In the German state of North Rhine-Westphalia, a coal mine will close in 2018. Aging coal infrastructure, low wholesale power prices, and a move away from the highly polluting power source all make renewable energy the political darling of the day.

But that doesn’t mean the Prosper-Haniel coal mine will be shutting down completely. According to Bloomberg , North Rhine-Westphalia State Governor Hannelore Kraft recently confirmed that a project to turn the coal mine into pumped storage will move forward after mining activities have stopped.

Pumped storage has been used for decades, but placing a pumped storage scheme at a retired mine is somewhat new. Here’s how it works: when electricity is plentiful and cheap—say, on a windy day when the Sun is shining and solar panels and wind turbines are working at their maximum—a pumped storage facility pumps water from a lower reservoir up to an upper reservoir. When electricity is scarce, the facility can release the water back down to the lower reservoir through a turbine, creating renewable hydroelectric power.

Traditionally, pumped storage has required fairly specific terrain. But a used coal mine can offer the right environment for pumped storage without natural elevation differences and reservoir capabilities, although mines do require extensive analysis to make sure they’re geologically fit for pumped storage. The University of Duisburg-Essen (UDE) is working with mine owner RAG AG to complete the project, and, in a press release from August 2016, UDE professor André Niemann said the mine site is “suitable for the infrastructure to implement an underground pumped-storage power station as a closed system."

“We assume a storage volume of 600,000 cubic meters,” Niemann added at the time. “This means that at full charge you would have an output of about 200 megawatts for four hours.” The mine itself is 600m (nearly 2,000 ft) deep and has been in operation since 1974. Initial development of the pumped-storage project is being supported by funding from the North Rhine-Westphalia government as well as the EU.

According to Bloomberg, North Rhine-Westphalia is the source of more than a third of Germany’s power generation, including many coal plants. But the state plans to make 30 percent of its power mix renewably sourced by 2025, so officials are seeking energy storage solutions.

The challenge: Going from idea to practice

The idea of using a mine for pumped storage isn’t totally new, of course, but earlier projects have had trouble getting off the ground. The most promising project that’s currently in development may be a potential site in North Queensland in Australia, where a company called Genex Power wants to build a 250MW, six-hour pumped storage site on an abandoned gold mine. According to a report written by Genex’s executive director in The AusIMM Bulletin this month, first-round feasibility studies were recently completed on that project, and construction is slated to commence in 2018.

Other projects seem more tentative. Northland Power, a Canadian green power company, has plans listed on its website for a pumped storage facility located in a former open-pit iron mine in Ontario, but the last updates about the project come from 2013, and the company didn’t respond to Ars’ request for comment. According to news reports from 2013, another old iron mine in upstate New York was potentially going to be used for pumped storage. The project reappeared in the news in 2015, with reports indicating that the company building the facility—Moriah Hydro Corp.—had applied for a license to begin construction from the Federal Energy Regulatory Commission (FERC). But Moriah Hydro Corp. doesn’t seem to have made any big moves in the last two years, it doesn’t seem to have a website, and the company didn’t return Ars’ phone calls.

A study was done in Minnesota in 2011 to assess the potential of the state’s abandoned iron mining pits for pumped storage, but others suggested that the weak energy density of such projects were prohibitive to getting them off the ground.

But six years later, support seems to be growing for such projects. Renewable Energy World reported last week that Virginia’s legislative bodies have passed bills this year to spur development of pumped storage facilities, which could be situated at abandoned coal mines. A combined bill now awaits the signature of Governor Terry McAuliffe.

So perhaps pumped storage using a retired mine has some maturing to do, but with the right political will and some good investment, maybe this kind of creative reuse could be possible in the future.