Power Grid of the Future Saves Energy

Cars and trucks race down the highway, turn off into town, wait at traffic lights and move slowly through side streets. Electricity flows in a similar way -- from the power plant via high voltage lines to transformer substations. The flow is controlled as if by traffic lights. Cables then take the electricity into the city centre. Numerous switching points reduce the voltage, so that equipment can tap into the electricity at low voltage. Thanks to this highly complex infrastructure, the electricity customer can use all kinds of electrical devices just by switching them on.

"A reliable power supply is the key to all this, and major changes will take place in the coming years to safeguard this reliability. The transport and power networks will grow together more strongly as a result of electromobility, because electric vehicles will not only tank up on electricity but will also make their batteries available to the power grid as storage devices. Renewable energy sources will become available on a wider scale, with individual households also contributing electricity they have generated," says Professor Lothar Frey, Director of the Fraunhofer Institute for Integrated Systems and Device Technology IISB in Erlangen.

In major projects such as Desertec, solar thermal power plants in sun-rich regions of North Africa and the Middle East will in the future produce electricity for Europe. The energy will then flow to the consumer via long high-voltage power lines or undersea cables. The existing cables, systems and components need to be adapted to the future energy mix now, so that the electricity will get to the consumer as reliably and with as few losses as possible. The power electronics experts at the IISB are working on technological solutions, and are developing components for the efficient conversion of electrical energy.

For energy transmission over distances of more than 500 kilometers or for undersea cables direct current is being increasingly used. This possesses a constant voltage and only loses up to seven percent of its energy over long distances. By comparison, the loss rate for alternating current can reach 40 percent. Additional converter stations are, however, required to convert the high voltage of the direct current into the alternating current needed by the consumer.

"In cooperation with Siemens Energy we are developing high-power switches. These are necessary for transmitting the direct voltage in the power grid and are crucial for projects like Desertec. The switches have to be more reliable, more scaleable and more versatile than previous solutions in order to meet the requirements of future energy supply networks," says Dipl.-Ing. Markus Billmann from the IISB. To this end, the research scientists are using low-cost semiconductor cells which with previous switching techniques could not be used for high-voltage direct-current transmission (HVDCT).

"At each end of a HVDCT system there is a converter station," explains the research scientist."For the converters we use interruptible devices which can be operated at higher switching frequencies, resulting in smaller systems that are easier to control." A major challenge is to protect the cells from damage. Each converter station will contain about 5,000 modules, connected in series, and if more than a few of them failed at the same time and affected their neighboring modules a chain reaction could be triggered which would destroy the entire station."We have now solved this problem. With our cooperation partners we are working on tailor-made materials and components so that in future the equipment will need less energy," says Billmann.

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Easy Fabrication of Non-Reflecting and Self-Cleaning Silicon and Plastic Surfaces

The most laborious part the fabrication process was excluded when the Aalto University's Microfabrication group developed a novel maskless method for fabrication of pyramid-shaped nanostructures on a silicon surface using deep reactive ion etching. The nanostructured silicon wafer can be further used as a template to create an ealstomeric stamp, which can be used to replicate the original non-reflective and self-cleaning nanostructure into the different polymers.

Smooth silicon surfaces are mirror-like and they reflect more than 50 percent of incoming light, while nanostructured silicon and polymeric surfaces are almost completely non-reflecting. The reflectance is reduced at broad wavelength range due to smooth refractive index transition from air to substrate because of the nanostructures, says Lauri Sainiemi from Microfabrication group.

Non-reflecting surfaces and their fabrication methods are hot research topics because they are needed in realization of more efficient solar cells. Similar nanostructured silicon and polymeric surfaces can also be utilized in chemical analysis, because low reflectance is needed in analysis procedure. The second beneficial property of the surfaces is self-cleaning, which is based on nanostructures, which are coated with a thin low surface energy film.

The applications of the developed nanofabrication methods for silicon and polymers range from sensors to solar cells. The biggest strength of the fabrication methods is their scalability and possibility to large scale industrial manufacturing. I believe that there is interest because our fabrication methods enable simple and low-cost manufacturing of nanostructures on large areas and the methods are compatible with single-crystalline, poly-crystalline and amorphous silicon as well as wide variety of different polymers, concludes Sainiemi.

The group has already developed surfaces for chemical analysis of drugs in collaboration with other research groups and that research will continue in future. An interesting novel field is the development of more effective self-cleaning and dirt-repellant surfaces that would especially benefit solar cell research. The fabrication of water-repellent surfaces is fairly straightforward, but liquids with low surface tension can still contaminate the surface. At the moment we are developing novel surfaces that also repel oily liquids.

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Researchers Aim to Harvest Solar Energy from Pavement to Melt Ice, Power Streetlights

"We have mile after mile of asphalt pavement around the country, and in the summer it absorbs a great deal of heat, warming the roads up to 140 degrees or more," said K. Wayne Lee, URI professor of civil and environmental engineering and the leader of the joint project."If we can harvest that heat, we can use it for our daily use, save on fossil fuels, and reduce global warming."

The URI team has identified four potential approaches, from simple to complex, and they are pursuing research projects designed to make each of them a reality.

One of the simplest ideas is to wrap flexible photovoltaic cells around the top of Jersey barriers dividing highways to provide electricity to power streetlights and illuminate road signs. The photovoltaic cells could also be embedded in the roadway between the Jersey barrier and the adjacent rumble strip.

"This is a project that could be implemented today because the technology already exists," said Lee."Since the new generation of solar cells are so flexible, they can be installed so that regardless of the angle of the sun, it will be shining on the cells and generating electricity. A pilot program is progressing for the lamps outside Bliss Hall on campus."

Another practical approach to harvesting solar energy from pavement is to embed water filled pipes beneath the asphalt and allow the sun to warm the water. The heated water could then be piped beneath bridge decks to melt accumulated ice on the surface and reduce the need for road salt. The water could also be piped to nearby buildings to satisfy heating or hot water needs, similar to geothermal heat pumps. It could even be converted to steam to turn a turbine in a small, traditional power plant.

Graduate student Andrew Correia has built a prototype of such a system in a URI laboratory to evaluate its effectiveness, thanks to funding from the Korea Institute for Construction Technology. By testing different asphalt mixes and various pipe systems, he hopes to demonstrate that the technology can work in a real world setting.

"One property of asphalt is that it retains heat really well," he said,"so even after the sun goes down the asphalt and the water in the pipes stays warm. My tests showed that during some circumstances, the water even gets hotter than the asphalt."

A third alternative uses a thermo-electric effect to generate a small but usable amount of electricity. When two types of semiconductors are connected to form a circuit linking a hot and a cold spot, there is a small amount of electricity generated in the circuit.

URI Chemistry Professor Sze Yang believes that thermo-electric materials could be embedded in the roadway at different depths -- or some could be in sunny areas and others in shade -- and the difference in temperature between the materials would generate an electric current. With many of these systems installed in parallel, enough electricity could be generated to defrost roadways or be used for other purposes. Instead of the traditional semiconductors, he proposes to use a family of organic polymeric semiconductors developed at his laboratory that can be fabricated inexpensively as plastic sheets or painted on a flexible plastic sheet.

"This is a somewhat futuristic idea, since there isn't any practical device on the market for doing this, but it has been demonstrated to work in a laboratory," said Yang."With enough additional research, I think it can be implemented in the field."

Perhaps the most futuristic idea the URI team has considered is to completely replace asphalt roadways with roadways made of large, durable electronic blocks that contain photovoltaic cells, LED lights and sensors. The blocks can generate electricity, illuminate the roadway lanes in interchangeable configurations, and provide early warning of the need for maintenance.

According to Lee, the technology for this concept exists, but it is extremely expensive. He said that one group in Idaho made a driveway from prototypes of these blocks, and it cost about$100,000. Lee envisions that corporate parking lots may become the first users of this technology before they become practical and economical for roadway use.

"This kind of advanced technology will take time to be accepted by the transportation industries," Lee said."But we've been using asphalt for our highways for more than 100 years, and pretty soon it will be time for a change."

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More Efficient Polymer Solar Cells Fabricated

"Our technology efficiently utilizes the light trapping scheme," said Sumit Chaudhary, an Iowa State assistant professor of electrical and computer engineering and an associate of the U.S. Department of Energy's Ames Laboratory."And so solar cell efficiency improved by 20 percent."

Details of the fabrication technology were recently published online by the journalAdvanced Materials.

Chaudhary said the key to improving the performance of solar cells made from flexible, lightweight and easy-to-manufacture polymers was to find a textured substrate pattern that allowed deposition of a light-absorbing layer that's uniformly thin -- even as it goes up and down flat-topped ridges that are less than a millionth of a meter high.

The result is a polymer solar cell that captures more light within those ridges -- including light that's reflected from one ridge to another, he said. The cell is also able to maintain the good electrical transport properties of a thin, uniform light-absorbing layer.

Tests indicated the research team's light-trapping cells increased power conversion efficiency by 20 percent over flat solar cells made from polymers, Chaudhary said. Tests also indicated that light captured at the red/near infrared band edge increased by 100 percent over flat cells.

Researchers working with Chaudhary on the solar cell project are Kai-Ming Ho, an Iowa State Distinguished Professor of Physics and Astronomy and an Ames Laboratory faculty scientist; Joong-Mok Park, an assistant scientist with the Ames Laboratory; and Kanwar Singh Nalwa, a graduate student in electrical and computer engineering and a student associate of the Ames Laboratory. The research was supported by the Iowa Power Fund, the Ames Laboratory and the Department of Energy's Office of Basic Energy Sciences.

The idea of boosting the performance of polymer solar cells by using a textured substrate is not a new one, Chaudhary said. The technology is commonly used in traditional, silicon-based solar cells.

But previous attempts to use textured substrates in polymer solar cells have failed because they require extra processing steps or technically challenging coating technologies. Some attempts produced a light-absorbing layer with air gaps or a too-thin layer over the ridges or a too-thick layer over the valleys. The result was a loss of charges and short circuiting at the valleys and ridges, resulting in poor solar cell performance.

But, get the substrate texture and the solution-based coating just right,"and we're getting more power out," Nalwa said.

The Iowa State University Research Foundation Inc. has filed a patent for the substrate and coating technology and is working to license the technology to solar cell manufacturers.

"This may be an old idea we're using," Chaudhary said,"but it's never before been successfully implemented in polymer solar cells."

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New Ultra-Clean Nanowires Have Great Potential in Solar Cell Technology and Electronics

Nanowires are one-dimensional structures with unique electrical and optical properties -- a kind of building blocks, which researchers use to create nanoscale devices. In recent years, there has been a great deal of research into how nanowires can be used as building blocks in the development of solar cells. One of the challenges is controlling the production of nanowires. The new ultra-clean nanowires are part of the solution. Ultra clean means that the electronic structure is perfectly uniform throughout the nanowires, which is a very important part in obtaining nano-electronic devices of high performance. This is achieved by growing the wires without the use of a metal catalysis like gold, and at the same time having a perfect crystal of only one single structural phase which until now have been impossible for these types of nanowires.

"The ultra-clean wires are grown on a silicon substrate with an extremely thin layer of natural oxide. The element Gallium, which is a part of the nanowire material, reacts with the oxide and makes small holes in the oxide layer, and here the gallium collects into small droplets of a few nanometers in thickness. These droplets capture the element Arsenic -- the other material in the nanowire and through a self-catalytic effect starts the growth of the nanowires without interference from other substances," explains Peter Krogstrup. The breakthrough is the result of a year's work in connection with his PhD.

Control over the cultivation of nanowires

Numerous experiments with different growing conditions have made the researchers wiser to physics behind the formation of the nanowires. A nanowire normally consists of both hexagonal and cubic crystal segments, but the new nanowires only consist of a perfect cubic crystal structure. This means that the path of the electrons through the wire is unaffected and thus suffers less energy loss which leads to a higher efficiency.

"This better understanding of the growing process gives us control over the cultivation of nanowires and the clean wires are the starting point for my current work developing a high efficiency solar cell based on nanowires. With these results we are a good step closer to this goal," explains Peter Krogstrup, pointing out that his nanowires are grown on a silicon substrate.

"The substrate is cheaper than the alternative substrates that many other researchers use. It is important because ultimately it is about getting as much energy as possible for as little cost as possible," explains Peter Krogstrup, whose research is conducted in collaboration with the company SunFlake A/S, which is located at the Nano-Science Center at the University of Copenhagen. The company is working to develop the solar cells of the future based on the nanostructures of Gallium and Arsenic.

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A New Twist for Nanopillar Light Collectors

"By tuning the shape and geometry of highly ordered nanopillar arrays of germanium or cadmium sulfide, we have been able to drastically enhance the optical absorption properties of our nanopillars," says Ali Javey, a chemist who holds joint appointments with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley.

Javey, a faculty scientist with Berkeley Lab's Materials Sciences Division and a UC Berkeley professor of electrical engineering and computer science, has been at the forefront of nanopillar research. He and his group were the first to demonstrate a technique by which cadmium sulfide nanopillars can be mass-produced in large-scale flexible modules. In this latest work, they were able to produce nanopillars that absorb light as well or even better than commercial thin-film solar cells, using far less semiconductor material and without the need for anti-reflective coating.

Theoretical and experimental works have shown that 3-D arrays of semiconductor nanopillars -- with well-defined diameter, length and pitch -- excel at trapping light while using less than half the semiconductor material required for thin-film solar cells made of compound semiconductors, such as cadmium telluride, and about one-percent of the material used in solar cells made from bulk silicon. But until the work of Javey and his research group, fabricating such nanopillars was a complex and cumbersome procedure.

Javey and his colleagues fashioned their dual diameter nanopillars from molds they made in 2.5 millimeter-thick alumina foil. A two-step anodization process was used to create an array of one micrometer deep pores in the mold with dual diameters -- narrow at the top and broad at the bottom. Gold particles were then deposited into the pores to catalyze the growth of the semiconductor nanopillars.

"This process enables fine control over geometry and shape of the single-crystalline nanopillar arrays, without the use of complex epitaxial and/or lithographic processes," Javey says."At a height of only two microns, our nanopillar arrays were able to absorb 99-percent of all photons ranging in wavelengths between 300 to 900 nanometers, without having to rely on any anti-reflective coatings."

The germanium nanopillars can be tuned to absorb infrared photons for highly sensitive detectors, and the cadmium sulfide/telluride nanopillars are ideal for solar cells. The fabrication technique is so highly generic, Javey says, it could be used with numerous other semiconductor materials as well for specific applications. Recently, he and his group demonstrated that the cross-sectional portion of the nanopillar arrays can also be tuned to assume specific shapes -- square, rectangle or circle -- simply by changing the shape of the template.

"This presents yet another degree of control in the optical absorption properties of nanopillars," Javey says.

Javey's dual-diameter nanopillar research was partially funded through the National Science Foundation's Center of Integrated Nanomechanical Systems (COINS) and through Berkeley Lab LDRD funds.

A paper describing this research appears on-line in the journalNANO Lettersunder the title"Ordered Arrays of Dual-Diameter Nanopillars for Maximized Optical Absorption." Co-authoring the paper with Javey were Zhiyong Fan, Rehan Kapadia, Paul Leu,Xiaobo Zhang, Yu-Lun Chueh, Kuniharu Takei, Kyoungsik Yu, Arash Jamshidi, Asghar Rathore, Daniel Ruebusch and Ming Wu.

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