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View Diary: Recent DOE Break-Through with Hydrogen Fuel Cells, should make them Affordable (275 comments)

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  •  nope (0+ / 0-)

    All devices are limited by the first and second laws. However, Carnot only applies (as is obvious from its simple derivation) to engines powered by the transfer of heat from a cold to hot reservoir. Electric motors, etc. have no Carnot limit. Of course, if you redefine the meaning of Carnot efficiency to simply being "obeying thermodynamics" then all devices are Carnot limited.  

    Michael Weissman UID 197542

    by docmidwest on Sun Apr 24, 2011 at 08:25:15 PM PDT

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    •  whoops- I meant hot to cold! duh nt (0+ / 0-)

      Michael Weissman UID 197542

      by docmidwest on Sun Apr 24, 2011 at 08:26:06 PM PDT

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    •  nope (0+ / 0-)

      When you have a peer-reviewed paper saying you're wrong, unless you can present a counter peer-reviewed paper, you're wrong.  Nothing against you personally.

      •  peer-reviewed paper (0+ / 0-)

        As I read that abstract (the journal was so obscure that we don't seem to have it in our major university library, so I wasn't going to pay for the full article) it was dealing with a purely semantic issue. The available work is given by the excess of G (Gibbs free energy) over the equilibrium value. That already takes into account the entropic second-law limits, via the -TS term in G. (There is no relevant T_H to even use in a Carnot expression, 1-T_C/T_H.) Peer reviewed papers are nice but we don't usually turn to them for the standard material we teach in sophomore courses.

        It ain't peer-reviewed, but (with a few awkward starts) the Wikipedia summary of the efficiency of fuel cells actually ends up giving a good description of the thermodynamics and some of the practical issues. It at least avoids getting hung-up on verbalisms.

        Michael Weissman UID 197542

        by docmidwest on Mon Apr 25, 2011 at 01:12:34 PM PDT

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      •  teachable thermodynamic moment (0+ / 0-)

        Maybe this will help. In this context (fixed p, T environment) the available work is given by the excess of G, the Gibbs free energy, above its equilibrium value. By the second law, all spontaneous changes are in the direction of lowering G. Combustion is such a change. Therefore it lowers G. In a burning system that process, in which G is lost, occurs before anything starts to extract work. Therefore it causes a loss of available  work. That's why combustion-based heat engines have intrinsic inefficiencies, in addition to practical problems.

        Michael Weissman UID 197542

        by docmidwest on Mon Apr 25, 2011 at 02:26:12 PM PDT

        [ Parent ]

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