Please allow me to share something that might be helpful to your Master and (maybe) future PhD.
Firstly, It would be very interesting if you can identify in the University that you want to do your Master's, which are the research fields and the corresponding Professors research areas of interest, so you could (maybe) easily filter the two topics you cited to a more specific subject.
Secondly, It would be interesting if you could describe a little bit more of what you are looking for with your master's dissertation. There are numerous topics in the renewable energy subject, such as: biofuels, solar energy, wind energy, biomass, etc. Besides, when you say thermodynamic fluids, or Thermal fluids, it could be inferred that you're talking about any topic-related to the main four intersecting fields: heat transfer, thermodynamics, fluid mechanics, and combustion. Also, do you intend to do a analytic, numerical, simulation, or experimental work?
Most master's theses are about application of scientific knowledge to solve an existent problem, say to a casestudy area. Thus, you could think about microgrids for rural electrification of a given casestudy using the available renewable energy resources. You can also think of energy business initiatives that could be appropriate for adoption of a specific renewable energy technology in a given community.