Ideal Thermophysical Properities Of Building Wall: Determination Method And Mechanism Interpretation

Building energy consumption is huge, and the thermophysical properties ofbuilding wall are important influencing factors. So selecting or developing anappropriate wall plays an important role in saving energy. Traditional researchesusually focus on the constant thermophysical properties wall and “trial anderror” method is used to determine the ideal thermophysical properties.However, this method will result in hgher calculation cost and cannot obtain theideal thermophysical properties easily by limited calculations. Besides, for thewall with the variable thermophysical properties, this method cannot obtain theideal thermophysical properties at all. In order to solve the addressed problems,some researches are carried out in this paper. The main academic contributionsare as follows:(1) An inverse problem approach of determining the ideal thermalconductivity (variable thermophysical properties) of passive building wall is putforward. The unknown ideal thermal conductivity of building wall is caclautedinversely according to the indoor thermal demond. For application, a passiveroom in Beijing is analyzed. The results show that the ideal thermal conductivityof the external wall approaches a square wave function and the indoor integrateduncomfortable degree in a year can reduce64.3%. This approach gives theguidelines for the designers of building materials and engineers of buildingenergy when they select the building materials.(2) In order to solve the limition of the traditional thermodynamics methodfor analyzing the ideal thermophysical properties of building wall, anentransy-dissipation-based impedance approach is put forward. Entropygeneration is recognized as a measure of the irreversibility in heat-workconversion process and it is unsuitable for the optimization analysis in heattransfer process (ideal thermophysical properties of building external wall is notcorresponding to the extremum of the entropy generation in the heat transferprocess of the building external wall). On the other hand, as the thermalresistance based on the entransy dissipation neglects the heat capacity, it cannot be used in the non-steady heat transfer problem. Based on this, considering theheat capacity, entransy-dissipation-based impedance in non-steady heat transferprocess is defined. It establishes the relation between the heat flow and theentransy dissipation and it provides a new optimization parameter foroptimizing the thermophysical properties. According to the analysis, it is foundthat for the active or passive building, the ideal thermophysical properties ofbuilding external wall are corresponding to the extremum of the entransydissipation in the heat transfer process of the building external wall.(3) Entransy-dissipation-based impedance approach is used to analyticallysolve the ideal thermophysical properties of building wall. As it is difficult toestablish the relationship between the input heat from wall and other relatedvariables in non-steady heat transfer process, the inverse problem approachcannot obtain the analytical ideal thermophysical properties of building wall.The entransy-dissipation-based impedance approach can solve this problem.This method takes the extremum of the entransy-dissipation-based impedance asthe optimization objectives, combining the corresponding restraints, and usesthe variational method to obtain the optimization criteria to determine the idealthermophysical properties of building external wall for active or passivebuilding. Based on the above optimization criteria, the optimization algorithmsfor determining the ideal thermophysical properties of the external wall aredesigned. According to the optimization for the ideal thermophysical propertiesof the previous passive building external wall in Beijing, it can be found that theoptimization results by the impedance method are almost the same with those bythe inverse problem method. In addition, the impedance method is superior tothe inverse problem method in the aspects of both optimization precision andspeed. The impedance method provides the guildline criteria for designing thethermophysical properties of building wall.