National Geographic

Renewableenergy supply``````EnergysurplusEnergydemand``````Energyload``````Time of day``````Waterpumped (^) tosurfaceEnergydeficitEnergydeficitWithout energy storage,renewable energy is wastedwhen demand is low.When there is moreenergy supply thandemand, excess poweris used to pump waterupward from the un-derground reservoirthrough existing shaftsto the surface reservoir.CLEANENERGY INA COAL MINE2THE CHALLENGE: Renewable energy fromthe wind and sun is abundant; storingit for future use has been a persistentpuzzle. An Australian project uses agiant lithium-ion battery, while one inNevada employs molten salt. Germany,which aims to run on 80 percent renew-able power by 2050, is turning to theinfrastructure of the very industry thecountry aims to replace—coal. The plan:Transform the Prosper-Haniel mine inBottrop into a battery. This would beamong the first underground projectsusing pumped hydropower, which hasproved to be an eicient energy storagetechnology. But even if clean, it’s expen-sive—and still in search of funding.IN THIS SECTIONShaping the GlobeMoving MountainsShe Stalks SharksArmadillo AmourILLUMINATING THE MYSTERIES AND WONDERS AROUND US EVERY DAYNATIONAL GEOGRAPHIC VOL. 234 NO. 1EXPLOREOn the surface,wind turbines andsolar panels gener-ate renewable energy.The coal mine, whichis set to close thisyear, is underneath.128 NATIONAL GEOGRAPHIC