THE AIR ENERGY BALANCE EQUATION PARADOX

In building physics applications, mainly related to the building energy performance simulations, the master equation is the inner air energy balance equation. Historically the followed approach in writing down such master equation was to adopt the simplifying hypothesis of incompressible fluid for the air assumed dry. This hypothesis, among others, reduces the general integral energy balance equation to the usual formulation which is reported in textbooks and manuals and has been used in available energy simulation programs, such as Energy Plus, Esp-r, TRNSYS, etc. Later the moist air problem, the indoor air humidity control, has been faced just by adding a new integral balance equation: the water vapour mass balance equation. In this work it is pointed out how this "layered" historical approach leads to a paradox: an incompressible fluid is used to describe an ideal gas in an ideal gas mixture. Herewith the general integral energy balance equation is directly written in terms of moist air under ideal gas hypothesis and it is shown how, under acceptable specific hypothesis, it is possible to obtain a similar but conceptually different final formulation, which mainly differs in the transient storage term.