Molten Aluminum Lakes Offer Power Storage for German Wind Farms

Aluminum ingots are stacked outside for cooling
Aluminum ingots are stacked outside for cooling at an Alcoa World Alumina Australia smelting plant, partially owned by Alumina Ltd., in Point Henry, Australia, July 30, 2008. Photographer: Carla Gottgens/Bloomberg News

Germany’s sprint toward renewable energy makes wholesale power cheap, even free, when the sun is shining and the wind is blowing. Absent that, prices spike -- this year the day-ahead price of a megawatt-hour has ranged from 52.68 euros ($65.35) to negative 4.13 euros.

The system doesn’t yet have dedicated storage equipment capable of holding large amounts of renewable energy.

Trimet Aluminium SE, Germany’s largest producer of the metal, is experimenting with one answer in a pilot project: using its vast pools of molten metal as virtual batteries.

Making aluminum is extremely power-intensive. To yield each ton of the flexible metal, derived from bauxite that’s been converted into molten aluminum oxide, Trimet needs roughly 14 megawatt-hours, or about 500 euros worth of power. Multiply that by 500,000, the number of tons of aluminum Trimet produced using electrolysis last year, and that’s a lot of power and money humming through the company’s plant near the Rhine-Herne Canal in the western German city of Essen.

For the most part, Trimet turns aluminum oxide into aluminum by way of electrolysis, the use of an electric current to stimulate a chemical reaction. Negative and positive electrodes in a tank separate the compound into aluminum and oxygen. The electrodes, in tandem with the liquid metal that settles to the bottom of the tank and the oxygen above, form an enormous battery.

Adjust Consumption

By varying the rate at which the metal is produced, the plant will be able to adjust the power consumption of the 290-megawatt smelter up and down by about 25 percent. Trimet can soak power from the grid when energy is cheap. It can then resell the power when demand is at its peak. The company can temporarily reduce its power consumption by slowing the electrolysis, cutting the energy drain.

Heribert Hauck, Trimet’s head of energy management, said the plant eventually will be able to store the aluminum equivalent of as much as 3,360 megawatt-hours over a two-day period -- enough to power more than 300,000 homes for a day. “The results can be transferred to the whole production process without any changes,” he said, estimating the energy conversion efficiency at as much as 90 percent.

Consumption of aluminum, almost half of which is produced in China, followed by Russia, Canada and the U.S., has been growing as the metal, long a primary component in jet aircraft bodies, has become increasingly common in cars and consumer electronics. U.S. manufacturers alone produce about 100 billion aluminum beverage cans per year.

Commercial Sense

Using the production process as a virtual battery is “an interesting option” for the industry, said Marian Klobasa, who heads the demand response and smart grids unit at Germany’s Fraunhofer Institute for System and Innovation Research ISI, an applied science researcher. If the volatility of the power market increases, Klobasa said by phone from Karlsruhe, “then it may make commercial sense and be economically worthwhile.”

Trimet’s Hauck estimates the full costs 70 euros to 150 euros to store one megawatt-hour based on 1,000 hours of use per year. That would make supply competitive at some periods of peak demand during the winter.

The principle Trimet is experimenting with may also work for other energy-intensive industrial processes, said Klobasa, including those used to manufacture cement, paper, and chemicals. Making chlorine, used to produce paper, plastic, fabric, paint, drugs, and antiseptics, also requires electrolysis.

Playing a similar game with chlorine production, Klobasa says, may be an “essential lever” for stabilizing the power market by helping match power demand with capricious supply from renewables.

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