Developing a transparent shading device as a daylighting system

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Developing a transparent shading device as a daylighting system Svetlana Olbina a; Yvan Beliveau b a M. E. Rinker, Sr. School of Building Construction, University of Florida, Gainesville, FL, US b Myers-Lawson School of Construction, Virginia Polytechnic Institute and State University, Blacksburg, VA, US Online Publication Date: 01 March 2009

To cite this Article Olbina, Svetlana and Beliveau, Yvan(2009)'Developing a transparent shading device as a daylighting

system',Building Research & Information,37:2,148 — 163 To link to this Article: DOI: 10.1080/09613210902723738 URL: http://dx.doi.org/10.1080/09613210902723738

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BUILDING RESEARCH & INFORMATION (2009) 37(2), 148 – 163

RESEARCH PAPER

Developing a transparent shading device as a daylighting system Svetlana Olbina1 and Yvan Beliveau2

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1

M. E. Rinker, Sr. School of Building Construction, University of Florida, 322 Rinker Hall, PO Box 115703, Gainesville, FL, 32611, US E-mail: solbina@u£.edu

2

Myers-Lawson School of Construction,Virginia Polytechnic Institute and State University, 250 South Main Street, Suite 300 (0156), Blacksburg,VA, 24061, US E-mail: [email protected]

A transparent shading device is developed as an alternative to opaque or translucent materials. The objectives are to create a new design of transparent blinds with improved daylighting performance and to simulate its daylighting performance. The application of a triangular cross-section for the slats and the use of clear plastic and a silver reflective coating as the materials for the new blinds utilize the principles of optics in this design. A case study analyses and compares the daylighting performance of the new transparent blinds, commercially available opaque blinds, and the previously patented transparent blinds. Results from a limited simulation of the three systems indicate the interior illuminance and daylighting performance are best for the new blinds. Keywords: alternative technology; daylight autonomy; daylighting; facade design; illuminance; optics; shading device; simulation; useful daylight illuminance Un dispositif pare-soleil transparent a e´te´ mis au point comme alternative aux mate´riaux opaques ou translucides. Les objectifs sont de cre´er des stores transparents de conception nouvelle pre´sentant des performances ame´liore´es en e´clairage naturel et de proce´der a` une simulation des performances de ce dispositif en e´clairage naturel. Ce design fait appel aux principes d’optique dans l’application d’une section transversale triangulaire aux lamelles et dans l’utilisation d’un plastique transparent et d’un reveˆtement re´fle´chissant argente´ comme mate´riaux de ces nouveaux stores. Une e´tude de cas analyse et compare les performances en e´clairage naturel de ces nouveaux stores transparents, des stores opaques disponibles dans le commerce, et des stores transparents de´ja` brevete´s. Les re´sultats d’une simulation limite´e des trois syste`mes indiquent que les performances concernant l’e´clairement inte´rieur et l’e´clairage naturel sont meilleures avec les nouveaux stores. Mots cle´s: technologie alternative; autonomie en lumie`re du jour; e´clairage naturel; conception de facades; e´clairement; optique; dispositif pare-soleil; simulation; e´clairement naturel utile

Introduction The shading device is an important component of any conventional facade system, as well as single- and double-skin curtain wall systems. The shading device

provides protection from direct sun (Bilgen, 1994; Klems and Warner, 1997; Galasiu et al., 2004) and overheating in summer, thus reducing the cooling loads for the building (Inoue et al., 1988; Pfrommer

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Developing a transparent shading device as a daylighting system

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et al., 1996; Sciuto, 1998; Selkowitz and Lee, 1998; Kuhn et al., 2000; Breitenbach et al., 2001; Athienitis and Tzempelikos, 2002). The shading device is also used to provide privacy (Tzempelikos, 2008) and protection from glare (Klems and Warner, 1997; Kuhn et al., 2000; Athienitis and Tzempelikos, 2002; Galasiu et al., 2004; Selkowitz and Lee, 2004). The venetian blind, as an optically complex shading device, transmits, reflects, and scatters direct sun. It also diffuses skylight and reflected light from the ground (Lee et al., 1998; Sciuto 1998; Tzempelikos, 2008). Proper application of the shading device is particularly important for preventing the greenhouse effect and overheating as well as for controlling the glare in buildings with curtain walls used as facade systems (Selkowitz and Lee, 2004). Large glass areas are broadly used in contemporary architecture since glass provides direct visual contact between the inside and the outside (Inoue et al., 1988; Klems and Warner 1997; Selkowitz and Lee, 1998; Kuhn et al., 2000; Tzempelikos, 2008), which has a significant psychological effect on building occupants (Inoue et al., 1988; Athienitis and Tzempelikos, 2002; Selkowitz and Lee, 2004). Glass also contributes to achieving a transparent appearance of the building. Glass transmits daylight which contributes to the well-being of the building occupants and provides energy savings for the building. However, the use of conventional shading devices in windows can obstruct the view to the outside and limit the amount of daylight that penetrates into the interior space (Illuminating Engineering Society of North America (IESNA) 1999; Kuhn et al., 2000; Kischkoweit-Lopin, 2002). A shading device designed as a daylighting system that also provides a view to the outside can mitigate this problem. Daylighting systems redirect diffuse or direct sunlight (Kischkoweit-Lopin, 2002), that is, they provide dynamic control of transmitted luminous flux to improve light distribution and reduce cooling loads in the space (Selkowitz and Lee, 1998; Beck et al., 1999; Kuhn et al., 2000; Breitenbach et al., 2001). Therefore, if a shading device is designed and utilized as the daylighting system, it can help in the distribution of the daylight into room areas that are far away from the windows and, therefore, do not have a sufficient amount of daylight. In addition to redirecting daylight deeper into a space and controlling solar gain, a daylighting system can also control glare, provide energy savings, and enhance privacy (Ouderkirk et al., 1999; Kischkoweit-Lopin, 2002). A transparent shading device is a shading system made of a transparent material such as glass or plastic. Along with providing transparency to the wall, which is one of the design goals when using a curtain wall system, a transparent shading device

needs to control solar gain. A transparent shading device, as a window component, can help in the transmission, reflection, and absorption of sunlight. As a result, a transparent system can provide a sufficient amount of daylight in the space and protect the space from overheating.

Problem statement

Commercially available shading devices are usually used for protection from overheating in summer, for glare protection, and for providing privacy. Commercially available conventional shading devices are also occasionally used as part of a daylighting system. For example, venetian blinds can perform poorly with respect to daylight levels (Moeck, 1998). However, the shading device may be designed to redirect sunlight to spaces where daylight is needed, such as those that are a great distance from the window. Shading device slats are usually made of non-transparent materials. If these slats are in a closed or nearly closed position, the direct view to the outside can be partly obstructed (Vine et al., 1998; Kuhn et al., 2000; Ruck et al., 2000), and consequently visual transparency of the facade is more difficult to achieve. Installed transparent shading devices are usually custom designed and made for a particular building. Mass-produced transparent systems are not yet available on the market. Several transparent shading devices are previously patented, but they are not yet in production for the market. There is also a problem associated with the control systems used to adjust the blinds’ tilt angle. A shading device is usually controlled manually by the building occupants based on the occupants’ personal preferences, which often do not meet requirements for thermal and daylighting performance (Lee et al., 1998; Selkowitz and Lee, 1998; Kuhn et al., 2000; Ruck et al., 2000; Guillemin and Morel, 2001; Athienitis and Tzempelikos, 2002). For example, problems occur if the occupants are absent from the room when the blinds need to be adjusted (Lee et al., 1998; Selkowitz and Lee, 2004). Also, occupants very often close the blinds completely to protect space from overheating and glare, but at the same time the amount of daylight in the space is reduced and the use of electrical lighting is increased (IESNA, 1999). As a result, the cooling loads are also increased. A balance between a sufficient amount of daylight and maximum overheating protection is difficult to achieve without the application of automatically controlled systems (Inoue et al., 1988; DiBartolomeo et al., 1996; Klems and Warner, 1997; Lee et al., 1998; Selkowitz and Lee, 1998, 2004). However, some level of user adjustment of the automated control is needed to avoid user rejection of the control (DiBartolomeo et al., 1996; Vine et al., 1998). 149

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Objectives

The purpose of this research is to study daylighting performance of a transparent shading device as part of a window system. Objectives of the research are as follows: .

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.

design a new shading device system that would do the following: .

function as a daylighting system

.

be made of transparent material

.

be feasible for mass production

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use an automatic control system

analyse the daylighting performance of this new transparent shading device

Figure 1 The Berlaymont Building in Brussels, Belgium, with moveable glass louvers as the outside facade layer. Source: Architects Pierre Lallemand & Steven Beckers/Berlaymont 2000.

Background Existing transparent shading devices

Shading devices made of transparent materials, such as glass or plastic, can have a useful application since they provide an opportunity for creating transparency of the windows and glass facades. A review is conducted to understand the principles of their design and performance. Knowledge about the installed transparent shading devices and the previously patented transparent shading devices is the background for the development of the new shading device system. Transparent materials, such as glass and plastic, and their properties, are studied to investigate the possibility of their use for the manufacture of blinds (Callister, 1985). There is a viable potential for manufacturing transparent blinds based on the appropriate design of the chemical and physical structure of the material and the blinds’ manufacturing process. Installed systems

Existing installed transparent shading systems tend to be custom made, designed, and manufactured only once for a specific building. The following building applications are a few examples that represent installed, custom-made transparent shading devices: .

150

The Berlaymont Building in Brussels, Belgium (Figure 1), has moveable glass louvers that are installed as the outside layer of the double-skin facade. The louvers’ position is adjusted based on the sun’s position. The louvers control heat gain in summer and heat loss in winter, and act as the light shelves allowing daylight penetration into the building (European Commission (DG TREN), 2005; Roger-France, 2005; Deneyer et al., 2008). The average daylight factor is around 6% and uniform daylighting is provided in the back of the 6 m-deep office. The louvers also provide views to the outside (Roger-France 2005).

Figure 2 The Gartner Design O⁄ce building in Gulden¢ngen, Germany, with moveable glass louvers. Source: Facade GARTNER, Josef Gartner GmbH. .

The new design office for the Gartner Cladding Co. in Guldenfingen, Germany, has glass louvers installed along the north and south elevations (Compagno, 2002; Wigginton and Harris, 2002; Koster, 2004). These louvers are an example of the custom-designed and custom-made transparent shading device (Figure 2). The glass louvers are used to redirect daylight onto the ceiling, protect space from direct sunlight, and provide visual contact with the outside (Compagno 2002; Wigginton and Harris, 2002). The louvers are moveable and automatically controlled.

.

Avax S.A. headquarters building in Athens, Greece, has the transparent blinds installed on the eastoriented facade (Tombazis and Preuss, 2001). These blinds are another example of the customdesigned and custom-made transparent shading

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performance level. The following examples present a few of the investigated systems:

Figure 3 Avax S.A. headquarters in Athens, Greece, with moveable glass panels/blinds as the outside facade layer. Source: Avax S.A. headquarters; Architects: A. N. Tombazis and Associates Architects, Athens. Image courtesy: Nikos Daniilidis.

device. Vertical shading devices consist of double laminated glass panels with a silk screen printed surface (Figure 3). These vertical blinds create an outside layer of the double-skin facade and are automatically controlled based on solar radiation. Previously patented systems

Previously patented transparent shading devices are investigated to obtain an understanding of their performance. An understanding of the design principles, such as the optics used and explained in the patented solutions, help in the design of a new transparent shading device, which is proposed by this research. The analysed systems offer a sufficient amount of daylight in the space and protection from glare and overheating (Seeger, 1969; Bartenbach, 1983; Murphy and Campbell, 1988; Lorenz, 2001; Koster, 2002). Some analysed systems do not essentially limit the view to the outside (Murphy and Campbell, 1988; Lorenz, 2001). Two basic types were analysed – moveable (Seeger 1969; Bartenbach 1983; Murphy and Campbell 1988; Moench 1991) and fixed (Boyd 1957; Wirth et al. 1998; Koster 2002) – to find out whether or not fixed blinds can achieve a required

.

The shading device patented by US Patent 4,517,960 consists of a plurality of slats that are made of a light, permeable material (Bartenbach, 1983). The slats have a flat, non-reflective base surface on a side oriented to the sun and a prismatic structure on the opposite side oriented away from the sun (Figure 4). The prismatic rods are parallel to the longitudinal axes and have two non-reflective surfaces that work only by total internal reflection. This shading device improves light transmittance and provides effective protection from sunlight. For the south orientation, this shading device does not require adjustment of the slat inclination during the day and requires little adjustment throughout the year (Bartenbach, 1983).

.

A prismatic transparent shading device patented by US Patent 4,773,733 blocks direct sun rays entering the space, permits the passage of indirect sun rays into the space, and permits a view through the shading device from the interior to the outdoor space (Murphy and Campbell, 1988). The shading device has a form of venetian blind comprised of prismatic, reflective, slatted panels that are rotatable depending on the sun’s movement (Figure 5). The prismatic slats are made from a light transmissive material with an index of refraction of approximately 1.5, such as glass, acrylic, and polycarbonate. The slats have two faces: the front flat face is oriented to the sun and

Figure 4 Previously patented transparent shading device (US Patent 4,517,960; Bartenbach, 1983): cross-section through the prismatic slat.

Figure 5 Previously patented transparent shading device (US Patent 4,773,733; Murphy and Campbell, 1988): partial crosssection through the three-faced prism. 151

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the rear face oriented away from the sun. The rear face has reflective prisms for total internal reflection of sunrays that strike the front face of the slat at a 308 angle from a normal to the front face (Murphy and Campbell, 1988).

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The transparent shading device patented by US Patent 6,367,937 has reflective lamellae that do not produce glare in the interior or exterior spaces and avoid multiple reflections that lead to overheating of the lamellae and, therefore, to heating of the interior space (Koster, 2002). The slats in this shading device are not moveable and the blinds are not retractable. Each slat consists of the following portions (Figure 6): .

one portion of teeth, close to the outside space, reflects the sun’s radiation back to the outside space by a single reflection for high sun incidence angles, such as in the summer, and works in favour of thermal comfort

.

a second portion of teeth, close to the interior space, illuminates the interior space for low sun incidence angles, such as in the winter, and works in favour of visual comfort by reflecting light into the interior

Each individual tooth has two sides: one exposed to the sun’s radiation, and a shaded side. Glare is avoided

since the tooth side exposed to the sun has a deflection function, while the shaded tooth side has a dimming function (Koster, 2002). The shaded tooth side, rather than the irradiated side, is seen from the interior space, and the shaded side becomes darker since it is not irradiated by the sun. No glare effect will occur when looking at the lamellae (Koster, 2002).

Daylighting performance of shading devices

This research focuses only on the daylighting performance of the shading devices, specifically illuminance, daylight autonomy (DA), and useful daylight illuminance (UDI), obtained by the application of transparent shading devices. Daylight has an important effect on the occupants of the buildings, particularly their behaviour and well-being in the space (Ruck et al., 2000; Galasiu and Veitch, 2006). As one of the sustainable strategies, daylight contributes to energy savings by reducing the use of electric lighting, which decreases the cooling loads in the building (Lee et al. 1998). Daylighting also has a significant aesthetic effect on the interior space. The appropriate use of shading devices can improve the daylight level in the space (Selkowitz and Lee, 1998; Kuhn et al., 2000; Breitenbach et al., 2001; Kischkoweit-Lopin, 2002). For these reasons, daylight performance parameters (illuminance, DA, and UDI) are used for the evaluation of the new transparent shading device proposed by this research.

Figure 6 Previously patented transparent shading device (US Patent 6,367,937; Koster, 2002): perspective section through three sun-protection lamellae. 152

Developing a transparent shading device as a daylighting system

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Illuminance is a measure of the quantity of light in a space. Illuminance levels in a space decrease asymptotically as the distance from the window increases toward the depth of the space. By definition, ‘illuminance is the density of luminous power’ (Stein and Reynolds, 2000, p. 1054) or ‘the luminous flux incident on a surface per unit area’ (Ruck et al., 2000, p. 8– 4). The unit of measure used to quantify lumen per square metre is lux (lx). The distribution of light illuminance shows ‘how lighting varies from point to point across a plane of surface’ (Ruck et al., 2000, p. 3 –5). Illuminance is measured across a horizontal work plane at a height of 0.8 m above the floor. The illuminance distribution in a room resulting from the application of the venetian blind is a complex function of the solar conditions and the slat tile angle (Klems and Warner, 1997; Lee et al., 1998). To evaluate the daylighting performance of a shading device, actual values of illuminance in the space obtained by the application of the shading device on a specific building need to be compared with values recommended by the standards and the literature (Nabil and Mardaljevic, 2006; Reinhart et al., 2006). The International Standard ISO 8995, CIE S 008/E, gives recommended maintained illuminances over the task area on the reference surface, which can be horizontal, vertical, or inclined. These values provide for visual safety at work and needed visual performance (Commission Internationale de l’Eclairage (CIE) Central Bureau, 2001). The ANSI/IES RP-1-1982 standard for office lighting recommends illuminance categories based on the area/activity, ranges of illuminance categories and types of activity, minimum values of illumination for safety, and weighting factors to be considered in selecting specific illuminance (IESNA, 1982). Reinhart et al. (2006) recommended the use of dynamic daylighting performance metrics, such as DA and UDI to evaluate the daylight performance of shading devices. Both DA and UDI are based on work plane illuminances (Reinhart et al., 2006). DA is defined as the percentage of the occupied times of the year when the minimum required illuminance level at the sensor point is provided by daylight only (Reinhart and Walkenhorst, 2001; Architectural Energy Corporation, 2006). Nabil and Mardaljevic (2006) defined UDI as a measure that determines when illuminance levels are useful for the occupant, that is, more than 100 lx (not too dark) and less than 2000 lx (not too bright). UDI is expressed as percentages of the occupied times of the year when it is achieved (100– 2000 lx), not sufficient (less than 100 lx), or exceeded (more than 2000 lx) (Reinhart et al. 2006). Mardaljevic (2006) suggested dividing achieved UDI into autonomous UDI (500– 2000 lx) and supplementary UDI (100– 500 lx). Supplementary electric lighting may be needed for the daylight illuminance

values from 100 to 500 lx, while daylight alone is sufficient for the illuminance levels from 500 to 2000 lx (Mardaljevic, 2006).

Methods In order to accomplish the research objectives, the following methodology is applied: .

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in the process of designing a new shading device: .

principles of optics are used to control daylight

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the simplest possible geometry for the slats is applied in order to achieve an economically viable and uncomplicated manufacturing process

.

transparent material and reflective coating are used for the slats

in the process of analysing the daylighting performance of the proposed transparent shading device: .

a case study is performed for the particular type of building at the specific location

.

daylight simulation is performed for the three shading device systems installed at the proposed building: a commercially available opaque system, a previously patented transparent system, and a new transparent system proposed by this research; the software Autodesk VIZ 4 TM (Autodesk, Inc., 2004) that calculates actual illuminance levels in the space as a result of the application of the specific types of the blinds in a proposed space is used

.

the results of daylighting simulation for the three types of blinds are compared to understand the performance and advantages of a new transparent shading device

Case study

In order to analyse the daylighting performance of the proposed new transparent shading device, a case study is conducted. The proposed office building used in the case study is located in Roanoke, Virginia, US (latitude 378, longitude 798), which has a moderate climate.

Figure 7 Perspective view of the o⁄ce building used in the case study. Source: authors. 153

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Olbina and Beliveau

Figure 8 Perspective view of the interior space of the o⁄ce building used in the case study. The window and facade are located on the left-hand side. Source: authors.

A simple office space, 18.3 m wide, 12.2 m deep, and 3.0 m high, is simulated for south and west facade orientations (Figure 7). The single-skin curtain wall has an area of 55.7 m2, with a transparent/window area of 29.3 m2. The window assembly has two parts: lower (1.41.5 m) and upper (1.40.5 m) (Figure 7). The window sill height is 0.9 m. The office space is divided into separate work spaces by cubicles, that is, by interior partitions 1.5 m high. Each cubicle contains an office desk, a chair, and a computer. Shelves and cabinets line the interior walls (Figure 8). The reflectance values of the interior surfaces and furniture are shown in Table 1.

oriented facade at 12.00, 14.00, and 16.00 hours). Simulations were conducted for three different sky conditions: clear, partly cloudy, and cloudy. The following shading device systems were tested: .

Commercially available opaque system: minivenetian blinds comprising slats with a curved cross-section were tested. The slat width is 25 mm and its thickness is 0.2 mm. The distance between the slats is 25 mm. The blinds are made of grey vinyl with a reflectance of 40% (value given by the manufacturer), and they are nontransparent for light. The blinds are in the horizontal position relative to the window and are installed on the inside of the window (Figure 9). The blinds are controlled manually by the occupants. The blinds are retractable. However, only the slat tilt angle is modified in this research. The slats’ tilt angle is adjusted based on sun and sky conditions and the occupant’s need for privacy. Three basic slat tilt angles are tested: completely open (08 tilt angle), nearly closed (458 tilt angle), and completely closed (908 tilt angle). A slat tilt angle is defined as an angle from the horizontal plane with the slats inclined downwards to provide a view of the ground from the interior space.

.

Previously patented transparent system: the shading device patented by US Patent 6,367,937 (Koster, 2002) is simulated as an example of a patented system (see the section titled ‘Previously patented transparent shading device systems’ and Figure 6). The slats are made of clear plastic with a transmittance of 100% and an index of refraction of 1.5. The blinds are simulated for a horizontal, fixed, and completely open position (08 tilt angle), and are installed in the cavity between two panes of glass.

Simulations are performed for one day each season (21 March, 21 June, and 21 December) and three times per day at two different orientations (south-oriented facade at 11.00, 12.00, and 15.00 hours; and west-

Table 1 Re£ectance values of the interior surfaces and furniture used in the case study Element

Material

Re£ectance (%)

Ceiling Walls Floor Columns Interior ¢nish of curtain wall Curtain wall mullions and transoms, window frame Interior doors Interior cubical partitions O⁄ce cabinets O⁄ce desks

Light grey paint Light grey paint 2 Green carpet White wood White wood Metal ^ bronze

84 55 42 77 77 48

Wood bass Beige fabric Wood burl oak Wood-oak plywood Wood bass Beige fabric

99 82 53 64

O⁄ce shelves O⁄ce chairs

154

99 82

Developing a transparent shading device as a daylighting system

Simulation engine

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The simulations were performed by using Autodesk VIZ 4 software (Autodesk, Inc., 2004). VIZ 4 calculates the illuminance levels as a result of the application of the specific type of shading device. It integrates two lighting algorithms in order to model lighting in the space: ray-tracing and radiosity. In VIZ 4, radiosity is used to render diffuse-to-diffuse inter-reflections, while ray-tracing is used for specular reflections. The user’s input includes the following data: .

three-dimensional geometry of the space and elements in the space

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materials for the interior and exterior finishes and furniture

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sun position and sky conditions

The sun angles (azimuth and altitude) as well as sun intensity depend on geographic location, date, and time. Autodesk VIZ 4 calculates daylight that derives from skylight that is scattered through the atmosphere.

Figure 9 Commercially available opaque shading device system used in the case study which consists of vinyl light-grey blinds. Source: authors.

In Autodesk VIZ 4 the sky is modelled as a dome of infinite radius placed around the scene. Daylight computes the illumination of a point in the scene with reference to all directions around the point where the sky is visible. The sky’s brightness is not constant over the sky dome, but rather it changes depending upon the position of the sun (Autodesk, Inc., 2004). The output of Autodesk VIZ 4 is a photorealistic threedimensional image of the space with illuminance levels defined by the range of colours. Pseudo Color Exposure Control as a lighting analysis tool maps illuminance values to pseudo-colours. Testing process

The testing process for the three analysed blind systems consists of the following tasks. Task 1: Making an input for the simulation

Figure 10 New transparent shading device proposed by the present research: three-dimensional view. The silver re£ective coating is applied on the hypotenuse of the slat. Source: authors.

.

New transparent system proposed by this research and further discussed in the section titled ‘Description of the new shading device system’ (also Figures 10 and 11).

Input for the simulation consists of independent, dependent, and shading device variables. The following independent variables are used in the simulation: date, time of day, building location, sun angle, sky conditions, building type, and site. Dependent variables used in the simulation are building spaces geometry, facade type, and window type. Shading device variables such as the slats’ geometry, width and thickness, distance between the slats, applied materials and coatings, and the slats’ tilt angle are also prepared as an input for the simulation. Task 2: Simulation of daylighting performance

Simulations of illuminance in the space are performed by using Autodesk VIZ 4 software. Input parameters 155

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prepared in the first step are used to test the daylighting performance of the three shading devices. Testing is repeated individually for each type of shading device to find out how the application of these shading devices in the building affects illuminance levels in the space. For each type of shading device multiple simulations are performed since the analysis is performed for different dates and different times per day, as well as for various sky conditions. If the blinds are adjustable (for example, the commercially available blinds and new blinds), then the shading device is tested for various blind tilt angles from completely open (08 tilt angle), to nearly closed (458 tilt angle), and finally to completely closed (908 tilt angle). This testing is helpful in establishing a control strategy for a shading device for different sun and sky conditions to maintain the required illuminance levels in the space. The following parameters are modified in the simulation process: .

time parameters: date and time of the day

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climate parameters: sun angle and sky conditions

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shading device variables: blind type (geometry and material) and slat tilt angle

The values of the remaining input variables are selected only once and remain constant in the simulation process.

Task 3: Presentation of the output of the simulations

The output of the computer simulation is the actual values of illuminance levels in the space as a result of the application of the specific shading device. In the third step of the testing process the output information is gathered, organized, and prepared for analysis. The output values of the illuminance are calculated for different combinations/values of the input variables. Since the output values are given in the format of the three-dimensional images, illuminance level measurements are read at the two sensor positions in the space from these images and recorded in the format of matrixes of the data. The two sensor positions are as follows (Figure 8): .

.

sensor positions 1 in the front of the room: the top of the office desk at a distance of 2.4 m from the facade sensor positions 2 in the back of the room: the top of the office desk at a distance of 8.7 m from the facade

The height of the top of the desks’ surfaces is 76 cm. The distance between the two sensor positions is 6.3 m. The required values of illuminance in this office space are determined based on the ANSI/IES and ISO 156

standards as well as the recommendations of Reinhart et al. (2006) and Nabil and Mardaljevic (2006). The required illuminance values range from a minimum of 500 lx to a maximum of 2000 lx. The actual illuminance values are then used to calculate the values of DA and UDI for each shading device. DA and UDI values are found for the two different facade orientations (south and west) and two sensor positions. An example of the calculated values of DA per year is presented in Table 6. DAs per year and UDIs per year for three types of blinds are compared to analyse the blinds’ performance and to understand the advantages of the proposed transparent shading device (Tables 7– 10).

Limitations This research was conducted only for a proposed office building located in Roanoke, Virginia, US. Only one type of shading device, that is, horizontal venetian blinds, was tested for climate conditions only on three days: 21 March, 21 June, and 21 December. The blinds were tested only for three times of the day: 11.00, 12.00, and 15.00 hours for a south orientation; and 12.00, 14.00, and 16.00 hours for a west orientation. The blinds were analysed only for two orientations: south and west. Only one view of the threedimensional space was drawn by using Autodesk VIZ 4, that is, only one fixed camera position at a task level was used in the simulation process. Only illuminance at the two sensor positions was measured by the simulations. Only two daylight performance parameters, DA and UDI, were calculated based on the illuminance values. The research does not analyse the occurrence of glare in the space. In this research, only theoretical testing of three blind systems, that is, computer simulation by Autodesk VIZ 4 software, was conducted. The objective of future research is to conduct experimental testing of the blinds to see how the experimental testing of the blinds corroborates with theoretical validation, that is, with the computer simulation of the blinds. The results and conclusions of this research were made on the basis of a small data set.

Results New shading device system

The result of accomplishing the first research objective was a new transparent shading device system. The main purpose of this system was to provide a sufficient amount and quality of daylight for the interior space. The principles of optics were used in the design of the new blinds. Light reflection, refraction, and total internal reflection in the glass prism were applied in the design of these blinds. The physical, thermal, and chemical properties of transparent materials, such as light transmission, refractive index, U-value,

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Developing a transparent shading device as a daylighting system

expansion coefficient, and maximum working temperature, were analysed when selecting the appropriate material for the blinds. The characteristics of the manufacturing process were investigated in order to produce the blinds simply and inexpensively. Transparent blinds, with a silver reflective coating applied on one surface of the blind, were designed and tested (Figure 10). These silvered blinds were developed to redirect incident light to the ceiling (Papamichael et al., 1994, cited in Selkowitz and Lee, 1998). The slat had a right triangular shape in cross-section. The hypotenuse dimension was 25 mm, the triangle legs were 18 mm, and the distance between the slats was 25 mm (Figure 11). The blinds were installed between two panes of glass. The blinds were designed for south orientation, thus, they were in a horizontal position relative to the window. However, in this research the blinds were also tested for a west orientation to investigate the opportunity of the application of blinds on west-oriented facades. The slats were made of clear plastic with a transmittance of 100% and an index of refraction of 1.5. A silver reflective coating, with a thickness of 0.5 mm and an internal reflectance (that is, the reflectivity for radiation hitting the coating from the plastic prism) of 94%, was applied on the hypotenuse outside surface. Daylight rays that strike the slats are first refracted through the plastic prism. Depending on the incident angle of the sun and tilt angle of the blinds, the refracted rays are additionally either reflected from

the silver coating to the upper part of the room – the ceiling – and from the ceiling to the depth of the room, or are reflected to the outside space (Figure 11). For example, in winter and at the 08 tilt angle, the sun’s rays are first refracted through the prism, then reflected from the silver coating and then refracted again and redirected to the interior space (Figure 11). Thus, the slats help improve daylight levels in the space. If the tilt angle of the slats changes to 908 in the winter, the sun’s rays are first refracted through the prism and then reflected either by the silver coating or by total internal reflection and redirected to the outside space (Figure 11). Therefore, the slats protect the interior from direct sun. The blinds are controlled automatically. They are not retractable and only the slat tilt angle can be modified. The control system adjusts the slats’ tilt angle based on the sun’s position, the solar irradiance on the facade, outside temperature and outside illuminance, the occupants’ presence in the room, and the occupants’ wishes for the inside illuminance levels, inside temperature, and a view to the outside. The blinds are activated by sensors that evaluate changes in ambient conditions and supply this information to the controllers for decision-making purposes. For example, the control system measures the actual illuminance level and computes the difference between the actual and desired illuminance to form a control action. Control action is initiated by the controllers and performed by an actuator, such as a DC motor.

Figure 11 New transparent shading device proposed by the present research: cross-section. The silver re£ective coating is applied on the hypotenuse of the slat. Light-re£ecting and -refracting situations are shown for three di¡erent slat tilt angles and for two sun angles of incidence. Source: authors. 157

Olbina and Beliveau

The DC motor adjusts the blinds’ position to drive the actual illuminance level back toward the set point. The user input for an automated control of the blinds includes the following parameters: the minimum and maximum work plane illuminance levels, the movement/adjustment frequency rate, and the degree to which the blinds can be opened and closed (DiBartolomeo et al., 1996). If the blinds are in the completely closed position, the direct view to the outside can be obstructed by the silver coating.

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The blinds were simulated for three slat tilt angles in this research (Figure 11): .

08 tilt angle (completely open blinds): the hypotenuse is parallel to the horizontal and the triangle legs are oriented upwards

.

458 tilt angle (nearly closed blinds): the angle between the hypotenuse and the horizontal is 458 and the triangle legs are oriented to the outside space

.

908 tilt angle (completely closed blinds): the angle between the hypotenuse and the horizontal is 908 and the triangle legs are oriented to the outside space

Simulation results

Table 2 presents one example of the matrix of data that includes actual values of illuminance in the space measured by simulations for the following:

.

three types of blinds (commercially available, previously patented, new)

.

one facade orientation (south)

.

a particular time (12.00 hours)

.

three sky conditions (clear, partly cloudy, cloudy)

.

two sensor positions in the space (sensor position 1: front of the room; and sensor position 2: back of the room)

.

optimized blind tilt angles, that is, the tilt angles that provide the best daylighting performance for the blinds, that is, illuminance values within the recommended range

Tables similar to Table 2 are created for the simulation results obtained for 11.00 and 15.00 hours for the south orientation. Table 3 presents the number of clear, partly cloudy, and cloudy days for each month that is simulated. These numbers are obtained from the weather statistical data for Roanoke, Virginia. The output results recorded in the matrixes of data (Table 2) and values from Table 3 are used to calculate daily, monthly, and annual values of DA and UDI. It is assumed that the user adjusts the tilt angles for the commercially available blinds and the automated control adjusted the tilt angle for the new blinds. Thus, the optimized tilt angles of the blinds, that is, the angles that provide the recommended illuminance levels at the

Table 2 Actual illuminances (lx) measured by simulations at 12.00 hours for the south orientation and optimized slat tilt angles Date

21 December

Sky conditions

Clear Partly cloudy Cloudy

21 March

Clear Partly cloudy Cloudy

21 June

Clear Partly cloudy Cloudy

158

Sensor position

1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back

Illuminance (lx) Commercially available blinds

Previously patented blinds

New blinds

600 650 850 700 450 400 1000 900 1300 1000 500 600 1300 1300 1300 1600 600 550

1600 850 1100 850 400 450 1600 900 1600 1000 550 500 1300 1300 1800 1700 500 550

1100 700 950 700 600 500 1300 900 1300 1100 700 550 1100 1000 1500 1700 1000 650

Developing a transparent shading device as a daylighting system

December, March, and June for the three types of the blinds and two sensor positions.

Table 3 Number of days per month for various sky conditions for Roanoke,VA, US Month

Sky conditions

Number of days

June

Clear Partly cloudy Cloudy Clear Partly cloudy Cloudy Clear Partly cloudy Cloudy

7 12 11 9 8 14 8 9 14

December

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March

DAs are calculated by using the following method. Any hour that exceeds the minimum required illuminance (500 lx) counts as 100% daylight, while any hour that does not provide a minimum illuminance (500 lx) counts as 0%. The daily, monthly, and annual values of DA for the south orientation are normalized as follows:

.

The daily values for DA are calculated as the average of the hourly values for DA. For example, Table 4 presents the daily values for DA for 21 December for the three different blinds, three sky conditions, and two sensor positions. Tables similar to Table 4 are also created for 21 March and 21 June. The monthly values for DA are calculated as the average of the daily values for DA, taking into consideration the number of clear, partly cloudy, and cloudy days for the specific month. For example, Table 5 shows the monthly values for DA for

Table 4 Daylight autonomy per day (%) (an example for 21 December) Blind type

Commercially available Previously patented New

Sensor position

1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back

The annual values for DA are calculated as the average of the monthly values for DA. Table 6 shows the annual values for DA for the three types of the blinds, two sensor positions, and south orientation. The monthly values of DA from Table 5 are used for the calculations of the annual DAs.

The daily, monthly, and annual values for DA for the west orientation are normalized in a same way as the values for DA for the south orientation.

sensor positions, are used in the calculations of DA and UDI. The blind systems are compared for these optimized slat tilt angles rather than the identical angles.

.

.

Daylight autonomy per day (%) on 21 December

UDI is calculated by using the following method. Any hour that falls in one of the following categories: insufficient (UDI,100 lx), achieved-supplementary (UDI 100 –500 lx), achieved-autonomous (UDI 500– 2000 lx), and exceeded (UDI . 2000 lx) accounts for 100% daylight for that particular category. For the remaining three categories the hour accounts for 0% daylight. For example, if the illuminance at the sensor position for a specific hour is 650 lx, it falls in the category UDI 500 – 2000 lx and that hour accounts for 100% daylight. For the categories UDI , 100, UDI 100 – 500, and UDI . 2000 lx, this specific hour counts for 0% daylight. The daily, monthly, and annual values of UDI are normalized in a same way as the DA values are normalized. These values of UDI are presented in the format of tables (similar to Tables 4– 6). Table 5 Daylight autonomy per month (%) Blind type

Commercially available Previously patented New

Sensor position

1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back

Daylight autonomy per month (%) December

March

June

70.0 55.0 45.3 55.0 100.0 70.0

70.0 70.0 70.0 70.0 100.0 100.0

100.0 100.0 87.7 100.0 100.0 100.0

Table 6 Annual daylight autonomy (%)

Clear sky

Partly cloudy

Cloudy

Blind type

Sensor position

Annual daylight autonomy (%)

100.0 100.0 66.7 100.0 100.0 100.0

100.0 100.0 100.0 100.0 100.0 100.0

33.3 0.0 0.0 0.0 100.0 33.3

Commercially available

1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back 1 ^ Front 2 ^ Back

80.0 75.0 67.7 75.0 100.0 90.0

Previously patented New

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Discussion

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The annual DA and annual UDI are compared for: .

the three types of blinds – to understand which blind seems to have the best daylighting performance

.

the two sensor positions in the room (for a specific blind type and facade orientation)

.

the two orientations (for a specific blind type and sensor position)

Values of the different daylighting performance parameters for the south orientation are shown in Table 7. The ratings of the three blind types for the south orientation according to the different daylighting performance parameters are shown in Table 8. The following criteria are used for the ratings. The higher the values for DA and UDI500 – 2000, the higher the rating of the blinds. The lower the values of UDI, 100, UDI100 – 500, and UDI. 2000, the higher the rating of the blinds. If a performance parameter leads to different ratings for the front and the back sensor position, the mean results for both sensor positions are compared. The results show that the new blinds (Figures 10 and 11) scored highest in all five daylighting performance parameters for the south orientation. The commercially available blinds (Figure 9) scored higher than the previously patented blinds (Figure 6) in four performance parameters. The new blinds provide sufficient daylight levels (DA) in the front of the room 100% of the time, that is, 20% more time than commercially available blinds and 32% more time than the previously patented blinds. The new blinds also provide the highest values of UDI500 – 2000 for 100% of the time in the front of the room, 15% more time than commercially available blinds, and 32% more time than the previously patented blinds. There is never too much daylight in the room as a result of the new blinds’ daylight control (that is, the UDI never exceeds 2000 lx). The new blinds do not provide useful daylight levels for 10% of the time (UDI100 – 500 ¼10%) during the year in the back of the room. A comparison of the daylighting performance parameters for the two sensor positions in the room (1 – front and 2 – back) with the south orientation shows that the new blinds and commercially available blinds have approximately 10% higher values for both DA and UDI500 – 2000 (that is, they provide a sufficient amount of daylight/useful daylight illuminance for a 10% longer period of time) for the front than for the back of the room. The previously patented blinds have 8% lower values of DA and 12% lower 160

values of UDI500 – 2000 in the front than in the back of the room. Values for the different daylighting performance parameters for the west orientation are shown in Table 9. The ratings of the three blind types for the west orientation according to the different daylighting performance parameters are shown in Table 10. The rating criteria used for the west orientation are the same as the rating criteria used for the south orientation. The results show that the new blinds also scored highest in all five performance parameters for the west orientation. The previously patented blinds scored higher than the commercially available blinds in four performance parameters. The new blinds provide sufficient daylight levels (DA) in the front of the room for 95% of the time, that is, approximately 20% longer than commercially available blinds and the previously patented blinds. The new blinds also provided the highest values of UDI500 – 2000 for almost 90% of the time in the front of the room; that is, approximately 15% more time than commercially available blinds and 20% more time than the previously patented blinds. The new blinds do not control daylight adequately for approximately 6– 7% of the time of the year when there is too much light in the room (the UDI is above 2000 lx levels). Also, the new blinds do not provide useful daylight levels 13% of the time of the year in the back of the room and 5% of the time in the front of the room. Comparison of the daylighting performance parameters for the two sensor positions in the room (1 – front and 2 – back) with the west orientation shows that the new blinds have approximately 10% higher values of both the DA and UDI500 – 2000 (that is, they provide a sufficient amount of daylight/useful daylight illuminance for a 10% longer period of time) for the front than for the back of the room. The commercially available blinds have 5% higher values of DA at the front of the room, while the value of their UDI500 – 2000 is 18% higher in the front than in the back of the room. The previously patented blinds have 6% lower values for DA and 2% lower values of UDI500 – 2000 in the front than in the back of the room. A comparison of the daylighting performance parameters for the two facade orientations shows that the new blinds and commercially available blinds perform better at the south than with the west orientation (considering the values for DA and UDI500 – 2000). For example, the new blinds have a 5% higher value for DA and a 10% higher value for UDI500 – 2000 for the south orientation than for the west orientation. This was expected because horizontal blinds perform better with the south orientation, while vertical blinds perform better with the west orientation.

Developing a transparent shading device as a daylighting system

Table 7 Values of the annual daylight performance parameters (%) for the south orientation Venetian blind type:

Commercially available

Previously patented

New

Sensor position

1 ^ Front

2 ^ Back

1 ^ Front

2 ^ Back

1 ^ Front

2 ^ Back

Daylight autonomy UDI, 100 UDI100 ^ 500 UDI500 ^ 2000 UDI. 2000

80.0 5.0 10.0 85.0 0.0

75.0 5.0 20.0 75.0 0.0

67.7 5.0 24.1 67.7 3.2

75.0 5.0 15.0 80.0 0.0

100.0 0.0 0.0 100.0 0.0

90.0 0.0 10.0 90.0 0.0

Note: UDI, useful daylight illuminance.

Table 8 Ratings of the three blind types for the south orientation

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Performance parameter

Rating First

Second

Third

Daylight autonomy UDI, 100

New New

UDI100 ^ 500 UDI500 ^ 2000 UDI. 2000

New New New/commercially available

Commercially available Commercially available/previously patented Commercially available Commercially available New/commercially available

Previously patented Commercially available/previously patented Previously patented Previously patented Previously patented

Note: UDI, useful daylight illuminance.

Table 9 Values of the annual daylight performance parameters (%) for the west orientation Venetian blind type:

Commercially available

Previously patented

New

Sensor point

1 ^ Front

2 ^ Back

1 ^ Front

2 ^ Back

1 ^ Front

2 ^ Back

Daylight autonomy UDI, 100 UDI100 ^ 500 UDI500 ^ 2000 UDI. 2000

74.8 10.0 15.2 74.8 0.0

69.8 10.0 20.2 56.7 13.1

75.9 5.0 19.1 69.8 6.1

82.1 5.0 12.9 72.2 9.9

95.0 5.0 0.0 89.2 5.8

87.1 5.0 7.9 79.8 7.3

Note: UDI, useful daylight illuminance.

Table 10 Ratings of the three blind types for the west orientation Performance parameter

Daylight autonomy UDI, 100 UDI100 ^ 500 UDI500 ^ 2000 UDI. 2000

Rating First

Second

Third

New New/previously patented New New New/commercially available

Previously patented New/previously patented Previously patented Previously patented New/commercially available

Commercially available Commercially available Commercially available Commercially available Previously patented

Note: UDI, useful daylight illuminance.

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The previously patented blinds seem to work better for the west orientation with the exception of the UDI500 – 2000 value at the back of the room, which is higher for the south orientation. Within the research limitations, and considering the five daylighting performance parameters, the new blinds seem to perform best when compared with commercially available blinds and previously patented blinds.

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Conclusions The research described in this paper has two major objectives: to design a new, transparent shading device; and to analyse its daylighting performance. The design of the new transparent shading device as a daylighting system is the most significant result of this research. The application of the triangular crosssection for the slats and the use of clear plastic and a silver reflective coating as the materials for the blinds utilize the principles of optics in the design. The blinds are adjustable and automatically controlled. The simple geometry of the blinds provides the opportunity for the uncomplicated and economically viable mass production of this shading device. A case study was performed to analyse the daylighting performance of a new, transparent shading device. Three different blind systems were simulated: a commercially available opaque (Figure 9), a transparent previously patented (Figure 6), and a new transparent (Figures 10 and 11). Their daylighting performance was compared to understand the behaviour of the new transparent blinds. Within the limitations of this research, the new transparent blinds seem to indicate a much better behaviour, that is, they provide higher values of daylight autonomy (DA) per year and achieved autonomous useful daylight illuminance (UDI500 – 2000) per year than both the commercially available blinds and the previously patented blinds. The new blinds provide sufficient daylight levels 100% of the time in the front of the room and 90% of the time in the back of the room simulated for a south orientation. There is never too much daylight in the room as a result of the application of the new blinds with the south orientation. The new blinds provide sufficient daylight for more time in the front of the room than in the back of the room, as expected. Also, the new blinds seem to have better daylighting performance at the south facade when compared with the west facade.

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