An anonymous reader quotes a report from IEEE Spectrum:Not only is the system more environmentally friendly 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."