I read a book back in college (it was actually the textbook in an elective course on alternate energy sources) which discussed energy sources such as wind, solar, hydro, and a number of other technologies. It was an interesting read, as it talked about some of the tricky limitations of different approaches. Solar, for example, runs into a number of challenges (even before the efficiency hurdles faced by photovoltaic) because of the atmosphere. Wind really only works if you can sustain at least a seven MPH wind. It's probably a bit dated now, but I'll look on my shelf tomorrow at work and see what it's called in case anyone's interested.