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The new solar coating, made from a special nanomaterial may not look like much, but it helps solar cells to be 42 percent more efficient, making them close to being cost competitive. Best of all it can be easily produced with existing infrastructure.  (Source: Rensselaer/Shawn Lin)
New coated cell 43 percent more efficient, can be easily produced with current production lines

Solar breakthroughs are relatively commonplace.  However, typically they are iterative -- small increases by a percent or two in efficiency.  Researchers at the Rensselaer Polytechnic Institute have invented a new solar cell that is anything but iterative as it blows away past offerings by a large margin; something RPI calls a "game-changer" for the solar business.

Against relatively cheap coal power, solar -- like nuclear and wind -- has struggled to compete from a purely economic standpoint.  Worse yet, it trails wind and nuclear in terms of how close it is to being cost competitive.  The light at the end of the tunnel is that solar have shown the highest gains in efficiency of any alternative energy source, making its future look very bright.

The new RPI solar cell is a normal cell covered in a special anti-reflective coating which traps sunlight from nearly every angle and part of the spectrum.  The new cell is near perfect; it absorbs 96.21 percent of the sunlight shined on it, while a normal cell could only absorb 67.4 percent.  That 43 percent efficiency boost, coupled with mass production, if properly implemented could place solar on the verge of competing unsubsidized with coal power, at last.

Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university’s Future Chips Constellation describes the breakthrough, stating, "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.  Our new antireflective coating makes this possible."

Most materials have a mixture of light absorbing (anti-reflective) and light reflecting properties, depending on the angle and wavelength of light.  For example, eyeglasses allow light to pass through on direct angles, but begin to reflect light at sharper angles.  Solar panels in their current form operate with similar mixed character.  In order to improve efficiency, mechanical components must be added to turn to panel to face the sun.  This system entails significant cost and loss of energy efficiency, as well as a great maintenance burden.

With Professor Lin's discovery, the world's first cost-efficient static solar arrays could be produced.  No matter what angle the sun was at, nearly all sunlight would be absorbed and converted to power.  Professor Lin describes, "At the beginning of the project, we asked ‘would it be possible to create a single antireflective structure that can work from all angles?’ Then we attacked the problem from a fundamental perspective, tested and fine-tuned our theory, and created a working device."

Rensselaer physics graduate student Mei-Ling Kuo helped Professor Lin investigate various antireflective coatings.  Their eventual choice was a nanomaterial, consisting of several fine anti-reflective sheets.  Normal antireflective coatings consist of one sheet, which absorbs light at a specific wavelength.  By stacking seven separate layers into a composite coat, they were able to absorb nearly the entire spectrum.  Furthermore, the staggered nature of the layers "bent" the flow of sunlight to a favorable angle, trapping it in the coating.  This means that if light manages to reflect off a lower layer, it will be sent back down by the upper layers.

Each layer was made from a special nanomaterial consisting of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle.  The material was grown through standard chemical vapor deposition techniques, and could be applied to the manufacturing of most standard solar cells, including III-V multi-junction and cadmium telluride cells.

On a microscopic level the nanomaterial looks like a forest of tiny, densely packed trees.  Each layer is 50 nm to 100 nm thick.

The team hopes to bring their technology quickly to market, as it will require little in the way of manufacturing line changes. The research is detailed in the paper "Realization of a Near Perfect Antireflection Coating for Silicon Solar Energy", published in the journal Optics Letters.

Besides Lin and Kuo, the other researchers listed as co-authors on the paper were E. Fred Schubert, Wellfleet Senior Constellation Professor of Future Chips at Rensselaer; Research Assistant Professor Jong Kyu Kim; physics graduate student David Poxson; and electrical engineering graduate student Frank Mont.

The research was funded with the help of funding from the U.S. Department of Energy’s Office of Basic Energy Sciences, as well as the U.S. Air Force Office of Scientific Research.



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My best guess is that...
By JonnyDough on 11/5/2008 10:21:17 AM , Rating: 2
you will still turn your solar panels to capture more light. Regardless of the angle, SOME sunlight is always going to be reflected. This tech may make it so that your panel only has to be two directional. Facing the east in the morning and west in the eve, without having to adjust for solstice/equinox. That should significantly lower the cost and maintenance of the gadgetry to tilt panels.




RE: My best guess is that...
By 3DoubleD on 11/5/2008 11:44:33 AM , Rating: 5
I just finished reading the paper and the film itself exhibits amazingly low reflections in an extremely broad spectrum of light (400-1600nm, although silicon is transparent to light above 1100nm anyway, but lower band gap solar cells could benefit).If a film like this was developed to be durable enough, there would be no need for a motorized system. At 60 degrees the graded index has a maximum reflection of 10% for all wavelengths mentioned above. At these angles, regular anti-reflection coatings found on solar cells give reflections in the 20% range (and even higher for long wavelengths). If you average the reflection of this film over all angles and wavelengths you get 32.6% for bare silicon (no solar cells are bare silicon), 18.8% for a quarter wavelength AR coating (typical solar cell coating), and 3.79% for this graded index. The efficiency of these films is simply given by 100%-R and translates to 67.4%, 81.2%, 96.21% respectively. This means the graded index is 22.2% more efficient than the anti-reflection coatings used on typical solar cells and 42.7% more efficient than bare silicon (a useless comparison when talking about the potential increase in efficiency of current solar cell devices).

With this in mind I would say that the purpose of this coating would be to eliminate the need for active sun tracking as regular anti-reflection coatings have nearly identical performance for incident sunlight (this film is better as it has lower IR reflectivity, but the difference isn't earth-shattering at incidence). Reducing the penalty of sunlight being off incident will make it possible for non-motorized solar power plants to produce more energy (even in the early morning and late afternoon, 60 degree reflections only 10%). Motorized systems increase the cost of implementing solar power plants and maintenance as well as reduce the output power from the cells (although less then the power gained from using them in the first place). With increased stationary solar cell performance we could see these systems with superior $/W performance and the elimination of motorized systems.

However, the article mentions there are barriers to using this commercially. First, this porous, nanorod film was found to be not robust enough for practical solar cell applications. Secondly, this experiment was not conducted on a solar cell but a piece of silicon. While the area covered by the graded index was not mentioned, it was referred to as a "dot". The paper also mentions the rods were formed using oblique angle deposition (not CVD as mentioned in this article, however, they said other deposition techniques were required to make the film, sputtering was mentioned). Oblique angle deposition may have difficulties scaling to commercial applications as it is a form of physical vapor deposition, although it wouldn't be impossible.

Finally, increasing the absorption of the cell will certainly lead to an increase in efficiency, but not by a factor of 22.2% (a 20% efficient cell that realizes the full impact of 22.2% more light would be 24.44% efficient). With increase absoption of light comes increased heat production. Solar cell efficiency decreases with increasing temperature. So our theoretical 20% cell would see an efficiency increase somewhere between 20 and 24.44%, but I would guess the heat contributions would be relatively small (so around 23% wouldn't be out of the question). This coating would have a much more pronounced impact on better quality solar cells such as multi-junction cells (eg. a 22.2% increase in overall light absorption could increase a top-of-the-line 40% multi-junction cell to 48.88%). Unfortunately, these cells aren't very competitive enough in the $/W category to make use of this (they are normally used on satellites and in solar concentrators).


RE: My best guess is that...
By randomly on 11/5/2008 12:06:23 PM , Rating: 4
Thanks for posting the 'reality check'. That info should have been in the original article. One gets so tired of the deceptively written weasel-word hype write ups.


RE: My best guess is that...
By swampjelly on 11/5/2008 2:19:12 PM , Rating: 2
I was about to say the same thing


RE: My best guess is that...
By 3DoubleD on 11/5/2008 2:37:34 PM , Rating: 5
I just realized I made an error my calculation of the maximum gain of having this graded index on a theoretical 20% solar cell. Previously I said that this new cell would give a 20% solar cell a 22.2% more light, thus the efficiency of the cell could be increased to a maximum of 24.44%. This was incorrect.

With the increased availability of light in the solar cell, efficiency will improve, but it will be much less than 24.44%. As I mentioned there are thermal issues, but there is one much more important issue. This would allow 22.2% more light across the solar spectrum (particularly low wavelengths in the near-IR regime). Light above ~1100nm subtracts from the efficiency of the solar cell (efficiency = Power Out/Power In, increasing the availability of 1100nm+ light does not increase Power Out but increases Power In, lowering efficiency). Increasing the amount of light exactly at the band gap will increase efficiency the most (however this is a very narrow regime). Increasing the amount of light above the band gap will increase efficiency but with diminishing returns as wavelength decrease (Power Out goes up, but Power In goes up faster).

To summarize, this film would have a moderate effect on the efficiency of solar cells. While reflections are reduced 22.2% more than cells readily available today, the full magnitude of this increase in collection efficiency cannot be directly applied to the efficiency of the solar cell. It would be most appropriate to say that this film will increase the POWER OUTPUT of the solar cell and thus improve power densities and $/Watt for solar power systems. There will be a small increase in efficiency, but the above terms are more applicable.


RE: My best guess is that...
By tastyratz on 11/5/2008 3:22:49 PM , Rating: 2
Someone give this man a 6.


RE: My best guess is that...
By JonnyDough on 11/5/2008 7:18:50 PM , Rating: 2
I'll second that, once I understand what the heck he just said. I'm no photonologist. :-P


RE: My best guess is that...
By Regs on 11/10/2008 10:37:12 AM , Rating: 2
And a job!


RE: My best guess is that...
By tygrus on 11/5/2008 7:36:25 PM , Rating: 1
Don't need to move panels .. Wrong.
90% effecient panel .. Wrong.

The major reason for decreased energy output when the sun is low is the apparent cross-sectional area of light. Just as shadows are larger in the morning and afternoon, so is the light spread. Use a torch and a flat rubber at the same distance, how much light is blocked when straight on, now rotate the rubber so it is almost end on (as if lit from the sun going down). The rubber blocks less light at high angle (not at 90 deg). You will increase power output by pointing these new panels towards the sun as the sun moves through the day.

Just because you can trap the light doesn't mean you can convert that energy to electricity. May be we can cool the PV panels with water for later use by the hot water service.

How much will these new panels cost per MWh/year (all else being equal) ?


RE: My best guess is that...
By 3DoubleD on 11/5/2008 9:35:58 PM , Rating: 3
Who said anything about a 90% efficient panel. The graded index film has a ~96% collection efficiency (meaning it only reflects ~4%). I clearly said the solar cell efficiency would only marginally go up.

You are correct that trapping the light doesn't mean it will be converted into electricity, but having more available will increase the power output (if thermal issues don't get out of hand, and they won't for a small 22.2% increase in collection efficiency). Water cooling for PVs is a great idea that should be implemented more often. This is a great way to increase the efficiency for a home or business installation. By heating your water from the waste heat generated in the solar cells you can lower your water heating power usage and enjoy increased PV efficiency due to cooler operation. This dramatically increases the efficiency of the system, as does any hybrid implementation in coal or nuclear power plants (nuclear plants see massive efficiency gains when the waste hot water is actually used instead of discarded).

That is true that the cross section of light decreases throughout the day as a function of the sine of the angle for a solar cell that is flat to the ground (sunrise being 0 degrees and sunset being 180 degrees). At larger angles from incidence (early morning, late afternoon) other factors come into play, such as increased scattering and absorption of light in the atmosphere. The power that can be collected during these hours is much smaller than peak daytime hours. Another important factor to consider is that solar cell arrays are used to supplement constant 24 hours power stations (eg coal, nuclear, ect). Peak power demand is during the day, which is normally when the sun is on or near incidence. Solar cells used in this capacity would not require expensive solar tracker motors, greatly enhancing their usefulness.

That said, every situation is different. One would have to do a complete cost analysis of each given situation to see whether it would be worth implementing motorized solar trackers or not. The point was that this brings more options to the table by making the non-motorized arrays more competitive and perhaps opening new markets. Of course this is assuming they can improve the technology to the point where it is practical for solar cell use (which it is not at this moment).

As for the costs, I don't think there is a number yet as this new graded index material isn't ready for practical use as it isn't durable enough. Depending on how those problems are addressed and the methods used to grow the film, it could be close to the cost of current AR coatings or much more. Only time will tell.


RE: My best guess is that...
By TheOtherBubka on 11/6/2008 11:31:18 AM , Rating: 2
3DoubleD I agree and also showed a more standard rationalization below. Somehow, I didn't see your post until today. The authors made a 3 layer graded Anti-relfection coating which they compared to a 1 layer anti-reflection coating and no coating. If you follow the pdf link I sent below, you will also see a standard 3 layer AR coating reflects very little over a very broad wavelength range without all the complexity of their deposition technology. The authors made the top 2 layers by scultptured thin film techniques. As I stated in my other post, not good material utilization and costly deposition techniques.


RE: My best guess is that...
By emboss on 11/5/2008 7:17:14 PM , Rating: 2
You'd still need to turn the panels to get good efficiency out of them even if you had no reflection. Simple geometry means that over a whole day, the efficiency of a fixed panel is only about half (season and geography depending) that of a tracking panel even if you assume zero reflective loss.

Also, the differences between a panel that flips between two orientations and a tracking panel would be pretty small. You've still got to have all the hinges, motors, etc for a two-orientation setup. The only "extra" bit on the tracking panel would be changing the timer to turn the structure one notch every x minutes, instead of turning the structure be a large number of notches every 6 hours.

The main benefit I see for these panel is in situations where moving the panel is not practical. So remote communication sites, solar panels on vehicles, etc need less space or can use more power. The angle independence is of limited use for "mains" solar power.


RE: My best guess is that...
By ShaolinSoccer on 11/6/2008 6:05:40 AM , Rating: 2
Would it be impractical to have two panels facing east and west? Or even 3 with one facing straight up? I was thinking of houses having this kind of setup on the roof. You can even put them under a clear panel so that rain runoff falls in certain directions. It would be terrific to be able to store up energy during the day then use it during the evening for free.


RE: My best guess is that...
By trisct on 11/6/2008 4:07:15 PM , Rating: 2
So, you would have solar panels that can't be washed or even swept clean, basically. That would mess up your non-reflective surface in a hurry.


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