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Posts Tagged ‘Karlsruhe Institute of Technology’

Wednesday, September 30, 2015 @ 04:09 PM gHale

Success of the energy turnaround will depend decisively on the extended use of renewable energy sources.

However, their efficiency is much smaller than that of conventional energy sources. The efficiency of commercially available photovoltaic cells, for instance, is about 20 percent. Having said that, an unconventional approach can increase the efficiency of the panels, said scientists of Karlsruhe Institute of Technology (KIT).

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Optical invisibility cloaks can guide sunlight around objects that cast a shadow on the solar panel.

Energy efficiency of solar panels has to hike significantly not only for the energy turnaround, but for enhancing economic efficiency. Modules mounted on roofs convert just one fifth of the light into electricity, which means about 80 percent of the solar energy is lost.

There are plenty of reasons of these high losses. Up to one tenth of the surface area of solar cells, for instance, remains covered by contact fingers that extract the current generated. At the locations of these contact fingers, light cannot reach the active area of the solar cell and efficiency of the cell decreases.

“Our model experiments have shown that the cloak layer makes the contact fingers nearly completely invisible,” said doctoral student Martin Schumann of the KIT Institute of Applied Physics, who conducted the experiments and simulations. Physicists of KIT around project head Carsten Rockstuhl, together with partners from Aachen, Freiburg, Halle, Jena, and Jülich, modified the optical invisibility cloak designed at KIT for guiding the incident light around the contact fingers of the solar cell.

Normally, invisibility cloak research aims at making objects invisible. For this purpose, light guides around the object that needs hiding. This research project did not focus on hiding the contact fingers visually, but on the deflected light that reaches the active surface area of the solar cell thanks to the invisibility cloak.

To achieve the cloaking effect, the scientists pursued two approaches.

Both approached end up based on applying a polymer coating onto the solar cell. This coating has to possess exactly calculated optical properties, i.e. an index of refraction that depends on the location or a special surface shape. The second concept is promising, as it can potentially integrate into mass production of solar cells at low costs. The surface of the cloak layer ends up grooved along the contact fingers. In this way, incident light refracts away from the contact fingers and finally reaches the active surface area of the solar cell.

By means of a model experiment and detailed simulations, the researchers found the two concepts can hide the contact fingers. In the next step, the researchers planned to apply the cloaking layer onto a solar cell in order to determine the efficiency increase. The physicists are optimistic that efficiency will improve by the cloak under real conditions.

“When applying such a coating onto a real solar cell, optical losses via the contact fingers are supposed to be reduced and efficiency is assumed to be increased by up to 10 percent,” Schumann said.

Monday, January 16, 2012 @ 02:01 PM gHale

For the first time, a superconducting current limiter is now working at a power plant that could help enhance intrinsic safety of the grid.

At the Boxberg power plant of Vattenfall, Germany, the current limiter, based on YBCO strip conductors, protects the grid against damage due to short circuits and voltage peaks. The new technology, co-developed by Karlsruhe Institute of Technology and made by Nexans SuperConductors, enhances the intrinsic safety of the grid and may help reduce the investment costs of plants.

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“For a long time, high-temperature superconductors were considered to be difficult to handle, too brittle, and too expensive for general industrial applications,” said project manager Wilfried Goldacker from Karlsruhe Institute of Technology. “The second generation of high-temperature superconductor wires based on YBCO ceramics is much more robust. Properties have been improved.”

Superconducting current limiters work reversibly. In case of current peaks after short circuits in the grid, no components end up destroyed. The limiter automatically returns to the normal state of operation after a few seconds. Consequently, the power failure is much shorter than in case of conventional current limiters, such as household fuses, whose components usually end up ruined and you have to replace them.

“Superconducting current limiters have a number of advantages for the stability of medium- and high-voltage grids,” said Mathias Noe, head of the Institute of Technical Physics of Karlsruhe Institute of Technology.

Reliable, compact current limiters enhance the operation stability of power grids and allow for a simplification of the grid structure. They end up protected against current peaks. In addition, decentralized energy generators, such as wind and solar systems, can integrate quite a bit easier into rids. Expensive components in the existing grid enjoy greater protection. In the future, components can undergo a design for smaller peak currents, and transformers will no longer be necessary. Investment costs of power plants and grids will be lower. Superconducting current limiters on the basis of YBCO can also apply to high-voltage grids of more than 100 kilovolts for better protection against power failures in the future.

YBCO stands for the constituents of the superconductor: Yttrium, barium, copper, and oxygen. An YBCO crystal layer of about 1 micrometer in thickness grows directly on a stainless steel strip of a few millimeters in width that gives the ceramics the necessary stability.

Below a temperature of 90° Kelvin or minus 183° Celsius, the material becomes superconductive. However, superconductivity collapses abruptly when the current in the conductor exceeds the design limits. This effect sees use by the current limiter. In case of current peaks in the grid, the superconductor loses its conductivity within fractions of a second and the current will flow through the stainless steel strip only, which has a much higher resistance and, thus, limits the current. The heat ends up removed by the cooling system of the superconductor. A few seconds after the short circuit, it returns to normal operation in the superconducting state. YBCO superconducting layers on stainless steel strips are more stable and operation-friendly than first-generation superconductors based on BSCCO ceramics. Moreover, their production does not require any noble metals, such as silver, and cost much less.

A field test is underway at the Vattenfall utility company.

Tuesday, December 20, 2011 @ 02:12 PM gHale

Talk about the ultimate physical security program, the invisibility cloak is becoming a reality thanks to progress made in metamaterials in nanotechnologies.

Technology allows for light waves to go around an object in such a way the object appears to be non-existent. This concept applied to electromagnetic light waves may also transfer to other types of waves, such as sound waves.

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Researchers from Karlsruhe Institute of Technology (KIT) have now succeeded in demonstrating for the first time an invisibility cloak for elastic waves. Such waves also occur in strings of a guitar or drum membranes.

“The key to controlling waves is to specifically influence their local speed as a function of the ‘running direction’ of the wave,” said Dr. Nicolas Stenger from the Institute of Applied Physics (AP).

In his experiment, he used a smartly microstructured material composed of two polymers: A soft and a hard plastic in a thin plate. The vibrations of this plate are in the range of acoustic frequencies at 100 Hz, which you can see directly from above.

The scientists found they can guide the sound around a circular area in the millimeter-thin plate in such a way that vibrations can neither enter nor leave this area.

“Contrary to other known noise protection measures, the sound waves are neither absorbed nor reflected,” said Professor Martin Wegener from the Institute of Applied Physics and coordinator of the DFG Center for Functional Nanostructures (CFN) at KIT. “It is as if nothing was there.”

The scientists explain their idea by the following story: A city, in the shape of a circle, suffers from noisy car traffic through its center. Finally, the mayor has the idea to introduce a speed limit for cars that drive directly toward the city: The closer the cars come to the city area, the slower they have to drive. At the same time, the mayor orders to build circular roads around the city, on which the cars can drive at higher speeds. The cars can approach the city, drive around it, and leave it in the same direction in the end. The time required corresponds to the time needed without the city. From outside, it appears as if the city was not there.

Wednesday, May 25, 2011 @ 03:05 PM gHale

“Seeing something invisible with your own eyes is an exciting experience,” said Physicist Joachim Fischer.

For about one year, Fischer and fellow Physicist Tolga Ergin have worked on refining the structure of the Karlsruhe Institute of Technology (KIT) invisibility cloak to such an extent that it is also effective in the visible spectral range.

In invisibility cloaks, light waves receive guidance by the material so they leave the invisibility cloak again as if they had never been in contact with the disguised object. Consequently, the object is invisible to the observer. The exotic optical properties of the camouflaging material are the result of using complex mathematical tools similar to Einstein’s theory of relativity.

These properties result from a special structuring of the material. It has to be smaller than the wavelength of the deflected light. For example, the relatively large radio or radar waves require a material “that can be produced using nail scissors,” said Professor Martin Wegener at KIT’s Center for Functional Nanostructures (CFN). At wavelengths visible to the human eye, materials have to be in the nanometer range.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These “logs” have precisely defined thicknesses in the range of 100 nm. These logs influence and guide the light waves normally deflected by the curvature so the reflected light corresponds to that of a flat mirror.

“If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum,” Fischer said.

Last year, the Wegener team presented the first 3D invisibility cloak. Until then, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When looking onto the structure from the third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could occur at wavelengths from 1500 to 2600 nm. This wavelength range is not visible to the human eye, but plays an important role in telecommunications. The breakthrough came from the use of the direct laser writing method (DLS) developed by CFN. With the help of this method, it is possible to produce minute 3D structures with optical properties that do not exist in nature, metamaterials.

In the past year, the KIT scientists continued to improve the already extremely fine direct laser writing method. For this purpose, they used methods that have significantly increased the resolution in microscopy. With this tool, they then succeeded in refining the metamaterial by a factor of two and in producing the first 3D invisibility cloak for non-polarized visible light in the range of 700 nm. This corresponds to the red color.

“The invisibility cloak now developed is an attractive object demonstrating the fantastic possibilities of the rather new field of transformation optics and metamaterials. The design options that opened up during the last years had not been deemed possible before,” Ergin said. “We expect dramatic improvements of light-based technologies, such as lenses, solar cells, microscopes, objectives, chip production, and data communication.”

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