The performance of indirect thermal storage systems is critically dependent on the degree of thermal contact between the energy storage medium and the energy transfer medium. For liquid-liquid systems, the energy transfer occurs across a heat exchanger for which the overall effectiveness is determined by both tube-side and storage-side convection coefficients. While the tube-side convection is essentially constant throughout a draw at a constant flow rate, the storage-side convection process depends intimately on the natural convection flow driven by the temperature difference between the two fluids. This temperature difference is inherently transient during the discharge process. In the present work, analytical models are developed which predict system behavior for constant and variable heat exchanger effectiveness. The accuracy of each model is quantified in relation to empirical data obtained by Liu et al. [1, 2] in a physical system motivated by application to integral collector storage (ICS) solar water heating devices. From analysis of the empirical data, discharge-averaged values in the constant effectiveness model and in the variable effectiveness model are determined for a range of empirical conditions. The results show that the initial flow transients generated by the start of the discharge process are flow rate dependent and have a significant impact on the observed overall heat transfer coefficients.

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