1、2009 ASHRAE 3ABSTRACTThe determination of off-normal solar optical properties of drapery fabrics is particularly useful in modelling the effec-tive solar optical properties of pleated drapery. Special sample holders were designed and fabricated to facilitate measure-ments using an integrating sphere
2、 installed in a commercially available spectrophotometer. Measurements were taken for eight of the nine fabric designations documented in the ASHRAE Handbook Fundamentals. Measurements were also obtained for a sheer fabric which does not fall into any of the customary fabric designations. Semi-empir
3、ical models were developed to quantify the variation of solar optical prop-erties with respect to incidence angle. Given solar optical properties obtained at normal incidence, these models can be used to characterise the off-normal beam-beam and beam-diffuse properties of a drapery fabric. The fabri
4、c models comprise a useful component of pleated drapery models and, in turn, a valuable tool for building energy simulation. The measurement technique described in this study can be used to obtain the off-normal solar optical properties of additional flat shading devices such as roller blinds and in
5、sect screens. INTRODUCTIONSolar gain is known to offset heating load but also mani-fests itself as increased peak cooling load and increased cool-ing energy consumption. The use of shading devices to control solar gain through windows is an important research topic. This is largely true because shad
6、ing devices such as venetian blinds, roller blinds and draperies offer a cost effec-tive strategy to actively accept or reject solar gain. Solar gain can be accepted when heating is required and rejected other-wise. The ability to control solar gain is especially important for the successful operati
7、on of well insulated, energy efficient buildings. The influence of shading devices can be calculated when optical and thermal properties of the individual glazing/shad-ing layers are known. The procedure takes advantage of the fact that there is no appreciable overlap between the solar and longwave
8、radiation bands. This leads to a two-step analysis. First, solar radiation models determine the fraction of incident solar radiation directly transmitted and the fraction that is absorbed in each layer. The absorbed solar radiation in each layer then serves as a source term in the second step the he
9、at transfer analysis. A building energy simulation might include this analysis in an hour-by-hour calculation. Since the location of the sun and the incidence angle change by the hour, the solar optical properties of the individual layers must be available at any given incidence and/or profile angle
10、.The off-normal solar properties of clear and tinted glass can readily be determined (e.g., Pettit 1979, Furler 1991). Several models have also been devised to characterize coated glass (e.g., Pfrommer et al. 1995, Roos 1997, Rubin et al. 1998, Rubin et. al. 1999). In general, shading layers may be
11、characterized by making the assumption that each layer, whether homogeneous or not, can be represented by an equivalent homogenous layer that is assigned spatially-averaged “effective” optical proper-ties. This approach has been used in a number of studies (e.g., Parmelee and Aubele 1952, Farber et
12、al. 1963, Pfrommer et al. 1996, van Dijk et al. 2002, Yahoda and Wright 2005) and has been shown to provide accurate characterization of venetian blinds (e.g., Kotey et al. 2008).When solar radiation is incident on a shading layer, some portion of the radiation passes undisturbed through openings De
13、termining Off-Normal Solar Optical Properties of Drapery FabricsNathan Kotey, PhD John L. Wright, PhD, PEng Michael Collins, PhD, PEngStudent Member ASHRAE Member ASHRAE Associate Member ASHRAENathan Kotey is a graduate student, John Wright is a professor, and Michael Collins is an associate profess
14、or in the Department of Mechan-ical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, CA.LO-09-001 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use
15、only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.4 ASHRAE Transactionsin the layer and the remaining portion is intercepted by the structure of the layer. The structure may consist of yarn, slats, o
16、r some other material. The portion of the intercepted radia-tion that is not absorbed will be scattered and will leave the layer as an apparent reflection or transmission. These scat-tered components are assumed to be uniformly diffuse. In addition, a shading layer will generally transmit longwave r
17、adiation (i.e., it is diathermanous), by virtue of its openness, and effective longwave properties are assigned accordingly.Using effective optical properties and a beam/diffuse split of solar radiation at each layer, the framework used to repre-sent multi-layer systems (Wright and Kotey 2006, Wrigh
18、t 2008) provides virtually unlimited freedom to consider differ-ent types of shading layers. This framework also delivers the computational speed needed in the context of building energy simulation.Techniques for characterising the off-normal properties of fabrics and pleated draperies are not readi
19、ly available (e.g., Keyes 1967, Kotey et al. 2007). The most widely used infor-mation originated with Keyes (1967) who used visual inspec-tion to characterise fabrics by yarn colour (yarn reflectance) as dark (D), medium (M) and light (L) and by weave as open (I), semi-open (II) and closed (III). Th
20、e yarn reflectance and open-ness factor of fabrics were conveniently represented on a chart allowing the user to estimate the shading effect of a pleated drape. Using fabric transmittance and reflectance as indepen-dent variables, a similar chart was generated by Moore and Pennington (1967). Keyes (
21、1967) then reconciled the two charts. Keyes (1967) universal chart, shown in Figure 1, is the basis of the interior attenuation coefficient (IAC) values that apply to glass-drapery combinations found in ASHRAE Handbook-Fundamentals (2005). This chart offers the possi-bility of using measured (beam-t
22、otal transmittance and beam-total reflectance at normal incidence) or eye-observed optical properties (openness and yarn colour) to estimate the shading effect of pleated draperies with 100% fullness. More recently, Hunn et al. (1991) designed an apparatus to measure the bidirectional transmittance
23、and reflectance distribution of fabrics. The measurements revealed the effect of textile properties (openness of weave, fibre cross section and fabric structure) on the distribution of sunlight. Such information is particularly useful in the context of daylighting simulation. Bidirectional solar opt
24、ical properties can be incor-porated into matrix layer calculation methods (e.g., Klems 1994a and 1994b) to predict the solar gain of glazing/shading systems. However, this experimental method and the associ-ated glazing/shading system layer system analysis are not well suited to building energy sim
25、ulation because of their complex-ity and because of the significant amount of CPU time required. The techniques that might be used to measure the off-normal solar optical properties of glazings cannot be applied to fabrics. This is due to the fact that fabric surfaces are rough and scatter incident
26、radiation. Nevertheless, the existing tech-Figure 1 Keyes Universal Chart (adopted from ASHRAE 2005).ASHRAE Transactions 5niques can be adapted. To achieve this, special sample holders were designed and fabricated to facilitate the measurement of off-normal solar optical properties of fabrics using
27、an integrat-ing sphere installed in a commercially available spectropho-tometer. The integrating sphere is particularly useful because it can resolve the undisturbed and scattered components of transmitted or reflected beam radiation. The sample holders were made from polished aluminium tubes with o
28、ne end trun-cated at a known angle, . The interior surface of each tube was painted black in order to absorb radiation scattered in reflec-tion during a transmittance measurement or scattered in trans-mission during a reflectance measurement. A similar technique was used by Pettit (1979) to measure
29、the off-normal transmittance of glazings. Pettits measurements compared favourably with results obtained from first principles. In the current study, spectral measurements of beam-beam transmittance, beam-diffuse transmittance and beam-diffuse reflectance were obtained at incidence angles, , rang-in
30、g from 0 to 60. These data showed that fabrics are generally not spectrally selective except for variation in the visible region corresponding to the colour of the fabric. Since the aim of the current study was to generate solar (spectral-averaged) optical properties for building energy simulation,
31、no spectral data are presented. The solar optical properties were calcu-lated using the 50-point selected ordinate method as described in ASTM E903-96 (1996). A second procedure was devised to repeat the beam-beam transmittance measurements, this time without the integrating sphere and at incidence
32、angle as high as 80. Having two sets of beam-beam transmission data offered an opportunity to compare and gain confidence in the new procedures.The direct measurement of off-normal solar optical properties of all drapery fabrics on the market is not a prac-tical option. A realistic approach is to de
33、velop models that require a small number of readily obtained measurements as input. Such an approach was used in determining the off-normal solar optical properties of coated and tinted glazings (e.g., Furler 1991, Roos 1997, Karlsson and Roos 2000). The models developed in this study can be applied
34、 as long as the user knows where the fabric is located on Keyes chart (Figure 1). OBJECTIVES AND APPROACHThe main objective of the current research was to develop semi-empirical models for off-normal solar optical properties of drapery fabrics. This objective was achieved by measuring the off-normal
35、 solar optical properties of a wide variety of fabric samples using special sample holders attached to an integrating sphere. A simple but pragmatic way to characterize fabrics is to assume that normalized transmittance and reflectance data share a common functional dependence with respect to inci-d
36、ence angle. In this study a cosine power function was chosen to represent this dependence. This was done for several reasons, including simplicity. The cosine function is symmet-rical, having zero gradient at = 0 (normal incidence). It has maximum and minimum values at = 0 and at = 90. Also, the sha
37、pe of the curve can be modified by changing the value of the exponent. Each model component was tuned using a set of integrating sphere measurements and, although the formu-lation appears to be primarily empirical, an effort was made to incorporate known or expected trends and limiting cases in orde
38、r to make the resulting models as general and reliable as possible. PRELIMINARY CONSIDERATIONSFabrics consist of strands of yarn that are woven or knit-ted. The yarn itself is made up of fibres that are twisted and plied. Strands of yarn can be woven loosely, leaving open areas, or woven tightly wit
39、h little or no open area. Further-more, strands of yarn that are loosely twisted and plied could have open areas between the fibres. Fabrics may be classified by weave as open, semi-open and closed. The colour of the yarn may be used to classify fabrics as light, medium or dark. See Figure 1. A port
40、ion of incident beam radiation will pass undis-turbed through the openings of the fabric. The remaining portion encounters the structure of the fabric and undergoes multiple reflections between the fibres as well as possible transmission through the fibres. The portion of the intercepted radiation t
41、hat is not absorbed by the fibres emerges in the forward or backward direction (Keyes 1967). The undisturbed radiation transmitted through the openings constitutes the beam-beam transmittance (specular transmittance), . The superscript “m” is used to denote a property of a fabric (i.e., a material).
42、 At normal incidence the beam-beam transmittance is equivalent to the openness factor (Keyes 1967), , which is defined as the ratio of the open area to the total area of the fabric. The portion of intercepted/scattered radiation that emerges in the forward direction constitutes beam-diffuse transmit
43、tance, , while the portion that emerges in the backward direction is the beam-diffuse reflectance, . These scattered components are assumed to be uniformly diffuse. The beam-total transmittance (direc-tional-hemispherical transmittance), , is the sum of and . It was assumed that fabrics do not exhib
44、it specular reflection, , and this was confirmed experimentally. The beam-total reflectance (directional-hemispherical reflec-tance), , is therefore equal to the beam-diffuse reflectance, . Accordingly, incident diffuse radiation is also assumed to be transmitted and reflected diffusely by the fabri
45、c. The corresponding diffuse-diffuse properties are and . MEASUREMENT TECHNIQUESpectrophotometerThe spectrophotometer used in this study is a double beam, direct ratio recording, rapid scanning instrument. It has a resolution of less than 0.05 nm for the ultraviolet and visible spectra (UV-VIS) and
46、less than 0.2 nm for the near infrared bbmAobbm 0=()=bdmbdmbtmbbmbdmbbm0bdmbtmbdm=ddmddm6 ASHRAE Transactionsspectrum (NIR); a repeatability characteristic of less than 0.025 nm for UV-VIS and less than 0.1 nm for NIR. In oper-ation, two detectors, a Photomultiplier Tube (PMT) and a lead-sulphide (P
47、bS) photoconductive sensor, are illuminated alter-nately by the sample and the reference beam. The PMT is used in the wavelength range of and the PbS detector responds in the wavelength range. There are a several accessories that can be attached to the spec-trophotometer. For the purpose of the curr
48、ent investigation the spectrophotometer was operated, in most cases, with the inte-grating sphere attachment. Integrating SphereIntegrating spheres are designed to measure, and distin-guish between, beam and scattered components of transmitted and reflected radiation. Light enters the sphere through
49、 a port and reflection from the interior surface must be purely diffuse. Light inside the sphere becomes uniformly distributed over the entire inner surface, eliminating any directional or spatial non-uniformity of the incoming radiation, and detectors measure this integrated signal. The detector signal is propor-tional to the rate at which radiant energy enters at the inlet port and the ratio between the two is called the “response of the sphere”. The surface of the sphere must be very highly reflec-tive to maximize the response of the sphere and produce a s
