UNSW researchers achieve 40.4% CPV cell result with outdoor testing

UNSW researchers, led by Martin Green, have racked up a number of headline grabbing efficiency records throughout PV technology’s evolution. The latest is a conversion efficiency record above 40%, achieved in outdoor testing. Martin Green told pv magazine that the next stage of research on the innovative triplejunction cell approach has a short-term goal of achieving 42% efficiency.

The UNSW cell structure in question comprised a monolithic stack of GaInP (1.88 eV), GaInAs (1.41 eV) and Ge (0.67eV), in combination with a commercial silicon CPV cell. Green says that further analysis of the Ge cell in the structure will now be carried out in the pursuit of the 42% efficiency goal.

“A detailed assessment of the full current generating capability of the Ge cell in a GaInP/GaInAs/Ge will also be undertaken to get an idea of how much further efficiency could be taken with relatively minor modification to present manufacturing processes for such cells," said Green.

To achieve the +40% efficiency result in sunlight, the UNSW team designed and build a CPV power tower that in effect splits the sun’s rays to optimize their conversion into electricity by the PV cell stack. This ensures that some sunlight normally wasted is absorbed by active PV layers more effectively.

“The new results are based on the use of focused sunlight, and are particularly relevant to photovoltaic power towers being developed in Australia,” said UNSW’s Green. Australian company RayGen Resources is developing such power tower technology. U.S. company Spectrolab also provided CPV silicon cells for the project. In terms of commercialization of the power tower technology, Green said that it might be only be 12 months away.

“The next stage is to design and build full scale prototypes that could be installed in a full scale PV power tower or dish concentrating system,” Green told pv magazine. “We expect that this could be completed within a year or so.”

The UNSW +40% record is below that achieved last week by French CPV company Soitec – which released its 46% CPV cell result. Green points out that the difference between the two results is that the Soitec achievement, while admirable, was achieved in test lab conditions, as opposed to outdoor testing with real sunlight.

“An as indicator of the difference, the group producing the 46% cell also recently produced a system using 52 similar cells that gave a very creditable, but significantly lower, 36.7% efficiency in outdoor testing, a record for a concentrating module of this size (830cm2),” said Green.

The UNSW result was presented at the Australian PV Institute’s Asia Pacific Solar Research Conference, today in Sydney. The record was achieved in testing carried out at the UNSW and then again at NREL testing carried out in Golden, Colorado.

Australia’s Renewable Energy Agency (ARENA) has supported the research project. ARENA CEO Ivor Frischknecht said that the result could have commercial implications for concentrating solar applications, as a rival to CSP molten salt technology.

“Instead of using mirrors to heat a medium like salt, the high tech receiver converts light to electricity directly, like a rooftop solar panel does,” Frischknecht said. “It has real potential for RayGen Resources’ concentrating PV power tower technology, currently being commercialized through another ARENA supported project.”

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