While battery innovations get a lot of attention, there's a simple, proven long-term storage technique that's been used in the U.S. since the 1920s.

It's called pumped hydro energy storage. It involves pumping water uphill from one reservoir to another at a higher elevation for storage, then, when power is needed, releasing the water to flow downhill through turbines, generating electricity on its way to the lower reservoir.

Pumped hydro storage is often overlooked in the U.S. because of concern about hydropower's impact on rivers. But what many people don't realize is that most of the best hydro storage sites aren't on rivers at all.

We created a world atlas of potential sites for closed-looped pumped hydro - systems that don't include a river - and found 35,000 paired sites in the U.S. with good potential. While many of these sites, which we located by satellite, are in rugged terrain and may be unsuitable for geological, hydrological, economic, environmental or social reasons, we estimate that only a few hundred sites are needed to support a 100% renewable U.S. electricity system.