Monday, 13 May 2013
Thursday, 13 September 2012
Parking Lot Science: Is Black Best?
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| Sharon Chen and Ana Paula Werle of the Berkeley Lab Heat Island Group use a reflectometer to take measurements of cool pavement solar-reflective coatings. (Photos by Roy Kaltschmidt/Berkeley Lab) |
In a typical city, pavements account for 35 to 50 per cent of surface area, of which about half is comprised of streets and about 40 per cent of exposed parking lots. Most of these streets and parking lots are constructed with dark materials.
"It's amazing how hot these pavements get and how we've let them cover most of our urban surfaces," said Haley Gilbert, a researcher in the Heat Island Group of Lawrence Berkeley National Laboratory. "Because dark pavements absorb almost all of the sun's energy, the pavement surface heats up, which in turn also warms the local air and aggravates urban heat islands."
To combat this problem, Berkeley Lab scientists have been studying "cool pavement" technologies. Like cool roofs, which are lighter-colored roofs that keep the air both inside and outside the building cooler by reflecting more of the sun's energy, cool pavements reflect as much as 30 to 50 per cent of the sun's energy, compared to only 5 percent for new asphalt (and 10 to 20 per cent for aged asphalt).
Now the Heat Island Group has converted a portion of a new temporary parking lot at Berkeley Lab into a cool pavement exhibit that will also allow them to evaluate the products over time. The parking lot provides an opportunity to feature cool pavement coatings that are applied directly to existing paved surfaces. Currently, the exhibit features six coatings donated by two manufacturers, Emerald Cities Cool Pavement and StreetBond. The Group is also coordinating with additional manufacturers to apply their technologies in the coming months.
Cool pavements can either be made from traditional pavement materials that are lighter in color and therefore have a higher solar reflectance, such as cement concrete, or can consist of cool-colored coatings or surface treatments for asphalt surfaces. The Heat Island Group works with both the asphalt and cement industries. "An ideal design goal would be a pavement with solar reflectance of at least 35 percent," Gilbert said. "How you get there will vary by project."
Sealcoats are a common maintenance practice for parking lots and schoolyards since the asphalt pavement structure degrades over time. Traditional sealcoats provide a protective layer, keeping water out and helping to slow the oxidation of the asphalt in the pavement structure, while restoring the aged asphalt surface to a jet-black color.
"The cool pavement coatings we're exhibiting can be applied to existing asphalt or cement," Gilbert said. "They can be used in lieu of a sealcoat, and are a good strategy for cities looking to introduce cool pavement technologies."
Cool pavements come in different hues, including green, blue and yellow, and their solar reflectance value depends on both color and material. "There are some colors that look dark but are actually more reflective in the near infrared spectrum," said Benjamin Mandel, another member of the research team. "These products have higher solar reflectance values than one might initially believe because they are specially designed to reflect invisible infrared light."
With the exhibit now open at Berkeley Lab, the scientists will be collecting data to see how the coatings fare over time. "At some point they will reach an equilibrium at which the solar reflectance won't degrade much anymore," Mandel said. "We're also very interested to see what happens when it rains, which may help the coatings self-clean and restore higher reflectance."
The benefits of cool pavements extend beyond just cooling the local ambient air. They can also impact global warming and energy loads. Dark roofs and dark pavements both contribute to global warming by absorbing large amounts of solar energy stored in sunlight, then radiating the energy back into the atmosphere in the form of heat.
Gilbert added: "Across an entire city, small changes in air temperature could be a huge benefit as it can slow the formation of smog. Just a couple of degrees can also reduce peak power demand, by reducing the energy load from air-conditioning."
Additionally more reflective parking lots could allow building owners and cities to save on energy needed to illuminate streets and parking lots. "Chicago has reported energy savings from using solar-reflective pavements in its alleys," said Gilbert. "Quantifying that would be something a business, such as Walmart, could literally take to the bank."
While many of these benefits have been confirmed by scientific models, field studies are needed to verify and quantify the results. Berkeley Lab is now leading a study in collaboration with the UC Pavement Research Center that will closely monitor the solar reflectance values and temperatures of 20 x 24 square-foot pavement sections of six different materials on a residential street on the UC Davis campus. The scientists hope to better understand how changes in solar reflectance over time affect heat transfer throughout the pavement structure. The results may assist policymakers and pavement professionals in making informed decisions regarding cool pavement requirements for building codes and project specifications.
Better data may also help sell cool pavement coatings since they tend to be more expensive than traditional sealants. Another hurdle is that the benefits of cool pavements are more for the public rather than the building owner. But Mandel points out that cool pavements can eventually pay for themselves. "The benefits are less immediately tangible than for cool roofs," he said. "But the initial cost premium can potentially be offset over the lifespan of the product with increased durability and less need for ongoing maintenance, which are factors we are working with manufacturers to investigate further."
Since cool pavements benefit the community, the Heat Island Group is focusing much of its technical assistance and outreach efforts on local governments. To that end, the Heat Island Group held a pavement seminar in June for local officials to learn about the benefits of cool pavements. In addition, schoolyards are a particular target because of the negative health implications of hot blacktops for schoolchildren. "It was shocking for us to hear that here in Berkeley, which has a pretty mild climate, students are not allowed to go out and play on the blacktop basketball courts on certain days," Gilbert said.
"People in California now know the term, 'cool pavements,'" said Gilbert. "That's a huge step forward, but we still need to do more to get the word out."
Wednesday, 29 August 2012
Synchronized Lasers Measure How Light Changes Matter
Berkeley Lab scientists and their colleagues have successfully probed the effects of light at the atomic scale by mixing x-ray and optical light waves at the Linac Coherent Light Source.
Light changes matter in ways that shape our world. Photons trigger changes in proteins in the eye to enable vision; sunlight splits water into hydrogen and oxygen and creates chemicals through photosynthesis; light causes electrons to flow in the semiconductors that make up solar cells; and new devices for consumers, industry, and medicine operate with photons instead of electrons. But directly measuring how light manipulates matter on the atomic scale has never been possible, until now.
An international team of scientists led by Thornton Glover of the US Department of Energy's Lawrence Berkeley National Laboratory used the Linac Coherent Light Source at the SLAC National Accelerator Laboratory to mix a pulse of superbright x-rays with a pulse of lower frequency, "optical" light from an ordinary laser. By aiming the combined pulses at a diamond sample, the team was able to measure the optical manipulation of chemical bonds in the crystal directly, on the scale of individual atoms.
The researchers report their work in the August 30, 2012 issue of the journal Nature.
Mixing x-rays with light in x-ray diffraction
X-ray and optical wave-mixing is an x-ray diffraction technique similar to that long used in solving the structures of proteins and other biological molecules in crystalline form. But in contrast to conventional diffraction, wave mixing selectively probes how light reshapes the distribution of charge in a material. It does this by imposing a distinction between x-rays scattered from optically perturbed charge and x-rays scattered from unperturbed charge.
"You can think of the electrons orbiting atoms in a material as belonging to one of two groups," says Glover. "The 'active' electrons are the outer, loosely bound valence electrons that participate in chemical reactions and form chemical bonds. The 'spectator' electrons are the ones tightly wrapped around the nucleus at the atom's core."
Glover explains that "because the x-ray photon energy is large compared to the electron binding energy, in a typical scattering experiment all electrons scatter with comparable strength and are therefore more or less indistinguishable." The core-electron signal usually swamps the weaker valence-charge signal because there are many more core electrons than valence electrons.
"So x-rays can tell you where atoms are, but they usually can't reveal how the chemically important valence charge is distributed," Glover says. "However, when light is also present with the x-rays, it wiggles some portion of the chemically relevant valence charge. X-rays scatter from this optically driven charge, and in doing so the x-ray photon energy is changed."
The modified x-rays have a frequency (or energy) equal to the sum of the frequencies of both the original x-ray pulse and the overlapping optical pulse. The change to a slightly higher energy provides a distinct signature, which distinguishes wave mixing from conventional x-ray diffraction.
"Conventional diffraction does not provide direct information on how the valence electrons respond to light, nor on the electric fields that arise in a material because of this response," says Glover. "But with x-ray and optical wave mixing, the energy-modified x-rays selectively probe a material's optically responsive valence charge."
Beyond the ability to directly probe atomic-scale details of how light initiates such changes as chemical reactions or phase transitions, sensitivity to valence charge creates new opportunities to track the evolution of chemical bonds or conduction electrons in a material - something traditional x-ray diffraction does poorly. Different components of the valence charge can be probed by tuning the so-called optical pulse; higher-frequency pulses of extreme ultraviolet light, for example, probe a larger portion of valence charge.
Because mixing x-ray and optical light waves creates a new beam, which shows up as a slightly higher-energy peak on a graph of x-ray diffraction, the process is called "sum frequency generation." It was proposed almost half a century ago by Isaac Freund and Barry Levine of Bell Labs as a technique for probing the microscopic details of light's interactions with matter, by separating information about the position of atoms from the response of valence charge exposed to light.
But sum frequency generation requires intense x-ray sources unavailable until recently. SLAC's LCLS is just such a source. It's a free-electron laser (FEL) that can produce ultrashort pulses of high-energy "hard" x-rays millions of times brighter than synchrotron light sources, a hundred times a second.
"The breadth of the science impact of LCLS is still before us," says Jerome Hastings, a professor of photon science at the LCLS and an author of the Nature article. "What is clear is that it has the potential to extend nonlinear optics into the x-ray range as a useful tool. Wave mixing is an obvious choice, and this first experiment opens the door."
Diamonds are just the beginning
Glover's team chose diamond to demonstrate x-ray and optical wave mixing because diamond's structure and electronic properties are already well known. With this test bed, wave mixing has proved its ability to study light-matter interactions on the atomic scale and has opened new opportunities for research.
"The easiest kinds of diffraction experiments are with crystals, and there's lots to learn," Glover says. "For example, light can be used to alter the magnetic order in advanced materials, yet it's often unclear just what the light does, on the microscopic scale, to initiate these changes."
Looking farther ahead, Glover imagines experiments that observe the dynamic evolution of a complex system as it evolves from the moment of initial excitation by light. Photosynthesis is a prime example, in which the energy of sunlight is transferred through a network of light-harvesting proteins into chemical reaction centers with almost no loss.
"Berkeley Lab's Graham Fleming has shown that this virtually instantaneous energy transfer is intrinsically quantum mechanical," Glover says. "Quantum entanglement plays an important role, as an excited electron simultaneously samples many spatially separated sites, probing to find the most efficient energy-transfer pathway. It would be great if we could use x-ray and optical wave mixing to make real-space images of this process as it's happening, to learn more about the quantum aspects of the energy transfer."
Such experiments will require high pulse-repetition rates that free electron lasers have not yet achieved. Synchrotron light sources like Berkeley Lab's Advanced Light Source, although not as bright as FELs, have inherently high repetition rates and, says Glover, "may play a role in helping us assess the technical adjustments needed for high repetition-rate experiments."
Light sources with repetition rates up to a million pulses per second may someday be able to do the job. Glover says, "FELs of the future will combine high-peak brightness with high repetition rate, and this combination will open new opportunities for examining the interactions of light and matter on the atomic scale."
"X-ray and optical wave mixing," by T.E. Glover, D.M. Fritz, M. Cammarata, T.K. Allison, Sinisa Coh, J.M. Feldkamp, H. Lemke, D. Zhu, Y. Feng, R.N. Coffee, M. Fuchs, S. Ghimire, J. Chen, S. Shwartz, D.A. Reis, S.E. Harris, and J. B. Hastings, appears in the August 30, 2012 issue of Nature. The work was principally supported by the U.S. Department of Energy's Office of Science.
Light changes matter in ways that shape our world. Photons trigger changes in proteins in the eye to enable vision; sunlight splits water into hydrogen and oxygen and creates chemicals through photosynthesis; light causes electrons to flow in the semiconductors that make up solar cells; and new devices for consumers, industry, and medicine operate with photons instead of electrons. But directly measuring how light manipulates matter on the atomic scale has never been possible, until now.
An international team of scientists led by Thornton Glover of the US Department of Energy's Lawrence Berkeley National Laboratory used the Linac Coherent Light Source at the SLAC National Accelerator Laboratory to mix a pulse of superbright x-rays with a pulse of lower frequency, "optical" light from an ordinary laser. By aiming the combined pulses at a diamond sample, the team was able to measure the optical manipulation of chemical bonds in the crystal directly, on the scale of individual atoms.
The researchers report their work in the August 30, 2012 issue of the journal Nature.
Mixing x-rays with light in x-ray diffraction
X-ray and optical wave-mixing is an x-ray diffraction technique similar to that long used in solving the structures of proteins and other biological molecules in crystalline form. But in contrast to conventional diffraction, wave mixing selectively probes how light reshapes the distribution of charge in a material. It does this by imposing a distinction between x-rays scattered from optically perturbed charge and x-rays scattered from unperturbed charge.
"You can think of the electrons orbiting atoms in a material as belonging to one of two groups," says Glover. "The 'active' electrons are the outer, loosely bound valence electrons that participate in chemical reactions and form chemical bonds. The 'spectator' electrons are the ones tightly wrapped around the nucleus at the atom's core."
Glover explains that "because the x-ray photon energy is large compared to the electron binding energy, in a typical scattering experiment all electrons scatter with comparable strength and are therefore more or less indistinguishable." The core-electron signal usually swamps the weaker valence-charge signal because there are many more core electrons than valence electrons.
"So x-rays can tell you where atoms are, but they usually can't reveal how the chemically important valence charge is distributed," Glover says. "However, when light is also present with the x-rays, it wiggles some portion of the chemically relevant valence charge. X-rays scatter from this optically driven charge, and in doing so the x-ray photon energy is changed."
The modified x-rays have a frequency (or energy) equal to the sum of the frequencies of both the original x-ray pulse and the overlapping optical pulse. The change to a slightly higher energy provides a distinct signature, which distinguishes wave mixing from conventional x-ray diffraction.
"Conventional diffraction does not provide direct information on how the valence electrons respond to light, nor on the electric fields that arise in a material because of this response," says Glover. "But with x-ray and optical wave mixing, the energy-modified x-rays selectively probe a material's optically responsive valence charge."
Beyond the ability to directly probe atomic-scale details of how light initiates such changes as chemical reactions or phase transitions, sensitivity to valence charge creates new opportunities to track the evolution of chemical bonds or conduction electrons in a material - something traditional x-ray diffraction does poorly. Different components of the valence charge can be probed by tuning the so-called optical pulse; higher-frequency pulses of extreme ultraviolet light, for example, probe a larger portion of valence charge.
Because mixing x-ray and optical light waves creates a new beam, which shows up as a slightly higher-energy peak on a graph of x-ray diffraction, the process is called "sum frequency generation." It was proposed almost half a century ago by Isaac Freund and Barry Levine of Bell Labs as a technique for probing the microscopic details of light's interactions with matter, by separating information about the position of atoms from the response of valence charge exposed to light.
But sum frequency generation requires intense x-ray sources unavailable until recently. SLAC's LCLS is just such a source. It's a free-electron laser (FEL) that can produce ultrashort pulses of high-energy "hard" x-rays millions of times brighter than synchrotron light sources, a hundred times a second.
"The breadth of the science impact of LCLS is still before us," says Jerome Hastings, a professor of photon science at the LCLS and an author of the Nature article. "What is clear is that it has the potential to extend nonlinear optics into the x-ray range as a useful tool. Wave mixing is an obvious choice, and this first experiment opens the door."
Diamonds are just the beginning
Glover's team chose diamond to demonstrate x-ray and optical wave mixing because diamond's structure and electronic properties are already well known. With this test bed, wave mixing has proved its ability to study light-matter interactions on the atomic scale and has opened new opportunities for research.
"The easiest kinds of diffraction experiments are with crystals, and there's lots to learn," Glover says. "For example, light can be used to alter the magnetic order in advanced materials, yet it's often unclear just what the light does, on the microscopic scale, to initiate these changes."
Looking farther ahead, Glover imagines experiments that observe the dynamic evolution of a complex system as it evolves from the moment of initial excitation by light. Photosynthesis is a prime example, in which the energy of sunlight is transferred through a network of light-harvesting proteins into chemical reaction centers with almost no loss.
"Berkeley Lab's Graham Fleming has shown that this virtually instantaneous energy transfer is intrinsically quantum mechanical," Glover says. "Quantum entanglement plays an important role, as an excited electron simultaneously samples many spatially separated sites, probing to find the most efficient energy-transfer pathway. It would be great if we could use x-ray and optical wave mixing to make real-space images of this process as it's happening, to learn more about the quantum aspects of the energy transfer."
Such experiments will require high pulse-repetition rates that free electron lasers have not yet achieved. Synchrotron light sources like Berkeley Lab's Advanced Light Source, although not as bright as FELs, have inherently high repetition rates and, says Glover, "may play a role in helping us assess the technical adjustments needed for high repetition-rate experiments."
Light sources with repetition rates up to a million pulses per second may someday be able to do the job. Glover says, "FELs of the future will combine high-peak brightness with high repetition rate, and this combination will open new opportunities for examining the interactions of light and matter on the atomic scale."
"X-ray and optical wave mixing," by T.E. Glover, D.M. Fritz, M. Cammarata, T.K. Allison, Sinisa Coh, J.M. Feldkamp, H. Lemke, D. Zhu, Y. Feng, R.N. Coffee, M. Fuchs, S. Ghimire, J. Chen, S. Shwartz, D.A. Reis, S.E. Harris, and J. B. Hastings, appears in the August 30, 2012 issue of Nature. The work was principally supported by the U.S. Department of Energy's Office of Science.
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