Thermal energy storage systems commonly involve a packed bed of solid spheres, through which a hot gas flows if the system is being charged, or a cold gas if it is being discharged. In a charging process, heat transfer from the hot gas increases thermal energy stored within the colder spheres; during discharge, the stored energy decreases as heat is transferred from the warmer spheres to the cooler gas. Consider a packed bed of 75 -mm-diameter aluminum spheres (=2700kg/m3,c=950J/kgK,K=240W/mK) and a charging process for which gas enters the storage unit at a temperature of Tzil=310C. If the initial temperature of the spheres is T=25C and the convection coefficient is h=80W/m2.K, how long does it take, in s, a sphere near the inlet of the system to accumulate 90% of the maximum possible thermal energy? What is the corresponding temperature, in C, at the center of the sphere? t=isT(t)=iC