Heat waves (HWs) are recognized as a serious threat for human health worldwide, with urban areas being more vulnerable due to the urban heat island (UHI) effect and population density. Yet, in the ...climate change context, HWs are becoming more frequent, stronger and longer, which, coupled with intensifying urbanization exacerbates thermal risk for urban residents. Despite the profound impact of this global phenomenon there is no clear consensus so far on possible synergies between UHIs and HWs. The study sheds light on the complex synergies between UHIs and HWs focusing on coastal sites. A quite challenging period comprising five HW episodes during summer 2012 in Athens (Greece) was selected for analysis. A positive feedback between UHIs and HWs was found, with intensification of the average UHI magnitude by up to 3.5 °C during HWs, compared to summer background conditions. Our results contribute significantly to understanding synergies between UHIs and HWs that may strongly increase thermal risk in cities and vulnerability of urban population.
► The role of passive cooling in the urban environment is reviewed. ► Earth cooling techniques and applications are analysed. ► Ventilative cooling techniques and applications are reviewed. ► The ...role of evaporative cooling as heat dissipation technique for buildings is discussed. ► The strengths and limitations of the use of passive cooling as heat dissipation technique are reviewed.
Passive cooling in the built environment is now reaching is phase of maturity. Passive cooling is achieved by the use of techniques for solar and heat control, heat amortization and heat dissipation. Modulation of heat gain deals with the thermal storage capacity of the building structure, while heat dissipation techniques deal with the potential for disposal of excess heat of the building to an environmental sink of lower temperature, like the ground, water, and ambient air or sky. The aim of the present paper is to underline and review the recent state of the art technologies for passive cooling dissipation techniques in the built environment and their contribution in the improvement of the indoor environmental quality as well as in the reduction of cooling needs. The paper starts with a short introduction in passive cooling and continues with the analysis of advanced heat dissipation techniques such as ground cooling, evaporative cooling, and night ventilation in the built environment. The various technologies are compared versus their contribution to energy efficiency and users’ comfort. Future trends and prospects are discussed.
Abstract
Urban Heat Island (UHI) phenomenon concerns the development of higher ambient temperatures in urban districts compared to the surrounding rural areas. Several studies investigated the ...influence of individual parameters in the UHI phenomenon, on the other hand, an exhaustive study that quantifies the influence of each parameter in the resulting UHI is missing in the related literature. This paper proposes a new index aimed at quantifying the hazard of the absolute maximum UHI intensity in urban districts during the Summer season by taking all the parameters influencing the phenomenon into account. In addition, for the first time, the influence of each parameter has been quantified. City albedo and the presence of greenery represent the most important characteristics with an influence of 29% and 21%. Population density, width of streets, canyon orientation and building height has a medium influence of 12%, 10%, 9% and 8% respectively. The remaining parameters have an overall influence of 11%. These results are achieved by exploiting three synergistically related techniques: the Analytic Hierarchy Processes to analyse the parameters involved in the UHI phenomenon; a state-of-the-art technique to acquire a large set of data; and an optimization procedure involving a involving a Jackknife resampling approach to calibrate the index by exploiting the effective UHI intensity measured in a total of 41 urban districts and 35 European Cities.
Radiative cooling is a well-researched area. For many years, surfaces relying on radiative cooling failed to exhibit a sub-ambient surface temperature under the sun because of the limited reflectance ...in the solar spectrum and the reduced absorptivity in the atmospheric window. The recent impressive developments in photonic nanoscience permitted to produce photonic structures exhibiting surface temperatures much below the ambient temperature. This paper aims to present and analyze the main recent achievements concerning daytime radiative cooling technologies. While the conventional radiative systems are briefly presented, the emphasis is given on the various photonic radiative structures and mainly the planar thin film radiators, metamaterials, 2 and 3D photonic structures, polymeric photonic technologies, and passive radiators under the form of a paint. The composition of each structure, as well as its experimental or simulated thermal performance, is reported in detail. The main limitations and constraints of the photonic radiative systems, the proposed technological solutions, and the prospects are presented and discussed.
Abstract
Understanding and evaluating the implications of photovoltaic solar panels (PVSPs) deployment on urban settings, as well as the pessimistic effects of densely populated areas on PVSPs ...efficiency, is becoming incredibly valuable. Thus, the deployment of low-efficiency, low-cost, and widely available PVSPs may diminish total solar reflectance, raising the risks of PVSPs-based urban heating, particularly during the summertime heatwaves. This study employs and assesses physical parameterizations that account for the impact of PVSPs on Sydney’s urban environment in the context of the mesoscale model weather research and forecasting (WRF). To account for the impacts of PVSPs, the parameterization presented in this paper assumes that PVSP arrays are parallel, detachable from roofs, and consist of a single layer. Results showed that increasing PVSPs can raise peak summer ambient temperatures by up to 1.4 °C and surface temperatures by up to 2.3°C at city-scale. Temperature variability was found between the city’s eastern and western parts due to the presence of PVSPs. In addition, local warming effects of PVSP were observed at urban district-scale as well. The large-scale deployment of PVSPs at local district-scale of the Sydney during a typical hot day caused air temperature to rise by 1.5 °C during the daytime and decrease by 2.7 °C at nighttime. The patterns of the city’s ambient temperature distribution were found to be strongly dependent on synoptic meteorological conditions and advection flow strength. The maximum increases in sensible heat flux and latent heat flux were 245.5 Wm
−2
and 11.5 Wm
−2
, respectively. Wind speed may be raised by up to 1.2 ms
−2
due to regional low effect over city domain. As a result, large-scale deployment of PVSPs promotes advective flow between the city and its environs. Modification of the PVSPs in Sydney results in an increase in planetary boundary layer (PBL) heights of up to 537.9 m above the city and may lower pollutant concentrations at ground level. The advent of sea breeze in the city’s eastern parts, which reduces the temperature of the coastal zone, along with inland westerly winds, which heat the city’s western zones, lessened the intensity of the urban heat island (UHI) phenomenon induced by PVSPs warming. The findings of this study can be used to help policymakers make informed decisions about the use of PVSPs systems. PVSPs with a high solar reflectance in wavelengths that do not convert solar energy to electricity can be considered as an alternative solution to reduce local warming in urban environments.
•Transpiration consumes ∼50% of solar radiation absorbed by the green facade canopy.•Net photosynthesis had a very low impact on the thermal effect of the green facade.•The green facade decreased the ...indoor operative temperature by up to 3.6 °C.•The green facade decreased the outdoor wet bulb globe temperature by up to 2.7 °C.
Vertical green facades (VGFs) are one of the most promising technologies for reducing building energy consumption and mitigating the urban heat island phenomenon. Many studies have investigated the cooling effects of VGFs; however, research on the thermal behavior of VGFs and the impacts on indoor and outdoor thermal environments are scarce, which limits the understanding and application of VGFs. Therefore, field measurements were conducted in the subtropical city of Guangzhou, China, during hot days. First, the thermal balance of the vegetation canopy was investigated. In particular, the net photosynthesis and transpiration of foliage were measured to estimate the thermal effect of plant physiological activities. Moreover, the operative temperature (OT) and wet bulb globe temperature (WBGT) were measured to assess the comprehensive effects of the VGF on the indoor and outdoor thermal environments, respectively. The results indicated that transpiration could consume approximately 50% of solar radiation absorbed by the vegetation canopy. Furthermore, the thermal effect ratio of net photosynthesis to transpiration was less than 5.5%, suggesting that omission of the thermal effect of the net photosynthesis of climbing plants in thermal balance calculations could result in an error lower than 2.9%; such a low error may be acceptable for most engineering applications of VGFs. The VGF caused a decline in room air temperature and mean radiation temperature, resulting in a peak OT reduction of 3.6 °C. Moreover, the peak WBGT in the outdoor environment could be reduced by up to 2.7 °C due to the shading effect and transpiration cooling of the VGF. These findings help advance our understanding of the thermal transfer process of VGFs and extend the application of VGFs from a single cooling purpose to comprehensive improvement of the thermal environment.
Display omitted
Daytime radiative cooling is regarded as the gold promise of future sustainable building energy systems and a breakthrough in the fight against local climate change. Despite the ...fervid research interest, most literature reports exceptional theoretical performances under ideal, desert-like conditions, but overlooks the cooling impairment that occurs under low atmospheric transparency (cloudy, humid, polluted conditions) and reduced sky access (packed urban contexts). Power recovery and stabilization call for decoupling of incoming and outgoing radiation at equal wavelengths. Enhanced directionality and high-contrast, broadband asymmetric transmission have been recently proposed to expand the applicability of radiative coolers over a wider spectrum of climates, weathers and terrains. This review offers itself as a first, timely synthesis of the current technological arena. Physical principles, materials and designs, collected from a variety of applicative fields, are detailed and discussed in terms of performance and feasibility, to inspire the transition into sustainable building cooling, worldwide. Major grey areas and serious concerns on potential violations of the 2nd law of thermodynamics reinforce the need for experimental demonstrations in future research.
By strongly reflecting solar radiation and being highly emissive within the atmospheric window, daytime radiative coolers can achieve sub-ambient temperature under direct sunlight. Radiative cooling ...performance is strongly coupled to specific climatic conditions since cooling efficiency is strongly affected by ambient air temperature, wind speed, and solar and ambient radiation intensity. In this paper, using a well-validated thermal model, the cooling performance of three radiative cooling materials with varying optical properties was evaluated under three distinct and representative climates. This analysis permits us to better understand the sensitivity of daytime radiative cooling materials to different climatic conditions, present strategies for selecting the ideal spectral properties of materials and investigate how to enhance cooling performance under adverse climatic conditions. It is shown that radiative cooling materials have better performance in hot and arid climates. Most radiative cooling materials exhibit the greatest response to changes in ambient radiation. Higher ambient air temperatures correspond to larger sub-ambient temperature of the surfaces, but this change is lower than that of the corresponding air temperature. Furthermore, by coupling a special optical grating window onto the surface of a radiative cooler, cooling performance can be significantly enhanced by asymmetrically reflecting incoming radiation but permitting outgoing emission. While an ideal material that only emits in the atmospheric window wavelengths presents the best performance under a large range of solar radiation, ambient radiation, and air temperature, the broadband ideal emitter exhibits higher cooling potential when coupled with the optical grating window.
•Radiative cooling materials have better performance in hot and arid climates.•Most radiative cooling materials exhibit the greatest response to changes in ambient radiation.•Higher ambient air temperatures correspond to larger sub-ambient temperature of the surfaces.•An AEMT window is a promising solution to improve the performance of radiative coolers under humid conditions.
The cities of desert climates are anticipated to recognize a synergy of urban heat island (UHI) and severe heat waves during summertime. To improve the urban thermal environment, the present study ...aims quantitatively explore a strategically designed network of vegetation patches called green infrastructure (GI) in subtropical desert cities such as Dubai. To achieve a more comfortable temperature environment, we built and simulated four GI situations with higher GI fractions, GI25, GI50, GI75, and GI100. Using a mesoscale urban model, the mosaic approach is utilized to test potential thermal improvement and urban climate impact, and a portion of each urban grid cell in the model domain is altered with various species of urban vegetation patches by 25%, 50%, 75%, and 100%. The daily peak reduction in ambient temperature at 17:00LT is similar to 0.0168 °C per unit of GI increase when compared to the untreated scenario; however, the maximum anticipated daytime summer temperature decline for GI25, GI50, GI75, and GI100 is 0.6 °C, 1.1 °C, 1.4 °C, and 1.7 °C, respectively. The associated reduction in nighttime ambient temperature per unit increase in the GI is 0.0432 °C, with a maximum temperature drop of around 2.4 °C for the GI100 scenario. Increased GI reduces the height of the planetary boundary layer (PBL) by up to 468 m, which might lead to greater pollution concentrations. While GI-based cooling has a significant influence on delayed sea breeze and humidity, it may raise the risk of heat discomfort in the indoor building environment. This study adds to our understanding of the potential for GI mitigation as well as the seasonal impact of developing GIs on the desert urban boundary layer.
•Next-generation radiative coolers require dynamic emissivity modulation.•Several modulation strategies are revised in this paper from different research fields.•Designs, materials, fabrication ...techniques and modulation performances are compared.•Most effective strategies and underexplored, yet promising, pathways are delineated.
Passive daytime radiative cooling represents one of the boldest answers to tackle the future cooling needs of the built environment and to mitigate urban heat island effects. Recent developments in the field targeted sub-ambience with several successful examples. On the other side, heating demands may get exacerbated unless effective countermeasures against overcooling are identified, especially in wintertime or heating-dominated climates. This review aims at collecting state-of-the-art technologies and techniques to dynamically control the heat transfer to and from the radiative emitter and ultimately modulate its cooling capacity. Potential solutions are selected from different applicative fields, including spacecraft thermal control, thermal camouflage and electronics. Environmentally-responsive solutions are analyzed in depth given their perfect match with radiative cooling design requirements. Among them, VO2-tuned Fabry-Perot resonators are given particular emphasis, owing to their proven applicability. Active solutions are presented for completeness, but in less detail. Underlying principles, structural composition and experimental/simulated results are detailed and discussed to identify prominent pathways towards technically and economically effective integration in the built environment.
PDF
