Comfort as the dead state of buildings: a preliminary discussion

Exergy is typically defined as the maximum work that can be extracted from a system when reaching equilibrium
with its "reference environment" - a large region unaffected by the interactions - and derives from Gibbs' original
concept of "available energy of a body and medium". The reference environment definition plays a key role in
exergy analysis but is still controversial, especially in the case of buildings; the most popular choice is the local
outdoor air, because readily available and largely unaffected by the presence of the building, but its fluctuating
conditions pose various challenges. The controversy around the reference environment arguably remains one of
the main blockers for the practical application of building exergy methods. However, going back to the origins,
Gibbs defined available energy not only for a "body and medium", but also for the case of a "body alone", a system
formed by subsystems in non-equilibrium conditions. Later, exergy too was defined for this second case, as the
subsystem contribution to the body available energy. In the case of the "body alone", the reference is the "dead
state" (or "thermostatic state" of the body), in place of the reference environment. The main idea of this article is
that building exergy analysis can be based not only, as currently, on the exergy definition originated by Gibbs' case
of the "body and medium", but alternatively on the exergy definition originated by the case of the "body alone",
for which a large environment is not needed. The outdoor reference environment can thus be substituted by an
indoor "dead state", making the analysis simpler and "warm" and "cool" exergies more relevant to practical heating
and cooling applications. The study discusses typical fluctuations of indoor conditions and the impact on exergy
values, and proposes a fixed dead state as the exergy reference.