I'm going to be choosing the topic of my masters thesis rather soon which will lay a lot of the ground work for my PhD. But I am wondering how general should I make my thesis with respect to other engineering areas. I'm looking to go specifically into energy and heat transfer of multi-phase/mixed flows and will most likely being doing my MS and PhD theses on fuel cell modeling and optimization. But lets say someday I would like to go into the area of solar thermal power or purhaps solar thermolysis. Would doing work only with fuel cells maybe make me sort of non-applicable to other fields such as solar thermal power? I guess a better way of stating the question would be, would doing work on one specific technology make me exempt from working on other types of technology even though they involve similar science? Of course this is all considering the industrial world and not academics.