An Active Breathing Wall to Improve Indoor Environment

To maintain good indoor air quality, this investigation proposes a prototype of an active breathing wall system, which is composed of a solar radiation absorption board, air heating cavity, porous filtration unit and fan device. The thermo-flow performance of a physical model is measured and compared against the computational fluid dynamics (CFD) modeling. Then an office environment with a breathing wall is simulated by CFD. This study finds different ventilation modes are formed within the room in different seasons. The system generally has good performance to enhance the outdoor air supply rate without causing moisture condensation in the winter. INTRODUCTION To conserve energy, modern buildings are designed to hold better air tightness. Most commercial buildings are equipped with mechanical ventilation, so the outdoor fresh air may only be introduced into indoor space through ventilation. For residential buildings which use natural ventilation, doors or windows shall be opened when outdoor air is needed. However, this can cause problems in dense towns or cities, because the outdoor noise and polluted traffic air may also be imported into the space if doors or windows are open. The appearance of breathing walls seems to provide a good solution to the dilemma. A breathing wall which contains a porous filtration unit can block the outdoor noise and filter the air permeating into the indoor space. However, the breathing wall shall be carefully designed to ensure its good performance. A breathing wall is also called dynamic insulation, through which the indoor and outdoor air is exchanged via air permeation through porous material. The earliest relevant concept was developed in 1960s, when the agricultural buildings were constructed using dynamically insulated ceilings (Graee, 1974). Since the late 20 century, Taylor and Imbabi (1996) carried out extensive research on breathing walls. The breathing wall is found being able to improve indoor health but still hold good energy efficiency (Imbabi, 2006). As the breathing wall is composed of porous unit that behaves as a filter (Taylor et al., 1999), most of coarse particles travelling together with the air can be removed. The outdoor air permeating through the breathing wall has an opposite direction with the heat loss, so most of the conduction heat loss can be recovered and thus providing good insulation. For example, if there is air penetrating into indoors in the winter, the penetrative air gets heated by the conduction heat loss within the breathing wall. Since the penetrative air is finally delivered to the indoor space, the breathing wall would recover the energy that originally would be lost to the outside if using a traditional solid wall. The above breathing wall sounds flawless; however, it may not work well if the following problems are not resolved. First, the aforementioned breathing wall can only provide dynamic insulation when the air permeates in a unique direction that should be opposite to the heat loss. If unfortunately there are cracks on the building envelopes, such as a small joint clearance in the windows or doors, the air will bypass the breathing wall because of larger flow resistance (Baker, 2003; Dimoudi et al, 2004). Second, the air permeation through a breathing wall is subject to the outdoor wind that is essentially chaotic in direction and strength. This makes hard to assure the expected moving direction of penetrative air. Finally, if the outdoor air temperature is extremely low in the winter, the penetrative outdoor air will result in low temperature of indoor space (Gan, 2000), which may be far from the comfortable range. In addition, the created low room surface temperature imposes great risks in condensing moisture in indoor spaces (Yoon and Hoyano, 1998). The above review reveals that although a breathing wall seems promising to conserve energy and improve indoor environment, the traditional passive breathing scheme may not work well. This investigation has thus proposed to design an active breathing wall by installing a fan inside the wall to remedy the existent problems. Such a breathing wall system is promising to be applied in dense city downtowns to improve air quality and keep indoor space quietly. In addition, the breathing wall is coupled together with an indoor space to fully evaluate the performance of the system using computational fluid dynamics (CFD) modeling. Proceedings of Building Simulation 2011: 12th Conference of International Building Performance Simulation Association, Sydney, 14-16 November.