicking off the damaged dye molecules. New chromophores would then be added.

Two elements are critical for the technology to mimic nature's self-repair mechanism: molecular recognition and thermodynamic metastability, or the ability of the system to continuously be dissolved and reassembled.

The research is an extension of work that Choi collaborated on with researchers at the Massachusetts Institute of Technology and the University of Illinois. The earlier work used biological chromophores taken from bacteria, and findings were detailed in a research paper published in November in the journalNature Chemistry(http://www.nature.com/nchem/journal/v2/n11/abs/nchem.822.html).

However, using natural chromophores is difficult, and they must be harvested and isolated from bacteria, a process that would be expensive to reproduce on an industrial scale, Choi said.

"So instead of using biological chromophores, we want to use synthetic ones made of dyes called porphyrins," he said.

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