bring free light to buildings: overview of daylighting...

Bring free light to buildings: overview of daylighting system

Tzu-Yu Huang1, 2, Hong Hocheng1, Ta-Hsin Chou1, 2 and Wen-Hsien Yang2 1Department of Power Mechanical Engineering, National Tsing Hua University, No. 101, Sec. 2, Kuang-Fu Rd., Hsinchu,

Taiwan 30013, R.O.C. 2Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, 195, Sec. 4, Chung Hsing Rd.,

Chutung, Hsinchu, Taiwan 31040, R.O.C.

Many countries devote to energy saving and carbon reduction for the sustainable development. In particular, the artificial lighting accounts for a great portion of the total energy consumption of an office building. It is well known that daylight is an essential resource providing plenty of energy and is ubiquitously available. Applying suitable daylighting system to introduce sunlight into the buildings offers vast potential for conserving energy. This chapter provides an overview of the daylighting system using the advanced submicron technology for buildings. The approaches of active tracking and passive daylighting system for saving the energy of artificial lighting will be reviewed. The major content is the sunlight-redirecting microstructure based on optical prismatic pattern on transparent substrate by roll-to-roll UV imprinting process. To achieve the suitable prismatic microstructure, the complete technological aspects including the selection of materials, optical design and imprint process are introduced. To estimate the effectiveness of sunlight-redirecting film, the experimental results are demonstrated and compared with the optical simulation.

Keywords: daylighting; redirect sunlight; imprinting; roll-to-roll

1. Introduction

1.1 Energy conservation

Currently, many countries dedicate giant efforts to energy saving and carbon reduction due to the grave threat of greenhouse effect. Sustainable development becomes the most important worldwide task. To achieve this goal, the “Green Building” is one of the essential means. In fact, the artificial light accounts for a great part of the total energy consumption of a common office building. In an efficient office building, using high-efficient fluorescent lamps and lighting control, which switched off lamps when the lighting intensity exceeds the norm, can save about 80% of the energy consumed by artificial light. Figure 1 shows the energy consumption chart of an office building [1]. It is well known that daylight is an essential resource providing plenty of energy and ubiquitously available; moreover, it will not drain away in the foreseeable future. Applying suitable daylighting systems to introduce sunlight into the buildings for free has vast potential for conserving energy.

Fig. 1 Primary energy index for the “standard” and the “efficient” office building [1].

1.2 Daylight usage

Daylight is preferred above artificial lighting because it has perfect colour rendering and a positive psychological effect on the human being. Hence, many architects attempt to introduce daylight into buildings wherever practical. It is not only about providing the delightful and comfortable space but also saving electricity by decreasing artificial light usage. Daylight is composed of two components. First, direct sunlight beam has high intensity and direction which varies with solar elevation angle. Second, diffuse skylight scattering in the atmosphere.

Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________

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The luminous efficacy, the ratio of luminous flux (in lumens) to power (usually measured in watts), of daylight which includes direct sunlight, 90-117 lm/W, and diffuse skylight, 110-175 lm/W, is higher than artificial light, 13.6-120 lm/W. The daylight has the highest luminous efficacy even in an overcast day [2-4]. According to the observation of International Commission on Illumination (CIE), the illuminance of cloudy sky, 5,000-20,000 lux, is lower but 10-50 times of the indoor illuminance. It makes the provision of daylight into a room and reduction of artificial light. However, there are three factors always to be considered to provide good lighting: quantity, quality of light, and its distribution. Intense sources of light (sunlight or electric light) can lead to severe glare which can be both irritating and debilitating. For this reason, controlling the admission of sunlight into a space requires the careful design of openings in a building [5]. The optimum window design should integrate all function. It needs to limit the view opening of window for shielding from sunlight, private space, and glare reduction, but, it will decrease the efficiency of daylight usage. To solve the above conflict of problem, it has to allocate the predominant function to a window. The upper window, transom, is designed to be sunlight redirection area which can admit sunlight into the room to increase deeper in-room daylight illuminance by daylighting systems. The lower window is designed to be vision transmission area which can shield from glare of sunlight, ventilate the room, and provide outside sight. Table 1 shows the window function division of different daylighting systems [6]. Table 1 Window function division and comparison of different daylighting systems [6].

Natural Light Type Direct Sunlight Diffuse skylight

System Type Mirror Louver or Blind

Light Guiding Shelf/ Light Shelf for Redirection

Laser Cut Panel/ Prismatic Panel

Sun-directing Glass

Light Shelf

Sketch

Win

dow

Fun

ctio

ns

Upp

er W

indo

w

Daylight Redirection

Y Y Y Y Y

Sunlight Shielding

N N N N N

Vision Transmission

A Y D N Y

Glare Protection

Y Y Y Y Y

Low

er W

indo

w

Sunlight Shielding

Y Y D D Y

Glare Protection

A D D D D

Vision Transmission

A A A A A

Room Ventilation

D D D D D

Day

light

ing

Eff

icac

y

Illuminance of Deeper Room

D Y Y Y D

Saving Potential

D Y Y Y D

P.S. Y = Yes, D = Depend, A = Adjustable, N = No.

2. Daylighting system techniques

The components of daylighting system are glazing and some other elements that enhances the delivery or control of natural light into a room. To improve the performance of daylight usage, there are some solutions: providing usable daylight at greater depths from the window wall than that is possible with conventional designs; increasing usable daylight for climates with predominantly overcast skies; increasing usable daylight for very sunny climates where


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control of direct sunlight is required; increasing usable daylight for windows that are blocked by exterior obstructions and therefore have a restricted view of the sky; transporting usable daylight to windowless spaces [5]. The daylighting system can be classified into two, one is based on daylight condition, direct sunlight and diffuse skylight, which is shown in Table 2 [5]; the other one is based on system mechanism, namely tracking and passive. The following is a brief introduction of daylighting systems which are based on system mechanism.

2.1 Tracking systems

The control of tracking daylighting system is divided into two types, manual and automatic, that can adjust the glazing to optimize the quantity and quality of the incident natural light. The automatic daylighting system consists of a sensor, measuring incident flux, and a control system acting according to the sensor signal which is capable of tracing the daylight. It is easier to design optical element, but increasing the system cost and consumption of electricity.

2.1.1 Solar canopy illumination system

Rosemann et al. proposed a solar canopy illumination system which attaches to the building directly above the windows to alter the orientation of direct sunlight into the core area of a building [7]. The sunlight is distributed within the building through a series of light guide which delivering the sunlight to the building and also efficiently incorporating artificial light sources so that they can provide supplemental lighting as necessary, as shown in Fig. 2. The tracking portion, in Fig. 3, of the system is comprised of the “adaptive butterfly array”, an array of thin and roughly square mirrors, which is protected from the weather by canopy. It enables the use of relatively cheap and lightweight materials. Each mirror in the array is supported by three fibers, the first and second which can be moved to position the mirror in any orientation in each of two corners of the mirror, and the third in the center of the opposite edge, fixed.

Fig. 2 Schematic drawing of the light guide [7]. Fig. 3 Top view of the solar canopy system showing the concentrating and redirecting optics [7].

2.1.2 Micro mirror array system

Viereck et al. demonstrated an electro-statically actuated micro mirror array between the panes of a double glazing window. The micro mirror array is composed of millions of micro mirrors [8]. Figure 4 shows the basic functionality of system. Each window is divided into two parts and using an intelligent sensor system, therefore, any required illumination configuration can be obtained. This diversity is not possible with standard blinds. The vision through the mirrors in the open-position is nearly undisturbed because of the small size of the micro mirrors. In other mirror positions, the window appears like a more or less tinted window which the outside vision is maintained. Furthermore, implementation between the two panes protects the mirrors against wind, weather and defilement that present high mechanical stability and long lifetime of microstructures, and the system is expected to be maintenance-free for many years. The drawing and SEM-micrograph of micro mirrors is shown in Fig. 5.

Fig. 4 Schematic cross section of a window within housed Fig. 5 Drawing and SEM-micrograph of micro mirrors in half micro mirror arrays in different situations [8]. opened position (left) and closed position (right) [8].


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2.1.3 ASZEN System

Kinney et al. indicated the ASZEN system which intercepts beam sunshine by horizontal and vertical reflective elements [9]. Figure 6 shows that sets of elements are moved together, but the elements can be adjusted in relation to one another. A single microcontroller controls sets of front end ASZEN light-redirecting systems in a coordinated manner to achieve uniform and glare-free natural lighting environment by directing sunlight across a ceiling and onto the tops of the far sidewall is shown in Fig. 7. ASZEN system connects with the rectangular light pipe, composed of specular reflective material with a reflectivity of 0.98 in the visible, carries flux above dropped ceilings well within a building, 20 meters or more.

Fig. 6 ASZEN front end illustration [9]. Fig. 7 ASZEN system as ceiling washer [9].

2.2 Passive systems

The passive daylighting system redirects the daylight by designing the optical structure of the glazing. The optical design of the passive daylighting system is more difficult for suitable all-daylight condition, because the daylight varies constantly in intensity, colour and luminance distribution by solar elevation angle. But, it lessens the cost of system and electricity. Therefore, the most important thing of passive daylighting system is how to overcome the varying daylight conditions by fixed optical structure, and provide the stable, uniform, and comfortable daylighting environment.

2.2.1 Laser cut panel

Edmonds et al. brought out a laser cut panel which is a daylight deflection system [10]. Figure 8(a) shows the laser cut panel which is produced by making narrow parallel laser cuts in a sheet of clear acrylic. Each cut surface deflects the daylight by refraction and total internal reflection is shown in Fig. 8(b). The orientation and deflective fraction, f, are determined by incidence angle, i, practical ratio of width to height, W/D, of rectangle ribs, and oblique angle of the panel. An advantage of the panel is that between the cuts, an outside vision of the window is maintained, while it is slightly distorted. On the other hand, the long fabrication time is its drawback.

(a) (b)

Fig. 8 Schematic laser-cut panel (a) cross section, (b) the deflection of daylight [10].

2.2.2 Sun-directing glass and microstructured components

Mueller demonstrated the sun-directing glass [11]. LUMITOP® is a double-glazing sealed vertically stacks of opaque concave acrylic elements [12]. It can redirect the direct sunlight incident angles from 15° to 65°, which represent the conditions in Middle Europe, toward the room ceiling [13, 14]. To avoid the obstruction of outside vision, the glazing is normally mounted above the view window. The sinusoidal surface of the acrylic elements could spread the outgoing light within a narrow horizontal, azimuthal angle, as shown in Fig. 9. These elements are well suited for installation between the double-pane window which protects the elements from contamination and weather [5]. The sun-directing glass has heavy weight due to its stacks of acrylic sinusoidal elements. Mueller et al. provided the new design of light-directing element in 2012 [13, 14]. They replaced the stacks of acrylic elements by acrylic prismatic microstructured system which had one third of thickness and weight of the LUMITOP. It leads to a saving of about 8 kg per m2. They used ZEMAX simulation software to design the micro optic structure. The


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one-sided prism or double-sided prism-prism combination could not achieve at the sunlight incident angles from 15° to 65°. Using the lens-like profile on the sun-facing panel side performs quite well. The angle of the daylight is widened considerably which will be redirected. By using lenses, the amount of glare is reduced to a negligible quantity.

Fig. 9 Work principle schema of sun-directing element [13]. Fig. 10 Simulation result of double-sided microstructured system [13].

3. Prismatic sunlight-redirecting microstructure

Prismatic daylighting system is to redirect the sunlight by refraction and total internal reflection, and is usually made of clear acrylic which is used in temperate climates. There are many applications of prismatic system, in fixed or sun-tracking arrangements. Its design may redirect a part of the direct sunlight downwards, causing glare. However with a correct profile and seasonal tilting, these downward beams can be avoided [5]. The authors designed the prismatic sunlight-redirecting microstructure capable for guiding sunlight into building deeper room which is a formed microstructure on transparent substrate by roll-to-roll process introduced hereafter. Since the sunlight-redirecting microstructure would disturb the outward vision due to the microstructures on the substrate, it is assembled on the transom of the building.

3.1. Inorganic-organic materials

Sunlight redirecting elements are commonly made of glass, thermoplastic polymer, or UV curing resin. However, it is difficult to emboss the microstructure onto a glass, and the weather resistance of the thermoplastic polymer and the UV curing resin are poor [15-17]. The inorganic-organic material is of interest. The inorganic-organic materials have high transmittance in the visible region. It consists with advantages of inorganic (weather resistance and wear resistance) and organic (flexibility) which is advantageous for fabricating micro optical devices [18-21]. A comparison of sunlight guide panel materials is listed in Table 2 [22]. Table 2 Comparison of sunlight-redirecting element materials [22].

Material PMMA Glass Inorganic-Organic Hybrid Material

Method of Patterning Injection Molding Hot-embossing Roll-to-roll Imprinting Processing Temperature 200 °C 600 °C 150 °C Hardness 4H >7H >6H Weather Resistance Worse Good Good Weight Light Heavy Moderate Microstructure Fabricability Good Worse Good Fabrication Cost Low High Low Material Cost Low Low High Large Area Production Moderate Difficult Easy

3.2. Optical Design and Analysis

Two types of polygonal prism to alter the direction of direct sunlight toward the room ceiling will be introduced to increase the brightness in the core of the building. The first type is the quadrangle, the second type is the inclined-curved complex. The parameters of type 1 includes top feature at 45° and pitch, height of 50 μm is shown in Fig 12(a). The features of type 2 include 18 μm-pitch and 21 μm-height. When sunlight beams shed on the prism surface, they refract at the interface between air and prism; then, the sunlight within the microstructure will be internal-reflected by the prism slopes. Finally, the sunlight through the microstructure at the right side surface, as shown in Fig. 12(b), is


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refracted toward the ceiling that becomes the outgoing light. There are many possible optical paths of sunlight. The optical path can be calculated by refraction equation, Snell’s law (n1Sinθ1 = n2Sinθ2), and reflection equation (θi = θr). For instance, the outgoing angle (θ5) of optical path condition in Fig. 11(b) is calculated as following Eq. (1-5). The type 1 prism can lead the high intensity sunlight into the deeper room, but cause harmful glare due to all inclined surfaces. And, the daylighting area by redirected sunlight will drift with the varying sunlight elevation angle during the day. (1) sin sin (2) 180° (3) (4) sin sin (5)

(a) (b)

Fig. 11 (a) Parameters and (b) optical paths of type 1 quadrangle prism. The modified prism, type 2, also includs features of two angles,A1 and A2, two heights, h and H, pitch P and curvature radius R which are shown in Fig. 12. The curved plane can diffuse the sunlight into a wide-angle spread of the outgoing sunlight which provides uniform redirected sunlight with stable daylighting area in a room, shown in Fig. 13. The range of outgoing angle is affected by A1, A2 and R. Furthermore, it influences the daylighting area and uniformity of daylighting. The design equation of type 2 is showed blow, 1 cot (6)

which H P cot A π cot A H cot A A π (7)

Fig. 12 The feature sizes of type 2 inclined-curved complex prism. An optical software TracePro® was used to simulate the outgoing angles of different elevation angle sunlight through the polygonal prisms in this study. The light source was assumed to be composed of parallel rays (see Fig. 14) because the sun is a far-flung star. The effective outgoing angle fell into the range of 90°-180°, from horizontal level of transom to room ceiling; thus, sunlight was redirected to the indoor ceiling by the polygonal prisms.


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(a)

(b)

Fig. 13 The possible optical paths of type 2 at (a) same and (b) different solar elevation angles.

Fig. 14 Simulation model of sunlight through the polygonal prism [22].

Fig. 15 Procedure for fabrication of the sunlight guide film by roll-to-roll process [27]

3.3 Roll-to-roll fabrication process

The planar UV-imprinting process can fabricate the sunlight-redirecting microstructure on glass and polyethylene terephthalate (PET) film to form the sunlight-redirecting panel and sunlight-redirecting film, respectively. To improve the productivity and throughput, the roll-to-roll technology was used to fabricate flexible optical element extensively [23-26]. Figure 15 shows the procedure for fabricating the sunlight-redirecting film of type 2 by this method. The roll


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mold was fabricated by diamond turning lathe, and used for UV cured embossing process. In embossing, the UV-curable resin was coated on PET film first, then the PET film was transferred by 40 N of tension and 3m/min of speed. Finally, the resin was embossed by roll mold and cured by UV light, and the power of light was 120 W/cm2. Figure 16 shows the SEM images of type 2 microsturcture. A reel of sunlight-redirecting film is shown in Fig. 17. Figure 18 shows the lamination of sunlight-redirecting film with adhesive layer which can easily install the sunlight-redirecting film for user [27].

(a) (b)

Fig. 16 SEM images of type 2 microstructure (a) top view and (b) cross-section view [27].

Fig. 17 Photograph of a reel of sunlight-redirecting film [27]. Fig. 18 Structure of sunlight-redirecting film [27].

3.4 Optical performance verification

The optical performance of type 1 and type 2 sunlight-redirecting microstructures were measured by the equipment that included light source, photo detector, and rotators, shown in Fig. 19 [27]. The experiments measured the outgoing light intensity at different solar elevation angles. The outgoing intensity at each angle was divided by the maximum outgoing intensity at different solar elevation angles. Figure 20 and figure 21 show a comparison of the outgoing light between the experimental and the simulation results of type 1 [22] and type 2 microstructure [27]. The outgoing angle of light with the maximum intensity at different solar elevation angles was above 90°. The results show that adequate indoor lighting by sunlight can be provided by these sunlight-redirecting microstructures.

Fig. 19 Optical measurement [27].


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(a) (b)

Fig. 20 Comparison of the experimental and the simulation results of type 1 at solar elevation of: (a) 40° and (b) 60° [22].

(a) (b)

Fig. 21 Comparison of the experimental and the simulation results of type 2 at solar elevation of: (a) 40° and (b) 60° [27].

4. Conclusions

The shortage of petrochemical energy and the increased greenhouse effect have caused serious concerns in human life. How to extend the energy supplies by energy conservation and deal with the greenhouse effect by carbon reduction is the important worldwide tasks. The artificial light consumes the greater part energy of an office building. Applying suitable daylighting system to introduce the natural and plenty of free-charge light is good alternative for conserving energy consumed by the artificial light. The daylighting system can be classified by active tracking and passive mechanisms. The former is capable of tracking the daylight and easier to design the optical parts, and avoiding the glare condition. But it increases the cost due to extra sensor, control system, etc., moreover, consumes additional electricity. The passive system redirects the daylight by refraction and reflection of the designed optical units. The optical design for all daylighting conditions of the passive system is more difficult because the daylight varies in the daytime with the solar elevation angle. Nevertheless, it lowers the cost, electricity consumption and is easier to use than the tracking one. The design method and fabrication of micro-prismatic structures are introduced in this chpater. It can further reduce the cost of daylighting package due to its miniaturization. The roll-to-roll process improves the productivity and promotes the commercialized use. To sustain the sunlight-redirecting in the outdoor application, the inorganic-organic materials with good weather resistance, wear resistance and flexibility were used. The optical measurement of these microstructures shows that the outgoing light was mainly above 90° at different solar elevation angles. It demonstrates that sufficient indoor lighting by the redirected sunlight can be provided by the proposed sunlight-redirecting microstructure. A significant energy saving can be achieved.

Acknowledgements The support by National Science Council and the Mechanical and Systems Research Laboratories of the Industrial Technology Research Institute of Taiwan are gratefully acknowledged.

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bring free light to buildings: overview of daylighting...

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