There are two technical challenges involved in building a renewable energy economy. The first one is familiar to most people: finding cost effective ways to capture the sun's energy, and there are lots of great designs and devices being developed to make solar and wind energy competitive with current prices for oil and coal.
But the other challenge is energy storage. Renewable energy, in most places, most of the time, is an intermittent energy source. The sun only shines during the day, and the wind doesn't blow all of the time. If you want to power a modern, 24-7 economy with renewable energy, you have to have a cheap way to store the cheap solar energy you generate during the day so that you can use it at night.
This article from Slate is a nice, quick survey of some proposed energy storage solutions. I'm attracted to the train idea. It's simple, and I like simple.
Energy storage technology: Batteries, flywheels, compressed air, rail storage. - Slate Magazine