Daylight availability in an office interior due to various fenestration options

Transcription

1 436 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Daylight availability in an office interior due to various fenestration options N. Ibrahim, A. Zain-Ahmed Universiti Teknologi MARA, Malaysia ABSTRACT Adoption of daylight as an energy efficiency strategy is especially relevant for a climate like Malaysia, as the country experiences long sunshine hours throughout the year. An important aspect of effective daylight strategy is the performance of the window in permitting daylight entry. This paper presents a study conducted to account daylight availability in an office interior under various fenestration options; namely window-wallratio (WWR), glass types and application of external shading devices of varying depth. The study used IES RADIANCE software and the Daylight Factor method to analyse its outcome. Daylight Factor (DF) was used as the basis to estimate the percentage of daylit floor area of a generic 4.5m deep office cell. The optimum WWR to achieve the maximum percentage of daylit floor area for a 4.5m deep room are 40%, 55% and 65% for clear, tinted and reflective glass, respectively. The results from the daylight availability study also shows that application of simple horizontal shades does not significantly affect the amount of daylight indoor. In fact, application of external shades improves the daylight distribution whereby the illuminance intensity in areas closer to the windows were reduced, providing a gentle daylit indoor. 1. INTRODUCTION Lighting is essential for the effective performance of visual tasks. To meet the energy efficiency challenge the common view is to utilise daylight as much as possible to minimise electricity consumption due to lighting power and generated cooling load due to articial lighting system. As a source of light, daylighting offers greater benefits over artificial lighting. It is natural and has the range of frequencies needed for the mind, body and soul. The luminous efficacy of daylight in Malaysia is excellent and could meet most of the required luminance during the day (Zain-Ahmed et al. 2002). This would result in significant savings in terms of energy consumption. The objective of the study is to estimate the impact of varying basic envelope design characteristics on daylight availability. 2. THE STUDY CONTEXT Malaysia is situated slightly north of the equator in Southeast Asia. Malaysia lies entirely in the equatorial zone, between latitude 1º to 7ºN and between longitude 100º to 120ºE. The climate is characterised by high temperature and humidity throughout the year. In terms of daylight, Malaysia receives plenty of natural light throughout the year. The Malaysian sky condition is classified as intermediate or average, whereby 85.7% of the time the sky is cloudy and 14.0% overcast (Zain-Ahmed 2000). Figures from energy audits (Suruhanjaya Tenaga 2005) and energy surveys conducted on offices in the country indicate electricity use in commercial buildings to be primarily attributed to air conditioning, artificial lighting and office equipment, with 52-60%, 18-42% and about 22% of the total consumption respectively. 3. ISSUES AFFECTING AMOUNT OF DAYLIGHT AVAILABILITY 3.1 Visibility The visibility aspect of lighting determines how much light is needed to perform a particular task. Its measurement is referred to as illuminance level, expressed in lux or footcandles (fc). Numerous studies by key institutions such as the IESNA (the Illuminating Engineering Society of North America) and CIBSE (Chartered Institute of Building Service Engineers) have resulted in illuminance recommendations (Institute for research in construction 2003). For offices, the IESNA recommends illuminance levels at 500 lux on the horizontal workplane. In Malaysia the recommended illuminance for an office environment is lux (Department of Standards 2001). When daylight is considered, the most commonly used illuminance measurement is the Daylight Factor (DF) (Moore 1991). This method requires the use of the CIE overcast sky model. The DF remains fairly constant for a given time of day and sky condition regardless of the degree of changes that may be affecting the absolute level of light available outdoor (Schiler & Japee 2001). This characteristic makes it practical and therefore used in many daylight studies. For Malaysian conditions researchers suggest the use of the CIBSE criteria to assess daylight performance (Denan PALENC Vol 1.indd 436 3/9/2007 1:25:12 µµ

2 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and ; Ibrahim 2006; Zain-Ahmed 2000), taking the 5% DF as the minimum illumination level for the working plan. 3.2 Visible transmittance (Tvis) The capacity of glass to admit daylight is expressed as visible transmittance, Tvis. It represents the fraction of the incident amount of light in the visible spectrum, at the normal incidence angle that passes through the glazing. This means that higher Tvis value will result in better daylight quantities indoor. 3.3 Human tolerance to daylight Daylight application only works if the building occupants comfort is met. It is therefore essential that daylight schemes take into account means of controlling visual discomfort. The options include provision of internal or external blinds, external shading devices and high performance glass. In a previous study conducted by the authors, it has been demonstrated that under the Malaysian climate, buildings with no external shadings require blinds applied at strategic time of the year to ensure a visually acceptable indoor condition (Ibrahim & Zain-Ahmed 2006). The study observed daily periods when occupants tolerance to daylight exposure exceeded, and estimated percentage of office hours 1 in a year that blinds is needed to provide comfort. Summary of the results according to the façade orientation is tabulated in Table 1. Table 1: Estimated percentage of office hours in a year that blinds required/used Alternatively the same study suggests the use of external shades to control visual discomfort. The recommended optimum external shades for a 40 WWR envelope according to the respective orientation is tabulated in Table 2. Table 2: Estimated percentage of office hours in a year that blinds required/used 1 Office hours was taken to be 8am to 5pm. 4. METHOD OF STUDY Simulation runs were performed on a model that represents an office space - 4m wide, 4.5m deep and 4m ceiling height. The base case model has a window-wallratio (WWR) of 40% with tinted glass, and no external shades (coded as Ti40uS). The DF values were generated using RADIANCE software under overcast sky condition (10K CIE Overcast Sky). The study was performed on a cell-model of 4.5m deep. DF value obtained at work plane height of 850 mm, of points equidistant to each other (at grid points of 0.5m intervals), along the central axis perpendicular to the window plane were recorded. These DF values were transferred to Microsoft Excel, and plotted against the depth of the room. 4.1 Study tool The study used RADIANCE software that is integrated in the IES <VE> programme. RADIANCE is a backward ray-tracing programme that accurately predicts light levels and rendering. It produces synthetic images that are realistic for all sky conditions. For illuminance study, RADIANCE generates readings in Lux level and, Daylight Factors (DF). 4.2 Variables WWR (window-wall-ratio) the base case model for the study is a building with 40% WWR. In addition to this, 5 options of WWRs were examined, ranging from to 25% and 65%, in steps of 10% increase. Glass types - There are numerous glass types in the market. In Malaysia, the most frequently used are 6mm thick glass with U-value of 5.6 W/m 2 K, shading coefficient between , and visible transmittance from 20-80% (Tang, Kristensen & Lojutin 2006). Three glazing types were examined namely clear glass, light green and reflective glass with Visible Transmittance value of 88, 75 and 30% respectively. External shades - External shades are often used as a practical solution to reduce glare conditions. It partially blocks the visibility of excessively bright sky. Horizontal projection is the most common and simple form of external shades applied in Malaysia buildings. Only a simple horizontal projection type was examined. There were four overhang depth tested, varying between 300mm and 1200mm, in steps of 300mm increment. 4.3 Data input Light transmittance (Tvis) is the primary term used to characterize daylight admittance properties of glass. Glass specification required for RADIANCE to perform the illuminance calculation is transmissivity at normal PALENC Vol 1.indd 437 3/9/2007 1:25:12 µµ

3 438 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and incidence (tn). The Tvis values for the three glass types (SG, Ti and Re) were converted into transmissivity, and used as input values 2. For clear glass, the transmissivity values are the same for all the spectrums, namely red (R- t n ), green (G- t n ) and blue (B- t n ). Table 3 lists the specifications of the three glass types under investigation. Table 3: Glass specification for illuminance study This classification was used as the basis to estimate the percentage of room area that could be daylit. The study used this value to represent the estimated amount of usable daylight. 5. RESULTS The percentage of daylit floor area for each case were estimated, and the results observed the following: changing WWR, for the three glass types (SG, Ti and Re) varying shades depth RADIANCE input includes data for surface properties. The material and the surface reflectance of the models were checked so that they meet the specification of an office interior. These specifications were also checked against input values adopted by other researchers who have conducted similar DF investigation using RADI- ANCE for Malaysian offices (Denan 2004; PJH 2003). 4.4 Estimation of daylight availability Based on literature review, the following classification for daylight assessment was formulated (refer also to Figure 1): DF exceeding 10% is categorised as adequate to overlit. It is considered that there is potential for a total daylight environment. DF between 5% and 10% is considered to be good quality daylight. It also means that there is an opportunity for full daylight environment. DF between 2% and 5% is considered to be insufficient, but with potential as supporting light source. In this case the current study assumes 60% of daylight availability. DF lesser than 2% is categorised as underlit, and considered to be fully artificial light dependent. Figure 1: Classification for daylight assessment 2 RADIANCE uses transmissivity (t n ) rather than transmittance (T vis ) as data input. t n = (sqrt of ( *T vis *T vis ) )/ /T vis. RA- DIANCE IES has an inbuilt tool that performed the conversion. Figure 2 and Table 4 are examples of the DF distribution graphs and tables produced for the study. Table 4: DF distribution table and estimated daylit floor area 5.1 Effect of glass types Clear glass [SGuS]: Generally the use of clear glass results in high illuminance level near the window. Usable daylight (DF>2) can penetrate the full room depth (4.5m), achievable with the least opening tested (25 WWR, h= 1.8m). Fully daylit environment (DF>5) can be experienced up to a maximum of 3.75m deep, achievable with a minimum of 40 WWR, h= 2.4m. The maximum percentage of floor area that could be daylit is estimated at 90%. This is achievable with the use of at least 40 WWR, with window height of 2.4m from floor. Tinted green glass [TiuS] Changing from clear glass to tinted glass has caused an overall drop of DF. Usable daylight (DF>2) is able to penetrate the full room depth (4.5m). This is achieved PALENC Vol 1.indd 438 3/9/2007 1:25:12 µµ

4 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and 439 with a minimum of 40% WWR, h= 2.4m. Full reliance on daylight (DF>5) can be achieved up to 3.75m deep, with a minimum of 55 WWR, h= 2.7m. This results in a maximum of 90% of the floor area of the perimeter being daylit. Reflective glass [ReuS] There is little opportunity for a fully daylit environment, whereby DF exceeding 5% only reaches up to a maximum of 1.25m depth. Maximum penetration of usable daylight is 3.75m, achieved with a minimum of 55 WWR, h= 3.75m. Table 5 lists the estimated daylit floor area according to the three glass types. Table 5: Optimum WWR for maximum percentage of floor area daylit 5.2 Effect of varied shades depth on the base-case model (Ti40uS) How external shades influence the DF were investigated by using the Ti40 model, with varied external shading depths, ranging from 300 mm to 1200 mm in steps of 200 mm deep. The DF distribution is illustrated in Figure 3, and summary of the results tabulated in Table 6. The room depth that could be fully daylit is gradually reduced with deeper shades. It can be seen that with application of external shades, the deepest point in the cell (at 4.5 m depth) continues to receive daylight exceeding 2% DF. This indicates that the opportunity for top-up lighting is not significantly affected by the external shades. It can also be observed that the external shades were able to reduce the DF value near window areas, and provide a more even DF distribution indoor. 6. DISCUSSION The study on the daylight availability shows the following: that application of simple horizontal shades under Malaysian climatic conditions does not significantly affect the amount of daylight indoor. In fact, application of external shades improves the daylight distribution whereby the illuminance intensity in areas closer to the windows were reduced, providing gentle daylight penetration indoor. none of the cases tested (with a maximum window height of 3m ) could provide a fully daylit perimeter zone. Maximum depth that can be fully daylit is 4m for clear and tinted glass, and 1.5m for reflective glass (refer to Table 6-24). It is thus recommended that to rely totally on daylight, for window height from floor (h) not exceeding 3m, the room depth should be kept to a maximum of 3.75m. Comparing performance of fenestration choices It is interesting to assess how the two options of providing a visually acceptable daylight condition perform in comparison to each other namely the choice of either to adopt an unshaded envelope with blinds or an envelope with external shading of optimum depth (fully daylit). The information/outcome summarized in Table 1, Table 2 and Table 6 were collated and used to estimate the percentage of daylit area for the two cases. The results are tabulated in Table 7. Table 2: Comparison of estimated percentage of daylit area Figure 3: Comparison of DF along central axis for Ti40uS with various shades depth Table 6: Summary of illuminance conditions for tinted glass, 40 wwr with varied shades depth [Ti40_uS-1800sh] From the table it can be observed that the envelopes with external shading perform better in all cases. The ranking of highest potential for daylight saving according to the orientation is North, followed by South, West and East. 7. CONCLUSION The study attempts to estimate the amount of usable daylight available with the aim of comparing the perfor- PALENC Vol 1.indd 439 3/9/2007 1:25:13 µµ

5 440 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and mance of several envelope fenestration characteristics. The study reaffirms common belief that application of external shadings is an effective envelope design choice for buildings in the tropic. REFERENCES Denan 2004, Assessment of window and lighting design in office buildings under daylight of a hot-humid climate, Malaysia, PhD Thesis, University of Wales. Department of Standards 2001, MS 1525: 2001 Code of Practice on Energy Efficiency and Use of Renewable Energy for Non-Residential Buildings, Malaysia. Ibrahim 2006, Design process strategies and methods for achieving energy efficient air-conditioned office buildings in Malaysia, Ph.D. Thesis, Curtin University of Technology. Ibrahim & Zain-Ahmed 2006, Wall parameters as means to control discomfort due to solar irradiance, ISESEE 2006: International Symposium & Exhibition on Sustainable Energy & Environment, eds. Zain-Ahmed, A, Sh Ahmed, S, Ibrahim, N, Sulaiman, F, Mohd Yunus, R & Abidin, MH, UPENA, Kuala Lumpur. Institute for research in construction 2003, Indoor environment research: COPE Project, from cope/02-4-reports_e.html#iaq Moore 1991, Concepts and practices of daylighting in architecture, Van Nostrand Reinhold, USA. PJH 2003, MECM Final Report (Energy), Putrajaya Holdings Sdn Bhd, Putrajaya. Schiler & Japee 2001, VITAL SIGNS: Interior illuminance, daylight controls and occupant response, University of Southern California, California. Retrieved 12 Nov 2004, from berkeley.edu/vitalsign/res/downloads/rp/daylight/day1_big.pdf Suruhanjaya Tenaga 2005, Building energy performance in Malaysia, EE Seminar - How to take advantage of it, Kuala Lumpur. Tang, et al. 2006, Design strategies for energy efficiency in new buildings (non-domestic), Ministry of Energy, Water and Communications, Selangor. Zain-Ahmed 2000, Daylighting and Shading for Thermal Comfort in Malaysia Buildings, Ph.D. Thesis, University of Hertfordshire. PALENC Vol 1.indd 440 3/9/2007 1:25:13 µµ