Pumped hydro is interesting but the reservoir requirements are huuuge. This post makes a very convincing case that scaling it up is not that simple:

http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

There is some discussion about the German rock variation in the comments. The density doesn't seem to help THAT much compared to water only.

Density isn't the only benefit of this system. At these scales, the water requirements alone are significant. This replaces many cubic meters of that water with rock.

The other big advantage is that you don't need a tall hill to put a reservoir on top of.

Sadly it seems the project has been at the "plan to get funding for a pilot" stage for at least 5 years now, and I still don't see how it'd seal.

Wonder how this works in relation to the earthquakes we're getting here in Oklahoma due to injection wells.

"to get the amount of energy stored in a single AA battery, we would have to lift 100 kg (220 lb) 10 m (33 ft) to match it. To match the energy contained in a gallon of gasoline, we would have to lift 13 tons of water (3500 gallons) one kilometer high (3,280 feet). It is clear that the energy density of gravitational storage is severely disadvantaged."

http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

It's almost incredible that a AA battery can do that much work, but I checked the math:

  Alkaline AA Battery =~ 3 Wh =~ 10000 Nm
  Gravitational Potential Energy =~ kg * m * 10 N/kg
  100 kg * 10 m * 10 N/kg == 10000 Nm == AA Battery

It also puts into perspective how much solar energy is available. A single 200 W panel (~5 sq ft or ~.5 m^2) can charge hundreds of AA batteries per day, and thus would require lifting more than 10000 kg by 10 m to store a single day's output from just that one panel!

Of course. Despite the title the article is only about off-grid storage, basically batteries. Though even off-grid storage can be on-grid if it's sometimes connected, as some plans for the future of plug-in hybrids call foresee.

Sadly you cannot very well use a miniature hydroelectric dam for off-grid energy storage on-board an electric car or electronic mobile device.