A lake isn't any good as an energy storage device without a Turbine. 

So far, Solar and wind have been mainly supplemental. Especially solar, giving increased energy during the day, when the load is greater anyway. You then use conventional generation at night for baseload. It doesn't make sense to have the battery backup(or whatever) before you have the excess generation in place to use it. In Europe they're doing some storage with molten salts that looks pretty promising.