The thermal energy storage unit in the form of the ceiling panel made of the gypsum-microencapsulated phase change material composite with internal U-shaped channels was optimised applying previously developed 1D-3D numerical simulator. Calculations were carried out for variable inlet velocities of air, different properties of the composite expressed by enthalpy-temperature curves, and summer climatic conditions in Central Poland (Warsaw). Optimal working temperature ranges of the composite were determined. Moreover, studies showed that for Central Poland, composites with wide ranges of the working temperature are preferred due to significant daily variations of ambient temperature during the whole summer.