Urban Overheating Research Articles

Urban Overheating and Cooling Potential in Australia: An Evidence-Based Review

Cities in Australia are experiencing unprecedented levels of urban overheating, which has caused a significant impact on the country’s socioeconomic environment. This article provides a comprehensive review on urban overheating, its impact on health, energy, economy, and the heat mitigation potential of a series of strategies in Australia. Existing studies show that the average urban heat island (UHI) intensity ranges from 1.0 °C to 13.0 °C. The magnitude of urban overheating phenomenon in Australia is determined by a combination of UHI effects and dualistic atmospheric circulation systems (cool sea breeze and hot desert winds). The strong relation between multiple characteristics contribute to dramatic fluctuations and high spatiotemporal variabilities in urban overheating. In addition, urban overheating contributes to serious impacts on human health, energy costs, thermal comfort, labour productivity, and social behaviour. Evidence suggest that cool materials, green roofs, vertical gardens, urban greenery, and water-based technologies can significantly alleviate the UHI effect, cool the ambient air, and create thermally balanced cities. Urban greenery, especially trees, has a high potential for mitigation. Trees and hedges can reduce the average maximum UHI by 1.0 °C. The average maximum mitigation performance values of green roofs and green walls are 0.2 °C and 0.1 °C, respectively. Reflective roofs and pavements can reduce the average maximum UHI by 0.3 °C. In dry areas, water has a high cooling potential. The average maximum cooling potential using only one technology is 0.4 °C. When two or more technologies are used at the same time, the average maximum UHI drop is 1.5 °C. The mitigation strategies identified in this article can help the governments and other stakeholders manage urban heating in the natural and built environment, and save health, energy, and economic costs.

SI: Survivability under Overheating: The Impact of Regional and Global Climate Change on the Vulnerable and Low-Income Population

The present special issue discusses three significant challenges of the built environment, namely regional and global climate change, vulnerability, and survivability under the changing climate. Synergies between local climate change, energy consumption of buildings and energy poverty, and health risks highlight the necessity to develop mitigation strategies to counterbalance overheating impacts. The studies presented here assess the underlying issues related to urban overheating. Further, the impacts of temperature extremes on the low-income population and increased morbidity and mortality have been discussed. The increasing intensity, duration, and frequency of heatwaves due to human-caused climate change is shown to affect underserved populations. Thus, housing policies on resident exposure to intra-urban heat have been assessed. Finally, opportunities to mitigate urban overheating have been proposed and discussed.

On the combination of quantum dots with near-infrared reflective base coats to maximize their urban overheating mitigation potential

Application of highly absorptive construction materials is proved to be one of leading causes of urban overheating in big cities. To avoid the excessive heat by the conventional construction materials, several advanced heat-rejecting coating technologies were developed during the last decades. The main idea behind heat-rejecting coatings is to have colder coatings with the same appearance and colour of conventional coatings. One of the existing technologies for heat-rejecting coatings are advanced coatings with high solar reflection in the infrared range or so-called cool coatings. Recently, re-emission of the visible-range light by nano-scale semiconductors, known as Quantum Dots (QDs), were introduced as another effective heat-rejecting technology. In this paper, we showed that QDs also demonstrate a very high solar transmission in the near-infrared range, and therefore, a highly near-infrared reflective base layer can significantly improve their cooling potential. The high transmission value in the near-infrared range is due to the low absorption coefficient in the wavelengths longer than absorption edge wavelength (i.e. the wavelength corresponding to the bandgap energy) in semiconductors. We show that surface temperature reduction potential of CdSe/ZnS QDs film through fluorescent cooling is about 2.5 °C, which could be increased by another 8.1 °C with a highly near-infrared reflective base layer in a typical sunny day.

Analyzing the local and climatic conditions affecting the urban overheating magnitude during the Heatwaves (HWs) in a coastal city: A case study of the greater Sydney region

Urban overheating coincides with Heatwaves (HWs) and the thermal stress might get amplified in cities. To predict the interactions between urban overheating and HWs, the surface energy balance response to HWs is crucial. HW is a regional phenomenon and the climatic conditions may influence the local conditions to alter the energy budget contrast between a city and its adjacent peripheral areas. The interactions between the urban overheating and HWs are explored in a coastal city (Sydney Australia), also in the proximity of dry landmass, while considering the site characteristics, distance from the coast, and the population density. A positive response between urban overheating and HWs is reported. Advective heat flux in the form of a dualistic circulation system is found responsible for exacerbating the urban overheating magnitude (ΔT) during the HWs and altering the available energy balance. Land-coastal distance is also found as an important contributor in magnifying the urban-suburban temperature contrast. Considering the future urbanization in western Sydney, surfaces capable of retaining higher moisture content are prescribed to reduce the occurrence of extreme HW events. Activation of the ventilation corridor for the coastal wind penetration in western Sydney is another recommendation of this study.

Holistic approach to assess co-benefits of local climate mitigation in a hot humid region of Australia

Overheated outdoor environments adversely impact urban sustainability and livability. Urban areas are particularly affected by heat waves and global climate change, which is a serious threat due to increasing heat stress and thermal risk for residents. The tropical city of Darwin, Australia, for example, is especially susceptible to urban overheating that can kill inhabitants. Here, using a modeling platform supported by detailed measurements of meteorological data, we report the first quantified analysis of the urban microclimate and evaluate the impacts of heat mitigation technologies to decrease the ambient temperature in the city of Darwin. We present a holistic study that quantifies the benefits of city-scale heat mitigation to human health, energy consumption, and peak electricity demand. The best-performing mitigation scenario, which combines cool materials, shading, and greenery, reduces the peak ambient temperature by 2.7 °C and consequently decreases the peak electricity demand and the total annual cooling load by 2% and 7.2%, respectively. Further, the proposed heat mitigation approach can save 9.66 excess deaths per year per 100,000 people within the Darwin urban health district. Our results confirm the technological possibilities for urban heat mitigation, which serves as a strategy for mitigating the severity of cumulative threats to urban sustainability.

Recent development and research priorities on cool and super cool materials to mitigate urban heat island

The urban heat island is increasing the temperature of cities up to 10 °C and has a very important impact on energy, environmental quality and health. Materials used in the building and urban fabric affect the urban thermal balance and contribute highly to urban overheating. The article presents the progress achieved on the design, development and implementation of mitigation materials presenting a low and very low surface temperature. The recent technological progress and developments concerning natural, light colour, IR reflective, PCM doped, thermochromic, fluorescent, photonic and plasmonic materials is presented. Experimental results on the cooling capacity and the thermal performance of conventional and advanced materials are described in a comparative way. It is demonstrated that innovative materials can exhibit sub-ambient surface temperatures and contribute highly to mitigate urban overheating. • New innovative mitigation materials are recently developed. • Innovative advanced materials present a superior performance. • New materials contribute to counterbalance urban overheating. • Super cool materials operate under sub ambient surface temperatures.

Modeling and analysis of heat emissions from buildings to ambient air

Heat emissions from buildings is a significant source of anthropogenic heat influencing the urban microclimate; however, they are usually oversimplified in urban climate and microclimate modeling. This study developed a bottom-up physics-based approach to calculate heat emissions from buildings to the ambient air and implemented the approach in EnergyPlus. A simple result verification was conducted by comparing the EnergyPlus simulated results against the spreadsheet calculations. Simulations covering 16 commercial building types, four climates, and two energy efficiency levels were conducted to understand and evaluate the building heat emissions and their temporal patterns as well as three major components: (1) building envelope (convective heat transfer to ambient air), (2) zones (air exfiltration and exhaust air), and (3) HVAC systems (relief air and heat rejection from condensers or cooling towers). The main findings are: (1)heat emissions are usually higher than the site energy use (about 2.5 times), and their dynamics should be considered; (2)building characteristics and their energy systems lead to differences in heat emission contributions from the three components, and their dynamics, for example, in the warehouse models, the envelope component accounts for 90.4%, while it is 12.7% for the large office models; (3) for most building typologies, the climate has a strong impact on heat emissions, for example, buildings with dominant heat emissions from the zone exhaust air and/or the HVAC reject heat, a general decrease in heat emissions in hotter climates is observed, while envelope-dominated buildings show the opposite; and (4)building technologies that reduce energy use in buildings may perform differently in reducing heat emissions. The developed heat emissions calculation method can be adopted in EnergyPlus and most other building energy modeling programs. It can provide dynamic building heat emissions as an input to urban climate computational fluid dynamics (CFD) models at a higher spatial and temporal resolution than is currently available, to improve the simulation accuracy of the urban microclimate and capture the urban heat island effect and urban overheating.

Quantifying the seasonal cooling capacity of ‘green infrastructure types’ (GITs): An approach to assess and mitigate surface urban heat island in Sydney, Australia

Green infrastructure (GI) can help moderate urban overheating and mitigate surface urban heat islands (SUHIs) by providing protection from solar radiation, cooling surfaces through evapotranspiration, and modifying heat exchanges through advection. However, more research is needed to determine the optimum amounts, compositions and configurations of GI necessary to provide maximum thermal cooling and mitigate urban warming more effectively. In this study, a recently developed ‘green infrastructure typology’ (GIT) scheme is applied in Sydney, Australia to analyse the interplay and cumulative cooling effects of natural and artificial elements in summer and winter by considering their morphological and spatial characteristics. Thermal-vegetation index (TVX), simple linear regression and correlation analyses were performed to identify factors that are more influential on the cooling effects of GI. After, a hierarchical cluster analysis was performed to group types with similar cooling capacity, representing potential mitigation strategies that could be implemented at local scale. Results presented here demonstrate that the cooling effect of different types of GI on surface temperatures mostly depend on the abundance and composition of natural and man-made features. As thermal performance of GI depends on the interplay between both, this study offers a more holistic and integrated approach to assess and counter urban overheating. Key insights derived from the analysis are presented to provide a better understanding on which aspects of GI and urban form should be considered to mitigate SUHIs more effectively in neighbourhoods urban precincts and streets.

Can quantum dots help to mitigate urban overheating? An experimental and modelling study

Application of fluorescent materials has proven to be an effective method for urban overheating mitigation due to their unique ability to re-emit a portion of the absorbed solar irradiation through photoluminescence (PL) effect. Herein, we introduced the idea of using quantum dots (QDs) as tunable fluorescent materials with potentially higher cooling capacity than their bulk counterparts and proposed a novel algorithm to model their thermo-optic behaviour under the sunlight. Our proposed method represents a major step forward in the understanding of the heat-rejection mechanism through PL effect and optimization of QDs fluorescent properties for urban overheating mitigation. Since it’s complicated to distinguish surface temperature reduction caused by reflection from that of PL effect, our developed algorithm could be used as a reliable tool for precise estimation of the PL effect contribution to heat dissipation.

On the Efficiency of Using Transpiration Cooling to Mitigate Urban Heat

Trees are considered to be effective for the mitigation of urban overheating, and the cooling capacity of trees mainly comes from two mechanisms: transpiration and shading. This study explores the transpiration cooling of large trees in urban environments where the sea breeze dominates the climate. In the experiment, sap flow sensors were used to measure the transpiration rate of two large trees located in Sydney over one year. Also, the temperature difference between the inside and outside of the canopy, as well as the vertical temperature distribution below the canopy, were measured during summer. In this experiment, the temperature under the canopies decreased by about 0.5 degrees from a 0.5 m height to a 3.5 m height, and the maximum temperature difference between the inside and outside of the canopy was about 2 degrees. After applying a principal component analysis of multiple variables, we found that when a strong sea breeze is the primary cooling mechanism, the sap flow still makes a considerable contribution to cooling. Further, the sea breeze and the transpiration cooling of trees are complementary. In conclusion, the characteristics of synoptic conditions must be fully considered when planting trees for mitigation purposes. Since the patterns of sea breeze and sap often do not coincide, the transpiration cooling of trees is still effective when the area is dominated by sea breeze.

On the cooling potential of irrigation to mitigate urban heat island

Urban overheating is the most documented phenomenon of climate change, causing severe energy problems. The study aims to evaluate the mitigation potential of irrigation as a response to the urban overheating and heatwaves in large cities, using simulation-based techniques. Mesoscale simulations are conducted for a 32.5 km × 22.5 km metropolitan region of Sydney, Australia, and it is modelled based on the moderate resolution imaging spectroradiometer (MODIS) 500 m resolution land surface dataset. To better represent the Sydney metropolitan area, the New South Wales Land Zoning dataset is used to remap the land surface. The weather research forecast model (WRF) combined with the urban canopy model (UCM) is used as the simulation tool. The results show that a daily irrigation scheme results in a maximum daily temperature drop of approximately 1.3 °C and an average daily air temperature decrease close to 0.5 °C. It is also found that the cooling effect due to the additional irrigation is strongly correlated to the ambient temperature, urban fraction, and the soil moisture state before irrigation. The irrigation induced cooling effect tends to be higher in the urban areas where the soil is drier before irrigation, or in a larger portion of the area that is covered with vegetation, or where the ambient temperature is higher.

The influence of changing location of trees in urban green spaces on insolation mitigation

Urban areas are often overheated, which potentially results in a range of negative effects on the urban microclimate. One way to mitigate urban overheating is to provide shading by tall vegetation (trees). Simulations of insolation with different tree locations were used for insolation mitigation in previous research. However, in urban green spaces potential tree locations can be selected with greater freedom. Therefore, the method for selecting tree locations has to rely on optimization, as there are many possible combinations of locations.The aim of this paper is to devise an approach for insolation mitigation by changing the location of trees in urban green spaces. The approach uses input data that describe the geometry of the built environment and the geometry of trees, as well as climate and weather data (direct radiation, diffuse radiation and cloud cover). The algorithm is applicable to any location and urban environment. The approach uses the Evolutionary algorithm, a method based on natural selection suitable for solving optimization problems. The evolutionary approach is compared to the simulation of average insolation values and the simulations and selection of random tree locations. It is also applied to a real-world example. The results indicate that the proposed approach can be used to mitigate insolation in urban green spaces.

Phosphorescent-based pavements for counteracting urban overheating – A proof of concept

The built environment is particularly sensitive to elevated temperatures across urban areas. Furthermore, engineered materials usually amplify surface overheating, exacerbating urban heat island and cooling energy needs. In the last decades, the scientific community introduced a new class of materials, i.e. cool materials, in the attempt of mitigating these phenomena. In this context, this work investigates the hygrothermal and radiative potential of phosphorescent-based paving solutions and benchmarks their performance against commercially available cool concrete. To this end, an extensive outdoor monitoring campaign was carried out in terms of surface/air temperature, relative humidity, reflected solar radiation and wind speed/direction during summer 2019. Phosphorescent-based fields were found to maintain lower superficial temperature than the reference during the hottest hours of the day, by up to 0.9 °C and 3.3 °C in terms of average and absolute values, respectively. Even though other cooling techniques have been found to have higher cooling effect, the outcomes of this study, reveal for a first time, (i) the promising thermal behavior of phosphorescent-based pavements, as well as (ii) their potential for developing advanced cool materials equipped with both radiation reflection and re-emission mechanisms.

Urban transformation and heat island: Potential of urban design alternatives to mitigate the effects of urban overheating in Austrian cities

Ongoing urbanization worldwide present a big challenge for the quality of urban life. This development poses great challengesfor cities due to the growing demand for more living space and supporting infrastructure, resulting in environmental pollution, higher anthropogenic waste heat and poor outdoor thermal comfort. To accommodate this rapid expansion of urban areas, the city authorities need to adopt a more climate-sensitive approach to urban transformation. In this regard, the present contribution investigates the potential of specific planning and adaptation strategies to attenuate the urban overheating for distinct urban locations in Vienna and Linz, Austria, over a hotsummer period. For this purpose, we applied the parametric modelling environment Rhinoceros 3D and a number of built-in algorithms in the Rhino’s plug-in Grasshopper for dynamic simulation of urban microclimate. The results were compared based on the meanradiant temperature (MRT) averaged over a 24-hour cycle and differentiated into day-and night-time shares. The results reveal a notable potential of selected greening measures to positively influence outdoor thermal conditions. The effectiveness of thesemeasures, however, seem to be time-dependent, whereby a more pronounced cooling effect was noted during the daytime, attributed to the solar shielding effect.

Heat mitigation technologies can improve sustainability in cities. An holistic experimental and numerical impact assessment of urban overheating and related heat mitigation strategies on energy consumption, indoor comfort, vulnerability and heat-related mortality and morbidity in cities

Urban overheating affects the energy, health, power, survivability and environmental quality of cities. We calculated the magnitude of overheating in Sydney, Australia, at close to 9 °C, which causes a cooling penalty of up to 16% and an increase in the indoor overheating levels of up to 56%. We developed and evaluated eight heat mitigation scenarios based on the use of reflective surfaces, additional greenery, an increase in the evapotranspiration rate and several combinations of these factors. We estimated that mitigation can decrease the peak ambient temperature by up to 2.9 °C, reduce the maximum annual cooling consumption by up to 1.5 TWh, decrease the CO2 emissions by as much as 1.21 MT CO2, reduce indoor overheating by up to 80%, decrease the heat-related morbidity by 1.07–1.49, and decrease the heat-related mortality anomaly by as much as 1.39 per 100,000 citizens. We estimated the associated water penalty range to be between 80 and 100 Gl/year or between 13 and 16.7% of the water consumption in Sydney.

Exploring the Synergies between Urban Overheating and Heatwaves (HWs) in Western Sydney

There is no consensus regarding the change of magnitude of urban overheating during HW periods, and possible interactions between the two phenomena are still an open question, despite the increasing frequency and impacts of Heatwaves (HW). The purpose of this study is to explore the interactions between urban overheating and HWs in Sydney, which is under the influence of two synoptic circulation systems. For this purpose, a detailed analysis has been performed for the city of Sydney, while considering an urban (Observatory Hill), in the Central Business District (CBD), and a non-urban station in Western Sydney (Penrith Lakes). Summer 2017 was considered as a study period, and HW and Non-Heatwave (NHW) periods were identified to explore the interactions between urban overheating and HWs. A strong link was observed between urban overheating and HWs, and the difference between the peak average urban overheating magnitude during HWs and NHWs was around 8 °C. Additionally, the daytime urban overheating effect was more pronounced during the HWs when compared to nighttime. The advective flux was found as the most important interaction between urban overheating and HWs, in addition to the sensible and latent heat fluxes.

Experimental and Theoretical analysis of the urban overheating and its mitigation potential in a hot arid city – Alice Springs

Urban overheating has become a growing concern in the Northern Territory of Australia, where residents are suffering from excessive heat and frequent heatwaves. The characteristics of the local climate of Alice Springs and the impact of mitigation strategies are investigated in this study. Experimental campaigns were performed to determine the characteristics of urban overheating. Advanced microclimate simulations were performed to develop mitigation scenarios. Further, ambient temperature measurements have been carried out for 11 months to assess the magnitude and characteristics of the urban heat island. During the night, the city was warmer than the surrounding desert environment with an average close to 4.1 °C, while it presented cooler temperatures by 2.7 °C during morning hours. Microclimate models developed to assess the influence of the proposed mitigation strategies found a possibility to decrease the maximum ambient temperature of Alice Springs up to approximately 1.1 °C using the combination of mitigation strategies.

Predicting the solar evaporative cooling performance of pervious materials based on hygrothermal properties

Pervious pavement has been regarded as an effective technique for both water storm management and overheating prevention. However, compared with its widely recognized permeability capability, its solar evaporative cooling performance remains under debate after post-occupancy evaluation; thus, it is essential to predict the solar evaporative cooling ability before practical use. Hygrothermal properties (i.e., albedo, water absorption, and water retention properties) are key factors that affect the solar evaporative cooling effect of pervious materials and could be extracted as predictive indicators. However, an approach for assessing the water absorption and retention properties has not been well studied. Therefore, this study aimed at characterizing a feasible method for testing water absorption and retention properties of pervious materials as well as the influence of albedo on the solar evaporative cooling performance. Five widely used pervious materials were selected as research subjects, and three hydrological test methods, including the integral immersion test, partial immersion test, and rainfall simulation test, were conducted for comparing the feasibility of measuring water absorption and retention properties. Furthermore, the solar evaporative cooling performance of pervious materials with high and low albedo was also investigated. The results reveal that the partial immersion test has the highest accuracy for evaluating the water absorption and retention properties of pervious materials. Although the high albedo could weaken the evaporation rate, pervious materials with high albedo could also achieve better solar evaporative cooling performance. The results of this study support the proper selection of pervious materials to alleviate urban overheating.

Urban overheating and impact on the built environment

The present article aims to analyse and discuss the quantitative and qualitative impact of ambient and urban overheating in the built environment and in particular : on the peak electricity demand, the cooling and heating energy demand of buildings, the electricity consumption during the hot period, the energy impact on the electricity generation and supply systems and the transmission networks, the direct and indirect effects on outdoor air environmental quality as well as the energy and environmental impact on low income and vulnerable population. The existing literature on the topic, more than 250 published articles, has been analysed, classified and discussed. All results and conclusion are tabulated.

Towards the next generation of green building for urban heat island mitigation: Zero UHI impact building

At present, cities are under the threats of global warming and urban heat island (UHI) phenomenon, where buildings are an important cause and also the sufferer of urban overheating. Therefore, building, especially the green building (GB), is predominantly provisioned to address the increasing energy consumption and carbon emissions in response to climate change, while to our best knowledge, it has been scarcely requested for UHI mitigation (UHIM). Therefore, this paper proposes the concept of GB-based UHIM system, or ‘zero UHI impact building’, or ‘zero-heat building’ or ‘microclimate neutral building’, aiming to achieve the zero-heat impact on surrounding environments through reasonably designing and operating buildings, or depending on innovative techniques to eliminate the excessive heats, on the basis of GB’s goals. In specific, this paper defines the boundary of the GB-based UHIM system, analyzes the interactions between building and urban heat fluxes, and the factors that may affect the use of UHIM techniques on building components. The potentials to establish the GB-based UHIM system are further analyzed in aspects of the UHI impacts on the GB benefits and the principle for heat mitigation. Moreover, the pathway to GB-based UHIM system implementation is further elaborated and suggestions for future direction and work are given. Overall, this paper presents the theoretical and practical foundation for the establishment of GB-based UHIM system and it is a significant move of building sector in response to the temperature increase.