I can understand how that seems to be confusing, as they are somewhat related but yet different. A method is only well understood when you apply it in real scenario. I am aware with the thematic analysis but rest I have just studied in theory, so my concepts aren’t that sharp either. As far as I know, phenomenology is considered as a methodology itself, while thematic analysis is just a method. Go on the link below, where you’ll find different questions and answers about research approaches and methodology on whole:
Find the literature (books, research papers) and then find the weakness (what is left out) in it. Finding a good topic is what will define your research. It is the basic step. Read a lot, don’t just do it in a buzz. Take your time. Meanwhile, you can read this blog, I found it quite useful:
Energy Management is a vast area, you first need to narrow it down. Which domain intrigues you the most, coal, petroleum, oil, renewable energies, natural gas or power production? You need to be particular about it and then look for a topic.