Solar power produces, per unit of energy, only about one-tenth as much carbon dioxide and other harmful emissions as does conventional power generation, a new study shows.

Solar panels don't release harmful gases during use, but making the solar cells does consume materials and energy—mainly from conventional power sources such as coal-fired power plants, which in turn produce emissions. Industrial techniques for making glass and other materials in solar panels also produce gases such as carbon dioxide.

In the 1970s, manufacturing a solar cell required about as much energy as the cell could produce over its 20-year lifetime, so using solar power provided little if any energy gain. Also, as recently as 10 years ago, total emissions from solar cells were about twice what the new study shows. "Solar power has been criticized in the past" for requiring too much energy to produce, says Vasilis M. Fthenakis of the Brookhaven National Laboratory in Upton, N.Y. "But what we find out is that those criticisms are not true with the new technologies."

Fthenakis and his colleagues compiled production records from manufacturers of four popular kinds of solar cells: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film cadmium telluride. They calculated that, for each unit of energy produced by solar cells, the net emissions of greenhouse gases and other pollutants due to the cells' manufacture were between 2 and 11 percent of what power plants in the United States and the European Union would emit to make the same amount of energy, the scientists report online and in the March 15 Environmental Science & Technology.

The new tally shows that net emissions from solar power have decreased significantly in recent years. "There have been studies before, but they've become outdated because technology has been changing," says Fthenakis, the study's lead scientist.

"It's a really solid piece of analysis," comments Robert M. Margolis, senior energy analyst at the National Renewable Energy Laboratory in Washington, D.C. "It's the most up-to-date analysis on solar that's out there."

Much of the improvement is from reducing energy and materials for making solar cells. Compared to those made in the 1970s, modern panels contain about one-third as much purified silicon, which is energy intensive to make. And thin-film solar cells trim back even further by depositing silicon or other materials in layers only a few thousandths of a millimeter thick.

These improvements in efficiency mean that today's solar panels can "pay back" in only 1 to 3 years the energy needed to make them, the study concludes.

Improvements in manufacturing efficiency could reduce emissions from solar power by another 50 percent within 5 to 7 years, the researchers say.