Decreasing g-value by Combining Internal Solar Screen and Exhaust Ventilation - A Numerical Study on what Parameters Affect the Possible Reduction

Abstract

In many modern buildings, using large glazed areas in the building envelope, the solar heat load needs to be limited. To do this without disturbing the architectural display of the building, internal solar shading is often the only alternative. However, using internal solar shading means a great part of the incoming solar load still ends up inside the building, causing an often large heat load. To decrease this problem, one solution is to put exhaust ventilation close to the windows or even connected to the cavities between solar screen and window. The purpose of this is to extract the heated air before it mixes with the air further inside the room. The purpose of this study is to investigate the possibilities to reduce the g-value of a window and solar screen by using the exhaust ventilation. The aim is to find a reduction factor of the g-value, applicable for these ventilated windows. By changing many design parameters, the aim is to see what parameters affect the efficiency of the system. The study is performed by creating a numerical model in COMSOL Multiphysics. The heat transfer and air movements around the screen and window are simulated. The study shows it is difficult to find a general reduction to the g-value by applying ventilation to the cavity. The efficiency of the system depends on numerous parameters. It is first and foremost dependent on exhaust flow but other parameters matter as well. Such parameters are: window height, distance between screen and window, location of exhaust device and which type of window or screen is used. With a correct and still reasonable design, results suggest that a reduction of the gvalue by -45 % should be possible.