Improve Outdoor Thermal Comfort Research Articles

Modeling the influences of layouts of residential townhouses and tree-planting patterns on outdoor thermal comfort in Bangkok suburb

In Bangkok, reducing the extreme heat in summer is a critical challenge for urban planners and researchers associated with the built environment. A significant cause of such extreme heat is the rapid expansion of residential development without corresponding planning and design. This work explores layout designs for residential townhouses, building geometries, and tree-planting patterns for enhancing outdoor thermal comfort in a Bangkok suburb. The microclimatic conditions of a total of 192 designs were simulated through modeling based on ENVI-met software. The climatic conditions were then transferred to the ENVI-met BioMET package to calculate the hourly physiological equivalent temperature (PET) values in a street canyon. The information was used to determine acceptable comfort conditions, as compared to an upper limit of outdoor thermal comfort from a prior work. It is found that the aspect ratio and street canyon orientation can significantly improve outdoor thermal comfort performance, as compared to increases in building density or decreases in tree-planting distance. Tree and building shade can reduce the PET value by 8.6 °C and 14.2 °C, respectively. For example, street trees can increase the outdoor thermal comfort conditions of an avenue canyon (H/W = 0.5–0.7) by up to 82%. However, building and tree shade cannot reduce the extreme conditions in E-W oriented canyons, as the overhead shade requires further protection from the angle of the sun's rays. The study's findings could assist in the development of new guidelines for the design and planning of residential townhouse projects for providing a cooler outdoor environment and comfortable conditions, especially in hot-humid areas of Bangkok.

The effect of Sustainable Urban Drainage Systems on outdoor comfort and runoff

Cities are facing numerous challenges such as Urban Heat Island Effect (UHIE) and more frequent flooding events, due to the increasing soil sealing. Greenery and water implemented in urban outdoor spaces have been promoted as effective strategies to counteract UHIE while Sustainable Urban Drainage Systems (SUDS) have been identified as convenient solutions to increase resilience to flooding. However, since many SUDS are green-based, they also benefit urban microclimate and outdoor thermal comfort. In this work, the effectiveness of SUDS as passive strategies to simultaneously improve outdoor thermal comfort and reduce stormwater runoff is assessed. The analysis is carried out on a neighbourhood in Ostia (Rome), selected as representative of Mediterranean climate. The case study area is modelled on ENVI-met in three different scenarios: the current condition is compared with two future developments in which green and water-based mitigation strategies are implemented or in which soil sealing continues based on current trend. Moreover, for each scenario, stormwater runoff has been determined using the Soil Conservation Service curve number method. Results show that the implemented mitigation strategies allow achieving improved thermal comfort conditions and at the same time they can reduce runoff.

Green Envelop Impact on Reducing Air Temperature and Enhancing Outdoor Thermal Comfort in Arid Climates

Today Urban Cities faces energy and environmental challenges due to increased population, higher urbanization. The building sector has a big responsibility as it acquires high consumption rates in global energy and environmental scenarios. It is thought that the built environment in Egypt is responsible for 26% of the total overall national energy consumption, 62% of the total electricity consumption and around 70% of resultant CO2 emissions. The increased use of electrical appliances causes Urban Heat Island effect (UHI), which affect major urban centres. Adding green elements to any urban area is proved to be an effective strategy with numerous benefits to enhance the city's ecosystem, also mitigate the urban heat island measures. In this research Green roofs/walls can regulate outdoor air temperature by 10°C and improve outdoor thermal comfort by 2 Predicted Mean Value (PMV) values. The modelling of green strategy models can take into consideration design developments in areas with hot and dry climatic zones. The properties of green walls can directly affect the results of thermal comfort as leafs absorbs, reflects and transmits solar radiation, and increases the evapotranspiration.

Quantifying the impact of urban trees on passive pollutant dispersion using a coupled large-eddy simulation–Lagrangian stochastic model

The use of shade trees is among favorable urban planning strategies to improve outdoor thermal comfort and promote energy efficiency. Meanwhile, the presence of trees alters flow patterns and turbulent transport and influences the dispersion of air pollutants. In this study, we investigated the effects of urban trees on air pollutant dispersion in urban canopy layers using a coupled large-eddy simulation–Lagrangian stochastic modeling framework. The dispersions of two-way traffic emissions were simulated using a set of twenty four scenarios with varying canyon and tree geometries. Results show that tall trees lead to the strongest modification of the canyon flow and pollutant concentration, except in narrow canyons. Trees can exacerbate canyon pollution level in certain built environment owing to the presence of isolated canyon vortices. Trees with high leaf area density are beneficial to reducing concentration in broad street canyons, while trapping of pollutants is manifest in narrow canyons. The participatory role of trees, in conjunction with the effect of urban morphology, is crucial and needs to be meticulously evaluated in urban planning for promoting environmental quality.

Optimization Design of Underground Space Overburden Thickness in a Residential Area Concerning Outdoor Thermal Environment Evaluation

Reasonable design of the overburden thickness of underground space (OTUS) can influence the outdoor thermal environment by affecting the ground plant communities. To optimize the design of the OTUS for improving the outdoor thermal environment, this study summarized the influence mechanism of the OTUS on the outdoor thermal environment and proposed a framework of the optimization design of underground space overburden thickness. A typical row layout residential area in Nanjing, China, was taken as the research object on which to perform a numerical study of the influence of plant communities formed by two types of plant collocations (a middle- and low-level plant collocation and a middle- and high-level plant collocation) on the outdoor thermal environment (airflow field, air temperature, relative humidity and thermal comfort) under three different ratios of trees to shrubs (2:3, 1:2, and 1:3), and to provide suggestions regarding the design of the OTUS according to the designer’s requirements. The conclusions were summarized as follows: (1) If a designer wants to enhance outdoor ventilation, the OTUS should be designed to satisfy the requirements for the middle- and low-level plant collocations and the overburden thickness of the 2/5 underground space development area should be set to 80~100 cm, the overburden thickness of the other 2/5 area should be set to 45~60 cm and the overburden thickness of the remaining 1/5 area should be set to 30~45 cm. (2) If a designer wants to reduce air temperature, increase relative humidity, and improve outdoor thermal comfort, the OTUS should be designed to satisfy the requirements for middle- and high-level plant collocations and the overburden thickness of the 1/4 underground space development area should be set to 80~100 cm, and the overburden thickness of the remaining 3/4 area should be set to 45~60 cm.

Study on the Prediction and Improvement of Indoor Natural Light and Outdoor Comfort in Apartment Complexes Using Daylight Factor and Physiologically Equivalent Temperature Indices

Urbanization has intensified population concentration, and the quality of residential environments has deteriorated owing to the accelerated construction of high-rise and high-density buildings. In this study, a quantitative analysis is conducted regarding the natural lighting and outdoor thermal comfort of apartment complexes, and satisfactory improvement measures for both factors are investigated. Natural lighting and outside microclimate modeling simulations and statistical analyses are performed on 27 cases using the layout planning factors of an apartment complex. In addition, outdoor thermal comfort analysis is performed by applying heat island mitigation factors (greening and reflective asphalt pavement) to cases satisfying the condition of daylight factor (DF ≥ 75%). To improve natural lighting, the azimuth (correlation coefficient −0.812) should be considered, and to improve outdoor thermal comfort, the aspect ratio should be considered (−0.402). The results of applying heat island mitigation factors suggest that greening can improve outdoor thermal comfort to a greater extent than reflective asphalt pavement. The significance of this study is that the measures to improve residential comfort have been determined by considering the factors affecting the residential environment.

The Effect of Building Facades on Outdoor Microclimate—Reflectance Recovery from Terrestrial Multispectral Images Using a Robust Empirical Line Method

Climate change and the urban heat island effect pose significant health, energy and economic risks. Urban heat mitigation research promotes the use of reflective surfaces to counteract the negative effects of extreme heat. Surface reflectance is a key parameter for understanding, modeling and modifying the urban surface energy balance to cool cities and improve outdoor thermal comfort. The majority of urban surface studies address the impacts of horizontal surface properties at the material and precinct scales. However, there is a gap in research focusing on individual building facades. This paper analyses the results of a novel application of the empirical line method to calibrate a terrestrial low-cost multispectral sensor to recover spectral reflectance from urban vertical surfaces. The high correlation between measured and predicted mean reflectance values per waveband (0.940 (Red) < rs > 0.967 (NIR)) confirmed a near-perfect positive agreement between pairs of samples of ranked scores. The measured and predicted distributions exhibited no statistically significant difference at the 95% confidence level. Accuracy measures indicate absolute errors within previously reported limits and support the utility of a single-target spectral reflectance recovery method for urban heat mitigation studies focusing on individual building facades.

Strategy to Improve Outdoor Thermal Comfort in Open Public Space of a Desert City, Ouargla, Algeria

The control of outdoor thermal comfort became a question of extreme importance in hot desert cities, knowing that people of these cities seek to shelter solar rays by all means especially during the hot season. The aim of this research is to study and analyze the effect of urban vegetation and shade on the microclimate and thermal comfort within an open public space in a hot desert city (the city of Ouargla in Algeria). As an element that minimizes and intercepts the sun’s rays, it generates solar energy shade and absorbs the fluxes of radiation, modifies the temperature, the relative humidity and minimizes the wind velocity and modifies its directions. For this, we have used Ray Man which is simulation software to calculate the indices of thermal comfort. The work consists in calculating these indices before and after the insertion of two types of vegetation within our case study (the martyr’s plaza in the traditional neighborhood of the city of Ouargla). The results obtained show the main role of vegetation by the improvement of thermal comfort.

Influence of aspect ratio and orientation on large courtyard thermal conditions in the historical centre of Camagüey-Cuba

Abstract The combined effect of climate change and Urban Heat Island (UHI) effect is leading to a rise in air temperature in urban areas, including those with heritage value. Urban morphology and its effect on sun shading conditions in tropical cities is crucial to reduce UHI and improve outdoor thermal comfort. This paper presents a temporal-spatial analysis of the effect of courtyards geometry on their outdoor thermal conditions in a warm-humid climate. The assessment is based on numerical simulations of the mean radiant temperature, by using the RayMan model. Large courtyards geometry (convent typology), in the historical centre of Camaguey, were modelled and analysed changing their height-to-width ratio and orientation. Our findings confirm the effect of varying courtyard tridimensional aspect ratios on outdoor thermal conditions. Aspect ratios higher than 1 are advisable, as they contribute to improve the courtyard thermal conditions in summer, by reducing the subzones in the courtyard where the Tmrt is above 45 °C. Orienting the courtyard's long axis away from the East-West results in a lower level of Tmrt, with reductions of up to 15.7 °C, for high aspect ratios. The obtained Tmrt patterns give information about the most suitable subzones within the courtyards, according to the time of day and season. The proposed design and usability recommendations could be included in renovation projects aimed at enhancing courtyards' thermal conditions and contributing to an improvement of the surrounding urban microclimate.

UHI effects and strategies to improve outdoor thermal comfort in dense and old neighbourhoods

Modelling techniques have received growing attention as a tool to investigate the thermal comfort within a city, on the basis of which decision makers can set-up appropriate mitigation strategies. This research aims at studying the effectiveness of strategies for reducing the urban heat island-associated effects in dense and old neighborhoods considering, in particular, green roofs, cool roofs, cool pavements, green areas and urban renewal actions. Computer simulation was selected as the major methodology in this research; ENVI-met software was used under different scenarios for a case study consisting in an old neighborhood in the city of Avola. The investigation focused on evaluating the efficacy of each strategy for a condition corresponding to a typical summer heat wave. The results highlight that the cool pavements allow relevant improvements at the height of 1.50 m, with a temperature decrease up 1.15°C, whereas the other scenarios, given the relatively high density of the buildings, are able to improve outdoor conditions only at higher elevations. Reported results represent a guideline for the choice of UHI mitigation method that can help stakeholders involved in new urban assessment of old neighborhoods in Mediterranean climate.

Impact of urban morphology on microclimate and thermal comfort in northern China

Abstract This work is an experimental study focusing on the impact of urban morphology on the urban heat island (UHI) intensity, microclimate conditions and thermal comfort in a newly-developed urban area in Tianjin city, China. According to the Koppen–Geiger climate classification system, the studied area is classified as hot summer continental climate, characterized by hot and humid summers as well as cold and dry winters. Air temperature, relative humidity (RH) and wind speed at 46 points within an 8-km2 area are measured during both winter and summer seasons. Based on measured results and climatic mapping, the impacts of urban constituents such as building, pavement, greenery and water area on UHI intensity and microclimate conditions are analyzed. Results show that UHI intensity reaches up to 4.5 °C during daytime and 5.3 °C at night in summer, and 2.6 °C during daytime and 5.0 °C at night in winter. Thermal comfort level at the measurement points is predicted using the Thermal Sensation Vote (TSV) model developed for the local climate. The cooling effect of trees is evident during both daytime and nighttime in summer, but negligible in winter due to the reduction of leaf area and evaporation. The presence of both greenery and water body result in an increase in RH in air. Trees tend to reduce wind speed and improve thermal comfort in winter. Radiant heat dissipated from buildings and roads is the main contributor to nighttime UHI in both summer and winter seasons. Based on research results, urban design recommendations are proposed so as to improve outdoor thermal comfort in urban areas located in temperate climate zone during summer and winter.

Wet Curtain Wall: A Novel Passive Radiant System for Hot and Dry Climates

This paper proposes a novel radiative passive system for hot and dry climates to improve outdoor thermal comfort. The solution consists in a curtain, characterized by two different finishing surfaces, one is waterproof and the other one is absorbent, immersed in a water tank by means of a mechanized system and subsequently used as a radiant surface with reduced temperature thanks to evaporative cooling effect. The effectiveness of the proposed system is verified by means of CFD simulations carried out on a sample application in outdoor space and supported by experimental measurements carried out on a small sample of curtain. The results showed that the proposed system can reduce the operative temperature up to 6°C with respect to the testing conditions (air temperature of 28.5°C and relative humidity of 50.5%) ensuring higher comfort levels and a limited water consumption to keep the curtain constantly wet (25 l/h for 130m2 of wet curtain).

Influence of changing trees locations on thermal comfort on street parking lot and footways

Locations of trees in street parking lots (SPL) impact outdoor thermal comfort and should be considered during the urban planning process. In this paper we developed a procedure for changing trees locations in order to improve outdoor thermal comfort on SPL and associated footways. Furthermore, a sensitivity test on the effect of different tree crown shapes on outdoor thermal comfort was carried out. We applied theprocedure on real-world SPL design in the City of Novi Sad (Serbia). A temporal analysis is performed for the heat wave period using Universal Thermal Climate Index (UTCI) calculations in the Ladybug software.The results showed improvement of outdoor thermal comfort on 77 % of all body locations in proposed SPL design with predetermined number of trees. The largest outdoor thermal comfort improvement was noticed in the afternoon hours with up to 3.3°C UTCI decrease on single body location. By adding trees to the SPL, heat stress was reduced on 84 % of all body locations with maximal UTCI decrease of 3.7°C on single body location. Furthermore, heat stress reduction by cylinder-shaped tree crowns showed to be more pronounced compared to the sphere-shaped and the cone-shaped tree crowns. Proposed procedure showed that the locations of trees as well as tree crown shapes are very important for the improvement of outdoor thermal comfort and creation of environmentally conscious SPL design.

COMPARISON ON COLORED COATING FOR ASPHALT AND CONCRETE PAVEMENT BASED ON THERMAL PERFORMANCE AND COOLING EFFECT

In tropical climate, pavement surface temperature could reach up higher than the air temperature due to solar-energy absorption by pavement during the daytime. Colored ground surface such as in pedestrian walkway, parking lot, and bicycle track may be able to reduce the heat absorbed by the pavements. This paper reports the thermal performance of five different colors of coating applied at asphalt and concrete pavement surface. In order to investigate the thermal performance and solar reflectance of the colored coating pavement, infrared thermometer and solar power meter as well as thermal imager procedure were used. From the statistical analysis, it was found that all the colored coating on asphalt and concrete sample demonstrate lower surface temperature compared to conventional or uncoated asphalt and concrete. The highest surface temperature reduction and solar reflectance recorded was for the white coated asphalt sample was 17°C and 0.61, while for the white coated concrete sample was 10°C and 0.78. ENVI-met simulation is used for evaluating the thermal impact of applying the samples in site study. This study can assist in choosing more appropriate colored coatings for ground surface of the urban environment (pedestrian walkway, parking lot, plaza, etc.), and thus contribute to the reduction of the air temperature due to the heat-transfer phenomena as well as improve outdoor thermal comfort and cityscape appearance.

Comparative analysis of green actions to improve outdoor thermal comfort inside typical urban street canyons

Urban microclimate analyses are being used more and more to address the planning decision process to create livable and healthy public spaces. The study, conducted in collaboration with the municipality of Bilbao (Basque Country, Spain), presents a comparative analysis of green actions to improve outdoor thermal comfort conditions. The evaluation was performed in three typical urban street canyons characterized by different geometric proportions and five urban greenery scenarios in typical summer day conditions. For each scenario, the mean radiant temperature, relative humidity, air temperature, surface temperature and wind speed have been analyzed using ENVI-met model. The study quantitatively confirms that the vegetation elements such as grass, green roofs and trees, improves the thermal comfort at pedestrian level. Thermal comfort is assessed using the PET (Physiological Equivalent Temperature) thermal index. The highest PET reduction occurs by combining the presence of trees and grass, which can lead to a reduction of about two PET thermophysiological assessment classes during the daily maximum values. Additionally, the work demonstrates how aspect ratio and ground surface materials can affect the intensity and the duration of discomfort period (PET>23°C). The outcomes will be used by the municipality of Bilbao to improve the actual planning recommendations.

Numerical modelling of mean radiant temperature in high-density sub-tropical urban environment

Outdoor thermal comfort has been a widely concerned issue in tropical and subtropical cities. In order to assess the conditions of outdoor thermal comfort, quantitative information on different spatial and temporal scales is required. This paper employs a numerical model (SOLWEIG – SOlar and LongWave Environmental Irradiance Geometry) to examine the spatial and temporal variations of mean radiant temperature (Tmrt), as an indicator of radiant heat load and outdoor heat stress in high-density sub-tropical urban environment in summer. The SOLWEIG model is found to simulate the six-directional shortwave and longwave radiation fluxes as well as Tmrt very well. Simulation results show that urban geometry plays an important role in intra-urban differences in summer daytime Tmrt. Open areas are generally warmer than surrounding narrow street canyons. Street canyons are sheltered from incoming direct solar radiation by shading of buildings while open areas are exposed to intense solar radiation, especially along the sunlit walls where high Tmrt is observed due to reflected short-wave radiation and long-wave radiation emitted from the sunlit building walls. The present study confirms that there are great potential in using urban geometry to mitigate high radiant heat load and daytime heat stress in the compacted urban environment. In high-density subtropical cities where high daytime Tmrt causes severe thermal discomfort in summer, dense urban structures are able to mitigate the extremely high Tmrt and improve outdoor thermal comfort. However, the shading strategy has to be cautious about air ventilation in such a dense urban environment.

On the development, optical properties and thermal performance of cool colored coatings for the urban environment

This paper reports the measured solar spectral properties and the thermal performance of 10 prototype cool colored coatings, developed at the National and Kapodistrian University of Athens, using near-infrared reflective color pigments in comparison to color-matched, conventionally pigmented coatings. These coatings are developed to be used in the urban environment to fight the heat island effect. The spectral reflectance and the infrared emittance were measured and the solar reflectance of the samples was calculated. The surface temperature of the coatings when applied to concrete tiles was monitored, using surface temperature sensors and a data logging system, on 24 h basis from August to December 2005 in an effort to investigate the ability of the cool colored coatings to maintain lower surface temperatures than conventionally pigmented color-matched coatings. The data obtained has been extensively analysed and indicate significant success in the development of these cool colored coatings. It was found that all the coatings containing infrared reflective pigments have solar reflectance values higher than those of standard coatings. Furthermore, it was demonstrated that cool colored coatings maintain lower surface temperatures than color-matched conventionally pigmented coatings. This temperature difference is mainly due to differences in solar reflectance. These cool colored coatings can be used on buildings (roofs and walls) and other surfaces in the urban environment. Thus, at building scale, the use of cool colored coatings with increased solar reflectance can improve building comfort and reduce cooling energy use, and at city city-scale it can contribute to the reduction of the air temperature due to the heat-transfer phenomena and therefore improve outdoor thermal comfort and reduce the heat-island effect.

Passive cooling of outdoor urban spaces. The role of materials

This paper presents the results of a comparative study aiming to investigate the suitability of materials used in outdoor urban spaces in order to contribute to lower ambient temperatures and fight heat island effect. The study involved in total 93 commonly used pavement materials outdoors and was performed during the whole summer period of 2001. The thermal performance of the materials was measured in detail using mainly infrared thermography procedures. The collected data have been extensively analysed using statistical techniques. Comparative studies have been performed in order to identify the major advantages and disadvantages of the materials studied. Materials have been classified according to their thermal performance and physical properties into `cool' and `warm' materials. The impact of color, surface roughness and sizing has been analysed as well. The study can contribute to selection of more appropriate materials for outdoor urban applications, and thus assist to fight the heat island effect, decrease the electricity consumption of buildings and improve outdoor thermal comfort conditions.