This paper proposes a ventilated window with a phase change material (PCM) heat exchanger as a new window application. In summer, night ventilation mode is operated to discharge energy stored in PCM by the ambient cold air, which can be reloaded again, when ventilation pre-cooled air is provided. Numerical model is built and verified by full-scale experiment to evaluate the PCM ventilation system. The nonlinear properties and hysteresis of PCM are set in the model. The conclusion is that the configuration optimization should be based on different climates. In the case study in Copenhagen, the heat exchanger with 10 mm plate thickness is optimized. It can cool down the ventilated air 6.5 degrees C on average in 3.9 h pre-cooling effective time with 3.19 MJ/day energy saving. The material cost saving is 16.87% compared to 20 mm plate thickness which has similar discharged heat amount. Nevertheless, the heat exchanger with 5 mm plate thickness has a faster thermal response and a higher cost saving ability, which is good for the climate when the period of outdoor air temperature suitable for night ventilation in a day is short. (C) 2018 Elsevier B.V. All rights reserved.