Studying the impact of local urban heat islands on the space cooling demand of buildings using coupled CFD and building energy simulations

Surface as well as air temperatures are due to the urban heat island effect higher in urban compared to their surrounding rural areas. These increased temperatures have a strong impact on the building energy performance in urban environments and thermal comfort as well as health of inhabitants of these environments. At smaller scales, local heat islands are formed within urban environments, which have the same negative impacts. In this study, we investigate the influence of local heat islands on the space cooling demand of buildings. Commonly, climate information from one weather station is used for building performance simulations within a whole city. This climate information cannot take local hot spots into account, what can lead to inaccurate space cooling demand predictions. Here, we model the local urban climate with CFD (Computational Fluid Dynamics) simulations. With CFD also the local convective heat transfer coefficients (CHTC) for the building surfaces can be predicted. These local CHTC can strongly vary locally due to differences in local wind speeds. The commonly used coefficients are mostly based on measurements at facades of stand-alone buildings, where the local wind speeds are higher compared to urban areas. This study shows a dependency of the space cooling demands on the local urban climate. Space cooling demands are higher in areas with high local temperatures, where the winddriven ventilation is decreased. Additionally to the higher local temperatures also the local CHTC are lower leading to lower heat losses from the buildings. It can be concluded that it is important to account for the local microclimate to accurately predict the space cooling demand of buildings in urban environments

Multiphysics Modeling of Materials, Assemblies, Buildings and Cities

There is growing evidence that heat waves are becoming more frequent under increased greenhouse forcing, associated with higher daytime temperatures and reduced night-time cooling, which might exceed the limits of thermoregulation of the human body and affect dramatically human health. Especially urban areas are affected, since these regions in addition experience an urban heat island (UHI) effect characterized by higher air temperatures compared to the surrounding rural environment. A necessary breakthrough is a shift away from a fragmented approach towards an integrated multiscale urban climate analysis. This type of research is a rather new domain of research and might be based on an all-physics understanding and modeling of the urban climate ranging from the scales of material and buildings, to the scales of a group of several buildings, street canyons, neighbourhoods, cities and urban regions, referred to as multiscale building physics. To adequately cover global and local urban heat island effect, regional and mesoscale climate analyses have to be downscaled to sub-kilometer resolution and linked with urban climate models at neighborhood and street canyon scales. Such a multiscale urban climate model allows to analyze the influence of urban and building parameters on thermal comfort and the building cooling demand. The importance of accounting for the local urban climate when quantifying the space cooling demands of buildings in an urban environment is demonstrated. The heat-moisture transport model for building materials allows the design of new building materials, which can help in the mitigation of local heat islands. With respect to evaporative cooling materials, we need to optimize their water retention and evaporative cooling by tailoring their pore structure. The understanding and information obtained from pore-scale investigations enables to understand macro-scale transport processes, and enabling us to explore the potential of new evaporative cooling materials at local urban scale

Integrated vegetation model for studying the cooling potential of trees in urban street canyons

Vegetation in cities provides natural cooling of the climate and is therefore increasingly integrated as an essential part of Urban Heat Island (UHI) mitigation strategies. In the present study, the influence of trees on the local climate in a street canyon is studied using an integrated vegetation model in OpenFOAM. Vegetation is modeled as porous medium providing the necessary source terms for the heat, mass and momentum fluxes. Additionally, a radiation model is developed to model the short-wave and long-wave radiative heat flux exchanges between vegetation and the surroundings. The study investigates the influence of transpirative and shaded cooling due to vegetation on the pedestrian comfort inside a street canyon. The study shows that both shading and transpiration have a direct positive influence on the temperatures measured in the street canyon. Moreover, the cooling due to shading is seen to be larger than the transpirative cooling, especially under the tree

Modelling the Urban Microclimate and its Influence on Building Energy Demands of an Urban Neighbourhood

In the past decades the portion of the population living in urban areas has continuously increased. Due to the high building density, the microclimate in urban areas changed significantly compared to rural areas. The temperatures measured in urban areas are, due to the urban heat island (UHI) effect, higher compared to the rural temperatures. The UHI intensities are increasing with higher building densities and growing cities. Space cooling and heating demands of buildings are strongly affected by the local microclimate at the building sites. Due to the climate change and the limited energy resources, energy saving and sustainability are nowadays important issues. A significant part of the global energy consumption is used for space cooling and space heating of buildings. Thus its minimization for buildings in urban areas has great energy saving potential. Most building energy simulation (BES) models were developed for stand-alone buildings and therefore do not consider effects of the urban microclimate. This can lead to inaccurate predictions of the space cooling and heating demands for buildings in urban areas. The aim of this paper is to investigate the urban microclimate and its potential influence on the energy demand of buildings in an urban context by conducting detailed flow, radiation and building energy simulations at the urban neighborhood scale. CitySim is used for the radiation and building energy simulations. In CitySim detailed radiation models for solar and longwave radiation are implemented that can account for the radiation exchange between neighbouring buildings. The flow around the buildings is modelled by running CFD (computational fluid dynamics) simulations using OpenFOAM. As a result it is shown, how the temperatures and wind speeds can strongly differ within different urban areas. Further an approach is presented, to consider the local microclimate in the building energy simulation tool CitySim

Genome-wide association study of school grades identifies genetic overlap between language ability, psychopathology and creativity

Cognitive functions of individuals with psychiatric disorders differ from that of the general population. Such cognitive differences often manifest early in life as differential school performance and have a strong genetic basis. Here we measured genetic predictors of school performance in 30,982 individuals in English, Danish and mathematics via a genome-wide association study (GWAS) and studied their relationship with risk for six major psychiatric disorders. When decomposing the school performance into math and language-specific performances, we observed phenotypically and genetically a strong negative correlation between math performance and risk for most psychiatric disorders. But language performance correlated positively with risk for certain disorders, especially schizophrenia, which we replicate in an independent sample (n = 4547). We also found that the genetic variants relating to increased risk for schizophrenia and better language performance are overrepresented in individuals involved in creative professions (n = 2953) compared to the general population (n = 164,622). The findings together suggest that language ability, creativity and psychopathology might stem from overlapping genetic roots.Peer reviewe

Genome-wide association study of school grades identifies genetic overlap between language ability, psychopathology and creativity

Cognitive functions of individuals with psychiatric disorders differ from that of the general population. Such cognitive differences often manifest early in life as differential school performance and have a strong genetic basis. Here we measured genetic predictors of school performance in 30,982 individuals in English, Danish and mathematics via a genome-wide association study (GWAS) and studied their relationship with risk for six major psychiatric disorders. When decomposing the school performance into math and language-specific performances, we observed phenotypically and genetically a strong negative correlation between math performance and risk for most psychiatric disorders. But language performance correlated positively with risk for certain disorders, especially schizophrenia, which we replicate in an independent sample (n = 4547). We also found that the genetic variants relating to increased risk for schizophrenia and better language performance are overrepresented in individuals involved in creative professions (n = 2953) compared to the general population (n = 164,622). The findings together suggest that language ability, creativity and psychopathology might stem from overlapping genetic roots

Employing statistical model emulation as a surrogate for CFD

International audienceThis work focuses on substituting a computationally expensive simulator by a cheap emulator to enable studying applications where running the simulator is prohibitively expensive. The procedure consists of two steps. In a first step, the emulator is calibrated to closely mimic the simulator response for a number of pre-defined cases. In a second step the calibrated emulator is used as surrogate for the simulator in the otherwise prohibitively expensive application. An appealing feature of the proposed framework contrary to other approaches is that the uncertainty on the emulator prediction can be determined. While the proposed framework is applicable in virtually all areas of natural sciences, we discuss the approach and evaluate its performance based on a typical example in the realm of computational wind engineering, namely the determination of the wind field in an urban area