Predicting Natural Light in Atria and Adjacent Spaces using Physical Models

Transcription

1 Predicting Natural Light in Atria and Adjacent Spaces using Physical Models Ibrahim Al-Turki P.O.Box 59035, Riyadh Saudi Arabia and Marc Schiler, Assoc. Prof. School of Architecture University of Southern California Los Angeles, CA Abstract-- The study intends to measure the daylight performance in atria and in the spaces adjacent to atria under exclusively clear sky conditions. It also illustrates the quantitative and qualitative available light in all orientations using physical models. A geometry of 1:1:2 for the atria was used. It was found that an atrium with same geometry provides sufficient illumination for the upper half portion of the spaces adjacent to atria, and low illumination level for the bottom half of the adjacent spaces. It was further found that physical model showed that the points immediately adjacent to the rear wall (two feet from the rear wall) in the adjacent spaces receive more illumination than the points five feet from the rear walls. 1. INTRODUCTION Atria have become popular factors in Architecture because they provide several simultaneous values. They resolve thermal problems in exterior walls. In a ddition to their characteristics in cooling a space, they act like private spaces isolated from the exterior noise. Daylighting, however, represents one of the esthetic values of an atrium. Due to a lack of good prediction tools, today's atria are overlit with excessive solar gains. Most daylighting calculations do not work well for atrium buildings. At present, the most sophisticated method used is still the physical scale model (Atif, Boyer & Degelman, 1994). Physical models are used in this research to measure the illumination level inside an atrium and in the adjacent spaces of an atrium building of four floors. 2. BACKGROUND The aim of this study is to provide insight on the relationship between light and form. For example, in some cases light is less desirable and heat gain is extremely undesirable. Therefore, the form of that space is different than when much light is needed. In spaces adjacent to atria, the study of the behavior of light is more interesting than when there is sidelighting opened directly to outside because several variables play major factors as to how the light behaves. There are direct sunlight, diffuse light from the sky, reflected light from the ground and the walls of the atrium, and reflected light from the walls, the ceiling and the ground of the adjacent spaces. 3. DESCRIPTION OF METHOD

2 A physical model of a scale of 1"=1'-0" (1:12) was used to simulate the daylight distribution inside the atrium and the adjacent spaces. An atrium of 20' width, 20' length, and 40' depth was modeled, (four-story) atrium. Shallower atria and courtyards have been modelled in other studies, but for the thermal considerations in hot arid climates, deeper atria are more appropriate. Adjacent spaces were modeled, as well, of 20' length, 20' width, and 10' height The critical issue in such situations is not the light in the atrium (which is usually sufficient), but the light available within the adjacent spaces. The physical model was built of foamcore panels which was covered with black sheets from the back to avoid any light leak. Reflectance materials for the atrium were used as follows: The reflectance of the ground = 30% The net reflectance of the walls = 50% Reflectance materials for the adjacent spaces were used as follows: The net reflectance of the ground = 30% The net reflectance of the walls = 50% The net reflectance of the ceiling = 70% Since one side of the atrium was modeled with adjacent spaces, the other three sides of the atrium should simiulate adjacent spaces, as well. Therefore, clear specular surfaces were added to those walls wherever windows would have been located, and an absorptive surface were placed behind these window surfaces. This approximated the absorptance of a full space behind the windows, but still allowed the specularly reflected component to be included whenever sun angles caused excessive reflection from the glass surfaces. Since there might actually be some slight contribution from the adjacent space back to the atrium, this was a conservative approximation. The difference becomes negligible (Al-Turki, 1994). Actual wall surfaces were the same on all four sides. A glazing area of 40% for all four floors was used (California Energy Commission, 1992). A grid pattern of one foot by one foot was drawn on the ground floor of the model to mark at which point measurements were to be taken. One of the four panels is removable to facilitate the process of placing the measuring devices. The physical model was fixed to a base made of plywood with a small 6" x 6" piece of plywood attached to the bottom of the base. The small piece of plywood was attached to a tripod to facilitate orienting the model in all directions (Al-Turki, 1994). Licor LI-210-S illuminance meters, which are designed to measure the illumination in foot-candles, were connected to a portable computer to read the illumination level from each licor using a computer program called "DATALIT", which was written at University of California, Los Angeles (UCLA), (Milne, 1986). 4. MEASURING THE RESULTS 4.1. The model was tested under exclusively clear sky conditions in Los Angeles, California, 34 N Latitude, where the tests took place.

3 4.2. Seven reference points were carefully selected and distributed on the ground floor of the atrium (Figure 1). Figure 1.Placing light sensors inside the atrium of 20' x 20' x 40' to measure the illumination level from the physical model Eighteen reference points were selected at the workplane level (3 feet) and located inside the adjacent spaces of 20' x 20' x 10' as follows (Figure 2): Two rows of six sensors each along the sides of the adjacent spaces with three feet distance between every two sensors, and one foot from the side walls One row of six sensors at the center of each space and along the depth with three feet distance between each two sensors (Figure 2.) 4.4. One light sensor was located completely outside the atrium to measure the total horizontal illumination. Figure 2. Locating light sensors inside the adjacent spaces of 20' x 20' x 10' of a four-floor atrium building to measure the illumination level in all orientations on 21 of each month The illumination level in the adjacent spaces was measured at the hours 9:00 a.m., noon, and 3:00 p.m. on the 21 of each month. On the other hand, the illumination level inside the atrium was measured every hour from 9:00 a.m. to 3:00 p.m. using the sundial diagram for the same latitude, and using the tilting device mentioned above The adjacent spaces were photographed using a 28 mm camera, and ASA 400 daylight film to

4 provide a permanent record of daylighting conditions inside the model (Schiler, 1990). 5. ANALYSIS 5.1. The Atrium Preliminary data indicated that the primary variation of the Daylight Factor (DF) for every reference point inside the atrium occurred due to the change in time and month. (See figures 4 and 5.) The geometry of the atrium was plotted showing the DF for each reference point. The DF was calculated by measuring the total horizontal illumination and the indoor illumination The Adjacent Spaces All the output data was plotted in graphs showing the geometry of each space. The x-axis and the y-axis show the geometry (width and length), and the z-axis shows the DF in that space at a particular time of the year. Figure 3. South facing wall of the adjacent spaces showing the illumination level for the four floors in June 21 at noon. Each graph shows the geometry of each space. Figure 3.A shows the fourth floor, figure 3.B the third floor, figure 3.C the second floor, and figure 3.D the first floor. The DF at the points two feet from the rear wall (2.3%) is higher than DF five feet from the rear wall (2.1% ) (Figure 3.D). 6. CONCLUSIONS

5 For this atrium with a ratio of 1:1:2, we observed several behaviors which were expected, and further observed consistent ratios beyond what we had expected Expected behaviors included: The adjacent spaces for the South facing wall received sufficient quality and quantity of light in all four floors, both summer and winter (Figure 3), (Al-Turki, 1994) The adjacent spaces for the North facing wall received good quality and quantity of light, mostly reflected from the opposite wall, only at Summer noon for all floors (Al-Turki, 1994) The East and West facing adjacent spaces act exactly the same at noon, in terms of quality and quantity of light. They act exactly the opposite at 9:00 a.m. and 3:00 p.m. (Al-Turki, 1994) Unexpected behaviors included: At 9:00 a.m. in winter, the floor of the atrium on the East side receives more light than the floor of the atrium on the West side. This is counter intuitive. The direct beam sunlight is striking the West wall. However, most of the light received on the atrium floor at that particular time is reflected light bouncing from the upper portion of the opposite wall. The same observation was found for the West side of the atrium at 3:00 p.m. (Figure 4), (Al-Turki, 1994). This does not appear to be true for the corners. Figure 4. The floor of the atrium on the East side receives more light (6% DF) than the floor on the West side (4% DF) on December 21 at 9:00 a.m. because of the low altitude angle of the sun in December. The same observation does not occur in July or March, when there are higher sun angles.

6 Figure 5. The floor of the atrium on the South side receives more light (7% DF) than the floor of the atrium on the North side (5% DF) on December 21 at noon. The low altitude angle causes that. However, on July 21 the direct sun beams hit the sensors that are one foot from the North wall At noon, in winter, the South side of the atrium the (North facing wall) received more illumination than the North side for the same reason as mentioned in (Figure 5), (Al-Turki, 1994) Unlike all other measurement methods, for interior adjacent spaces, the physical model showed that the points immediately adjacent to the rear wall of the space (two feet from the rear wall) received more light than the point five feet from the rear wall, which is the traditional U.S. measurement station. This is from the light reflected from the rear wall (=50%) in a sidelighted condition (Figures 3, 6 & 7), (Al-Turki, 1994). Figure 6. Daylighting Factor (DF) curve using the lumen method calculations. Notice that only three reference points are calculated (MAX, MID & MIN), where the MIN point is five feet from the rear wall.

7 Figure 7. Daylight Factor (DF) measurements, which obtained from the tested physical model, proved that the points five feet from the rear wall are not the lowest illumination. Points two feet from the rear wall are lower. 7. RECOMMENDATIONS 7.1. Graphing the daylight factor in the spaces adjacent to atria shows that some spaces do not get sufficient illumination level and part of these spaces need artificial lighting. From an architectural perspective, there is a strong relationship between light and form in a space; therefore, the adjacent spaces could be formed upon the available functional light in each floor (see Al-Turki, 1994 for suggested form modifications). This allows us to arrange the function in a space without using any artificial light which resultantly saves a lot of energy To improve the quantity of light in the first and second floors in winter, the glazing area could be increased. At the same time, the glazing area in the third and fourth floors could be decreased. (Figure 3 shows sufficient light on the fourth floor in June, and the fourth floor receives an increase from low angle sun in December, see Al-Turki, 1994). However, the total glazing area for all floors should remain the same so that the function of the cooling and heating systems will remain the same (Al-Turki, 1994) Another way of increasing the quantity of light is using light reflectors to supply more light to the bottom two floors. The reflectors could be of any high reflectance and specular materials. They should be tilted toward the areas which require more light. The depth of the reflectors should be carefully chosen so that they do not block the light coming to the other sides. (Al-Turki, 1994). BIBLIOGRAPHY Al-Turki I (1994) Qualitative and Quantitative Natural Light in Atria and Adjacent Spaces. University of Southern California, Los Angeles, California. Atif MR, Boyer LL, Degelman LO (1994) Development of Atrium Daylighting Prediction: From an Algorithm to a Design Tool. Journal of Illuminating Engineering Society. Vol. 24, No 1: California Energy Commission (1992) Energy Efficiency Standards for Residential and Nonresidential Buildings: Standards and Regulations. California Energy Commission.

8 Milne M (1986) DATALIT Computer Software. University of California, Los Angeles, California. Schiler M, editor (1990) Simulating Daylighting With Architectural Models. DNNA. Reviewer #1: In section 3 Description of Method, it is stated that specular surfaces were modeled on 3 of the 4 sides to simulate fenestration. I believe that this was a grave error... The comment from the reviewer indicates that the substitution was misunderstood. The specular surfaces were not covering the entire wall, but rather only in the position of the windows. The absorptivity behind the specular surface approximated the amount of diffused light reflected back out of the room. The specularity was added to account for the phenomenon which occurs when direct beam sunlight comes to within 15 of the surface, at which point the glass reflects most of the light. This phenomenon is well documented, and in no way invalidates the tests. Indeed, the inclusion of the phenomenon makes the tests more accurate than most. Reviewer #2: Point #1: The paper... does not advance the state of the art. The paper clearly presents careful and useful information. The phenomenon of the increased illuminances close to the reflecting walls is not properly addressed in other papers. Of course, the authors would love to advance the state of the art further, but do not wish to hold publication until everything is understood about all possible atrium configurations. Point #2: The paper does not explain rationale for selection of atrium and adjacent space geometry. Good point. See new version of Section 3 for requested explanation. Point #3: Although Section 7.1 Recommendations says there is a strong relationship between light and form in a space, only one geometry is analyzed and presented. For the backup information on the relationship between form and light, as well as some suggested modifications to the base form, see Al-Turki, Only one geometry was presented because only this geometry was completely tested, and it was considered the critical geometry. (See new Section 3) Point #4: Paper discusses analysis (Section 5.1) but does not present the data. Supporting data is presented in figures 4 and 5 (but was not referred to in the text, our apologies.) Point #5: Paper does not attempt to resolve or explain apparent contradictions (Figures 4 & 5) or at least to substantiate findings with multiple readings to minimize possiblities of errors. There are two sets of contradictions. The contradictions to conventional wisdom are specifically noted at the beginning of 6.2 Unexpected Behaviors. The explanation has been rewritten for the sake of clarity. The authors were perhaps not clear enough about distinguishing between measurements of the East side (floor) of the atrium as opposed to the East facing wall of the atrium, as opposed to the West wall of the atrium. Indeed, there is verbal confusion between East wall (facing West), East side of the atrium floor, and West wall. Our apologies. We think that this has been clarified. This contradiction was one of the major points of the paper, and was part of why the paper was considered to have improved the state of the art.

9 The second contradiction occurs between the behavior of the space in front of the walls and the behavior of the corners. The reviewer's observation is quite correct, the phenomenon was not explained. It was consistently measured and observed, so that there is no doubt about the fact that it occurs. We have not come to a satisfactory explanation, ourselves, yet. Indeed, we have added it as a consideration for future examinations, and are looking at all of the recorded times and dates for clues. Back to Papers Published