In what engineering field can i study renewable energies in the USA?
Renewable energy is an application of engineering, and requires expertise from various disciplines. For example, wind energy requires civil, mechanical and electrical engineering. 

I don't know how things work in the US, but in the UK my advice would be to identify which discipline interests you the most and pursue that in a department that includes renewables on the syllabus. Then either look for a job in renewables, or do a masters to learn more about that industry. However, I don't know how applicable that advice is to your situation.