Thermal energy storage systems commonly involve apacked
Chapter 5, Problem 5.12(choose chapter or problem)
Thermal energy storage systems commonly involve apacked bedof solid spheres, through which a hot gasflows if the system is being charged, or a cold gas if it isbeing discharged. In a charging process, heat transferfrom the hot gas increases thermal energy stored withinthe colder spheres; during discharge, the stored energydecreases as heat is transferred from the warmerspheres to the cooler gas. Consider a packed bed of 75-mm-diameter alumi-num spheres (??2700 kg/m3, c?950 J/kg?K, k?240 W/m?K) and a charging process for which gasenters the storage unit at a temperature of Tg,i?300C.If the initial temperature of the spheres is Ti?25Cand the convection coefficient is h?75 W/m2?K, howlong does it take a sphere near the inlet of the system toaccumulate 90% of the maximum possible thermalenergy? What is the corresponding temperature at thecenter of the sphere? Is there any advantage to usingcopper instead of aluminum?
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