Exergy efficiency

Exergy efficiency (also known as the second-law efficiency or rational efficiency) computes the efficiency of a process taking the second law of thermodynamics into account.

From the second law of thermodynamics it can be demonstrated that no system can ever be 100% efficient. When calculating the energy efficiency of a system, the figure found gives no indication of how the system compares to a thermodynamically perfect one operating under the same conditions. In comparison, the rational efficiency of a system can reach 100% because the work output is compared to the potential of the input to do work. The energy efficiencies of a heat engine are always smaller than its exergy efficiency.

The exergy B balance of a process gives:

with exergy efficiency defined as:

For many engineering systems this can be rephrased as:

Where ${\displaystyle \Delta G_{T}^{0}}$ is the standard Gibbs (free) energy of reaction at temperature ${\displaystyle T}$ and pressure ${\displaystyle p_{0}=1\mathrm {bar} }$ (also known as the standard Gibbs function change), ${\displaystyle {\dot {W}}_{net}}$ is the net work output and ${\displaystyle {\dot {m}}_{fuel}}$ is the mass flow rate of fuel.

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