Radial diffusers are used for the loading and unloading of thermal energy storages and have a significant influence on the development of the thermal stratification. Three-dimensional flow effects during the development of the thermal stratification in a hot water storage tank are discussed. An unequally distributed outflow was observed, which is accompanied by stronger mixing compared to a rotationally symmetric outflow. The reasons for this behavior will be discussed followed by a presentation of possible solutions in the third part of the publication.