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Determining off-normal solar optical properties of insect screens.

Article Excerpt
INTRODUCTION

Building energy consumption constitutes approximately 40% of the total energy used in the US (DOE 2004). It has also been estimated that about 25 billion dollars per year is tied to the thermal performance of windows (DOE 2004). Due to rising costs of conventional hydrocarbon fuels, there is an increased interest in conversion from conventional to clean and environmentally friendly sources of energy. The first step in this conversion is conservation. Improved thermal performance of windows, therefore, offers great potential.

In pursuit of energy conservation, shading devices such as venetian blinds, roller blinds, and draperies can be used to control solar gain. Their potential for reduction of cooling load and annual energy consumption is recognised to be large (e.g., Grasso and Buchanan [1982] and Harrison and van Wonderen [1998]). In addition, insect screens are frequently attached to windows, particularly in residential buildings, and recent studies have revealed that insect screens have a significant influence on solar gain (e.g., Brunger et al. [1999]). By using a solar simulator-based test method, Brunger et al. (1999) noted that a typical insect screen attached to a window reduced its solar heat gain coefficient (SHGC) significantly. Their test results showed that when the insect screen was placed on the outdoor side of a double-glazed window, SHGC was reduced by 46%. On the other hand, SHGC was reduced by 15% when the insect screen was placed on the indoor side of the window. In light of the aforementioned observations, an insect screen can be classified as a shading device and could be used to control solar gain.

Shading layers are often characterised by 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 characterization of shading layers was used in a number of studies (e.g., Parmelee and Aubele [1952], Farber et al. [1963], Pfrommer et al. [1996], and Yahoda and Wright [2005]) and was shown to provide accurate optical properties of venetian blinds (Kotey et al. 2008).

The use of effective optical properties and a beam/diffuse split of solar radiation in multilayer systems provide virtually unlimited freedom to consider different types of shading layers. This approach also delivers the computational speed needed in the context of building energy simulation.

The most recent characterisation of screens is reported in the EnergyPlus reference manual (DOE 2007). Off-normal solar property models were developed for building energy simulation using analytical and ray tracing techniques. The models are based on the orthogonal crossed cylinder geometry with known wire diameter, wire centre-to-centre spacing, and wire reflectance. The models assume that the wire diameter and wire spacing are the same in both directions. For a unit of incoming beam radiation with known direction, the models account for both undisturbed flux going through the openings of the screen and intercepted flux. More specifically, the undisturbed flux constitutes the beam-beam transmittance. The portion of the intercepted flux that is not absorbed is scattered and leaves the layer as an apparent reflection or transmission. The portion of the intercepted flux not absorbed and emerging in the forward direction gives rise to beam-diffuse transmittance, and the flux emerging in the reverse direction is the beam-diffuse reflectance. The beam-diffuse transmittance model was "empirically" formulated by curve-fitting results from an optical ray tracing algorithm. The ray tracing algorithm is based on the assumption that the wire reflectance is diffuse. The beam-diffuse reflectance is a function of the beam-beam transmittance, the wire reflectance, and the beam-diffuse transmittance. The diffuse-diffuse transmittance and reflectance models are simply hemispherical integrations of the beam-total (beam-beam plus beam-diffuse) transmittance and beam-diffuse reflectance, respectively.

A measurement technique was devised to obtain the off-normal solar optical properties of flat shading devices (Kotey et al. 2009a, 2009b). It involves the use...

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