Here's why coal mines could be crucial cogs in the transition to renewables

Seen by many as Europe's economic powerhouse, Germany is also looking to be a leader when it comes to green energy.

Its Federal Ministry for Economic Affairs and Energy has described the expansion of renewable energy as "one of the central pillars" in Germany's energy transition, also known as the "Energiewende".

Last year, for example, renewables were responsible for 29 percent of gross electricity generation in Germany.

With renewables, finding reliable ways to store energy is crucial, because sources such as wind and solar do not promise a constant stream of power. If storage solutions can be found, then the transition to renewable sources of energy will be that much smoother.

As Germany looks to move to renewable sources of energy, some innovative ideas are being developed as mining companies rethink their business models.

The Prosper-Haniel coal mining facility in Germany's western state of North-Rhine Westphalia is slated to close in 2018. Efforts are now underway to extend its life once coal is no longer mined. The plan? An underground, pumped storage system.

According to the U.S. Department of Energy, pumped storage hydropower (PSH) "works like a battery." With a PSH system, water is pumped from a lower reservoir to an upper one "for storage and later generation", the DOE says.

Usually, schemes make use of a reservoir on a hill and another in a valley. According to those developing underground pumped storage facilities, conventional schemes are limited by "topographical conditions."

With an underground pumped storage plant, the lower reservoir can be placed below ground, with "drop heights" of up to 1,200 meters.

Andre Niemann, from the University of Duisburg-Essen's Center for Water and Environmental Research, works on the scheme in North-Rhine Westphalia.

"The basic idea is a classical pump storage scheme: you have an upper reservoir and then a lower reservoir," he told CNBC.

Niemann said that the upper reservoir was set to be built on existing infrastructure, while the lower reservoir would be located more than 500 meters deep in the rock.

He added that after investigating a number of facilities, it was concluded that a capacity of 200 megawatts would be suitable for the site.