Development of a daylight discomfort detector for control of
shading
Citation for published version (APA):
Zonneveldt, L., & Aries, M. B. C. (2009). Development of a daylight discomfort detector for control of shading. In Proceedings of the CISBAT 2009 International Scientific Conference "Renewables in a Changing Climate: from Nano to Urban Scale, 2-3 September 2009, Lausanne, Switzerland (pp. 4-). LESBO EPFL.
Document status and date: Published: 01/01/2009
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Development of a Daylight Discomfort Detector for
control of shading
Zonneveldt, L., Aries, M.B.C.
Indoor Environment and Health Group, TNO Built Environment and Geosciences, P.O. Box 49, 2600 AA, Delft, the Netherlands, e-mail: laurens.zonneveldt@tno.nl
Abstract
Shading control is a key parameter in real energy saving of electricity for lighting in workspaces. Daylight responsive controls may be very efficient, but improper blind control can strongly reduce the effect. This paper presents the results of an investigation of a simple approach to develop a sensor that could be placed on a desk. This sensor supports blind control from the user perspective, because maintaining the user’s visual comfort is necessary in work spaces. Discomfort always leads to extra energy use. This paper presents the status of ongoing research with the final objective of creating an easy to understand and easily commissionable shading control system providing maximum daylight and preventing the user(s) from glare.
Keywords: daylight, visual comfort, luminous thresholds, shading control
1 Introduction
Fully glazed office buildings have a high risk of visual and thermal discomfort, especially under sunny sky conditions. Lindsay and Littlefair (1992) found a strong correlation between the amount of sunshine, the sun position, and Venetian blind use. Shading has to be used to prevent discomfort and very often additional electric lighting will be switched on to compensate for high luminance ratios in the interior.
The use of electric lighting is not preferred due to its energy consumption, being one of the several practical challenges related to the use of shading on fully glazed facades (Altan et al., 2009). Another maybe even larger problem is the loss of an outside view which is often the result of shading being fully-closed unnecessarily. A window view provides information about time and weather, decreases the feeling of claustrophobia, and can have a positive contribution to health. The positive effects and the preference of people for natural over built or urban views is shown in many window studies (e.g., Ulrich, 1984; Tennessen and Cimprich, 1995; Hartig et al., 2003; Chang and Chen, 2005). A good view should normally include the foreground and the skyline (Littlefair, 1996), but care needs to be taken to control the glaring effects of the sky.
These days, there are several reasons why shading is often closed. The layout of offices makes it possible to have many people in one room. Multiple users means many viewing directions and therefore more chance that at least one person may experience visual discomfort. In many modern office buildings, sun shading is automatically controlled. Even though the sun usually strikes only fractions of the facade, the entire facade will be closed. Often the shading is even made out of one piece of fabric, or the blinds are facade-wide. With automated shading, there is often one sensor per sun orientation which means that all shading for that single orientation will be closed at the same time. Once the
blinds are closed, they usually remain closed, as electric lighting in the office room presents no immediate need to open the shading as soon as the sun is gone. Some office employees would like to open the blinds or shading for daylight or an outside view, but as some shading systems are either hard to handle or control, or the mechanisms are hard to reach, employees may still refrain from changing the shading.
The fact is that daylight contains information about time and weather conditions, which can be interesting enough to have any resulting discomfort accepted. Osterhaus (2001) reported that discomfort glare from daylight appears to be tolerated to a much higher degree if there is a pleasant view from a window causing the glare. On the other hand, if visual and ergonomic conditions are inappropriate, visual tasks may lead to physical or psychological complaints (e.g., difficulty focussing, double images, glare, headache). Interestingly, most of the reasons for closure can be easily avoided in case visual comfort criteria are met, and shading is automatically controlled based on personal requirements. In the literature review of Galasiu and Veitch (2006) was concluded that only very few investigations have looked specifically at the issue of occupants’ acceptance, preferences or satisfaction with photo-controlled shading systems. Recently, Dounis and Caraiscos (2009) emphasize focussing on the design of agent-based intelligent control systems in building environments. Multi-agent control systems (MACS) attempt to manage the user’s preferences for thermal and illuminance comfort, indoor air quality, and energy conservation (Dounis and Caraiscos, 2009). Wienold and Christoffersen (2006) developed an evaluation method to assess glare from daylight that could eventually lead to a shading control system. Integrated lighting control schemes, in combination with good fenestrations products, will be needed to allow for more comfortable and cost-effective utilisation of natural light (Osterhaus, 2005).
2 Method
An initial conceptual detector was used to collect data in relation to entering daylight, in both the quantity and direction of the light. Measurements were made in a laboratory test room in the Netherlands, under different weather conditions. The measurements took place between January 15th and February 16th, 2009, when the sun angle was limited in the Netherlands and likely to cause visual discomfort. The outcome for both sunny and overcast days (extreme daylight conditions) was tested against available maximum luminance criteria. Human maximum visual comfort threshold criteria were investigated in the same test facility during prior research by means of a simple light sensitivity test (Aries, 2005). This information will form the basis for further analysis with an improved and more accurately calibrated discomfort detector. The current prototype has been used to assess the feasibility of such a detector.
2.1 Sensor design
The Daylight Discomfort Detector [DDD] measures illuminances distributed over six sections, each with an aperture of 30° by 30° (horizontally and vertically). Each section is provided with a Hagner SD2 illuminance photo-sensor. The sections of the DDD are numbered as shown in Figure 1 (N11 to N32). The prototype detector is made of black, mat cardboard and mounted on an also black, mat sheet. The Hagner detectors are mounted on a metal ring and the entire formation is made light-tight by means of black
tape. Dimensions of the DDD are 150 by 150 mm and originate by the dimensions of a Hagner SD2 detector. The current dimensions are no issue for the first prototype measurements, but will be mineralized in future products.
Figure 1. Daylight Discomfort Detector: schematic top and side view (left), and real-time model (right)
In this experiment, a second, alternative photo-sensor is located aside from the DDD. This cubic unit contains six Hagner SD2 detectors for determination of the light direction (Figure 2). The concept of cubic illumination was originated by Cuttle (1997). Cubic illuminance describes the directional qualities of the light reaching a chosen measurement point in terms of the illuminance on the six sides of a tiny cube centred at that point.
2.2 Test facility
The measurements took place in a daylighting laboratory located in Eindhoven, the Netherlands (51º N / 5º E). The façade of the test room faces true West. The test was conducted in the period 15 January to 16 February 2009, when there was both direct and indirect sun. The set-up (see also Figure 2) contained the following elements:
• The Daylight Discomfort Detector (first prototype), abbreviated as DDD; • The cubic measurement device for light direction determination;
• A Hagner SD2 illuminance detector on the horizontal work plane;
• A horizontal and vertical Hagner SD2 illuminance detector near the daylight opening;
• A data acquisition equipment (16-channel Multilab TU/e-manufactured, serial number 3804 301), including software purposively written in LabView
• A luminance camera (LMK 96-2 CCD camera TechnoTeam, serial number DXP 1330), with accompanying software and data storage.
Detectors on the horizontal work plane and the detectors near the daylight opening provide information about lighting conditions in the room next to weather conditions.
Figure 2 shows the entire set-up. Every five minutes, the luminance camera registered the
situation, including the sky condition.
During the experiments, the sensor was oriented true west, meaning that the line between sectors N21 and N22 (see Figure 1) pointed west. The experimental facility has an unobstructed view. The DDD was only slightly obstructed by a white, vertical window post (see Figure 2).
Figure 2. Measurement set-up including the Daylight Discomfort Detector, the cubic light direction detectors on the metal stand, the horizontal illuminance detector on the work plane (just right of the metal stand), the detectors near the daylight opening (partially visible on the right side of the set-up), and data acquisition equipment
3 Results
3.1 Sunny day
For the purpose of this paper, January 19th was selected as representative of a typical sunny day. That this day was sunny is apparent from the results of the sensor connected to the window, but also from the cubic sensor and the sensor on the desk (see Figure 3, left graph). As the test room has a West orientation, direct sun only appears on the façade in the afternoon. In winter, this happens around 12h40 for this location in the Netherlands.
Glare will result later when the angle of the direction towards the sun with the normal on the façade becomes smaller. Measurement results made with the comfort sensor (DDD) on this sunny day are presented in Figure 3 (right). This figure shows the signals for the six viewing directions. It is clear that there are significant differences between the directions. This is especially true for the channel representing the angle between -30 and 0 degrees with the normal (green line). In this interval direct sun incidence is present, and therefore high intensities are measured. It indicates that this is the sector where glare from daylight will occur, and shows that in such situations measures should be taken to prevent glare.
Figure 3. Daylight measurements on a sunny day (January 19th, 2009), with illuminances on the glass and horizontal work plane (left), and illuminances as measured by the DDD in six viewing directions (right)
3.2 Overcast day
January 16th was selected as representative of a typical overcast day for the purpose of this paper. Here daylight levels are much lower compared to the sunny day, as can be seen in Figure 4 (left). The strong variations in daylight levels are a result of layers of inhomogeneous clouds moved over each other. These variations are typical for daylight under overcast skies, but are hardly noticed by human observers. The lower luminances and the absence of a strong luminance difference both indicate that there is no risk of glare on a day like this.
Figure 4. Daylight measurements on an overcast day (January 16th, 2009), with illuminances on the glass and horizontal work plane (left), and illuminances as measured by the DDD in six viewing directions (right). The levels are much lower with variations due to layers of clouds and variable thickness of clouds, compared to levels on a sunny day. The DDD shows no extreme peaks.
3.3 Measurement of the direction of the incoming daylight
Next to the Daylight Discomfort Detector measurements, measurements were made with a cubical sensor. The direction of the incoming daylight is more accurately measured with this cubical sensor. In this cubical sensor six illuminance detectors are mounted on the surface of a cube. By subtracting the signals of the opposite pairs of detectors (left from right, back from front and bottom from top) the three components of the direction
vector of the daylight can be found. This vector gives both the direction and strength of the daylight at the location of the sensor. The top of the vector can be plotted (as a dot) in a three dimensional graph for each measurement. The distribution of these dots shows how the daylight is distributed.
On the clear, sunny day (January 19th) the direction of the daylight varies with the position of the sun, resulting in a spreading pattern of the direction vector (Figure 5, left). This is a second indication that a risk of glare is present in this afternoon, when there is direct sunlight available. On days like this, solar shading has to be used in the afternoon. On the overcast day (January 16th), the direction of the daylight is perpendicular to the facade during the entire day (Figure 5, right). This shows a low glare risk. On an overcast sky the solar shading remains unused.
Figure 5. Direction of the daylight on January 19th (sunny sky condition) and January 16th (overcast sky condition)
4 Conclusion and discussion
In this paper we investigated a simple approach to develop a sensor to assess visual comfort in the work environment. The two sensors evaluated both show a possible application in the office environment. The discomfort sensor DDD gives a direct indication of possible glare by luminance ratios, whereas the cubical sensor gives less specific information as it does not give luminance data. The advantage of the cubical sensor is the measurement of the direction of the daylight; the disadvantage is that it needs a free view in all directions. This makes its use in an office environment unpractical. The discomfort sensor, once it is miniaturised, can easily be attached to the back of an LCD display or any other suitable support on a work surface, preferably in the main viewing direction of the office employee.
This paper presents the status of ongoing research with the final objective of creating an easy to understand and easily commissionable shading control system providing maximal daylight and preventing the user(s) from glare. Continuing research focuses on sensor improvement as well as on the development of glare detection criteria. These criteria should be based on luminance ratio detection capacity in combination with human discomfort acceptance thresholds. The criteria can be implemented in software to regulate
sun shading. Compared to CCD sensors, this discomfort sensor approach is relatively simple and requires less analysis and commissioning.
Human maximum visual comfort threshold criteria were investigated in the same test facility during prior research by means of a simple light sensitivity test (Aries, 2005). The aim of this test was to find an indication for the human upper (and lower) luminance limits with regard to visual comfort. Under daylight conditions in summer, test people (N=30) accepted on average a maximum luminance of 1650±680 cd/m². In winter, the accepted average luminance was significantly lower: 1390±880 cd/m² (N=16). These data can form the basis for further research with the succeeding, more accurately calibrated version of the DDD.
5 Acknowledgements
This project was is initiated and financed within the TNO ‘Energy and Comfort systems’ Program. The authors would like to thank the unit Buildings Physics and Systems of Eindhoven Technical University for using their laboratory facilities for the experiments.
6 References
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