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Article Excerpt
INTRODUCTION
Draperies provide privacy, reduce glare, and improve aesthetics. Draperies can also be used to reduce solar gain, peak cooling load, and annual energy consumption. The energy performance for windows with shading devices can be modeled using a two-step procedure. In the first step, solar radiation is considered. This requires the determination of the solar optical properties of each layer in the glazing/shading system. Layers that are not uniform (e.g., shading layers) are assigned spatially averaged, or "effective," solar optical properties. Assigning effective optical properties to a shading layer allows it to be treated as a homogeneous, planar layer that can be placed at any location with respect to the glazing system. The effective properties can then be used as part of a multilayer analysis that considers beam and diffuse components of solar radiation as they interact with a multilayer assembly (e.g., Wright and Kotey [2006]). This procedure provides calculated values of system solar transmission and absorbed solar components. The absorbed solar components appear as energy source terms in the second step-the heat transfer analysis (e.g., Wright [2008]).
Researchers have used several ways to quantify the reduction of solar gain when draperies are present. Keyes (1967), for example, characterised 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). Keyes then developed a chart that expressed measured shading coefficient (SC), defined as the ratio of solar gain through a window to the solar gain through a standard layer of clear glass as a function of yarn reflectance and weave openness when a drapery was combined with both regular plate and heat-absorbing glass. If the solar optical properties of the fabric are not well known, the Keyes method can still be used to obtain an approximate SC of the glass-drapery combination.
Having acknowledged that fabric colour and weave openness alone were not sufficient to accurately determine the SC of the glass-drapery combination, Moore and Pennington (1967) developed a chart that expressed the SC as a function of fabric solar optical properties. They measured the solar optical properties of fabrics, draperies, and glass-drapery combinations using various techniques. They also measured the SC using a solar calorimeter. Furthermore, they developed equations to calculate the SC using solar optical properties as inputs. The effective solar properties of the drapery were estimated by applying a multiplicative factor to the solar properties of the fabric at normal incidence. This factor accounted for the effect of folding and the variation of incidence angle. Their calculations agreed well with experimentally determined SC values.
By careful analysis of fabric transmittance and reflectance, yarn reflectance, and openness factor, Keyes (1967) was able to reconcile the yarn reflectance-openness chart with the fabric reflectance-transmittance chart. The Keyes (1967) universal chart is the basis of the interior attenuation coefficient (IAC) data for glass-drapery combinations found in the 2005 ASHRAE Handbook-Fundamentals (ASHRAE 2005). This chart correlates measured optical properties with eye-observed values to determine the IAC, thus making it a convenient tool for designers. However, optical properties measurements carried out by Moore and Pennington (1967) revealed that the solar properties could differ from the visible properties. In such situations, using visual judgement to predict shading effects could give inaccurate results.
The first attempt to quantify the cumulative effect of folding (or pleating) and the directional nature of incident radiation on the solar gain through draperies was carried out by Ozisik and Schutrum (1960). To determine the effectiveness of 100% fullness draperies in reducing solar gain, Ozisik and Schutrum tested draperies of different fabrics in combination with regular and heat-absorbing glass using a solar calorimeter. Their results were presented in terms of the solar heat transfer factor K, defined as the ratio of the solar gain to insolation. Note that K is identical to the solar heat gain coefficient (SHGC) currently used. They showed that K was independent of incidence angle for incidence angles ranging from 0[degrees] to 50[degrees]. For incidence angles greater than 50[degrees], they suggested a decrease in K by 10% for each 10[degrees] increase in incidence angle. They also proposed a reduction of 10% in K for incident diffuse radiation. Furthermore, they presented the variation of K with solar optical properties of fabrics at normal incidence and observed that the reflectance was the dominant property influencing solar gain. In addition to the solar gain tests, they performed a series of tests to investigate the effect
of pleating on the solar optical properties of draperies. They measured the angular transmittance and reflectance of both fabrics and draperies with a pyrheliometer. Their results showed that the trans-mittance of the drapery at normal incidence was almost the same as that of the fabric. However, at 45[degrees] incidence, the transmittance of the drapery was 20% lower. For incident diffuse radiation, both transmittance and reflectance of the drapery were 20% lower than the fabric values.
Yellot (1965) determined experimentally the solar heat gain factor (SHGF), defined as solar gain through a standard clear glass, and the SC of draperies using an outdoor solar calorimeter. He also measured the solar optical properties of fabrics as well as glass-fabric combinations using a custom-made instrument. The measurements were taken at incidence angles ranging from 26[degrees] to 90[degrees]. His experiments showed that the SHGC for a typical glass-fabric combination decreased as the incidence angle increased, although the SC remained nearly constant. To explore the effects of varying surface solar azimuth on reflectance, Yellot used a reflectometer to measure the reflectance of a typical light-coloured fabric and drapery. His results showed that although the reflectance of both fabric and drapery varied with surface solar azimuth, there was very little difference between the two reflectances for a given surface solar azimuth.
The results of the preceding studies (Keyes 1967; Moore and Pennington 1967; Ozisik and Schutrum 1960; Yellot 1965) are useful in predicting the solar gain through windows with draperies. However, they are limited to single-glazed windows.
Few data can be found in the literature for comparison against results of the research presented in this study. The work of Farber et al. (1963) is of particular interest because it includes a model to determine the effective solar optical properties of draperies using a simplified rectangular configuration. Farber et al. assumed that the fabric is diffusely reflecting and diffusely transmitting and that the reflectance and transmittance for beam radiation vary with incidence angle. Their calculation involved separate treatments of...
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