The characteristics and potential of $Na_2HPO_4 · 12H_2O$ have been studied for a thermal energy storage system. Based on the phase diagram of $Na_2HPO_4 · 12H_2O$ system, the best physicochemical properties for recycling found to be 37-38 wt. % of $Na_2HPO_4$.
In addition, using double pipe heat exchanger, heat transfer from the inner pipe to the phase-change material in a cylindrical annuals has been studied experimentally. Using water bath or collectors as a heating source, heat storage performance has been tested with the condition of 0.5-3.0 l/min in flow rate and 50-70℃ in temperature. Tap water or cold water was used to withdraw the stored heat with the flow rate range of 0.5-3.0 l/min.
Heat transfer properties in the phase change material during heating cycle can be characterized by the conduction for the first stage and the convection in the later stage. The rate of heat storing and withdrawing were found to be about $1.8\times 10^5-3.2\times10^5$ cal/hr and $1.0\times 10^5-1.25\times 10^5$ cal/hr. respectively.
Melt thickness (R) of phase-change material for the thermal energy storage system has been correlated in terms of Fourier, Stefan and Reynold numbers ;
$R = 0.00225 (N_{Fo})^{1.38}(N_{Ste})^{0.737} (N_{Re})^{0.307}$
Phase-change energy storage system found to be more attractive than that of water or rock storage systems on the basis of economical feasibility study.
