1、 Author Huang is a Ph.D. candidate in the Department of Mechanical and Mechatronics Engineering at the University of Waterloo, Waterloo, Ontario Canada. Authors Collins and Wright are professors in the Department of Mechanical and Mechatronics Engineering at the University of Waterloo, Waterloo, Ont
2、ario Canada. Off-Normal Solar-Optical Performance of Pleated Drapery: Simulation versus Measurement Ned Huang Mike Collins, PhD John Wright, PhD Student Member ASHRAE Member ASHRAE Member ASHRAE ABSTRACT In recent years, significant advances have been made in modeling fenestration with shading attac
3、hments. Most shading devices have great potential for reducing both peak building cooling load and annual energy consumption through the control of solar gains, and the ability to quantify their impact is important. As part of an ASHRAE sponsored research project, several new models were developed f
4、or various types of shading devices. One of the most complex of these was the pleated drapery model. This model uses off-normal solar-optical fabric properties to predict the off-normal solar-optical properties of the pleated drapery. In doing so, the model assumes that the system could be represent
5、ed as a series of uniformly arranged rectangular pleats. The work presented here aims to validate model performance. A Broad-Area Illumination Integrating Sphere (BAI-IS) was used to perform solar transmittance measurements on pleated drape samples. The BAI-IS is capable of measuring optical propert
6、ies of thick and non-uniform samples. Five pleated drape samples composed of fabrics with different transmittance and reflectance were used in measurements. Results were compared to the model output for different incidence angles. Predicted transmittances were generally within 0.05 of measured value
7、s although there could be an overprediction as much as +0.11 for normal incidence test cases of high transmittance test samples. This discrepancy can be attributed to the geometric difference between the model and the test samples. INTRODUCTION Background Sustainability has become an important pursu
8、it. New buildings are being designed to have good insulation, allowing little heat transfer. Solar radiation is a natural and renewable source of light and heat for buildings. Window areas that are subject to high solar heat gain may cause overheating of a well-insulated building. As well, solar hea
9、t gain is usually the largest and most variable heat gain that affects cooling loads of a building. This is especially true given the current architectural trend toward highly glazed facades in commercial buildings. Indoor space conditioning of a building would be much simpler if window areas could
10、be replaced by walls. Yet windows create aesthetically pleasing spaces in any building design. The key is to find an acceptable and optimized balance among competing factors of building design (e.g., comfort, daylightlighting, energy conservation, indoor environmental quality, view, and etc.). One o
11、ption is to use a complex fenestration system (CFS), i.e., a window system that incorporates one or more shading elements. CFS has become essential in meeting multiple objectives of building design, including high building energy-efficiency and lower peak energy demand. CFS is conventional, economic
12、al and is commonly used to regulate sunlight and solar heat gain in high performance buildings. As energy efficiency requirements are increasingly demanding and indoor environmental quality remains a high priority, the ability to accurately predict window energy performance and quantify the impact o
13、f shading devices becomes more important than ever before. Ongoing Research and the Pleated Drape Model A generalized mutil-layer framework (Wright 2008) has been developed to predict center-glass energy performance indices of glazing systems with shading devices. The impact of a fenestration system
14、 on energy consumption can be calculated if the solar optical and thermal properties of individual layers in a CFS are known. Individual layer models for determining the solar optical and thermal properties of each layer in the window system have been developed through ASHRAE sponsored research proj
15、ects (Barnaby et al. 2009). Effort has also been made to implement shading layer models into building simulation software (e.g., Wright et al. 2011). Of particular interest is the pleated drape layer model developed by Farber et al. in 1963 and refined by Kotey et al. in 2009. Figure 1 Configuration
16、 of pleated drapery model showing solar angles (Kotey et al. 2009). When beam radiation is incident on a drapery, a fraction of it can be transmitted unobstructed through fabric openings (beam-beam transmission) with the rest being scattered forward (beam-diffuse transmission) and backward (beam-dif
17、fuse reflection) through multiple reflections whin the drapery layer. The pleated drape model (Koteys et al., 2009), which is geometrically represented as a series of uniformly arranged rectangular pleats (Figure 1), calculates the effective solar properties of pleated drape layer based on these bea
18、m and diffuse radiation components. The model uses angle-dependent properties of the flat fabric in conjunction with drapery geometry and solar angles to calculate the effective solar properties for both incident beam and diffuse radiation. Therefore, the off-normal solar-optical properties of a ple
19、ated drapery layer can be determined based on the off-normal solar-optical properties of flat fabric, folding ratio, and incident angle, . Koteys et al. (2009) provides a detailed formulation of this model. The present work aims to validate the pleated drape layer model. A Broad-Area Illumination In
20、tegrating Sphere (BAI-IS) was used to perform measurements on pleated drape samples. In this study, total solar transmittance, t, was measured. The BAI-IS is capable of measuring optical properties of thick and non-uniform samples. Pleated drape samples composed of fabrics with different transmittan
21、ce, t,f, and reflectance, t,f, are used in measurements. Finally, results are discussed and compared to the model output for different . EXPERIMENT TRANSMITTANCE OF A DRAPE LAYER Flat Fabric Measurements Using Commercial UV-Vis-NIR Spectrophotometer The pleated drape model relies on the solar optica
22、l properties of flat fabric as input. A commercially produced spectrophotometer, which is designed for photometric measurements in the 250-2500 nm range, was used to measure the required properties. The spectrophotometer is equipped with a 110 mm diameter integrating sphere. An integrating sphere is
23、 a hollow sphere with its inner surface coated with a layer of high reflectance material. An integrating sphere collects and integrates, spatially and directionally, all incoming radiation. Its inner surface is assumed to be Lambertian. An integrating sphere usually has at least one inlet port to ad
24、mit light and an exit port where detectors are located. One particular technical guide (Labsphere 2013) provides a good discussion on integrating sphere theory and applications. Construction of Drape Samples A drape sample frame has been built to support fabrics and for making pleats. The frame is d
25、esigned to hold strings (fishing line) vertically that enable a piece of soft fabric to fold and wrap around these strings in order to form rectangular pleats. The arrangement of strings will determine the pleat size and folding ratio (Fr) of a sample, which is defined as the ratio of fabric width t
26、o the width of window area to be covered. So to cover the whole window area, a minimum of Fr = 1.0 is required (i.e., flat fabric). Figure 2 illustrates various folding ratios. For rectangular pleats, Fr = 1 + w/s where w is pleat depth and s is pleat spacing. Most common folding ratios for drapes r
27、ange from 2.0 to 3.0. For this study Fr = 2.0 (s = 2 cm and w = 2 cm) was used. Figure 2 Illustration of folding ratios (Fr) in terms of rectangular pleats. Note that pleats do not naturally stay in a rectangular shape. Therefore, it is almost impossible to make the folds perfectly square. Although
28、efforts have been made to tighten the strings and make the pleats as close to square as possible, some smooth irregularity can be observed in the test sample folds. Pleated Drape Layer Measurements Using the BAI-IS System While the commercial spectrophotometer is easy to use and has excellent capabi
29、lities, it cannot measure the solar optical properties of thick and/or spatially non-uniform samples. The commercial spectrophotometer has a small integrating sphere, and therefore a small inlet port. The small inlet port cannot capture all the scattering light. This is known as out-scattering loss.
30、 Also, the narrow beam of incident light source cannot irradiate a representative (broad) sample area. The Broad-Area Illumination Integrating Sphere (BAI-IS) system is a custom-built spectrophotometer specifically designed to overcome the limitations of the commercial spectrophotometer. First, it h
31、as a larger integrating sphere with an inlet port area that is big enough to cover a representative area of a non-uniform sample. Second, the radiant source illuminates a large sample area, allowing in-scattering gain to offset out-scattering loss. The BAI-IS system consists of the following compone
32、nts and sub-systems: radiant source system, sample mount structure, integrating sphere and monochromator, and control and data processing system. Figure 3 shows a schematic layout of the BAI-IS system. Figure 3 A schematic layout of the BAI-IS system that consists of several sub-systems. Test Matrix
33、 Keyes universal chart (Keyes 1967) categorized fabrics into nine groups, by weave openness (Open_I, Semi-open_II, and Closed_III) and yarn color (Dark_D, Medium_M, and Light_L). Openness, Ao, is defined as the beam-beam transmittance at = 0. Keyes chart does not cover sheer fabrics (Ao 50%) that is
34、 also a popular choice for draperies. For the purpose of this study, three more groups (S_D, S_M, and S_L) have been added for fabrics with very high Ao. Classification using the Keyes chart and the three additional groups for sheer fabrics are shown in Table 1. Samples chosen for experimentation we
35、re I_D, III_L, S_M, and S_L. Table 1. Classification of Drapery Fabrics by Openness (Ao) and Yarn Color Dark (D) Medium (M) Light (L) Sheer (S) ( 50% open) S_D S_M S_L Open Weave (I) (25 50% open) I_D I_M I_L Semi-open Weave (II) (7 25% open) II_D II_M II_L Closed Weave (III) (0 7% open) III_D III_M
36、 III_L RESULTS Each sample was measured from = 0 to = 60, with 10 increment. For = 0, incident light is normal to the draped layer surface (i.e., solar altitude angle and surface azimuth angle are both zero for the vertical pleated drape layer). The light source is placed at the same height as the p
37、leated samples so the solar altitude angle stays at 0. Therefore, is equivalent to the horizontal surface azimuth angle, H, for these experiments. Table 2 and Figure 4 show results of t measured using the BAI-IS system (dots) versus predictions of the pleated drape model (solid line). Both predictio
38、ns and measurements follow the expected trend that, in general, t decreases with increasing . As well, the results show that rate of decrease depends on the solar optical properties of the fabric. Fabrics with high t,f (Sheer_Red and Sheer_White) have a high rate of decreasing t versus drapes made o
39、f fabrics with low t,f (Yellow and White). For most cases, the difference between prediction and measured t is within 0.05 except at = 0 and = 10, where the model overpredicts by as much as +0.11 for the Sheer_Red fabric. The authors attribute this overprediction to the irregularity observed in the
40、pleated samples. For instance, the light source shines on a slightly curved surface instead of on a perfectly flat surface. Then, at normal incidence for example, = 0 is only true for the surface area near the center of each pleat. However, increases along the curved surface away from the pleat cent
41、er, near the folds. In other words, is actually greater than 0 for surface area away from pleat center and increases toward the folding lines with rounded corners. As a result, the incident angle is not constant across the surface, and the “true” or “representative” would be higher than zero. Total
42、solar transmittance consists of two components: beam-beam transmittance through opening, bb, and scattered transmittance, bd. bb has a much stronger dependence on than bd does. That is why high transmitting fabrics (mainly due to high Ao) have higher rate of decreasing t with increasing . As a resul
43、t, the integrated effect due to irregularity in the sample is prominent for high Ao fabrics such as sheer fabrics and minimal for fabrics with low Ao (low bb and relatively high bd), as shown in Figure 4 (a), (b) for low Ao and (d), (e) for high Ao). In general, as increases, the fabric layer bb red
44、uces and bd becomes more dominant. In addition, to be transmitted some incident radiation has to pass through multiple layers of fabrics at high , further enhancing the dominance of bd. When bd dominates, as is the case in low Ao fabrics or high , the effect of sample irregularity on t diminishes. 0
45、 . 00 . 20 . 40 . 60 . 81 . 00 10 20 30 40 50 60t (de gr ee )Y ellow Mo d e lMe as u re dF ab ri c T _ b t0 . 00 . 20 . 40 . 60 . 81 . 00 10 20 30 40 50 60t (de gr ee )Wh it e M o d e lMe as u re dF ab ri c T _ b t(a) (b) 0 . 00 . 20 . 40 . 60 . 81 . 00 10 20 30 40 50 60t (de gr ee )Gr e y_O p en Mo
46、 d e lMe as u re dF ab ri c T _ b t(c) 0 . 00 . 20 . 40 . 60 . 81 . 00 10 20 30 40 50 60t (de gr ee )S hee r _R ed M o d e lMe as u re dF ab ri c T -b t0 . 00 . 20 . 40 . 60 . 81 . 00 10 20 30 40 50 60t (de gr ee )She er_ Wh it e M o d e lMe as u re dF ab ri c T _ b t(d) (e) Figure 4 Comparison of B
47、AI-IS transmittance test results to pleated drape model predictions Table 2. Results of Predicted and Measured t of Pleated Drape Layer Yellow (a) White (b) Grey Open (c) Sheer Red (d) Sheer White (e) Angle Model Measured Model Measured Model Measured Model Measured Model Measured 0 0.190 0.158 0.25
48、8 0.191 0.374 0.286 0.607 0.498 0.700 0.608 10 0.175 0.155 0.242 0.185 0.330 0.268 0.550 0.488 0.647 0.587 20 0.161 0.155 0.227 0.180 0.289 0.247 0.498 0.453 0.600 0.560 30 0.147 0.150 0.212 0.174 0.250 0.219 0.446 0.421 0.553 0.543 40 0.134 0.143 0.198 0.165 0.212 0.182 0.392 0.380 0.505 0.508 50 0
49、.124 0.134 0.184 0.150 0.176 0.143 0.338 0.340 0.455 0.471 60 0.113 0.114 0.170 0.133 0.123 0.072 0.262 0.252 0.390 0.363 Other Considerations As discussed above, the measurements and predictions agree reasonably well except for cases with both high Ao and low . The discrepancy is caused by the geometric difference between model and test sample. One could simply “correct” the discrepancy between model and measurements by assigning a representative for the model. For example, the representative i
