Thermodynamic considerations in the evaluation of thermal energy storage systems

In this paper the thermodynamic factors to be considered in evaluating the performance of thermal energy storage systems are discussed. The factors considered imply that an analysis methodology must be capable of: determining important analysis quantities, obtaining appropriate measures of efficiency, pinpointing losses, assessing the effects of stratification, evaluating storages for cooling capacity, assessing the performance of subprocesses, accounting for temporal and spatial variations in the temperature of the surroundings, accounting for time duration of storage, and assessing consistency in data utilization and analysis methodologies. As an illustrative example, a simple closed tank storage with heat transfers by heat exchanger is considered; energy and exergy efficiencies are obtained for a complete storing cycle, and for the individual charging, storing and discharging processes. The objective of the reported work is the development of rational and meaningful methodologies for the evaluation and comparison of the performance of thermal energy storage systems