The work studies the turbulent fluid flow under the effect of natural convection within a solar chimney power plant (SCPP). This study is carried out by numerical simulation using the Saturne code coupled to the Syrthes code. Geometric modifications were brought upon the chimney outlet. Divergent shape with different exit sections was set at the tower outlet. The study is undertaken for two types of solar chimney power plant. In the first configuration, only soil below the collector is considered as a storage system. In the second one, the storage system is composed of the soil and a 10 cm thick tub filled with water, covering the entire surface of the collector. The boundary conditions will be defined according to meteorological data, for a typical day available on the site of Adrar, Algeria. The results allow us to focus on the storage system influence on the SCPP performance and their operation duration after sunset and to determine the geometric modification effect on the plant energy efficiency.