Technology

Pumped hydro is both very geologically limited and environmentally detrimental.

If you are willing to live with the very considerable impact and are willing to do a costly megaproject, one possibility that I've raised before: it'd be possible to go implement Atlantropa, but instead of using it (exclusively) to generate hydroelectric power, as its creator envisioned, use it for pumped storage. The world will never need more energy storage than that could provide.

https://en.wikipedia.org/wiki/Atlantropa

Atlantropa, also referred to as Panropa,[1] was a gigantic engineering and colonisation idea that German architect Herman Sörgel devised in the 1920s, and promoted until his death in 1952.[2][3] The proposal included several hydroelectric dams at key points on the Mediterranean Sea, such as the Strait of Gibraltar and the Bosporus, to cause a sea level drop and reclaim land.

The central feature of the Atlantropa proposal was to build a hydroelectric dam across the Strait of Gibraltar, which would have generated enormous amounts of hydroelectricity[4] and would have led to the lowering of the surface of the Mediterranean Sea by as much as 200 metres (660 ft), opening up large new areas of land for settlement, such as in the Adriatic Sea. Four other major dams were also proposed:[5][6][7]

• Across the Dardanelles to hold back the Black Sea
• Between Sicily and Tunisia to provide a roadway and to lower the inner Mediterranean further
• On the Congo River below its Kasai River tributary, to refill the Chad basin around Lake Chad, provide fresh water to irrigate the Sahara, and create a shipping lane to the interior of Africa
• Extending the Suez Canal and locks to maintain connection with the Red Sea

Sörgel saw his scheme, which was projected to take more than a century, as a peaceful pan-European alternative to the Lebensraum concepts that later became one of the stated reasons for Nazi Germany's conquest of new territories. He envisioned Atlantropa as a way of providing land, food, employment, electric power, and, most of all, a new vision for Europe and neighbouring Africa.

There are two very considerable issues there:

• First, dropping the Mediterranean Sea by 200 meters is going to have a very large impact on the coasts of northern Africa and southern Europe. Sörgel considered that desirable, but obviously there are going to be a lot of people who don't like such a change.

• Second, if it's permitted to build structures in this new area -- as was originally intended -- then a rupture of the dams would produce cataclysmic flooding; we would essentially have recreated the Zanclean flood:

  Ninety percent of the Mediterranean Basin flooding occurred abruptly during a period estimated to have been between several months and two years, following low water discharges that could have lasted for several thousand years.[3] Sea level rise in the basin may have reached rates at times greater than ten metres per day (thirty feet per day). Based on the erosion features preserved until modern times under the Pliocene sediment, Garcia-Castellanos et al. estimate that water rushed down a drop of more than 1,000 metres (3,000 ft) with a maximum discharge of about 100 million cubic metres per second (3.5 billion cubic feet per second), about 1,000 times that of the present-day Amazon River.

  The Royal Air Force bombed two dams in Germany during World War 2 to flood an industrial area in Germany. Russia just blew up a hydroelectric dam in Ukraine that caused a mess and water to drop upstream by 2 meters. If such a dam were to be attacked in a war like that, it would be horrendous. We'd be talking about a water depth difference a hundred times that and a far larger area.

EDIT: And a third, I suppose -- if you take water out of the Mediterranean via evaporation and pumping, it will eventually wind up elsewhere, and we live in an era where sea level rise is already a concern, so it'll cause sea level rise elsewhere. Would eliminate concerns about sea level rise for the Mediterranean, though...

at least it works at scale relevant to grids. there are other interesting devices that store high grade heat in things like molten silicon or sand, then convert it to electric energy again, but it's rather at prototype scale now i think. power to hydrogen is fine if it's replacing hydrogen from natural gas, but it's wack for storage of energy