If at some point the government requires power plants to capture their emissions of carbon dioxide, a key challenge will be what to do with the stuff.

Some have advocated storing the heat-trapping greenhouse gas deep underground.

Engineers at Pennsylvania State University have come up with a clever alternative: Turn it back into fuel. They combine the carbon dioxide with water vapor to make methane, the primary component of natural gas, which can be burned in a generator.

The concept is not new, but the chemical reaction, which also yields oxygen, requires a lot of energy. Some previous attempts have tried using ultraviolet light, with limited success.

The Penn State team says it improved the reaction speed by a factor of least 20, just using regular sunlight. The key is that the gas conversion takes place in the presence of a high-tech catalyst: an array of titanium dioxide nanotubes coated with copper and platinum.

The researchers describe their results in a recent issue of the journal Nano Letters.

Team leader Craig Grimes, a professor of electrical engineering, says that in theory the process could be a "closed loop." Burn methane, produce carbon dioxide, convert to methane, and burn again.

"If you're going to capture the CO2," Grimes says, "then what the heck, you might as well do something with it and turn it into a fuel that you can use."

The initial efforts were on an experimental scale. Grimes projects that one square meter of the catalyst material would yield 500 liters of methane per hour; so commercial-scale production would require a large solar farm.

But if someday there were a tax on carbon emissions, this idea could be a lot more than hot air.

- Tom Avril