Investigation on the dynamic characteristics of a direct contact thermal energy storage charging process for use in conventional air-conditioning systems

This study proposes a 3-dimensional (3D) mathematical model to simulate the heat transfer process in a direct contact latent heat thermal energy storage (TES) tank for conventional air-conditioning systems. The thermal performance, including the congealing point and congealing latent heat of a new phase change material (PCM), which is developed for use in conventional air-conditioning systems with a direct contact TES tank, is studied. To improve the charging performance of direct contact TES system and further understand the freezing process, a 3D numerical model was developed in ANSYS FLUENT. The volume of fluid (VOF) method tracked the volume fraction of each of the phases. The Navier-Stokes equations were solved using a finite-volume formulation, and energy equation was modeled by using an enthalpy-based formulation. The method provided a comprehensive model of the dynamic and thermal aspects of the impact process. To validate the analytical model an experimental cool storage air-conditioning system with a direct contact TES tank was designed and setup. The effects of the heat transfer fluid (HTF) inlet temperature, the flow rate of the liquid PCM, the complete charging time, cold storage capacity of the direct contact storage tank and distribution of temperature in the direct contact cold storage device were investigated. The results indicate that the charging capacity increases more rapidly when the PCM flow rate is greater. The complete charging time can be reduced by increasing the flow rate of the liquid PCM and decreasing inlet temperature of the HTF when charging the same quality of PCM. The charging capacity is larger with a lower HTF inlet temperature, however, changing the HTF inlet temperature does not appreciably change the total storage capacity.