Unsteady-state exergetic performance comparison of externally and internally insulated building envelopes

摘要

In the exergy analysis of the built environment, particularly for a system with large heat capacity and low thermal conductivity, such as building envelopes, consideration of physics associated with the entropy and exergy storages is increasingly important. However, steady-state exergy analyses neglect storage-related physics. Therefore, they do not provide insights into transient processes. Moreover, the results may not be thermodynamically correct because the integrity of the physics does not hold. In this study, we first examined why unsteady-state exergy analysis is required for systems in the built environment. Then, we conducted the unsteady-state exergy analysis of externally and internally insulated building envelopes under a 24-h periodic environmental boundary condition. This was based on a methodology that numerically solves the energy, entropy, and exergy equations, where thermodynamic integrity, including exergy storage, is fully established. We compared the spatiotemporal exergetic behavior of the two envelopes from macroscopic and microscopic perspectives. In terms of exergy flow, storage, and consumption, the exergetic behavioral difference and its physical rationale were detailed. The spatiotemporal exergetic behavior of the two envelopes demonstrated a sharp contrast. In particular, the wasted exergy flow from the outer surface into the surrounding environment was significantly larger in the internally insulated envelope than in the externally insulated envelope. Additionally, the exergy stored in the internal layer, which positively affects indoor air conditioning, was much larger in the externally insulated envelope than that in the internally insulated envelope. The insights obtained can contribute to improving building thermal design and creating more efficient operation strategies for energy systems.