Performance Evaluation And Optimazation Of A Dual-Airflow Window

Building energy conservation and indoor air quality improvement are the two main challenges in the sustainable building development. Ventilated windows have shown great potential in conserving energy in buildings due to their energy exchange ability and in improving indoor air quality by providing fresh air into indoors. This investigation studied the potential of energy conservation and indoor air quality improvement of the dual-airflow window.This investigation first conducted experimental measurements to study the dynamic energy performance of the dual-airflow window under the real weather conditions in Harbin. The study compared the dual-airflow window with a low-e (low-emissivity) window and a blinds window for their energy demand under the two kinds of condition: one is without window ventilation, and the other is with a ventilation rate of 40m3/h through the windows. The results demonstrated that the dual-airflow window can save energy under both the conditions. Without ventilation, the energy saving of the dual-airflow window was achieved by its three-layer glazing system. With ventilation, the energy efficiency of the dual-airflow window was improved by its energy recovery ability.Although the experimental measurements were most reliable and realistic, they are very expensive and time consuming. Our investigation has further developed a network model that can be used to numerically study the window performance. The model can account for the two-dimensional heat transfer in the window system and the model has been implemented it into EnergyPlus, a building energy simulation program. The two-dimensional assumption of heat transfer and the modified EnergyPlus program were validated by the energy demand measured in a test cell with the window under the actual weather conditions. The results show that the modified EnergyPlus can predict the building energy demand in a building with the dual airflow window at an acceptable accuracy.In order to optimize the window design, this investigation used the orthogonal method to evaluate the importance of 13 design parameters related to the dual airflow window. The parameters are outdoor air supply rate, window cavity width, window width, window height, window glass pane thermal conductivity, window glass pane thickness, window glass pane solar heat gain coefficient, window glass pane emissivity, thermal conductivity of window frame, window frame width, window orientation, shading coefficient, and window blinds position. The outdoor air supply rate, window height, solar heat gain coefficient, and window orientation were found to be the most important and were further studied by using the listing method to identify their optimal design values. The investigation was conducted for five climate zones in China.With the optimal design, this investigation compared the energy performance of two buildings with the dual-airflow windows with those with the three-layer conventional windows and single-supply-airflow windows in the five climate zones in China and the seven climate zones in the United States, respectively. One building was an apartment with a floor area of 80m2, and another was a house with a floor area of 200m2. The results showed that the dual-airflow performed better for severe cold or cold climate zones, while it had limited energy saving potential in the hot and humid climate zones.To save building energy demand, most residential buildings only used infiltration to delute contaminant concentration. However, infiltration is not stable and may not be sufficient to maintain an acceptable indoor air quality. To achieve acceptable indoor air quality requirement, mechanical ventilation is necessary. This study used a multizone airflow network model, CONTAM, to study indoor air quality by using CO2 as an indicator. The simulations used infiltration, ventilation by bathroom/kitchen exhaust fan, ventilation through a supply-airflow window, and ventilation through a dual-airflow window. The infiltration and ventilation through an exhaust fan cannot maintain an acceptable indoor air quality, but the ventilation by a supply-airflow window or by the dual-airflow window is sufficiently good..Considerring energy conservation and indoor air quality together, the dual-airflow window should be used.