Asymptotic Behavior of a Storage Unit Undergoing Cyclic Melting and Solidification Processes

Abstract

In this paper, the asymptotic behavior of a thermal latent-energy storage system undergoing periodic charge/ discharge cycles is numerically investigated. The system consists of a cylindrical tank, which is randomly packed with spheres having uniform sizes and encapsulating paraffin as phase change material. The working fluid flowing through the bed is pure air. In the main part of this study, the entrance air temperature is supposed to vary in a sinusoidal way with a one-day period. The related governing equations are solved by a control-volume-based finite difference method. First- and second-law-based efficiency indicators are used to characterize the performances of the system. The effects of the phase change material melting point temperature on the energy efficiency and the irreversibility of the system are investigated. It is shown that in all situations, an asymptotic regime of charge/ discharge cycles is reached. For a zero-cycle-average Stefan number, the bed proves to behave like a quasi-perfect reject band filter (i.e., it yields a quasi-constant outlet temperature signal). In such a case, the energy efficiency reaches its maximum value, which also corresponds to a maximum of irreversibility. Thus, this indicates that because of such opposite trends, the design of practical systems should be based on a sound compromise, on a case-by-case basis, and between energy and exergy efficiencies on one side and utilization requirements on the other side. Finally, the predictive capability of such a model is assessed in the situation in which a realistic inlet temperature of the working fluid is considered. The predicted time evolution of the outlet temperature of the working fluid proves to be in good agreement with that reported in the selected reference