Capturing the Sun

With a prestigious Truman Fellowship at the Sandia National Laboratories, a Caltech graduate student continues his quest to create solar fuel.

William Chueh has traveled thousands of miles throughout the United States to pursue his passion of nature photography, often hiking deep into remote canyons to snap the perfect picture. But when it came time to choose a graduate school, he decided to stay put at his undergraduate alma mater, Caltech, summoning his love of nature and concern for the environment as a guide.

In 2005, Chueh, then a Caltech senior, applied to graduate school in the Institute's Division of Engineering and Applied Science, and in his application, he recalled an event that would inform his research path. Two years earlier, he wrote, he had made a trip to the iconic Yosemite Valley. He was anticipating pristine views, but what he found instead was a curtain of smog thrown up by thousands of cars and buses passing through. His goal as a graduate student, Chueh wrote, was to help fix that problem. Today, he's made significant progress toward that goal.

This month, after five years in the lab of Professor of Materials Science and Chemical Engineering Sossina Haile, Chueh wrapped up his doctoral research—work that included developing a novel method of using solar energy to generate fuel. That breakthrough recently earned him a prestigious Truman Fellowship at the Sandia National Laboratories in Livermore, California. Chueh is the first Caltecher to receive the three-year, $800,000 fellowship, which will give him the freedom and funding to pursue a line of research that may lead to crucial advances in the production of abundant, clean energy.

Chueh started his freshman year at Caltech shortly after 9/11, amid considerable discussion about America's critical need to wean itself off fossil fuels. One of the recommended areas of research concerned ways to improve energy conversion and storage, and Chueh got hooked on the subject.

"If you throw fuel and oxygen in an engine, it burns in an inefficient and dirty way," Chueh says. "But if you use electrochemistry and do it in a more controlled manner, then you will have better efficiency and lower emissions." During his senior year, he assisted in a research project led by Haile, who had been studying ways of improving fuel cells, which convert fuel into electricity through a chemical reaction.

One of the problems with many fuel cells concerns temperature.  Some can only operate at such high temperatures that they must be encased in expensive ceramic materials to withstand the heat, while those that can operate at close to room temperature need precious, scarce metals such as platinum to work. Another problem is that they need fuel—typically hydrogen derived from fossil fuels—to generate electricity.

Chueh holds three samples of metalized, thin-film cerium oxide, which he and Haile used to study the fundamental chemistry for generating fuel from the sun's heat.

Tackling the first problem, Haile had been investigating materials that would also allow fuel cells to work at lower temperatures. One of them, cerium oxide (CeO2), is derived from the element cerium—which is classified as a rare earth metal, but is actually as common as copper. Cerium oxide plays an important role in a car's catalytic converter, helping to turn smog-causing molecules into carbon dioxide.

Shortly after Chueh joined Haile's lab as a graduate student, he and Haile started talking about whether CeO2 could also play a role in using the heat of the sun to convert a chemical "cocktail," consisting primarily of carbon dioxide (CO2) and steam, into a gas mixture of carbon monoxide and hydrogen known as "synthesis gas." This "syngas," as it's commonly called, can then be converted into liquid fuels through a decades-old process involving a series of chemical reactions.

"I was pessimistic at first," Chueh says. For a while he held off on testing the idea, but at Haile's urging, he decided to run the necessary experiments during winter break in 2007, when everyone else in the lab was on vacation. "It worked right off the bat," Chueh says. "I'm very cautious, though, so I repeated it before I told her about it. We were all very excited by the results."

Currently, says Chueh, "I'm working on experiments to demonstrate that this is not just a laboratory curiosity, but a solution that could potentially work on a larger scale." The process could also be used in a variety of applications, including the production of fuel for transportation and for running factories.

Chueh took this photograph of the sensational fall colors in Yosemite Valley in 2006.

"William is a truly remarkable researcher, combining exceptional experimental talent with deep theoretical insight," says Haile. "This has allowed him to transform a loosely defined idea from a few sketches on a piece of paper to a meaningful scientific and technological breakthrough. I look forward to learning of his latest discoveries as he moves on to the next stage of his career."

At Sandia, a government-owned facility that develops technologies that support national security, Chueh will continue to study the cerium oxide–reaction to try to determine exactly what is happening at the molecular level while the catalyst is working. "Once we have a more detailed picture of that, we will be able to better understand why it works," he says, and possibly come up with ways to improve it.

Chueh says that he's "convinced that in the years to come, we'll see scaled-up plants that are actually producing a good amount of fuel from this kind of process. This system would work best in the desert, where there's lots of sun."

While Chueh acknowledges that there are numerous other solar research projects that could prove to be as beneficial as the cerium process, he says,  "Every system has its advantages and drawbacks. In the end, a solution to the energy problem will not come from a single technology but from a wide range of technologies. This gives consumers and policy makers one additional option."

As for the nature photography that started it all, Chueh didn't have much time for his hobby during graduate school, but he's looking forward to taking it up again.

"I'm hoping to go to the eastern Sierra in the fall when all the aspens turn yellow and then orange," he says. "In nature photography, I love finding order in chaos, and that's what we essentially do in science.

"Deep down, I have a great appreciation for the environment," Chueh says. "When I saw that smog-filled Yosemite Valley, that's when I thought, 'I've got to do something before all this gets wiped out.'"

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Mike Rogers
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Caltech-led Team Gets up to $122 Million for Energy Innovation Hub

Caltech will partner with Lawrence Berkeley Nat. Lab. and other CA institutions to develop method to produce fuels from sunlight

PASADENA, Calif.-As part of a broad effort to achieve breakthrough innovations in energy production, U.S. Deputy Secretary of Energy Daniel Poneman today announced an award of up to $122 million over five years to a multidisciplinary team of top scientists to establish an Energy Innovation Hub aimed at developing revolutionary methods to generate fuels directly from sunlight. 

The hub will be directed by Nathan S. Lewis, George L. Argyros Professor and professor of chemistry at the California Institute of Technology (Caltech). 

The Joint Center for Artificial Photosynthesis (JCAP), to be led by Caltech in partnership with the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab), will bring together leading researchers in an ambitious effort aimed at simulating nature's photosynthetic apparatus for practical energy production. The goal of the hub is to develop an integrated solar energy-to-chemical fuel conversion system and move this system from the bench-top discovery phase to a scale where it can be commercialized.

"The Energy Innovation Hubs have enormous potential to advance transformative breakthroughs," says Deputy Secretary Poneman. "Finding a cost-effective way to produce fuels as plants do-combining sunlight, water, and carbon dioxide-would be a game changer, reducing our dependence on oil and enhancing energy security.  This Energy Innovation Hub will enable our scientists to combine their talents to tackle this bold and highly promising challenge."

Lewis, who will lead the multi-institutional team, says, "The sun is by far the largest source of energy available to man, but we must find a way to cheaply capture, convert, and store its energy if we are to build a complete clean energy system. Making fuels directly from sunlight presents an exciting opportunity to focus the efforts of teams of leading scientists onto developing the breakthroughs that are required to obtain a safe and secure energy future for all nations."

The hubs are large, multidisciplinary, highly collaborative teams of scientists and engineers working over a longer time frame to achieve a specific high-priority goal. They are managed by top teams of scientists and engineers with enough resources and authority to move quickly in response to new developments.

On the Caltech campus, the center will be housed in the Jorgensen Laboratory building.

"Caltech is honored to be chosen by the Department of Energy to lead its new Energy Innovation Hub, and I am confident that this bold public-private partnership envisioned by President Obama will ultimately help develop significant clean energy solutions and create green jobs," says Caltech President Jean-Lou Chameau. "Caltech's history of solving the most difficult, multidisciplinary, scientific problems, and the strong commitment to energy innovation through our new Resnick Sustainability Institute, make us uniquely suited to help make fuels from the sun an efficient and economical part of our nation's energy strategy."  

JCAP research will be directed at the discovery of the functional components necessary to assemble a complete artificial photosynthetic system: light absorbers, catalysts, molecular linkers, and separation membranes. The hub will then integrate those components into an operational solar fuel system and develop strategies to move from the laboratory toward commercial viability. The ultimate objective is to drive the field of solar fuels from fundamental research, where it has resided for decades, into applied research and technology development, thereby setting the stage for the creation of a direct solar fuels industry.   

Other members of the hub leadership team include: Bruce Brunschwig (Caltech); Peidong Yang (UC Berkeley/Berkeley Lab); and Harry Atwater, Caltech's Howard Hughes Professor, professor of applied physics and materials science, and director of the Resnick Institute, which will work in conjunction with the new center to foster transformational advances in energy science. Atwater and Lewis are both founding board members of the Kavli Nanoscience Institute based at Caltech.

The JCAP Proposal Leadership team included Heinz Frei and Elaine Chandler of Berkeley Lab, as well as Eric McFarland of the University of California, Santa Barbara and Jens Norskov of the SLAC National Accelerator Lab.  Also involved at Caltech will be Harry Gray, the Arnold O. Beckman Professor of Chemistry; Jonas Peters, the Bren Professor of Chemistry; and Michael Hoffman, the James Irvine Professor of Environmental Science.

In addition to the major partners, Caltech and Berkeley Lab, other participating institutions include SLAC, Stanford University; UC Berkeley; UC Santa Barbara; UC Irvine; and UC San Diego.

Selection was based on a competitive process using scientific peer review.  The selection process for the Fuels from Sunlight Hub was managed by the Department of Energy Office of Science, which will have federal oversight responsibilities for the artificial photosynthesis Hub.

The hub will be funded at up to $22 million this fiscal year.  The hub will then be funded at an estimated $25 million per year for the next four years, subject to congressional appropriations.  More information on the hubs can be found at: http://www.energy.gov/hubs/.

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Jon Weiner
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Something in the Air

For the past month, Caltech scientists have been zigzagging across the Los Angeles basin. Using an orange and white DeHavilland Twin Otter aircraft packed with instruments, the researchers have been sampling the air, measuring particles and pollutants to help policymakers improve air quality and dampen the impacts of climate change.

"We want to understand very thoroughly where these particles come from, what they're made of, how they evolve, and eventually how they're removed," says chemical engineer John Seinfeld, who leads the Caltech group. The flights are just one element of a project dubbed CalNex—the nexus of pollution and climate over California—run by the National Oceanic and Atmospheric Administration (NOAA).

CalNex is one of the largest air-quality experiments ever done, says Jose-Luis Jimenez, a professor at the University of Colorado and a former Caltech postdoc. The project involves three other aircraft—including NOAA's Lockheed WP-3D Orion, a plane with a 100-foot wingspan whose resume includes missions into dozens of hurricanes—and the Atlantis, a research vessel operated by the Woods Hole Oceanographic Institution. There are also two ground stations, one in Bakersfield and the other on the Caltech campus. (You may have noticed the main part of the Caltech station: two towers of scaffolding and the huddle of trailers on the vacant lot north of the Holliston parking structure.) Such studies are so expensive that they only occur about once a decade in southern California—the crew on campus alone includes more than 60 people from around the world.

Every morning in May, the Caltech team gathers at Ontario International Airport, checking their equipment for the day's four-hour flight. Typically carrying up to 20 passengers when it operates as a commuter plane, the twin-engine turboprop is so stuffed with gizmos and computers that there's only room for one researcher—usually a grad student—who monitors all the instruments, fixing them if needed.

 

"The best part is getting to see all the instruments in action," says Andrew Metcalf, one of the graduate students who got to fly. Because he can watch the data being collected in real time, he gets a better sense of what each data point on the screen means—important when trying to analyze the information later. For most flights, the plane heads west over Pasadena and toward Long Beach, then crisscrosses back east—occasionally going as far as Palm Springs and the Salton Sea—following the changing chemistry of the particles as they travel with the eastward wind. The plane usually flies at 1,000 feet—as low as the FAA will allow. To measure how the air changes with elevation, the pilot sometimes executes missed approaches—a maneuver in which the plane approaches the runway but doesn't land—over many of the small airports that dot the L.A. basin. On occasion, the plane flies north to Bakersfield and the San Joaquin Valley to see how the air differs from that above the Los Angeles basin.

An inlet pipe jutting from the front of the plane collects the air and channels it through tubes to the instruments, which are lined in racks on one side of the plane. The devices collect an assortment of data, such as the size distribution of particles and their chemical constituents.

Grad student Jill Craven gets to the airport at 6 a.m., having to boot up her mass spectrometer, a powerful but temperamental instrument. "When it breaks down, I get really stressed out," she says. "Field campaigns are wonderful because you're not in the lab. The hard part is that you're under pressure to perform in a month, because we only have four weeks to collect data for the entire year."

Airsickness can also be a challenge. "I flew the first flight and I got really sick," Metcalf recalls. As it happened, there was no airsickness bag on board that day, and one of the pilots had to sacrifice his lunch bag. "It was about a week and a half before I got up the nerve to try it again. Now I take motion-sickness drugs to help me out." Still, it's much more fun to be up in the air than cooped up in a lab, he says. "It's exciting to fly around and see exactly what's out there in the L.A. basin."

So what is out there? It will be years before scientists finish analyzing all the data. The results, however, will have a global impact. CalNex is designed to help untangle the complex ways in which particles affect air quality and climate. For example, tiny particles are bad for air quality, but they can also scatter sunlight, counteracting the warming effect of greenhouse gases. "If you go anywhere in the world," Seinfeld explains, "particles in the air are a mixture of the same kitchen sink of compounds. A large urban area like Los Angeles, with sources ranging from traffic, industry, and ships to vegetation, is the perfect staging area to study how such particles are formed and how they evolve."

Seinfeld, the Nohl Professor and professor of chemical engineering, leads the Caltech team, which includes Richard Flagan, the McCollum-Corcoran Professor of Chemical Engineering and professor of environmental science and engineering.

View our narrated slideshow of the Calnex plane and its instrumentation.

Writer: 
Marcus Woo
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Schooling Fish Offer New Ideas for Wind Farming

The quest to derive energy from wind may soon be getting some help from California Institute of Technology (Caltech) fluid-dynamics expert John Dabiri-and a school of fish.

As head of Caltech's Biological Propulsion Laboratory, Dabiri studies water- and wind-energy concepts that share the theme of bioinspiration: that is, identifying energy-related processes in biological systems that may provide insight into new approaches to-in this case-wind energy.

"I became inspired by observations of schooling fish, and the suggestion that there is constructive hydrodynamic interference between the wakes of neighboring fish," says Dabiri, associate professor of aeronautics and bioengineering at Caltech. "It turns out that many of the same physical principles can be applied to the interaction of vertical-axis wind turbines."

The biggest challenge with current wind farms is lack of space. The horizontal-axis wind turbines most commonly seen-those with large propellers-require a substantial amount of land to perform properly. "Propeller-style wind turbines suffer in performance as they come in proximity to one another," says Dabiri.

In the Los Angeles basin, the challenge of finding suitable space for such large wind farms has prevented further progress in the use of wind energy. But with help from the principles supplied by schooling fish, and the use of vertical-axis turbines, that may change.

Vertical turbines-which are relatively new additions to the wind-energy landscape-have no propellers; instead, they use a vertical rotor. Because of this, the devices can be placed on smaller plots of land in a denser pattern. Caltech graduate students Robert Whittlesey and Sebastian Liska researched the use of vertical-axis turbines on small plots during a class research project supervised by Dabiri. Their results suggest that there may be substantial benefits to placing vertical-axis turbines in a strategic array, and that some configurations may allow the turbines to work more efficiently as a result of their relationship to others around them-a concept first triggered by examining schools of fish.

In current wind farms, all of the turbines rotate in the same direction. But while studying the vortices left behind by fish swimming in a school, Dabiri noticed that some vortices rotated clockwise, while others rotated counter-clockwise. Dabiri therefore wants to examine whether alternating the rotation of vertical-axis turbines in close proximity will help improve efficiency. The second observation he made studying fish-and seen in Whittlesey and Liska's simulation-was that the vortices formed a "staircase" pattern, which contrasts with current wind farms that place turbines neatly in rows.

Whittlesey and Liska's computer models predicted that the wind energy extracted from a parcel of land using this staggered placement approach would be several times that of conventional wind farms using horizontal-axis turbines. Once they've identified the optimal placement, Dabiri believes it may be possible to produce more than 10 times the amount of energy currently provided by a farm of horizontal turbines. The results are sufficiently compelling that the Caltech group is pursuing a field demonstration of the idea.

Dabiri has purchased two acres of land north of Los Angeles, where he is establishing the Caltech Field Laboratory for Optimized Wind Energy (FLOWE). The pilot program at the site will feature six vertical turbines on mobile platforms.

Dabiri and his team will systematically move the turbines around, testing various configurations to find the most efficient patterns.

"Our goal is to demonstrate a new technology that enables us to extract significantly more wind energy from a given parcel of land than is currently possible using existing methods," says Dabiri. "We want to take advantage of constructive aerodynamic interference between closely spaced vertical-axis wind turbines. Our results can potentially make better use of existing wind farms, allow for wind farms to be located closer to urban centers-reducing power transmission costs-and reduce the size of offshore installations."

Three of Dabiri's turbines are being provided in partnership with Windspire Energy. In exchange for the use of the turbines, Dabiri will share his research results with the company. Each Windspire turbine stands approximately 30 feet tall and 4 feet wide, and can generate up to 1.2 kW of power.

"This leading-edge project is a great example of how thinking differently can drive meaningful innovation," says Windspire Energy President and CEO Walt Borland. "We are very excited to be able to work with Dr. Dabiri and Caltech to better leverage the unique attributes of vertical-axis technology in harvesting wind energy."   

Three turbines from another manufacturer have been purchased; the six turbines give the pilot facility a total power capacity of 15 kW, enough to power several homes.

"This project is unique in that we are conducting these experiments in real-world conditions, as opposed to on the computer or in a laboratory wind tunnel," says Dabiri. "We have intentionally focused on a field demonstration because this can more easily facilitate a future expansion of the project from basic science research into a power-generating facility. Our ability to make that transition will depend on the results of the pilot program."

The initial phase of the study will attempt to demonstrate which configuration of units will improve power output and performance relative to a horizontal-axis wind turbine farm with a similar sized plot of land.

"In the future, we hope to transition to power-generation experiments in which the generated power can be put to use either locally or via a grid connection," Dabiri says.

The American Recovery and Reinvestment Act provided partial funding for this project.

For more information on FLOWE, visit: http://dabiri.caltech.edu/research/wind-energy.html.

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Jon Weiner
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Caltech-Led Team Designs Novel Negative-Index Metamaterial that Responds to Visible Light

Uniquely versatile material could be used for more efficient light collection in solar cells

PASADENA, Calif.—A group of scientists led by researchers from the California Institute of Technology (Caltech) has engineered a type of artificial optical material—a metamaterial—with a particular three-dimensional structure such that light exhibits a negative index of refraction upon entering the material. In other words, this material bends light in the "wrong" direction from what normally would be expected, irrespective of the angle of the approaching light.

This new type of negative-index metamaterial (NIM), described in an advance online publication in the journal Nature Materials, is simpler than previous NIMs—requiring only a single functional layer—and yet more versatile, in that it can handle light with any polarization over a broad range of incident angles. And it can do all of this in the blue part of the visible spectrum, making it "the first negative index metamaterial to operate at visible frequencies," says graduate student Stanley Burgos, a researcher at the Light-Material Interactions in Energy Conversion Energy Frontier Research Center at Caltech and the paper's first author.

"By engineering a metamaterial with such properties, we are opening the door to such unusual—but potentially useful—phenomena as superlensing (high-resolution imaging past the diffraction limit), invisibility cloaking, and the synthesis of materials index-matched to air, for potential enhancement of light collection in solar cells," says Harry Atwater, Howard Hughes Professor and professor of applied physics and materials science, director of Caltech's Resnick Institute, founding member of the Kavli Nanoscience Institute, and leader of the research team

What makes this NIM unique, says Burgos, is its engineering.

"The source of the negative-index response is fundamentally different from that of previous NIM designs," he explains. Those previous efforts used multiple layers of "resonant elements" to refract the light in this unusual way, while this version is composed of a single layer of silver permeated with "coupled plasmonic waveguide elements."

Surface plasmons are light waves coupled to waves of electrons at the interface between a metal and a dielectric (a non-conducting material like air). Plasmonic waveguide elements route these coupled waves through the material. Not only is this material more feasible to fabricate than those previously used, Burgos says, it also allows for simple "tuning" of the negative-index response; by changing the materials used, or the geometry of the waveguide, the NIM can be tuned to respond to a different wavelength of light coming from nearly any angle with any polarization. "By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with a nearly isotopic refractive index tuned to operate at visible frequencies."

This sort of functional flexibility is critical if the material is to be used in a wide variety of ways, says Atwater. "For practical applications, it is very important for a material's response to be insensitive to both incidence angle and polarization," he says. "Take eyeglasses, for example. In order for them to properly focus light reflected off an object on the back of your eye, they must be able to accept and focus light coming from a broad range of angles, independent of polarization. Said another way, their response must be nearly isotropic. Our metamaterial has the same capabilities in terms of its response to incident light."

This means the new metamaterial is particularly well suited to use in solar cells, Atwater adds. "The fact that our NIM design is tunable means we could potentially tune its index response to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells," he explains. "And the fact that the metamaterial has a wide-angle response is important because it means that it can 'accept' light from a broad range of angles. In the case of solar cells, this means more light collection and less reflected or 'wasted' light."

"This work stands out because, through careful engineering, greater simplicity has been achieved," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

In addition to Burgos and Atwater, the other authors on the Nature Materials paper, "A single-layer wide-angle negative index metamaterial at visible frequencies," are Rene de Waele and Albert Polman from the Foundation for Fundamental Research on Matter Institute for Atomic and Molecular Physics in Amsterdam. Their work was supported by the Energy Frontier Research Centers program of the Office of Science of the Department of Energy, the National Science Foundation, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek, and "NanoNed," a nanotechnology program funded by the Dutch Ministry of Economic Affairs.

Writer: 
Lori Oliwenstein

Solar Decathlon

Students from Caltech and the Southern California Institute of Architecture (SCI-Arc) have been chosen to build a green house near the White House. They will compete against teams from around the world in the biennial Solar Decathlon, sponsored by the U.S. Department of Energy. Taking place in October 2011 on the National Mall in Washington, D.C., the two-week event will challenge teams of university students to design and build the most affordable, attractive, and energy-efficient house they can.

A panel of scientists, engineers, and other experts from the DOE's National Renewable Energy Laboratory selected the SCI-Arc/Caltech team as one of the 20 teams whose proposals and concept designs were the best. Now that they've been chosen, the students have 18 months to design, test, and build a state-of-the-art, solar-powered house—and they have to figure out how to transport it across the country and assemble it in the few days leading up to the event. "I can't think of any other project that I've done in the past that even compares in magnitude,"says Fei Yang, one of the student leaders at Caltech.

For the first time, the Decathlon will require the houses to cost $250,000 or less. The houses, which will be open to the public, must be within 600 and 1,000 square feet. The Decathlon's 10 "events" range from completing certain tasks, such as heating 15 gallons of water in under 10 minutes, to being judged by a panel of experts on such subjective aspects as the aesthetics of the architecture.

SCI-Arc first approached Caltech's chapter of Engineers for a Sustainable World, of which Yang is president, to recruit Techers to join the team. Yang, a junior mechanical engineering major, and Ben Kurtz, a junior physics major who helps lead the Caltech crew, say they jumped at the opportunity to get their hands dirty. "I think the difference between this and almost any other research project you can do at Caltech is that this is an immediate opportunity to affect the way things are done in the real world, and the way people live," Kurtz says. Ideally, they hope a contractor will be sufficiently impressed with their design to build and market their house.

As first-time participants coming from small schools, the SCI-Arc and Caltech team members are definitely underdogs, Yang says. Seven faculty members have already offered their guidance and expertise in a series of reading courses, and a new class is in the works for the next term. At this point, fewer than 20 Caltech students are involved. But as the team members proceed past the initial planning stages and toward actual construction, they will need more than a hundred people—and they're recruiting, Yang says. "We need as much help as we can get," adds Kurtz. "Anybody who wants to help is welcome."

Some of the features of the SCI-Arc-Caltech prototype are angled walls that minimize the amount of heating from the sun and a tilted roof that maximizes energy production from solar panels.

The team is sponsored by the Resnick Institute, which is providing financial support and helping with advising and fundraising.

Listen to an audio podcast about this project.

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Marcus Woo
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Caltech Researchers Create Highly Absorbing, Flexible Solar Cells with Silicon Wire Arrays

PASADENA, Calif.—Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has created a new type of flexible solar cell that enhances the absorption of sunlight and efficiently converts its photons into electrons. The solar cell does all this using only a fraction of the expensive semiconductor materials required by conventional solar cells.

"These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials," says Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech's Resnick Institute, which focuses on sustainability research.

The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. "We've surpassed previous optical microstructures developed to trap light," he says. 

Atwater and his colleagues—including Nathan Lewis, the George L. Argyros Professor and professor of chemistry at Caltech, and graduate student Michael Kelzenberg—assessed the performance of these arrays in a paper appearing in the February 14 advance online edition of the journal Nature Materials.

Atwater notes that the solar cells' enhanced absorption is "useful absorption."

"Many materials can absorb light quite well but not generate electricity—like, for instance, black paint," he explains. "What's most important in a solar cell is whether that absorption leads to the creation of charge carriers."

The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. "High absorption plus good conversion makes for a high-quality solar cell," says Atwater. "It's an important advance."

The key to the success of these solar cells is their silicon wires, each of which, says Atwater, "is independently a high-efficiency, high-quality solar cell." When brought together in an array, however, they're even more effective, because they interact to increase the cell's ability to absorb light.

"Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires make the array very absorbing," he says.

This is a photomicrograph of a silicon wire array embedded within a transparent, flexible polymer film.
Credit: Caltech/Michael Kelzenberg

This effect occurs despite the sparseness of the wires in the array—they cover only between 2 and 10 percent of the cell's surface area.

"When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires," explains Kelzenberg. "So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration—an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells."

Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”

In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.

Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce.

The composite nature of these solar cells, Atwater adds, means that they are also flexible. "Having these be complete flexible sheets of material ends up being important," he says, "because flexible thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells."

Atwater, Lewis, and their colleagues had earlier demonstrated that it was possible to create these innovative solar cells. "They were visually striking," says Atwater. "But it wasn't until now that we could show that they are both highly efficient at carrier collection and highly absorbing."

The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. "The structures we've made are square centimeters in size," he explains. "We're now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell."

Atwater says that the team is already "on its way" to showing that large-area cells work just as well as these smaller versions.

In addition to Atwater, Lewis, and Kelzenberg, the all-Caltech coauthors on the Nature Materials paper, "Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications," are postdoctoral scholars Shannon Boettcher and Joshua Spurgeon; undergraduate student Jan Petykiewicz; and graduate students Daniel Turner-Evans, Morgan Putnam, Emily Warren, and Ryan Briggs.

Their research was supported by BP and the Energy Frontier Research Center program of the Department of Energy, and made use of facilities supported by the Center for Science and Engineering of Materials, a National Science Foundation Materials Research Science and Engineering Center at Caltech. In addition, Boettcher received fellowship support from the Kavli Nanoscience Institute at Caltech.

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Lori Oliwenstein
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Caltech Opens "Green" Building Offering New Opportunities in Information Science

Annenberg Center for Information Science and Technology combines multiple academic efforts under one roof

Pasadena, Calif., Oct. 30, 2009-New ideas in architecture will foster new ideas in information science as the California Institute of Technology (Caltech) opens its groundbreaking "green" building, the Walter and Leonore Annenberg Center for Information Science and Technology.

Designed by Los Angeles architectural firm Frederick Fisher and Partners, the Annenberg Center will serve as home to interdisciplinary research and instruction that address the growth and impact of information as it relates to all scientific and engineering practices.

Researchers in this emerging academic discipline come from five of Caltech's six academic divisions. The aim of the new facility is to bring physicists, biologists, engineers, and computer scientists together to foster collaboration and interdisciplinary research and teaching.

"The choice we made to bring together researchers from across disciplines has already led to new research and hiring practices," says Ares Rosakis, chair of the Division of Engineering and Applied Science (EAS) and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech.

In a move to help cultivate the potential wealth of new ideas, the building was created-inside and out-with sustainability in mind.

"A workplace should be a source of inspiration," says Rosakis. "As with other newly renovated Caltech buildings, the innovative design of this building inspires its residents."

The building-one of three new "green" constructions on the Caltech campus-is seeking a gold certification from the Leadership in Energy and Environmental Design (LEED). The coveted designation would be a result of the building's water and energy efficiency (30 percent reduction in water use; 28 percent reduction in energy use); use of recycled materials (more than 20 percent of materials have recycled content and have local or regional origins); and indoor environmental quality. The bamboo found throughout the building adds to the list of sustainable materials in use.

The construction of this state-of-the-art building was funded primarily by a $25-million gift from the Annenberg Foundation, founded in 1989 by Walter H. Annenberg and later led by his wife, Leonore. The foundation encourages the development of more effective ways to share ideas and knowledge, exemplified in the new building by its cross-disciplinary structure.

Funding also came from the Gordon and Betty Moore Foundation and Stephen D. Bechtel, Jr.

"Information science will provide the intellectual foundation for the exploitation of information across diverse disciplines," says Caltech president Jean-Lou Chameau. "It will result in the education of a new generation of leaders and the creation of innovative scientific, technological, and business models. The Annenberg Center will be the catalyst for this bold venture undertaken by the Caltech faculty."

Other highlights of the new building include

  • quantity and quality control for storm water;
  • individual HVAC controls and operable windows; and
  • a minimum of 75 percent of spaces with daylight access.

Faculty members are housed on the second and third floors of the Annenberg Center, where offices are designed with floor-to-ceiling windows to take advantage of the exterior views. Each of the rooms in the building has sensors that save on the use of energy for lighting, heating, and air-conditioning.

Spaces within the building include

  • 16 faculty offices, with an additional four for visiting faculty;
  • 16 studios for students;
  • 14 postdoctoral offices;
  • 33 graduate-student offices;
  • 14 administrative offices;
  • two lounge areas;
  • three kitchens;
  • five classrooms;
  • an auditorium;
  • a computer lab; and
  • a conference room.

The second-floor atrium contains unique bamboo screens, a staircase, and a skylight that gives the area an open feeling conducive to casual, collegial collaboration.  

The building uses furniture of classic and contemporary design to create an inspiring and inviting atmosphere. The lounge area has a spiral staircase connecting its two levels, and creating a spacious open area with an abundance of natural light.

The exterior landscaping of the Annenberg Center follows Caltech's goal to improve campus sustainability by including a multitude of native drought-resistant plants and hardscaping.

For a visual tour of the center, visit http://images.caltech.edu/slideshows/IST/.

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Jon Weiner
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Caltech Researchers Show How Organic Carbon Compounds Emitted by Trees Affect Air Quality

Research provides first-ever glimpse of role of epoxides in atmospheric chemistry

PASADENA, Calif.—A previously unrecognized player in the process by which gases produced by trees and other plants become aerosols—microscopically small particles in the atmosphere—has been discovered by a research team led by scientists at the California Institute of Technology (Caltech).

Their research on the creation and effects of these chemicals, called epoxides, is being featured in this week's issue of the journal Science.

Paul Wennberg, the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering and director of the Ronald and Maxine Linde Center for Global Environmental Science at Caltech, and John Seinfeld, the Louis E. Nohl Professor and professor of chemical engineering, have been studying the role of biogenic emissions—organic carbon compounds given off by plants and trees—in the atmospheric chemical reactions that result in the creation of aerosols.

"If you mix emissions from the city with emissions from plants, they interact to alter the chemistry of the atmosphere," Wennberg notes.

While there's been plenty of attention paid to the effect of emissions from cars and manufacturing, less is understood about what happens to biogenic emissions, especially in places where there are relatively few man-made emissions. That situation is the focus of the research that led to this Science paper. "What we're interested in," Wennberg explains, "is what happens to the chemicals produced by trees once they are emitted into the atmosphere."

In these studies, the research team focused on a chemical called isoprene, which is given off by many deciduous trees. "The king emitters are oaks," Wennberg says. "And the isoprene they emit is one of the reasons that the Smoky Mountains appear smoky."

Isoprene is no minor player in atmospheric chemistry, Wennberg notes. "There is much more isoprene emitted to the atmosphere than all of the gases—gasoline, industrial chemicals—emitted by human activities, with the important exceptions of methane and carbon dioxide," he says. "And isoprene only comes from plants. They make hundreds of millions of tons of this chemical . . . for reasons that we still do not fully understand."

"Much of the emission of isoprene occurs where anthropogenic emissions are limited," adds Caltech graduate student Fabien Paulot, the paper's first author. "The chemistry is very poorly understood."

Once released into the atmosphere, isoprene gets "oxidized or chewed on" by free-radical oxidants such as OH, explains Wennberg. It is this chemistry that is the focus of this new study. In particular, the research was initiated to understand how the oxidation of isoprene can lead to formation of atmospheric particulate matter, so-called secondary organic aerosol. "A small fraction of the isoprene becomes secondary organic aerosol," Seinfeld notes, "but because isoprene emissions are so large, even this small fraction is important."

Up until now, the chemical pathways from isoprene to aerosol were not known. Wennberg, Seinfeld, and their colleagues discovered that this aerosol likely forms from chemicals known as epoxides.

The name is apt. "These epoxides are nature's glue," says Wennberg. And, much like the epoxy you buy in a hardware store—which requires the addition of an acid for the compound to turn into glue—the epoxides found in the atmosphere also need an acidic kick in order to become sticky.

"When these epoxides bump into particles that are acidic, they make glue," Wennberg explains. "The epoxides precipitate out of the atmosphere and stick to the particles, growing them and resulting in lowered visibility in the atmosphere." Because the acidity of the aerosols is generally higher in the presence of anthropogenic activities, the efficiency of converting the epoxides to aerosol is likely higher in polluted environments, illustrating yet another complex interaction between emissions from the biosphere and from humans. 

"Particles in the atmosphere have been shown to impact human health, as they are small enough to penetrate deep into the lungs of people. Also, aerosols impact Earth's climate through the scattering and absorption of solar radiation and through serving as the nuclei on which clouds form. So it is important to know where particles come from," notes Seinfeld.

The research team was able to make this scientific leap forward thanks to their development of a new type of chemical ionization mass spectrometry (CIMS), led by coauthor and Caltech graduate student John Crounse. "These new CIMS methods open up a very wide range of possibilities for the study of new sets of compounds that scientists have been largely unable to measure previously, mainly because they decompose when analyzed with traditional techniques."

In general, molecules identified and quantified using mass spectroscopy must first be converted to charged ions. They are then directed into an electric field, where the ions are sorted by mass. The problem with traditional ionization techniques is that delicate molecules, such as those produced in the oxidation of isoprene, generally fragment during the ionization process, making their identification difficult or impossible. "This new method was originally developed in order to allow scientists to make atmospheric measurements from airplanes. It is able to ionize gases, even fragile peroxide compounds, while still preserving information about the size or mass of the original molecule," says Wennberg.

That makes determining the individual gases in a complex mixture much easier—especially when, as it turned out, you're looking at a chemical you weren't expecting to find.

Wennberg and colleagues also used oxygen isotopes—oxygen atoms with different numbers of neutrons in their nucleus, and thus different masses—to gain insight into the chemical mechanism yielding epoxides. Epoxides have remained unindentified so far because they have the same mass as another chemical that had been anticipated to form in isoprene oxidation, peroxide. "The oxygen isotopes separated the peroxides from epoxides and further showed that as the epoxides form, OH is recycled to the atmosphere," comments Paulot. "Since OH is the atmosphere detergent, cleaning the atmosphere of many chemicals, the recycling has important implications for the overall oxidizing capacity of the atmosphere."

The identification of a major photochemical pathway to formation of epoxides helps to explain just how tree emissions of organic carbon compounds influence the air in both city and rural settings. While trees aren't exactly the "killers" that Ronald Reagan was once so famously derided for calling them, their isoprene emission levels can—and often probably should—"be a part of the criteria we use when buying and planting trees in a polluted urban setting," notes Wennberg. In fact, he points out, the South Coast Air Quality Management District in Southern California already does this with its list of "approved" trees that don't emit large amounts of organic carbon compounds into the atmosphere.

In addition to Wennberg, Paulot, Crounse, and Seinfeld, other authors on the Science paper, "Unexpected epoxide formation in the gas-phase photooxidation of isoprene," are Henrik Kjaergaard of the University of Otago in New Zealand and the University of Copenhagen in Denmark; former Caltech postdoctoral scholar Andreas Kürten, now at Goethe University in Germany; and Caltech postdoctoral scholar Jason St. Clair.

Purchase of the mass spectrometer used in this study was funded by a Major Research Instrumentation Award from the National Science Foundation. Additional support for the work described in the Science article came from Caltech trustee William Davidow and by grants from the Office of Science, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the Royal Society of New Zealand, and NASA.

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Lori Oliwenstein
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Caltech Researchers Link Tiny Sea Creatures to Large-scale Ocean Mixing

Mechanism, which focuses on impact of drag and water viscosity, shows how swimming animals affect the ocean environment

PASADENA, Calif. —Using a combination of theoretical modeling, energy calculations, and field observations, researchers from the California Institute of Technology (Caltech) have for the first time described a mechanism that explains how some of the ocean's tiniest swimming animals can have a huge impact on large-scale ocean mixing.

Their findings are being published in the July 30 issue of the journal Nature.

"We've been studying swimming animals for quite some time," says John Dabiri, a Caltech assistant professor of aeronautics and bioengineering who, along with Caltech graduate student Kakani Katija, discovered the new mechanism. "The perspective we usually take is that of how the ocean—by its currents, temperature, and chemistry—is affecting the animals. But there have been increasing suggestions that the inverse is also important-how the animals themselves, via swimming, might impact the ocean environment."

Specifically, Dabiri says, scientists have increasingly been thinking about how and whether the animals in the ocean might play a role in larger-scale ocean mixing, the process by which various layers of water interact with one another to distribute heat, nutrients, and gasses throughout the oceans.

Dabiri notes that oceanographers have previously dismissed the idea that animals might have a significant effect on ocean mixing, saying that the viscosity of water would damp out any turbulence created, especially by small planktonic animals. "They said that there was no mechanism by which these animals could impact large-scale ocean mixing," he notes.

But Dabiri and Katija thought there might be a mechanism that had been overlooked—a mechanism they call Darwinian mixing, because it was first discovered and described by Charles Darwin. (No, not that Darwin; his grandson.)

"Darwin's grandson discovered a mechanism for mixing similar in principle to the idea of drafting in aerodynamics," Dabiri explains. "In this mechanism, an individual organism literally drags the surrounding water with it as it goes."

Using this idea as their basis, Dabiri and Katija did some mathematical simulations of what might happen if you had many small animals all moving at more or less the same time, in the same direction. After all, each day, billions of tiny krill and copepods migrate hundreds of meters from the depths of the ocean toward the surface. Darwin's mechanism would suggest that they drag some of the colder, heavier bottom water up with them toward the warmer, lighter water at the top. This would create instability, and eventually, the water would flip, mixing itself as it went.

What the Caltech researchers also found was that the water's viscosity enhances Darwin's mechanism and that the effects are magnified when you're dealing with such minuscule creatures as krill and copepods. "It's like a human swimming through honey," Dabiri explains. "What happens is that even more fluid ends up being carried up with a copepod, relatively speaking, than would be carried up by a whale."

"This research is truly reflective of the type of exciting, without-boundaries research at which Caltech engineering professors excel—in this case a deep analysis of the movement of fluid surrounding tiny ocean creatures leading to completely revelatory insights on possible mechanisms of global ocean mixing," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech.

To verify the findings from their simulations, Katija and collaborators Monty Graham (from the Dauphin Island Sea Laboratory), Jack Costello (from Providence College), and Mike Dawson (from the University of California, Merced) traveled to the island of Palau, where they studied this animal-led transport of water—otherwise known as induced drift—among jellyfish, which are the focus of much of Dabiri's work.

"From a fluid mechanics perspective, this study had less to do with the fact that they're jellyfish, and more to do with the fact that they're solid objects moving through water," Dabiri explains.

Katija's jellyfish experiments involved putting fluorescent dye in the water in front of the sea creatures, and then watching what happened to that dye—or, to be more specific, to the water that took up the dye—as the jellyfish swam. And, indeed, rather than being left behind the jellyfish—or being dissipated in turbulent eddies—the dye travelled right along with the swimming creatures, following them for long distances.

These findings verified that, yes, swimming animals are capable of carrying bottom water with them as they migrate upward, and that movement indeed creates an inversion that results in ocean mixing. But what the findings didn't address was just how much of an impact this type of ocean mixing—performed by impossibly tiny sea creatures—could have on a large scale.

After a series of calculations, Dabiri and Katija were able to estimate the impact of this so-called biogenic ocean mixing. And, Dabiri says, it's quite a significant impact.

"There are enough of these animals in the ocean," he notes, "that, on the whole, the global power input from this process is as much as a trillion watts of energy—comparable to that of wind forcing and tidal forcing."

In other words, the amount of power that copepods and krill put into ocean mixing is on the same scale as that of winds and tides, and thus their impact is expected to be on a similar scale as well.

And while these numbers are just estimates, Dabiri says, they are likely to be conservative estimates, having been "based on the fluid transport induced by individual animals swimming in isolation." In the ocean, these individual contributions to fluid transport may actually interact with one another, and amplify how far the ocean waters can be pulled upward.

In addition, says Dabiri, they have yet to consider the effects of such things as fecal pellets and marine snow (falling organic debris), which no doubt pull surface water with them as they drift downward. "This may have an impact on carbon sequestration on the ocean floor," says Dabiri. "It's something we need to look at in the future."

Dabiri says the next major question to answer is how these effects can be incorporated into computer models of the global ocean circulation. Such models are important for simulations of global climate-change scenarios.

The work described in the Nature paper, "A viscosity-enhanced mechanism for biogenic ocean mixing," was supported by grants from the National Science Foundation's Biological Oceanography, Ocean Technology, Fluid Dynamics, and Energy for Sustainability programs, and by the Office of Naval Research, the Department of Defense's National Science and Engineering Graduate Fellowship, and the Charles Lee Powell Foundation.

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Lori Oliwenstein
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