The trade-off here is these seem to be substantially simpler and cheaper than batteries. No fancy metals, no wear cycles, no risk of spontaneous fire. In fact, the up-front environment impacts seem significantly less than that of battery storage. These are still a good solution when the power supply you're using is in excess - as is common with most natural sources (solar, wind, hydro, etc).

Lastly, it's worth remembering that not everything is competing against an "optimal" solution. This may be a _very_ viable system for infrastructure that simply sheds excess energy. Shedding of excess energy results in 0% efficiency gains. Additionally, this type of storage potentially offsets the need to run a secondary system for peak/off-cycle loads. That in itself can be a massive energy savings.

-----

Finally, it seems that power plants are actually terribly inefficient at converting an energy source to electric. Coal - 33%, gas - 42%, and combined cycle - 60%.

http://needtoknow.nas.edu/energy/energy-sources/fossil-fuels....