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Absorption
  • Absorbance
    Quotient of the rate of solar radiation absorbed by a surface by the rate of solar radiation incident on this same surface. Symbol: a. a) Absorbance : Absorbance is defined as the ratio of absorbed radiant energy to incident radiant energy. Q=α (dimensionless) incident absorbed Q b) Reflectance Reflectance is defined as the ratio of reflected radiant energy to incident radiant energy. reflected incident Q Q=ρ (dimensionless) c) Transmittance Transmittance is defined as the ratio of transmitted radiant energy to incident radiant energy. incident transmitted Q Q=τ (dimensionless) According to the criteria of the energy regulation of Brazilian buildings, the solar absorbance of surfaces is a parameter used to classify the energy performance level of the envelope. In order to obtain the maximum level of efficiency, inclusive, the regulation limits the weighted average absorbance to 0, 4 for facades and apparent roofs of buildings located in Bioclimatic Zones 2 to 8. On the other hand, nationally there are few references of precise data for quantifying the solar absorption of building materials. Based on a bibliographic review, the absorptance parameter is analyzed in the context of the regulation through a case study. A building's surface absorbance values are varied, generating the respective final envelope efficiency level ratings. In Brazil, solar radiation is responsible for an important portion of the thermal load of buildings and its impact on this load depends, among other factors, on a property called solar absorbance (α) and defined by NBR 15220-1 as the “ quotient of the rate of solar radiation absorbed by a surface by the rate of solar radiation incident on this same surface” (ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS, 2005a). The absorbances of the building envelope are associated with the impact of solar radiation on the building, as they indicate which fraction of the solar energy that reaches the building is actually absorbed by its envelope, affecting its heat gain and internal temperatures, and which fraction is reflected, with no effect on the thermal conditions of the building. In addition, energy consumption with refrigeration systems can be significantly reduced by limiting the solar heat gain through its envelope, which depends on the intensity of incident solar radiation and the absorbance of the external surface (DORNELLES, 2008). Buildings that have highly reflective external surfaces allow to decrease the solar heat gain through them, while maintaining the capacity of the building to lose heat quickly after sunset (MULHALL; AYNSLEY, 1999). In addition, the use of roofs with low solar absorptance can considerably decrease the air temperature inside buildings and, on a mesoclimatic scale, decrease the temperatures of the urban environment, which, in turn, reduce the potential for the occurrence of solar radiation. urban heat island phenomenon. On the other hand, the low solar absorbance of the constructive envelope allows to reduce the annual use of energy in climates where the use of refrigeration predominates. Different from what is disclosed by many researchers, the solar absorbance of opaque surfaces is not influenced solely by the color they present. Colors are visual sensations and can vary between different observers and are not reliable indicators of the physical properties of a surface. The various physical characteristics that the surface presents form a set of factors that will directly influence its absorbance, among them the chemical composition of the material that composes it, characterized by its absorptivity, the roughness of the material (defined by its geometric micro-irregularities), surface undulation and its maintenance over time, due to the effects of degradation (RORIZ; DORNELLES; RORIZ, 2007). Energy regulation of Brazilian buildings (RTQ-C) and absorbance In Brazil, buildings are responsible for the consumption of almost 45% of available energy (BRASIL, 2008). At the end of 2009, the RTQ-C, Technical Regulation for the Quality of the Energy Efficiency Level of Commercial, Service and Public Buildings, came into effect. This is the first Brazilian legal instrument designed to regulate energy consumption in buildings (BRASIL, 2009a). RTQ-C is provisionally voluntary and the date for it to become compulsory has yet to be defined. It affects a considerable number of buildings, as it applies to commercial, public and service buildings, with a total usable area greater than 500m2 and/or served by high voltage, whether already built or in the design phase. Awards a label that indicates the level of energy performance of the various systems in the building, from A (most efficient) to E (least efficient). The facade and roof systems that make up the building envelope can be analyzed, in addition to the lighting and artificial air conditioning systems in specific areas or the entire building. Other systems and devices that demonstrate energy and/or water savings in the building may also be scored to improve the rating level. In a complete assessment, the envelope accounts for 30% of the overall building rating. One of the parameters used for its evaluation is the solar radiation absorbance of the surfaces that compose it. In almost all of the national territory, the total absorbance of the envelope is limited to a certain value so that the envelope of the building can reach the maximum classification levels _ respectively “A” or “B”. Difficulty in obtaining accurate data of absorbance It is understood that the exact quantification of the absorbance data of each material that usually makes up the envelope of buildings is not always possible or feasible, which can make the application of RTQ-C difficult. Preferably, the absorbance should be measured in the laboratory, but the number of those who are prepared to carry out this type of analysis is probably insufficient to meet the demand generated by the RTQ-C. Some test methods require the use of large samples, which can require complex and economically expensive logistics for parts to be transported and data to be obtained. In cases where there is a need for on-site measurement, the cost of the procedures may make the measurement unfeasible. In general, manufacturers of coating materials do not present absorbance data within their technical specifications. Currently, the specialized technical literature has only a few average values of solar absorbance and reflectance, most of the time quite generic, inaccurate and outdated. Dornelles (2008), in his doctoral thesis, discloses precise solar absorbance data for 72 different facade paints commercially available in Brazil, based on spectrophotometer measurements, which is considered the most accurate method for quantifying this physical property of the surfaces. However, because it is a high-cost equipment and not very accessible to researchers and designers, it is difficult to obtain precise values of solar absorbance of the elements that make up the envelope of buildings. Furthermore, there is little reference data in the only Brazilian standard that addresses this parameter, NBR 15220-2 (ABNT, 2005b). Therefore, there is no absorptance database that includes the variety of Brazilian materials and building systems. Consequently, due to the unavailability of these data, approximations in the absorbance values can be made frequently during the use of the evaluation methodology proposed in the RTQ-C. This context hinders the application of the methodology and may compromise the correct evaluation of the envelope performance.
  • Closure
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  • Right foot
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  • Shaft
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