Compact and flexible thermal storage

Engineerblogger
June 8, 2012

These zeolite pellets can bind steam within their pores, generating heat.  © Fraunhofer IGB

Biogas plants, combined heat and power plants don’t just generate electricity, they also produce heat. However, unlike the electricity they yield, the heat generally dissipates unused. A new technology is set to change this: It will allow the heat to be stored lossfree in the smallest of spaces for lengthy periods of time, for use as and when required.

There’s a growing trend towards generating electricity from biogas. But these systems would be considerably more effective if better use could be made of the heat that is produced in the process. Roughly half of the total energy content of the fuel is released as heat, which typically dissipates into the atmosphere unused. Large quantities of heat likewise escape from combined heat and power plants, not to mention many industrial installations. The root of the problem lies in the fact that the heat is not generally used at the time it is generated – and options for storing it are limited. Traditionally, water tanks have been used for this purpose, but they can only absorb a finite quantity of heat. And of course, the heat can only be stored for short periods of time, because although the water tanks are insulated, the water gradually loses its heat to the surrounding atmosphere.

Working together with industrial partners such as ZeoSys GmbH in Berlin, scientists from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart are currently developing a new type of thermal storage system. This new system can store three to four times the amount of heat that water can, so it only requires storage containers around a quarter the size of water tanks. Moreover, it is able to store the heat loss-free over lengthy periods of time and can even operate at temperatures well in excess of 100 degrees Celsius. The new system contains zeolite pellets, from the Greek zeō, meaning ‘boil’ and lithos, meaning ‘stone’. Normally this material is used as an ion exchanger, for example to soften water. Because zeolites are porous, they have a huge surface area: A single gram of these pellets boasts a surface area of up to 1000 square meters. When the material comes into contact with water vapor, it binds the steam within its pores by means of a physicochemical reaction, which generates heat. The water is in reverse removed from the material by the application of heat and the energy is stored, but not as a result of the material becoming palpably warm – as when water tanks are used. What is stored is the potential to adsorb water and in the process release heat; the term ‘sorptive thermal storage’ is frequently used to describe these systems. And provided the dried zeolite material is prevented from coming into contact with water, it can store the heat for an unlimited amount of time.

Mobile test facility with a storage volume of 750 liters

Although the basic principle has been widely understood for some time, it had never before been translated into a broad-based technical application for storage systems. “We took the principle and confirmed it was technically feasible,” says Mike Blicker, group manager, heat and sorption systems in the IGB. Initially, the researchers used a 1.5- and then a 15-liter reactor to demonstrate that the process really does work. Blicker explains: “First we developed the process engineering, then we looked around to see how we could physically implement the thermal storage principle – i.e. how a storage device has to be constructed, and at which locations heat exchangers, pumps and valves are needed.” The institute’s development partners were responsible for the material testing side of the project, investigating which of the various zeolites would be best suited for the purpose, how big the zeolite pellets needed to be, and whether or not the material would remain stable even after numerous storage cycles. They proved that heat could be stored and discharged many thousands of times without the system showing significant signs of wear and tear. The researchers subsequently up-scaled their operations to the current test facility, which has a storage volume of 750 liters and is mounted in a transportable container, along with all the additional equipment it requires. Its mobility allows the scientists to test the system in a variety of locations under realistic conditions.

The next stage of their work will be to reduce production costs, further optimize the system and adapt it for a variety of applications. Ultimately, the goal is to be able to store heat both in industrial installations and in small combined heat and power plants such as those used in larger residential buildings. To start with, priority will be given to industrial applications. “It would be ideal if we were able to devise a modular system that would allow us to construct each storage device to suit the individual requirement,” says Blicker. The Fraunhofer researchers will be using a model system to demonstrate the principles of sorptive thermal storage at ACHEMA 2012 in Frankfurt from June 18 through 22.

Source: Fraunhofer-Gesellschaft

Cheaper LED Lightbulbs Are on the Way: Silicon backing should make high-efficiency lighting more affordable.

Engineerblogger
June 8, 2012

Cheaper on silicon: Osram has made high-performance white LEDs by growing gallium-nitride on 15-centimeter silicon wafers, as seen here. Credit: Osram

Cost has been a major barrier in keeping people from buying energy-efficient LED lightbulbs. While they last much longer than $1 incandescent bulbs or $4 compact fluorescents, their sticker-price is daunting: a 60-watt equivalent LED bulb costs $15 to $25.

Now one of the world's largest LED makers, Osram Opto Semiconductors, says it has perfected a technique that could significantly cut the production cost of LEDs.

White LEDs are typically made by coating blue gallium-nitride LEDs with yellow phosphors. Manufacturers normally grow the gallium-nitride in thin layers on top of costly sapphire substrates. Osram is making the devices on silicon substrates instead. Silicon substrates cost a third as much as sapphire and could get even cheaper, since they're made in larger pieces.

The company says its silicon-based white LEDs produce 127 lumens for each watt of power, with a power efficiency of 58 percent, comparable to state-of-the-art commercial LEDs grown on sapphire. Peter Stauss, a project manager at Osram, says researchers are now testing and optimizing the devices, and expect to start selling them in the next two to three years.

Osram joins a few other companies in the race to bring cheaper gallium-nitride-on-silicon LEDs to market. China's Lattice Power claims to have already started commercial production using the same technique. Last month, California startup Bridgelux announced that it has teamed up with Toshiba to make LEDs on silicon. Meanwhile, U.K.-based Plessey Semiconductors plans to produce gallium-nitride-on-silicon LEDs by the end of this year using technology acquired from Cambridge University spinoff CamGaN. Philips and Samsung are also said to be pursuing the silicon approach.

Osram has not said how much cheaper its new LEDs will be, but Bridgelux and Plessey both claim that the silicon approach could cut LED production costs by 75 percent or more. Bridgelux also predicts that its process could bring the cost of a 75-watt equivalent LED lightbulb, which now costs $40, down to under $5.

Compared to sapphire, it's easier and cheaper to make large-diameter silicon wafers, which cuts cost. Another appealing reason to switch to silicon is to take advantage of existing machines that are used to fabricate silicon computer chips. Bridgelux has already demonstrated devices grown on 20-centimeter-wide silicon wafers.

The challenge with growing gallium-nitride on silicon is that the two materials expand and contract at different rates. LEDs are made at high temperatures, and when the materials are cooled, the light-emitting gallium-nitride layer cracks because of tension with the silicon underneath. To get around this problem, Osram and others grow additional thin films of materials such as aluminum gallium-nitride around the gallium-nitride, balancing out the tension produced during cooling.

While many LED makers are opting for silicon, California startup Soraa is using gallium-nitride substrates. This also reduces the mismatch in the crystal structure between the two layers, says Soraa CTO Michael Krames, resulting in a material with 1,000 times fewer defects than that grown on silicon. The resulting LEDs are more expensive than silicon-backed ones, but they are also 10 times brighter, which helps make up for the extra cost. "It's really the performance of the LED that drives the overall cost of the [lightbulb]," Krames says. "Also, the substrate is a small part of the overall product cost."

Right now though, Stauss says, the cost advantage of silicon substrates is more favorable to LED manufacturers. "Gallium-nitride substrates might be a choice when they are much cheaper, but today there's no real opportunity in using them," he says.

Steve Denbaars, a professor of materials and electrical and computer engineering at the University of California, Santa Barbara, and a cofounder of Soraa, believes that both gallium-nitride and silicon substrates have a fair shot at replacing sapphire. While gallium-nitride might have a performance edge as well, he says, "there is a chance silicon will beat sapphire solely because you have a lot of silicon [fabrication facilities] sitting around the world."

Source: Technology Review

Photovoltaic Cells Tap Underwater Solar Energy

Engineerblogger
June 8, 2012

Power density of GaInP and crystalline silicon cells, underwater, as a function of depth. (U.S. Naval Research Laboratory) 

Scientists at the U.S. Naval Research Laboratory, Electronics Science and Technology Division, dive into underwater photovoltaic research to develop high bandgap solar cells capable of producing sufficient power to operate electronic sensor systems at depths of 9 meters.

Underwater autonomous systems and sensor platforms are severely limited by the lack of long endurance power sources. To date, these systems must rely on on-shore power, batteries or solar power supplied by an above water platform. Attempts to use photovoltaics have had limited success, primarily due to the lack of penetrating sunlight and the use of solar cells optimized more towards the unimpeded terrestrial solar spectrum.

"The use of autonomous systems to provide situational awareness and long-term environment monitoring underwater is increasing," said Phillip Jenkins, head, NRL Imagers and Detectors Section. "Although water absorbs sunlight, the technical challenge is to develop a solar cell that can efficiently convert these underwater photons to electricity."

Even though the absolute intensity of solar radiation is lower underwater, the spectral content is narrow and thus lends itself to high conversion efficiency if the solar cell is well matched to the wavelength range. Previous attempts to operate solar cells underwater have focused on crystalline silicon solar cells and more recently, amorphous silicon cells.

High-quality gallium indium phosphide (GaInP) cells are well suited for underwater operation. GaInP cells have high quantum efficiency in wavelengths between 400 and 700 nanometers (visible light) and intrinsically low dark current, which is critical for high efficiency in lowlight conditions.

The filtered spectrum of the sun underwater is biased toward the blue/green portion of the spectrum and thus higher bandgap cells such as GaInP perform much better than conventional silicon cells, states Jenkins.

Preliminary results at a maximum depth of 9.1 meters reveal output to be 7 watts per square meter of solar cells, sufficient to demonstrate there is useful solar power to be harvested at depths commonly found in nearshore littoral zones.

Source: U.S. Naval Research Laboratory

Researchers produce 3-D configurations that could lead to new microchips and other devices

Engineerblogger
June 8, 2012

Researchers at MIT have found a new way of making complex three-dimensional structures using self-assembling polymer materials that form tiny wires and junctions. The work has the potential to usher in a new generation of microchips and other devices made up of submicroscopic features.

Researchers at MIT have found a new way of making complex three-dimensional structures using self-assembling polymer materials that form tiny wires and junctions. The work has the potential to usher in a new generation of microchips and other devices made up of submicroscopic features.

Although similar self-assembling structures with very fine wires have been produced before, this is the first time the structures have been extended into three dimensions with different, independent configurations on different layers, the researchers say. The research is published this week in the journal Science.

Caroline Ross, the Toyota Professor of Materials Science and Engineering at MIT, says there has been “a lot of interest” among semiconductor researchers in finding ways to produce chip features that are much narrower than the wavelength of light — and hence narrower than what can be achieved using present light-based fabrication systems. Self-assembly based on polymers has been an active area of research, Ross says, but “what we did in this paper was push it into the third dimension.”

She and her colleagues began by creating an array of tiny posts on a substrate of silicon; they then coated the surface with materials called block copolymers, which have a natural tendency to assemble into long cylindrical structures. By carefully controlling the initial spacing of the posts, Ross explains, the researchers were able to set the spacing, angles, bends and junctions of the cylinders that form on the surface. What’s more, she says, “Each of the two layers of cylinders can be independently controlled using these posts,” making it possible to create complex 3-D configurations.

Amir Tavakkoli, a visiting graduate student from the National University of Singapore and lead author of the Science paper, says many researchers have tried to produce complex arrangements of nanoscale wires through self-assembly. But earlier attempts used complex processes with many steps, and had failed to control the resulting configurations well. The new system is simpler, Tavakkoli says, and “not only controlled the alignment of the wires, but showed we can even have sharp bends and junctions” at precisely determined locations.

“It wasn’t expected to be possible,” says MIT graduate student Kevin Gotrik. “It was a surprising result. We stumbled upon it, and then had to figure out how it works.”

There were a number of barriers to overcome in making the system practical, Gotrik says. For example, the posts fabricated on the surface are the key to controlling the whole self-assembly process, but they need to be quite a bit taller than they are wide, which could lead some to topple over; the MIT team ultimately found materials and shapes that would be stable. “We explored a wide range of conditions,” Gotrik says.

Graduate student Adam Hannon says the team used computer simulations of the structures in order to explore the effects of different post configurations on the double-layer 3-D structure. These simulations were compared with the most promising structures observed in the laboratory to get greater insight into how to control the resulting structures that formed.

So far, the MIT team has only produced two-layer configurations, but Alfredo Alexander-Katz, an assistant professor of materials science and engineering, says, “I think it would be feasible to go to three layers” while still maintaining full control over the arrangement of structures on each layer.

A key enabling technology was the MIT lab’s capability, using electron-beam lithography, to make 10-nanometer-wide cylindrical posts with precisely controlled positioning. These posts, in turn, guide the positioning of the self-assembling cylinders. Karl Berggren, an associate professor of electrical engineering, says it’s as if the lithography puts down an array of pillars, and these pillars then control the complex, multilevel routing of crisscrossing highways.

In earlier work, the MIT researchers had demonstrated that this self-assembly method could be used to create wires that are much finer than those that can be made by existing photolithography techniques for producing microchips — and thus help lead the way to next-generation devices that pack even more wires and transistors into a given area of silicon chip material. “In principle, this is scalable to quite small dimensions,” Ross says, far smaller than the 15-nanometer width of the cylinders produced so far — which is already less than half the width of the finest wires in existing microchips.

The basic technologies involved are compatible with existing manufacturing equipment in the semiconductor industry, the researchers say. But this is basic research that is probably still far from actual chip production, they caution. Within the next year the team hopes to use this methodology to produce a simple electronic device.

The technique is not limited to producing wires on a silicon chip, Ross and her colleagues say. The same method could be used to create 3-D arrays of other kinds of materials — such as proteins or DNA molecules, for example — in order to create biological detectors or drug-delivery systems.

Craig Hawker, a professor of chemistry and biochemistry at the University of California at Santa Barbara, says this is a “far-reaching finding,” which “goes a long way to fulfilling the demands of the International Technology Roadmap for Semiconductors, which calls for a robust, commercially viable nanopatterning technique.”

Hawker adds, “The robustness and power of this approach may also lead to applications outside lithography and microelectronics, with impact in water purification, membranes and organic photovoltaics.” He says this work is “a spectacular example of multidisciplinary work, with advances in chemistry, physics and nanotechnology seamlessly combined to address a critical technological and important societal problem.”

The work was supported by the Semiconductor Research Corporation, the FENA Center, the Nanoelectronics Research Initiative, the Singapore-MIT Alliance, the National Science Foundation, Tokyo Electron and Taiwan Semiconductor Manufacturing Company.

Source: MIT

Researcher develops durable plastic that may replace metals: Steel-Strength Plastics -- and Green, Too!

Engineerblogger
June 8, 2012

As landfills overflow with discarded plastics, scientists have been working to produce a biodegradable alternative that will reduce pollution. Now a Tel Aviv University researcher is giving the quest for environmentally friendly plastics an entirely new dimension — by making them tougher than ever before.

Prof. Moshe Kol of TAU's School of Chemistry is developing a super-strength polypropylene — one of the world's most commonly used plastics — that has the potential to replace steel and other materials used in everyday products. This could have a long-term impact on many industries, including car manufacturing, in which plastic parts could replace metallic car parts.

Durable plastics consume less energy during the production process, explains Prof. Kol. And there are additional benefits as well. If polypropylene car parts replaced traditional steel, cars would be lighter overall and consume less fuel, for example. And because the material is cheap, plastic could provide a much more affordable manufacturing alternative.

His research has been published in the journal Angewandte Chemie.

Better building blocks

Although a promising field of research, biodegradable plastics have not yet been able to mimic the durability and resilience of common, non-biodegradable plastics like polypropylene. Prof. Kol believes that the answer could lie in the catalysts, the chemicals that enable their production.

Plastics consist of very long chains called polymers, made of simple building blocks assembled in a repeating pattern. Polymerization catalysts are responsible for connecting these building blocks and create a polymer chain. The better the catalyst, the more orderly and well-defined the chain — leading to a plastic with a higher melting point and greater strength and durability. This is why the catalyst is a crucial part of the plastic production process.

Prof. Kol and his team of researchers have succeeded in developing a new catalyst for the polypropylene production process, ultimately producing the strongest version of the plastic that has been created to date. "Everyone is using the same building blocks, so the key is to use different machinery," he explains. With their catalyst, the researchers have produced the most accurate or "regular" polypropylene ever made, reaching the highest melting point to date.

Using resources more efficiently

By 2020, the consumption of plastics is estimated to reach 200 million tons a year. Prof. Kol says that because traditional plastics aren't considered green, it's important to think creatively to develop this material, which has become a staple of daily life, with the least amount of harm to the environment. Cheaper and more efficient to produce in terms of energy consumption, as well as non-toxic, Prof. Kol's polypropylene is good news for green manufacturing and could revolutionize the industry. The durability of the plastic results in products that require less maintenance — and a much longer life for parts made from the plastic.

Beyond car parts, Prof. Kol envisions a number of uses for this and related plastics, including water pipes, which he says could ultimately conserve water use. Drinking water for the home has been traditionally carried by steel and cement pipes. These pipes are susceptible to leakage, leading to waste and therefore higher water bills. But they are also very heavy, so replacing them can be a major, expensive operation.

"Plastic pipes require far fewer raw materials, weighing ten times less than steel and a hundred times less than cement. Reduced leaking means more efficient water use and better water quality," Prof. Kol explains. The replacement of steel water pipes by those made of plastic is becoming more common, and the production of plastics with even greater strength and durability will make this transition even more environmentally-friendly.

Prof. Kol holds the Bruno Landesberg Chair in Green Chemistry at TAU.

Source: Tel Aviv University

‘Armored Caterpillar’ Could Inspire New Body Armor

Engineerblogger
June 8, 2012

A mantis shrimp, which has a fist-like club that can accelerate underwater faster than a 22-caliber bullet. Photo credit: Silke Baron

Unique structure of fist-like club of mantis shrimp could transform materials used to create military body armor and vehicle and aircraft frames.

Military body armor and vehicle and aircraft frames could be transformed by incorporating the unique structure of the club-like arm of a crustacean that looks like an armored caterpillar, according to findings by a team of researchers at the University of California, Riverside’s Bourns College of Engineering and elsewhere published online today, June 7, in the journal Science.

The bright orange fist-like club of the mantis shrimp, or stomatopod, a 4-inch long crustacean found in tropical waters, accelerates underwater faster than a 22-caliber bullet. Repeated blows can destroy mollusk shells and crab exoskeletons, both of which have been studied for decades for their impact-resistant qualities.

The power of the mantis shrimp is exciting, but David Kisailus, an assistant professor at the Bourns College of Engineering, and his collaborators, were interested in what enabled the club to withstand 50,000 high-velocity strikes on prey during its lifespan. Essentially, how does something withstand 50,000 bullet impacts?

They found that the club is a highly complex structure, comprised of three specialized regions that work together to create a structure tougher than many engineered ceramics.

The first region, located at the impacting surface of the club, contains a high concentration of mineral, similar to that found in human bone, which supports the impact when the mantis shrimp strikes prey. Further inside, highly organized and rotated layers of chitin (a complex sugar) fibers dispersed in mineral act as a shock absorber, absorbing energy as stress waves pass through the club. Finally, the club is encapsulated on its sides by oriented chitin fibers, which wrap around the club, keeping it intact during these high velocity impacts.

“This club is stiff, yet it’s light-weight and tough, making it incredibly impact tolerant and interestingly, shock resistant,” Kisailus said. “That’s the holy grail for materials engineers.”

Kisailus said the potential applications in structural materials are widespread because the final product could be lighter weight and more impact resistant than existing products.

For example, with electric cars less weight will reduce power consumption and increase driving range. With airplanes, less weight would reduce fuel costs and better impact resistance would improve reliability and cut repair bills.

But Kisailus is primarily focused on improving military body armor, which can add 30 pounds to a service member’s load. His goal is to develop a material that is one-third the weight and thickness of existing body armor.

Kisailus and James C. Weaver, who worked with Kisailus as a post-doctoral scholar and is now at Harvard University, began work on the mantis shrimp when Kisailus arrived at UC Riverside in 2007. They were later joined at UC Riverside by Garrett W. Milliron, a Ph.D. student, and Steven Herrera, an undergraduate student.

Kisailus, who studies the structures of marine animals for inspiration to develop new materials, has also worked with snails such as the abalone and chiton, as well as sea urchin.

Those animals were all studied for their defensive prowess, in other words their exterior protection from predators. The club of the mantis shrimp interested Kisailus because it’s an offensive tool.

“We have been studying these other organisms when we should have been studying this guy because he literally eats them for breakfast,” Kisailus said.

The force created by the mantis shrimp’s impact is more than 1,000 times its own weight. It’s so powerful that Kisailus needs to keep it in a special aquarium in his lab so it doesn’t break the glass.

Also, the acceleration of the club creates cavitation, meaning it shears the water, literally boiling it, forming cavitation bubbles that implode, yielding a secondary impact on the mantis shrimp’s prey.

Kisailus and Pablo Zavattieri, of Purdue University, one of the co-authors of Science paper, just received additional $590,000 in funding from the Air Force Office of Scientific Research to continue work on the stomatopod. They want to further understand the structure of club and continue work designing materials inspired by that structure.

Since this project was multidisciplinary, Kisailus and his UC Riverside team continued working with Weaver after he moved on to Harvard University, and others including Ali Miserez, Nanyang Technical University in Singapore; Kenneth Evans-Lutterodt and Elaine DiMasi, Brookhaven National Laboratory in Upton, NY; and Brook Swanson, Gonzaga University.

“The team we put together was excellent: having experts in zoology, mechanics, modeling and synchrotron x-ray characterization gave us multiple views of the same problem, making it a very thorough investigation,” Kisailus said.

Source: University of California, Riverside

Additional Information:

• The Stomatopod Dactyl Club: A Formidable Damage-Tolerant Biological Hammer

New nanomaterials method answers tough challenges

Engineerblogger
June 8, 2012

This image shows channels etched using sequential infiltration synthesis, which scientists at Argonne have used to create features that have high aspect ratios – that is, they are far deeper than wide. These crevasses will permit the creation of a new generation of semiconducting materials.

When searching for the technology to boost computer speeds and improve memory density, the best things come in the smallest packages.

A relentless move toward smaller and more precisely defined semiconductors has prompted researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory to develop a new technique that can dramatically improve the efficiency and reduce the cost of preparing different classes of semiconducting materials.

The new discovery meets certain requirements of the international semiconductor “roadmap” all the way out to 2022—leapfrogging an anticipated ten years of progress with a single set of experiments.

Most semiconductor patterns are currently made using a process known as photolithography, in which portions of a thin film are selectively removed to create a pattern. The pattern in this film, known as a resist, is etched into the semiconductor by exposure to an ionized gas. This gas also etches away the resist itself, reducing the number of times the film can be used. Especially durable resists are known as hard masks.

The drive to create smaller and smaller semiconductor components is often limited by a phenomenon known as domain collapse, said Argonne nanoscientist Seth Darling. Conventional lithography -- the technique used to make patterns in materials -- attempts to create features that are separated like the teeth of a comb. However, gaps in the resist that are too deep tend to collapse inwards, which makes the material useless.

“Engineers have tried many ways of avoiding this collapse, but the industry is constantly running up against it,” Darling said.

In 2010, Darling and his colleagues developed a technique known as sequential infiltration synthesis (SIS), which used gases to grow hard inorganic materials inside a soft polymer film. The work was supported by the DOE Office of Science through Argonne’s Center for Nanoscale Materials and the Argonne-Northwestern Solar Energy Research Center.

One of the most notable benefits of SIS is that it eliminates the need for hard masks in photolithography, according to Darling. “Hard masks are a real pain when it comes to semiconductor processing – they’re expensive, complicated, reduce pattern quality and add extra steps,” he said.

According to Darling, sequential infiltration synthesis has already been identified by leading semiconductor companies as a technology with the potential to overcome several different limitations.

In a recent experiment, Darling and his Argonne colleagues showed that SIS can actually eliminate pattern collapse, enabling the fabrication of materials that have patterns with higher “aspect ratios,” which measures the height of a feature divided by its width.

Generally speaking, lithography seeks to create patterns with higher aspect ratios while using as little resist as possible. “Usually, you need a certain thickness of the resist in order for the process to work,” Darling said. “This new process enables us to do away with a lot of that problem.”

“One of the biggest advantages of this new study is that we’ve shown the possibility of using SIS for photolithography, which is one of the most industrially important processes,” Darling said. “As there’s more and more demand for better electronics, the sizes of these semiconductors need to keep getting smaller and smaller, and it becomes that much more important for us to meet and exceed the benchmarks that we’ve set for ourselves.”

Source: Argonne National Laboratory

Additional Information:

• A paper based on the research appeared online in the April 10 issue of Advanced Materials and can be found online at http://onlinelibrary.wiley.com/doi/10.1002/adma.201104871/abstract.