A quantitative infrared thermography method for the assessment of windows thermal transmittance FRANCESCO BIANCHI, GIORGIO BALDINELLI, FRANCESCO ASDRUBALI Department of Engineering University of Perugia Via Duranti, 67 06125 Perugia ITALY bianchi.unipg@ciriaf.it Abstract: - The window is one of the weakest elements of buildings envelope and it is characterized by threedimensional heat exchange that makes its study, in terms of overall thermal performance, quite complex. The thermal field on window surface is defined by different parts such as transparent zone (glass) and frame (wood, aluminum or PVC) and their matching generates thermal bridges, downgrading the insulation properties of the entire window. The hot box test is a laboratory setup which allows the evaluation of the window thermal transmittance;the paper shows the comparison between the results obtained by hot box measurements and the thermal transmittance assessed by an original experimental methodology based on quantitative infrared thermography, conducted on a wooden window. Key-Words: window thermal transmittance, hot box test, quantitative infrared thermography. 1 Introduction The thermal properties of windows constitute a key factor as far as buildings insulation. Over recent years, the European Commission, through the Construction Products Regulation [1] and the Energy Performance of Buildings Directive [2], encouraged the development of more energy efficient building materials and components. Some priority sectors have been identified to foster efficiency, such as the product group "Windows". According to the Ecodesign Directive 2009/125/EC [3], windows are in the indicative list of energyrelated products which will be considered in priority for the adoption of implementing measures, as this product group has significant sales and trade in the EU, as well as a significant environmental impact and potential for improvement. As a matter of fact, windows are among the components of the building envelope that are developing faster. Beyond the traditional windows with double or triple glazing, new innovative products are appearing on the market providing excellent overall performance: quadruple (or quintuple) glazing [4], vacuum glazing [5], selective and low-emissivity coatings [6, 7], electrochromic windows [8], aerogels [9] and others. A comprehensive review of the various innovative products and an analysis of the future development can be found in [10]. The assessment of the thermal transmittance of windows is extremely important in the common market; it is mainly performed through numerical simulations [11, 12] and, when possible, using complex experimental setups. The paper presents a new methodology for the evaluation of the insulation characteristics of these components, based on the capture of infrared thermography images. The proposed technique is validated through laboratory measurements: in particular, a calibrated hot box apparatus is used, which gives the possibility to compare the infrared thermography results with the thermal resistance value obtained from a Standard procedure. 2 Hot box measurements The hot box facility is useful to evaluate thermal performance of inhomogeneous elements; it is composed by two chambers, a hot and a cold one, separated by a wall where the sample to be tested is installed (Fig. 1). The functioning principle of this apparatus is the measurement of heat flux provided to the hot chamber, in order to maintain a fixed temperature once that cold chamber is conditioned and steadystate conditions are achieved. Therefore, it is not necessary to install probes on the surface of sample, avoiding the uncertainty of the installation position ISBN: 978-960-474-377-3 137
linked to the lack of homogeneity of the constructing elements. Several measurement on the sample window and its elements were performed in this experimental setup [14, 15], including the campaign aimed to define a methodology forthermal bridge analysis by means of quantitative infrared thermography[16]. The sample analyzed is a window with a wooden frame (dimensions 1.20 m x 1.40 m), with a lowemission double glazing (4-15-4). Figure 3 shows the window in the hot box apparatus with some probes (thermocouples and heat flow meters) installed just for monitoring purposes. Fig. 1 - Scheme of the hot box apparatus: 1) specimen; 2) heating system; 3) refrigeration system; 4) fan; 5) central movable panel; 6) baffle. At the Department of Engineering of Perugia University a calibrated hot box set-up was built (Fig. 2), following the recommendations of the Standard EN ISO 8990 [13], as well as tips gathered from a literature review. Fig. 3 The analyzed window installed in the hot box. The thermal transmittance obtained through the hot box measurement resulted equal to 1.62 W/m 2 K, a value that fits with the thermal performance of similar wooden windows. In table 1 the main acquired and calculated data from the testare reported. Table 1 Data from the hot box test. Hot room temperature 292 K Cold room temperature 273 K Heat flowentering the hot chamber 69.3 W Heat flow through the sample 30.5 W/m 2 Total surface resistance 0.18 m 2 K/W Thermal transmittance 1.62 W/m 2 K Fig. 2 Hot box facility at Department of Engineering of Perugia University. 3 Quantitative infrared thermography methodology 3.1 Image acquisition The hot box facility allows to capture infrared thermography images on the hot side, keeping closed the cold side. The infrared camera used for the analysis is a FLIR B360. Table 2 shows itsmain features [17]. ISBN: 978-960-474-377-3 138
Table 2 Characteristics of the infrared camera used. Field of view 25 x 19 Focal length 18 mm Spatial resolution 1,36 mrad f number 1,3 Thermal sensitivity < 0,06 C a +30 C/60 mk FPA (Focal Plane Array), Type of detector microbolometer without cooling Spectral range 7,5 13 μm IR Resolution 320 x 240 pixel Object temperature range From -20 C to +120 C Precision ±2 C o ±2% of reading The quantitative analysis proposed needs the retrieve of some parameters involved in the infrared thermography measurement. The surface emissivity of the transparent and opaque surfaces have been measured according to the indications of ASTM C1371 [18] that suggests to use an instrument to obtain the hemispherical emissivity. The device consists of a circular head with a diameter of 50 mm, heated by an electric power supplier up to a temperature of 355 K. During the measurement, the sample surface and the measurement head surface remain separated by an air layer of about 4.3 mm, confined by the circular crown of the plastic cylinder that encases the entire head and facilitates its use. The instrument includes a heat flow differential gauge made of two pairs of sensors: a pair with the same surface treatment of the head (black-opaque, high-emissive), the other with a golden low-emissive treatment (Fig. 4). From the knowledge of the radiation heat flow and the surfaces temperatures (the sample temperature must be kept close to the ambient temperature) the emissivity of the sample can be determined. The instrument needs a significant heat dissipation through the analyzed object at the aim of maintaining a certain temperature difference between the sensor and the sample surface; for this reason, it results accurate for highly conductive materials, rather than for thermal insulating ones. Other technique to evaluate the surface emissivity is by mean of infrared thermography comparative method. The temperature registered by the instrument is strictly linked to the surface emissivity; in each thermography camera, the user has the possibility to vary the emissivity of the objects captured, to obtain the real temperature value. On the other hand, if the object temperature is known, for instance by means of a thermocouple contact measurement, its emissivity can be obtained, correcting the value until the camera registers the same value of the thermocouple (Fig. 5). Figure 4 Instruments for the surface emissivity measurement (calibration phase). The differential sensor returns the difference between the voltage signals produced by the two thermopile pairs. Through this measurement it is possible to assess the radiation heat exchange since the heat flux registered by the first pair includes both the conduction and radiation heat transfer, while the second pair of sensors evaluates the only contribution of the conduction; the small thickness of the cavity does not allow the activation of convection. Figure 5 Infrared thermography image for the evaluation of the surface emissivity: the red circle highlights the thermocouple fixed to the aluminum surface and the point detected by the infrared camera. This methodology is affected by numerous error sources that have to be controlled and reduced: the main uncertainty factors are connected to the camera detector, which acquires the heat due to the absolute temperature of the surfaces analyzed, together with the energy reflected by the surface itself coming from the closer objects. This noisy contribution could be diminished raising the surface temperature to a value 10 C higher respect to the other objects, so enhancing the percentage of the energy emitted against the energy reflected. The ISBN: 978-960-474-377-3 139
procedure results therefore suitable only for highemissive samples. The reflected temperature for the window analysis,was evaluated lying a rubbed aluminum foil on the object surface, setting at the same time the detector emissivity to the unitary value (Fig. 6). Because of the aluminum high reflectivity, the rub and the infrared thermography camera detector setting, the thermogram obtained gives the reflected temperature, linked to the radiative heat sources present in the surrounding environment. Fig. 7 Infrared thermography thermal field of the wooden window Fig. 6 Infrared image of the measurement of reflected temperature. Finally,the humidity and the air temperature of the laboratory during the acquisition period were taken in account too. All the previous evaluations are necessary to improve the quantitative thermography accuracy, minimizing the sources of errors and uncertainty [19]. Due to the narrow space existing between the sample and the position of the infrared camera, the window was divided in nine sectors and for each of themten images were acquired with an angle suitable to avoid the narcissus effect [20]. The temperature values of each pixel of the ten images were averaged and composed to create an overall thermal field of the surface of the wooden window (Fig. 7). 3.2 Quantitative infrared analysis: Methodology The following methodology is implemented to check at the same time the value of the window thermal performance andits weakest points in terms of heat insulation, for product optimization purposes. The temperature values related to each pixel constitute the basis of the methodology for the calculation of the thermal transmittance of the overall sample. Even if the hot chamber was opened during the acquisition period, the temperature of the laboratory was kept constant to achieve steady state conditions between the laboratory itself and the cold chamber.therefore, the heat flow and the thermal field could be considered constant, this condition allows to state that the heat flux through the sample is equal to the heat flux exchanged between the air laboratory and the sample hot surface (Fig. 8). For each pixel it is possible to calculate the temperature difference between the sample surface and the laboratory air measured with thermocouples. The heat flow that passes for each pixel is expressed by the following equation: q h A ( T T ) (1) k = c k c k,sc where: q k h c is the heat flux of the k th pixel [W]. is the laboratory convective coefficient [W/m 2 K]. ISBN: 978-960-474-377-3 140
A k T c T k,sc is the assigned surface of the k th pixel [m 2 ] is the laboratory air temperature [K]. is the surface temperature revealed by the pixel of infrared camera [K]. Fig. 8 Steady state condition: heat flow through the sample. The convective coefficient h c is the variable that influences more significantly the results [21, 22]. At the aim of properly defining the convective coefficient, a surface temperature probe and a heat flow meter were installed on the transparent zone on the window.it wasassumed that the convective coefficient remained constant on the overall surface of the window. The evaluation of the entire heat flowtransmitted through the overall window is expressed by the sum of the singular contributions calculated by equation (1): n q = (2) tot q k k =1 where n is the total number of pixels composing the whole image. The evaluation of thermal transmittance U is therefore implemented dividing the total heat transferred (q tot ) by the product of the sample surface (A tot ) and the difference between the laboratory average temperature and the average cold room temperature (T c and T f respectively): 4 Discussion and results (3) Table 3 reports the values obtained by the quantitative thermographic investigation. A particular attention was given to the value of the convective coefficient which was determined, as stated previously, by experimental tests. As a matter of fact, it results close to the standard convective coefficient for indoor environments [23], thus, it is possible to compare directly the transmittance values derived from the two approaches, since the thermal performance acquired from the hot box method is standardized according to the thermal surface resistance. In accordance with equations (1) and (2), the heat flow passing through the entire thermal image was calculated to obtain, according to equation (3), the thermal transmittance value: 1.54 W/m 2 K. The percentage difference of the window thermal performance measured with the two approaches is about 5%, thus confirming the reliability of the proposed method. It is also possible to determine the measurement uncertaintiesfollowing the indications of the Standards [24]. The results show a value of 7.9% for the hot box test and 12.5%or the thermographic technique. Table 3 Results of the infrared thermography analysis on the window. Convection coefficient 7,75 W/m 2 K Heat flux 61,43 W Laboratoryaverage temperature 22,72 C Cold room average temperature -1,05 C Total sample surface 1,68 m 2 Thermal transmittance (infrared thermography) 1,54 W/m 2 K Thermal transmittance (hot box) 1,62 W m 2 K 5 Conclusion Windows are generally the weakest part of buildings envelope and can therefore contribute to reduce buildings global performance in terms of energy consumption and users comfort. Pushed by national and EU recommendations, researchers and enterprises are studying and developing more and more performing products, such as multiple glazing, vacuum glazing and advanced low-e coatings. New products need to be optimized and their thermal transmittance needs to be certified. A common approach is the use of numerical simulations; alternatively complex experimental setups, such as hot box test rigs, are used. ISBN: 978-960-474-377-3 141
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