Paper
Motivation
The application of the steady-state assumption to systems with high thermal inertia may decrease the reliability of results due to significant transient effect. However, in the field of exergy analysis, there is no proper unsteady-state exergy analysis method, which hinders the ability to obtain reliable results.
Analysis
To address this problem, fully unsteady-state exergy analysis in heat conduction process is formulated. This study presents the partial differential equation of the unsteady-state exergy balance and explains how it can be solved numerically.
The developed unsteady-state exergy analysis method maintains physical integrity by accounting the exergy storage in the exergy balance. Through this method, it becomes possible to capture the spatiotemporal exergetic behavior of the system.
Utilizing the unsteady-state exergy analysis method, the exergetic behavior of two building envelopes with different insulation methods was compared:
1.
Internally insulated case (CI: concrete–insulation configuration)
2.
Externally insulated case (IC: insulation–concrete configuration)
Spatiotemporal exergy consumption rate result
In CI configuration, the exergy is significantly consumed than the IC configuration. Specifically in the concrete layer of CI configuration (0~10 cm) the exergy is significantly consumed. In contrast in the IC configuration most exergy consumption is occured in externally located insulation layer (0~6 cm).
Spatiotemporal Node stored exergy result
Consequently, the stored exergy in concrete layer much larger in IC configuration than in the CI configuration. The stored exergy of concrete in IC configuration can contribute to thermal comfort by exchanging the exergy with human body. During the daytime, cool exergy of concrete layer can be released the cool exergy in the form of radiation, and during the night time warm exergy can interact with our body.
Conclusion
We hope that the development of unsteady-state exergy analysis and its application to building envelopes will broaden our perspective from a fragmentary energy analysis view to a more comprehensive thermodynamic understanding of the built environment.