Evaluating Urban Forms for Comparison Studies in the Massing Design Stage

We introduce five performance indicators to facilitate the comparison of urban massingdesign in the early design stages. The five simple indicators are based on existing studies andcover three main performance areas that are sensitive to urban form changes: solar, ventilation,and connectivity potentials. The first three indicators-the non-solar heated façade to floor areaindex, daylight façade to floor area index, and photovoltaics envelope to floor area index-measurethe solar potential. The frontal area index measures the ventilation potential and the route-directnessindex measures the connectivity potential. The indicators are simple to use, as they only requireurban geometry data for their calculation. We demonstrate the indicators in two case studies;variations in the values of these indicators show that they are sensitive to urban form changesand can be used in comparative studies to identify better performing urban forms among massingdesigns. We implement the indicators as an open-source Python library, Pyliburo, that designers andresearchers can readily access and integrate into their existing design workflows. Keywords:performance-based urban design; performance indicators; connectivity; urban ventilation; solar analysi

Pedestrian-Level Urban Wind Flow Enhancement with Wind Catchers

Dense urban areas restrict air movement, causing airflow in urban street canyons to be much lower than the flow above buildings. Boosting near-ground wind speed can enhance thermal comfort in warm climates by increasing skin convective heat transfer. We explored the potential of a wind catcher to direct atmospheric wind into urban street canyons. We arranged scaled-down models of buildings with a wind catcher prototype in a water channel to simulate flow across two-dimensional urban street canyons. Velocity profiles were measured with Acoustic Doppler Velocimeters. Experiments showed that a wind catcher enhances pedestrian-level wind speed in the target canyon by 2.5 times. The flow enhancement is local to the target canyon with little effect in other canyons. With reversed flow direction, a “reversed wind catcher” has no effect in the target canyon but reduces the flow in the immediate downstream canyon. The reversed wind catcher exhibits a similar blockage effect of a tall building amid an array of lower buildings. Next, we validated Computational Fluid Dynamics (CFD) simulations of all cases with experiments and extended the study to reveal impacts on three-dimensional ensembles of buildings. A wind catcher with closed sidewalls enhances maximum pedestrian-level wind speed in three-dimensional canyons by four times. Our results encourage better designs of wind catchers to increase wind speed in targeted areas

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

Thermal stratification (neutral, unstable and stable) plays an important role in determining the transport processes in and above urban street canyons. This paper summarizes the recent findings of the effect of thermal stratification on the transport of momentum, heat, and pollutants in the two-dimensional (2D) urban street canyons in the skimming flow regime. Special attention is paid to the results from large-eddy simulations (LESs), while other experimental and numerical results are referred to when necessary. With increasing Richardson number, Ri, the drag coefficient of the 2D street canyon as felt by the overlying atmosphere decreases in a linear manner. Under neutral and stable stratification, a nearly constant drag coefficient of 0.02 is predicted by the LESs. Under unstable stratification, the turbulent pollutant transport is dominated by organized turbulent motions (ejections and sweeps), while under stable stratification, the unorganized turbulent motion (inward interactions) plays a more important role and the sweeps are inhibited. The unstable stratification condition also enhances the ejections of turbulent pollutant flux, especially at the leeward roof-level corner, where the ejections dominate the turbulent pollutant flux, outweighing the sweeps. With increasing Ri, both the heat (area active scalar source) and pollutant (line passive scalar source) transfer coefficients decrease towards a state where the transfer coefficients become zero at Ri≈0.5. It should be noted that, due to the limit of the 2D street canyon configuration discussed in this paper, great caution should be taken when generalising the conclusions drawn here.Singapore. National Research FoundationSingapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Modelin

Impact of urbanization patterns on the local climate of a tropical city, Singapore: An ensemble study

The effect of urbanization and urbanization pattern on the thermal environment and local rainfall is investigated in the tropical coastal city, Singapore. The Weather Research and Forecasting (WRF) model is employed with 5 one-way nested domains and the highest horizontal resolution is 300 m. The urban effect is taken into account by a single-layer urban canopy model. Several scenarios with idealized urbanization patterns are designed and simulated for an ensemble of 28 members. In the asymmetric urbanization scenarios, in which either the southern or northern part of Singapore is urbanized while the other part is forest, the magnitude of urban heat island (UHI) intensity is higher than that in the symmetric urbanization scenario, in which the urban and forest land use is homogeneously distributed in Singapore. The anthropogenic heat (AH) associated with the urban areas will exacerbate the UHI intensity. Most of the rainfall in the examined cases occurs from late morning to afternoon when the sea breeze blows northeastward. The results suggest that sea breezes have stronger influence on the rainfall than the urbanization pattern since the downwind part always gets more rainfall than the upwind part. The urbanization and associated AH can have two opposite effects on the rainfall amount: increasing rainfall through increasing buoyancy by AH and decreasing rainfall through reducing evaporation by converting greenery to impervious surfaces. The ultimate effect is dependent on the relative strength of these two influences

Development and evaluation of a building energy model integrated in the TEB scheme

The use of air-conditioning systems is expected to increase as a consequence of global-scale and urban-scale climate warming. In order to represent future scenarios of urban climate and building energy consumption, the Town Energy Balance (TEB) scheme must be improved. This paper presents a new building energy model (BEM) that has been integrated in the TEB scheme. BEM-TEB makes it possible to represent the energy effects of buildings and building systems on the urban climate and to estimate the building energy consumption at city scale (~10 km) with a resolution of a neighbourhood (~100 m). The physical and geometric definition of buildings in BEM has been intentionally kept as simple as possible, while maintaining the required features of a comprehensive building energy model. The model considers a single thermal zone, where the thermal inertia of building materials associated with multiple levels is represented by a generic thermal mass. The model accounts for heat gains due to transmitted solar radiation, heat conduction through the enclosure, infiltration, ventilation, and internal heat gains. BEM allows for previously unavailable sophistication in the modelling of air-conditioning systems. It accounts for the dependence of the system capacity and efficiency on indoor and outdoor air temperatures and solves the dehumidification of the air passing through the system. Furthermore, BEM includes specific models for passive systems, such as window shadowing devices and natural ventilation. BEM has satisfactorily passed different evaluation processes, including testing its modelling assumptions, verifying that the chosen equations are solved correctly, and validating the model with field data.French National Research Agency (ANR). MUSCADE project (ANR-09-VILL-003)European Commission Framework Program (FP7/2007–2013) (BRIDGE Project grant 211345

Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation

A validated large-eddy simulation model was employed to study the effect of the aspect ratio and ground heating on the flow and pollutant dispersion in urban street canyons. Three ground-heating intensities (neutral, weak and strong) were imposed in street canyons of aspect ratio 1, 2, and 0.5. The detailed patterns of flow, turbulence, temperature and pollutant transport were analyzed and compared. Significant changes of flow and scalar patterns were caused by ground heating in the street canyon of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed a change in flow regime (from wake interference flow to skimming flow). The street canyon of aspect ratio 1 does not show any significant change in the flow field. Ground heating generated strong mixing of heat and pollutant; the normalized temperature inside street canyons was approximately spatially uniform and somewhat insensitive to the aspect ratio and heating intensity. This study helps elucidate the combined effects of urban geometry and thermal stratification on the urban canyon flow and pollutant dispersion.Singapore National Research Foundation (Singapore-MIT Alliance for Research and Technology (SMART)

Cost-effective Planning of Decarbonized Power-Gas Infrastructure to Meet the Challenges of Heating Electrification

Building heat electrification is central to economy-wide decarbonization efforts and directly affects energy infrastructure planning through increasing electricity demand and reduces the use of gas infrastructure that also serves the power sector. However, the simultaneous effects on both the power and gas systems have yet to be rigorously evaluated. Offering two key contributions, we develop a modeling framework to project end-use demand for electricity and gas in the buildings sector under various electrification pathways and evaluate their impact on co-optimized bulk power-gas infrastructure investments and operations under deep decarbonization scenarios. Applying the framework to study the U.S. New England region in 2050 across 20 weather scenarios, we find high electrification of the residential sector can increase sectoral peak and total electricity demands by up to 62-160% and 47-65% respectively relative to business-as-usual trajectories. Employing demand-side measures like building envelope improvements under high electrification, however, can reduce the magnitude and weather sensitivity of peak load as well as reduce combined power and gas demand by 29-31% relative to the present day. Notably, a combination of high electrification and envelope improvements yields the lowest bulk power-gas system cost outcomes. We also find that inter-annual weather-driven variations in demand result in up to 20% variation in optimal power sector investments, which highlights the importance of capturing weather sensitivity for planning purposes

Next-generation HVAC: Prospects for and limitations of desiccant and membrane-based dehumidification and cooling

Recently, next-generation HVAC technologies have gained attention as potential alternatives to the conventional vapor-compression system (VCS) for dehumidification and cooling. Previous studies have primarily focused on analyzing a specific technology or its application to a particular climate. A comparison of these technologies is necessary to elucidate the reasons and conditions under which one technology might outperform the rest. In this study, we apply a uniform framework based on fundamental thermodynamic principles to assess and compare different HVAC technologies from an energy conversion standpoint. The thermodynamic least work of dehumidification and cooling is formally defined as a thermodynamic benchmark, while VCS performance is chosen as the industry benchmark against which other technologies, namely desiccant-based cooling system (DCS) and membrane-based cooling system (MCS), are compared. The effect of outdoor temperature and humidity on device performance is investigated, and key insights underlying the dehumidification and cooling process are elucidated. In spite of the great potential of DCS and MCS technologies, our results underscore the need for improved system-level design and integration if DCS or MCS are to compete with VCS. Our findings have significant implications for the design and operation of next-generation HVAC technologies and shed light on potential avenues to achieve higher efficiencies in dehumidification and cooling applications

Ultrafine particles in cities

Ultrafine particles (UFPs; diameter less than 100 nm) are ubiquitous in urban air, and an acknowledged risk to human health. Globally, the major source for urban outdoor UFP concentrations is motor traffic. Ongoing trends towards urbanisation and expansion of road traffic are anticipated to further increase population exposure to UFPs. Numerous experimental studies have characterised UFPs in individual cities, but an integrated evaluation of emissions and population exposure is still lacking. Our analysis suggests that the average exposure to outdoor UFPs in Asian cities is about four-times larger than that in European cities but impacts on human health are largely unknown. This article reviews some fundamental drivers of UFP emissions and dispersion, and highlights unresolved challenges, as well as recommendations to ensure sustainable urban development whilst minimising any possible adverse health impacts