It's really a badly written article about an interesting topic - and I'm not sure what the real story is. Considering the graphic: http://images.bwbx.io/cms/2014-11-26/tech_aluminumgraphic49_...
it looks like they are indeed releasing energy back to the grid.

Additional article: http://www.aluinfo.de/index.php/gda-news-en/items/trimet-dev...

It looks more like they are adapting the process to handle energy fluctuations. The current process most likely requires a steady current and they are adapting the process to ramp up during times of cheap energy and throttle it during times of expensive energy. Still no battery, more like a balancer, adaptively throttling energy consumption.

Edit

More information from a paper abstract [1]:

High Frequency Power Modulation - TRIMET Smelters Provide Primary Control Power for Stabilizing the Frequency in the Electricity Grid

Scope	The strong growth of renewable energy in Germany leads to high prices fluctuations, varying with the availability of these energy resources. To deal with this situation, TRIMET is using the tool “power modulation” since 2008 to compensate these strong price effects. Simultaneously the strong growth of renewable energy sources leads to a need of primary regulation energy for stabilizing the electricity grid at 50 Hz. In this context, TRIMET is the first electricity consumer world wide, which provides primary regulation energy by modulating a consumer. Traditionally this source of energy is only provided by power plants or energy storage plants, e.g. pumped energy storage or stationery batteries.

http://www.programmaster.org/PM/PM.nsf/ApprovedAbstracts/281...

Now were getting closer to the real thing.

Edit II [2]:

TRIMET Aluminium SE is developing a process that can turn electro-
lytic furnaces for the production of primary aluminum in collabora-
tion with conventional power plants into energy storage reservoirs
for renewable energy. Aluminum has a high energy density. This
makes its production energy-intensive and aluminum smelters re-
cipients of base-load electricity. Excess energy is stored in the light
metal. The principle behind the new process is the grid-commutated operation of the production equipment. To ensure this, the electrolysis cells are modified so that their output can be increased or reduced for several hours, as needed. The ope- rating point is selected in such a way that the output can be raised or lowered by more than 25 percent for hours. This makes it possible to provide enormous storage capacity. Using this method, TRIMET aluminum smelters could provide outputs of up to 130 megawatts with future capacities of up to 6,000 megawatt hours. This corresponds to the daily energy consumption of two million people.

The “virtual battery” relies on the improved flexibility of aluminum elec- trolysis. With this method, TRIMET is revolutionizing the basic principle of a manufacturing process that has existed for more than 120 years. An electrolytic furnace is similar to a water container whose walls are made of ice. To ensure that the tank does not melt, the ambient temperature must be kept very low. But it must not reach a temperature that is too cold, or the water in the tank will freeze. Similarly, the temperature of the electrolytic furnace must be harmonized in order to stabilize the fur- nace walls under fluctuating power supply levels. The process developed by TRIMET regulates this balance through the clever use of exhaust heat, a form of thermal insulation that can be adjusted in an innovative way.

The process offers significant advantages over other storage systems. At 85 percent, the efficiency factor of the aluminum storage reservoir approaches that of pump storage reservoirs, far higher than those of compressed-air or hydrogen storage reservoirs. what’s more, the system does not require new power lines, since it is
integrated into the existing high-voltage grid.

[2] http://www.trimet.de/fileadmin/pdf/trimet-at-a-glance-2013.p...