Say energy storage and most imagine EV lithium-ion batteries. But a range of "long duration" concepts that store power for weeks rather than hours are coming to market, among them one called high-density hydro that uses a mud-brown slurry pumped through a long loop of plastic pipe on a hillside to store energy until it's needed. With first systems now being built, the technology could change the way renewables-powered grids work in the future.