Here’s what you’ll learn when you read this story:For more than a century, pumped hydropower has been the go-to technology for long-term energy storage. To pull off this long-term storage, green energy engineers have relied on a decades-old technology known as pumped hydropower. Called “geochemical energy storage,” this process pumps water into impermeable rock within the Earth and stores it there under pressure. “The water in a GES system, minus what is lost on evaporation, is reused over and over in a closed-loop system,” Quidnet states on its website . However, long-term storage is a different commercial regime than short-term storage, which relies on lithium-ion and ( increasingly ) iron-air batteries, so lower efficiencies are still useful.