Heat recovery performance was investigated in a latent heat storage unit utilizing $Na_2HPO_412H_2O$ as a heat storage material. A system unit consisted of a vertical cylindrical pipe (140mmI.D.×540mmH) with an internalheat transfer tube (35 mm I.D.) at the center. The storage system contained 11 kg $Na_2HPO_412H_2O$ and 800g excess water in shell side. The degree of subcooling and phase separation of $Na_2HPO_412H_2O$ was also studied using a test tube. Results obtained from test-tube experiments indicated that the degree of subcooling ranged from $17\circ{C}$ to $24\circ{C}$ with cooling rate. It was also found that the optimum content for avoiding the formation of $Na_2HPO_4$ $7H_2O$ was 37.5-37.75 wt% $Na_2HPO_4$. Main results obtained from heat transfer experiments are as follows. First, the induction time needed for solidification was more affected by the flow rate than by the inlet temperature of heat transfer fluid. However, the reason behind this result has yet to be clarified. Second, the heat recovery rate sharply decreased during the discharge stage because the solidified layer formed acted as a thermal resistance. Third, it was observed that the crystals of $Na_2HPO_412H_2O$ grew in dendritic shapes and dendritically crystallized portions acted as fins. As a result, the measured value of thermal resistance during the discharge stage was always smaller than the estimated value. Fourth, The thickness of crystallized heat storage material has been correlated in terms of Fourier, Stefan, and Reynolds number.
$R=0.182Fo^{0.50}Ste^{0.62}Re^{0.18}$
