1,721,174 research outputs found
Misura della diffusività termica mediante applicazione di una sollecitazione termica ciclica alternante raffreddamento e riscaldamento
No full textA new test method is presented to quantify the thermal diffusivity, devised for on-site non-destructive measurements of solids accessible from one side only.A periodic thermal disturbance is supplied to the inspected solid by a thermoelectric unit based on the Peltier effect. This can alternate heating and cooling stages and provide, if properly controlled, a null net heat flux. A steady-periodic temperature field can be quickly produced within the solid. The diffusivity of the material is then estimated by monitoring the propagation of the temperature cycles along the optically accessible surface of the solid, adjacent to the thermal input surface area. A portable camera for infrared thermography is used for non-intrusive surface temperature measurement.Tests on thermally thin specimens of materials with well known properties give measures in encouraging agreement with the nominal values.At the current stage of development, the attention is focused onto obtaining the adherence of the test system to the theoretical model on which the method is based. Ease of operation and portability of the testing equipment are also pursued
Measurement of thermal diffusivity by a modification of the Angstroem's method using thermally short specimens
No full textA modification of the Angstroem's method for measurement of thermal diffusivity is presented. The Angstroem's method relies upon the steady-periodic propagation of temperature waves along the specimen. The diffusivity is recovered from combination of phase shift and amplitude decay of the waves, measured by infrared thermography, under the hypothesis of virtually semi-infinite specimen, that is neglecting the reflection of temperature waves at the end of the specimen.In this work, a condition is exploited in which the specimen is not considered semi-infinite. Testing of a large set of materials is made possible, as well as recovering the diffusivity value from either amplitude decay or phase shift, independently. A procedure to estimate the diffusivity is developed from the mathematical model of the 'thermally short' specimen and it is subsequently validated by numerical simulation
Atti del Quinto Congresso Nazionale dell'AIGE -Associazione Italiana Gestione EnergiaUniversità diModena e Reggio Emilia, Facoltà di Ingegneria diModena8-9 giugno 2011
No full textAtti del Quinto Congresso Nazionale dell'AIGE -Associazione Italiana Gestione EnergiaUniversità diModena e Reggio Emilia, Facoltà di Ingegneria diModena8-9 giugno 201
Coupling of solar reflective cool roofing solutions with sub-surface phase change materials (PCM) to avoid condensation and biological growth
Full text linkCool roofs are effective solutions to counter the overheating of building roofs, inhabited spaces below and urban areas in which buildings are located thanks to their capability of reflecting solar radiation. Nonetheless, the relatively low surface temperatures that they induce can cause condensation of humidity and leave the surface wetted for large part of the day, thus promoting the growth of bacteria, algae and other biological fouling; this can cause a quick decay of the solar reflective performance. Biological growth is countered by surface treatments, which however may be toxic and forbidden in many countries and may also vanish quickly. It can also be countered by lowering the thermal emittance and thus decreasing heat transfer by infrared radiation to the sky and the consequent night undercooling, but this can decrease the performance of cool roofs. An alternative approach, which is analyzed in this work, is to embed in the first layer below the cool roof surface a phase change material (PCM) that absorbs heat during the daytime and then releases it in the nighttime. This can increase the minimum surface temperatures, thus reducing the occurrence humidity condensation and, with this, the biological growth. In this work, preliminary results on the coupling of a cool roof surface with a PCM sublayer are presented, being obtained by theoretical investigation on commercial materials and taking into account the time evolution pattern of the environmental conditions
The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues
Full text linkOverheating of buildings and urban areas is a more and more severe issue in view of global warming combined with increasing urbanization. The thermal behavior of urban surfaces in the hot seasons is the result of a complex balance of construction and environmental parameters such as insulation level, thermal mass, shielding, and solar reflective capability on one side, and ambient conditions on the other side. Regulations makers and the construction industry have favored the use of parameters that allow the forecasting of the interaction between different material properties without the need for complex analyses. Among these, the solar reflectance index (SRI) takes into account solar reflectance and thermal emittance to predict the thermal behavior of a surface subjected to solar radiation through a physically rigorous mathematical procedure that considers assigned air and sky temperatures, peak solar irradiance, and wind velocity. The correlation of SRI with the heat released to the urban environment is analyzed in this paper, as well as the sensitivity of its calculation procedure to variation of the input parameters, as possibly induced by the measurement methods used or by the material ageing
Misure di diffusività termica mediante il metodo di Angstroem applicato a campioni termicamente corti
No full textIl metodo di Angstroem e le sue diverse varianti prevedono che la diffusività termica di un materiale venga determinata sulla base delle modalità di propagazione periodica stabilizzata di onde termiche in un campione con elevato allungamento. In generale, si adottano condizioni di lavoro tali da poter considerare il campione virtualmente infinito, cioè tali che le onde termiche vengano smorzate completamente prima di raggiungere l'estremità del campione opposta a quella a cui sono state applicate. In questo modo, la formulazione matematica del problema rimane relativamente semplice e se ne possono ricavare agevoli procedure di stima della diffusività.La condizione di campione termicamente infinito a volte non può essere raggiunta, specialmente se si devono analizzare materiali molto conduttivi e disponibili in campioni di dimensioni ridotte. Inoltre, i fenomeni di riflessione delle onde termiche che si instaurano in un campione termicamente corto possono essere proficuamente sfruttati per filtrare gli effetti dello scambio termico tra superfici del campione ed ambiente di prova. Per questi motivi, si è studiato un modello matematico per la condizione di campione termicamente corto e si sono da questo sviluppate procedure specifiche per la stima della diffusività.Nel presente articolo si fornisce una presentazione dettagliata del modello matematico sviluppato e dei risultati sinora ottenuti
THERMAL BEHAVIOR OF SUNSPACES OBTAINED BY CONVERSION OF EXISTING BALCONIES
No full textLa riduzione del fabbisogno energetico per riscaldamento di edifici esistenti può essere conseguita, tra le altre cose, attraverso la realizzazione di serre solari ottenute per conversione di balconate.
Alle latitudini italiane, l’utilizzo delle serre solari non è mai stato particolarmente diffuso, probabilmente a causa della mancanza di esperienze architettoniche specifiche e della scarsa diffusione di sistemi di controllo ambientale ed energetico, atti a massimizzare i benefici invernali e minimizzare le penalizzazioni estive dei sistemi passivi di captazione degli apporti solari. Tuttavia, con l’attuazione delle norme tecniche di supporto alla direttiva EPBD (Energy Performance Building Directive), le procedure di calcolo della resa energetica e quindi, indirettamente, le caratteristiche progettuali delle serre solari sono state stabilite anche con elevato livello di dettaglio e permettono, attraverso varie metodologie, di calcolare il beneficio energetico atteso.
Negli edifici esistenti e anche in quelli di nuova costruzione è in generale difficile, architettonicamente e costruttivamente, la correzione dei ponti termici dati dai solai passanti di balconi e logge, che possono essere quindi causa di importanti dispersioni termiche. Ecco che un possibile approccio sia al contenimento delle dispersioni termiche invernali, sia alla efficace captazione degli apporti solari, può essere dato dalla realizzazione di serre solari che vadano a chiudere perimetralmente i balconi o le logge, eventualmente associate a sistemi di ventilazione meccanica controllata (VMC) per sfruttare l’energia termica captata ai fini del preriscaldo dell’aria di ventilazione.
Il presente studio mira ad analizzare il comportamento termico ed il beneficio energetico di serre solari realizzate tramite la chiusura di balconi a solaio passante delle tipologie più comuni nell’edilizia italiana, con particolare attenzione alla riqualificazione energetica di edifici esistenti. L’analisi è condotta simulando matematicamente il comportamento termico delle serre solari tramite le procedure specificate nelle norme tecniche attualmente disponibili, tenendo in considerazione parametri quali le condizioni climatiche locali, l’esposizione, le peculiarità architettoniche del fabbricato, le proprietà degli elementi trasparenti e quelle delle superfici opache irradiate
An index for the overall performance of opaque building elements subjected to solar radiation
Full text linkThe thermal behavior of an external opaque building element depends on the combination of several physical characteristics related to insulation level, thermal inertia, external radiative properties. Concerning the insulation level, parameters like the R-value and its inverse, the U-value or thermal transmittance, are commonly considered in building codes, but they are defined with reference to steady-state conditions and cannot describe the behavior of the element when it is subjected to the cycle of solar radiation. On the other hand, parameters like periodic thermal transmittance, decrement factor and time shift represent the dynamic response of the element resulting from its thermal inertia, so they are often considered but do not include the capability of returning solar energy to the atmosphere. In this regard, a few additional parameters are relevant such as solar reflectance and thermal emittance of the external surface, which are unrelated to both insulation and inertia.In order to rate the overall dynamic behavior of an external opaque building element subjected to the cycle of solar radiation and constant indoor temperature, a "solar transmittance index" (STI) is proposed. STI includes in a single performance parameters the effects of both the radiative properties of the external surface and the thermo-physical properties of the materials under the surface. The utilization of such single performance parameter may be greatly helpful in defining requirements and policies to prevent building overheating, reduce cooling energy demand and mitigate the fallouts of the urban heat island effect
The lock-in heating-cooling method for the measurement of the thermal diffusivity of solid materials
No full textA new test method is presented for the on-field nondestructive measurement of the thermal diffusivity of solid materials. A periodic thermal disturbance is supplied to the inspected material by a thermoelectric source based on the Peltier effect. This can alternate heating and cooling stages and provide, it properly controlled, a harmonic disturbance with null net heat flux. A steady-periodic temperature field can thus be induced within the specimen. The diffusivity of the material is then estimated by monitoring the propagation of the temperature cycles along the optically accessible surface of the specimen, adjacent to the thermal input surface area. A camera tor infrared thermography is used for nonintrusive surface temperature measurement. At the current stage of development, the focus is on the accurate reproduction of the theoretical model on which the method is based. Ease of operation and portability of the test equipment are also par,sited. However, tests on thin specimens of materials with known properties give measurements in encouraging agreement with the nominal values
Cooling Heat Islands
No full textAccording to the United Nation report “World Urbanization Prospects”, 54% (about 3.9 billion) of the world’s population residing in urban areas in 2014. The urban population is expected to grow to 66% (about 6.3 billion) by 2050. In addition to encountering many challenges of global climate change, urban inhabitants face many problems associated with urban heat islands (UHI) such as higher outdoor summertime temperatures, higher urban air pollutions, higher demands for air conditioning, and higher heat stress-related mortalities, to name a few. Although the migration to urban areas is caused by macro and global economic drives and little can be done to reverse the trend in the short-term, the policy makers can design effective urban growth policies to mitigate the detrimental effects of UHIs. The Third International Conference on Countermeasures to Urban Heat Island held in Venice, Italy in October 2014 brought together the community of scholars and policy makers to focus on development of effective countermeasures to UHIs.
Over 160 papers discussing advances in both policy and science of UHI countermeasures were presented in the Venice conference. A group of high-quality papers focusing on issues related to the effect of UHI on buildings energy use are published in a special issue of Energy and Buildings. Fifteen other high-quality papers related to urban climate are collected in this special issue of Urban Climate. Papers included in this special issue address:
Quantification and monitoring of urban heat islands and effects on urban climate
Sismanidis; Keramitsoglou, and Kiranoudis discuss a satellite-based system for continuous monitoring of surface UHIs
Kaloustian and Diab present the results of an analysis on the effects of urbanization on the UHI in Beirut
Polydoros and Cartalis assess the impact of urban expansion to the state of thermal environment of peri-urban areas using indices
Rasul, Balzter, and Smith analyze the spatial variation of the daytime surface urban cool island during the dry season in Erbil, Iraqi Kurdistan, from Landsat 8
Analysis of the effects of urban-scale countermeasure technologies
Noro and Lazzarin present the results of simulation analysis of the effect of UHI mitigation strategies in Padua, Italy
Ruiz, Sosa, Correa, and Cantón discuss suitable configurations for forested urban canyons to mitigate the UHI in the city of Mendoza, Argentina
Musy, Malys, Morille, and Inard evaluate the use of SOLENE-microclimate model to assess adaptation strategies at the district scale
Duarte, Shinzato, Gusson, and Alves evaluate the impact of vegetation on urban microclimate to counterbalance built density in a subtropical changing climate
Yoshida, Hisabayashi, Kashihara, Kinoshita, and Hashida evaluate the effect of tree canopy on urban thermal environment
Lobaccaro, and Acero present a comparative analysis of green actions to improve outdoor thermal comfort inside typical urban street canyons
Botham-Myint, Recktenwald, and Sailor discuss the thermal footprint effect of rooftop urban cooling strategies
Simplified methods to quantify the effect of UHI countermeasure
Touchaei and Akbari discuss a method to evaluate the seasonal effect of increasing albedo on urban climate and energy
Pomerantz, Rosado and Levinson outline a simple tool for estimating city-wide annual electrical energy savings from cooler surfaces
Innovative methods to improve urban outdoor thermal comfort
Kleerekoper, van dan Dobbelsteen, van den Ham, Hordijk, and Martin explore a new climate adaptation technique by proposing colored façades to create drafts in urban settings
Castaldo presents results of a thermal-energy analysis of natural “cool” stone aggregates as passive cooling and global warming mitigation technique
Editors of this special issue, Cooling Heat Islands, hope that this collection will further contribute to development of policy plans to counter UHI. The efforts in cooling heat islands is ever expanding; the Fourth International Conference on Countermeasures to Urban Heat Island to be held in Singapore from May 30th to June 1st, 2016 has attracted substantially more attention from both scholars and policy makers with over 300 abstracts submitted as of the date of this editorial (31 October 2015).
Change only happens with good data and analyses, strong leadership, and persistence. Let us work towards cooling our urban heat islands that saves us money in reducing air conditioning energy use, improves ambient air quality and comfort, reduces heat stress related mortality, and cools the globe all with little or no incremental cost. A deal hard to pass
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