Daylight can be considered as one of the main factors of sustainable architecture. Light is one of the effective strategies in building energy optimization. Using a developed system of daylight designing decreases building’s electrical consumption significantly. It also affects indoor lighting quality [1
]. Using daylight has many economic and bio-environmental advantages [2
]. Electric energy consumption is the most important effective factor in buildings’ carbon emission [3
], which consist 30–40 percent of the total energy consumed in commercial buildings [4
]. By the developed designing of daylight, cooling load decreases significantly [5
]. Weak designing of the buildings’ natural light and improper internal air quality caused negative lasting influences on residents’ health. Daylight quality is one of the most important influential factors for students’ performance. The literature shows not only the significant influence of daylight on the visual system, but also its positive role in increasing physical and mental health and decreasing stress. Using daylight is the base and infrastructure of sustainable architecture of high efficiency schools. By taking advantage of daylight in schools or the educational environment, classroom light amount and physical health level will increase, stress will decrease, success will improve and, as a result, students efficiency increases as observed and shown in the results of various research works [6
]. The light shelf is one of the passive systems for daylight controlling. In fact, the light shelf is a horizontal plate made of light-colored and reflective materials that reflects daylight and is placed above human eye level in the upper half of the window. It decreases the light severity near the window and increases the light penetration depth; distributes daylight more properly in space and also decreases glare by reflecting daylight to the ceiling and reflecting it into the space. Most of the time, the light shelf is accompanied with shading or an external shelf to perform better [9
]. Some of the most important parts of the light shelf performance have been studied in Table 1
, which include some of technical issues and considerable data.
The conducted studies investigating and measuring light shelf efficiency have almost all been done for a determined date and have investigated daylight by simulation or measurement. The light shelf has been less implemented in the annual examination of daylight and visual comfort in educational spaces. Furthermore, limited numerical studies on the form of light shelf actually exist for Iran. This article shows that according to the literature, the hypothesis that the light shelf increases natural light is confirmed and proven to be very useful for educational spaces. The purpose of this article is to examine the light shelf dimensions’ and orientations’ influence on daylight performance and visual comfort during the year by using daylight simulation and the developed analysis in an educational space in Tehran. The article is going to answer the following questions.
How much do internal and external light shelf dimensions affect classroom annual daylight in Tehran? Do the light shelf external rotation angle and window orientation affect the annual natural light performance rate? Does the glare possibility decrease by using the light shelf, which plays an important role in the visual comfort of the students?
The present study aimed to investigate the influence of a light shelf on daylight performance and visual comfort and achieved significant results in this respect. Unfortunately, the light shelf is not used in Iran, and the importance of daylight is neglected by both construction rules and the groups of architects and designers. In this respect, the results of this study can play an important role in architectural engineering in Iran. Regarding the research questions, the variations of the most important results of simulating 100 different light shelf models were presented. According to Table 4
, the series of performance results presented all belong to the daylight and visual comfort.
The light shelf has a better performance at the southern orientation compared to other orientations. By increasing the size of the internal and external light shelves, daylit areas will be distributed more appropriately and will be increased by 40% compared to the shelf-less conditions. For example, according to Figure 3
, (Ext 1.2_Int 1) has 87% daylit and is the most efficient option with a very good natural light distribution. On the other hand, there are limitations in building internal and external shelves because of their high dimensionality. It addition, they can interfere with classroom conditions. However, even a little increase in the size of internal and external light shelves will result in increased daylight and reduced overlit, and there will be little partial daylit (Figure 3
). This increase will also result in the unified suitable distribution of natural light. Considering the design limitations and conditions, designers can use light shelves with suitable dimensions. Regarding the rotation angle of the external light shelf to the horizon (Figure 3
), the range of allowed changes will be smaller, since at the southern front, a 30-degree angle for the external shelf may result in overlit and disturb visual comfort, especially at larger sunlight angles.
At the southern facade, paying enough attention to visual comfort is very important, especially in educational environments and classrooms. That is why analyzing visual comfort is also important for where we use light shelves. At the initial stage of measuring visual comfort, conducted for a specified date and time, it became clear that the light shelf prevents visual disturbance by creating a shadow (Figure 9
). In the annual evaluation and according to the changes in sunlight seasonal direction and severity, notable results were obtained. At the southern orientation (Figure 8
), 3.77% of the time, the annual intolerable glare reaches 1.48%, and the disturbing glare drops to 0.06% from 1.48%. Additionally, the sum of these values is equal to 330 h adapted to the occupancy schedule. According to Figure 6
, intolerable glare is limited to 10 a.m. in the winter.
Unfortunately, it was not possible to study all cases and all sizes of light shelves and their visual comfort efficiency, since simulating the annual glare is time consuming; furthermore, different sizes of internal and external shelves can have different effects according to the direction of sunlight, especially in the winter. Therefore, we had to confine the calculations to a sample model of the light shelf for the eastern and western orientations. Their impact on visual comfort was clearly observable, although because of their role in breaking the sunlight and creating shadow, all light shelves influence visual comfort. Light shelves are not efficient in the northern orientation. They play the opposite role here, preventing daylight from entering and increasing partial daylit. This is because the north facade does not receive direct sunlight. Here, the best option is not to use light shelves. Without the light shelf, there will be 70% daylight, while the presence of one will reduce this amount (Figure 4
). It must be noted, however, that we used diffuse reflection material for light shelves; materials with a higher degree of reflection, like mirrors, will probably produce different results in the northern orientation.
Daylight performance is relatively the same for eastern and western orientations because of the symmetry of the Sun’s east-west movement direction and the amount of time these two fronts receive direct sunlight. As shown in Figure 5
, an increase in the size of the light shelf will reduce overlit and increase partial daylit. However, for types of Ext 1.2, we face increasing daylit areas; this might be due to the broad surface for transferring light. Regarding the fact that at the west side, the amount of sunlight in summer is high and disturbing, the light shelf acts as both a light controller and shading, reducing overlit and increasing daylit. This is very important at the west side; however, using openings is not recommended. If needed, the light shelf can act as a controller, especially for distributing daylight, producing shadow near the window and reducing glare. Its performance is however impaired when it comes to transferring daylight to the back of the classroom. The rotation angle of the external light shelf does not play an important role in these two directions.
Visual comfort is very important in eastern and western orientations, especially because of the oblique sunlight. The eastern front was evaluated because it is widely accessed during early hours in the morning. The difference between using shelves or not using shelves (shown at a specific hour in Figure 9
) regarding DGP is small, but in calculating the annual amount of glare (Figure 8
) for the eastern orientation, the intolerable hours without the light shelf is 5.89%, which reaches 3.32%, with a reduced amount of disturbing glare. The total number of hours is 278, and the light shelf reduces intolerable and disturbing glare in this orientation. Because of the Sun’s movement direction and direct sunlight at the beginning and at the end of the day for the eastern and western orientations, it is very important to be extremely careful in choosing the dimensions and position of the light shelf, so that it does not interfere with visual comfort and proper daylight distribution.
There were limitations in studying the models, since simulating the annual amount of daylight is time consuming, especially in the case of the annual amount of glare. Accordingly, we were limited to studying two cases and comparing them with cases without the light shelf in both orientations. We tried to use large amounts of data, so that the light shelf performance can be studied more conveniently and accurately. Light shelves perform pretty well for the southern orientation, both in terms of daylight, causing a unified light distribution, and visual comfort, reducing the number of hours of visual disturbances. Similar studies support these results [9
]. In addition to causing a unified distribution of daylight, light shelves can act as shading; they overshadow half of the window, which does not increase the cooling load in summer nor prevent sunlight from entering in winter. Further research can focus on the role of blinds and shading devices and their combination with light shelves in daylight distribution and energy consumption. In addition, because of different latitudes, the performance of the light shelf can differ in different regions and cities in Iran. This might especially require further research for areas with hot and dry climates or cold climates regarding the buildings’ cooling and heating issues.