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MIT Associate Professor Marc Baldo (left) and postdoctoral student Shalom Goffri show off some organic solar concentrators. See more electricity pictures.

© Donna Coveney/MIT

Could luminescent solar concentrators make solar power more affordable?

­If you've installed solar panels on your roof, chances are you did it more to help the environment than to save money on your power bill. With the current cost of solar techn­ology, it can t­ake many years to even start to recoup your investment. A new take on an existing solar-power technology could change that.

Solar pan­els use semiconductors, usually silicon-based, to turn sunlight into electricity. These semiconductors work as photovoltaic cells -- when sunlight hits the cell, the silicon absorbs the energy in the light waves. This energy excites the electrons in the silicon, which break loose. Free-flowing electrons become electricity (see How Solar Cells Work to learn more). This setup is inherently expensive because, first, semiconductors are costly, and second, because it's not terribly efficient. Some of the sun's energy is lost to heat, and a lot of it doesn't hit the solar cells at all because the sun isn't stationary. This means it takes a whole lot of cells to generate any significant amount of electricity.

­A solution to this problem currently being tried out is the use of solar trackers. These are moving panels of mirrors that trace the sun a­cross the sky, throwing the light onto the solar cells. This gets more sunlight into the system, but solar trackers are extremely costly, and not just because they move. This method of directing large amounts of sunlight tends to overheat the silicon cells, which require the installation of expensive cooling systems.

Another solution to the efficiency problem in solar panels is the solar concentrator. This is a less expensive method, and it's the one Massachusetts Institute of Technology researchers recently improved upon in a way that could revolutionize solar power technology. We could be installing these new luminescent solar concentrators as windows in our houses - windows that generate the power to run our lights, our air conditioners and our appliances. And we could be doing it very soon.

In this article, we'll find out what a luminescent solar concentrator (LSC) does, how it works and why it's more efficient and less expensive than older solar technologies. We'll see how LSCs might be used and when they could hit the commercial market.

First, how exactly do you concentrate sunlight?

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Solar concentrators can be used to increase efficiency of existing solar panels.

Nicolle Rager Fuller/NSF

Squeezing the Light

A solar concentrator does exactly what its name implies: It takes the sunlight that strikes a wide area and bunches it together. The coolest part of the system is that it doesn't just concentrate the sunlight; it also directs that sunlight to a very specific, smaller location.

Unlike a solar tracker, a solar concentrator is stationary. The main components in the traditional design are plastic, dye molecules and solar cells. A variety of dye molecules are sprayed onto a­ sheet of plastic. At the outer edges of the plastic are solar cells.

The combination of the plastic and dyes works as a waveguide. A waveguide is any device that traps light and then moves those light waves along a path to a particular destination. In this case, when light hits the plastic, the dyes absorb it. The sun's energy is thereby transferred to the dye, causing the electrons in those molecules to jump to a higher energy level. When the electrons fall back to a lower energy level, the dye molecules release that energy into the plastic sheet, where it gets stuck. In a process called total internal reflection, the light can escape the plastic. It just bounces around in the material, ultimately making its way to the outer surface. At the outer surface, the solar cells are waiting to absorb the light and generate electricity.

A solar concentrator doesn't require a cooling system, and there are no moving parts, making it less expensive than a solar tracker. There's a drawback to the traditional design, though. While the light energy bounces around in the plastic, it sometimes gets reabsorbed into the dye molecules and ends up emitted as heat. This energy, then, never makes it to the solar cells.

The researchers at Massachusetts Institute of Technology (MIT) made a few primary changes to the system to make it more efficient and even less expensive. They call it the luminescent solar concentrator (LSC). First, they traded in the plastic for glass. Glass is easier to manufacture, and it opens up some new possibilities in terms of applications -- more on this later. They also added a new element that eliminates the loss of energy to re-absorption.

The LSC is composed of a sheet of glass coated with particles of dye. It basically works like the old plastic version except for two additions. First, a type of aluminum called tris(8-hydroxyquinoline) is added to the mix of dye molecules. These aluminum molecules cause the dyes to emit light waves at a frequency the dyes can't absorb. In this way, no light is lost to re-absorption as it makes its way to the solar cells at the edges of the glass.

In this system, 10 times more of the sunlight that hits the panel is converted to electricity compared to a traditional solar panel [Source: Economist]. Each solar cell is exposed to much more sunlight, meaning fewer silicon cells are needed and the cost goes way down.

How far down? Researchers aren't putting a dollar amount on the product, but it's sure to be cheaper than solar trackers. With increased efficiency, it'll also be able to generate more electricity per dollar than the solar panels you see on people's roofs now. Retrofitted onto current solar-panel systems, luminescent solar concentrators could increase efficiency by 50 percent [Source: ScienceDaily]. By far the most amazing aspect of the product, though, comes from the use of glass: By making windows out of these LSCs, the glass that lets sunlight into our homes and offices could also generate the power we need to run those spaces.

We're not quite there yet, though. The biggest obstacle to getting these solar-power windows onto our homes is longevity. In fact, the LSC prototype only lasts about three months [Source: TreeHugger]. The MIT group is working to get the panel to maintain stability for the 10 or so years people expect these types of devices to last. But it's pretty close -- we can expect luminescent solar concentrators to be available for sale within three years [Source: MIT].

For more information on luminescent solar concentrators and solar power in general, check out the links on the next page.

Lots More Information

Sources

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  • Economist.com. "Guiding Light."July 10, 2008. http://www.economist.com/science/displaystory.cfm?story_id=11703131
  • McGee, Tim. "MIT Solar Concentrator Improves Solar Cell Efficiency and Design Options." TreeHugger.com. July 10, 2008. http://www.treehugger.com/files/2008/07/mit-solar-concentrator-innovation.php
  • National Science Foundation. "A Colorful Approach to Solar Energy." NSF.gov. July 10, 2008 http://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=111903 ScienceDaily. New 'Window' Opens On Solar Energy: Cost Effective Devices Available Soon. ScienceDaily.com. July 11, 2008. http://www.sciencedaily.com/releases/2008/07/080710142927.htm
  • Thomson, Elizabeth A. "Fact Sheet: MIT's Solar Concentrators." MIT News. July 10, 2008. http://web.mit.edu/newsoffice/2008/solarcells-faq-0710.html

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