Evaluation of Stratification in Thermal Energy Storages

Abstract

Thermal stratification in water-based storages can be destroyed by mixing, heat diffusion, and thermal conduction. For this reason, the evaluation of stratification in water-based thermal energy storages is important for assessing their performance. The most promising indicators were identified and assessed based on their suitability for use in practical applications. The selected stratification indicators were calculated for four simulated storage scenarios comprising a fully stratified, a fully mixed, and two realistic storages. It was found that most indicators had severe limitations in their application. For this reason, a new indicator called internal exergy destruction was proposed, which can be used in combination with the overall exergy efficiency for assessing the performance and stratification of thermal energy storages. The main benefit of internal exergy destruction is that it can be used to compare storages with different heat loss coefficients. In addition, it separates the effects of mixing from the heat losses and is easily applied to real-life storages.