In some of the major European power grids, the penetration of variable renewable energy sources (VRES) has reached a level where their variability can no longer be easily absorbed by the grid as minor noise in the daily fluctuations of supply and demand. Taking for example the German grid (~ 60 GW load), the spot market prices now routinely "flat-line" at or below zero during nights and week-end on particularly blustery days. This forces wind farms to shut down (curtailment) during times when they would be at their most productive. Because the production potential of wind & solar is each highly synchronised across a relatively large geographic area, this trend will only increase as more wind and solar capacity is added to the same grid and profitably selling renewable energy "as-produced" will become increasingly difficulty. For the next stages of integration into the grid, wind & solar resources might have to be combined with storage, moving away from de...