Architecture? Civil Engineering? Environmental Engineering?
It depends. I recommend staying broad so you can be a desirable candidate in other career fields. Environmental Engineering, Environmental Science, or Civil Engineering can work just fine.
Depends what you want to do. Develop new fuel cell technologies? Then chemical engineering or chemistry. Develop new, high efficiency photovoltaics? Then EE or physics. Bio-to-fuel conversion? Chemistry, biology, chemical engineering, bio engineering. Design hydro power plants? Then civil engineering or architecture. Wind power? Probably mechanical engineering or electrical engineering.
You can come at this from the area of interest side, and choose an appropriate major. Or choose a major, and that will focus what segments of renewable energy you can pursue. It’s such a broad area given all the different technologies involved. And i would say all of the suggested majors here and in the other posts cover broad fields. You’ll be fine should your career interests change.
Most kinds of engineering are applicable in some way, though in different ways. Of course, any industry has plenty of other job functions that do not require an engineering background.