Wind doesn't always blow. Sun doesn't always shine. Storing elecricity for when it is needed.....
Simple solution in an integrated system would be to use the surplus electricity to split water into its components Hydrogen and Oxygen - e.g. An enhanced project, which can then be stored until needed and recombined either as fuel for cars or in a power station when energy demand exceeds the sun/wave/wind power being produced.
The thing about burning hydrogen with oxygen is that the only products are water and heat.
Quack!
GtD.