Parametric study on the dynamic heat storage capacity of building elements

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 sufficientamount of thermal mass is needed in the building to store the heat. Assuming a 24h-period harmonic oscillationof the indoor air temperature within a range of thermal comfort, the analytical solution of one-dimensionalheat conduction in a slab with convective boundarycondition 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 potentialof 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 100mm thick concrete slab was found to increase with increasing heat transfer coefficientsas high as 30W/m2K. In contrast the heat storage capacity of a thin gypsum plaster board was found to be constant when the heat transfer coefficient exceeded 3W/m2K. Additionally, the optimal thickness of an elementdepended greatly on the heat transfer coefficient. For thin, light elements a significant increase in heat capacitydue 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.