Exergy (source code)
= Exergy
{title2=$A-A_{\mathrm{eq}}$}
{wiki}
= Availability
{synonym}
For a <closed system> with fixed composition in an environment at <temperature> $T_R>0$ and <pressure> $p_R$, define $A=U+p_RV-T_RS$, where $U$ is <internal energy>, $V$ is <volume (thermodynamics)> and $S$ is <entropy>. The <exergy> is this <availability> relative to the equilibrium reference state, $A-A_{\mathrm{eq}}$. When the combined system and environment are isolated except for the specified exchanges, $\Delta S_{\mathrm{tot}}=-\Delta A/T_R$, so the <Second law of thermodynamics> makes $A$ decrease toward its constrained minimum. Away from equilibrium, the environmental $T_R,p_R$ must not be replaced by the system's instantaneous values.
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