Thermal energy storage is one of the many strategies that are effective in alleviating the electrical power supply and demand imbalance issues on the electric grid, and buildings are a good place to implement such storage solutions because of their high electricity consumption. This thesis presents a novel energy storage solution by incorporating phase change material (PCM) in the building supply-air duct. The in-duct PCM storage has various advantages compared to PCMintegrated walls including more effective heat transfer (forced convection and greater temperature differentials). During off-peak hours, the system runs at a supply-air temperature below the material’s solidification point to charge the PCM with cooling energy. During on-peak hours, a higher supply-air temperature is utilized so that the stored energy can be discharged into the supply-air. This shifts a portion of building’s cooling load from the on-peak hours to the off-peak hours. A numerical model for the melting and solidification of PCM in the duct was developed and modified using experimental data. Whole building energy simulations were conducted by coupling the PCM numerical model with EnergyPlus' DOE prototypical building model in a Simulink co-simulation platform. Simulations were performed for three cities in different climate zones over a three-month cooling season (June to August) and the PCM storage reduced the onpeak energy consumption by 20-25%. The electricity cost and payback period were determined using current time-of-use electricity rates.