Elsevier

Building and Environment

Volume 47, January 2012, Pages 232-242
Building and Environment

Development of a calorimeter for determination of the solar factor of architectural glass and fenestrations

https://doi.org/10.1016/j.buildenv.2011.07.017Get rights and content

Abstract

This work presents the development of a calorimeter used to determine the Solar Factor of glazing and windows, including shading devices or not. Solar Factor is an index used around the world for comparing the thermal performance of fenestrations. The development of the calorimeter includes its project, construction, instrumentation particularities and calibration. The calorimeter has two systems of thermal gain measurement: the first one depends on the temperature difference of the fluid used for refrigeration of the main cavity (employed in tests with elements in full-scale); the second system is applied in a secondary cavity, where heat flow transducers are used to measure the solar gain through fenestrations. During the calibration stage, a new formulation for the determination of the Solar Factor was proposed and applied. After this, a reference glass sample (3 mm clear monolithic glass) was tested simultaneously in the two cavities. All tests were conducted under outdoor conditions. The measurement surface was always maintained in vertical position and facing north. The results of Solar Factor measurements were compared to theoretical values determined by ISO 9050. The uncertainty of measurement (absolute) was on average ±0.04 for the secondary cavity, and ranged between ±0.10 and ±0.16 in the main cavity. In general, experimental values showed good agreement with theoretical values. Therefore, the calorimeter can be used for research purposes or as an alternative to determine the Solar Factor of new products, which are not covered by the calculation procedures presented in the existing standardization.

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.SF(θ,λ)=τ(θ,λ)+Nα(θ,λ)

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

References (32)

  • LBNL, Lawrence Berkeley National Laboratory, WINDOW 6 research version, website:...
  • WIS, Advanced Window Information System, website:

  • J.H. Klems et al.

    A comparison between calculated and measured SHGC for complex fenestration systems

    ASHRAE Transactions

    (1996)
  • M.R. Collins et al.

    Calorimetric analysis of the solar and thermal performance of windows with interior louvered blinds

    ASHRAE Transactions

    (2004)
  • J.S. Harrison et al.

    A test method for the determination of window solar heat gain coefficient

    ASHRAE Transactions

    (1994)
  • J.H. Klems et al.

    Calorimetric measurements of inward-flowing fraction for complex glazing and shading systems

    (February 17–21, 1996)
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