The effect of reference point selection on dynamics exergy analysis in buildings (Case study: Tehran)

One of the necessities for exergy analysis is the correct choice of reference environment. The reference environment is very important in low-temperature exergy analyses because a slight temperature change can greatly affect the calculated values. In this research, a room in Tehran was simulated, and energy analysis performed. Following that, dynamic exergy analysis was performed with two approaches. In one approach, indoor air was defined as the reference environment, while in the other approach, outdoor air temperature was defined as the reference environment for exergy analysis. Conceptually, the outdoor air was an environment that was available for free and unlimited at any time. So, choosing outdoor air temperature as the reference environment for exergy analysis was more accurate, but it had more effort in exergy calculations. Exergy analysis was performed accordingly, and then the two approaches were compared. The comparison showed that in most of the modelling period, when the reference environment was chosen to be indoor, the calculated hourly exergy destruction was less than when the reference environment was outdoor. Results showed that, due to the simplification in choosing the indoor as the reference environment, the error of calculation of the hourly exergy destruction reached a maximum of 60%, high enough to avoid the simplification for dynamic exergy analysis. However, the results showed that calculations’ annually-averaged error was limited to 1.5%, an acceptable level for state exergy analysis.