A background and understanding of mechanical or electrical engineering is good preparation.
Currently, only a handful of undergraduate degree programs in renewable energy engineering are offered in the US.
Be careful when enrolling in programs in this area. Because this is an emerging area of study, the industry does not yet have training requirements. As a result, the quality and depth of information provided by programs vary widely. You should investigate the schools you are interested in.
Source: Illinois Career Information System (CIS) brought to you by Illinois Department of Employment Security.