Luis "Lou"’s Answer
Alternative energy is a pretty broad field, so there isn't one specific engineering field that covers it.
If you look at it from the point of view of the type of energy you are dealing with: alternatives that deal with mechanical or thermal energy transfers usually are the field of Mechanical Eng.'s; stuff like Solar, wind, geothermal, waves, tides and hydrologic. If the energy goal is large (i.e.: a dam or wind farm) you will probably need a Civil Eng. to design and build the big stuff.
If you are thinking photovoltaic, fuel cells or battery technology, a Chemical or Electrical Eng. is probably most helpful.
That is not to say that you can be a Mechanical and do chemistry or electrical stuff, or vice versa.
So, no matter what the type of energy you are dealing with, there is always some overlap on the engineering disciplines, so pick the one you are more interested on and good luck saving the World.