Development of Optical Light Pipes for Office Spaces
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1 Development of Optical Light Pipes for Office Spaces Liliana O. Beltrán, Ph.D., Betina Martins Mogo, M.Sc. College of Architecture, Texas A&M University, College Station, Texas, 77843, USA ABSTRACT: This paper presents the preliminary findings of the daylighting performance of an optical light pipe system placed in the ceiling plenum of a deep plan office space (1m x 6m x 3m). The south-facing light pipe system can efficiently deliver daylight at the back of the space; between 5m to 1m from window plane for locations between latitudes 24-5ºN-S under clear sky conditions. Two identical large scale models (1:4) were used to assess the daylight performance of the light pipe over long-term periods. Results showed that the light pipe can introduce adequate light levels for visual tasks in office environments in climates with predominantly clear sky conditions. Light levels of 3-1,5 lux can be achieved at the back of the space (24 ft from window wall) for solar azimuths 6º East and West of true South (Northern Hemisphere) or true North (Southern Hemisphere) under clear sky conditions. Keywords: daylighting, light pipe, optical systems, deep plan, scale models 1. INTRODUCTION In recent years, the interest in daylighting has grown due mainly to its potential to save energy, and to provide healthy environments for occupants. The use of daylight is an efficient strategy to offset artificial illumination and to create more visually amenable spaces, in spite of its design challenges. However, in multi-story commercial buildings with deep open plan configurations (3-4 ft.), daylight coming from the perimeter only reaches the first 15 ft. from the window plane. Beyond this area, daylighting levels decrease abruptly to levels below the recommended ones for office task lighting. This paper intends to demonstrate that a passive horizontal light pipe can be able to provide adequate light levels at the back of a space under clear and partly clear sky conditions without introducing additional solar heat gains to the building. 2. LIGHT PIPE 2.1 Background Light pipes are not new as a concept; we can trace their origins to ancient Egypt where daylight was introduced into massive structures through reflective shafts covered with thin layers of gold. In recent years light pipes have been explored because of their potential to introduce daylight further into the building core. One of the first developments of a passive horizontal light pipe suitable for deep plan office buildings was developed by LBNL [1]. In this study, the light pipe was shown as a promising daylighting technology for new and existing multi-story buildings in climates with predominantly sunny skies. Other researchers adapted these light pipes to locations at low latitudes (3ºN, 14ºN) [2, 3], where the light pipes were oriented to face the sun towards the East or West limiting the light pipes daylight performance. Another daylighting system was developed using anidolic (non imaging) elements to collect light rays from the sky and redirect the emitted light in a 6 m room. This anidolic ceiling is suitable for locations with predominantly overcast skies, This anidolic ceiling is suitable for locations with predominantly overcast skies due to the high illuminance measured in experimental test room (>1 Klux). For locations with sunny skies, the source of light may become a source of glare for occupants and may require solar protection systems [4]. A daylight system, that has been extensively studied in detail for single story and top floor spaces [5] as well as for multi-story buildings, is the vertical light pipe which uses active systems to redirect sunlight to the lower levels [6, 7]. Other light pipes integrate electric lighting, as backup lighting along with heliostats and tracking mirrors to efficiently redirect sunlight at higher costs [8, 9]. The horizontal light pipes proposed in this study are suitable for south-facing facades in latitudes between 24º-5ºN/S in predominantly sunny sky locations. Because sunlight is beamed into the space through the horizontal shaft, the central and the side reflectors are designed to redirect sun rays from oblique sun angles (early morning or late afternoon). 2.2 The Site The experimental setup, is placed in a hot and humid location at latitude 3 36 N. The site is located on the roof of the four-story Architecture building in our campus, and consists of two identical large scale models (reference and test models) that represent typical office spaces of the region (Figure 1). The surrounding obstructions to both windows of the experimental setup were checked with a fish-eye lens and sun path diagrams. Figure 2 shows that the models will be shaded early in the mornings until 8:am in winter and until 7:3am in summer, and in the afternoons after 4:3pm in summer.
2 of the light pipe consists of a 15 ft. long diffuser located at the ceiling plane with a translucent Mylar polyester film with a VT of 7%. Eq 6 SS 83 Figure 1: South side of experimental setup: test model (left) and reference model (right). WS 36 Cut-off angle Glazing " " 31 2 " 6" " 2" Summer Reflector Equinoxe Reflector Winter Reflector Figure 3: Section of light pipe aperture with angles of central reflectors. Figure 2: Fish-eye view of site obstructions of light pipe test model. In our location, the annual percentage of sunny and partly cloudy days is 81% based on the year 23 (see Table 1). Table 1: Cloudiness for Brazos County, TX, for the year 23 (CL=Clear, PC=Partly Cloudy, CD=Cloudy) A Equinoxes 4 Summer Solstice 78 B Cloudiness Mean Number of Days Annual (days) Annual (%) (Clear, Partly Cloudy, Cloudy) CL PC CD CL+PC Brazos County, TX % 2.3 Light Pipe Prototype The light pipe has a trapezoidal shape in plan and has been tapered in its longitudinal section towards the rear of the room [1]. The pipe is coated with a 95% specular reflective film. The light pipe was designed to capture, transport and distribute daylight over the last 15 ft of the room. Sunlight is captured through a small glazing area (visible transmittance, VT of 88%) of 5.5 ft 2 ; which accounts for a Window Floor Ratio (WFR) of less than 1% and a Window Wall Ratio (WWR) of less than 2%, compare to a standard sidelight window for this region which has a WFR of 24% and a WWR of 5%. Sun rays are redirected by a reflector system, which includes a central and two side reflectors to improve collimation of incoming sun rays and to reduce the number of interior reflections within the pipe section (see Figures 3 and 4). The distribution component at the back end Section A-B Figure 4: Floor plan and section of light pipe aperture with angles of side reflectors. 3. METHODOLOGY 3.1 Scale model photometry The experimental setup consists of two scale models that simulate south-facing deep open plan office space of 1 ft high, 2 ft wide and 3 ft long (6 ft 2 ). The models were constructed at scale 1:4; the reference model (RM) introduced daylight only through sidelight windows, while the test model (TM) received additional daylight through the horizontal light pipe. In both models the interior surface reflectance were.8 for the ceiling,.47 for the walls,.23 for the floor, and.34 for the office furniture and photometric sensor holders. The windows represent
3 double-pane spectrally selective low-e glazing with closed white Venetian blinds, with visible transmittance (VT) of 77% and 2% respectively; with an overall VT of 15% (Overall VT = Glass VT x Blinds VT). The models use clear glass (VT=88%) and three layers of diffusing white paper; with an overall VT of 14%. In order to have visual access to the models, three viewports at eye level (5 ft) were provided on each scale model: one at the back (north-facing wall) and two on the side (east-facing wall). The models did not include shading devices. A comparative daylighting analysis of the two scale models was done [11]. The daylight performance of the light pipe was assessed both quantitatively (illuminance and luminance levels) and qualitatively (visual inspection and photography). Workplane illuminance measurements were taken at twelve interior reference points in each scale model, and four exterior reference points (Figure 5). Three parallel lines of four cosine- and color corrected LI-COR photometric sensors (LI-21SA) were placed to measure illuminance levels every minute. Sensors were placed at the height of 28", at equal distances (6 ft. to 24 ft.) from the window wall, at the centerline, and 5 ft. on either side of the centerline. Outside the model, four sensors were placed in pairs to take global and diffuse illuminance. One pair of sensors was mounted on a post next to the models to take vertical and horizontal global illuminance. The other pair was positioned under a static shadow band to measure vertical and horizontal diffuse illuminance (no direct sun). To evaluate the light contribution of the light pipe by itself, the lower window of the test model was covered at different times with a black cloth. These tests enabled us to understand the efficiency of the system, visualize the amount of sunlight redirection, and detect the presence of specular reflections due to the reflective and diffusing films. Time-lapse sequences were recorded to observe the variations of sun penetration throughout the day of the light pipe without diffusers. Figure 5: Arrangement of sensors inside and outside both models. 3.2 Visual Assessment Glare, contrast, and visual comfort were assessed by photographic documentation (Figure 6), luminance measurements, High Dynamic Range (HDR) images, and visual observation. A survey to nineteen participants was conducted on February 3 and 4, to determine the effectiveness of the light pipe system. The procedure involved observation of the interior of the two models (reference model followed by the test model) through the lateral viewports, and answering a questionnaire. Figure 6: Interior view of test model taken from North wall. 4. RESULTS 4.1 Quantitative Assessment As expected, illuminance measurements in the test model with the light pipe under clear and partly cloudy sky conditions are much higher than overcast or partly cloudy sky conditions. Table 2 shows that illuminance levels between 9:am and 3:pm (more than 6 hours) during clear days is over 3 lux. Illuminance levels due to light pipe and lower window (>1,1 lux) doubles the amount introduced by the lower window only (55) around noon hours at the back of the space (Sensor 4), as depicted in Figures 7 and 9. Daylight is redirected to the back of the space between solar azimuths 12º and 24º. The illuminance levels at 18 ft and 24 ft (Sensors 3 and 4) are fairly similar under sunny and overcast conditions (Figures 7 and 8), which gives an indication of light uniformity in this area. Figure 1 shows workplane illuminance variations in TM throughout nine days under clear and partly cloudy sky conditions. Notice that illuminance values can reach over 1, lux between 1:am and 2:pm. Figure 11 displays the daylight distribution due to the light pipe only in the TM (without the contribution of the lower window). Light levels are distributed mainly at the back of the space reaching over 9 lux under the pipe and over 3 lux along both sides at 5 ft away from it. The efficiency (ε) of the light pipe is defined as the ratio between the illuminance inside the model equipped with the light pipe and a reference model without light pipe. The overall efficiency of the light pipe at the back of the space (18 ft-24 ft) varied from to 2.4 times the amount of daylight provided by a sidelight window with blinds (Figures 12 to 15). The light pipe is a highly efficient daylighting system, considering that the WWR of the light pipe is less than 2% and the WWR of the lower window is 5%.
4 Adding more light pipes to the space will not affect the overall solar heat gains, since the glazing area of the light pipe is less than 4% of the sidelight window glass. On the other hand it can introduce higher and more uniform illuminance levels throughout the space. Under overcast conditions, the daylight factors (DF) at the back of the space in the TM is increased by 8-98% compared to the RM (see Figure 16). Table 2: Workplane illuminance (lux) at 24 ft with light pipe and lower window contribution Light pipe Jan. 25 Feb. 14 8:am - 4:pm :am - 3:pm :am - 2:pm :am - 1:pm :pm Illuminance (lux) TM - Sensor 4 RM - Sensor 4 TM - Sensor 3 RM - Sensor 3 Interior illuminance / Exterior Vertical Illuminance RM 4 TM Hours Figure 9: Interior illuminance over exterior global vertical illuminance ratio at 24 ft in RM and TM on a sunny day (Feb.14). Illuminance (Lux) Hours Julian Day 116 Julian Day 117 Julian Day 118 Julian Day 121 Julian Day 45 Julian Day 35 Julian Day 25 Julian Day 26 Julian Day 89 Figure 1: Workplane illuminance (lux) in TM at 24 ft from window wall, under clear and partly cloudy sky conditions. Figure 7: Workplane illuminance (lux) in reference model (RM) and test model (TM) at the back of the space against solar azimuth on a sunny day (Feb.14). LIGHT PIPE with MYLAR (t = 82%), Feb 14, 12: pm (solar time) Ext Glob Horizontal 947 lux 25 TM - Sensor 4 RM - Sensor 4 TM - Sensor 3 RM - Sensor 3 Illuminance (lux) Figure 8: Workplane illuminance (lux) in RM and TM at the back of the space against solar azimuth on an overcast day (Feb. 2). 6' 12' From Window 38 18' 78 24' ' ' Width = 2 ft 15' Figure 11: Workplane illuminance distribution across the space with light pipe only, without lower window contribution in Test model on Feb. 14 at 12:pm (solar time). Illuminance (Lux)
5 2. 2. Efficiency 1.5 Efficacy Figure 12: Efficiency of light pipe TM compared to RM against solar azimuth on a sunny day (Feb.14).. Figure 15: Efficiency of light pipe TM compared to RM against solar azimuth on a variable sunny day (Feb.4) % Reference Model Test Model Efficiency Daylight Factor (%) % +8% +98% Figure 13: Efficiency of light pipe TM compared to RM against solar azimuth on an overcast day (Feb.2). Efficacy Figure 14: Efficiency of light pipe TM compared to RM against solar azimuth on a partly cloudy day (Feb.3) Sensors Figure 16: Average daylight factor (DF) in reference and test model along centre line under overcast sky conditions. 4.2 Qualitative Assessment Figure 17 is a time lapse sequence of images taken in the TM on March 8th. Notice that early in the morning half of the side walls, as well as the sensors #3 and #4 (at 18 ft and at 24 ft respectively), are wellilluminated. Towards noon hours, the light starts to concentrate directly below the light pipe, and the sun patch gets brighter on the back wall and recedes from the side walls. Consequently, the sensors at 24 ft get more light as opposed to the sensors at 18 ft that enter in a penumbra. At noon, the sun patch is at the center of the back wall; hence, the light reflected from it reaches the sensors in the middle row, especially the one at 24 ft.
6 Figure 19: Responses to question: What is your impression of the overall lighting conditions? Figure 17: Time lapse sequence from 9:3am to 12:pm in TM, on March 8. Figure 18 depicts luminance ratios inside the test model. The luminance ratio between the desk and one side wall, and the ratio between a VDT screen and the light pipe, are 1:1.4 and 1:8 respectively. These values are within the recommended lighting standards for office spaces [12]. The luminance ratio between the desk and the window (with closed blinds) is 1:24.4; which is over the maximum recommended luminance ratios. 5. CONCLUSION Results from this preliminary study have demonstrated that the light pipe is an effective daylighting system to provide lighting levels of more than 3 lux at the back of a deep plan space (15 ft to 35 ft) for more than six hours. The efficacy (ε) of the light pipe around noon hours is over 2. while in the mornings is over. The light pipe will not be introducing additional solar heat gains to the space due to its small area (WWR >2%). As opposed to other remote lighting systems the light pipe is designed to integrate with sidelight windows a daylighting system that provides natural light to deep spaces (>3 ft) and creates a more uniform and comfortable visual space for occupants in multi-story buildings. At the moment new studies are undertaken to optimize the current geometry of the light pipe using advanced ray-tracing software; and to integrate the light pipe with energy efficient electric light sources and daylight controls, and other optical daylighting systems, i.e. light shelves, automated light reflectors, and external shading devices. Currently, a final report is being prepared summarizing the findings of the annual performance of the light pipe. ACKNOWLEDGEMENT Figure 18: Luminance ratios in test model between a VDT screen (9 cd/m 2 ) and three reference points, March 1, 1:pm. Results from survey showed a preference from the participants towards the test model: 95% found satisfactory the overall lighting, 86% found satisfactory the light at the back of the space, nobody complained about glare due to the light pipe, and 1% expressed their preference to work in a similar office to the test model. On the other side, 95% said that they would turn the electric lights on, if they would have to work in an office space similar to the reference model. The first impression about the overall lighting conditions of the models was 95% satisfactory for the space with the light pipe, while for the reference space 58% of the respondents agreed that it was too dim (Figure 19). The authors wish to thank the College Research Interdisciplinary Council of the College of Architecture at Texas A&M University for funding this study and to the many colleagues at the College of Architecture for their valuable input. REFERENCES [1] L. Beltrán, E. Lee, K. Papamichael and S. Selkowitz, Proc. ASES 1994, San Jose, California, (1994). [2] S. Chirarattananon, S. Chedsiri, and L. Renshen, Solar Energy 69-4, (2) [3] V. Garcia, and I. Edmonds, Proc. ISES Solar World Congress, Goteborg, Sweden (23). [4] G. Courret, J. Scartezzini, D. Franzioli and J. Meyer, Energy and Buildings, 28 (1998) [5] L. Shao, A. Elmualim and I. Yohannes, Light. Res. & Technology, 3, 1 (1998)
7 [6] L. Whitehead, J. Scott, B. Lee and B. York, Proc. Daylighting Conference (1986). [7] A. Mingozzi, S. Bottiglioni and R. Casalone, Proc. Lux Europa (21). [8] A. Rosemann, M. Mossman and L. Whitehead, Proc. ASES 26, Denver, Colorado (26). [9] M.V. Lapsa, D.L. Beshears, L.C. Maxey and C.D. Ward, Proc. ASES 27, Cleveland, Ohio (27). [1] L. Beltrán, E. Lee and S. Selkowitz, Journal of the Illuminating Engineering Society, 26, 2, (1997) [11] B. Martins Mogo, MS Thesis, Texas A&M University, (25). [12] Illuminating Engineering Society of North America IESNA, Lighting Handbook 9th ed. New York, (2).
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