A novel window-based cooling unit filled with phase change materials (PCMs) is developed in an office building. At night, outdoor coolness is stored in the unit by natural ventilation and it is actively released to indoor environment during daytime. Thermal performances of this cooling unit during both cool storage and release periods are numerically investigated with a transient 3-D model. The results show that during cool storage period most parts of the PCM slabs with optimum thickness of 5 mm can freeze completely within 8 h from 21:00 to 4:00 and the thinner the PCM slab is, the less solidification time will cost under the same inlet air velocity. Also, PCM with the same thickness solidifies faster with increasing the inlet air velocity. During cool release period the cooling unit can decrease indoor temperature up to 3.3 degrees C within 1 h and can maintain the temperature for 2 h during 5 consecutive days (June 19-24). Additionally, the inlet air velocity in day mode should be controlled to satisfy the effective draft temperature (EDT) stipulated for thermal comfort. The transient EER can be as high as 8.7 for inlet air velocity of 0.8 m/s during the long period simulation. Moreover, the cooling unit can save about 1.9 kW hour, which economizes 0.95 RMB within 1 h, compared with a traditional air conditioner with the same capacity. Also, carbon dioxide emission is decreased by 1.89 kg in 1 h. The proposed cooling unit has the advantages of convenient installation for existing buildings and the energy saving potential particularly applying natural ventilation in night mode. (C) 2015 Elsevier B.V. All rights reserved.