Relying on coal power, the German city of North-Rhine Westphalia used to generate a third of the nation’s power. Built in the 1970s, the 2,000-foot-deep Prosper-Haniel hard coal mine powered Germany’s industrial revolution. But in November 2016, Germany – among other countries like Poland and Denmark – adopted a strategy of cutting coal to reduce carbon emissions, leading to the impending closure of the Prosper-Haniel mine in 2018, only to transform it into a 200-MW pumped-storage hydroelectric reservoir. When completed, the mine will store enough power for 400,000 homes, according to North-Rhine Westphalia’s Governor Hannelore Kraft.


The Coal Mine. Goseteufel

The decision to turn the mine into a Hydroelectric Reservoir may step in to solve two of the most challenging obstacles created by the country’s shift to renewable energy. Germany’s most populous city contributes a fifth of the country’s economic output. Bearing that in mind, the decision provides a new economic activity in a city where generations of workers have relied on fossil fuel for as long as they have lived. On a wider scale, it facilitates the expansion of renewable energy by helping to maintain electric capacity despite the absence of wind and sun shine.


A diagram of Prosper-Haniel

A Diagram of the Hydroelectric Reservoir. University of Duisburg-Essen

One of the advantages of having energy stored in an underground mine is increased capacity. While flat land energy storage (FLES) requires the excavation of an underground chamber, there are many inactive underground mines that could potentially act as energy storage facilities. At times when compensation is needed for the lack of wind and solar power, up to one million cubic meters of water could be plunged as deep as 1,200 meters, turning turbines at the foot of the colliery’s mine shafts. The Prosper-Haniel mining complex will be comprised of 26 kilometers of horizontal shafts.

It’s been years since researchers have started suggesting underground energy storage as a solution for renewable energy transitions. A Stanford professor along with his colleague released a study in late 2015 that suggested storing solar energy underground to both heat up and cool houses at a low cost. They argued that summer’s thermal energy gathered in rooftop solar collectors could be stored in soil or rocks. With the help of hydroelectric facilities, excess or low-cost electricity could be used to make ice for cooling.