With an electrical efficiency of 6.7%, the Fraunhofer Institute for Solar Energy Systems ISE set a new world record for fluorescent collectors. Colored plates filter out part of the sunlight and redirect it to solar cells mounted on the plate edge. Fluorescent collectors have been developed by the researchers at Fraunhofer ISE as early as the 1980s. Now using new materials and techniques, improvements in the economic viability of the product are anticipated.

“First now, solar cells exist which have the high efficiencies needed,” says Jan Christoph Goldschmidt, project leader at Fraunhofer ISE. “For some time now, we have again begun investigations on fluorescent collectors and performed experiments that show our calculations are correct.”

For their experiments, the scientists at Fraunhofer ISE used over 20 year old fluorescent collectors on PMMA basis. A 2 x 2 cm² high stack made up of one red and one yellow fluorescent collector was exposed to direct sunlight. Mounted on the plate edges, 4.8 cm² gallium indium phosphide (GaInP) solar cells convert the redirected sunlight into electricity. 6.7% of the sunlight incident on the stacked plates was converted into electricity. For practical applications, the area of the fluorescent collectors must be increased considerably in order that it is larger than the solar cell area. First then, will the system be economically profitable.

The further development is already in progress: “The largest loss factor up to now is due to the loss cone,” explains Goldschmidt. “Over 25% of the light converted by the dye was lost through the surface of the plates. For larger systems this loss is still greater. In another experiment using a much greater collector area, we were able to increase the efficiency by 20% by employing novel filter layers with photonic structures. With only one plate, we achieve an efficiency of over 3% - even with a solar cell area amounting to one twentieth of the collector area. Next, we will combine both concepts – a stack made up of several colors and photonic structures – into one system.”

Fraunhofer ISE holds back their expectations that their technology will ready in the short-term to produce electricity on a large scale. The researchers are, however, confident that the new technology can be used, for example, in facades and windows to simultaneously generate electricity. Before practical applications are possible, however, comprehensive development work is still necessary.

Back in 1976, Prof. Adolf Goetzberger formulated the basic idea of the fluorescent collectors: Sunlight is collected on large area plates. In the plate it is then converted by a fluorescent dye and transported by total internal reflection to solar cells mounted on the plate edges. When the edge area is smaller than the surface area of the plates, then the sunlight can be concentrated. As a result, fewer of the costly solar cells are needed, as compared to if the entire surface area of the plate were to be covered.
System schematic
Source: Fraunhofer ISE

Contact:
Dipl- Phys. Jan Christoph Goldschmidt
Project Leader
Phone: +49 (0) 7 61 / 45 88 - 54 75
Fax: +49 (0) 7 61 / 45 88 - 92 50
E-Mail: jan.christoph.goldschmidt@ise.fraunhofer.de
Fraunhofer-Institute for Solar Energy Systems ISE
Heidenhofstrasse 2
79110 Freiburg