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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. [ABSTRACT FROM AUTHOR]

Local and global climate change increases the ambient temperature of cities by several degrees with important consequences on energy consumption, health and the economy. Advanced urban mitigation technologies contribute to decrease the ambient temperature and counterbalance the impact of urban heat islands. The present paper analyses and presents in a comparative way the mitigation potential of the known mitigation technologies using performance data from about 220 real scale urban rehabilitation projects. The average and peak temperature drop of reflective technologies, greenery, evaporative systems, earth to air heat exchangers and their combinations is calculated and presented. The mitigation potential of the main systems like cool roofs, cool pavements, green roofs, urban trees, pools and ponds, sprinklers, fountains, and evaporative towers, is analysed. It is found that the potential of the main mitigation technologies is considerable and can counterbalance UHI effects partly or fully. The average peak temperature drop calculated for all projects is close to 2 K, while the corresponding decrease of the average ambient temperature is close to 0.74 K. Almost 31% of the analysed projects resulted in a peak temperature drop below 1 K, 62% below 2 K, 82% below 3 K and 90% below 4 K. [ABSTRACT FROM AUTHOR]

Abstract: Heat island phenomenon rises the temperature of cities, increases the energy demand for cooling and deteriorates comfort conditions in the urban environment. To counterbalance the impact of the phenomenon, important mitigation techniques have been proposed and developed. Pavements present a very high fraction of the urban areas and contribute highly to the development of heat island in cities. The use of cool pavements presenting substantially lower surface temperature and reduced sensible heat flux to the atmosphere, appears to be one of the most important proposed mitigation solutions. The present paper investigates and describes the actual state of the art on the field of cool pavements. The main thermal and optical parameters defining the thermal performance of pavements are analyzed. Almost all of the developed technologies, where data and results are available, are considered while emphasis is given on the presentation of reflective and permeable/water retentive pavements. The main technological achievements on both fields are reviewed while existing applications are described and performance data are given when available. The existing results clearly show that the mitigation and cooling potential of cool pavements is very significant and can highly contribute to decrease temperature on the urban environment. [Copyright &y& Elsevier]

Abstract: The present paper is a review article aiming to present the actual state of the art on the development and the assessment of cool materials (i.e. materials with high solar reflectance and infrared emittance) for buildings and urban structures. The research in this field is roughly divided in four phases and includes the development and assessment of: (a) highly reflective and emissive light colored materials, (b) cool colored materials, i.e. colored materials with increased near infrared and thus overall solar reflectance compared to similarly colored conventional ones, (c) phase change materials and (d) dynamic cool materials. All major aspects related to each technology are described and the benefits and impacts of these materials at building, city and global scale are reported. The wide use of such materials can significantly contribute to the mitigation of the heat island effect and the improvement of urban environmental quality. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to study the urban micro-climatic conditions in the historic centre of Tirana in order to integrate the information in the rehabilitation of specific open spaces. The process included field measurements of the thermal characteristics in the examined area, bioclimatic design and use of passive cooling techniques (cool materials, green spaces, water, solar control & earth to air heat exchangers). Advanced simulation tools were used to evaluate and optimize the planed microclimatic modifications. CFD calculations with model PHOENICS have been applied for the simulation of the air flow and the temperature field at the concept design stage. The proposed design solutions included the increase of vegetation (planting of trees), the use of shading, as well as the use of cool materials with appropriate thermal and optical properties. In parallel, earth to air heat exchangers, are proposed to supply cool air during the summer period and preheat the ambient air during the winter. Simulations have shown that the use of the proposed bioclimatic measures can decrease the peak summer ambient temperatures up to 3°C, while surface temperatures are decreased seriously. In parallel, the calculation of appropriate bioclimatic indices showed that the thermal comfort conditions were significantly improved with the application of interventions mainly because of the increase of shading as well as because of the use of construction materials with high emissivity and reflectivity values. [Copyright &y& Elsevier]

High-spatial resolution multispectral satellite images collected over the metropolitan Athens area in Greece were used to generate (a) a shortwave albedo map depicting the albedo spatial variations across the metropolitan area, (b) a fractional vegetation cover map showing the spatial distribution of urban vegetation and (c) a daytime and night-time land surface temperature (LST) map depicting the spatial variations of the surface temperature across the city. From LST maps, cooling and heating regions were identified and analysed to reveal relationships between surface heat islands and urban surface characteristics. Based on the data acquired with the use of satellite images and in order to better define the heat island problem and the mitigation measures that need to be taken, the most common building and paving materials used in the urban fabric of Athens were examined. Their optical properties were measured using a UV/VIS/NIR spectrophotometer fitted with an integrating sphere, an emissometer and their thermal performance was evaluated. Furthermore, measurements of the spectral reflectance help explore the possibility of increasing the near-infrared reflectance of materials in order to increase their total solar reflectance. The solar reflectance index of the samples was calculated in order to characterise them as 'cool' or 'warm'. Cool materials, with high albedo and thermal emittance values, attain lower surface temperatures when exposed to solar radiation, reducing the transference of heat to the environmental air. [ABSTRACT FROM AUTHOR]

[Display omitted] • A total of 95 papers systematically reviewed cool material as a mitigation strategy for urban heat island (UHI). • Based on the type of reflection and the spectral range, the analysis found 7 material technologies (UV–Vis-NIR). • Optical properties such as reflectance (ρ), absorptance (α), and emittance (ε) are key factors in cool materials design. • Technological setbacks constitute a key limitation to scale cool material technologies and low-cost alternatives. • DRC and super cool materials represent the most efficient innovations amongst current technologies. The challenging urban expansion represents a pressing need to implement climate mitigation strategies for the urban heat island (UHI) effects and significant threats to the environmental quality, energy consumption, and human health. Thus, this manuscript systematically evaluates 95 relevant studies through a robust evidence-based analysis centered on the latest advances in cool materials, their potential, technological innovation, and global trends. The results showed studies in 21 countries, 81 cities, and 16 climate zones, of which the majority belong to Humid subtropical climates (Csa) including 29 % of studies, Oceanic climates (Cfb) with 15 %, and Mediterranean climates (Cfa) with 13 %. Spectrophotometry analysis is used in 48 % of the studies, while the most studied technologies are the Daytime Radiative Cooling (DRC); 22 %, followed by Thermochromics (TC); 19 %, Cool-Colored (CC); 17 %, Phase Change Materials (PCM); 17 %, Retroreflectives (RR); 11 %, Light-colored (LC); 5 % and Fluorescents (FL); 5 %. Finally, DRC reported the highest cooling power of up to 136 W/m2, however, the latest developments advocate for climate-dependent dynamic modulation techniques of Super Cool Materials (SCM) with Passive Daytime Radiative Cooling (PDRC) attributes and engineered spectral properties to dissipate urban heat to outer space in different meteorological conditions and wavelengths levels of the atmospheric window (8–13 μm). [ABSTRACT FROM AUTHOR]

In recent years a steadily increasing interest has been noticed, concerning the development of advanced materials for both buildings and the urban environment. The aim of the present work is to examine the performance of inorganic coatings as a passive solar technique that contributes to buildings’ energy efficiency. A number of inorganic coatings are developed. The materials used for the coatings’ developed are characterized using XRD and DTA to verify their composition. Their optical properties, i.e. the solar reflectance and infrared emittance are then measured while the samples were also exposed to the outdoor environment for surface temperature measurements. Finally the energy efficiency of the specific coatings are evaluated in an insulated and non-insulated building using ESP-r modelling tool. [ABSTRACT FROM AUTHOR]

Abstract: Commonly used materials in the construction of the building façades located in warm climates, often entrap a great amount of heat from the incoming solar radiation, leading to a respective thermal increase in the interior. The aim of the present study is to examine mineral-based coatings as a passive solar technique that contributes to buildings’ energy efficiency. A series of mineral based samples are developed and tested. Their thermal performance is estimated by infrared thermography and surface temperature measurements. Their optical properties are also measured while the energy efficiency is quantified using simulation techniques. The results revealed that renders based on hydraulic and/or hydrated lime with additions of calcium carbonate powder, as well as mineral paints comprised either of lime wash or silicate minerals can behave as cool coatings and contribute to a significant reduction of the cooling demand in warm climates. [Copyright &y& Elsevier]