Development of a calorimeter for determination of the solar factor of architectural glass and fenestrations
Highlights
► We developed a calorimeter to determine the solar factor of glazing and windows. ► We show calorimeter project, construction, instrumentation and calibration. ► A reference glass sample (3 mm clear monolithic glass) was used for calibration. ► Tests results were compared with theoretical values determined by ISO 9050. ► Experimental values showed good agreement with theoretical values.
Introduction
In hot-climate countries like Brazil, solar heat gain through fenestrations is largely responsible for the increase of thermal load inside buildings. Apart from implicating on users’ comfort, this thermal gain influences power consumption. Considering that houses and buildings are increasingly being built with air conditioning systems, the control of solar heat gain through fenestrations is important to make buildings more efficient, reducing individual energy consumption and demand peaks in the electrical system, especially during the summer.
In Brazil the concern about the impact that fenestrations cause on energy consumption is still incipient. Most of the regulations directed to the sector are related to the constructive aspects of windows and there are practically no national regulations related to their energy efficiency. It was only in 2009 that an official resolution was applied in this field, when the Brazilian government instituted a regulation, initially voluntary, for the certification of energy efficiency levels in commercial and public buildings [1], [2]. With this regulation, the knowledge over properties of windows and their components has become essential for a global evaluation of the energy efficiency of buildings.
One of the window properties mentioned above is the Solar Factor (SF), also known as Solar Heat Gain Coefficient (SHGC). The Solar Factor is one of the most important indexes of energy performance of windows and fenestrations. It represents the fraction of heat gain due to solar radiation that the fenestration directly transmits, added to the portion that is absorbed and re-emitted to the interior of the building by the fenestration itself. Its definition is expressed in Eq. (1), where τ and α are the optical properties (transmittance and absorptance) of each element, and N is the fraction of absorbed heat flow that reaches the interior of the building [3]. These optical properties are, in turn, dependent on the incidence angle (θ) and wavelength. The Solar Factor is given as a dimensionless number between zero and 1, which can be specified only for glass or be indicated for the whole of the window. This information, associated with computer simulation and other analytical processes, enables the elaboration of better and more efficient building designs.
During the last decades, research centers around the world have engaged in efforts to characterize the phenomenon of the passage of solar radiation through windows, especially for situations in which shades are used [4], [5], [6], [7], [8]. The creation of mathematical models for some situations is difficult due to the large number of variables involved, since each type of window and shade has specific characteristics. Nevertheless, some of these calculation mathematical models have been implemented in computer programs [9], [10], in spite of their limitations. Direct measurements continue to be important, therefore, to compare and validate calculated results [7], [11], [12].
Calorimeters are devices commonly used in researches related to the evaluation of the thermal performance of windows [8], [11], [13], [14], [15], [16], [17]. Through these devices, it is possible to perform measurements to determine the amount of heat that crosses the glazing under real use conditions or test-specific situations. In some cases, measurement results are used to assist the certification process of windows [18].
In Brazil, the Federal University of Santa Catarina (UFSC) has continuously dedicated efforts for the development of experimental devices to determine the Solar Factor, aiming to characterize fenestration systems [19], [20], [21]. Marinoski [22], [23] continued this line of research, presenting an improvement to the existing tests. These last studies were used as a basis for the construction of a new measuring device [24].
This paper describes the project and assembly of a calorimeter developed in Brazil for the test of glazing samples and real-size windows. The method for determination of the Solar Factor applied to the equipment and calibration tests performed in field using a reference glass sample are also presented.
Section snippets
Project and manufacturing of components
Due to the complexity of the project of the final device, subprojects were prepared for each of the basic components of the equipment (shelter trailer, heat absorbers, cooling system, electrical and hydraulic system, and monitoring system), which are presented below.
Assembly of the calorimeter
The construction of the calorimeter started from the assembly of each of the components manufactured separately. Some of these components were produced simultaneously to speed up the construction process. Before assembling, all the monitoring sensors were calibrated and tested.
During the assembly phase, the absorption surface of the two cavities was painted with a dye with known properties (absorptivity = 0.95, and emissivity = 0.86) [27]. In the SC the dye was applied on the surface of the
Reference glass
For the calibration of the calorimeter a 3 mm thick colorless glass sample was used as the test element. This material has a well-characterized behavior in relation to the passage of solar radiation, and is commonly used as reference for the comparison to other types of glazing and shading elements [3], [18].
The glass was supplied by the company Cebrace Cristal Plano S/A, and the two plates used (one for the MC and another for the SC) were taken from the same batch of material. For the
Conclusions
This paper presented the project and construction of a solar calorimeter for the determination of the SF in glass and windows (with or without shading) under environmental conditions observed in field. The device employs two different systems for measuring the heat flow that penetrates through the elements tested. The first system determines the heat gain as a function of the temperature difference of the cooling liquid of the main cavity (used for tests with samples of 1500 mm × 1200 mm
Acknowledgments
The authors would like to acknowledge the financial support provided by the Brazilian research agencies: CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Ministério da Educação), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico - Ministério da Ciência e Tecnologia) and the Program of Inovation at R&D FURNAS/ANEEL (Ciclo 2002–2003). We are also grateful to LABSOLAR (Laboratory of Solar Energy), LABTERMO (Laboratory of Thermal Sciences) and POLO - Research
Nomenclature
- Aface_i
- area of each external face of the main cavity (m2);
- AW
- window area exposed to solar radiation (m2);
- ATfaces
- total area of all external faces of the main cavity (m2);
- cwater
- specific heat of water (J/l °C);
- Fluxi
- heat gain measured at each fluxmeter (W/m2);
- hin
- thermal heat transfer coefficient toward inside of glass (W/m2K);
- hout
- thermal heat transfer coefficient toward outside of glass (W/m2K);
- Inc Ang
- incidence angle of solar radiation (°);
- MC or mc
- main cavity;
- mmc
- mass flow rate of water in the main
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