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Research&Development of Solar Selective Coatings Pays Off

07 Nov 08 | General, Solar Electric

From just up the road a short distance, researchers at RPI (Rensselaer Polytechnic Institute) have discovered a process that greatly increases the absorption of sunlight by photovoltaic panels and allows those panels to use the entire solar spectrum from nearly any incident angle.

“To get maximum efficiency when converting solar power into electricity, you want a solar panel that can absorb nearly every single photon of light, regardless of the sun’s position in the sky,” said Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university’s Future Chips Constellation, who led the research project. “Our new antireflective coating makes this possible.”

It is possible by using nano technology to create seven layers nano rods.  Each layer is designed to transmit a specific wave length (color) of light.  The result is an absorption efficiency of greater than 96%.  This is indeed great news for PV cell producers, as the current light absorption efficiency is about 67 percent for the typical PV panel.

The seven layers, each with a height of 50 nanometers to 100 nanometers, are made up of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle – each layer looks and functions similar to a dense forest where sunlight is ‘captured’ between the trees.

The major implication for solar manufactures is smaller more powerful PV cells can be produced with less raw material.  Is this the breakthrough the solar industry has been waiting for?  Maybe.  In any case, it certainly seems like a step in the right direction.

The one problem I see with all of this is the efficiency of the photovoltaic cell itself.  A PV cell is essentially a large exposed transistor.  When a photon strikes a P-N junction, one of four things happens; it bounces off, it passes through to the other side, it gets converted to heat, or it knocks an electron free.  Of course the first situation is mostly cured by the selective coating.  The last situation is the desired outcome.  Conversion to heat remains a problem.

Currently manufactured PV cell technology has roughly a 15% efficiency from insolation rate to electricity production.  As we learned above, some of this efficiency loss is due to reflection of light from the surface of the PV cell.  A comparison of the total light reaching the PN junction (67% of the available sunlight) compared to the output of the PV cell, shows that the actual conversion efficiency of the PN junction is about 22%.  The remainder either passes through the PV cell substrate or generates heat.  The selective coating applied to a PV cell will increase the heat in the PN juction by 25-30%.

Heat is a major problem to semiconductors.  Heat reduces efficiency and lifespan of a traditional silicone based PV cell.  The computer industry has gone to great lengths to improve the heat tolerances of the semiconductors used in computer chips, therefore, this is not an insurmountable problem.  It does, however, need to be addressed in cells that will use the selective coating developed at RPI.

It will likely take several years for this technology to make it onto the general market.  In the mean time, every watt of installed PV is one less watt generated by fossil fuels.

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07 Nov 08 | General, Solar Electric
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