Concentrated sunshine – utilized efficiently

(f.l.t.r) Dr. rer. nat. Klaus-Dieter Rasch, Hansjörg Lerchenmüller, Dr. rer. nat. Andreas W. Bett

Hansjörg Lerchenmüller (Spokesperson)
Dr. rer. nat. Andreas W. Bett*
Dr. rer. nat. Klaus-Dieter Rasch**

Soitec Solar GmbH
*Fraunhofer Institut für Solare Energiesysteme ISE
**AZUR SPACE Solar Power GmbH

The future of our energy supply is to be found in renewable sources of energy. Photovoltaics will continue to increase in importance worldwide. Yet solar power is still relatively expensive. How can sunlight be converted into electricity at competitive costs?

Hans Lerchenmüller, Dr. Andreas W. Bett und Dr. Klaus-Dieter Rasch discovered the answer: they increased the energy efficiency of the utilization of sunlight by stacking three small solar cells made of different materials – and then ensured that sunlight was concentrated on the small parts. Hansjörg Lechermüller is the Managing Director of Soitec Solar GmbH in Freiburg, Andreas Bett is Deputy Director of the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, and Klaus-Dieter Rasch is head of AZUR SPACE Solar Power based in Heilbronn.

The advantage of the multi-junction solar cells is to be found in the more efficient utilization of sunlight: each of the three cells converts a different part of the solar spectrum. The result is the highest efficiency for every type of solar cell. Yet solar cell production is still comparatively expensive. Consequently, the multi-junction cells were used in the past only for generating electricity onboard satellites. The corresponding technology was developed by experts from Fraunhofer ISE and AZUR Space Solar Power. By concentrating the light, the technology is now also affordable for applications on Earth since they considerably lower the need for expensive materials for the cells.

The researchers surrounding Andreas Bett created a new photovoltaic module concept based on the multi-junction cell technology. In this solution, special lenses integrated in a glass plate concentrate sunlight by a factor of 500 and focus it onto the small stacked cells. Each of stacked cells is only three millimeters in diameter. Soitec developed a series-ready product from the new module principle and successfully transferred it from the laboratory to industrial series production.

The efficiency of power generation as achieved by the multi-junction solar cells is enormous. Fraunhofer researchers succeeded in producing a triple-junction solar cell with an efficiency of 41.1 percent: a world record and a far better yield than the best conventional cells. The rapid transfer of this cell technology from the laboratory to industry by AZUR SPACE Solar Power will soon make the yield of the systems already being produced by Soitec even more efficient. In practical use in concentrator PV power plants they already achieve a system efficiency of over 25 percent – twice as high as power plants consisting of conventional silicon flat plate modules.

Since Concentrating Photovoltaic (CPV) technology only utilizes direct solar radiation, it is ideally suited to use in sunny regions – like southern Europe or the American Southwest. In future, they could be used in large solar power stations. According to experts, approximately 2 gigawatts will be installed in these types of power stations by 2015. Concentrix Solar GmbH, now Soitec Solar GmbH, was established in 2005 as a spin-off from the Fraunhofer ISE. The company has already installed over 3 megawatts at CPV power plants in Spain, France, South Africa, and the USA. Additional installations are planned. Over the next few years, the company will also build a large module factory in California. The close cooperation between Fraunhofer researchers and experts in industry has put the solar technology from Freiburg and Heilbronn in the number one position in the world.

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