I'm having some difficulty understanding exactly what the difference between the definitions of these values are. As I understand it, in terms of solving given problems, you have to use the equation for specific exergy (NOT flow) when solving a problem that isn't a "flow" problem (ie: otto cycle, diesel cycle). However, I don't really understand the difference conceptually. My understanding is: exergy of a state refers to the maximum theoretical energy that can be extracted from it usefully in a given environment, where e = (u-u0) + p0(v - v0) - T0(s-s0) + (V^2)/2 + gz Meaning what you can get from it is its internal energy above the normal, the "potential" energy it has from displacing the atmosphere due to expansion, a loss based on its entropy, and its KE + GPE. For flow exergy, the equation is much the same as above, only the (u-u0)+p0(v-v0) is replaced with (h-h0). I thought these were the same thing...? When, mathematically, would these two formulae give different answers? I've been given an explanation based on a piston engine having to push against the atmosphere as stuff in it changes, while (say) a rankine cycle is all self contained and never acts to displace the atmosphere. This seems like it's the right concept to explore, but I don't see how to apply it to understanding two equations which mathematically look identical to me.