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Stacking Cells Could Make Solar as Cheap as Natural Gas<br>
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<a class="moz-txt-link-freetext" href="http://www.technologyreview.com/news/529651/stacking-cells-could-make-solar-as-cheap-as-natural-gas/">http://www.technologyreview.com/news/529651/stacking-cells-could-make-solar-as-cheap-as-natural-gas/</a>
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A novel manufacturing method could make it practical to stack solar
cells and convert more of the energy in sunlight into electricity. <br>
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By <a
href="http://www.technologyreview.com/contributor/kevin-bullis/">Kevin
Bullis</a> on August 6, 2014<br>
<p>When experts talk about future solar cells, they usually bring up
exotic materials and physical phenomena. In the short term,
however, a much simpler approach—stacking different semiconducting
materials that collect different frequencies of light—could
provide nearly as much of an increase in efficiency as any radical
new design. And a new manufacturing technique could soon make this
approach practical.</p>
<p>The startup <a href="http://www.semprius.com/" target="_blank">Semprius</a>,
based in Durham, North Carolina, says it can produce very
efficient stacked solar cells quickly and cheaply, opening the
door to efficiencies as high as 50 percent. (Conventional solar
cells convert less than 25 percent of the energy in sunlight into
electricity.)</p>
<p>Semprius has come up with three key innovations: a cheap, fast
way to stack cells, a proprietary way to electrically connect
cells, and a new kind of glue for holding the cells together. In
its designs, Semprius uses tiny individual solar cells, each less
than a millimeter across. That reduces costs for cooling and also
helps improve efficiency.</p>
The conventional way to stack semiconductors is to grow layers on
top of each other. But not all semiconductors can be combined this
way, because their crystalline structure doesn’t allow it (see “<a
href="http://www.technologyreview.com/news/529476/adaptive-material-could-cut-the-cost-of-solar-in-half/">Adaptive
Material Could Cut the Cost of Solar in Half</a>”). Semprius grows
semiconductor materials in the conventional way but also stacks
several different combinations, resulting in a solar panel that can
capture more energy from sunlight.<br>
Semprius has demonstrated cells made of three semiconductor
materials stacked on top of a fourth solar cell that would not have
been compatible otherwise. It has made two versions of the device
this year, one with an efficiency of 43.9 percent and the other,
using slightly different materials, with an efficiency of 44.1
percent.
<p>In addition to being fast and precise, the approach also makes it
possible to reuse the expensive crystalline wafers that
multijunction solar cells are grown on. Eventually the company
hopes to stack two multijunction devices, for a total of five or
six semiconductors with a “very high performance, beyond 50
percent efficiency,” says <a
href="http://www.semprius.com/company/leadership.html"
target="_blank">Scott Burroughs</a>, vice president of
technology at Semprius. He says the company hopes to achieve this
in three to five years.</p>
<p>The one catch is that the cells will be more costly than
conventional ones. <a
href="http://www.nrel.gov/pv/performance_reliability/research_staff.html#Kurtz"
target="_blank">Sarah Kurtz</a>, a principal scientist at the
National Renewable Energy Laboratory, says costs won’t come down
until production happens at a large scale.</p>
<p>With economies of scale, however, such cells could improve the
economics of solar power. At a scale of 80 to 100 megawatts a year
of manufacturing capacity, a cell with 50 percent efficiency would
make it possible to reach costs of less than five cents per
kilowatt-hour, Burroughs says. The U.S. Energy Information
Administration estimates that new natural-gas power plants will
produce electricity at 6.4 cents per kilowatt-hour.<br>
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<p><a class="moz-txt-link-freetext" href="http://www.technologyreview.com/news/529651/stacking-cells-could-make-solar-as-cheap-as-natural-gas/">http://www.technologyreview.com/news/529651/stacking-cells-could-make-solar-as-cheap-as-natural-gas/</a>
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<p><br>
Ken<br>
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