Those are fascinating plans. Excess power to pump the water out, water let back in runs turbines to create power when needed. I wonder what environmental impact this might have, being not too far off the coastline. I wish they'd given the dimension of the "island."

'tis strange I should be old and neither wise nor valiant. From "The Maid's Tragedy" by Beaumont & Fletcher

I'm reminded of my Great Battery of Kimberly and Great Battery of Aberdeen Diaries here five and three years ago.

"The future is already here -- it's just not very evenly distributed" William Gibson

It's gonna take a pretty big island to do anything useful. Here's a paper that mentions an existing pumped hydroelectric plant in Colorado.

http://www.ceri-mines.org/documents/LargeScaleElectricalEnergyStorageinColorado-BarnesgroupCU_001.pd f

This one has a head of 374 meters (elevation difference between upper and lower reservoirs, and an area of about 7 hectares, and the resulting system can generate 330 MW of power and store 1,300 MWh of energy. That's comparable to the power you get from a smallish coal plant...

With an island, the head would be something like maybe 10 meters at most, I would think. Or less, even, so the area would have to be proportionally greater...

It's a peculiar idea. Similar ideas were drawn up for the Dutch and the Danes too, under the name "energy islands".

It's peculiar because it's a lot of investment for a little bit of balancing capacity. And in the case of Belgium, Netherlands and Denmark, they already have lots of cheap balancing, because they're each well-connected to their neighbours, and several of their neighbours are much bigger than they are, so the balancing requirement is relatively small.

I note that the Belgian Standaard says in its photo caption:

Hoewel er nog geen concreet plan is, is er wel al een ontwerptekening.

- Although there is no concrete plan, there is a design drawing.

Integrating storage with offshore wind might make sense if transmission costs to shore were a high proportion of system costs, and if they were (more than) proportional to their capacity. Transmission costs can be expensive, but this proposed island is only 3 or 4 km offshore, and the construction costs of the island must be huge by comparison.