In modern, extensively glazed office buildings, due to high solar and internal loads and increased comfort expectations, air conditioning systems are often used even in moderate and cold climates. Particularly in this case, passive cooling by night-time ventilation seems to offer considerable potential. However, because heat gains and night ventilation periods do not coincide in time, a sufficient amount of thermal mass is needed in the building to store the heat. Assuming a 24 h-period harmonic oscillation of the indoor air temperature within a range of thermal comfort, the analytical solution of onedimensional heat conduction in a slab with convective boundary condition was applied to quantify the dynamic heat storage capacity of a particular building element. The impact of different parameters, such as slab thickness, material properties and the heat transfer coefficient was investigated, as well as their interrelation. The potential of increasing thermal mass by using phase change materials (PCM) was estimated assuming increased thermal capacity. The results show a significant impact of the heat transfer coefficient on heat storage capacity, especially for thick, thermally heavy elements. The storage capacity of a 100 mm thick concrete slab was found to increase with increasing heat transfer coefficients as high as 30 W/m2K. In contrast the heat storage capacity of a thin gypsum plaster board was found to be constant when the heat transfer coefficient exceeded 3 W/m2K. Additionally, the optimal thickness of an element depended greatly on the heat transfer coefficient. For thin, light elements a significant increase in heat capacity due to the use of PCMs was found to be possible. The present study shows the impact and interrelation of geometrical and physical parameters which appreciably influence the heat storage capacity of building elements.
28th Aivc Conference: Crete, Island, 2007
Ventilations; Air conditioning systems; Passive cooling; Thermal comfort