Exergy transfer

A more reasonable definition of the efficiency is, however, the comparison of the exergy change of the water and the exergy transferred from the working medium, in our case ammonia. This efficiency is defined as... 

Fig. 11.8. Schematic exergy vector diagrams for exergy transfer between an exergy-absorbing process and an exergy-releasing process (a) the combined process is not feasible, (b) the combined process is feasible.

The above equation is analogous with the entropy balance of the second law. The first term in this expression shows the exergy transfer accompanying heat when the temperature at the heat transfer medium is not constant. 

Figure 4.16. Mechanism of exergy transfer for a general system.

In accordance with the second law, the exergy loss is positive in an irreversible process and vanishes in a reversible process. The change in exergy of a system can be positive, negative, or zero. When the temperature of a process where heat transfer occurs is less than the temperature of the environment, the transfer of heat and exergy flows in opposite directions. Work and the accompanying exergy transfer can be in the same or opposite directions. 

In an open system, flow exergy is exergy transfer due to mass flow and flow work, and the specific flow exergy exr is... 

This equation shows that the rate of exergy transferred into the control volume must exceed the rate of exergy transferred out, and the difference is the exergy destroyed due to irreversibilities. Exergy concepts for some steady-state processes are ... 

In a steam power plant, exergy transfers are due to work, heat, and exergy loss within the control volume. 

Exergy loss Thermodynamic loss due to exergy transfer to the environment. 

The exergy transfer Eq associated with heat transfer Q is... 

The exergy transfer associated with the transfer of energy by work W is given by... 

Exergy transfer across the boundary of a control volume system can be associated with material streams and with energy transfers by work or heat. The general form of the exergy balance for a control volume involving multiple inlet and outlet streams is... 

The exergy transfer associated with the time rate of energy transfer by work Wcv other than flow work is... 

In all real processes, exergy loss always accompanies exergy transfer. At absolute source temperature T, the exergy transfer accompanying heat transfer from that resource becomes... 

Change in the total exergy = [Exergy transfer (Total exergy in — Total exergy out)] — Total exergy lost (destroyed). 

The first term on the right-hand side of equation above shows the exergy transfer accompanying heat and work. The second term is the exergy lost (exergy destroyed) due to irreversibilities. 

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