04 May 2010

White-hot energy

STORING energy is one of the biggest obstacles to the widespread adoption of alternative sources of power. Batteries can be bulky and slow to charge. Hydrogen, which can be made electrolytically from water and used to power fuel cells, is difficult to handle. But there may be an alternative: magnesium. As school chemistry lessons show, metallic magnesium is highly reactive and stores a lot of energy. Even a small amount of magnesium ribbon burns in a flame with a satisfying white heat. Researchers are now devising ways to extract energy from magnesium in a more controlled fashion.

Engineers at MagPower in White Rock, British Columbia, for example, have developed a metal-air cell that uses water and ambient air to react with a magnesium fuel supply, in the form of a metal anode, to generate electricity. Doron Aurbach at Bar-Ilan University, Israel, has created a magnesium-based version of the lithium-ion rechargeable cell, a type of battery known for its long life and stability. It would be ideal for storing electricity from renewable sources, says Dr Aurbach. And Andrew Kindler at the California Institute of Technology in Pasadena is developing a way for cars to generate hydrogen on board by reacting magnesium fuel with steam. The reaction produces a pure form of hydrogen suitable for fuel cells, leaving behind only magnesium oxide, a relatively benign material, as a by-product.

But there is, of course, a catch. Although magnesium is abundant, its production is neither cheap nor clean, says Takashi Yabe of the Tokyo Institute of Technology. Various industrial methods are used to extract magnesium, ranging from an electrolytic process to a high temperature method called the Pidgeon process, but the energy cost is high. Producing a single kilogram of magnesium requires 10kg of coal, says Dr Yabe.

To change this, he is developing a process using only renewable energy. Dr Yabe’s solution is to use concentrated solar energy to power a laser, which is used to heat and ultimately burn magnesium oxide extracted from seawater—where, he says, there is enough magnesium to meet the world’s energy needs for the next 300,000 years. A solar-pumped laser is necessary, he says, because concentrated solar energy alone would not be enough to generate the 3,700˚C temperatures required. Dr Yabe calls his approach the Magnesium Injection Cycle.

The pure magnesium can then be used as a fuel (its energy density is about ten times that of hydrogen). When the magnesium is mixed with water, it produces heat, boiling the water to produce steam, which can then drive a turbine and do useful work. The reaction also produces hydrogen, which can be burned to produce even more energy. The byproducts are water and magnesium oxide, which can then be converted back into magnesium using the solar laser.

The trouble is that concentrated solar collectors tend to be huge and costly, and solar-pumped lasers are normally very low powered. Dr Yabe’s trick is to use relatively small Fresnel lenses—transparent and relatively thin planar lenses made up of concentric rings of prisms. These are commonly found in lighthouses to magnify light in a way that would normally require a much larger, thicker lens. His other trick is to boost the output power of the lasing material, neodymium-doped yttrium aluminium garnet. It normally only absorbs about 7% of the energy from sunlight, but when doped with chromium this figure increases to more than 67%.

Dr Yabe has built a demonstration plant at Chitose, Japan, in partnership with Mitsubishi. It is capable of producing 80 watts of power from the laser, enough to cut steel and extract 70% of the magnesium in seawater. The process will, says Dr Yabe, become commercially viable when the laser power reaches 400 watts, which could happen later this year. “As a starting point we are planning to use 300 lasers to produce 50 tonnes of magnesium per year,” he says. After that, it is just a small matter of convincing the world to start thinking about a magnesium economy instead of hydrogen one, he adds.

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