Turning windows into powerplants

April 15, 2011 by David L. Chandler
Turning windows into powerplants
Richard Lunt, one of the researchers who developed the new transparent solar cell, demonstrates its transparency using a prototype cell. Photo: Geoffrey Supran

If a new development from labs at MIT pans out as expected, someday the entire surface area of a building’s windows could be used to generate electricity — without interfering with the ability to see through them.

The key technology is a based on organic molecules, which harnesses the energy of infrared light while allowing visible light to pass through. Coated onto a pane of standard window glass, it could provide power for lights and other devices, and would lower installation costs by taking advantage of existing window structures.

These days, anywhere from half to two-thirds of the cost of a traditional, thin-film solar-power system comes from those installation costs, and up to half of the cost of the panels themselves is for the glass and structural parts, said Vladimir Bulović, professor of electrical engineering in the Department of Electrical Engineering and Computer Science. But the transparent photovoltaic system he developed with Richard Lunt, a postdoctoral researcher in the Research Laboratory of Electronics, could eliminate many of those associated costs, they say.

A paper by Bulović and Lunt describing their new system has been published online in the journal , and will appear in a forthcoming issue of the print edition.

Previous attempts to create transparent have either had extremely low efficiency (less than 1 percent of incoming solar radiation is converted to electricity), or have blocked too much light to be practical for use in windows. But the MIT researchers were able to find a specific chemical formulation for their cells that, when combined with partially infrared-reflective coatings, gives both high visible-light transparency and much better efficiency than earlier versions — comparable to that of non-transparent organic photovoltaic cells.

In a new building, or one where windows are being replaced anyway, adding the transparent solar cell material to the glass would be a relatively small incremental cost, since the cost of the glass, frames and installation would all be the same with or without the solar component, the researchers say, although it is too early in the process to be able to estimate actual costs. And with modern double-pane windows, the photovoltaic material could be coated on one of the inner surfaces, where it would be completely protected from weather or window washing. Only wiring connections to the window and a voltage controller would be needed to complete the system in a home.

In addition, much of the cost of existing solar panels comes from the glass substrate that the cells are placed on, and from the handling of that glass in the factory. Again, much of that cost would not apply if the process were made part of an existing window-manufacturing operation. Overall, Bulović says, “a large fraction of the cost could be eliminated” compared to today’s solar installations.

This will not be the ultimate solution to all the nation’s energy needs, Bulović says, but rather it is part of “a family of solutions” for producing power without greenhouse-gas emissions. “It’s attractive, because it can be added to things already being deployed,” rather than requiring land and infrastructure for a whole new system.

Fine-tuning the cells

The work is still at a very early stage, Bulović cautions. So far, they have achieved an efficiency of 1.7 percent in the prototype solar cells, but they expect that with further development they should be able to reach 12 percent, making it comparable to existing commercial solar panels. “It will be a challenge to get there,” Lunt says, “but it’s a question of excitonic engineering,” requiring optimization of the composition and configuration of the photovoltaic materials.

The researchers expect that after further development in the lab followed by work on manufacturability, the technology could become a practical commercial product within a decade. In addition to being suitable for coating directly on glass in the manufacture of new windows, the material might also be coated onto flexible material that could then be rolled onto existing windows, Lunt says.

Using the window surfaces of existing buildings could provide much more surface area for solar power than traditional solar panels, Bulović says. In mornings and evenings, with the sun low in the sky, the sides of big-city buildings are brightly illuminated, he says, and that vertical “footprint” of potential light-harvesting area could produce a significant amount of power.

A prototype of the MIT researchers' transparent solar cell is seen on top of a promotional item for MIT's 150th anniversary celebrations. Photo: Geoffrey Supran

Max Shtein, associate professor of materials science and engineering at the University of Michigan, says, “This work demonstrates a useful effect, and is based on very sound science and engineering.” But he adds that “it is but one of the many other methods by which a similar functionality could be achieved,” and says the biggest uncertainty at this point is that because they are so new, “the lifetime of organic PV cells is a bit of an unknown at this point, though there is some hope.”

In addition, Shtein says, “The potential of this technology is good if projected far into the future,” but only if the efficiency can be improved as the researchers expect it can.

As added benefits, the manufacturing process for the MIT researchers' solar cells could be more environmentally friendly, because it does not require the energy-intensive processes used to create silicon solar cells. The MIT process of fabricating solar cells keeps the glass panes at ordinary room temperature, Bulović noted. Installations of the new system would also block much of the heating effect of sunlight streaming through the windows, potentially cutting down on air conditioning needs within a building.

The research was funded by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy.

Vladimir Bulovic on OLED Displays

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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4 / 5 (5) Apr 15, 2011
During the summer this would be practically ideal, since it attacks the cooling bill from both ends: Absorbing IR to prevent it from heating the inside of the building, and of course turning IR into electricity to power the cooling systems.

So in a sense, you could double the efficiency number, because every percent you absorb is a percent you don't have to cool down.
2 / 5 (4) Apr 15, 2011
During the summer this would be practically ideal, since it attacks the cooling bill from both ends: Absorbing IR to prevent it from heating the inside of the building, and of course turning IR into electricity to power the cooling systems.

So in a sense, you could double the efficiency number, because every percent you absorb is a percent you don't have to cool down.

IR doesn't pass through glass.
3.7 / 5 (3) Apr 15, 2011
Even if they would reach only 4%, the possible surface area for most office buildings is enormous! It could reduce the electrical bill by alot and if new house designs around the world would use this technique than a huge % of solar powered world becomes possible.
2.5 / 5 (2) Apr 15, 2011
IR doesn't pass through glass.

But the IR will heat the exterior of a building up, some of which will heat what's inside.

5 / 5 (1) Apr 15, 2011
Yay what did Kingsix talk about just days ago? Its world of tomorrow... Today!
3 / 5 (6) Apr 15, 2011

IR doesn't pass through glass.

Yes it does. "IR" isn't just a single wavelenght of light. It's a whole spectrum, some of which goes through, some of which doesn't. Typically the shorter wavelenghts pass through, and the longer don't.

That means the IR heat comes in through the window, but doesn't come out because it is absorbed and re-transmitted at a longer wavelenght. Commonly known as the greenhouse effect.
not rated yet Apr 15, 2011
I didn't read when this would be practical, but I always am weary of getting my hopes up, because you hear of these amazing advances, but they take 10+ years to ever become practical or never happen. This is still exciting none-the-less. Infrastructure is one of the leading (if not number 1?) contributor to pollution, so this is good news for sure.
not rated yet Apr 15, 2011
what about putting that coating on the ceiling lights? most of these lamps get pretty warm, so they must be generating infrared, maybe in higher concentration than sun light
5 / 5 (3) Apr 15, 2011

You have it backwards. Long-wave light (like IR and visible frequencies) passes through glass; short-wave (like UV) tends to be blocked.

And you misunderstand how greenhouses work. They don't trap radiant energy. They trap warm air (preventing it from convecting upward.) Air itself isn't a very good conductor of heat, and it doesn't radiate very efficiently either; that's why it tends to warm up in a greenhouse -- because it comes in contact with the ground and the plants and other opaque objects that absorb incoming sunlight and heat up as a result.

Do not confuse the convection-blocking, air-trapping mechanics of greenhouses, with the radiation-blocking mechanics of the atmospheric greenhouse effect. These effects may both have "greenhouse" in their description, but they are fundamentally very different (and in case of the atmosphere, the reference to "greenhouse" is actually misleading, and unfortunate.)
not rated yet Apr 15, 2011
Puzzling is the reluctance to innovative, promising technology and /or engineering. Give this the priority and spirit, the space race enjoyed.

Of course, your first line of attack is to discredit credible science with labels, such as "junk science". I'm sure, when other countries have gone ahead - (aka past space race, for example) - the incentive to becoming energy independent will have outstripped the energy industries' conflict of interests and/or profits.

Harbor the ship called Hope. You will know when this ship departs America for lands of lesser greed for all humankind.
Apr 16, 2011
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not rated yet Apr 16, 2011
a company called NanoSolar was supposedly making a paint that would turn the entire surface of a house into a solar collector. Where are they in their progress?
not rated yet Apr 16, 2011
But the IR will heat the exterior of a building up, some of which will heat what's inside.
That is true. Still the amount of IR that passes through the glass is negligible.
not rated yet Apr 16, 2011
the amount of IR that passes through the glass is negligible
Depends on type of glass.
not rated yet Apr 18, 2011
the amount of IR that passes through the glass is negligible
Depends on type of glass.
How about common window glass? If you don't consider that answer to be precise enough, then how about the glass in your windows? Or even better, the glass in the windows of every person who reads this. I'm fairly confident that our windows don't differ enough for your statement to be relevant in this case.


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