The researchers, from the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, develop solutions to make the conversion from sunlight to electricity cheaper and more efficient for the future. To this end, they and their team have been working for over ten years on solar modules which concentrate solar radiation by a factor of 500 onto tiny solar cells. This procedure reduces the area of the semiconductor material required and enables the use of novel, extremely efficient solar cells.
They received recognition last year for their development of a metamorphic triple-junction solar cell which achieved a record efficiency of 41.1%. Together with a concentrating optic, the highly efficient multi-junction solar cells were implemented in concentrator modules, which Fraunhofer says are now "market ready" at 29% efficiency in the form of Concentrix Solar's (a Fraunhofer spin off) FLATCON module.
Today Concentrix Solar employs over 60 people and operates a production line for concentrator modules in Freiburg, shipping its highly-concentrating solar PV modules primarily to Southern Europe and the USA. It has installed more than 600 kW of concentrator systems in Spain.
The CPV technology - optics and material
“We substitute costly semiconductor material with inexpensive optics," explains Dr. Andreas Bett, head of the department of materials – Solar Cells and Technologies. "In addition, we use extremely efficient solar cells and thus reduce the solar electricity costs." This technology produces more power per area than conventional flat plate PV technology. Under favourable conditions, electricity costs of just ten to fifteen cents per kilowatt are possible for systems located in Southern Europe, say the company.
CPV technology however is still young, and technology development is a delicate balance. The sector does have the theoretical advantage of low costs and high efficiency, but flat plate PV continues to improve in these areas and the cost of silicon is currently low, removing one of the earlier selling points of CPV. CPV technologies also require highly efficient tracking and a perfect balance of optics and technology; a high wire act with significant potential but requiring perfect timing.
Due to the concentrating optics, the systems have to make use of direct solar radiation and therefore the systems need to track the sun. Since the radiation scattered by clouds or water droplets can not be concentrated, such systems are not particularly suitable in countries like Germany or as roof top systems, but rather as large commercial power stations in sunny countries having a large proportion of direct radiation - like Southern Europe. Due to the two-axis tracking mechanism installed in the systems, a high power output can be achieved from morning, and until the evening hours.
An advantage of concentrator systems is their modular construction, which allows them to be scaled up to kilowatt or even gigawatt range. And according to the company, the capital needs and investment for building up an automated series production are comparatively low.
The 41.1% multi junction cell
Three subcells made out of gallium indium phosphide, gallium indium arsenide and germanium are stacked on top of one another. Each of the III-V compound semiconductors utilise a different wavelength range in the solar spectrum.
Special Fresnel lenses concentrate the incident sunlight by a factor of 500 and focus it onto the tiny 3 mm² cells, and to avoid overheating, each cell is mounted on a copper plate which dissipates the heat well enough to make a passive cooling of the cells sufficient.
Prof. Eicke R. Weber, Director of Fraunhofer ISE is convinced: “We expect that high efficiency concentrator technology – in addition to photovoltaics using crystalline silicon and the classic thin-film technology – will become established as a third technology for cost-efficient generation of solar electricity in the sunny regions of the world.”