Hydroelectricity is unfashionable because:<p>a) there aren't that many places left to build conventional hydro.<p>b) of the places that are left, there's been a lot of (IMO justified) opposition on the basis that flooding wilderness and disrupting river ecosystems is bad (also, flooding rainforest tends to result in a shedload of CO2 and methane, so the climate effects of conventional hydro are nontrivial).<p>c) Because of and b, new hydro is pretty much dead in developed countries and difficult to get funding for in the developing world.<p>d) until very recently, there's been little need for more pumped hydro because it's cheaper to build peaking gas plants, and a wash for the environment because the energy has (mostly) been coming from coal or gas anyway.<p>e) with the introduction of more and more wind and solar into the grid, the need for energy storage to match supply and demand has become much greater.<p>f) Hence, there's increasing interest in new pumped storage projects around the world, or other changes to hydro to better match the peakiness of energy prices. These can be either completely new systems, or modifications to existing ones. For instance, rather than running a 100MW generator 24 hours a day, you might put an extra 300MW of capacity but only run it at peak times.<p>g) Pumped hydro isn't the only game in town for energy storage. Aside from lithium-ion batteries (which is more economically competitive as a storage technology than some seem to think), there's things like iron flow batteries, thermal storage, and, yes, hydrogen.