Renewable Energy Focus, Nov/Dec 2016, by David Blood. - scroll to pages 237-238.
http://www.e-ditionsbyfry.com/olive/ODN/RRF/Default.aspx?href=RRF/2016/11/01

"David Blood takes a look at the energy storage systems market and explores why they are so integral to renewable energy generation. Energy storage systems (ESS) are growing in importance as renewable energy sources like wind power, wave power and solar are being commissioned and connected to the grid at an ever-increasing rate. Energy Storage Systems are needed to help the grid deal with the instability and unpredictability of such power generation systems and match them to equally variable consumer demand."