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Determining off-normal solar optical properties of roller blinds.

Article Excerpt
INTRODUCTION

The capability of window shading devices--roller blinds in particular--to reduce solar heat gain through windows has been an important research topic for many years. The ability to accurately quantify the reduction in cooling load that these devices deliver would be an asset to architects, engineers, and building designers in general.

Several studies have shown that roller blinds can significantly reduce energy costs associated with windows. Grasso and Buchanan (1979, 1982) reported a 60% reduction in energy costs when a light-coloured opaque roller blind was used during the cooling season. A light-coloured translucent roller blind yielded an annual cost reduction of 50%. During the heating season, they found that roller blinds had the potential of reducing energy cost since they reduce heat transfer through the window. For climates with net seasonal energy loss, an average of 34% reduction in energy cost was realised when conventional roller blinds were attached to a window. They also noted that the percentage reduction in the energy cost during the heating season was insensitive to the type and colour of the roller blind used but was sensitive to the proximity of the roller blind to the window-the roller blinds tended to be more effective in reducing heat transfer when installed closer to the window. The energy saving potential of roller blinds has also been examined by means of calorimetric measurements (e.g., Ozisik and Schutrum [1959], Grasso and Buchanan [1982], and Harrison and van Wonderen [1998]). Such measurements are time consuming and expensive.

With the advent of several computational techniques, the energy saving potential of roller blinds can be readily calculated if the solar optical and the thermal properties of individual layers of a glazing/shading system are known (e.g., Wright and Kotey [2006], EnergyPlus [DOE 2007], and van Dijk et al. [2002]). The procedure takes advantage of the fact that there is no appreciable overlap between the solar and the longwave radiation bands. This leads to a two-step analysis. First, solar radiation models determine the fraction of incident solar radiation that is directly transmitted and the fraction that is absorbed in each layer of the glazing system. The solar radiation absorbed in each layer then serves as a source term in the second step-the heat transfer analysis. For building energy simulation, the two-step analysis is done on a time-step basis. Since the location of the sun and therefore the incidence angle, [THETA], changes by the hour, solar optical properties of the individual layers of any glazing/shading system must be available for any given angle of incidence.

Solar optical properties of glazings, including coated and tinted glazings, can readily be estimate at any given value of [THETA] (e.g., Furler [1991], Pfrommer et al. [1995], Roos [1997], and Rubin et al. [1998, 1999]). The off-normal solar optical properties of roller blinds, however, are not readily available. Normal incidence solar optical properties are easily obtained, however, and a means of estimating the off-normal and diffuse properties from these is highly desirable.

Shading layers are often characterised by making the assumption that each layer, whether homogeneous or not, is represented by an equivalent homogenous layer with spatially averaged "effective" optical properties (e.g., Parmelee and Aubele [1952], Farber et al. [1963], Pfrommer et al. [1996], and Yahoda and Wright [2005]). Such an approach has been shown to provide accurate characterisation of venetian blinds (e.g., Kotey et al. [2008]).

Careful consideration of solar radiation incident on a shading layer with some openness reveals that some portion of the radiation passes undisturbed through openings while the remaining portion is intercepted by the structure of the layer. The structure may consist of yarn, slats, or some other material. A portion of the intercepted radiation is absorbed and the rest is scattered, leaving the layer as an apparent reflection or transmission. These scattered components are assumed to be uniformly diffuse.

The use of effective optical properties and a beam/diffuse split of solar radiation at each layer of a multilayer system provides virtually unlimited freedom to consider different types of shading layers. This approach also delivers the high computational speed needed in building energy simulation tools.

A recent study by Kotey et al. (2009a) used specially designed sample holders attached to an integrating sphere of a commercially available spectrophotometer to measure the off-normal solar optical properties of drapery fabrics. The integrating sphere is particularly useful since it can separate the undisturbed and scattered components of incident beam radiation. Kotey et al. measured the spectral beam-beam trans-mittance, beam-diffuse transmittance, and beam-diffuse reflectance at incident angles ranging from to 60[degrees] and then calculated the corresponding solar properties (ASTM 1996). Having obtained the solar properties at varying [THETA], cosine power functions were fitted to normalised forms of the measured data. The cosine power function was chosen because it is symmetrical about [THETA]=0 . Furthermore, the shape...

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