Unsteady-state exergy analysis on two types of building envelopes under time-varying boundary condition

In the built environment, the thermal exergy behavior is very sensitive to the change of environmental temperature, because the temperature difference between the reservoir and a system of interest is small. Moreover, the transient characteristics become very important for the building envelope, which is primarily affected by the environmental temperature changes and has a relatively large heat capacity. Most of the exergy analyses have been performed under steady-state assumption. However, it may miss some important details of the transient process. Thus, when the transient transfer process becomes important, the unsteady-state exergy analysis should be conducted. In this study, we propose complete energy, entropy, and exergy equations in their partial differential forms. By solving them numerically, we examined the transient exergy process inside the building envelope composed of concrete and insulation layers under time-varying boundary condition. Using this new methodology, we can improve the temporal and spatial resolution of the exergy analysis and thus provide more complete information about exergetic behavior