The aim of this paper is a presentation of a new solar air conditioning unit for office spaces at Tunisia and a simulation study of the behaviour of two stages. The first one is the dehumidification stage which is mainly composed of desiccant wheel. The second one is the regeneration stage which is mainly composed of flat plate solar collector, storage tank and heat exchanger. Thus, mathematical models for each of those components are developed which are based mainly on thermal and mass balances. A finite differences method is applied to the partial differential equations. The simulation study shows that minimizing the value of the distributed hot water mass flow for the storage tank is very interesting. This can be beneficial as it reduces thermal losses and makes the heating of the inlet cold water entering the storage tank effective. In addition, increasing the value of the hot water temperature produced by flat plate solar collector is interesting because it ensures an efficient heating of the water contained in the storage tank. Finally, the studied regeneration stage can successfully ensure the regeneration of the hygroscopic wheel used in the corresponding solar air conditioning unit.