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  • richardmitnick 1:46 pm on December 22, 2015 Permalink | Reply
    Tags: , , Solar Energy   

    From PNNL: “Creating a Super Lattice: Zipping Electrons, Jumping Holes, and the Quest for Solar Fuels” 

    PNNL Lab

    December 2015
    Web Publishing Services

    Designing a superlattice of chromium and iron oxides produces a material that allows electrons excited by sunshine to freely move away from their “holes,” answering a fundamental question for material scientists working to create the materials that produce fuels from sunlight. No image credit

    Imagine having solar panels turn out fuel, essentially storing the sun’s energy for a rainy day. Scientists are searching for a material that can handle the job. The material must excite electrons when struck by sunlight, easily transport the electrons to where they are needed, and use those electrons to create fuel—and, it must be a material that isn’t in short supply. Rare metals such as platinum need not apply. Hematite, an oxide of readily available iron, is a popular choice. It meets all the requirements but one—it doesn’t let the electrons zip along. Dr. Tiffany Kaspar at Pacific Northwest National Laboratory and her colleagues may have found a way to let the electrons flow—by layering on the oxide of another abundant metal: chromium.

    Why It Matters: Solar energy must be used when it is generated, or it is lost. Storing the energy as fuel could allow solar power to play a larger role on the nation’s energy stage. In the simplest case, solar energy would split water, H2O, to generate hydrogen, H2, fuel. This work shows how one of the challenges to solar fuels could be overcome with earth-abundant minerals. This work also shows how abrupt interfaces between hematite and chromium oxide can be controlled in such a way as to move the electricity without requiring added energy.

    Methods: When you shine light on hematite, electrons are excited, leaving behind “holes,” which act as the positive charge to the electron’s negative charge. Unfortunately, in hematite the electrons tend to fall back into their “holes.” If the electrons and holes could be quickly separated after the electron was excited, both could move on. Ideally, the holes would migrate to the material’s surface, where they can catalyze the production of fuel.

    To create a material where the electrons and holes are forced to separate, the team produced an artificial crystal structure called a superlattice. The team built a thin layer of hematite and then added a layer, three atoms deep, of chromium oxide. They added another layer of hematite, and then chromium oxide, like stacking up the layers on a cake. The abrupt interface between each distinct layer is key to separating the electrons and holes: the electrons prefer to remain in the hematite, while the holes are driven to the chromium oxide layers. The layers were created using the molecular beam epitaxy instrument at EMSL, a DOE scientific user facility.

    Now, when light strikes the surface of the superlattice, the interfaces are such that they drive the excited electrons to the hematite and the holes to the chromium oxide. As an added benefit, the superlattice stack generates an internal voltage that is epxected to drive holes to the material’s surface, where they can react to create fuels.

    The ability of these superlattice stacks to separate electrons and holes was first predicted in 2000 by Kaspar’s colleague Dr. Scott Chambers, but no practical applications were envisioned at the time. Further study led to an understanding of the interfacial properties between the hematite and chromium oxide layers. This work proved relevant to the recent interest in using hematite to produce solar fuels, prompting Kaspar and colleagues to create and test the superlattice stacks.

    Solar panels don’t produce electricity on overcast days, so the energy they produce when the sun shines needs to be stored. Scientists are making progress in the quest for materials that are both readily available and efficient. Stock photo: Dollar Photo Club

    What’s Next? Kaspar and her team are now conducting photoelectrochemical studies to take the next step: split water to produce fuel.

    See the full article here .

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    Pacific Northwest National Laboratory (PNNL) is one of the United States Department of Energy National Laboratories, managed by the Department of Energy’s Office of Science. The main campus of the laboratory is in Richland, Washington.

    PNNL scientists conduct basic and applied research and development to strengthen U.S. scientific foundations for fundamental research and innovation; prevent and counter acts of terrorism through applied research in information analysis, cyber security, and the nonproliferation of weapons of mass destruction; increase the U.S. energy capacity and reduce dependence on imported oil; and reduce the effects of human activity on the environment. PNNL has been operated by Battelle Memorial Institute since 1965.


  • richardmitnick 9:09 am on December 13, 2015 Permalink | Reply
    Tags: , , , , Solar Energy   

    From phys.org: “Vast desert sun farm to help light up Morocco” 


    December 13, 2015
    Jalal Al Makhfi

    Solar mirrors at the Noor 1 Concentrated Solar Power plant, outside the central Moroccan town of Ouarzazate. No image credits.

    On the edge of the Sahara desert, engineers make final checks to a sea of metal mirrors turned towards the sun, preparing for the launch of Morocco’s first solar power plant.

    The ambitious project is part of the North African country’s goal of boosting its clean energy output with what it says will eventually be the world’s largest solar power production facility.

    Morocco has scarce oil and gas reserves, and is the biggest importer of energy in the Middle East and North Africa.

    The plant is part of a vision to move beyond this heavy dependency and raise renewable energy production to 42 percent of its total power needs by 2020.

    About 20 kilometres (12 miles) outside Ouarzazate, half a million U-shaped mirrors—called “parabolic troughs”—stretch out in 800 rows, slowly following the sun as it moves across the sky.

    Spread over an area equivalent to more than 600 football pitches, they store thermal energy from the sun’s rays and use it to activate steam turbines that produce electricity.

    King Mohamed VI launched construction of the plant, called Noor 1, in 2013, at a cost of 600 million euros ($660 million) and involving roughly 1,000 workers.

    Its start of operations by the end of this month was set to coincide with the conclusion of high-stakes COP21 global climate talks in Paris.

    “Construction work has finished,” said Obaid Amran, a board member of Morocco’s solar power agency.

    “We are testing components of the production units with a view to connecting them to the national grid at the end of the year.”

    Morocco is boosting its clean energy output with what it says will eventually be the world’s largest solar power production facility.

    The project’s next phases—Noor 2 and Noor 3—are to follow in 2016 and 2017, and a call for tenders is open for Noor 4.

    A million homes

    Once all phases are complete, Noor will be “the largest solar power production facility in the world”, its developers say, covering an area of 30 square kilometres (11.6 square miles).

    It will generate 580 megawatts and provide electricity to a million homes.

    The solar power project will also help reduce the country’s greenhouse gas emissions.

    The energy ministry estimates that its first solar power plant will allow the country to reduce CO2 emissions by 240,000 tonnes per year initially, and by 522,000 tonnes with the second two phases.

    That is equivalent to nearly one percent of Morocco’s CO2 emissions of around 56.5 million tonnes in 2011, according to World Bank figures.

    The so-called “greenhouse effect” is a natural phenomenon—an invisible blanket of gases including small amounts of carbon dioxide (CO2)—that has made Earth warm enough for humans to survive on it comfortably.

    King Mohamed VI launched construction of the solar plant, called Noor 1, in 2013, at a cost of 600 million euros ($660 million) and involving roughly 1,000 workers.

    But human activities such as burning coal and oil inject additional CO2 into the atmosphere, leading to global warming.

    Humanity’s annual output of greenhouse gases is higher than ever, totalling just under 53 billion tonnes of CO2 in 2014, according to the UN.

    Morocco, to host next year’s COP22, aims to reduce its greenhouse gas emissions by 32 percent by 2030 as it develops renewable energy production.

    “We have a project to introduce 6,000 megawatts to the existing electricity production nationwide,” Energy Minister Abdelkader Amara said recently.

    “Two thousand megawatts will come from solar energy and 2,000 megawatts from wind and hydroelectric power.”

    Morocco started producing electricity at Africa’s largest wind farm in its southwestern coastal region of Tarfaya last year.

    “Things have been going well so far,” the minister said. “We’re likely to go beyond 2,000 megawatts by 2020 in the area of wind power.”

    But Rabat has not abandoned fossil fuels altogether—last December, Amara announced a multi-billion-dollar project to step up Morocco’s search for natural gas to produce electricity.

    See the full article here .

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

  • richardmitnick 12:45 pm on August 8, 2015 Permalink | Reply
    Tags: , , , Solar Energy   

    From PBS via SETI Institute: The Sun, a Crash Course Video 

    Phil Plait takes us for a closer (eye safe!) look at the two-octillion ton star that rules our solar system. We look at the sun’s core, plasma, magnetic fields, sunspots, solar flares, coronal mass ejections, and what all of that means for our planet.

    See the full article here.

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  • richardmitnick 9:18 am on August 7, 2015 Permalink | Reply
    Tags: , India, , Solar Energy   

    From MIT Tech Review: “India, Solar Technology, and the Monkey Problem” 

    MIT Technology Review
    M.I.T. Technology Review

    August 7, 2015
    Richard Martin

    In central Karnataka state, 120 miles north of Bangalore, the lush jungle of India’s west coast gives way to dry scrubland. Sunflowers, onions, chilis, and groundnuts grow in parched fields. In scattered, populous villages, concrete buildings alternate with ramshackle thatched huts. Cows nose through the garbage, and wooden carts drawn by horned oxen crowd the streets. Rough brick-producing factories belch black smoke into the air. Much of the scene appears as it did a century ago. But in a walled compound just beyond the town of Challakere sits an installation that could hold one of the keys to India’s energy future.

    The project, run by the Bangalore-based Indian Institute of Science (known as IISC), is a test array for concentrated solar power. Rows of shallow parabolic troughs, made of specially coated aluminum, stretch for more than the length of two and a half football fields. Above them are water pipes set to catch sunlight reflected from the troughs. When the project begins operation in a few weeks, the water in the pipes will be heated to 200 °C (392 °F); the hot water will go to a heat exchanger attached to a small turbine that will produce 100 kilowatts of electricity.

    A part of the Solar Energy Research Institute for India and the United States (SERIIUS), this small solar array will be used to test various reflective materials and heat-transfer fluids (including, for instance, molten salt in addition to water) from multiple manufacturers. Dozens of small wireless sensors will collect data and send it via the Internet to a dashboard at IISC, where it can be analyzed and catalogued. The objective, says Praveen Ramamurthy, a professor of materials engineering at IISC, is to find the combinations of components that best suit conditions in India, which under the National Solar Mission of Prime Minister Narendra Modi is poised to become one of the world’s largest solar markets in the next seven years.

    The Indian subcontinent, as has often been noted, is a world unto itself, encompassing the rainforests of Assam, the deserts of Rajasthan, and the Himalayan plateaus of Ladakh. Finding solar panels that will stand up to these extreme conditions will be critical to Modi’s goal of building 100 gigawatts of solar capacity by 2022. “Nobody is testing for the aging [of solar equipment] in India,” says Ramamurthy. “We get solar panels, but they’re certified for moderate climates in the U.S. and Europe, and we just adapt.”

    Concentrated solar power, in which reflective troughs heat up water that is then fed to heat exchangers and turbines to produce electricity, could play a key role in India’s energy future.

    India’s solar mission is important not only to India but to the entire world. The world’s third-largest emitter of greenhouse gases, India is an energy-starved, coal-dependent country, where more than 300 million people, according to official estimates, live without electricity and millions more have only spotty service from the grid. Modi has pledged to create dozens of “ultra mega solar power parks,” of 500 megawatts and above, to feed power to the grid, even as the National Institute for Rural Development embarks on a program to bring rooftop solar panels to thousands of India’s impoverished villages. Piyush Goyal, the minister of power, has said that the government’s energy policies will reduce annual carbon dioxide emissions by 550 million tons. Whether India can industrialize and provide universal electricity access while reining in greenhouse-gas emissions will help determine whether the world can avoid catastrophic climate change.

    The Challakere test array will eventually include solar photovoltaic installations, as well as concentrated solar. Ramamurthy’s own research focuses on developing polymers to encapsulate solar panels and seal them against high temperatures and humidity, which tend to rot the adhesives that hold conventional solar panels together. Dust and degradation are also major problems in India. And then there are the monkeys.

    Like many places in India, IISC’s leafy Bangalore campus abounds with tribes of monkeys that like to lick the dew off solar panels and chew the electrical cables. Various methods have been tried to drive them off, but so far none have worked, including an ultrasonic monkey repeller that actually seems to attract the primates. “We’ve tried giving them food to lure them away, but they just sit there,” says an exasperated Ramamurthy. “I don’t know what to do.”

    While solar PV is expected to provide the majority of solar power generation in India, concentrated solar is also of keen interest, as it can be put to a variety of non-electricity applications. Those brick factories in Karnataka, for instance, are mostly illegal, and they bake the bricks using firewood. That causes deforestation and heavy emissions of carbon dioxide. Using concentrated solar power to bake bricks would be a huge boon to the environment.

    In other words, the work being carried out at Challakere will help India, whose energy sector in many ways has progressed little since the 1960s, leapfrog its way to a 21st-century solar industry.

    See the full article here.

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  • richardmitnick 8:48 am on August 7, 2015 Permalink | Reply
    Tags: , , Solar Energy   

    From Discovery: “Massive Solar Power Plant Going Up in China” 

    Discovery News
    Discovery News

    Aug 6, 2015
    Tracy Staedter


    China, which is on track to build the equivalent of one new 600-megawatt coal-fired power plant every 10 days for the next 10 years, is also committed to boosting the amount of emission-free energy it produces.

    By 2030, the government announced this week, it wants to reduce fossil-fuel energy by 20 percent.

    To that end, the country is already constructing its largest solar power plant to date — a 10-square-mile facility in the Gobi desert that’s expected to have a capacity of 200 megawatts, reports via National Geographic. Two hundred megawatts is enough to supply electricity to one million households.

    Named Delingha, the concentrating solar thermal project — similar to Ivanpah in California’s Mojave Desert — will sprawl across 6,300 acres of vacant land in the Qinghai province once completed.

    The satellite images above, courtesy of NASA, show just how much progress has been made. On the left, the new power plant is just beginning to creep over the desert on October 15, 2012. On the right, the surface area covered has tripled by May 22, 2015.

    Six fields of mirrors will direct sunlight to a tall tower, each of which will generate about 135 megawatts of electricity when fully operational. Overall, the plant will reduce carbon emissions by 20 millions tons over its lifetime.

    “China’s carbon dioxide emission will peak by around 2030 and China will work hard to achieve the target at an even earlier date,” Chinese Premier Li Keqiang said after meeting French President Francois Hollande in Paris to agree upon a U.N. climate deal.

    Delingha is set to go online in 2017 with other solar power plants in China following. Since Chinese solar panel manufacturers are the main source of the technology in the world, we can only hope that their direct involvement in this technology will improve solar panel efficiency over time. And reduce fossil-fuel emissions, to boot.

    See the full article here.

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  • richardmitnick 4:55 am on July 21, 2015 Permalink | Reply
    Tags: , , , Solar Energy,   

    From livescience: ” Is a Mini Ice Age Coming? ‘Maunder Minimum’ Spurs Controversy” 


    July 18, 2015
    Elizabeth Goldbaum

    Temp 0
    Image of the sun taken by NASA’s Solar Dynamics Observatory on July 18, 2015. Credit: NASA Solar Dynamics Observatory

    A scientist who claims waning solar activity in the next 15 years will trigger what some are calling a mini ice age has revived talk about the effects of man-made versus natural disruptors to Earth’s climate.

    Valentina Zharkova, a professor of mathematics at Northumbria University in the United Kingdom, used a new model of the sun’s solar cycle, which is the periodic change in solar radiation, sunspots and other solar activity over a span of 11 years, to predict that “solar activity will fall by 60 percent during the 2030s to conditions last seen during the ‘mini ice age‘ that began in 1645,” according to a statement.

    At the National Astronomy meeting in Llanduno, north Wales last week, Zharkova said that a series of solar phenomena will lead to a “Maunder Minimum,” which refers to the seven decades, from 1645 to 1715, when the sun’s surface ceased its heat-releasing magnetic storms and coincided with the Little Ice Age, a period of chillier temperatures, from around 1550 to 1850 in Europe, North America and Asia, according to NASA.

    “The upcoming Maunder Minimum is expected to be shorter than the last one in 17th century (five solar cycles of 11 years),” Zharkova told Live Science in an email. “It will be lasting about three solar cycles.”

    However, many scientists are not convinced. Georg Feulner, the deputy chair of the Earth system analysis research domain at the Potsdam Institute on Climate Change Research, has studied the effect a solar minimum might have on Earth’s climate. His research has shown that temperature drops correlated to a less intense sun would be insignificant compared with anthropogenic global warming, according to the Washington Post.

    Regarding the Maunder Minimum predicted by Zharkova, Feulner said, “The expected decrease in global temperature would be 0.1 degrees Celsius at most, compared to about 1.3 degrees Celsius since pre-industrial times by the year 2030,” Feulner told the Post. Furthermore, this isn’t the first time research has predicted waning heat from the sun, to which experts also said that man-made global warming won’t be trumped.

    Solar cycles and the Maunder Minimum

    Solar cycles rise and fall over an 11-year cycle, though each cycle is unique. The sun can emit extreme ultraviolet and X-ray emissions that heat the part of the sky where planes fly. “Although the change in total solar irradiance seems too small to produce significant climatic effects, there is good evidence that, to some extent, the Earth’s climate heats and cools as solar activity rises and falls,” wrote David Hathaway, a solar physicist with NASA’s Ames Research Center, in a 2010 review paper published in the journal Living Reviews in Solar Physics.

    The Maunder Minimum was named by solar astronomer John Eddy in 1976 after E.W. Maunder, an English scientist who, along with German scientist Gustav Spörer, first noticed the decrease in solar activity in the 1890s, according to the New York Times.

    “I have re-examined the contemporary reports and new evidence which has come to light since Maunder’s time and conclude that this 70-year period was indeed a time when solar activity all but stopped,” Eddy wrote in the Times.

    Eddy looked through historical documents dating all the way back to Galileo to find any mention of visual observations of sun spots — everything he found corroborated, though to double check, he looked to some hard data.

    Carbon-14, the radioactive isotope that is associated with living things, correlates with solar activity. The isotope is produced in the upper atmosphere when cosmic rays hit nitrogen-14 and convert it to carbon-14. Increased solar activity reduces the amount of cosmic rays that penetrate the atmosphere, decreasing carbon-14 formation. Eddy determined that the carbon-14 measurements in tree rings indicated a period of lower solar activity from 1450 to 1540, during a period Eddy called the Spörer Minimum.

    In a paper detailing the study published in the journal Science in 1977, Eddy pointed out that both the Maunder Minimum and the Spörer Minimum happened during the coldest intervals of the Little Ice Age.

    The Little Ice Age

    The Little Ice Age saw rapid expansion of mountain glaciers, especially in the Alps, Norway, Ireland and Alaska. There were three cycles of particularly chilly periods, beginning around 1650, 1770 and 1850, each separated by slight warming intervals, according to NASA. Although the Maunder Minimum corresponds with the first of the three cooling periods, the connection between solar activity and terrestrial climate are topics of on-going research, according to NASA. [See Photos of Greenland’s Gorgeous Glaciers]

    Some historical records peg the onset of the Little Ice Age earlier, to around the year 1300, which includes the Spörer Minimum. Records are more robust for the later part of the millennia-long cooling, with figures like Charles Dickens’ writing about white Christmases, and records of Mary Shelly spending an unusually cold summer in 1816 indoors, where she and her husband shared horror stories, one of which became “Frankenstein,” according to climate scientist Michael Mann in Volume 1 of the Encyclopedia of Global Environmental Change (Wiley, 2002).

    “The Little Ice Age may have been more significant in terms of increased variability of the climate, rather than changes in the average climate itself,” Mann wrote. Furthermore, the most dramatic climatic extremes happened with year-to-year temperature changes, rather than prolonged multiyear periods of cold.

    Mann points to atmospheric circulation patterns, like the North Atlantic Oscillation, to explain some of the regional variability during the Little Ice Age. Although the coldest year in Europe and over much of the Northern Hemisphere was 1838, temperatures were relatively mild over significant portions of Greenland and Alaska during the same year. A large volcanic eruption in Cosigüina, Nicaragua, in 1838 may have emitted aerosols that circulated through the atmosphere, deflecting incoming solar radiation and cooling the air.

    Also, Dickens’ white Christmases may have benefited from the 1815 eruption of the volcano Tambora in Indonesia.

    Although solar activities can align with changes in temperatures, there are many processes that contribute to climatic variations, and human-induced climate change will likely prove too big a force for muted solar activity to influence.

    See the full article here.

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  • richardmitnick 2:38 am on February 14, 2015 Permalink | Reply
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    From phys.org: “Getting two for one: ‘Bonus’ electrons in germanium nanocrystals can lead to better solar cells” 


    February 14, 2015
    Ans Hekkenberg

    The material is illuminated with photons. In some of the germanium nanocrystals, the photons cause electrons to be excited, and thus form an electron-hole (e-h) pair. There are two possibilities. (1) The incoming photon has an energy in the range between once and twice the bandgap energy. One e-h pair is formed. (2) The incoming photon has an energy of more than two times the bandgap energy. The excess energy of the electron – the ‘kinetic’ energy of the electron which is excited high up in the conduction band – is sufficient to create a second e-h pair in the same nanocrystal. In that way, carrier multiplication is achieved. Credit: Fundamental Research on Matter (FOM)

    Researchers from FOM, the University of Amsterdam, the Delft University of Technology and the University of the Algarve have discovered that when light hits germanium nanocrystals, the crystals produce ‘bonus electrons’. These additional electrons could increase the yield of solar cells and improve the sensitivity of photodetectors. The researchers will publish their work in Light: Science & Applications today.

    In nanocrystals, the absorption of a single photon can lead to the excitation of multiple electrons: two for one! This phenomenon, known as carrier multiplication, was already well known in silicon nanocrystals. Silicon is the most commonly used material in solar cells. However, the researchers found that carrier multiplication also occurs in germanium nanocrystals, which are more suitable for optimizing the efficiency than silicon nanocrystals. Their discovery could lead to better solar cells.

    Semiconductor physics

    Germanium and silicon are examples of semiconductors: materials that have an energy bandgap. When these materials absorb light, electrons from the band below this energy gap (valence band) leap to the band above the gap (conduction band). These excited ‘hot’ electrons and the holes they leave behind can be harvested to form an electrical current. They form the basic fuel for a solar cell.

    Nanocrystals and carrier multiplication

    If an absorbed photon contains more energy than an electron requires to leap over the bandgap, the excess energy can be used to excite a second electron. Earlier research has shown that a bandgap energy from 0.6 to 1.0 electronvolts is ideal to achieve this carrier multiplication.

    Nanocrystals are extremely small, about a thousand times smaller than the width of a human hair. Due to their size, the energy structure of the crystals is dramatically different from that of bulk material. In fact, the bandgap energy depends on the nanocrystal size. Bulk germanium has an energy bandgap of 0.67 electronvolts. By tuning the germanium nanocrystals’ size, the researchers can change the bandgap energy to values between 0.6 and 1.4 electronvolts. This is within the ideal range for optimizing carrier multiplication, or the amount of ‘bonus electrons’.

    Performing the experiment

    To investigate carrier multiplication in nanocrystals, the researchers used an optical technique called pump-probe spectroscopy. An initial laser pulse, called the pump, emits photons that excite the nanocrystal by creating one free electron in the conduction band. A second pulse of photons, called the probe, can then be absorbed by this electron.

    The researchers found that if the energy of the pump photon is twice the bandgap energy of the germanium nanocrystals, the probe light is absorbed by two electrons instead of one. This effect is the well-known fingerprint of carrier multiplication. In other words, if the pump photon carries sufficient energy, the hot electron contains enough excess energy to excite a second electron in the same nanocrystal. Using this carrier multiplication, germanium nanocrystals can help achieve the maximum efficiency of solar cells.

    See the full article here.

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    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

  • richardmitnick 8:51 am on July 21, 2014 Permalink | Reply
    Tags: , , , , Solar Energy   

    From M.I.T.: “Steam from the sun” 


    July 21, 2014
    Jennifer Chu | MIT News Office

    New spongelike structure converts solar energy into steam.

    A new material structure developed at MIT generates steam by soaking up the sun.


    On the left, a representative structure for localization of heat; the cross section of structure and temperature distribution. On the right, a picture of enhanced steam generation by the DLS structure under solar illumination. Courtesy of the researchers

    The DLS that consists of a carbon foam (10-mm thick) supporting an exfoliated graphite layer (B5-mm thick). Both layers are hydrophilic to promote the capillary rise of water to the surface. Courtesy of the researchers

    The structure — a layer of graphite flakes and an underlying carbon foam — is a porous, insulating material structure that floats on water. When sunlight hits the structure’s surface, it creates a hotspot in the graphite, drawing water up through the material’s pores, where it evaporates as steam. The brighter the light, the more steam is generated.

    The new material is able to convert 85 percent of incoming solar energy into steam — a significant improvement over recent approaches to solar-powered steam generation. What’s more, the setup loses very little heat in the process, and can produce steam at relatively low solar intensity. This would mean that, if scaled up, the setup would likely not require complex, costly systems to highly concentrate sunlight.

    Hadi Ghasemi, a postdoc in MIT’s Department of Mechanical Engineering, says the spongelike structure can be made from relatively inexpensive materials — a particular advantage for a variety of compact, steam-powered applications.

    “Steam is important for desalination, hygiene systems, and sterilization,” says Ghasemi, who led the development of the structure. “Especially in remote areas where the sun is the only source of energy, if you can generate steam with solar energy, it would be very useful.”

    Ghasemi and mechanical engineering department head Gang Chen, along with five others at MIT, report on the details of the new steam-generating structure in the journal Nature Communications.

    Cutting the optical concentration

    Today, solar-powered steam generation involves vast fields of mirrors or lenses that concentrate incoming sunlight, heating large volumes of liquid to high enough temperatures to produce steam. However, these complex systems can experience significant heat loss, leading to inefficient steam generation.

    Recently, scientists have explored ways to improve the efficiency of solar-thermal harvesting by developing new solar receivers and by working with nanofluids. The latter approach involves mixing water with nanoparticles that heat up quickly when exposed to sunlight, vaporizing the surrounding water molecules as steam. But initiating this reaction requires very intense solar energy — about 1,000 times that of an average sunny day.

    By contrast, the MIT approach generates steam at a solar intensity about 10 times that of a sunny day — the lowest optical concentration reported thus far. The implication, the researchers say, is that steam-generating applications can function with lower sunlight concentration and less-expensive tracking systems.

    “This is a huge advantage in cost-reduction,” Ghasemi says. “That’s exciting for us because we’ve come up with a new approach to solar steam generation.”

    From sun to steam

    The approach itself is relatively simple: Since steam is generated at the surface of a liquid, Ghasemi looked for a material that could both efficiently absorb sunlight and generate steam at a liquid’s surface.

    After trials with multiple materials, he settled on a thin, double-layered, disc-shaped structure. Its top layer is made from graphite that the researchers exfoliated by placing the material in a microwave. The effect, Chen says, is “just like popcorn”: The graphite bubbles up, forming a nest of flakes. The result is a highly porous material that can better absorb and retain solar energy.

    The structure’s bottom layer is a carbon foam that contains pockets of air to keep the foam afloat and act as an insulator, preventing heat from escaping to the underlying liquid. The foam also contains very small pores that allow water to creep up through the structure via capillary action.

    As sunlight hits the structure, it creates a hotspot in the graphite layer, generating a pressure gradient that draws water up through the carbon foam. As water seeps into the graphite layer, the heat concentrated in the graphite turns the water into steam. The structure works much like a sponge that, when placed in water on a hot, sunny day, can continuously absorb and evaporate liquid.

    The researchers tested the structure by placing it in a chamber of water and exposing it to a solar simulator — a light source that simulates various intensities of solar radiation. They found they were able to convert 85 percent of solar energy into steam at a solar intensity 10 times that of a typical sunny day.

    Ghasemi says the structure may be designed to be even more efficient, depending on the type of materials used.

    “There can be different combinations of materials that can be used in these two layers that can lead to higher efficiencies at lower concentrations,” Ghasemi says. “There is still a lot of research that can be done on implementing this in larger systems.”

    See the full article here.

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