A Cool Trick for Solar Cells
By employing a trick normally used to cool high-performance computer chips, IBM researchers have found a way to make concentrated photovoltaic cells that are more efficient in converting the sun’s energy into electricity.
The researchers have shown that it is possible to increase the concentration of light on photovoltaic cells by about ten times without causing them to melt. This, they say, makes it possible to boost the amount of usable electrical energy produced by up to five times.
IBM is not known for its work in solar energy, but that has changed recently, with the rising cost of fuel and the growing interest in renewable alternative energies, says Supratik Guha, lead scientist of photovoltaic research at IBM’s T.J. Watson Research Center, in Yorktown Heights, NJ. “About a year and a half ago, we decided to start looking at photovoltaics,” he says.
The principle behind concentrated photovoltaic cells is to use a large lens to focus light onto a relatively small piece of photovoltaic semiconductor material. The benefit is that only a fraction of the semiconductor material is used, thereby reducing costs.
There are a number of companies marketing such technologies, but one of the main challenges is in coping with the vast amounts of heat produced by the focused sunlight, says Guha. “You’re really heating the chip up. As you raise the temperature of the chip, its efficiency drops, so you’ve got to keep the temperature down.” There are generally two ways to do this: either by using passive heat sinks–metal blocks that draw the heat away from the cell–or, for higher-temperature systems, by using water cooling, in which water is pumped through a metal heat sink to draw the thermal energy away more efficiently.
In many respects, this is a problem very similar to cooling computer chips–something with which IBM has a long history, says Guha. State-of-the-art chips now kick out about 100 watts per square centimeter, which is similar to what concentrated photovoltaic cells have to endure, he says.
So Guha adapted an IBM-developed material currently used for chips to improve the heat transfer between the photovoltaic cell and a water-cooled heat sink.
“If you place the chip on a copper heat sink, the interfacial heat transfer isn’t good enough to keep the temperature down,” says Guha. This is because microscopic indentations in both surfaces means that there will be relatively little surface contact between the faces. So photovoltaic companies tend to use various organic pastes to act as thermal interfaces. The problem is that such materials aren’t particularly efficient at transferring heat.
IBM’s solution is to place an ultrathin layer of liquid metal, a compound of gallium and indium, between the two surfaces. “The main benefit here is that it’s a metal, so it has a very high thermal conductivity,” says Guha. And because it’s a liquid, it is possible to make this layer extremely thin, typically around 10 micrometers.
Using this simple solution, Guha and his colleagues have demonstrated that they can focus the equivalent of 2,300 times the sun’s natural energy on a one-centimeter-square photovoltaic chip. Without cooling, this would melt steel, he says. The photovoltaic cell temperature would be in excess of 1,500 °C, and therefore would simply vaporize. With the liquid metal and water-cooling system, the IBM photovoltaic material remains at 85 °C.
“I’m sure there will be interest from [concentrated photovoltaic] companies, and it’s definitely something we would want to investigate,” says Stephen Bates, CEO of Whitfield Solar, in Reading, U.K. “But it would have to be exceptionally low cost because the industry is so incredibly price sensitive,” he says.
However, despite the promise of this approach, IBM is not planning on branching out into the solar-energy market. “We don’t plan to make [concentrated photovoltaic] systems,” says Guha. Instead, IBM is in talks with solar-cell companies about licensing the technology, he says.
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