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May 23, 2013  | Tom Cheyney  | 0 Comments

Power of concentration

Soitec goes four deep on multijunction CPV cell, BrightSource’s Ivanpah CSP plant gathering steam

Few tricks of solar energy efficiency enhancement are more alluring than concentration. For those of us who toyed with a magnifying glass to aim the sun’s beams on a leaf or a bug when we were kids (mea culpa), the idea is relatively simple—using optical or reflective media to focus and increase our star’s intensity on a limited area. In the solar industry, there are two schools of concentration: concentrated photovoltaics, which seeks to extract more energy out of less cell area, and concentrating solar power, which harnesses the sun’s heat to spin power-generating turbines. Although the concentrator clique does not (yet) account for much of the overall solar installed capacity, two leaders in the respective fields of CPV and CSP—Soitec and BrightSource—continue their pioneering efforts.

Soitec’s latest advancement comes from the lab, not the farm. Along with its Franco-German technology partners at Fraunhofer ISE, Helmholtz Zentrum Berlin and CEA-LETI, the company has been developing a new breed of III-V CPV cells, which the team believes can reach holy grail-esque 50% conversion efficiencies within a few years. The current versions of the cells have reached peak efficiencies of 43.6% (and fill factors of at least 85.6%) at a concentration level of 319 suns, according to press releases from Soitec and Fraunhofer. (The latter PR includes a pair of informative IV and EQE charts.) What really sets these devices apart is their novel four-junction design—an industry first which represents a pn-junction more than conventional triple-junction cells.

“Two dual-junction [sub]cell structures are first grown on separate III-V compound semiconductor substrates and then fused together so efficiently that the interface promotes the current flow through the four-junction solar cell device,” explains the Fraunhofer release. Because of the wafer-bonding capabilities developed by Soitec and Leti, the research group has been “able to combine lattice mismatched crystals which with conventional technology cannot be grown on top of each other without deteriorating material quality,” notes ISE’s Frank Dimroth. If all goes according to plan, 50%-efficient 4J CPV cells may arrive as soon as 2015, the partners claim.

Far from the cloistered clean spaces of those European CPV labs, the Ivanpah Solar Electric Generating Station (SEGS) is increasingly heating things up in the Mojave Desert southwest of Las Vegas, BrightSource Energy’s latest informative update says that the 377MW project is 92% completed and on schedule to come online later this year. Some 500 heliostats are being placed daily, steam “blows” are taking place, and all three generating units are moving closer to activation. When the Curator last checked in with Ivanpah in early March, the site’s Unit 1 had achieved the “first flux” milestone; more than a month later, Unit 2 has also fluxed.

Back at the 95%-complete first station, all of the 53,527 heliostats have been installed in the field surrounding the power tower, and the crew continues to move into start-up mode, BrightSource says. Over the past month or so, the team, has been “atop the tower,” where it has “continued the boiler’s ‘load ascension’ program to support steam blows,” according to the update. “Once the steam blows are complete, the team will remove the temporary steam blow piping and reconnect the piping to design conditions. The next step will be for the boiler to admit steam to the steam turbine. Once the steam turbine generator is synchronized to the grid, the plant will generate electricity to the grid.”

(For a dose of awe-inspiring megasolar grandeur, check out the most recent Flickr slideshow of mostly aerial images from Ivanpah SEGS. And for those who want to see Ivanpah first hand, the CSP Today USA 2013 conference, held June 25-27 in Sin City, is sponsoring a road trip out to the site for a limited number of attendees.)


Sources: Soitec, PRNewswire, Fraunhofer ISE, BrightSource, Flickr, CSP Today, SolarCurator,

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