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Daylight performance of anidolic ceiling under different sky conditions
Solar Energy 81 (2007) 151–161 www.elsevier.com/locate/solener
Daylight performance of anidolic ceiling under different sky conditions S.K. Wittkopf
*
School of Design and Environment, Department of Architecture, National University of Singapore, 4 Architecture Drive, Singapore 117566, Singapore Received 27 July 2005; received in revised form 13 April 2006; accepted 14 April 2006 Available online 24 May 2006 Communicated by: Associate Editor Jean Rosenfeld
Abstract The daylight performance of anidolic ceiling for diffuse daylight is the subject of this article. The performance is assessed over a wide range of sky conditions following the new CIE Standard General Sky and compared between locations in Singapore, Japan and the United Kingdom. The criteria illuminance ratio (IR) and daylight glare index (DGI) are used to quantify the daylight performance of a default ribbon window fac¸ade with and without an anidolic ceiling. The difference is expressed in new terms IR improvement factor (IR IF) and DGI reduction. These factors are charted over all 15 sky conditions and various sun altitudes and may serve as general references indicating conditions under which anidolic ceilings perform best. The application of these new terms are demonstrated for Singapore, Fukuoka and Sheffield, representing three main latitude bands and different sets of prevailing sky types. It can be concluded that the daylight performance improvement through anidolic ceilings is most significant in Singapore. 2006 Elsevier Ltd. All rights reserved. Keywords: Daylight performance; Anidolic ceiling; Computational simulation
1. Introduction The demand for a sustainable environment and in particular sustainable architecture is ubiquitous. It is well known that energy consumption in buildings is too high and contributes significantly to the hazardous global warming. Buildings in hot climates are commonly designed to block the solar radiation; the window is either tinted or shaded extensively, which in turn also blocks the daylight from entering the interior efficiently leaving the rear room areas gloomy. Daylight in interiors contributes significantly to the human well being (Lieberman, 1991). It is also reported that daylight helps to improve the productivity in work i.e. in schools (Smiley, 1996). A human perception oriented daylighting design must consider functional and biological needs, with the visual connection to the outside
*
Tel.: +65 6516 3481; fax: +65 6779 3078. E-mail address: [email protected]
0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.04.002
being a predominant demand to provide for sense of orientation, time and security (Lam, 1977). An advanced use of daylight is characterized by efficiently re-directing daylight into the interior while keeping the solar heat out. This helps to reduce the need for electrical lighting, being one major building load. The many daylighting systems around have been classified by the IEA SHC Task 21 Subtask A on Performance Evaluation of Daylight Systems (Ruck et al., 2000) and Task 31 on Daylighting Buildings in the 21st Century. A system matrix differentiates daylight systems by shading task (shading or non-shading) and daylight preference (diffuse or direct). A checklist has been developed which includes further features such as ability to reduce glare or to allow view outside, etc. to support in determining the appropriate requirements and systems. Successful applications of daylighting systems have been investigated and their performances are broadly qualified. This paper focuses on one anidolic ceiling, classified as non-shading device and designed for diffuse daylight.
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Anidolic devices are advanced light re-directing devices through which a homogenized daylight distribution in the interior is achieved, i.e. reducing the daylight factor closer to the window and increasing it in the rear part of the room. The most prominent feature is the external scoop, which is usually located in the upper part of the fac¸ade, followed by a funnel (anidolic ceiling) and exit aperture. This system is shaped after the non-imaging optics to allow a maximum of light being collected and re-distributed internally with a minimum number of reflections (Welford and Winston, 1989). The inner surfaces need to have a very high reflectance of more than 90% to minimize the absorption occurring at every reflection. Several anidolic ceilings have been monitored in temperate climate regions of overcast sky conditions and the improvement in the rear of the room is significant (Scartezzini and Gourret, 2002). Concerning tropical climates mainly sunlight re-directing systems have been introduced and their application may prove successful (Edmonds and Greenup, 2002). The likely performance of anidolic ceilings for diffuse daylight over the complete range of possible sky conditions is subject of this article. It helps to identify under which sky condition the performance is maximized or marginal. The terms illuminance ratio (IR) and daylight glare index (DGI) are used to quantify the daylight performance. Illuminance ratio is more appropriate for all sky types including clear and partly cloudy, since the daylight factor is only used in conjunction with overcast skies. A new term illuminance ratio improvement factor (IF) was introduced to quantify the performance improvement over a reference fac¸ade without an anidolic ceiling. This factor is charted for all sky conditions and may serve as a reference to decide whether the application makes sense or not. For example, there are certain assumptions that they perform well under partly cloudy sky conditions with relative high zenithal luminance such as in the tropical region, but these assumptions have not yet been verified or quantified.
2. Spatial distribution of daylight The wide range of daylight conditions is represented by the 15 different sky types described in the CIE Standard General Sky (CIE, 2003). This new standard extends the previous set of three sky types (overcast, intermediate and clear only) by adding five subtypes for each type which allows a more accurate representation of the various spatial distributions of daylight. This set of 15 sky types has already been used to characterize the standard skies for various maritime climates (Tregenza, 1999). The sky luminance distribution depends mainly on (a) the gradation between horizon and zenith and (b) the scattering around the sun spot (Fig. 1). Certain indices are used to characterize the different sky types and to calculate the luminosity of any sky patch (Kittler and Darula, 2002). Table 1 lists the main characteristics of the 15 different sky types.
Fig. 1. Stereographic view of the hemisphere with superimposed circles denoting luminance gradation between horizon and zenith and scattering around the sun disk.
3. Computational simulation of the 15 sky types – the virtual sky dome method The performance comparison for different sky conditions and locations would require at least one fac¸ade system at each location. And furthermore a consensus is required on the definition of real sky conditions to ensure similar test conditions at each site. Computational simulation helps to build the different sky conditions and sun altitudes which in turn ensure better comparison. The virtual sky dome is one method to generate a CIE general sky compliant representation of the daylight conditions for use in 3D-CAD based lighting simulation software (Wittkopf, 2004). A VSD imitates the spatial luminance distribution of the sky vault by 145 distinct light sources whose distribution over the hemisphere follows the conventions of sky patch luminance measurements and whose individual luminous flux is calculated using the set of equations for the 15 sky types (Kittler and Darula, 2002; Kittler et al., 2006). A tool has been developed that calculates the luminous flux values for any sun position, sky type and radius of the sky dome. These VSD data is converted into ASCII file formats eventually representing the spatial distribution of daylight for a particular sky type, time and location in a wider range of 3D CAD based light simulation software. Fig. 2 shows charts illuminance values on different planes using a VSD representing a particular date, time, and location. The VSD method has been applied for use with the light simulation software Lightscape and the concordance of daylight factors with a reference case (Tregenza, 1999) has verified the universal application of the VSD method (Wittkopf, 2005; Wittkopf et al., 2006). However the simulation of the optical system of the anidolic devices requires an algorithm that accounts for accurate photometric analysis and true reflections. A standard Radiosity based algorithm would fail in modeling the reflections correctly since they are by default simplified to be ideal diffuse irrespective of the incident angle. Instead,
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Table 1 Main characteristics of the 15 different sky types Type
Gradation
Indicatrix
Description of luminance distribution
Group
1
I
1
Overcast skies
2
I
2
3 4 5 6
II II III III
1 2 1 2
7 8 9 10 11 12 13 14 15
III III IV IV IV V V VI VI
3 4 2 3 4 4 5 5 6
CIE standard overcast sky, alternative form, steep luminance gradation towards zenith, azimuthal uniformity Overcast, with steep luminance gradation and slight brightening towards the sun Overcast, moderately graded with azimuthal uniformity Overcast, moderately graded and slight brightening towards the sun Sky of uniform luminance Partly cloudy sky, no gradation towards zenith, slight brightening towards the sun Partly cloudy sun, no gradation towards zenith, brighter circumsolar region Partly cloudy sky, no gradation towards zenith, distinct solar corona Partly cloudy, with the obscured sun Partly cloudy, with brighter circumsolar region White-blue sky with distinct solar corona CIE standard clear sky, low illuminance turbidity CIE standard clear sky, polluted atmosphere Cloudless turbid sky with broad solar corona White-blue turbid sky with broad solar corona
Partly cloudy skies
Clear skies
Fig. 2. Partly cloudy sky type 8 as stereographic view and illuminance values across different plane orientations for Singapore 21 June.
Photopia, an optical design and analysis software has been selected that produces comprehensive performance evaluations for non-imaging optical designs. It is based on the backwards ray tracing algorithm and provides data of various commercially available reflector materials. Subsequently the universal VSD method was applied for use within this software. 4. Anidolic ceiling performance simulation Various VSD and fac¸ade designs are set up for predicting the performance of the system under different sky types and sun altitude. The following chapter lists several simulation runs with gradually increasing level of detail. The first simulation run starts by comparing the illuminance ratio across different fac¸ade configurations, so as to introduce the improvement factor of the anidolic ceiling. The second limits the fac¸ade configuration to the default and anidolic ceiling only, but extends the illuminance ratio assessment
across all sky types as to detect trends and preferences. The third simulation run includes the daylight glare index assessment, so as to verify any trends. The next two simulation runs complement the default sun altitude of 50 by 30 and 70, so as to check on the superiority of trends, being it more sensitive to the sky type or sun altitude. Simulation run 4 approaches this with illuminance ratio and simulation run 5 with daylight glare index assessment respectively. 4.1. Simulation run 1: Comparison of 3 different fac¸ade designs The first simulation run is to introduce the impact of the anidolic ceiling on the illuminance ratio by comparing it to other fac¸ade configurations. The reference fac¸ade comprises a single glazing ribbon window commonly used in Singapore, as can be seen in Figs. 3 and 4. It is complemented with a second upper ribbon as clerestory windows
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Fig. 6. Elevation of configuration 3: anidolic ceiling.
Fig. 3. 3 D View and elevation of default fac¸ade configuration: Ribbon window.
Fig. 4. 3D View and elevation of default fac¸ade configuration: Ribbon window.
in the second configuration (Fig. 5). The third configuration is the anidolic ceiling comprising an external light collector (replacing the clerestory window), anidolic ceiling and an exit aperture located deeper in the room (Fig. 6). The dimensions follow the dimensions of tested and established mock-up models (Scartezzini and Gourret, 2002). A forth setup looks at the shading effect of the external collector only.
The room receives diffuse daylight from the sky vault only, since the fac¸ade faces north and the sun is due south at an altitude of 50. A sky type of uniform luminance distribution was selected (type no. 5). The room dimensions are 6 · 6 m and 3.21 m floor-to-ceiling height, as can be seen in Fig. 3. The reflectance and transmission values are listed in Table 2. The reading plane for the Illuminance Ratios is 0.72 m above the floor. All illuminance ratios show an extreme inhomogeneous distribution across the room depth (Fig. 7). This is expected since the room is deep and illuminated from one side only. The illuminance ratio of the reference case (ribbon window) drops to almost a quarter at a distance from the window, which is equal to the ceiling height (rear area). It is further halved at the rear wall of the room with a distance equal to approx. two times ceiling height. This trend is about the same for additional clerestory windows or the design with the external collector shading the window with the absolute values being above and respectively below the reference case. Of course, additional clerestory windows bring more light in deeper areas but also in those already overexposed areas, thus not improving the negative imbalanced distribution at all. The anidolic ceilings however makes a significant contribution towards leveling the illuminance ratio. It reduces the unnecessary peak closer by the window through the shading effect of the external anidolic collector and increases the necessary levels in the rear of the room, through re-directing the oversupply of light into areas of need. With the exit aperture of the anidolic ceiling being located 4 m away from the window, the illuminance ratios starts to increase from here and maintains on a relatively high level. This improvement is best quantified as illuminance ratio improvement factor (IR IF = IR design/IR reference design) charted over the relevant rear room area. Fig. 8 Table 2 Reflectance and transmittance of surfaces
Fig. 5. Elevation of configuration 2: additional clerestory window.
Surface type
Properties
Walls Ceiling Floor Anidolic system Window glass Glass over exit aperture
50% 80% 20% 90% 92% 92%
perfectly diffuse reflectance perfectly diffuse reflectance perfectly diffuse reflectance perfectly specular reflectance transmittance transmittance
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Illuminance Ratio Comparison 30% Reference Add. clerestory window
25%
Illuminance Ratio
Add. anidolic ceiling 20%
External collector as shading
15%
10%
5%
0.125 0.375 0.625 0.875 1.125 1.375 1.625 1.875 2.125 2.375 2.625 2.875 3.125 3.375 3.625 3.875 4.125 4.375 4.625 4.875 5.125 5.375 5.625 5.875
0%
Distance from Window (m) Fig. 7. Illuminance ratios across all four fac¸ade designs with the anidolic ceiling being the only solution to improve the illuminance ratio distribution significantly. Illuminance Ratio Improvement Factor (IR IF) Comparison 2.6 2.4
IR IF
2.2 2.0
Add. anidolic ceiling
1.8
Add. clerestory window External collector as shading
1.6 1.4 1.2 1.0
5.875
5.625
5.375
5.125
4.875
4.625
4.375
4.125
0.8
Distance from Window (m)
Fig. 8. Improvement factor over reference design with the anidolic ceiling scoring an average improvement of 2.2.
charts the improvement factor with the reference illuminance ratio case being factor 1. Clearly the improvement of the anidolic ceiling outperforms the other designs by far. The improvement factor is about 2 underneath the exit aperture and peaks at 2.6 even deeper inside the room. The average IR IF between the exit aperture and rear wall is around 2.2. 4.2. Simulation run 2: Comparison of IR IF across all 15 sky types The previous simulation run was focusing on sky type 5 (uniform luminance distribution) only. This simulation run extends it over the remaining 14 sky types, so as to identify
sky types that result in higher or lower improvement factors. The designs with additional clerestory windows and external collector as shading device are no longer considered. The focus is on the improvement factor of the anidolic ceiling over the default ribbon window design. Fig. 9 compares the average IR IF between the exit aperture and rear wall of the room across all sky types. The improvement factor across the various sky types is quite different and ranges between 3.5 for overcast sky type 2 and 1.9 for clear sky type 12 respectively. All clear skies (types 11–15) result in significant lower improvements, with an average factor of around 2. Conversely, overcast skies (types 1–5) provide the highest improvements; even their lowest still exceeds all of the clear skies. The deviation is
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S.K. Wittkopf / Solar Energy 81 (2007) 151–161 IR IF Comparison 4.0
Overcast Skies
Partly Cloudy Skies
Clear Skies
3.5
IR IF
3.0 2.5 2.0 1.5 1.0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Sky Type
Fig. 9. Illuminance ratio improvement factor across all sky types.
Table 3 Grouped ranking of sky types in terms of IR IF
Table 4 Daylight glare index and corresponding subjective perception levels
Rank
IR IF range
Sky type
Nos. of skies
1 2 3 4
>3.5 Around 3 >2.5 < 3 2–2.5
2 (overcast) 1, 3 (overcast) 3, 6, 7, 8 (overcast and partly cloudy) All clear and remaining partly cloudy skies
1 2 4 8
also quite different between the group of skies, lowest again within the group of clear skies (types 11–15) and highest for overcast skies (types 1–5). Table 3 ranks groups of sky types by their improvements. 4.3. Simulation run 3: Comparison of DGI across all 15 sky types The third simulation run looks at the daylight glare index (DGI) instead of IR. Glare is an important factor affecting visual comfort. Windows can be extremely bright sources of daylight and the resulting high and uneven distribution of brightness in the field of view can cause discomfort or even disability glare. The user usually responds to daylight glare through pulling blinds and switching on electrical lights. This compromises the view to the outside neglecting biological needs and furthermore results in an unnecessary high consumption of electricity. The window itself is in most of the cases not the immediate cause of glare but rather the visible luminous sky. With clouds are brighter than a deeps blue clear sky, the risk of daylight glare increases under partly cloudy and overcast skies. A common calculation of daylight glare is provided by Chauvel (1982) based on Hopkinson (1972) equations for general glare. Nazzal (2004) has developed it into a new daylight glare index which is used in this simulation run. Baker et al., 1993) grouped daylight glare indices into comprehensive subjective perception levels, which are also used in this simulation run and listed in Table 4.
DGI
Subjective perception level
<16 16–20 20–24 24–28 >28
Imperceptible Perceptible Acceptable Uncomfortable Intolerable
The user position and field of view for the glare assessments is centered at 5 m distance from the window, with the view line towards the center of the window. It thus reflects the worst-case, which is deemed sufficient for this study. The height of 1.25 m corresponds to a sitting position. As such a vertical plane is defined for which the illuminance is recorded (Fig. 10). Two values are obtained for the calculation of DGI, with the first one being the direct illuminance from the window only and the second one representing the true illuminance by direct light and indirect inter-reflections from the room surfaces. Table 5 and Fig. 11 show DGI reduction for the reference case (ribbon window) and anidolic ceiling across all sky types. The DGI improvement varies between 18% and 4% for clear sky type 1 and partly cloudy sky type 8 respectively. Expressed in quantitative terms the DGI is improved by an average of 11%, 7% and 9% for overcast, partly cloudy and clear skies respectively. In 60% of sky types the implementation of the anidolic ceiling improves the subjective appreciation of glare by at least one level, i.e. from ‘uncomfortable (24.7)’ to ‘acceptable (20.3)’ in the case of sky type 1. For the rest 40% the glare remains within the same perception level. However, despite an average improvement of approx 8%, 73% of the sky types are still in the uncomfortable or intolerable level. They are mainly partly cloudy and clear skies, which remain in unacceptable range. This is mainly due to the window and view configuration of this particular room which is indeed critical. Unlike the previous simulation
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Fig. 10. Position of the viewing point represented by the viewing plane at 5 m distance from the window.
Table 5 DGI values across all 15 sky types comparing anidolic ceiling and reference case DGI
Sky type 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Reference room Anidolic device Decrease
24.7 20.3 4.4
23.2 20.9 2.3
26.4 22.3 4.1
29.0 27.0 2.0
29.4 26.8 2.6
27.3 25.7 1.5
30.7 28.7 2.1
28.3 23.7 4.7
28.5 26.6 1.9
26.0 24.9 1.2
10%
15%
7%
9%
6%
12%
7%
16%
7%
28.9 26.3 2.6 2.5 9% 9%
28.8 26.7 2.2
18%
27.0 25.9 1.1 2.2 4% 7%
32.5 28.5 4.0
Decrease (%)
27.1 25.4 1.7 2.9 6% 11%
7%
4%
DGI Improvement
0.2 Overcast Skies
Partially Cloudy Skies
Clear Skies
0.18 0.16
DGI Decrease
0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 1
2
3
4
5
6
7 8 9 Sky Type
10
11
12
13
14
15
Fig. 11. DGI reduction of anidolic ceiling over reference room charted across all sky types.
run, there seems to be no obvious trend for daylight glare index. According to Table 6, there is no clear group at top or bottom of the ranking.
Table 6 Ranking of sky types with reference to the DGI improvement Rank
DGI decrease
Sky type
Nos. of skies
1 2 3 4
>15% >10 < 15% >5 < 10% <5
1, 4 (overcast), 11 (clear) 2 (overcast), 9 (partly cloudy) 3, 5, 6, 7, 10, 12, 13, 14 (all) 8 (partly cloudy), 15 (clear)
3 2 8 2
4.4. Simulation run 4: Comparison of IR IF for different sun altitudes This simulation run explores the relationship between the IR IF and sun altitude. It extents simulation run 2 by additional sun altitudes of 30 and 70. Fig. 12 shows a general trend whereby the IR IF increases with sun altitude. This trend is expected since the brighter sky being closer to the zenith results in better light incident angle for the collector and conversely less light entering the vertical window. However, overcast skies seem to be the least affected compared to clear and particularly partly cloudy skies.
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S.K. Wittkopf / Solar Energy 81 (2007) 151–161 IR IF comparison for different sun altitude
4
70 deg altitude
Improvement Factor
3.5
50 deg alitude 30 deg altitude
3
2.5
2
1.5
1 1
2
3
4
5
6
7 8 9 Sky Type
10
11
12
13
14
15
Fig. 12. Illuminance ratio improvement for different sun altitude.
4.5. Simulation run 5: Comparison of DGI reduction for different sun altitude
Improvement trend for sky groups
Average IR IF
3.5
This simulation run explores the relationship between the DGI reduction and sun altitude. It extents simulation run 3 by additional sun altitudes of 30 and 70. Unlike in simulation run 4 there seems to be no obvious trend relating DGI reduction to sun altitude. Instead Fig. 14 suggests that it is more related to the type of sky, with the overcast types on average resulting in higher improvements.
3
2.5 Overcast Partly cloudy
2
Clear
1.5 30
50
70
Sun altitude
5. Adaptation of IR IF and DGI reduction for sites of different latitude
Fig. 13. IR IF trend.
Lam et al. (1999) have thoroughly evaluated six sky luminance prediction models using measured data from Singapore. It is shown that the choice of sky model significantly affected the accuracy of prediction. Three models were found to give good predictions, however with some scatter. Tregenza (1999) has compared the set of 15 standard skies with large samples of measured sky luminance
On average overcast skies seem to produce the highest improvement regardless of the sun altitude. The trend can be seen in Fig. 13. Clear skies, on the other hand, depend highly on sun altitude to make up for high improvements. This trend is the same for partly cloudy skies, but on a higher level.
DGI Decrease (%) for different sun altitude 0.2
70 deg altitude 50 deg altitude 30 deg altitude
DGI Decrease
0.15
0.1
0.05
0 1
2
3
4
5
6
7
8
9
10
11
12
Sky Type
Fig. 14. DGI reduction for different sun altitude.
13
14
15
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Table 7 Prevailing sky types for different locations Site
Location
Climate classification (Ko¨ppen)
Prevailing sky types and their relative frequencies
Singapore Fukuoka Sheffield
1.5N, 104E 33.5N, 130E 53.5N, 1.5W
Af (Tropical wet) CfA (Humid subtropical) Cfb (Marine west coastal)
1 (33%), 4 (28%), 8 (28%), 13 (11%) 1 (42%), 3 (27%), 4 (6%), 5 (18%) 1 (16%), 3 (40%), 8 (20%), 11 (24%)
distributions from Singapore, Fukuoka, Garston and Sheffield. It was found that a basket of about four sky types was adequate to represent the skies occurring at each site. Here, three locations are selected representing low, mid and high latitude regions roughly corresponding to high, mid and low sun altitudes. Table 7 lists their prevailing sky types and frequencies. 5.1. Singapore Singapore’s location near to the equator results in high sun altitudes, with a maximum of 87, 68, 89, and 65 for spring equinox, summer solstice, autumn equinox, and winter solstice respectively. Thus an average of about 70 is adequate to represent the likely IR IF. The tropical hot and humid climate is characterized by a high vertical extent of cloud with totally clear skies uncommon in most months. Overcast skies make up about 60%, followed by partly cloudy type 8 with distinct solar corona and some occurrence of a white-blue clear sky type 11. In general these are excellent conditions for anidolic ceilings. This is reflected in Fig. 15a by a high illuminance ratio improve-
ment of 3.3 after considering the different weightings of the sky types. Similarly Fig. 15b shows an average DGI reduction of 14%. 5.2. Fukuoka Fukuoka’s latitude is about 30 different from Singapore. The sun altitude peaks at 56, 80, 57, and 33 for spring equinox, summer solstice, autumn equinox, and winter solstice respectively. Here an average of about 50 is about adequate to represent the likely IR IF. The climate is humid subtropical with a high overcast sky occurrence of 70%. After considering different weightings of the sky types an average IR IF of 2.5 and average DGI reduction of 11% can be derived (Fig. 16a and b). This is significantly less than that for Singapore. 5.3. Sheffield Sheffield is considered a high latitude location with a maximum sun altitude capped significantly lower than the previous sites. The altitude peaks at 36, 60, 38, and as
IR IF for Singapore
DGI reduction (%)
4.0
IR IF
3.5 3.0 2.5
IR IF Weighted Average
2.0 1.5 1 (33%)
(a)
4 (28%)
8 (28%)
20 18 16 14 12 10 8 6 4 2 0
13 (11%)
DGI reduction for Singapore
DGI Weighted Average
1 (33%)
(b)
Sky type
4 (28%)
8 (28%)
13 (11%)
Sky type
Fig. 15. IR and DGI improvement across best-fit sky types for Singapore.
IR IF for Fukuoka
DGI reduction for Fukuoka
4.0
20
IR IF
DGI reduction (%)
IR IF
3.5
Weighted Average
3.0 2.5 2.0
10 DGI
5
Weighted Average
0
1.5 1 (42%)
(a)
15
3 (27%)
4 (6%)
Sky type
5 (18%)
1 (42%)
(b)
3 (27%)
4 (6%)
Sky type
Fig. 16. IR and DGI improvement across best-fit sky types for Fukuoka.
5 (18%)
160
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low as 13 for spring equinox, summer solstice, autumn equinox, and winter solstice respectively. An average of about 30 is about adequate to represent the likely IR IF here. The climate is tempered maritime with rain occurring at any time of the year and the dominant sky type is partly cloudy. The improvement factor of an anidolic ceiling is here at its lowest. However the dominant partly cloudy sky makes up for losses through the significantly higher latitude and the average improvement of 2.4 is just below the subtropical Fukuoka, as can be seen in Fig. 17a. Similarly Fig. 17b shows the DGI reduction for this location with the average of 7.5% being significantly lower than the previous ones.
5.4. Comparing all sites The dominant factors for the improvement factor of an anidolic ceiling are the dominant sky types and sun altitude. An ideal mix would be a partly cloudy sky preferably type 8 complemented by a high sun altitude. Singapore is closest to this situation with very high sun altitude and after all 28% occurrence of type 8. Fukuoka’s disadvantage is the too high occurrence of overcast skies which minimizes the positive effect of the subtropical sun altitude. This ratio is better for Sheffield where the predominantly partly cloudy skies make up for the disadvantageous low sun altitude. Fig. 18a and b shows average and range of IR IF and DGI reduction for all sites and clearly anidolic
ceilings perform much better in tropical Singapore than in high latitude climates. 6. Conclusions The aim of this research was to establish to what extend anidolic ceiling improve the illuminance ratio and daylight glare index in interiors. A further investigation into energy savings is recommended and subject of future publications. Methodologically the new set of CIE standard skies was taken to simulate a maximum range of daylight conditions. These standard skies have been turned into virtual sky domes, enabling advanced light simulation software to accurately model the daylight propagation through the anidolic ceiling. New terms illuminance ratio improvement factor and daylight glare index reduction have been introduced to quantify how much the daylighting improves if a standard ribbon fac¸ade is complemented with an anidolic ceiling. The results suggest that anidolic ceilings are an excellent mean to improve both the illuminance ratio and daylight glare in buildings, particularly in Singapore. The improvement is much higher as Fukuoka and Sheffield, representing locations of mid and high latitude. In particular the results go to show that: • Singapore is an optimal location where integrated anidolic devices are used to their full capacity. They can greatly improve the illuminance ratio in deep rooms by DGI reduction for Sheffield
IR IF for Sheffield 20
3.5
IR IF
3.0
Weighted Average
2.5 2.0
DGI reduction (%)
IR IF
4.0
DGI
15
Weighted Average
10 5
1.5
0 1 (16%)
(a)
3 (40%)
8 (20%)
1 (16%)
11 (24%)
(b)
Sky type
3 (40%)
8 (20%)
11 (24%)
Sky type
Fig. 17. IR and DGI improvement across best-fit sky types for Sheffield.
DGI reduction (High, low, average) across sites
IR IF (High, low, average) across sites 20
4.0
DGI reduction
IR IF
3.5 3.0 2.5
15
10
2.0 1.5
(a)
5
0
Sheffield
Fukuoka
Singapore
(b)
Sheffield
Fukuoka
Fig. 18. IR IF and DGI reduction average and range across all sites.
Singapore
S.K. Wittkopf / Solar Energy 81 (2007) 151–161
•
•
•
•
•
factor of 3.3 and reduce glare by 14%. Fukuoka and Sheffield, although 20 apart in latitude score a much lower IR IF of 2.4. Fukuoka’s disadvantage is the too high occurrence of overcast skies which minimizes the positive effect of the subtropical sun altitude. This ratio is better for Sheffield where the predominately partly cloudy skies make up for the disadvantageous low sun altitude. The improvements are generally high in overcast regions, regardless the sun altitude. The dependency reverses for clear sky regions, with a generally lower improvement, but steeper rising with sun altitude. The relationship is more balanced for partly cloudy skies. A chart like in Fig. 13 can be used as a rough guide of the illuminance ratio improvement factor for various sky types and sun altitudes. One can easily anticipate that high latitude regions with low sun position and dominant clear sky result in low improvements, probably not enough to justify an anidolic ceiling. On the other hand, low latitude regions with overcast or partly cloudy skies are identified to yield the best improvement. An anidolic ceiling provides a more homogenized daylight distribution in the interior compared to other fac¸ade options, such as additional clerestory windows. It reduces the disturbing overprovision of daylight closer by the window through the shading effect of the external collector and increases the necessary levels in the rear of the room through re-directing the oversupply of light into areas of need. The application of the new terms illuminance ratio improvement factor (IR IF) and daylight glare index reduction (DGI R) helps to quantify and thus compare the applicability of anidolic systems in different locations. Applying the VSD method for the new CIE general sky enables an accurate representation of the daylight conditions of any site, which is necessary for accurate IR IF and DGI R simulation.
Given these positive results, subsequent research should be undertaken to pave the way for anidolic ceiling in Singapore. A solar architecture of this kind would contribute to energy conservation towards a sustainable build environment. Acknowledgement This research is funded by an academic research grant from the National University of Singapore and in equal terms from National Environment Agency of Singapore. The author would like to thank Professor J.-L. Scartezzini
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for sending detailed information on anidolic ceilings and M. Jongewaard for his help in implementing the VSD method in the software Photopia; furthermore research assistant E. Yuniarti for her help in computational simulation and data post-processing.
References Baker, N., Fanchiotti, A., Steemers, K., 1993. Daylighting in Architecture. A European Reference Book. James and James, London, UK. Chauvel, P., 1982. Glare from windows: current views of the problem. Lighting Research & Technology 14 (1), 31–46. CIE S 011/E, 2003. Spatial Distribution of Daylight – CIE Standards General Sky. Commission International de l’Eclairage, Vienna. Edmonds, I.R., Greenup, P.J., 2002. Daylighting in the tropics. Solar Energy 73 (2), 111–121. Hopkinson, R.G., 1972. Glare from daylight in buildings. Applied Ergonomics 34, 206–215. Kittler, R., Darula, S., 2002. IE general sky standard defining luminance distributions. In: Proceedings of Canadian Conference on Building Energy Simulation (IBPSA-Canada), Montreal, Canada. Kittler, R., Darula, S., Wittkopf, S., 2006. Outdoor illuminance levels in the tropics and their representation in the Virtual Sky Dome. Architectural Science Review 49 (3), in press. Lam, W.C., 1977. Perception and Lighting as Formgivers for Architecture. McGraw-Hill, New York, USA. Lam, K.-P., Ng, E., Mahdavi, G., Ullah, M.B., 1999. Evaluation of six sky luminance prediction models using measured data from Singapore. Lighting Research & Technology 73 (2), 123–135. Lieberman, J., 1991. Light-medicine of the future: how we can use it to heal ourselves now. Bear and Co. Publ., Santa Fe, USA. ¨ ., Onaygil, S., 2004. Subjective experience of Nazzal, A., Gu¨ler, O discomfort glare in a daylit computerized office in Istanbul and its mathematical prediction with the DGIN method. Bulletin of the Istanbul Technical University 54 (3). Ruck, N. et al., 2000. Daylight in Buildings – A Sourcebook on Daylighting Systems and ComponentsPublication of the IEA SHC programme. Lawrence Berkeley National Laboratory, USA. Scartezzini, J.-L., Gourret, G., 2002. Anidolic daylighting systems. Solar Energy 73 (2), 123–135. Smiley, F., 1996. Students delight in daylight. International Association for Energy Efficient Lighting Newsletter 5 (2), 11–12. Tregenza, P., 1999. Standard skies for maritime climates. Lighting Research & Technology 31 (3), 97–106. Welford, W.T., Winston, R., 1989. Non-Imaging Optics. Academic, Pres, New York, USA. Wittkopf, 2004. A method to construct virtual sky domes for use in standards CAD-based light simulation software. Architectural Science Review 47 (3), 275–286. Wittkopf, 2005. Evaluation of virtual sky domes for the prediction of daylight performance with Radiosity-based light simulation software. Architectural Science Review 48 (2). Wittkopf, S., Soon, L.K., Yuniarti, E., Grobe, L., 2006. Virtual Sky Domes—Making the CIE/ISO Standard General Sky available for CAD-based light simulation software. In: Proceedings IBPSA-Canada’s 4th Biennial Building Performance Simulation Conference, Toronto, Canada.
Daylight performance of anidolic ceiling under different sky conditions S.K. Wittkopf
*
School of Design and Environment, Department of Architecture, National University of Singapore, 4 Architecture Drive, Singapore 117566, Singapore Received 27 July 2005; received in revised form 13 April 2006; accepted 14 April 2006 Available online 24 May 2006 Communicated by: Associate Editor Jean Rosenfeld
Abstract The daylight performance of anidolic ceiling for diffuse daylight is the subject of this article. The performance is assessed over a wide range of sky conditions following the new CIE Standard General Sky and compared between locations in Singapore, Japan and the United Kingdom. The criteria illuminance ratio (IR) and daylight glare index (DGI) are used to quantify the daylight performance of a default ribbon window fac¸ade with and without an anidolic ceiling. The difference is expressed in new terms IR improvement factor (IR IF) and DGI reduction. These factors are charted over all 15 sky conditions and various sun altitudes and may serve as general references indicating conditions under which anidolic ceilings perform best. The application of these new terms are demonstrated for Singapore, Fukuoka and Sheffield, representing three main latitude bands and different sets of prevailing sky types. It can be concluded that the daylight performance improvement through anidolic ceilings is most significant in Singapore. 2006 Elsevier Ltd. All rights reserved. Keywords: Daylight performance; Anidolic ceiling; Computational simulation
1. Introduction The demand for a sustainable environment and in particular sustainable architecture is ubiquitous. It is well known that energy consumption in buildings is too high and contributes significantly to the hazardous global warming. Buildings in hot climates are commonly designed to block the solar radiation; the window is either tinted or shaded extensively, which in turn also blocks the daylight from entering the interior efficiently leaving the rear room areas gloomy. Daylight in interiors contributes significantly to the human well being (Lieberman, 1991). It is also reported that daylight helps to improve the productivity in work i.e. in schools (Smiley, 1996). A human perception oriented daylighting design must consider functional and biological needs, with the visual connection to the outside
*
Tel.: +65 6516 3481; fax: +65 6779 3078. E-mail address: [email protected]
0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.04.002
being a predominant demand to provide for sense of orientation, time and security (Lam, 1977). An advanced use of daylight is characterized by efficiently re-directing daylight into the interior while keeping the solar heat out. This helps to reduce the need for electrical lighting, being one major building load. The many daylighting systems around have been classified by the IEA SHC Task 21 Subtask A on Performance Evaluation of Daylight Systems (Ruck et al., 2000) and Task 31 on Daylighting Buildings in the 21st Century. A system matrix differentiates daylight systems by shading task (shading or non-shading) and daylight preference (diffuse or direct). A checklist has been developed which includes further features such as ability to reduce glare or to allow view outside, etc. to support in determining the appropriate requirements and systems. Successful applications of daylighting systems have been investigated and their performances are broadly qualified. This paper focuses on one anidolic ceiling, classified as non-shading device and designed for diffuse daylight.
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Anidolic devices are advanced light re-directing devices through which a homogenized daylight distribution in the interior is achieved, i.e. reducing the daylight factor closer to the window and increasing it in the rear part of the room. The most prominent feature is the external scoop, which is usually located in the upper part of the fac¸ade, followed by a funnel (anidolic ceiling) and exit aperture. This system is shaped after the non-imaging optics to allow a maximum of light being collected and re-distributed internally with a minimum number of reflections (Welford and Winston, 1989). The inner surfaces need to have a very high reflectance of more than 90% to minimize the absorption occurring at every reflection. Several anidolic ceilings have been monitored in temperate climate regions of overcast sky conditions and the improvement in the rear of the room is significant (Scartezzini and Gourret, 2002). Concerning tropical climates mainly sunlight re-directing systems have been introduced and their application may prove successful (Edmonds and Greenup, 2002). The likely performance of anidolic ceilings for diffuse daylight over the complete range of possible sky conditions is subject of this article. It helps to identify under which sky condition the performance is maximized or marginal. The terms illuminance ratio (IR) and daylight glare index (DGI) are used to quantify the daylight performance. Illuminance ratio is more appropriate for all sky types including clear and partly cloudy, since the daylight factor is only used in conjunction with overcast skies. A new term illuminance ratio improvement factor (IF) was introduced to quantify the performance improvement over a reference fac¸ade without an anidolic ceiling. This factor is charted for all sky conditions and may serve as a reference to decide whether the application makes sense or not. For example, there are certain assumptions that they perform well under partly cloudy sky conditions with relative high zenithal luminance such as in the tropical region, but these assumptions have not yet been verified or quantified.
2. Spatial distribution of daylight The wide range of daylight conditions is represented by the 15 different sky types described in the CIE Standard General Sky (CIE, 2003). This new standard extends the previous set of three sky types (overcast, intermediate and clear only) by adding five subtypes for each type which allows a more accurate representation of the various spatial distributions of daylight. This set of 15 sky types has already been used to characterize the standard skies for various maritime climates (Tregenza, 1999). The sky luminance distribution depends mainly on (a) the gradation between horizon and zenith and (b) the scattering around the sun spot (Fig. 1). Certain indices are used to characterize the different sky types and to calculate the luminosity of any sky patch (Kittler and Darula, 2002). Table 1 lists the main characteristics of the 15 different sky types.
Fig. 1. Stereographic view of the hemisphere with superimposed circles denoting luminance gradation between horizon and zenith and scattering around the sun disk.
3. Computational simulation of the 15 sky types – the virtual sky dome method The performance comparison for different sky conditions and locations would require at least one fac¸ade system at each location. And furthermore a consensus is required on the definition of real sky conditions to ensure similar test conditions at each site. Computational simulation helps to build the different sky conditions and sun altitudes which in turn ensure better comparison. The virtual sky dome is one method to generate a CIE general sky compliant representation of the daylight conditions for use in 3D-CAD based lighting simulation software (Wittkopf, 2004). A VSD imitates the spatial luminance distribution of the sky vault by 145 distinct light sources whose distribution over the hemisphere follows the conventions of sky patch luminance measurements and whose individual luminous flux is calculated using the set of equations for the 15 sky types (Kittler and Darula, 2002; Kittler et al., 2006). A tool has been developed that calculates the luminous flux values for any sun position, sky type and radius of the sky dome. These VSD data is converted into ASCII file formats eventually representing the spatial distribution of daylight for a particular sky type, time and location in a wider range of 3D CAD based light simulation software. Fig. 2 shows charts illuminance values on different planes using a VSD representing a particular date, time, and location. The VSD method has been applied for use with the light simulation software Lightscape and the concordance of daylight factors with a reference case (Tregenza, 1999) has verified the universal application of the VSD method (Wittkopf, 2005; Wittkopf et al., 2006). However the simulation of the optical system of the anidolic devices requires an algorithm that accounts for accurate photometric analysis and true reflections. A standard Radiosity based algorithm would fail in modeling the reflections correctly since they are by default simplified to be ideal diffuse irrespective of the incident angle. Instead,
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Table 1 Main characteristics of the 15 different sky types Type
Gradation
Indicatrix
Description of luminance distribution
Group
1
I
1
Overcast skies
2
I
2
3 4 5 6
II II III III
1 2 1 2
7 8 9 10 11 12 13 14 15
III III IV IV IV V V VI VI
3 4 2 3 4 4 5 5 6
CIE standard overcast sky, alternative form, steep luminance gradation towards zenith, azimuthal uniformity Overcast, with steep luminance gradation and slight brightening towards the sun Overcast, moderately graded with azimuthal uniformity Overcast, moderately graded and slight brightening towards the sun Sky of uniform luminance Partly cloudy sky, no gradation towards zenith, slight brightening towards the sun Partly cloudy sun, no gradation towards zenith, brighter circumsolar region Partly cloudy sky, no gradation towards zenith, distinct solar corona Partly cloudy, with the obscured sun Partly cloudy, with brighter circumsolar region White-blue sky with distinct solar corona CIE standard clear sky, low illuminance turbidity CIE standard clear sky, polluted atmosphere Cloudless turbid sky with broad solar corona White-blue turbid sky with broad solar corona
Partly cloudy skies
Clear skies
Fig. 2. Partly cloudy sky type 8 as stereographic view and illuminance values across different plane orientations for Singapore 21 June.
Photopia, an optical design and analysis software has been selected that produces comprehensive performance evaluations for non-imaging optical designs. It is based on the backwards ray tracing algorithm and provides data of various commercially available reflector materials. Subsequently the universal VSD method was applied for use within this software. 4. Anidolic ceiling performance simulation Various VSD and fac¸ade designs are set up for predicting the performance of the system under different sky types and sun altitude. The following chapter lists several simulation runs with gradually increasing level of detail. The first simulation run starts by comparing the illuminance ratio across different fac¸ade configurations, so as to introduce the improvement factor of the anidolic ceiling. The second limits the fac¸ade configuration to the default and anidolic ceiling only, but extends the illuminance ratio assessment
across all sky types as to detect trends and preferences. The third simulation run includes the daylight glare index assessment, so as to verify any trends. The next two simulation runs complement the default sun altitude of 50 by 30 and 70, so as to check on the superiority of trends, being it more sensitive to the sky type or sun altitude. Simulation run 4 approaches this with illuminance ratio and simulation run 5 with daylight glare index assessment respectively. 4.1. Simulation run 1: Comparison of 3 different fac¸ade designs The first simulation run is to introduce the impact of the anidolic ceiling on the illuminance ratio by comparing it to other fac¸ade configurations. The reference fac¸ade comprises a single glazing ribbon window commonly used in Singapore, as can be seen in Figs. 3 and 4. It is complemented with a second upper ribbon as clerestory windows
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Fig. 6. Elevation of configuration 3: anidolic ceiling.
Fig. 3. 3 D View and elevation of default fac¸ade configuration: Ribbon window.
Fig. 4. 3D View and elevation of default fac¸ade configuration: Ribbon window.
in the second configuration (Fig. 5). The third configuration is the anidolic ceiling comprising an external light collector (replacing the clerestory window), anidolic ceiling and an exit aperture located deeper in the room (Fig. 6). The dimensions follow the dimensions of tested and established mock-up models (Scartezzini and Gourret, 2002). A forth setup looks at the shading effect of the external collector only.
The room receives diffuse daylight from the sky vault only, since the fac¸ade faces north and the sun is due south at an altitude of 50. A sky type of uniform luminance distribution was selected (type no. 5). The room dimensions are 6 · 6 m and 3.21 m floor-to-ceiling height, as can be seen in Fig. 3. The reflectance and transmission values are listed in Table 2. The reading plane for the Illuminance Ratios is 0.72 m above the floor. All illuminance ratios show an extreme inhomogeneous distribution across the room depth (Fig. 7). This is expected since the room is deep and illuminated from one side only. The illuminance ratio of the reference case (ribbon window) drops to almost a quarter at a distance from the window, which is equal to the ceiling height (rear area). It is further halved at the rear wall of the room with a distance equal to approx. two times ceiling height. This trend is about the same for additional clerestory windows or the design with the external collector shading the window with the absolute values being above and respectively below the reference case. Of course, additional clerestory windows bring more light in deeper areas but also in those already overexposed areas, thus not improving the negative imbalanced distribution at all. The anidolic ceilings however makes a significant contribution towards leveling the illuminance ratio. It reduces the unnecessary peak closer by the window through the shading effect of the external anidolic collector and increases the necessary levels in the rear of the room, through re-directing the oversupply of light into areas of need. With the exit aperture of the anidolic ceiling being located 4 m away from the window, the illuminance ratios starts to increase from here and maintains on a relatively high level. This improvement is best quantified as illuminance ratio improvement factor (IR IF = IR design/IR reference design) charted over the relevant rear room area. Fig. 8 Table 2 Reflectance and transmittance of surfaces
Fig. 5. Elevation of configuration 2: additional clerestory window.
Surface type
Properties
Walls Ceiling Floor Anidolic system Window glass Glass over exit aperture
50% 80% 20% 90% 92% 92%
perfectly diffuse reflectance perfectly diffuse reflectance perfectly diffuse reflectance perfectly specular reflectance transmittance transmittance
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Illuminance Ratio Comparison 30% Reference Add. clerestory window
25%
Illuminance Ratio
Add. anidolic ceiling 20%
External collector as shading
15%
10%
5%
0.125 0.375 0.625 0.875 1.125 1.375 1.625 1.875 2.125 2.375 2.625 2.875 3.125 3.375 3.625 3.875 4.125 4.375 4.625 4.875 5.125 5.375 5.625 5.875
0%
Distance from Window (m) Fig. 7. Illuminance ratios across all four fac¸ade designs with the anidolic ceiling being the only solution to improve the illuminance ratio distribution significantly. Illuminance Ratio Improvement Factor (IR IF) Comparison 2.6 2.4
IR IF
2.2 2.0
Add. anidolic ceiling
1.8
Add. clerestory window External collector as shading
1.6 1.4 1.2 1.0
5.875
5.625
5.375
5.125
4.875
4.625
4.375
4.125
0.8
Distance from Window (m)
Fig. 8. Improvement factor over reference design with the anidolic ceiling scoring an average improvement of 2.2.
charts the improvement factor with the reference illuminance ratio case being factor 1. Clearly the improvement of the anidolic ceiling outperforms the other designs by far. The improvement factor is about 2 underneath the exit aperture and peaks at 2.6 even deeper inside the room. The average IR IF between the exit aperture and rear wall is around 2.2. 4.2. Simulation run 2: Comparison of IR IF across all 15 sky types The previous simulation run was focusing on sky type 5 (uniform luminance distribution) only. This simulation run extends it over the remaining 14 sky types, so as to identify
sky types that result in higher or lower improvement factors. The designs with additional clerestory windows and external collector as shading device are no longer considered. The focus is on the improvement factor of the anidolic ceiling over the default ribbon window design. Fig. 9 compares the average IR IF between the exit aperture and rear wall of the room across all sky types. The improvement factor across the various sky types is quite different and ranges between 3.5 for overcast sky type 2 and 1.9 for clear sky type 12 respectively. All clear skies (types 11–15) result in significant lower improvements, with an average factor of around 2. Conversely, overcast skies (types 1–5) provide the highest improvements; even their lowest still exceeds all of the clear skies. The deviation is
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S.K. Wittkopf / Solar Energy 81 (2007) 151–161 IR IF Comparison 4.0
Overcast Skies
Partly Cloudy Skies
Clear Skies
3.5
IR IF
3.0 2.5 2.0 1.5 1.0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Sky Type
Fig. 9. Illuminance ratio improvement factor across all sky types.
Table 3 Grouped ranking of sky types in terms of IR IF
Table 4 Daylight glare index and corresponding subjective perception levels
Rank
IR IF range
Sky type
Nos. of skies
1 2 3 4
>3.5 Around 3 >2.5 < 3 2–2.5
2 (overcast) 1, 3 (overcast) 3, 6, 7, 8 (overcast and partly cloudy) All clear and remaining partly cloudy skies
1 2 4 8
also quite different between the group of skies, lowest again within the group of clear skies (types 11–15) and highest for overcast skies (types 1–5). Table 3 ranks groups of sky types by their improvements. 4.3. Simulation run 3: Comparison of DGI across all 15 sky types The third simulation run looks at the daylight glare index (DGI) instead of IR. Glare is an important factor affecting visual comfort. Windows can be extremely bright sources of daylight and the resulting high and uneven distribution of brightness in the field of view can cause discomfort or even disability glare. The user usually responds to daylight glare through pulling blinds and switching on electrical lights. This compromises the view to the outside neglecting biological needs and furthermore results in an unnecessary high consumption of electricity. The window itself is in most of the cases not the immediate cause of glare but rather the visible luminous sky. With clouds are brighter than a deeps blue clear sky, the risk of daylight glare increases under partly cloudy and overcast skies. A common calculation of daylight glare is provided by Chauvel (1982) based on Hopkinson (1972) equations for general glare. Nazzal (2004) has developed it into a new daylight glare index which is used in this simulation run. Baker et al., 1993) grouped daylight glare indices into comprehensive subjective perception levels, which are also used in this simulation run and listed in Table 4.
DGI
Subjective perception level
<16 16–20 20–24 24–28 >28
Imperceptible Perceptible Acceptable Uncomfortable Intolerable
The user position and field of view for the glare assessments is centered at 5 m distance from the window, with the view line towards the center of the window. It thus reflects the worst-case, which is deemed sufficient for this study. The height of 1.25 m corresponds to a sitting position. As such a vertical plane is defined for which the illuminance is recorded (Fig. 10). Two values are obtained for the calculation of DGI, with the first one being the direct illuminance from the window only and the second one representing the true illuminance by direct light and indirect inter-reflections from the room surfaces. Table 5 and Fig. 11 show DGI reduction for the reference case (ribbon window) and anidolic ceiling across all sky types. The DGI improvement varies between 18% and 4% for clear sky type 1 and partly cloudy sky type 8 respectively. Expressed in quantitative terms the DGI is improved by an average of 11%, 7% and 9% for overcast, partly cloudy and clear skies respectively. In 60% of sky types the implementation of the anidolic ceiling improves the subjective appreciation of glare by at least one level, i.e. from ‘uncomfortable (24.7)’ to ‘acceptable (20.3)’ in the case of sky type 1. For the rest 40% the glare remains within the same perception level. However, despite an average improvement of approx 8%, 73% of the sky types are still in the uncomfortable or intolerable level. They are mainly partly cloudy and clear skies, which remain in unacceptable range. This is mainly due to the window and view configuration of this particular room which is indeed critical. Unlike the previous simulation
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Fig. 10. Position of the viewing point represented by the viewing plane at 5 m distance from the window.
Table 5 DGI values across all 15 sky types comparing anidolic ceiling and reference case DGI
Sky type 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Reference room Anidolic device Decrease
24.7 20.3 4.4
23.2 20.9 2.3
26.4 22.3 4.1
29.0 27.0 2.0
29.4 26.8 2.6
27.3 25.7 1.5
30.7 28.7 2.1
28.3 23.7 4.7
28.5 26.6 1.9
26.0 24.9 1.2
10%
15%
7%
9%
6%
12%
7%
16%
7%
28.9 26.3 2.6 2.5 9% 9%
28.8 26.7 2.2
18%
27.0 25.9 1.1 2.2 4% 7%
32.5 28.5 4.0
Decrease (%)
27.1 25.4 1.7 2.9 6% 11%
7%
4%
DGI Improvement
0.2 Overcast Skies
Partially Cloudy Skies
Clear Skies
0.18 0.16
DGI Decrease
0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 1
2
3
4
5
6
7 8 9 Sky Type
10
11
12
13
14
15
Fig. 11. DGI reduction of anidolic ceiling over reference room charted across all sky types.
run, there seems to be no obvious trend for daylight glare index. According to Table 6, there is no clear group at top or bottom of the ranking.
Table 6 Ranking of sky types with reference to the DGI improvement Rank
DGI decrease
Sky type
Nos. of skies
1 2 3 4
>15% >10 < 15% >5 < 10% <5
1, 4 (overcast), 11 (clear) 2 (overcast), 9 (partly cloudy) 3, 5, 6, 7, 10, 12, 13, 14 (all) 8 (partly cloudy), 15 (clear)
3 2 8 2
4.4. Simulation run 4: Comparison of IR IF for different sun altitudes This simulation run explores the relationship between the IR IF and sun altitude. It extents simulation run 2 by additional sun altitudes of 30 and 70. Fig. 12 shows a general trend whereby the IR IF increases with sun altitude. This trend is expected since the brighter sky being closer to the zenith results in better light incident angle for the collector and conversely less light entering the vertical window. However, overcast skies seem to be the least affected compared to clear and particularly partly cloudy skies.
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S.K. Wittkopf / Solar Energy 81 (2007) 151–161 IR IF comparison for different sun altitude
4
70 deg altitude
Improvement Factor
3.5
50 deg alitude 30 deg altitude
3
2.5
2
1.5
1 1
2
3
4
5
6
7 8 9 Sky Type
10
11
12
13
14
15
Fig. 12. Illuminance ratio improvement for different sun altitude.
4.5. Simulation run 5: Comparison of DGI reduction for different sun altitude
Improvement trend for sky groups
Average IR IF
3.5
This simulation run explores the relationship between the DGI reduction and sun altitude. It extents simulation run 3 by additional sun altitudes of 30 and 70. Unlike in simulation run 4 there seems to be no obvious trend relating DGI reduction to sun altitude. Instead Fig. 14 suggests that it is more related to the type of sky, with the overcast types on average resulting in higher improvements.
3
2.5 Overcast Partly cloudy
2
Clear
1.5 30
50
70
Sun altitude
5. Adaptation of IR IF and DGI reduction for sites of different latitude
Fig. 13. IR IF trend.
Lam et al. (1999) have thoroughly evaluated six sky luminance prediction models using measured data from Singapore. It is shown that the choice of sky model significantly affected the accuracy of prediction. Three models were found to give good predictions, however with some scatter. Tregenza (1999) has compared the set of 15 standard skies with large samples of measured sky luminance
On average overcast skies seem to produce the highest improvement regardless of the sun altitude. The trend can be seen in Fig. 13. Clear skies, on the other hand, depend highly on sun altitude to make up for high improvements. This trend is the same for partly cloudy skies, but on a higher level.
DGI Decrease (%) for different sun altitude 0.2
70 deg altitude 50 deg altitude 30 deg altitude
DGI Decrease
0.15
0.1
0.05
0 1
2
3
4
5
6
7
8
9
10
11
12
Sky Type
Fig. 14. DGI reduction for different sun altitude.
13
14
15
S.K. Wittkopf / Solar Energy 81 (2007) 151–161
159
Table 7 Prevailing sky types for different locations Site
Location
Climate classification (Ko¨ppen)
Prevailing sky types and their relative frequencies
Singapore Fukuoka Sheffield
1.5N, 104E 33.5N, 130E 53.5N, 1.5W
Af (Tropical wet) CfA (Humid subtropical) Cfb (Marine west coastal)
1 (33%), 4 (28%), 8 (28%), 13 (11%) 1 (42%), 3 (27%), 4 (6%), 5 (18%) 1 (16%), 3 (40%), 8 (20%), 11 (24%)
distributions from Singapore, Fukuoka, Garston and Sheffield. It was found that a basket of about four sky types was adequate to represent the skies occurring at each site. Here, three locations are selected representing low, mid and high latitude regions roughly corresponding to high, mid and low sun altitudes. Table 7 lists their prevailing sky types and frequencies. 5.1. Singapore Singapore’s location near to the equator results in high sun altitudes, with a maximum of 87, 68, 89, and 65 for spring equinox, summer solstice, autumn equinox, and winter solstice respectively. Thus an average of about 70 is adequate to represent the likely IR IF. The tropical hot and humid climate is characterized by a high vertical extent of cloud with totally clear skies uncommon in most months. Overcast skies make up about 60%, followed by partly cloudy type 8 with distinct solar corona and some occurrence of a white-blue clear sky type 11. In general these are excellent conditions for anidolic ceilings. This is reflected in Fig. 15a by a high illuminance ratio improve-
ment of 3.3 after considering the different weightings of the sky types. Similarly Fig. 15b shows an average DGI reduction of 14%. 5.2. Fukuoka Fukuoka’s latitude is about 30 different from Singapore. The sun altitude peaks at 56, 80, 57, and 33 for spring equinox, summer solstice, autumn equinox, and winter solstice respectively. Here an average of about 50 is about adequate to represent the likely IR IF. The climate is humid subtropical with a high overcast sky occurrence of 70%. After considering different weightings of the sky types an average IR IF of 2.5 and average DGI reduction of 11% can be derived (Fig. 16a and b). This is significantly less than that for Singapore. 5.3. Sheffield Sheffield is considered a high latitude location with a maximum sun altitude capped significantly lower than the previous sites. The altitude peaks at 36, 60, 38, and as
IR IF for Singapore
DGI reduction (%)
4.0
IR IF
3.5 3.0 2.5
IR IF Weighted Average
2.0 1.5 1 (33%)
(a)
4 (28%)
8 (28%)
20 18 16 14 12 10 8 6 4 2 0
13 (11%)
DGI reduction for Singapore
DGI Weighted Average
1 (33%)
(b)
Sky type
4 (28%)
8 (28%)
13 (11%)
Sky type
Fig. 15. IR and DGI improvement across best-fit sky types for Singapore.
IR IF for Fukuoka
DGI reduction for Fukuoka
4.0
20
IR IF
DGI reduction (%)
IR IF
3.5
Weighted Average
3.0 2.5 2.0
10 DGI
5
Weighted Average
0
1.5 1 (42%)
(a)
15
3 (27%)
4 (6%)
Sky type
5 (18%)
1 (42%)
(b)
3 (27%)
4 (6%)
Sky type
Fig. 16. IR and DGI improvement across best-fit sky types for Fukuoka.
5 (18%)
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low as 13 for spring equinox, summer solstice, autumn equinox, and winter solstice respectively. An average of about 30 is about adequate to represent the likely IR IF here. The climate is tempered maritime with rain occurring at any time of the year and the dominant sky type is partly cloudy. The improvement factor of an anidolic ceiling is here at its lowest. However the dominant partly cloudy sky makes up for losses through the significantly higher latitude and the average improvement of 2.4 is just below the subtropical Fukuoka, as can be seen in Fig. 17a. Similarly Fig. 17b shows the DGI reduction for this location with the average of 7.5% being significantly lower than the previous ones.
5.4. Comparing all sites The dominant factors for the improvement factor of an anidolic ceiling are the dominant sky types and sun altitude. An ideal mix would be a partly cloudy sky preferably type 8 complemented by a high sun altitude. Singapore is closest to this situation with very high sun altitude and after all 28% occurrence of type 8. Fukuoka’s disadvantage is the too high occurrence of overcast skies which minimizes the positive effect of the subtropical sun altitude. This ratio is better for Sheffield where the predominantly partly cloudy skies make up for the disadvantageous low sun altitude. Fig. 18a and b shows average and range of IR IF and DGI reduction for all sites and clearly anidolic
ceilings perform much better in tropical Singapore than in high latitude climates. 6. Conclusions The aim of this research was to establish to what extend anidolic ceiling improve the illuminance ratio and daylight glare index in interiors. A further investigation into energy savings is recommended and subject of future publications. Methodologically the new set of CIE standard skies was taken to simulate a maximum range of daylight conditions. These standard skies have been turned into virtual sky domes, enabling advanced light simulation software to accurately model the daylight propagation through the anidolic ceiling. New terms illuminance ratio improvement factor and daylight glare index reduction have been introduced to quantify how much the daylighting improves if a standard ribbon fac¸ade is complemented with an anidolic ceiling. The results suggest that anidolic ceilings are an excellent mean to improve both the illuminance ratio and daylight glare in buildings, particularly in Singapore. The improvement is much higher as Fukuoka and Sheffield, representing locations of mid and high latitude. In particular the results go to show that: • Singapore is an optimal location where integrated anidolic devices are used to their full capacity. They can greatly improve the illuminance ratio in deep rooms by DGI reduction for Sheffield
IR IF for Sheffield 20
3.5
IR IF
3.0
Weighted Average
2.5 2.0
DGI reduction (%)
IR IF
4.0
DGI
15
Weighted Average
10 5
1.5
0 1 (16%)
(a)
3 (40%)
8 (20%)
1 (16%)
11 (24%)
(b)
Sky type
3 (40%)
8 (20%)
11 (24%)
Sky type
Fig. 17. IR and DGI improvement across best-fit sky types for Sheffield.
DGI reduction (High, low, average) across sites
IR IF (High, low, average) across sites 20
4.0
DGI reduction
IR IF
3.5 3.0 2.5
15
10
2.0 1.5
(a)
5
0
Sheffield
Fukuoka
Singapore
(b)
Sheffield
Fukuoka
Fig. 18. IR IF and DGI reduction average and range across all sites.
Singapore
S.K. Wittkopf / Solar Energy 81 (2007) 151–161
•
•
•
•
•
factor of 3.3 and reduce glare by 14%. Fukuoka and Sheffield, although 20 apart in latitude score a much lower IR IF of 2.4. Fukuoka’s disadvantage is the too high occurrence of overcast skies which minimizes the positive effect of the subtropical sun altitude. This ratio is better for Sheffield where the predominately partly cloudy skies make up for the disadvantageous low sun altitude. The improvements are generally high in overcast regions, regardless the sun altitude. The dependency reverses for clear sky regions, with a generally lower improvement, but steeper rising with sun altitude. The relationship is more balanced for partly cloudy skies. A chart like in Fig. 13 can be used as a rough guide of the illuminance ratio improvement factor for various sky types and sun altitudes. One can easily anticipate that high latitude regions with low sun position and dominant clear sky result in low improvements, probably not enough to justify an anidolic ceiling. On the other hand, low latitude regions with overcast or partly cloudy skies are identified to yield the best improvement. An anidolic ceiling provides a more homogenized daylight distribution in the interior compared to other fac¸ade options, such as additional clerestory windows. It reduces the disturbing overprovision of daylight closer by the window through the shading effect of the external collector and increases the necessary levels in the rear of the room through re-directing the oversupply of light into areas of need. The application of the new terms illuminance ratio improvement factor (IR IF) and daylight glare index reduction (DGI R) helps to quantify and thus compare the applicability of anidolic systems in different locations. Applying the VSD method for the new CIE general sky enables an accurate representation of the daylight conditions of any site, which is necessary for accurate IR IF and DGI R simulation.
Given these positive results, subsequent research should be undertaken to pave the way for anidolic ceiling in Singapore. A solar architecture of this kind would contribute to energy conservation towards a sustainable build environment. Acknowledgement This research is funded by an academic research grant from the National University of Singapore and in equal terms from National Environment Agency of Singapore. The author would like to thank Professor J.-L. Scartezzini
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for sending detailed information on anidolic ceilings and M. Jongewaard for his help in implementing the VSD method in the software Photopia; furthermore research assistant E. Yuniarti for her help in computational simulation and data post-processing.
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