Abstract

Packed-bed thermal energy storage (PBTES) systems have arisen as a promising alternative to store heat at high temperatures with low implementation costs.The literature has focused on improving its design to reduce thermal losses, enhance structural integrity, and increase storage capacity by analyzing thermocline development.The integration of PBTES systems has been a challenge due to the thermocline; such thermal gradient implies assessing the temperature limits of the applications to decide how to manage the PBTES charge and discharge processes.Nevertheless, this component's lack of monitoring indicators leads to undesired operation once integrated into the system.This article proposes a novel degradation parameter as a monitoring indicator for the PBTES operation.Through a second law of thermodynamics, different sources of exergy losses are collected into this parameter and compared to an ideal exergy stored/released rate.This article analyzes in depth the actual limits of the charging degradation and contrasts it to the utilization factor, a metric to evaluate the amount of useful energy discharged each cycle.This contrast is performed with a multi-objective optimization varying the cut-off temperature at charge/discharge and the charge/discharge temperature.The maximum limit for the charging degradation is around 0.35 to maximize the utilization factor when the system discharges from temperatures close to the environment.For that operational condition, the charging degradation limit can also be predicted with the cut-off charging temperature; therefore, its value can be set before its operation and used as a decision variable to manage the system's operation.