Re: Waste heat below 100oC...?
Quite right, has to be a delta T, presumably across the film.
Also: at a few hundreds of nm thick, unless there is a decent heat sink on one side the film will reach a steady state temp somewhere btw the driving side and ambient - probably a lot closer to the source temp, so your delta T will be less than surface temperature to ambient.
There appears to be a requirement for an external electric field. Assuming we are talking about a static field this may not require much energy... But depending on field strength and dielectric breakdown considerations it may drive up the solution size / decrease mass and volumetric efficiency.
The power density numbers and efficiency for the system are given without any real context, but seem very high for this sort of system. My assumption is that the the volumetric efficiency refers to the film itself... And it takes a hell of a lot of micrometer-scale film to make up the cubic centimeter needed to hit the claimed whole number Joule energies (over what time frame?). The easiest way to make an efficiency number look good is to draw your system boundary as tightly as possible around your magic widget and ignore the balance of plant hardware needed to make it run.
Example: Does the efficiency number include the electric field apparatus? Inefficiencies in the power conversion electronics? Does the efficiency number reflect use of a hypothetical infinite heat sink?
What's the thermal conductivity and heat capacity of the film? Other salient material properties one would need to engineer a material and interface it with others? Guess we will have to wait for and buy the paper to see... But:
Old man rant: the final paper in all likelihood will not provide enough information to make an intelligent decision about any of these real-world factors. Early in my career you could design experiments and systems off of published resilts, at least in the more reputable IEEE, Physical Review, Nature, etc. Now almost nothing is disclosed... The intent of a scientific paper is now to serve as an advertising rag for whatever septic think tank is performing the work rather than a vehicle for effective peer review.