In my experience, we can calculate the exergy efficiency based on exergy (physical and chemical ones) at inlets and outlets of your simulated domain. To calculate physical and chemical exergy, in Fluent you need to extract the results related to composition, entropy, enthalpy... To do that in Fluent you can find the tool at Results-> reports -> Surface Integrals.
I would like to suggest reading Exergy efficiency in Wikipedia. Exergy efficiency, also known as the second-law efficiency or rational efficiency, computes a system's effectiveness relative to its performance in reversible conditions. It is defined as the ratio of the thermal efficiency of an actual system compared to an idealized or reversible version of the system for heat engines. It can also be described as the ratio of the useful work output of the system to the reversible work output for work-consuming systems.
The article "Exergy efficiency" in Wikipedia is not correct. Have a look at the following references:
Tsatsaronis, G., "Thermoeconomic Analysis and Optimization of Energy Systems," Progress in Energy and Combustion Systems 19 (1993), pp. 227-257
A. Bejan, G. Tsatsaronis, and M. Moran, Thermal Design and Optimization, J. Wiley, New York, 1996
Lazzaretto, A. and Tsatsaronis, G., “SPECO: A Systematic and General Methodology for Calculating Efficiencies and Costs in Thermal Systems”, Energy – The International Journal 31, (2006), pp.1257-1289.