Once a hub of mining activity, today northern Germany's Harz mountains are a sleepy backwater, more likely to attract hikers and winter sports enthusiasts than industrial magnates.

But that may be about to change.

Engineers, together with the state government of Lower Saxony, are looking into the potential to turn former mines into a form of 'green battery.'

The idea would be a world-first engineering feat and could provide part of the answer to Germany's quest to boost renewable energies and make them more reliable.

Marko Schmidt, an engineer for Lower Saxony's Energy Research Center, has come up with a plan that would house a hydroelectric pumped storage station in abandoned mine shafts deep under the Harz.

Let gravity do the work

Wind energy is an increasingly popular alternative

The principle is simple.

At times of excess supply, wind energy can be used to pump water into elevated storage tanks or reservoirs within the moutains.

When the wind is low and not meeting demand, the stored water can flow down to tanks at a lower elevation, powering turbines along the way, much like a dam.

"Wind energy does not always blow at the same strength so the output of a wind farm usually goes up and down," Marko Schmidt told Deutsche Welle.

"Demand for electricity also varies, so it is good to have backup systems," he added.

Subterranean advantage

Above ground, these kinds of pumped-storage stations are already in use in several locations across Germany and around the world.

Schmidt's innovation – which would be a world first – is to site the facility below ground, adapting the existing infrastructure of dormant mines.

The engineer cited six potential locations for these plants in the Harz mountain region, with one specific pilot site identified in the abandoned Wiemannsbucht mine shaft in Bad Grund, a town in the western Harz mountains.

The fact that the plant would be concealed from the view of local residents is its biggest advantage, says the engineer.

"We presented our idea to the region and were very pleasantly surprised to find that people responded to it very positively. They identified with the project," he said.

"The tradition of mining is so great in the Harz region, that they want to see the mines back in use again, so there are practically no critics of the project," he added.

Nationwide, there is potential for 60-100 similar plants

Few critics

So far, the project hasn't met with any official planning objections, either. It has also been well received by renewable energy experts who admire the fact that it has been tailored to suit local conditions.

"I think this is really a brilliant idea and a very nice example of how easy it can be, under some local conditions, to achieve a 100 percent renewable energy supply," said Stefan Gsänger, the Secretary General for the World Wind Energy Association.

"Renewable energies are everywhere and you just need to find an appropriate way to harness them where you are," Gsänger added.

Schmidt has estimated in his study that the pilot plant in Bad Grund could be built within the next three to five years for between 170 and 200 million euros.

The storage capacity for such a plant would be up to 400 megawatts of electricity, enough to power around 40,000 households for a day.

If the project goes ahead, it is estimated that up to 200 people would be employed for its construction, and up to 7 others would be needed to run the plant, providing the region with jobs as well as clean energy.

Awaiting investors

Although no investors have come forward yet, the engineer says several parties are interested in the scheme.

He is confident that the model will be deployed not just in the Harz, but in other areas of Germany too, where surface installations of pumped storage facilities have met local resistance due to concerns about environmental impact.

"It is absolutely possible that this project will be realized in the near future, it's very realistic," Schmidt said.

"In the whole of Germany there are a further 60-100 sites which are also possible."

Author: Charlotte Chelsom-Pill

Editor: Nathan Witkop