A new method for the thermal characterization of transparent and semi-transparent materials using outdoor measurements and dynamic simulation
Introduction
Windows and glazing can offer to building occupants visual relief, insulation against heat and cold, control of light and ventilation. Windows and other fenestrations are of great importance adding esthetic qualities and beauty to the building design. However, in recent decades, fenestrations have been often considered for another type of concern: their influence on the building energy consumption. This aspect may have a direct influence on the design and performance of lighting and air-conditioning systems. For this reason, research on new materials and their properties have grown recently, as for example glazing with the integration of silica aerogel in monolithic form or the use of electro-chromic smart windows.
A problem of these new technologies is that the semi-transparent part is composed by different materials with different physical properties that sometime are not yet defined. To know the behavior of the materials under real operating conditions is primarily important to determine the actual efficiency of the device.
External thermal loads in buildings depend mainly on thermal transmittance (U) of the envelope. In addition to U value, another parameter named Solar Heat Gain Coefficient (SHGC) has to be considered to identify the thermal behavior of glazing systems. SHGC is defined as the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. It is expressed as a number between 0 and 1. Usually, the center-of-glass SHGC is considered, which describes the effect of the glazing alone. Moreover, the manufacturer referenced values are given considering solar irradiance at normal incidence on the glazing.
In this paper a method is proposed to use Solar Test Boxes (STB) to evaluate the overall energy performance of innovative glazing systems suitable for energy saving purposes, consisting of layers of different materials whose thermal characteristics cannot be easily retrieved.
To prove the capability of the method the thermal characteristics of a light diffusing insulating glass (LDIG) were evaluated. The method consists in a short-term outdoor monitoring of STB thermal behavior and in the construction of a calibrated dynamic simulation model for the boxes. The thermal properties, U and SHGC, can be evaluated finding the best match between the experimental data and the calibrated model simulation data. Section 2 gives an overview of the actual methods and models. In Section 3 the STB are described and the short-term monitoring campaign is reported. Section 4 briefly describes the dynamic building simulation program used by the method while in Section 5 the measurement methodology is described. Finally, Section 6 presents the results obtained during the outdoor campaign and describes the data analysis for the implementation of the method.
Section snippets
Models and measurement methods overview
Many modeling attempts have been made to characterize fenestrations with particular characteristics. Indeed many variables such as thermal properties (thermal conductivity, specific heat, thermal diffusivity) of the various materials that constitute the glaze system and their optical characteristics (transmittance, absorbance and reflectance at short wavelengths) are involved in the calculation and it is difficult to build mathematical models to determine the solar heat gain through different
STB description
Two Solar Test Boxes (STBs) were built at the ESTER laboratory of the University of Rome “Tor Vergata” [23] with the objective of making comparative analysis of thermal and lighting performance of innovative transparent material with respect to a double glass reference pane and to investigate the possibility of a complete thermal characterization of an unknown sample. The boxes were designed with a linear scale factor of 1:5 and a surface scale factor of 1:25 with respect to a real room. The
Dynamic building simulation software
The dynamic building simulation (DBS) software IDA Indoor Climate and Energy (IDA ICE) was used to develop the method. IDA ICE is a tool for building simulation of energy performance, indoor air quality and thermal comfort in dynamic conditions. It covers a large range of phenomena, such as the integrated airflow network and thermal models, CO2 and moisture calculation, and vertical temperature gradients.
The DBS software was developed by the Swedish Company EQUA Simulation AB. The model library
Methodology
The proposed method consists in the combination of experimental data collection and modeling and it can be divided in two phases, calibration phase and evaluation phase. The flux diagram of Fig. 4 highlights the various steps of the procedure. Each phase is based on the fine tuning of the STB simulation model built with the DBS tool using the temperature profiles measured inside the reference STB, equipped with the reference double pane, and the test STB, hosting the test sample, during a
Experimental data
Fig. 5 shows the trend of direct and diffuse solar irradiance data collected during the monitoring campaign. Only the first 2 days, as shown in the graph, could be considered of clear sky conditions while for the others, few passing clouds perturbed the solar irradiance trend during the central hours of the day. Despite that all 4 days were used for calibration.
In Fig. 6 the trends of inside air temperature of the reference STB and of the test STB together with the outside air temperature are
Conclusions
A new test method is proposed for the thermal characterization of transparent and semi-transparent materials in outdoor conditions. The procedure implementation is quite simple and it does not require complex hardware. Moreover it allows the evaluation of materials under operating conditions, meaning the determination of their true performance. To be able to have a quantitative evaluation of the overall energy performance it is necessary to model the thermal behavior of STB in unsteady state
Acknowledgements
The authors wish to thank Maria Josè Varazi, Alessandro Aiello and Davide Musella for the contribution to the STB construction and testing. Thanks to Dr. Andreas Schilhan for providing the Okalux sample from Okalux GmbH.
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