Investigation Of Thermal Load And Thermodynamic Evaluation Method For Building Envelope

The thermal load of air conditioning system is formed of heat gain from indoor heat resource,fresh air and building envelope.The heat gain from indoor heat resource and fresh air depend on the function of the building.It is irregularity but easy to estimate.The heat gain from building envelope is related to the shape of the building,the construction of the envelope and its insulation performance.Its calculation is complicated and it is essential for building energy efficiency.The thermal load calculation of building envelope includes cooling and heating sizing load,and annual cumulative cooling and heating load.The main methods for thermal load calculation of building envelope include hourly calculation according to current codes or simulated by software.The accuracy of the method provided by the codes need to be improved,while the efficiency of software calculation needs to be enhanced.The main factors of building envelope which decide the thermal load of building envelope have been analyzed by dimension theory and the comprehensive heat transfer coefficient of building envelope has been established.EnergyPlus has been applied to calculate the thermal load of multiple building samples to get the comprehensive heat transfer coefficient of building envelope.Then the comprehensive calculating models for cooling and heating sizing load and annual cumulate cooling and heating load of building envelope in typical cities have been established.The comprehensive cooling sizing load calculation model has been validated and its accuracy is verified.Finally,a new thermodynamic evaluation method for the life cycle thermal performance of building envelopes is proposed.The main contents of the paper are as follows:(1)The cooling load of building envelope mainly comes from solar radiation.The factors decide the cooling load includes heat transfer coefficient,thermal inertia index,solar radiation absorption coefficient,window-wall ratio and roof area ratio.At the same time the heat transfer process is dynamical.Thus the computation of cooling load is complex and time-consuming.All the factors are analyzed by dimension theory to define the comprehensive heat transfer coefficient which is the basis of comprehensive thermal load calculation model of building envelope.Then dimensionless building shape factor is proposed to evaluate the influence of bui lding shape on the thermal load.It reflects the influence of the building shape on the heat gain from temperature difference and solar radiation,so it is more reasonable to evaluate the influence of envelope shape on load than by shape coefficient.(2)The equivalent window-wall ratio(EWWR)is defined to evaluate the impact of the overall window-wall ratio on building energy consumption.It is considered that a building that has the same EWWR would have the same solar radiation heat gain from windows on summer design day.The increasing EWWR will leads to increasing solar radiation heat gain from windows.Therefore,if the influence of solar radiation heat gain from windows on thermal load is taken into account,the EWWR is more reasonable than window-wall ratio to evaluate the building envelope performance.Equivalent coefficients of 6mm+9A+6mm windows in Nanning,Changsha,Nanjing,Beijing and Changchun are calculated to provide basic data for evaluating the performance of building envelope.At the same time,the limit value of equivalent window-wall ratio in typical cities is calculated according to the Chinese codes of energy-saving design for residential buildings.(3)The factors such as heat transfer coefficient,solar heat gain coeffici ent of external windows,solar radiation absorption coefficient of building envelope surface,building orientation,equivalent window-wall ratio and shape factor are analyzed to study their impact on building envelope thermal load.The percentage of cooling sizing load of external walls,roofs and windows is analyzed.(4)EnergyPlus is used to calculate the thermal load of hundreds of building samples to obtain the comprehensive heat transfer coefficient.Then the comprehensive calculation models are obtained with MATLAB by fitting the data.The models include cooling and heating sizing load and annual cumulative cooling and heating load calculation models of Changchun,Beijing,Nanjing,Changsha and Nanning.According to the building shape factor,the buildings are divided into two groups.The two groups of buildings have different comprehensive thermal load calculation models.The scatter plots of the thermal load calculated by the comprehensive models are compared with the results from EnergyPlus.The results show that the comprehensive calculation models are accurate enough and can be applied in practical engineering.Finally,the practicability and rationality of the models are elaborated in detail.(5)The ratio of exergy to energy(REXE)is applied to evaluate the utilization degree of high-quality energy during building life cycle.The model of energy consumption and REXE is established to optimize the design of building envelope and to evaluate the rationality of energy utilization.The critical value of REXE is proposed to evaluate the building life cycle efficiency.Five public buildings in Yiyang,Hunan Province have been studied to analyze the relationship between energy consumption and REXE.The Gauss curve shows that different buildings can be classified into four energy-saving zones.The buildings in different zones have their own energy consumption characteristics and energy-saving measures.Generally,the comprehensive calculation models of building envelope thermal load are established in the paper,which can quickly and accurately estimate the thermal load based on thermodynamic parameters of building envelope.A new thermodynamic evaluation method has been proposed in the paper for building life cycle energy efficiency.