Effect Of Thermal Insulation On Thermal Environment Of Naturally Ventilated And Intermittently Conditioned Buildings

There are two major kinds of operation mode of buildings,i.e.,natural ventilation and intermittent air-conditioner operation,in hot summer and cold winter zone in China,which is determined by its climate characteristics of the zone,living habits of the people and the local economic conditions.Therefore,more and more studies focus on the effective improvement of thermal environment and energy efficiency of buildings of this zone,but the basis is that the indoor and outdoor heat transfer process in buildings can be evaluated accurately.Due to the specific thermal inertia of buildings themselves,indoor thermal environment and air-conditioning energy consumption is not only influenced by the outdoor climate,but also affected by the early stage of the heat transfer process in a season,this leads to that under actual varies of climate conditions,thermal process and energy consumption of buildings during several typical days is difficult to represent the building thermal behavior throughout a year.Therefore,there is a need to evaluate the thermal behavior throughout a whole year in naturally ventilated and intermittently conditioned buildings.However,convective coupling between ventilation,room air and surface in these two buildings results a difficulty to solve the building heat transfer process,especially under actual changeful climate conditions,leading to that the existing method is not suitable to tackle such complex physical problems.The key to evaluation of the building thermal environment and energy consumption during the whole year is to explore a method which can rapidly and accurately obtain the heat transfer characteristic considering the convective coupling between ventilation,room air and surface.In this study,firstly,the physical process of the wall heat transfer is analyzed.Secondly,a description of this kind of wall heat transfer control equations is given on the basis of some simplifications,and the corresponding analytical solution is given.Finally,combined with Matlab program,the target of quick and accurate calculation of the coupling heat transfer process of buildings is achieved.Using this method,building thermal characteristics during the next cycle affected by the last cycle is considered,even if the calculation is for the whole year.This can lay the foundation for analysis of annual thermal behavior characteristics in natural ventilated and intermittently conditioned buildings.Thermal insulation performance of envelope is an important factor affecting the indoor thermal environment in naturally ventilated rooms,but most studies focus on the effect of insulation performance on the energy consumption of air-conditioned spaces,and almost no related studies focus on naturally ventilated buildings.The effects of the thermal insulation of walls and windows on indoor thermal environment in a naturally ventilated room for different window-wall ratio was analyzed in this study.The concept of disadvantage coefficient,advantage coefficient and intensity of thermal discomfort were introduced to evaluate the potential and the effect of increasing the thermal insulation performance of walls and windows on improving indoor thermal environment.Both the potential prediction and effect analysis results show that intensity of thermal discomfort in summer and winter decrease significantly with the increase of the insulation resistance of external wall generally,but for a building with large window-wall ratio,insulation performance of walls almost has no effect on the thermal environment in winter.Decreasing the heat transfer coefficient of the window is an effective way to improve indoor thermal environment in winter,while it is negligible in summer.For the whole year,it can decrease the intensity of thermal discomfort linearly.According to the analysis of the degree of indoor thermal comfort improvement,the insulation thermal resistance of 1.0 m2??C/W is an appropriate value for thermal insulation of external walls in this zone.The heat transfer coefficient of the window should be reduced as low as possible,preferring to the national ‘Energy saving design standard for residential buildings in hot summer and cold winter',this study suggests that the window heat transfer coefficient is preferable to 2.3 W/(m2??).Based on these,this study builds a prediction model of the degree of thermal discomfort improvement when the external walls or windows insulated as suggested.In order to choose effective way to improve indoor thermal environment,the influences of internal thermal mass and air change rate at night on the indoor thermal environment were also analyzed and a sensitivity analysis was carried out,to compare the effects of variation of these parameters,including thermal insulation od external walls and windows,internal thermal mass and air change rate at night,on the thermal environment.The results indicates that a critical window-wall ratio 0.6 can be found for relative sensitivity of external walls and windows.When the window-wall ratio is less than 0.6,the external wall thermal insulation should be mainly strengthened,if the window-wall ratio greater than 0.6,the opposite is true.Increasing air change rate at night is the most effective way to improve indoor thermal environment.Compared with other three parameters,the impact of the internal thermal mass almost can be neglected.When the air-conditioner operated intermittently,the process of absorbing and releasing heat through thermal mass is unavoidable to appear.To analyze the influence of this process on energy consumption,the present study measures heating process in a phytotron to evaluate the relationship between energy consumption and internal thermal mass.The results show that due to the existence of the internal thermal mass,a substantial share of energy heat goes to heat it,instead of improving the level of human thermal comfort.Therefore,the larger the thermal mass is,the greater the heating energy consumption.Dynamic thermal behaviors of building walls with real climate conditions may be different from those with only the representative day's climate conditions due to the time varying nature of the local climate conditions,and this will lead to the variation of the cooling load of intermittently operated air-conditioning systems.This study analyzed the effects of insulation location and thermal resistance on heat transfer of building walls and energy consumption using the climate data of Shanghai over three summer and winter seasons(June,2011 to March,2014).The results indicates that in terms of capacity configuration of air-conditioning systems,energy consumption and system operation stability,inside insulation could result in better performance.Heat dissipation and storage of the wall's inside layer during non-working times are the key factors affecting the transmission loads of the intermittently operated air-conditioning systems.In addition,the results show that the insulation thermal resistance of 1.0 m2??C/W is an appropriate value for thermal insulation in this zone.