ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
Scope
"INTRODUCTION Solar gain is known to offset heating load but also manifests itself as increased peak cooling load and increased cooling energy consumption. The use of shading devices to control solar gain through windows is an important research topic. This is largely true because shading devices such as venetian blinds@ roller blinds and draperies offer a cost effective strategy to actively accept or reject solar gain. Solar gain can be accepted when heating is required and rejected otherwise. The ability to control solar gain is especially important for the successful operation of well insulated@ energy efficient buildings. The influence of shading devices can be calculated when optical and thermal properties of the individual glazing/shading layers are known. The procedure takes advantage of the fact that there is no appreciable overlap between the solar and longwave 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 ?C the heat 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. 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 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 properties. This approach has been used in a number of studies (e.g.@ Parmelee and Aubele 1952@ Farber et 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 in the layer and the remaining portion is intercepted by the structure of the layer. The structure may consist of yarn@ slats@ or some other material. The portion of the intercepted radiation that is not absorbed will be scattered and will leave the layer as an apparent reflection or transmission. These scattered components are assumed to be uniformly diffuse. In addition@ a shading layer will generally transmit longwave radiation (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 represent multi-layer systems (Wright and Kotey 2006@ Wright 2008) provides virtually unlimited freedom to consider different 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 readily available (e.g.@ Keyes 1967@ Kotey et al. 2007). The most widely used information originated with Keyes (1967) who used visual inspection 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). The yarn reflectance and openness 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 independent variables@ a similar chart was generated by Moore and Pennington (1967). Keyes (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 possibility of using measured (beam-total transmittance and beamtotal 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 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 optical properties can be incorporated 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 associated glazing/shading system layer system analysis are not well suited to building energy simulation because of their complexity and because of the significant amount of CPU time required. The techniques that might be used to measure the offnormal solar optical properties of glazings cannot be applied to fabrics. This is due to the fact that fabric surfaces are rough and scatter incident radiation. Nevertheless@ the existing techniques 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 an integrating sphere installed in a commercially available spectrophotometer. 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 one end truncated at a known angle@ ?? The interior surface of each tube was painted black in order to absorb radiation scattered in reflection during a transmittance measurement or scattered in transmission during a reflectance measurement. A similar technique was used by Pettit (1979) to measure the off-normal transmittance of glazings. Pettit's measurements compared favourably with results obtained from first principles. In the current study@ spectral measurements of beambeam transmittance@ beam-diffuse transmittance and beamdiffuse reflectance were obtained at incidence angles@ ?? ranging 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@ no spectral data are presented. The solar optical properties were calculated 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 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 practical option. A realistic approach is to develop models that require a small number of readily obtained measurements as input. Such an approach was used in determining the offnormal 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 as long as the user knows where the fabric is located on Keyes' chart (Figure 1)."