A Model of Direct Contact Heat Transfer for Latent Heat Energy Storage

Direct contact heat transfer is an attractive method to reduce the cost of heat exchange for latent heat thermal energy storage systems. However, current performance information is insufficient to allow an accurate appraisal of its economic and technical feasibility. In a direct contact heat transfer system, an immiscible fluid is bubbled through the storage media and heat is transferred between the phases as the droplets rise. An analytical model is presented for predicting the temperature of the rising droplets from information in the literature. The drop size is calculated from empirical correlations in the jetting formation region and rise velocity is characterized by a creeping-flow surface cell model which accounts for the hindering effects of neighboring droplets. The viscosity of the crystallizing solution in the rise velocity equation is approximated by an expression for concentrated suspensions, where the percentage of solids is taken as the percentage of crystallization. Dispersed phase holdup is predicted with the rise velocity. Calculation of the rate of heat transfer to the dispersed immiscible fluid droplets is based on three different internal hydrodynamic approximations: rigid, internally circulating, and wall-mixed spheres. The predictions of the circulating drop case agree reasonably well in the latent heat region with previous data on a similar system. However, because the model is also sensitive to the estimates used for drop size, continuous phase viscosity, and interfacial tension, the heat transfer mechanism cannot be conclusively identified, and experimental research is required to establish the validity of the model.