(63c) Energetic Self Sufficiency: Theory and Application

            The concept of energetic self-sufficiency, for an open thermodynamic system, is first presented and its application to hydrogen production is discussed.

Let Ω be a steady-state open thermodynamic system with inlets in S_I; outlets in S_O; participating chemical species in S_C; participating elements in S_E; heat transfer rates Q_j with j in S_Q entering (Q_j >0) or exiting (Q_j <0) the system at temperatures T_j respectively, and shaft work rates W_j, j in S_W entering (consumed by) the system (W_j >0) or exiting (produced by) the system (W_j <0). This system has to satisfy the overall mass, element, energy, and entropy conservation laws. These latter two laws are often referred to as the 1^st and 2^nd laws of thermodynamics, which at steady-state require that the energy entering the system must equal the energy exiting the system, and that the total entropy change of both the surroundings and the system is greater than or equal to zero. Then

Definition. The system Ω is called energetically self-sufficient iff no heat is transferred from the surroundings to the system, heat is possibly transferred from the system to the surroundings (Q_j<0, j in S_Q) at the uniform surroundings temperature T₀, no shaft work is utilized (consumed) by the system, and possibly shaft work is generated (produced) by the system (W_j<0, j in S_W).

The considered application of this concept involves the thermodynamic assessment of hydrogen production from natural gas so that no carbon dioxide is emitted from the system. The concept of thermodynamic self-sufficiency is used to establish rigorous limits on the amount of hydrogen that can be produced.