Study Of Spatial And Temporal Law Of Heat Rejection From Air-Conditioning System Of Public Building In Urban Space

With the rapid urbanization, for the higher standards of working and living demand of city residents, buildings in a city, especially the public buildings as the most important component are developing in a direction of the higher density and higher energy consumption. As a result, the heat rejection from building airconditioning system will increased, especially the heat rejection of water-cooled refrigeration unit was released through the cooling tower both in the forms of sensible heat and latent heat, which would ultimately affect the thermal climate in the scope of building and even in the scale of urban. This study is proposed to analysis the temporal and spatial distribution of the heat rejection from building air-conditioning system in the urban area by the method of field measurements and numerical simulations.Firstly, this study selected four public buildings as the measured objects which located in Shenzhen and Beijing. By the hourly dynamic field measurement of the parameters of air conditioning refrigeration system, the measurement results showed that total heat rejection of measured public buildings air conditioning system by unit building area is in a range of 70-100 W/m2, where the proportion of sensible heat rejection is less than 14%, and the proportion of latent heat rejection is more than 86%. Heat rejection from the public building air-conditioning system through the cooling tower is mainly in the form of latent heat which has a significant increase for the surrounding air relative humidity, while the surrounding air temperature has a less increase. By the further analysis of the relationship between the atmospheric conditions and heat rejection of building air conditioning system, found that the high temperature and high humidity climate condition are conducive to heat rejection of building air conditioning system, and dereased the heat rejection coefficient of air conditioning system.Secondly, the building load simplified calculation model for predicting the urban thermal climate is the the basic conditions for modeling the heat rejection of building air conditioning system by the developed urban thermal climate dynamic prediction model. This study considering the characteristic of outdoor thermal climate, and by the analysis of building thermal process, the heat balance equations of building envelope are set up, and consider the indoor thermal interference and ventilation, respectively. Then a building load simplified calculation model for the urban thermal climate prediction is obtained. By applying the theoretical calculation method, measured data, and the similar building simulation software to certify this building load simplified calculation model. The certify result indicated that the error is in the range of precision which shows that the accuracy of this model is high, and simplified assumptions are rationality according to the practical building.The heat rejection model of building air conditioning system is an important module of the urban thermal climate dynamic prediction model. By considering the heat rejection of public building air conditioning refrigeration system was released through the cooling tower in the actual situation, based on the thermodynamic basic theorem, and test data of running parameters or performance curve of various types of water chiller that provided by the equipment sample, the mathematical model of various types of water chiller could be established. According to the principle of heat and mass transfer, the mechanical ventilation wet cooling tower mathematical model is established. By combining with the established building load calculation model, and through the mathematical coupling relationship between the above models, the heat rejection model of building air conditioning system was established eventually which detailed considering the different forms of heat rejection. By compare with the test data, the results indicated that the simulation results of heat rejection were well consistent with measured data both in value and time variation. This heat rejection model of building air conditioning system is used in the actual public building has a high reliability.Based on the above developed models and original urban thermal climate dynamic prediction model, an urban thermal dynamic prediction model which could simulate the heat rejection of building air conditioning system is developed, and applied to analysis the complex and dynamic relationships between the heat rejection of building air conditioning system and urban thermal climate. By analyzing the main influence factors, firstly, the single factor numerical test method is used to study the variation law of the heat rejection of building air conditioning system, outdoor air temperature and humidity, urban heat and wet island intensity when these factors are in the different levels. The results indicate that the building load and heat rejection of air conditioning system, and atmospheric temperature and humidity, urban heat and wet island island intensity are decreased when respectively decreasing the building density, floor area ratio, average windward area ratio and window shading coefficient, while raising the air conditioning set temperature and relative humidity, and raising heat rejection height position. Ultimately, orthogonal test is applied to investigate the effect of several factors changed simultaneously on the heat rejection of air conditioning system. The results show that the air conditioning set temperature is the most important and significant factor, while the significance of average windward area ratio is the lowest.In the end, the devolped urban thermal climate dynamic prediction model is applied to a Shenzhen actual complex urban area. Combined with the platform of geographic information system technology, the time and space distribution law of the building load, heat rejection of air conditioning system, atmospheric temperature and humidity, urban heat and wet island island intensity is analyzed by the simulation study in a standard summer day in the complicated urban region where is mainly composed of public buildings. The simulation results showed that the double peaks of heat rejection of air conditioning system have a lasting impact on the outdoor thermal weather. In addition, the urban heat and wet island island intensity of building block showed a strong positive correlation with the sensible and latent heat rejection of air conditioning system, respectively.