Study On The Influence And Optimization Of Time Lags And Decrement Factor Characteristics On The Exterior Wall Peak Heat Transfer

With the advancement of urbanization,the new building area is increasing every year,which leads to the increase of building energy consumption year by year.The improvement of people’s demand for indoor space comfort is that air-conditioning system has become an important factor affecting building energy consumption.In general,air-conditioning equipment and systems are generally designed according to the building peak cooling load.If the peak cooling load is too large,it will not only increase the initial investment and later maintenance costs of the equipment,reduce the energy efficiency of the equipment and system,but also directly affect the thermal comfort of indoor personnel.Therefore,reducing the building peak heat transfer cooling load is of great significance to reduce the capacity of HVAC units,improve economic benefits and room thermal comfort,and implement the policy of energy conservation and emission reduction.As the main part of the building envelope,the exterior wall thermal properties will directly affect the building cooling load.Time lags and decrement factor(TLDF)are the characterization parameters of wall thermal storage.It not only reduces the solar-air peak temperature,but also delays the time when the peak temperature enters the room.According to the heat transfer formula,the inner surface peak temperature is the main parameter for calculating the wall peak heat transfer capacity.The wall peak heat transfer capacity will affect the building envelope peak cooling load.Based on the above,this paper will study the influence of TLDF characteristics on the exterior wall peak heat transfer.The main contents and results of this paper are as follows:1)Taking a high-rise residential building in Changsha as an example,this paper explores the influence of different wall combinations TLDF characteristics on the exterior wall peak cooling load.According to the wall materials and structure,a variety of exterior walls with the same heat transfer coefficient and different time lags and decrement factor are constructed.Several groups of wall combinations are obtained by orthogonal experimental design method,and then the peak cooling load of each combination is simulated.The results show that there are great differences in the peak heat transfer cooling load of exterior walls with different combinations.It shows that the difference of TLDF characteristics between wall combinations has an impact on the building peak heat transfer,in the case of the same heat transfer coefficient.2)In order to analyze the TLDF characteristics of wall combinations,according to the difference of the influence of time lags and decrement factor on the external disturbance temperature wave,the weight decrement factors(WDF)and the standard deviation of the inner surface peak temperature occurrence time(SD)are defined.WDF is used to evaluate the decrement factor of each wall combination.The larger the WDF is,the better the attenuation characteristics are.SD is used to evaluate the time lags of each wall combination.The larger the SD is,the greater the wall combination delay time difference is,and the better the delay characteristics are.The results shows that the larger the WDF and the larger the SD,the smaller the exterior wall peak cooling load.3)Based on the above findings,we propose an optimal combination method of TLDF characteristics to minimize the heat transfer peak value of the exterior wall.The method can achieve "staggered peak superposition" by adjusting the time lags of exterior walls on different orientations,and the exterior wall peak heat transfer can be reduced after the superposition.The Peak Shifting Indicator(PSI)is introduced to adjust the time lags of each facing wall.According to the formula,the programming calculation is carried out to find out the minimum PSI combination of the total heat transfer peak value and the best time lags of the building facing wall.By using the TLDF stability calculation method and the transfer function method to calculate the decrement factor,the optimized wall combination can be obtained.4)This method is used to calculate the heat transfer of an experimental cabin in Changsha,and compare the total heat transfer peak value difference of the wall before and after optimization.According to the calculation,the total heat transfer peak value is 326.17 W before optimization and 254.35 W after optimization,and the total heat transfer peak value is reduced by 22.1%.In order to verify the correctness of the optimization design method,Energy Plus is used to simulate the total heat transfer peak value before and after optimization,and compared with the theoretical calculation value.The simulation results show that the total heat transfer peak value before and after optimization is 354.98 W and 282.57 W respectively,and the total heat transfer peak value after optimization is reduced by 20.4%,which is basically consistent with the theoretical calculation results.