Studies On Natural Ventilation Potentials Based On Urban Climates And Air Pollution

The building sector is one of the largest energy consumers.With the rapid development of China’s economy and the continuous improvement of people’s living standards,the energy consumption of building operations will keep growing.Therefore,energy-efficiency in buildings has played a positive and significant effect on the realization of emission reduction targets.Natural ventilation can achieve effective passive cooling,using outdoor air to remove indoor waste heat and humidity.When the outdoor climatic conditions are suitable,it can create a comfortable indoor thermal environment without consuming energy.While,outdoor fresh air affects the interior,the dilution of pollutant is beneficial to the physical and mental health of occupiers.Hence,natural ventilation is an energy-efficient passive measure to create a comfortable and healthy indoor thermal environment.However,the utilization of natural ventilation relies on urban environmental conditions,including urban climate conditions,local wind fields and outdoor air pollution.There is a lack of systematic methods to comprehensively evaluate building natural ventilation potential from various environmental factors.Climate conditions mainly include temperature,humidity,radiation and other factors.Since natural ventilation is essentially a direct exchange of indoor and outdoor air,the difference in temperature and moisture content between indoor and outdoor reflects the ability of outdoor weather conditions to improve the indoor thermal environment.Using heating degree days（HDD18）and cooling degree days（CDD26）as the first-layer indexes,relative humidity,solar radiation and wind speed as the second-layer indexes,a hierarchical climate zoning method was established.Using this method,the hot summer and cold winter zone is divided into 7 sub-climate zones,which improves the spatial accuracy of the sub-climates and shows higher consistency in each sub-climate zone.The zoning results provide a scientific basis for policymakers in this region to propose more reasonable ventilation or other passive strategies.Based on the results of climate zoning,the quantitative relation between different climate conditions and building natural ventilation potential is determined.The urban wind environment determines the pressure on the surface of the building envelope,forming a certain pressure difference between indoor and outdoor,promoting the air circulation.The pressure difference determines the ventilation rate of the room,reflecting the degree of the utilization of nature resources.Based on satellite images and the principle of solar shadows,a building information model was established using the GIS system.Cluster analysis was applied to recognize typical building types.According to the building distributions in real communities,the basic community morphologies are formed by arranging typical building types with appropriate spacing.Then,the basic community morphologies are combined with roads in the street canyon to establish partial urban layouts.Through CFD simulations,the strength of air circulation inside the block is closely related to the building density.According to the flow field simulation results of various urban layouts under different winds,combined with the calculation of the ventilation rate based on the wind pressure coefficient,the relationship between the urban building layout and the building natural ventilation rate is determined.Outdoor air pollution will be introduced into the indoor environment through natural ventilation.When the concentration of air pollutants in the indoor environment caused by outdoor sources exceeds the standard,occupiers will close the external windows in consideration of the impact on health,which limits the utilization of natural ventilation.Large-scale field surveys on the shape and distribution of buildings in the city were conducted,and the road layout information and construction site information were obtained using satellite images.Selecting indexes that represent the layout of urban buildings,roads,and construction sites makes city information parameterized.Meanwhile,weather and air pollution observation data during the test period was obtained.Then,using a series of variables concerning background particle pollution level,weather conditions,urban morphology and urban pollution sources as predictors,and outdoor monitoring data of particle concentration as a response,the artificial neural network models were trained to create a community-scale model for rapid estimations of particle concentrations.Verifying its predicting performance,the average relative error between the predicted value of PM₁₀ and the measured value is 17.56%,and for PM_2.5 is 16.04%,the model shows high accuracy.At the same time,the indoor particle concentrations in the corresponding locations were measured.When natural ventilation is applied only,there is a high correlation between indoor and outdoor particle concentrations.In the central urban area of Chongqing,the permeability coefficient is between 0.60 and 0.85,and the concentrations of indoor-sourced particles are normally between 0 and 10μg·m^-3.Using these models can realize the dynamic control of natural ventilation based on the degree of outdoor air pollution.Finally,a comprehensive evaluation system and synthetic analysis methods for building natural ventilation potential are proposed.The systematic framework considers the mutual influence of multiple factors,containing climate conditions,local wind environment and outdoor air pollution,using upper-level urban data,to achieve a comprehensive assessment of the building natural ventilation potential in the early planning and design stage.Taking the Chongqing benchmark building as an example,the non-heating-and-cooling period can be extended from 48%to 68%after using natural ventilation.When natural ventilation with the maximum of 10 h^-1 air changes is applied in an actual case-study building in Chongqing,there is no need for heating and cooling system 70.2%of the time.The non-heating-and-cooling period is shortened by 3.4%considering the urban layout,as the building is located in a very dense area.When considering the actual situation of air pollution,the available natural ventilation period is shortened to 52.6%.