For high temperature industrial processes, or usual thermo-mechanical energy conversion plants, the results of the
exergy analysis show very low differences if the higher or the lower, possible value for the ambient temperature T ° is
chosen. This is not true if energy processes at a temperature close to ambient temperature are considered, for instance
thermal solar systems, or low temperature district heating systems.
The paper revises the derivation of flow and nonflow exergy if T° and P° vary in time, highlighting the case of periodic
variations. As a result, besides the terms analogous to the well known ones, in the expression of the exergy balance of a
generic control volume, two additional terms appear, that take into account exactly the available work (exergy)
destruction related to the variation of ambient condition during the considered time interval. Then, the theoretical
opportunity of taking advantage of the effects of variable T° (like exergy increment of thermal storag es, or creation of
“hot” and “cold” reservoirs without work expense) are discussed and their physical limitations are outlined.
