Storing energy in flywheels is old. But at a cost competitive with the power grid is uncommon. And low losses over half a day storage, using affordable technology, hasn't been done, I believe. Links to my enabling technologies:
A cheap flywheel material storing much energy, and how to process the wheel
http://saposjoint.net/Forum/viewtopic.php?...k=t&sd=a#p22398
http://saposjoint.net/Forum/viewtopic.php?...k=t&sd=a#p31298
Here a low-loss hydrostatic axial bearing:
http://saposjoint.net/Forum/viewtopic.php?...&p=33472#p33178
http://saposjoint.net/Forum/viewtopic.php?...&p=33472#p33201
There a low-loss roller bearing for horizontal axis:
http://saposjoint.net/Forum/viewtopic.php?...&p=33472#p33263
http://saposjoint.net/Forum/viewtopic.php?...&p=33472#p33273
And this shall achieve low loss in normal air:
http://saposjoint.net/Forum/viewtopic.php?...&p=33472#p33461
From the cost I evaluated, it would be cheaper to build power plants to produce the mean daily electricity consumption only, and provide the peak consumption over such flywheels.
Also nice if a country lacks electricity production capability, like Japan now. Or if a country wants to close some plants in the future, like Germany.
The designs are to operate immediately after an earthquake with 2G upwards acceleration and 3G sidewards. Useful as an emergency supply, when power plants shut off and lines break. Few units can supply a hospital or a factory.
Cheap storage capability for half a day means that Solar electricity (I mean: Solar thermal electricity) becomes available all the day and dependable in favourable places like California, Neguev, Atacama and many more.
Affordable storage over a few days - as it now seems - makes wind energy dependable in places like Scotland, Brittany, Galicia, Patagonia and more.
Marc Schaefer, aka Enthalpy