en_daylighting performance for office buildings in hong kong

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Building and Environment 43 (2008) 14461458 www.elsevier.com/locate/buildenv

An analysis of daylighting performance for ofce buildings in Hong KongDanny H.W. Li, Ernest K.W. TsangBuilding Energy Research Group, Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China Received 11 July 2007; accepted 20 July 2007

Abstract Daylighting has often been recognized as a useful source of energy savings and visual comforts in buildings. Occupants expect good daylight in their working spaces. The quality and quantity of natural light entering a building depend on both internal and external factors. In Hong Kong, commercial building accounts for the major building energy use and electric lighting is one of the major electricity-consuming items. This paper studies the daylighting performance and energy implications for ofce buildings. A total of 35 commercial buildings have been selected in the survey. Key building parameters affecting daylighting designs are presented. Two typical ofce blocks were further analysed based on a lighting simulation program. The daylighting performance was evaluated in terms of daylight factor, room depth and glare index. It has been found that the daylighting performance for ofce buildings is quite effective. About one-third of the ofce areas that are near the perimeter regions have an average daylight factor of 5%. For inner region of deep plan ofces, some innovative daylighting systems such as light redirecting panels and light pipe could be used to improve the daylighting performance. In general, the ofce building envelop designs are conducive to effective daylighting and proper daylight linked lighting controls could save over 25% of the total electric lighting use. r 2008 Elsevier Ltd. All rights reserved.Keywords: Daylighting; Daylight factor; Glare index; Ofce buildings

1. Introduction Daylighting is an effective approach in creating a pleasant visual environment and a useful source of energy savings in commercial buildings. Daylight is considered as the best source of light for good colour rendering and its quality is the one light source that most closely matches human visual response. The amount of daylight entering a building is mainly through window openings that provide the dual function not only of admitting light for indoor environment with a more attractive and pleasing atmosphere, but also allowing people to maintain visual contact with the outside world. People desire good natural lighting in their working environments [1]. Articial lighting is one of the major electricity-consuming items in many non-domestic buildings,Corresponding author. Tel.: +86 852 27887063; fax: +86 852 27887612. E-mail address: [email protected] (D.H.W. Li).

accounting for about 2030% of the total building energy load [2]. Recently, there has been an increasing interest in incorporating daylight in architectural and building designs as a means to reduce energy use in buildings. Proper lighting controls integrated with daylighting have a strong potential for reducing energy demand in non-domestic building by exploiting daylight more effectively [35]. In Hong Kong, fenestration designs are mainly to minimize solar radiant heat gain and meet the current code of practice for overall thermal transfer values (OTTV) standard. Many commercial buildings are high-rise constructed in densely built business district and the shading effects from nearby buildings lead to severe sky obstructions, more particularly for rooms on the lower oors. The natural light entering a building depends on both internal and external factors. Indoor environment includes the size and position of the windows, the depth and shape of the rooms, and the colours of the internal surfaces [6]. Externally, the light reected from the streets and opposite

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ARTICLE IN PRESSD.H.W. Li, E.K.W. Tsang / Building and Environment 43 (2008) 14461458 1447

facades can be important sources of interior lighting [7,8]. Innovative daylighting technologies such as laser cut penal [9], light shelf [10], light pipes [1113], anidolic light-duct [14] are appropriate devices to transport natural light from outdoor into the deep plan rooms to enhance daylighting schemes. Good visual performance and less building energy use may result if a proper daylighting scheme is employed. In Hong Kong, however, daylighting strategies are not commonly incorporated in ofce buildings. The lack of information on the suitability of daylighting and its potential to save energy is the main reason for such an unenthusiastic response to daylighting designs. This paper presents the key variables affecting the interior daylight level via a survey of 35 buildings in Hong Kong, and studies the daylighting performance and energy implications to air-conditioned commercial buildings using computer simulation techniques. 2. Building survey and parameters affecting daylighting The daylighting performance of ofce buildings was determined through a survey of 35 ofce blocks completed in different years. The selection was based on the following conceived criteria: (i) Within a reasonable geographical spread over the territory. (ii) Year of completion should cover the past four decades (i.e. 19622004). (iii) Inclusion of some low-rise of 10-story or less ofce buildings. (iv) The sample should include common glazing types, viz. clear, tinted, reective and low-e glass. (v) All buildings are air-conditioned Data of the selected ofce buildings were collected from the original design documents including the building and oor layout drawings wherever possible. Further information was obtained through site visits and discussions with the practicing architects, engineers, surveyors and building management staffs. In order to retain the individual building anonymity, they are designated Building 1, Building 2 and so on according to the year of completion. The 35 ofce buildings selected are located in the three main territories in Hong Kong, 18 on Hong Kong Island, 11 in Kowloon Peninsula and the remaining 6 in the New Territories. Over half of the buildings were completed during 1980s to mid-1990s that was the booming period of the local economy as well as the building industry. The daylighting performance of a building depends very much on a good understanding of the interior and exterior building parameters. It is envisaged that the sample in the survey can give a good indication of the general daylighting characteristics of the ofce buildings in Hong Kong for building professionals to consider. In the following sections, ve key building parameters affecting daylighting performance including building area and orientation,

window area, glass type, shading and external obstruction are briey described.

2.1. Building area and orientation Traditionally, daylighting efciency is often evaluated in terms of daylight factor (DF), which by denition is the ratio of the internal illuminance to the illuminance simultaneously available on a horizontal plane from the whole of an unobstructed sky, expressed as a percentage. For computation proposes, a DF is split into three components, namely the sky component (SC), the externally reected component (ERC) and internally reected component (IRC). The calculation of the average DF is based on the theory of the split-ux principle that divides the ux entering the interior through window over its lower parts of the room surface areas and total internal surface areas. The bigger the oor and internal room surface areas, the smaller will be the average DF. In a side-lit room, interior daylight may be more nearly proportional to the amount of daylight falling on the window, rather than to the external horizontal illuminance. More vertical outdoor illuminances contribute to higher amount of indoor daylight levels. In Hong Kong, it has been found that there is an orientation effect on outdoor illuminance [15]. If a multistory building is to be completely lit by daylight, small interior surface areas tend to have a large average DF. However, average DF alone offers no information on the diversity of illuminance within the space. In a side-lit room, there will be limits on its overall plan depth. If the depth of the room from window to back wall is too long comparing with the room width and height, then rear half of the room tends to look shadowy compared to the brightly-lit front half and supplement electric lighting will be required for most of daytime. Higher window heads and wider spaces allow deeper plan room designs. Perimeter zones and narrow room layouts are, therefore, favourable to daylighting. Previous work on the daylight availability suggested that in more than half of the working year, daylighting alone could provide sufcient lighting level in the perimeter zones of most ofce buildings [16]. Table 1 shows a summary of the building envelop information for the 35 selected buildings. The gross oor area (GFA) is from 1287 to 98197 m2 and the number of storeys from 6 to 48. In terms of GFA per storey, it ranges between 76.5 and 3159 m2. Occupants expect good natural lighting in their working environment and most likely to be located near the windows with a clear view outside. The working spaces in the form of cellular and open plan ofces are generally designed along the perimeter zones of the buildings with major areas facing a less obstructed view. The building shapes for these 35 buildings are mainly square which represents the typical high-rise ofce building form in the urban district of Hong Kong [17]. Due to symmetrical layout, it makes little difference to orientate square buildings.

ARTICLE IN PRESS1448 D.H.W. Li, E.K.W. Tsang / Building and Environment 43 (2008) 14461458 Table 1 Summary of daylight parameter for surveyed buildings Building and site condition Building number Floor area (m2) 3604 17,400 12,636 9765 34,595 6150 44,054 46,357 55,911 11,610 7129 13,216 24,237 8316 12,610 24,237 1287 98,197 60,961 29,722 23,107 12,118 91,800 33,852 2041 2164 17,409 55,625 1529.8 58,481 Number of storey 7 14 20 7 37 6 21 48 47 12 10 17 25 11 6 13 6 48 47 27 11 15 48 26 20 24 17 40 20 33 Year built Window to wall ratio 0.29 0.50 0.40 0.46 0.34 0.31 0.35 0.62 0.60 0.32 0.37 0.29 0.50 0.45 0.39 0.45 0.27 0.65 0.50 0.45 0.40 0.36 0.45 0.33 0.24 0.40 0.51 0.41 0.18 0.55 External reectancea Mean obstructed angle (1) 30 8 44 18 63 14 62 69 70 54 40 28 20 19 42 32 53 60 42 50 38 14 68 36 29 55 34 59 73 44 Glazing information Glazing typeb Shading Coefcient Visible transmittance Shading typec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

1962 1970 1971 1972 1976 1978 1979 1979 1982 1983 1984 1984 1985 1985 1985 1987 1987 1989 1989 1989 1989 1991 1991 1993 1995 1995 1996 1997 1997 1998

Light Light Light Light Light Light Light Light Light Medium Light Light Medium Light Light Light Light Medium Light Light Medium Light Medium Light Light Light Light Light Medium Light

Single glazing clear glass Single glazing clear glass Single glazing clear glass Single glazing clear glass Single glazing tinted glass Single glazing clear glass Single glazing tinted glass Single glazing tinted glass Single glazing tinted glass Single glazing clear glass Single glazing clear glass Single glazing tinted glass Single glazing reective glass Single glazing clear glass Single glazing clear glass Single glazing tinted glass Single glazing clear glass Single glazing reective glass Single glazing reective glass Single glazing tinted glass Single glazing tinted glass Single glazing tinted glass Single glazing reective glass Single glazing tinted glass Single glazing clear glass Single glazing reective glass Single glazing reective glass Single glazing reective glass Single glazing reective glass Low-e IGU #2

0.9 0.9 0.9 0.9 0.72 0.9 0.65 0.72 0.72 0.9 0.9 0.7 0.4 0.9 0.9 0.5 0.9 0.25 0.25 0.45 0.6 0.65 0.25 0.55 0.9 0.37 0.38 0.41 0.41 0.36

0.88 0.88 0.88 0.88 0.51 0.88 0.41 0.51 0.51 0.88 0.88 0.49 0.26 0.88 0.88 0.27 0.88 0.14 0.14 0.25 0.3 0.41 0.14 0.23 0.88 0.26 0.2 0.26 0.26 0.41

c c nil c b nil nil c c c nil nil nil c c nil c d nil nil nil nil nil nil b nil nil nil nil a

ARTICLE IN PRESSD.H.W. Li, E.K.W. Tsang / Building and Environment 43 (2008) 14461458 Table 1 (continued ) Building and site condition Building number Floor area (m2) 94,757 46,654 3168 47,170 15,713a

1449

Glazing information Year built Window to wall ratio 0.45 0.42 0.50 0.44 0.31 External reectancea Mean obstructed angle (1) 58 76 71 65 68 Glazing typeb Shading Coefcient Visible transmittance Shading typec

Number of storey 30 39 21 33 29

31 32 33 34 35

1998 1999 1999 2004 2004

Light Light Light Light Light

Single glazing low-e glass Low-e IGU #2 Single glazing reective glass Low-e IGU #2 Single glazing reective glass

0.38 0.25 0.31 0.37 0.3

0.2 0.21 0.16 0.62 0.12

b b nil b c

Light and heavy represent the light and heavy colour nishing, respectively. For low-e IGU, the sign # represents the coating is attached to which window surface. #2 is the inner surface of exterior glass. c The type of shading device are as follows: Type aSiden only, Type bOverhang only, Type cBoth siden and overhang, Type dRecessed windows.b

2.2. Glass type Glass type controls the amount of daylight penetrating into an interior in terms of light transmittance. In daylight calculation, light transmittance is directly proportion to the daylight factor. In Hong Kong, most buildings use single glazing. Double glazing is mainly for acoustic issue rather than thermal or visual problems. It can be observed from Table 1 that 32 out of the 35 buildings use single glazing. Only three recently completed buildings adopt low emission insulated glass unit (Low-e IGU) with double glazing. Four types of glass have been found, namely clear, tinted, reective and low-e with 56 mm thickness. Clear glass provides a high transmission of daylight with typical visible transmittance (VT) of 0.88 but it also allows a large amount of solar heat (high shading coefcient) to pass through into a building. Tinted glass absorbs a considerable amount of infrared with some reduction of visible light. The VT ranges from 0.23 to 0.51. Reective glass absorbs more heat than tinted glass and offers good reecting characteristic in the infrared region with a certain reduction of VT. The VT can be of 0.12 and the rooms may look gloomy with low DF under overcast skies. This may cause visual discomfort to occupants. Low-e glass contains a thin coating of metal oxide substantially cutting down heat gain without proportionally reducing daylight transmittance. It should be pointed out that low-e glass is effective in terms of minimizing solar heat gain when there is short wave radiation (i.e. East and West orientations). In some countries, for incidence cold climate regions in North hemisphere, it is desirable to have solar heat gain for South facing windows which may require the use of glazing of other type of windows. The VT shown in Table 1 ranges between 0.2 and 0.62 that is generally more than that in reective glass for the present study. With such quite good VT values, occupants can enjoy more natural light as well as maintain a good visual contact with the outdoor

environment. Based on the 35 surveyed buildings, the fenestration systems between 60 and 70 s were largely single glazing with clear glass. Thereafter, in the 80 s and early 90 s, tinted glass was the main glazing type used. Afterwards, buildings completed in late 80 and 90 s tended to have curtain walls with reective glazing. Currently, more buildings used the low-e glass. 2.3. Window area For a given glazing type, the critical factor determining the daylight entering a building is the window area. According to the Hong Kong Building Regulation, the required window area should be more than one-tenth of the oor area of the room. Window area is commonly represented by the window-to-wall ratio (WWR) that is dened as the ratio of the total area of windows to the overall gross fac ade areas including windows. As indicated, the window area of the 35 buildings varies a great deal, with the lowest WWR of 0.18 appearing in Building 29 and the highest of 0.65 found in Building 18. With a typical oor-to-oor height of 3.4 m, these represent window heights of about 0.62 and 2.2 m. Clear glass windows tend to have small areas and large WWRs are mainly for low-e glass. The mean WWRs for clear glass, tinted glass, reective glass and low-e glass are, respectively, 36.4%, 42, 44% and 46.5%. 2.4. Shading Shading devices shade the window from direct sun penetration but allow diffuse daylight to be admitted. Exterior shading devices frequently found in Hong Kong for ofce buildings include overhangs and side-ns which are more effective to block the direct sunlight and solar heat than interior shading devices such as venetian blinds and curtain blinds. According to Table 1, buildings

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completed from 60s to mid-80s tend to have overhangs and side-ns with clear glass. Between late 80 and 90s, external shading devices were not popular because more large scale prestigious building projects in up-market commercial districts had a tendency to use curtain walling. Since 1998, resurgent shading device has been found and such a design has become popular again in recent years. A plausible explanation is the introduction of the code of practice for overall thermal transfer value (OTTV) in 1995 for controlling building-envelope designs [18]. To meet the current Hong Kong OTTV standard of 30 W m2 [19] which stands for the average heat gain through the building envelop of ofce buildings, shading devices such as metal overhangs were employed in a number of newly constructed buildings.

3. Computer simulation approach In order to gain more ideas about the indoor daylighting performance, daylight factors (DF) inside the ofce spaces at low, middle and high oor levels for two buildings (Buildings 20 and 30) were simulated. The interior daylight levels depend on a number of design features and computer-based lighting programs are appropriate tools for daylighting analysis. A lighting computer package namely, RADIANCE was used for the study. 3.1. Building description Building 30 is a high-rise block with 33 ofce oors built in 1998. Fig. 1 shows the layout plan. It is a deep plan building with the room depth of about 14 m. The oor area is 1772 m2 and the ofce spaces are located along the four perimeter zones. The staircases, metre rooms, services lifts, air-conditioning plant rooms and toilets are placed in the interior core. External shading device is side-n and the WWR and VT are 0.55 and 0.41, respectively. The building is slightly shaded by surrounding obstructions and the y values for northeast, southeast, southwest and northwest are 221, 571, 641 and 321, respectively. The exterior environment and the fac ade designs appear to be quite suitable for daylighting schemes. Building 20 was constructed in 1989. It is a 31-storey commercial building of which 27 storeys are ofce areas. The internal dimensions are of 43 m 32 m with the room depths between 10 and 12 m. The oor-to-oor height is 3.5 m and no external shading device is being used. The product of WWR and TV is only 0.11 (0.45 0.25) which is half of that for Building 30. It is heavily obstructed by a nearby building in the east with y4821. Ofce spaces are built along the north, south and west perimeter zones with y values ranging between 171 and 641. Staircases, metre rooms, mechanical plant rooms, toilets and lift lobbies are located at the interior and east perimeter zones. The typical oor layout plan is shown in Fig. 2. 3.2. Simulation setting RADIANCE [20] is a computer simulation package for simulating and visualizing lighting in and around architectural environments using the backward ray-tracing technique. It is a well-established lighting program that has been used by a number of researchers [21,22]. It has been reported that RADIANCE simulations can produce more close prediction to real building measurements comparing with a number of daylighting software packages [23]. The simulation models contain two parts: the building blocks including exterior obstructions, which are comprised of opaque and transparent material, and selfluminous source that represents the sky. The built-in Gensky sub-program was used to generate the CIE overcast sky conditions for indoor illuminance

2.5. External obstruction External obstruction inuences the daylighting performance in two aspects. First is the amount of the sky being obstructed or unobstructed. Second is the colour of the external surface nish that can be regarded as the reected luminance from the obstructing buildings. The former depends on the height of neighbouring buildings and the separations. When buildings are located close to each other, blockage of natural daylight can be severe, particularly for the lower oors. For convenience, the effect of external obstruction can be evaluated by using angle of vertical obstruction (y). The latter, in practice, is to assume an average fraction of the luminance of the sky. The choice of colours mainly depends on the preference of the architects and clients. The colours of the external surface nish can be grouped into light and medium. Surrounding buildings with light colour surface nish tend to give large ERC and IRC values. However, high reectance from nearby obstructions may cause glare problems. From Table 1, it has been found that 29 and 6 buildings have, respectively, light and medium colour surface nish. In metropolitan Hong Kong, vast of tall buildings built very closely together are not uncommon. For the present study, y is measured from the lowest ofce oor level of the surveyed building to the highest level of its surrounding buildings. The mean y is the average value for the four principal facades of a given surveyed building. It can be observed from Table 1 that the smallest and largest mean y values are 81 and 761, respectively. Over 25% of the buildings have a quite good access to the SC with the mean y of 301 or less. These buildings are mainly located near the coast with at least one fac ade facing the panoramic view of the harbour. Around 30% of the buildings have the average sky obstruction between 601 and 801. Being constructed in high-density business districts, such buildings are surrounded considerably by large obstructions. The mean y values ranging from 301 to 601 constitute the remaining 45% of the buildings.


Fig. 1. Typical oor plan with simulation points for Building 30.

calculations. The key simulation parameters for daylight illuminance determinations are the number of reections and resolution of the inter-reection calculation, which are referred to the ambient parameters. Convergence tests were conducted to obtain the settings for ambient parameters such that the accuracy and simulation time of the raytracing calculations in RADIANCE can be reduced to an acceptable level. Table 2 summarises the ambient parameter settings for the two buildings. Geometrically, the models generated for the simulations were close to the actual dimensions of the two buildings that were obtained from the architectural drawings. Common interior reectances for ceilings, walls and oors were used. Externally, two circular ground planes with a common reectivity of 0.2 and radii of 2000 and 1500 m were set for Buildings 30 and 20, respectively. The non-luminous external objects including roads and surrounding blocks were modelled in RADIANCE. The physical dimensions were determined from a map and on-site measurements, and the reectances were obtained according to their colours and textures.

4. Daylighting performance The daylight illuminances for the 1636 set points in a typical oor (as shown in Fig. 1) for Building 30 were simulated. Three oors at 33/F, 17/F and 1/F representing, respectively, high, middle and low levels were considered. Figs. 36 present the DFs along the centerline for the three levels faced northeast, southeast, southwest and northwest. Without large external obstructions, all orientations at the top oors are able to see a large amount of the sky, and thus enjoy more daylight. The simulated DFs are in the range of 8.48.8% at the point of 0.5 m away from the windows for various orientations. The presence of obstructing buildings blocks certain parts of the sky, and hence reduces the daylight received at lower oors. It can be seen that DF decreases from 8.7% at 33/F to 5.3% at 1/ F for the southwest zone. With y=221, there is almost no change in DF at different oor levels for northeast zone. As SC and IRC become smaller, the DF dwindles rapidly away from the window facades. At room depth of 7 m (half room depth), the DFs reduce to less than 0.7% for all


Fig. 2. Typical oor plan with simulation points for Building 20.

Table 2 Ambient parameters setting for simulation Ambient parameters Ambient Ambient Ambient Ambient Ambient division sampling accuracy resolution bounces Building 20 1024 512 0.08 2048 5 Building 30 2048 1024 0.08 2048 6

oors and zones. Likewise, the daylight illuminances at 31/ F, 18/F and 5/F (ofce areas are between 5/F and 31/F) for Building 20 were simulated. Totally, 876 set points for each oor (as shown in Fig. 2) were used for the computation and Figs. 79 present DF along the centerline at the three oor levels facing west, south, and north. The DF is around 5.5%, 0.5 m away from the windows on the top and middle oors for the south and west zones. As the shading effect is not severe, the DFs at 5/F for these two zones slightly reduce to 5%. As the points are far away from the apertures, the simulated DFs drop rapidly. The DF can be less than 0.2% near the internal core. Similar characteristics can be found for the north zone but with lower DF levels due to the large y of 641 appearing at the north orientation. The DF ranges from 0.1% to 5.4% depending on the oor level and distance from the window. The daylighting performance for Building 20 can be improved

by using a mix of glazing type. For incidence, in North facing facades, much larger glazing areas of clear type can be used to replace the tinted glass such that it can improve the DF. The average DF is used as the measure of general illuminance from diffuse daylight in a room. As pointed out in literatures [24], an average DF of 5% or more will ensure that an interior looks substantially daylight and electric lighting is not normally to be used during daytime (i.e. ofce hours). An average DF below 2% generally makes a room look dull and articial lighting is likely to be in frequent use. Based on these criteria, the ofce areas at each oor were divided into three regions, namely perimeter, intermediate and inner. The perimeter region starts at the fac ade with an average DF of 5%. The intermediate region starts at the inner border of the perimeter regions with a mean DF of 2%. The inner region will be the remaining part of the room towards the rear wall and this area is illuminated mainly by electric lighting. Accordingly, the areas of the three regions were estimated and Table 3 shows the results for the two buildings. As expected, the perimeter region areas for the two buildings drop gently from the top oor to the low oor but the reserve is seen for the inner region. The information can also give the likely electric lighting energy savings. Assuming that during the working hours for ofce (i.e. 9:0017:00) no electrical power is consumed by


Fig. 3. Daylight factors at the various oors facing northeast for Building 30.

Fig. 4. Daylight factors at the various oors facing southeast for Building 30.

articial lighting in perimeter region and 50% of the working time daylighting alone will be adequate for the intermediate region. Accordingly, it was deduced that the building lighting energy could be signicantly decreased by around 25% and 20% for Buildings 30 and 20, respectively. Further savings in cooling energy and heat rejection could be obtained due to less sensible heat gains generated by articial lighting ttings. The results agreed with the ndings in our early studies [25]. During cold winter days, heating energy if required, may be increased when less electric light ttings are used.

The average daylight alone offers no information on the diversity of illuminance within the space. In a side-lit room, the uniformity of the daylighting will depend mainly on the dimensions of the rooms and the surface reectances. If a multistory building is to be lit by daylight, there will be limits on its overall plan depth. Lynes [26] proposed the following equation to justify the daylight uniformity of a room. L L 2 o , W H 1 Rb (1)


Fig. 5. Daylight factors at the various oors facing southwest for Building 30.

Fig. 6. Daylight factors at the various oors facing northwest for Building 30.

where L is the depth of the room from window to back wall (m), W the width of the room measured parallel to the window (m), H the height of window head above oor level (m) and Ra the average reectance of surface in the rear half of the room (away from the window) (dimensionless) If L is too long comparing with the room width and height, then the rear half of the room tends to look gloomy and supplementary electric lighting will be required. The limiting depths at various zones for the two buildings were determined and are presented in Table 4. The limiting depths range from 8.05 to 8.24 m for Building 20 and

9.63 m for Building 30. It indicates that the limiting depths are in the inner region. The whole perimeter and intermediate zones are appropriate to be lit by daylight and electric lighting will be required for the inner zone even though daylight illuminance is sufcient. To increase the daylight illuminance and improve the uniformity inside the rear part of the ofce buildings, light guiding systems such as light redirecting panels [9] and light pipes [27] would be the appropriate devices. Glare from windows can arise from excessive contrast between the luminance of the visible sky and the luminance


Fig. 7. Daylight factors at the various oors facing west for Building 20.

Fig. 8. Daylight factors at the various oors facing south for Building 20.

of the internal surfaces within the eld of view. The degree of glare discomfort is measured in terms of daylight glare index (DGI) as follows: DGI 10 log10 0:478n X i1

L1:6 O0:8 s , Lb 0:07o0:5 Lw s

(2)

where Ls is the average luminance of each glare source in the eld of view (cd/m2), Lb the average luminance of the background excluding the glare source (cd/m2), Lw the average luminance of the window (cd/m2), os the solid

angle of the source seen from the point of observation (sr), O the solid angle subtended by the source, modied of the position of the light source with respect to the eld of view and Guths position index P(sr), n the number of glare sources. Eq. (2) is included in the RADIANCE package to predict the DGI. The DGI at the rear walls at various oors for the two buildings were calculated based on RADIANCE program and Table 5 shows the ndings. It can be seen that the DGI for Building 30 is of a smaller variation, ranging between 15.9 and 19.6. In general, the


Fig. 9. Daylight factors at the various oors facing north for Building 20.

Table 3 Areas for different daylight zones Building 20 High oor Perimeter zone (m2) Intermediate zone (m2) Inner zone (m2) Middle oor Perimeter zone (m2) Intermediate zone (m2) Inner zone (m2) Low oor Perimeter zone (m2) Intermediate zone (m2) Inner zone (m2) Building 30

Table 5 Daylight glare index for Buildings 20 and 30 Direction Daylight glare index Low oor 62 265 562 62 237 589 62 177 650 410 171 1155 342 188 1206 239 274 1223 Building 20 North South West Building 30 Northeast Southeast Southwest Northwest Middle oor High oor

14 13.8 10.9 19.6 16.1 17.6 18.4

16.3 18.5 16.4 18.6 16.7 15.9 17.8

19.1 19 17 19.4 17.8 17.5 19.3

Table 4 Room depth at different orientations for Buildings 20 and 30 Room depth (m) Building 20 North South West Building 30 Northeast Southeast Southwest Northwest

great deal, ranging from 10.9 to just over 19. Generally, large DGIs appear in high oor levels. With the threshold DGI of 22 [28], there will be no discomfort glare for the two buildings under overcast skies. 5. Conclusions

8.05 8.05 8.24 9.63 9.63 9.63 9.63

DGI for the northwest and northeast zones are higher than those for the southeast and southwest zones. For Building 20, the situation is quite different. The DGI varies in a

The daylighting performance of 35 fully air-conditioned ofce buildings in subtropical Hong Kong was investigated. Five key building parameters affecting the daylighting designs namely, building area and orientation, glazing type, shading devices and colour of external surface nishing were presented. In Hong Kong, the building layout and orientation depend on the site boundary and view out rather than daylighting consideration. From 1960 to 1970s window area was small with clear glass. Window area became larger when tinted glass was introduced in the 1980s. The 1990s were the period where


curtain walling ofce complexes with reective glass were predominant. Currently, with the advances in glazing products, more high-rise ofce buildings adopted the low-e glass. External shading devices were widely used for ofce buildings constructed between 1960s and mid-1980s and vanished afterwards. With the implementation of code of practice for overall thermal transfer values; metal overhangs were popular for ofce buildings constructed after late 1990s. Shading effects due to surrounding buildings were taken into account. Most of the 35 surveyed buildings have a mean vertical obstructing angle between 301 and 601. The daylighting performance of the ofce buildings was further illustrated based on two surveyed buildings, namely Buildings 30 and 20 via computer simulation techniques. With room parameters for good daylighting schemes and small external obstructions, Building 30 has a good daylighting performance. The DF is over 8.5%, 0.5 m away from the window and around 24% of zone area at the top oor achieving an average DF of 5%. As the SC and IRC reduced with the presence of obstructing buildings, the DF drops rapidly at low oor levels and away from the window. Similar characteristics have been found for Building 20 but with lower DF values due to larger external obstruction effect and the less-effective room parameters for daylighting designs. The likely energy savings for the two buildings due to daylighting were estimated. Based on the simulated results and the assumptions made, around 25% and 20% of total electric lighting energy can be saved for Buildings 30 and 20, respectively. The glare issue is also considered. It has been found that the glare index was less than the threshold limit of 22. The ndings suggested that the building fac ade designs are conducive to daylighting and proper daylight linked lighting controls have potential for signicant energy savings. With more reliable data via eld measurements and computer simulations are available, more architectural designs and construction practices would incorporate daylighting schemes in their buildings to enhance more energy-efcient building developments.

Acknowledgements The work described in this paper was supported by a Grant from the City University of Hong Kong (Project No. CityU 7001567). EKW Tsang is supported by a City University of Hong Kong Studentship. The authors would like to thank the student assistants for their help with site survey.

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