Determination of active and reactive thermal resistance of one-layer building envelopes

Abstract

Introduction. The subject of the study is the individual characteristics of a 0.51 m thick external single-layer building envelope made of solid ceramic bricks. The paper focuses on the heat engineering parameters of the wall, namely, the calculation of active and reactive thermal resistances. We determine the differences between the two types of resistances. We also provide an example of calculating the thermal boundary layer in which all temperature fluctuations occur and determining the amount of heat absorbed and released by the envelope. Materials and methods. We give consideration to taking into account the two components of thermal resistance based on wave functions — thermal and temperature waves. Active thermal resistance is determined at any point of the building envelope with a fixed time value t (stationary heat transfer mode). The coordinate is recorded when determining total resistance. To calculate the thickness of the envelope thermal boundary layer, the temperature differential from −30 to 40 °С outside the premises is considered, the temperature inside the premises is assumed to be 18 °С. The temperature differential value is calculated from the ratio of the difference between current temperatures and the initial value. The required heat quantity and heat output are calculated using standard thermal physics formulas. Results. The difference between active and reactive thermal resistances, which together make up total thermal resistance, was proved. Active resistance is always 1.57 times less than total resistance. In this case, the active resistance will drop as the temperature differential decreases, and will increase when the outside temperature is higher than the temperature inside the premise. The thermal boundary layer thickness is always less than half of the envelope thickness. Conclusions. Using this method, it is sufficient to calculate the active thermal resistance of the building envelope to determine the remaining values. In addition, the greater the temperature differential, the thicker the temperature boundary layer, i.e. all temperature changes occur only in this layer while the rest of the envelope functions as a thermal accumulator. When the outside ambient temperature drops, all accumulated heat will be transferred into the premise. Such an envelope can be used to heat the premise or to direct this heat to various envelope elements.