Energy storage and battery banks Why is it not possible to store windfarm energy in battery banks? A lot of energy isn't being used when wind farms are at peak and we have no way of storing it. This is wasteful. So why don't we use  battery banks?
 A: We can. But they don't scale well: to get grid-level storage, you need to be able to scale to gigawatts of power, and gigawatt-hours of energy. And to be able to cycle hundreds, or thousands, of times.
To date, we have one technology which will do that, which is pumped storage hydro, which typically has a round-trip efficiency of 75%. So even though that's worse than a decent battery's round-trip efficiency, its scalability means that it dominates grid storage.
But that's just one small part of the picture. Behind the question of storage, is the physics question of how do you balance an electricity grid. The grid typically has very low capacitance, so electricity in and electricity out must balance at every second. To manage that balance, you can either adjust the amount going in, or the amount going out, or both.
There are lots of ways to integrate high wind penetrations into the grid: this is a solved problem technically. See, for example, the work in Energy Policy by Delucchi & Jacobson; or the book by Gregor Czisch on renewable scenarios.
There are more ways to do virtual storage than direct storage. For example, delaying consumption of 1GWh of electrical energy at 1GW power, for one hour, is equivalent to storing it at 100% efficiency for one hour.
In the UK, a lot of energy is used for domestic hot water use. So thermal storage and delayed heating of that thermal storage, can act as a virtual storage for electrical heating of water. To put some numbers on that, UK domestic hot water storage is currently about equivalent to 60GWh @ 30GW for 24 hours.
Similarly, with 20 million cars, if they were all electrified, you might have 20 million 50kWh batteries, which is 1TWh of electrical storage. V2G (vehicle to grid) studies often refer to a round-trip efficiency of about 75%, which comes from 10% loss on charge, 10% loss on discharge, and a few percent on transmission.
To find out more, there's a wealth of literature on integrating renewables into the grid.
The paper "Energy-storage technologies and electricity generation", Hall & Bain, Energy Policy 2008 available as a pdf here, sets out some of the issues of batteries and the grid.
