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The new windows will be formed of stacked tinted thin plates of glass which each target a specific wavelength of light, which they gather and emit intensely to cells at their edges.  (Source: Donna Coveney)

An artistic rendition detailing how the new system could easily be applied to existing panels to make them more efficient.  (Source: Nicolle Rager Fuller, NSF)

Marc Baldo, associate professor of electrical engineering and computer science (left), and Shalom Goffri, postdoc in MIT's Research Laboratory of Electronics (right), show off examples of the new solar window layers.  (Source: Donna Coveney)
A new solar design will soon be able to contribute significantly to powering city buildings

There is much ongoing research into making photovoltaic solar power, common among commercial business and residential installations, more efficient.  While many focus on the cells themselves, MIT researchers are focusing on a different approach by changing the places where cells can be deployed and how light gets to them.

MIT researchers built upon previous research into colored dyes from the 1970s and created special glass panels.  Each panel absorbs a different wavelength of light and carries it to solar cells.  The result: the first potentially viable solar windows.

The new research is reported in the July 11 issue of the journal Science.  In it, the researchers detail how the build up their novel "solar concentrator".  Explains Marc A. Baldo, leader of the work and the Esther and Harold E. Edgerton Career Development Associate Professor of Electrical Engineering, "Light is collected over a large area [like a window] and gathered, or concentrated, at the edges."

By clustering solar cells around the edges of the specially prepared sheets of glass, the new method provides a unique alternative to expensive rooftop solar cells.  They are also much more efficient than their rooftop brethren.  The special glass panels concentrate light 40 times standard sunlight before delivery directly to the cell.  Further different designs to absorb different wavelengths are available, so by using windows with several stacked glass panes, absorption can be optimized across the entire visible wavelength.

The system is so simple to manufacturer that the inventors expect it to be deployed within 3 years at little cost over standard window costs.  Furthermore, it can be added to existing solar panels to help concentrate sunlight for virtually no additional cost, and would increase their efficiency by a modest 50 percent, according to the authors.

Baldo's team includes Michael Currie, Jon Mapel, and Timothy Heidel; all graduate students in the Department of Electrical Engineering and Computer Science, and Shalom Goffri, a postdoctoral associate in MIT's Research Laboratory of Electronics.  The U.S. Department of Energy (DOE), a sponsor of the research is very pleased with their progress.

Dr. Aravinda Kini, program manager in the Office of Basic Energy Sciences in the DOE's Office of Science, states, "Professor Baldo's project utilizes innovative design to achieve superior solar conversion without optical tracking.  This accomplishment demonstrates the critical importance of innovative basic research in bringing about revolutionary advances in solar energy utilization in a cost-effective manner."

In the paper, Professor Baldo addresses current methods for improving efficiently, stating that, "track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain.  He comments that additionally "solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighboring concentrators."

As he points out, even methods today considered to promote efficiency are clumsy and expensive.  His team's process is extremely simple in comparison with no moving parts.  The windows are painted, in essence, with a series of two dyes.  Combined, the dyes absorb light of a specific wavelength and then retransmit it at the edges of the window. 

The previous work in the 1970s involved a similar principle but lost too much energy during transportation.  The MIT engineers were able to take their expertise in lasers and organic light-emitting diodes and apply it to making a mixture of two special dyes which work much better.  Says Jon Mapel, "We made it so the light can travel a much longer distance.  We were able to substantially reduce light transport losses, resulting in a tenfold increase in the amount of power converted by the solar cells."

The research was sponsored by the National Science Foundation in addition to the DOE.  Professor Baldo is associated with MIT's Research Laboratory of Electronics, Microsystems Technology Laboratories, and Institute for Soldier Nanotechnologies.

Mr. Mapel, Mr. Currie and Mr. Goffri have founded a company Covalent Solar, which looks to make good on the researchers' promise to bring the technology to market within 3 years.  The company is progressing nicely, winning the Energy category ($20,000) and the Audience Judging Award ($10,000) in MIT's $100K Entrepreneurship Competition.  The new design is especially heavy to the tinted-glass heavy urban settings and follows a trend of incorporating wind and solar power into less obtrusive urban-ready designs.

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RE: This is cool if...
By Etsp on 7/17/2008 3:10:47 PM , Rating: 2
But like I said, that was probably done so they could demonstrate how it works, it's so much easier to understand it if you can see the effect taking place. You can describe what it does with one picture, instead of a series of similes and metaphors, punctuated with technical terms many people wouldn't be able to understand.

RE: This is cool if...
By TheOtherBubka on 7/17/2008 10:55:02 PM , Rating: 3
Okay...sorry this is going to be longish.

1. This technology does absorb visible radiation. The dyes absorb radiation and re-emit radiation of a longer, wavelength which undergoes total internal reflection (waveguides) and continues to the edges of the window where it is emitted/collected/you see it glow.

2. As you can see, the windows are colored. Most people like 'neutral' transmittance for 'windows' and not to look at the outside world in colors.

3. If they do make a 'multilayer' glass, architects need to design that into the frame of the building to support the extra weight for the skyscraper. More window weight, stronger materials, higher cost. Same for your roof in the depiction they claim.

4. Typical 'low E' windows let in about 33% of the solar radiation, absorb about 33%, and reflect about 33%. The energy they let in is highly skewed toward the visible. This technology as is, states nothing about reducing the IR that would be transmitted through the window. If they want to reflect the IR, then you are still going to need a 'low E' tehcnology behind it to keep long wavelength radiation (aka heat) in/out depending on the season. Thus higher cost compared to conventional windows. See solar heat gain. The more solar heat gain, the bigger AC units you need, then more power required to cool.

5. Yes this technology concentrates light. But only light of certain wavelengths. Let's say the dye absorbs strongly over 300-550 nm as is common in Graetzel (dye sensitized solar cells). If you absorb all of that radiation, the maximum output power available (no light getting through) is ~400 W/m^2 of glass under direct sunlight. If not direct radiation, then decrease maximum accordingly. The light is then re-emitted at a longer wavelength (less energy) plus photovoltaic diode losses in the cells and you are down to regular PV efficiencies which is where they get to. Also, if you want 30% light throughput so people can 'see through the window', take 30% off of your efficiency (roughly).

6. If you are lucky enough to be able to see the paper, you will see there were a lot of estimations in their efficiency measurements. Furthermore, this technology with 2 of the best concentrator materials known (GaInP and GaAs which are also very expensive in solar costs), was estimated to produce 6.8%.

7. Their power conversion efficiencies went down with increasing concentration and not up as conventional concentrators. Important if you consider a 1.5m x 2.0m x 3mm glass pane on a side of a building. Concentration ratio of roughly 142. Thinner glass since you can laminate this technology between them? Then higher concentration.

8. Not to say this isn't a worthwhile idea, just a long, long way to go with high promises and 'theoretical' results such as 'low cost organic PV packaging.' Yes. You will pay for expensive packaging for 1 OLED TV. But will you pay for 20 for your windows in your house? Imagine a skyscraper.

RE: This is cool if...
By Etsp on 7/18/2008 12:42:57 AM , Rating: 2
1. This technology does absorb visible radiation. The dyes absorb radiation and re-emit radiation of a longer, wavelength which undergoes total internal reflection (waveguides) and continues to the edges of the window where it is emitted/collected/you see it glow.
Only if they choose to use dyes that create this effect on visible wavelengths of light when it goes into production. Because like you said
Yes this technology concentrates light. But only light of certain wavelengths.
Yes, the technology is certainly capable of absorbing visible light, but it is also capable of absorbing UV light without affecting visible light. That was the only point I was trying to make. :)

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