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
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