Free cooling is an effective way to make good use of night cold energy and helps to reduce air-conditioning energy consumption in the daytime. This research proposed integrating a new-type of shell-and-tube based phase change material (PCM) thermal storage system with conventional air-conditioner to increase cooling coefficient of performance (COP). The proposed PCM thermal storage unit uses two different kinds of media (water and air) for the heat transfer fluids (HTF). Water is used for charging loop while air is used for discharging loop. The two HTFs better fit the heat transfer needs of nighttime's free cooling harvest and daytime's condensing water use by the air-conditioning system. A numerical model for the PCM thermal storage unit has been developed, particularly with consideration of staged natural convection effects in PCM melting process. The numerical model equipped with a new PCM staged effective thermal conductivity has been validated by experimental data. Numerical study has evaluated the effects of HTF inlet temperature, mass flow rate and conductive fin height on the PCM thermal storage system's performance. Modeling results show that HTF mass flow rate and fin height need to be designed through an optimization process according to the cooling load profile and achieve best performance of the PCM thermal storage system. Effectiveness of this proposed PCM thermal storage system is generally higher than 0.5. Case study of replacing conventional cooling tower by the proposed PCM thermal storage system for a water-cooled air-conditioner shows a COP value increase of about 25.6%. Published by Elsevier Ltd.