To appreciate the size of the problem of replacing fossil fuels with renewable, non-harming substitutes have a look at this:
References for the diagram are here
In Plan B 3.0: Mobilizing to Save Civilization
(W. W. Norton, 2008, online as a single PDF file
), Lester Brown crunches the numbers and recommends we embark on a "WWII" style effort to build windmills. (In the 1940's we stopped building cars and produced phenomenal quanties of planes, tanks, an ships.) He's also friendly to solar.
What I find lacking in most discussions of current energy challenges is an appreciation of the shear amount of fossil fuels we use. To me, the conversation must include how we change our behavior to use less and the affect that will have on society. In other words, conservation and renewables are only part of the solution. The rest comes from not demanding so much from Earth.