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One of the big challenges of making 100 percent renewable energy [sic] a reality is long-term storage,” says Julian Hunt, an engineering scientist at the International Institute for Applied Systems Analysis in Austria. Hunt and his collaborators have devised a novel system to complement lithium-ion battery use for energy storage over the long run: Mountain Gravity Energy Storage, or MGES for short. Similar to hydroelectric power, MGES involves storing material at elevation to produce gravitational energy. The energy is recovered when the stored material falls and turns turbines to generate electricity. The group describes its system in a paper published November 6 in Energy.

“Instead of building a dam, we propose building a big sand or gravel reservoir,” explains Hunt. The key to MGES lies in finding two mountaintop sites that have a suitable difference in elevation — 1,000 meters is ideal. “The greater the height difference, the cheaper the technology,” he says. The sites will look similar, with each comprised of a mine-like station to store the sand or gravel, and a filling station directly below it. Valves release the material into waiting vessels, which are then transported via cranes and motor-run cables to the upper site. There, the sand or gravel is stored — for weeks, months, or even years — until it’s ready to be used. When the material is moved back down the mountain, that stored gravitational energy is released and converted into electrical energy.

Not only is the system more environmentally friendly [sic] than pumped-storage hydropower and dams, but it’s more flexible to meet varying energy demands.

“Hunt estimates that the annual cost of storing energy via this system will vary between $50 to $100 per megawatt hour (MWh),” the report adds. “And he says that the energy expended to transport materials to the upper sits will be offset by the amount of gravitational energy the system produces.”