Energy and exergy efficiencies assessment for a stratified cold thermal energy storage

Cold Thermal Energy storage systems (TES) present opportunities for offsetting peak demand from chillers. An assessment of the TES system at the University of Idaho was performed to quantify the current exergy efficiency of the single phase, water based, cold TES that typically operates under the full capacity. Internal temperatures within the TES vary from 7 °C and 15.5 °C throughout the summer months. Measurements were taken on internal temperatures and flow rates and were used to create a model in TRNSYS. Comparison of Richardson, Fourier Peclet, and Biot numbers show the influence of internal and external conditions on TES performance and stratification characteristics. The Richardson number indicates that the turbulence effect of the inlet geometry is very small and it has negligible effect on the stratification within the TES. The Peclet and Fourier numbers show that prediction of the stratification profile of a TES can be done with minimal measurements. The Biot number shows that the temperature variation only occurred vertically within the TES. Using the data obtained, as well as an analysis of the model, showed the TES is determined to have an overall energy efficiency of 75% and an exergy efficiency of 20%. The individual layers of the TES were evaluated to conclude that the upper half of the TES is the most exergy efficient layers of the TES and steps to maximize their usage should be taken without reducing the overall CAPACITY.