So, I've read in books and on Wikipedia (see for instance http://en.wikipedia.org/wiki/Carnot...cs)#Applicability_to_fuel_cells_and_batteries) that the Carnot efficiency cannot be applied to a fuel cell because it is not a heat engine that produces work, operating between two heat reservoirs. However, I have managed to come across the following article, which claims to "dispel the misconception that an ideal fuel cell is potentially more efficient than an ideal heat engine". (Taken from the abstract of "Thermodynamic comparison of fuel cells to the Carnot cycle", International Journal of Hydrogen Energy, Volume 27, Issue 10, October 2002, Pages 1103-1111 Andrew E. Lutz, Richard S. Larson, Jay O. Keller) I don't know if copying and pasting from a research journal is allowed here, and I don't think I'd do justice by trying to summarize what it says, so I hope the people that are interested in discussing this topic have access to the article (it's available at ScienceDirect, if you have access to that.) So, is the article right? If it is, can the Carnot efficiency be applied to any physical system that produces work, and not just fuel cells? I'd appreciate to hear your thoughts, thanks.