Dynamic exergy analysis for the built environment: fixed or variable reference?

Exergy is a combination of the first and second law of thermodynamics through a reference environment, also called "reference state" or "dead state", and quantifies the quality of energy. Classically, exergy analysis is used for optimising stationary processes, and the reference state is defined as a set of constant properties corresponding to the average outdoor conditions. In the case of buildings, stationary models are not accurate and a dynamic analysis is often necessary. However, the choice of the reference environment for dynamic exergy assessments is critical and still highly controversial; the main dilemma is the selection between a fixed (constant in time) or a variable (fluctuating with the outdoor conditions) definition. Although a fixed-reference selection can be justified with analytical considerations, the vast majority of authors adopt a variable reference environment, coincident with the local climate. This research proposes an alternative fixed reference and compares the numerical impact of fixed and variable reference definitions on the dynamic exergy analysis of the built environment. The exergy stored in the building envelope and in a domestic hot water tank of a case study is assessed in typical winter and summer conditions. The main advantages and criticalities of different approaches found in the literature are considered in the definition of the reference state that is finally proposed as the best option. Further debate, based on both theoretical and practical considerations, is needed to achieve a common agreement for the reference state of building exergy analysis.