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Solar-powered laser: Two Fresnel lenses focus sunlight on a ceramic crystal to produce laser light. The hope is to use such powerful lasers to generate heat and hydrogen from magnesium and water.

The idea, says Takashi Yabe, a professor of mechanical engineering and science at the Tokyo Institute, is to make a powerful laser capable of combusting the magnesium content of seawater. In the process, large amounts of heat and hydrogen are given off.

Magnesium has great potential as an energy source because it has an energy storage density about 10 times higher than that of hydrogen, says Yabe. It is also highly abundant, with about 1.3 grams found in every liter of seawater, or about 1,800 trillion metric tons in our oceans, he says.

Moreover, the magnesium oxide resulting from the reaction can be converted back into magnesium, says Yabe. The catch? Recycling the magnesium oxide back into magnesium requires temperatures of 4,000 kelvins (3,726 ºC)–hence the need for a laser to generate such temperatures on a small spot.

But for a magnesium combustion engine to function as a practical source of energy, the lasers need to be powered by a renewable energy source, such as solar power.

Solar-pumped lasers already exist: they work by concentrating sunlight onto crystalline materials such as neodymium-doped yttrium aluminium garnet, causing them to emit laser light. Until now, however, most solar-pumped lasers have relied on extremely large mirrors to focus the sunlight on the crystal.

Yabe and his colleagues have developed a compact laser that offers a threefold improvement in efficiency over previous designs, in terms of how much power it can deliver compared with the available sunlight.

This is partly due to the use of Nd:YAG crystals that are additionally doped with chromium, enabling them to absorb a broader range of light. Adding the chromium makes a greater proportion of the spectrum available, says Yabe: “Thus the efficiency from sunlight to laser is greatly enhanced.”

The other innovation of Yabe’s laser is the use of a small Fresnel lens instead of large mirror lenses. Fresnel lenses reduce the size and amount of material needed to build a lens by breaking it into concentric rings of lenses. Typically, 10 percent of incident light is focused on the crystal, whereas with the Fresnel, it’s around 80 percent.

“In our case, we used only 1.3 meter squared and achieved 25 watts,” says Yabe. Although this is only a threefold increase, the laser output exponentially increases with the increasing area. “So we are expecting 300 to 400 watts with the four-meter-squared Fresnel lens,” he says.

It’s an unusual approach, says Sunita Satyapal, head of the Department of Energy’s hydrogen-storage team, in Washington, DC. But it’s not the first time that metals, such as magnesium, and water have been explored as a means of hydrogen production, she says.

What is needed now is a total-efficiency budget for the entire system, says Satyapal: “The key issue is cost and total efficiency.” There are much simpler ways of generating hydrogen using sunlight, such as by employing solar cells to split water using electrolysis, she adds.