It's more efficient and simpler to use that spare energy to pump water uphill:

http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

That pumped water requires a lot of space up there, so viable spots are fairly limited. It also is fairly difficult to transport compared to hydrogen.

If we're talking about the US, we have tons of open space. Really, tons. Americans don't appreciate just how much open space is in this country. If it's really a concern, it can be done with water towers rather than a conveniently-placed hill.

As for transport, that's what a power grid is for. Transporting hydrogen loses more efficiency than transmission losses.

Batteries are way ahead on every objective measure, and getting better every year.

I'm afraid the physics just don't work out for Hydro.

See the Hydro section of Sustainable Energy - without the hot air by David JC MacKay:

http://www.withouthotair.com/c8/page_55.shtml

Even though the land mass of the US is larger than the UK, it's hard to see hydro being more than MacKay's 2kW per day per person...

That's two 40W light bulbs left on all day.

That's not the same thing. In a standard hydroelectric dam, you need an existing river to create electricity.

The goal of pumped storage isn't to create new electricity, but to store large amounts of unused baseload power. This can be done with a standard hydroelectric dam by pumping the water back up into the lake behind the dam, but it can also be an entirely artificial reservoir.

Um have you seen a pumped storage power plants they are major civil engineering constructions.

You could probably add a small hydrogen storage facility to every wind farm in the UK for less than the cost of the public inquiry into a new pumped storage scheme.

yes but the number of places that are suitable are limited and it might be politicaly sensitive eg flooding Wales and Scotland to provide power for the English.

And farmers might object to you taking the water they use for irrigation