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Japanese team revives solar lasers in quest for clean fuels

05 Oct 2007

Researchers have developed a solar-powered laser that could be used to convert magnesium chloride in the sea into an emission-free fuel.

The idea of using solar energy to power lasers is not new. Current designs work by using a system of mirrors to concentrate sunlight into an Nd:YAG crystal, but these lasers are not widely used because they require huge mirrors to collect the light – and even then achieve only low efficiency.

To address these issues, Takashi Yabe and colleagues at the Tokyo Institute of Technology experimented with using a Fresnel lens instead of mirrors as light collectors. They also found that doping the Nd:YAG crystal with small amounts of chromium significantly increases the power output of the laser.

The laser demonstrated by the team produces a power output of 24 W at 1064 nm. The design, which incorporates a 1.3 m2 Fresnel lens, offers an unprecedented slope efficiency of 12% above a threshold solar input of 500 W. (The slope efficiency is defined as the ratio of output power to input power above the threshold value.)

The team has also been working on a larger system with a 4 m2 lens, and a prototype is now being tested. "It's too early to give an exact figure, but I think we should get an output between 150–200 W. Eventually, we hope to achieve an output of 1 kW."

One key challenge was to find an efficient way to dope the laser crystal, with theory suggesting that adding chromium ions to an Nd:YAG crystal would improve power levels. "We used a method developed by Kunio Yoshida of the Osaka Institute of Technology to mass-produce co-doped YAG ceramics. An addition of 0.1% chromium during the fabrication process almost doubled the laser's power output." said Yabe.

The choice of a Fresnel lens as a solar collector might seem surprising, since high chromatic dispersion makes most lenses unsuitable for focusing broadband solar radiation. "We set about calculating the chromatic dispersion of a Fresnel lens, and found it to be quite small. This was really quite strange," said Yabe.

"No one had tried using these lenses in solar lasers before," he added. "Fresnel lenses are ideal for our lasers - they are thin and can be built in large sizes."

The researchers say that a number of these lasers could be connected together to build an engine that converts magnesium chloride in the oceans to magnesium, which spontaneously burns to produce heat and hydrogen gas. The Japanese believe that magnesium could be a clean, emission-free fuel, but extraction from the sea and subsequent storage has proved a challenge.

"We think this laser may have the solution," said Yabe. "It is cost-efficient and capable of producing the energy needed to break the compound in seawater into elemental magnesium." And while solar lasers can only operate at top efficiency for only a few hours a day, they could offer a cost-effective and efficient way to produce environmentally friendly fuel in places that enjoy sunlight all year round.

Whether or not magnesium takes off as a fuel of the future, the researchers believe that this laser could also have applications in laser cutting and welding, nanoparticle fabrication, laser-assisted chemical reactions and in the refinement of metals.

The researchers reported their work in Applied Physics Letters.

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