Traditional pumped hydro and modular pumped hydro are storage at the margin, as, indeed, is consumption-shifting at the demand side.

The "zero order" is the portfolio effects allowed by a subcontinent wide loose grid of electricity superhighways ~ night falls at different times across the continent, the wind is often blowing one place when it is not blowing another, diurnal patterns of onshore and offshore wind are distinctive ~ even with just wind and solar, a European sub-continent-wide portfolio is far more stable than any individual national portfolio would be.

The first order storage is turning off conventional hydro when the volatile renewable power portfolio is producing to current consumption and ramping up conventional hydro when the volatile renewable power portfolio is producing below current consumption.

Another first order storage for scheduled load is biocoal, which can be stored in the form known as a "pile of the stuff" ... a renewable biomass feedstock for biocoal has a fixed annual budget, but the amount that can be deployed in a day is driven by generating capacity, with the annual budget determining how many generation days are available to supplement the volatile renewable portfolio. In a steady state, that is likely best converted to electricity with direct carbon fuel cells, but as a transition, existing coal thermal plants that are presently obsolete due to the need to refrain from CO2 emissions provide a massive per day back up capacity.

If the first order storage falls short, then the storage capacity of the dammed hydro can be stretched by conventional pumped hydro.

And then if that falls short, then the modular pumped hydro, or ammonia energy storage, or one of the other pure energy stores come into the frame. There is ample technical capacity in existing storage technologies to cope with any shortfall in the first order storage capacities, so which of those to use is an issue of which is the least cost per stored kW over the storage period typically required.

If all you need is sufficient storage capacity to cover the time required to bring biocoal thermal up from cold to generating (in this setting, you do not have thermal plants as "spinning reserve"), ammonia would seem appealing, since its main generation would be for petroleum-independent fertilizer feedstock with the stored power a diversion from output of a energy consumer that is in any event already one of your consumption-shifting electricity consumers ~ that's a big part of the promise of the newer solid-state ammonia production technologies, that in reducing the fixed cost of ammonia production, you reduce the requirement to run the production facilities 24/7 and you have an industry that is far better adapted to have excess capacity that is brought online and taken offline in reaction to smart-grid electricity pricing.

If you need longer term storage, modular pumped hydro is attractive ~ the limit of power stored per metric ton of water is determined by the rise, and while the rise is constrained with conventional pumped hydro to the original fall of the river that was dammed, the modular pumped hydro can take advantage of substantially higher rises, with the pipe and the upper reservoir entirely passive, and with all of the active equipment down by the lower reservoir down in the valley, where its easier to get to for maintenance.


I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.