EFFECTS OF AUTOMATICALLY CONTROLLED BLINDS ON VISUAL ENVIRONMENT AND ENERGY CONSUMPTION IN OFFICE BUILDINGS Takashi INOUE 1, Masayuki ICHINOSE 1 1: Department of architecture, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-851, JAPAN ABSTRACT The desirable control of automatically controlled blinds for energy conservation in office buildings by solar-shading and daylighting was investigated through questionnaires, preliminary experiments, measurements using a two-dimensional color analyzer, and simulations. Based on the results of these investigations, it was shown that an appropriate control of blinds could provide a more comfortable visual and thermal environment and large energy savings by shading solar radiation, making efficient use of natural light, and managing the changes of external light environment. It was also indicated that a proper introduction of daylight was not directly related to an increase in air-conditioning load, but on the contrary, energy saving could be expected by the synergistic effect due to the reduction of illumination power and cooling load. 1. INTRODUCTION Recently it has become popular for the façade of a building to be made entirely of glass, and high-performance window systems such as air-flow, double-skin and others have been adopted for the purpose of improving energy saving and a pleasant indoor environment. In such window systems, solar irradiation is generally shielded by automatically controlled blinds, however at present the control tends to be focused exclusively on shielding sunlight. The purpose of this study is to describe how to improve not only energy efficiency but also the internal environment by reducing the cooling load caused by solar radiation, reducing lighting energy by introducing daylight and managing the changes of external light environment using the transparency of the windows. 2. REQUIREMENTS OF OFFICE WORKERS CONCERNING AUTOMATICALLY CONTROLLED BLINDS Air duct 245 Lighting High transmission double glazing 12-A12-12mm Automatic control blinds Glazing 8mm [Clear weather] [Cloudy or rainy weather] Minimum angle for protection Figure 1: Western façade of the building. 3 Control slat angle to shade direct sunlight Figure 2: Cross-section of window. Slat angle : horizontal 27
2.1 Buildings to be surveyed In order to examine office workers views concerning office windows, a survey was carried out in a super-high-rise building (Fig. 1: Tokyo, 3 stories, approx. 18, m2) which has many windows facing west. This building is equipped with air-flow windows with automatically-controlled blinds (Fig. 2). The protective angle of the blinds is controlled at 2 minute intervals to prevent direct sunlight, above a threshold value, from coming through into the working area. The control takes into account the calculated values of solar altitude, azimuthal angle and level of illumination of direct sunlight which is detected by an illumination detection sensor set up on the roof of the building. 2.2 Results of the survey On most days the blind slats set in the windows facing west are totally closed for a relatively long time before sunset. However, when we proposed that a control system by which the office workers could feel the change of time of day or the weather and the change of season could be introduced, more than 7 % of the office workers supported the idea as shown in Fig. 3. In reply to the survey, most of the respondents chose such expressions as It gives a feeling of openness; It will broaden our view; and also It makes us feel the change of weather; It gives us a sense of passage of time; and It gives us a feeling of better mental and physical health. Obviously it seems that they have a desire to bring the outside environmental changes inside. In the past, keeping a uniform and homogeneous indoor environment was considered to be the appropriate policy. However, this survey suggests that the office workers sense of comfort and degree of satisfaction will be improved by introducing an awareness of changes in the outside world. In order to respond to these desires, it seems appropriate to provide a feeling of openness by keeping the blinds open by applying an appropriate blind control so long as it does not cause problems for the office workers, and also to reflect changes such as a sunset glow to the indoor office. What do you think of a blind control system which allows you to feel more change of time of day, weather and transition of season? It will broaden our view It gives a feeling of openness It will lighten the room It gives a sense of passage of time It makes us feel the change of the weather It gives contrast in the room It gives change of color of light It gives a feeling of better mental and physical health No opinion It creates glare It enables the other people to see inside our office 18% It creates a sense of warmth Disagree It makes sense of cold 8% Agree It causes reflected sunlight It makes glare on PC screen 74% It affords a beautiful night view Other 5 1 15 2 25 Response number [-] Figure 3: Results of questionnaire survey. 3. INVESTIGATION OF CRITERION FOR JUDGMENT OF SHIELDING SUNLIGHT ACCORDING TO THE RESULTS OF ANNUAL SOLAR IRRADIATION DATA Figure 4 shows a histogram of illumination level of direct sunlight plotted against time through 27 as actually measured on the roof of the building. Even in daytime, the illumination level is less than 5 lx during about half of the total time, and it is evident that the blinds can be opened for a long while even when the sun is located in a position where sunlight would hit the windows. According to our studies so far, it is appropriate to set a criterion for judgment of sunlight shielding (hereafter referred to as the threshold value) of about 15 lx, but because the fraction of the time when the illumination level is 1~2 lx is quite small, the influence of the difference of the criteria on the result of blind control will be relatively small. 28
Occurrence frequency [Hour / year] 4 3 2 1 Figure 4: Histogram of illumination level of direct sunlight in daytime. 4. INVESTIGATION OF OPENING CONTROL IN THE EVENING Opening control in the evening means keeping the blind slats in the horizontal position or curling up the blinds for a specific time so people can look at the beautiful sunset colors just before sunset. We investigated to what extent people tolerate dazzle which may be caused by the setting sun where the solar altitude is low, and what criterion must be established in order to realize proper control. Furthermore, the indoor light environment including color shade, in the case where the blinds are retracted in the evening was examined by actual measurements. 4.1 Investigation of threshold value As shown in Fig. 5, we asked student subjects to stand facing south in a room with the opening in the west window, and asked them to estimate how dazzling the evening sun was. Figure 6 shows that after the illumination level of the direct sunlight dropped to about 23 lx at 16:25, the frequency of the no dazzle response increased, and when the illumination level fell to about 12 lx at 16:29, none of the subjects expressed concern over dazzle. Based on these facts, we concluded that the threshold value for opening the blinds in the evening should be about 1 lx~2 lx. Figure 5: Subject experiment for threshold value. 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 Time 1 2 8 6 4 2 5~lx 45~5lx 4~45lx 35~4lx 3~35lx 25~3lx 2~25lx 15~2lx 1~15lx 5~1lx ~5lx 15:4 15:5 16: 16:1 16:2 16:3 Time 16 12 8 4 2 Figure 6: Result of subject experiment. 1 8 6 4 2 2 4 6 8 1 Solar altitude Time after sunset [minute] 1 9 8 7 6 5 4 3 2 1 sunset 5 1 15225 3 5~lx 45~5lx 4~45lx 35~4lx 3~35lx 25~3lx 2~25lx 15~2lx 1~15lx 5~1lx 1~5lx ~1lx Figure 7: Change of sunlight around sunset. 29
4.2 Changing process of direct sunlight and diffused illumination in the evening Referring to the above mentioned results of measurement of annual solar irradiation, the frequency when the illumination level of direct light is about 2 lx was examined. It can be seen from Fig. 7 that when the solar altitude becomes less than 2 degrees (about 1 minutes before the sunset), the proportion of the illumination level which is lower than 2 lx is greater than 95 %, so controlling the illumination by setting the ordinary protective angle horizontal and at the same time letting the blinds retract could be a proper way to afford outside view. It is also evident that even after sunset brightness can be maintained by diffused light for 1~15 minutes. 4.3 Distribution of color temperature by the light transmitted through window area Many white globes made of white foam polystyrene were located within the study room as shown in Fig. 8, and spatial distribution of light was detected by measuring both brightness of the surface of the white globes and color temperature by a two-dimensional color analyzer (Fig. 9). By using the measuring apparatus, it is possible to know the distribution of measured values as a graphical image as shown in Fig. 1. If we select an arbitrary evaluating area (Fig. 11), it is possible to extract the average value of the data of that area. Pixel data in the window side were extracted to obtain the results. 19 13 7 1 Height from floor [mm] 25 75 175 275 375 475 575 675 Distance from window [mm] Figure 8: Sectional layout of white globes in the study room. Figure 9: 2D color analyzer. (KONICA MINOLTA CA-2W) 1cm White globe Color analyzer Upper Window Figure 11: Measuring procedure of spatial distribution. Figure 1: Spatial distribution of radiance of white globe. 25 35 25 2 6 2 3 25 4 5 15 5 1 15 5 4 1 2 3 4 5 6 7 Distance from window [mm] Figure 12: Spatial distribution of sphere radiance and color temp. at daytime and in the evening. 1 4 Radiance 5 4 Color temp. 12 8 4 9 7 5 3 Color temp. [K] Radiance [ /m 2 ] 21
Spatial distribution of brightness and color temperature when the blinds are retracted (17:) is shown in Fig. 12. Brightness decreases as the distance from the window area increases. Color temperature also decreases as the distance from the window area increases, and at the window it decreases as the distance from the ceiling decreases. This is due to the fact that as the evaluation area approaches the window, the fraction of diffused light, which has a higher color temperature than direct light, increases and in the vicinity of the ceiling the diffused light is interrupted by the protective effect of eaves. 4.4 Investigation in consideration of working environment Intensity and direction characteristics of transmitted light through the window on sunny days when white blinds are simultaneously used were measured using the apparatus equipped with the integration sphere (Fig. 13), and the results are shown in Fig. 14. It is evident that as slat angle increases, the intensity of transmitted light decreases and the fraction of upward light increases. In cases where the slat angle is the same, as the solar altitude comes down and as more light is irradiate on the surface of the blinds, the above-mentioned fraction increases. These results of actual measurements can be used in the following simulation. (a Appearance (b) Cross-section Figure 13: Proposed integral sphere. 1 8 6 4 2 1.8.6.4 External vertical Transmitted Slat angle 15 3 45 6 close 可視光透過率上比下比 Downward Upward Figure 14: Intensity and ratio of upward / downward of transmitted light through windows. Figure 15 shows the indoor color temperature as a function of time, assuming that required intensity of luminance on the desk is 75 lx and the short fall in illumination is offset with artificial light (fluorescent light, daytime white color: color temperature 6 K). Color temperature data for both artificial light only and natural light only are also shown for comparison. When black blinds are used, the influence of artificial light is relatively large especially after 14:, and the quality of natural light taking on a red tinge is not transferred, while the value is changing in parallel to that of natural light in the case where white blinds are used. Color temp.[k] Barium sulfate pigment 7cm 12 1 8 25cm Illuminance meter Figure 15: Change of color temp. in the office. Upward Downward 5 4 3 2 1.2 Visible transmissivity 11: 12: 13: 14: 15: 16: 17: 18: slat; horizontal slat; protective angle retracted Daylight (Black BL) Daylight (White BL) Combination (White BL) Combination (Black BL) 6 Artificial lighting 4 11: 12: 13: 14: 15: 16: 211
Figure 16 shows the spatial distribution of color temperature in the case of protective angle at 15:3. When bright colored blinds are employed, it became clear that a tinge to the natural light is introduced and more so as the point of measurement approaches the window. Based on the facts mentioned above, it seems appropriate to choose bright colored blinds not only for introducing daylight, but also for fully reflecting the change of tinge of natural light to the indoor environment. By combining the dimming control technique, tinge of natural light can be introduced to the room even when the slats are kept to the protective angle. 6 Artificial lighting Combination Artificial lighting Color temp.[k] 5 4 Daylighting Combination Daylighting 2 4 6 8 2 4 6 8 White color of blind Figure 16: Influence of color of blinds on spatial distribution of color temp. 5. INFLUENCE ON AIR-CONDITIONING LOAD AND LIGHT ENERGY Black color of blind So far the effectiveness of specifications and control system of blinds to introduce natural light in a positive way has been emphasized, but attention must also be paid to the increase of energy consumption due to the introduction of natural light. Figure 17 shows the calculated values of the annual air conditioning load and energy consumption for illumination assuming a general office building. Specification of windows was examined with regard to blind color and automatic controlling for each time interval. Weather data, solar irradiation and illumination level at time intervals of 1 minute were used. It became clear that a proper introduction of natural light is not directly related to an increase in air-conditioning load, but on the contrary, energy saving can also be expected by the synergistic effect due to the reduction of illumination power and cooling load. Figure 17: Calculated annual energy consumption for air-conditioning and lighting. 6. CONCLUSION Lighting High reflectivity, White BL Heat-absorbing, White BL High-transmission, White BL Clear, White BL Clear + Low-E, White BL Clear + Low-E, White / Gray BL Clear + Low-E, Gray BL Clear + Low-E, Black BL 1 Primary energy [MJ/m2 ] Heating Cooling 3 2 1 2 3 4 5 6 It has been quantitatively shown that changes in natural light can be fully reflected to the indoor environment even when the solar irradiation is shielded, and it has been verified by simulation that such a control system, in which the problem of energy saving is taken into consideration, can be used in practice. REFERENCE 1. Paul BAKER, Takashi INOUE, et al., Advanced Envelopes: Methodology Evaluation and Design Tools, IEA-ECBCS-Annex 32, Vol.3, International Energy Agency, 2, ISBN97574173 2. Takashi INOUE, Energy saving effect of combination control of solar shading and daylighting for office buildings, Proceedings of International Building Physics 2, P.323 33. 212