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).
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.
“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.