Case for Solar Cells

March 18, 2008


It takes power to make power —even with a solar grand plan. From the mining of quartz sand to the coating with ethylene-vinyl acetate, manufacturing a photovoltaic (PV) solar cell requires energy—most often derived from the burning of fossil fuels. But a new analysis finds that even accounting for all the energy and waste involved, PV power would cut air pollution—including the greenhouse gases that cause climate change—by nearly 90 percent if it replaced fossil fuels.

Environmental engineer Vasilis Fthenakis, a senior scientist at Brookhaven National Laboratory in Upton, N.Y., and his colleagues examined the four most common types of PV cells: multicrystalline silicon, monocrystalline silicon, ribbon silicon and thin-film. (Other contenders, such as amorphous silicon or superefficient multijunction cells were excluded for lack of data or lack of widespread application to date.) Even taking into account the low efficiency of thin-film solar cells or the energy needed to purify silicon for the other types of PV, all proved to entail significantly fewer emissions in their entire life cycle than the fossil fuels needed to produce an equivalent amount of electricity. In fact, most of their dirty side derived from the indirect emissions of the coal-burning power plants or other fossil fuels used to generate the electricity for PV manufacturing facilities.

These four types of solar cells pay back the energy involved in their manufacture in one to three years, according to an earlier analysis by the same team. And even the most energy-intensive to produce—monocrystalline silicate cells with the highest energy conversion efficiency of 14 percent—emit just 55 grams of globe warming pollution per kilowatt-hour—a fraction of the near one kilogram of greenhouse gases emitted by a coal-fired power plant per kilowatt-hour.Even though thin-film solar PVs employ heavy metals such as cadmium recovered from mining slimes, the overall toxic emissions are “90 to 300 times lower than those from coal power plants,” the researchers write in Environmental Science & Technology.

The energy benefits of solar photovoltaics will only improve as the technology continues to boost its efficiency at converting sunlight to electricity or proves to last longer than the 30 years anticipated by manufacturers. “There is no reason for this not to last a lot more than 30 years,” Fthenakis says.

If solar energy begins to power its own production—a so-called PV breeder cycle, in which PV-generated electricity goes to produce more PV cells—the outlook is even sunnier. “I think 30 percent of the energy consumption in the [manufacturing] facilities is easily met from the land they have available [on] the roof and in the parking lot,” Fthenakis says.And, as Fthenakis and colleagues argued in a recent article in Scientific American, if storage technologies such as compressed air improve, then PV could provide the majority of electricity needs in the U.S. “With storage,” Fthenakis says, “it is feasible to go to 100 percent”.

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