A packed bed of solid spheres acts as a thermal energy storage system. In a charging process, heat transfer from the hot gas increases thermal energy stored within the spheres. Consider two beds, one with aluminium spheres ($$\rho = 2700 kg/m^3$$, C = 950 J/kg-K, k = 240 W/m-K) and other with copper spheres ($$\rho = 8900 kg/m^3$$, C = 400 J/kg-K, k = 380 W/m-K). Select the correct graph for the temperature rise of a sphere at the inlet of the bed