Thermal Performance of Stratified Chilled Water Storage Tanks

Abstract

The mechanisms which contribute to a loss of capacity in stratified storage tanks are usually grouped into heat transfer through the tank walls, conduction across the thermocline, and the flow dynamics of the charge and discharge process. In this paper, we use analytical solutions of the unsteady one-dimensional energy equation to show that the flow dynamics are generally orders of magnitude more important than the other factors. Heat transfer through the tank walls constitutes a loss which is less than a couple percent of the tank capacity for reasonably sized, underground or above-ground, insulated tanks. The importance of convective fluid motion relative to one-dimensional conduction across the thermocline is quantified with a single parameter. Values for this parameter can be estimated from measurements of in-tank temperature profiles or outlet temperature profiles. The model solutions are also used to calculate an estimate of the maximum first law efficiency for stratified storage. Published data shows current storage tanks generally operate at efficiencies of 50 to 80%. The efficiency of a storage-based cooling system is shown to be the product of storage efficiency and chiller efficiency. Storage tank losses must therefore be offset by improvements in chiller efficiency if these systems are to compare favorably with conventional cooling systems on the basis of energy efficiency.