Friday, June 27, 2008

Bringing Sunlight Inside

Mechanical Engineers Create High-tech Solar Panels

May 1, 2007 — Photovoltaic panels have a new design: concentric circles that focus the sun's rays on miniaturized modules. Having the panels automatically sense sunlight and turn towards it also makes these high-tech solar cells more efficient.

Solar energy technology is advancing daily. Now, a new, high-tech system is working to efficiently harness the power of the sun and drastically reduce harmful carbon dioxide emissions.

Today, there are more than 76 million residential buildings and nearly 5 million commercial buildings in the United States. Combined, they use two-thirds of all electricity consumed in the United States and produce 35 percent of all carbon dioxide emissions.

Anna Dyson, an architectural scientist from Rensselaer Polytechnic Institute in Troy, New York, is leading the way to make solar energy a real alternative to pollution-emitting fossil fuels. Her system contains rows of thin lenses that track the sun's movement. Sunlight floods each lens and is focused onto a postage-stamp sized, high-tech solar cell. Dyson says, "Really, what we want to do is be capturing and transferring that energy for usable means."

Conventional solar systems are about 14 percent efficient. This system has a combined heat and power efficiency of nearly 80 percent. "What they're doing is very efficiently capturing and transferring that light into electricity and the solar heat into hot water," Dyson explains.

"We basically have a system that can sense where the sun is at any time, and then the modules will basically be facing directly perpendicular to the incoming sun rays," she says. The lenses will be nestled between window panes and all of the pieces will be made of glass.

Michael Jensen, Ph.D., a mechanical engineer from Rensselaer Polytechnic Institute says reducing dependency on fossil fuels is critical. Dr. Jensen explains, "We use fewer fossil fuels, then we are going to put less CO2 into the atmosphere. We are going to decrease the effects on global warming."

This system will also lower the lighting needs of buildings, as it will provide usable light inside. It could supply as much as 50 percent of the energy needed for a building to operate. The system is set to be installed in the Center for Excellence and Environmental Energy Systems in Syracuse, New York, in 2008, and in the Fashion Institute of Technology in New York City by 2009.

BACKGROUND: A team of different types of scientists at Rensselaer Polytechnic Institute has developed a radical new solar energy technology that promises to collect and distribute solar energy more efficiently. Rows, or stacks, of pivoting lenses incorporated into a glass building facade track the movement of the sun across the sky, focusing its rays onto high-tech solar cells. The new system uses high-tech solar-concentrator technology and advanced materials. The full-size prototype will be incorporated into a new building at The Center of Excellence in Syracuse, New York.

HOW IT WORKS: The key breakthrough is the miniaturized concentrator solar cell, which uses a lens with concentric grooves to focus collected light. Even though it is only the size of a postage stamp -- compared to the usual solar collector area that spans 4 x 4 feet -- the cell is much more efficient in collecting and reusing solar energy. The lens focuses incoming sunlight onto the solar cell. Microchannels at the base of the module transfer energy in the form of heat and light to wires contained inside. Each vertical stack of lenses rolls and tilts like a track blind, keeping the surface of the lenses faced to incoming sunlight as the sun changes position in the sky throughout the day. Incorporating these new cells into arrays could make solar energy an option that is competitive with other energy sources, reducing our dependency on fossil fuels.

ABOUT SOLAR CELLS: The solar cells on calculators and satellites are photovoltaic cells or modules: groups of cells electrically connected and packaged together. Photovoltaics convert sunlight directly into electricity. Photovoltaic cells are made of semiconductor materials like silicon. When light strikes the cell, a certain portion of the light is absorbed by the semiconductor material. The energy of the absorbed light knocks electrons in the semiconductor material loose, allowing them to flow freely. Photovoltaic cells also all have one or more electric fields that act to force the freed electrons to flow in a certain direction. This flow of electrons is a current. By placing metal contacts on the top and bottom of the photovoltaic cell, the current can be drawn off to be used. For example, the current can power a calculator. However, conventional photovoltaic panels made from silicon to provide electricity are expensive, and thus not cost-competitive with electricity from the power grid.

Wednesday, June 18, 2008

Perfecting A Solar Cell By Adding Imperfections

ScienceDaily (Jun. 18, 2008) — Nanotechnology is paving the way toward improved solar cells. New research shows that a film of carbon nanotubes may be able to replace two of the layers normally used in a solar cell, with improved performance at a lower cost. Researchers have found a surprising way to give the nanotubes the properties they need: add defects.


Currently, these solar cells, called dye-sensitized solar cells, have a transparent film made of an oxide that is applied to glass and conducts electricity. In addition, a separate film made of platinum acts as a catalyst to speed the chemical reactions involved.

Both of these materials have disadvantages, though. The oxide films can't easily be applied to flexible materials: they perform much better on a rigid and heat resistant substrate like glass. This increases costs and limits the kinds of products that can be made. And expensive equipment is necessary to create the platinum films.

Jessika Trancik of the Santa Fe Institute, Scott Calabrese Barton of Michigan State University and James Hone of Columbia University decided to use carbon nanotubes to create a single layer that could perform the functions of both the oxide and platinum layers. They needed it to have three properties: transparency, conductivity, and catalytic activity.

Ordinary carbon nanotubes films are so-so in each of these properties. The obvious ways of improving one, though, sacrifice one of the others. For example, making the film thicker makes it a better catalyst, but then it's less transparent.

Previous theory had suggested that materials may function better as catalysts when they have tiny defects, providing sites for chemicals to attach. So the researchers tried exposing the carbon nanotubes to ozone, which roughs them up a bit. Very thin films, they found, became dramatically better catalysts, with more than ten-fold improvement.

Thin, transparent nanotube films catalyze the reduction of triiodide, a reaction important for the dye-sensitized solar cell, with a charge-transfer resistance as measured by electrochemical impedance spectroscopy that decreases with increasing film thickness. Moreover the catalytic activity can be enhanced by exposing nanotubes to ozone to introduce defects. Ozone-treated, defective nanotube films could serve as catalytic, transparent, conducting electrodes for the dye-sensitized solar cell.  Other applications include batteries, fuel cells, and electroanalytical devices.

In fact, the performance gets close to that of platinum. "That's remarkable," Trancik says, "because platinum is considered pretty much the best catalyst there is."

In order to address the trade-off between transparency and conductivity, the researchers tried another trick on a bottom layer of tubes: they created carbon nanotubes that were longer. This improved both conductivity and transparency.

The carbon nanotube films might be used in fuel cells and batteries as well.

"This study is an example of using nanostructuring of materials -- changing things like defect density and tube length at very small scales -- to shift trade-offs between materials properties and get more performance out of a given material," Trancik says. "Making inexpensive materials behave in advanced ways is critical for achieving low-carbon emissions and low cost energy technologies."

The researchers published their results recently in Nano Letters. They are currently in the process of filing a patent application for their techniques.


Adapted from materials provided by Santa Fe Institute, via EurekAlert!, a service of AAAS.

Special Coating Greatly Improves Solar Cell Performance

ScienceDaily (Feb. 26, 2008) — The energy from sunlight falling on only 9 percent of California's Mojave Desert could power all of the United States' electricity needs if the energy could be efficiently harvested, according to some estimates. Unfortunately, current-generation solar cell technologies are too expensive and inefficient for wide-scale commercial applications.


A team of Northwestern University researchers has developed a new anode coating strategy that significantly enhances the efficiency of solar energy power conversion. A paper about the work, which focuses on "engineering" organic material-electrode interfaces in bulk-heterojunction organic solar cells, is published online in the Proceedings of the National Academy of Sciences.

This breakthrough in solar energy conversion promises to bring researchers and developers worldwide closer to the goal of producing cheaper, more manufacturable and more easily implemented solar cells. Such technology would greatly reduce our dependence on burning fossil fuels for electricity production as well as reduce the combustion product: carbon dioxide, a global warming greenhouse gas.

Solar cell schematic showing a glass substrate (A) coated with the transparent conducting anodes (B), followed by the nickel oxide electron-blocking/hole transport layer (C), which in turn is then coated with the polymer-fullerene light absorbing/charge transporting layer (D), then an additional interfacial layer of LiF (E) and, finally, the device is completed by vapor deposition of the aluminum cathodes (F). (Credit: Michael Irwin, Northwestern University)

Tobin J. Marks, the Vladimir N. Ipatieff Research Professor in Chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering, and Robert Chang, professor of materials science and engineering in the McCormick School of Engineering and Applied Science, led the research team. Other Northwestern team members were researcher Bruce Buchholz and graduate students Michael D. Irwin and Alexander W. Hains.

Of the new solar energy conversion technologies on the horizon, solar cells fabricated from plastic-like organic materials are attractive because they could be printed cheaply and quickly by a process similar to printing a newspaper (roll-to-roll processing).

To date, the most successful type of plastic photovoltaic cell is called a "bulk-heterojunction cell." This cell utilizes a layer consisting of a mixture of a semiconducting polymer (an electron donor) and a fullerene (an electron acceptor) sandwiched between two electrodes -- one a transparent electrically conducting electrode (the anode, which is usually a tin-doped indium oxide) and a metal (the cathode), such as aluminum.

When light enters through the transparent conducting electrode and strikes the light-absorbing polymer layer, electricity flows due to formation of pairs of electrons and holes that separate and move to the cathode and anode, respectively. These moving charges are the electrical current (photocurrent) generated by the cell and are collected by the two electrodes, assuming that each type of charge can readily traverse the interface between the polymer-fullerene active layer and the correct electrode to carry away the charge -- a significant challenge.

The Northwestern researchers employed a laser deposition technique that coats the anode with a very thin (5 to 10 nanometers thick) and smooth layer of nickel oxide. This material is an excellent conductor for extracting holes from the irradiated cell but, equally important, is an efficient "blocker" which prevents misdirected electrons from straying to the "wrong" electrode (the anode), which would compromise the cell energy conversion efficiency.

In contrast to earlier approaches for anode coating, the Northwestern nickel oxide coating is cheap, electrically homogeneous and non-corrosive. In the case of model bulk-heterojunction cells, the Northwestern team has increased the cell voltage by approximately 40 percent and the power conversion efficiency from approximately 3 to 4 percent to 5.2 to 5.6 percent.

The researchers currently are working on further tuning the anode coating technique for increased hole extraction and electron blocking efficiency and moving to production-scaling experiments on flexible substrates.

The PNAS paper is titled "p-Type Semiconducting Nickel Oxide as an Efficiency-enhancing Anode Interfacial Layer in Polymer Bulk-heterojunction Solar Cells."


Adapted from materials provided by Northwestern University, via EurekAlert!, a service of AAAS.