Rotary desiccant units for air-conditioning and ventilation use the principle of adsorption open cycles. In this study, a thermodynamic analysis is first developed that emphasises the open character of the cycle, and especially the consumption of liquid water. Then, the system is ＇closed＇ with the help of virtual open systems that exchange mass hows with the cooling unit and only heat fluxes with heat sources. The entropic analysis is developed, first for these virtual systems showing that they are irreversible, second for each component of the unit where heat losses are taken into account. The equations are derived in such a manner that the different irreversibilities can directly be compared. In addition, the concept of entropic mean temperature is introduced and applied to the two virtual systems because it is very helpful for representing the variations of the entropy productions in these systems. In Part 2, these analyses will be applied to experimental data showing that they bring lots of information.