This article presents the results of numerical and experimental studies on the impacts of the selected geometrical features of liquid solar collectors on their thermal efficiency. The experiments were carried out while meeting the requirements of the ISO 9806:2017 standard. Selected changes in the geometrical features were analysed by using fully functional prototypes of modified solar collectors. The correlations between the design and performance properties of the solar collectors were determined in accordance with the changes in the shape of the thermal efficiency η(T*m) curve.
