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Sunlight used to smelt zinc

August 4, 2005 By Mark Peplow This article courtesy of Nature News.

Solar technique could lead to cleaner, cheaper hydrogen.

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Scientists have found a way to harness the Sun's energy to extract zinc metal, which can then be used to produce hydrogen simply by pouring water over it.

With improvements, the process may prove a cleaner, more efficient way of producing hydrogen for fuel-cell-powered vehicles, which would emit nothing more polluting than water.

Current methods of producing hydrogen gas rely either on the fossil fuels they purport to replace, or water-splitting technology that has so far been too inefficient to deliver cheap hydrogen.

It has long been known that metals such as zinc can release hydrogen from water. But purifying the metal is the hard part. The traditional method of obtaining zinc involves many chemical steps, baths of acid and masses of electricity.

We have a lot of zinc powder available here for everyone.
Michael Epstein
Weizmann Institute
Researchers at the solar-powered plant at the Weizmann Institute of Science in Rehovot, Israel, have found a better way to deliver the metal. They use 64 seven-metre-wide mirrors to focus a beam of sunlight onto a tower containing the mineral zinc oxide and wood charcoal. The beam delivers 300 kilowatts of power, heating the chemical reactor up to 1,200 °C and delivering up to 50 kilograms of powdered zinc per hour.

"We have a lot of zinc powder available here for everyone," laughs Michael Epstein, part of the Weizmann team. "And this could be done on a very large scale," he adds. "We can imagine solar plants around the Mediterranean producing zinc."

As a bonus, he adds, the zinc should also be useful for making batteries.

Epstein will present results from the SOLZINC project, which includes researchers from Switzerland, Sweden and France, on 8 August at the International Solar Energy Society conference in Orlando, Florida.

Cleaning up

The process isn't yet entirely clean. The zinc-forming reaction also releases carbon monoxide from the charcoal, which eventually converts to the greenhouse gas carbon dioxide in the atmosphere.

In a full-scale industrial process, the carbon monoxide could be harnessed to help produce even more hydrogen from water. But this too would produce carbon dioxide.

For now the process produces as much carbon dioxide as extracting the same amount of hydrogen from natural gas, Epstein says. But, he adds, the carbon in his reaction is a renewable resource rather than a fossil fuel.

Eventually, the team hopes to replace charcoal with agricultural waste. And, if they can get the solar mirrors to heat things up to 1,800 °C, they would be able to extract zinc without any carbon.

Transport friendly

"It's an interesting option," says John Maddy, a hydrogen-power expert at the University of Glamorgan in Pontypridd, Wales. The work could be useful in sunny climes, he says. But, he adds, transporting either zinc or hydrogen over long distances is a major hurdle. "I'm more of a advocate of local resources," says Maddy.

The team is trying to produce other, lighter metals, such as magnesium, in the same way, although these require hotter temperatures to extract.

If a clean way can be found to make these low-density metals, Epstein suggests, they could be used to produce hydrogen right in the tank of a car. That would remove the need to transport the gas altogether.


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