The NPR series Power Hungry:  Reinventing the U.S. Electric Grid continues with stories on The Challenge:  Constant Current from Fickle Winds and The Grid May Be Smart, But Will It Be Green?    

The problem of green energy is that when the wind does not blow, the sun does not shine, there is no power being generated.  That the amount of energy cannot be controlled as a coal or natural gas plant can be.  I think this problem will be short lived because of advancements in batteries will help store the access energy that is produced – allowing the grid operators to have the control they need to meet demands.

Mark Viquesney