Recently, Thermal energy storage (TES) air conditioning system has been introduced in many buildings because it will contribute to electric-load levelling. This system can also improve the operating efficiency of heat source equipment. For TES system, a wide range of materials can be used as thermal storage media, i.e. water, concrete, soil, ice and other phase change materials. In Japan, the water TES air conditioning system is commonly used, and this study focuses on a temperature-stratified water TES tank where water of different temperature is stored utilizing density difference without mixing. The configuration, size, location and number of diffusers used as inlet and outlet are very important in order to design the temperature-stratified water TES tank of high performance. Since the disk-shaped diffuser commonly used for this system is generally expensive, this study aims to propose new type low cost diffusers, and several kinds of half-pipe diffuser were studied. In this paper, a temperature-stratified water TES tank of length 2.3m, width 1.3m, and depth 5.8m is analyzed, where the disk-shaped diffuser, half-pipe diffuser, no-cover half-pipe diffuser, and punched-metal half-pipe diffuser are provided. By conducting unsteady CFD calculation varying both the configuration and installation depth of diffuser, thermal storage performance is evaluated based on temperature gradient. According to these CFD analyses, it is shown that the difference in thermal storage performance is not significant among disk-shaped diffuser, half-pipe diffuser, and punched-metal half-pipe diffuser. However, it is considerably affected by installation depth.