Exergy calculations based on a fixed standard reference environment vs. the actual ambient conditions: gas turbine and fuel cell examples

Two approaches for calculating exergy values were compared: ne based on the actual local temperature, pressure and relative humidity, and the other one based on the fixed reference conditions of 25°C, 1 atm and 70% relative humidity. In the former, exergy values represent the actual potential of the system, while the latter is chosen by some commercial software. The objective was to investigate to which degree the choice of environment influence on the results of the exergy analysis. For power plants and chemical conversion, the effects were modest with respect to overall efficiency (which, however, can be assessed without exergy). Details of the exergy analysis were, however, influenced by the choice of environment. Irreversibilities (exergy destruction) and flow exergies were more affected, and the distribution of exergy losses between irreversibility and discharge was different. For elevations above the sea level, the flow exergies can get negative values.