A Review of Light Shelf Designs for Daylit Environments
Abstract
:1. Introduction
2. Overview
- (a)
- Geometrical (width, length, mounting height, tilt angle and shape).
- (b)
- Reflectance type (specular or diffuse).
- (c)
- Material type (metal perforated or not, transparent).
- (d)
- Position adjustment (rotation, operation).
- (e)
- Building data (room dimensions including ceiling shape).
- (f)
- Climatic conditions.
- An increase in illuminance especially in non-daylit areas.
- An increase in uniformity.
- The improvement of visual comfort.
- The provision of sufficient shading.
2.1. Performance Analysis of Static Horizontal Light Shelves
2.2. Performance Analysis of Flat Tilted Light Shelves
2.3. Performance Analysis of Curved Light Shelves
2.4. Performance Analysis of Active Light Shelves
3. Discussion
4. Conclusions
- 1
- Light shelves perform differently under various sky conditions. The least favorable condition is that associated with overcast sky conditions. This situation results in an overall reduction in interior daylight illuminances although a slight increase of uniformity can be achieved. However, it seems that the use of an anidolic shelf can increase the daylight factors in comparison to a reference facade with conventional double glazing. In the deeper parts of the room, this increase was 1.7 times the value of the reference case when an urban environment was simulated [61].
- 2
- Light shelves performed best when installed in south-oriented facades.
- 3
- Light shelves are usually placed above eye level in an effort to prevent glare form their upper surface. However, in the Collège la Vanoise in Modane, France, south window sills are used as specular light shelves having a tilt towards the south to avoid glare. Because of this tilt, the light shelf presents a seasonal selectivity to reflected sunlight.
- 4
- 5
- Uniformity of daylight illuminance do increase although luminance contrast can be increased giving rise to glare problems. The maximum uniformity increase that was reported is 178.6% in comparison with the reference case (i.e., window without any shading system) under clear sky conditions [63]. Under overcast conditions, uniformity is also improved but this is due to the reduction of illuminance levels in the area near the window. Another way to increase uniformity is to use an optically treated upper surface [47]. By using small mirrored surfaces oriented properly, uniformity can increase by 29.9–34.3% for the external light shelf and 10.4–13.7% for the internal, compared to the existing flat mirror-type light shelf.
- 6
- In very broad terms, the maximum value of an external light shelf depth () can be defined by [33,34] using the equation:
- 7
- In terms of work-plane illuminances, when a light shelf is used there is an overall decrease of illuminance values between 20% and 60% in comparison with a reference case (unshaded, no light shelf) while its effectiveness decreased after 6–7 m from the window. However, light shelves can increase work-plane illuminance levels in the secondary non-daylit zone ranging from 10% to over 70% when highly reflective materials or mirror surfaces are used. This relative increase is inversely proportional to the window size. [40,45,50,53,56,59,63,67,69].
- 8
- A slight tilt of a light shelf can increase the light flux entering the room especially during summer months in the northern hemisphere, increasing at the same time the cooling load. The increase of the latitude decreases this angle [47]. In [60] Lee et al. using a light shelf that is 0.6 m wide with a tilt angle equal to 30° upwards for buildings in Korea achieved savings for both lighting and cooling. They suggest that during winter the light shelf should be removed to save energy. It seems that the equation (4) proposed by Moore [50] give results compatible with those found by Meresi’s work [51] while diverging slightly from those of Warrier and Raphael [46].
- 9
- It seems that the shape of the ceiling can affect the distribution of illuminance. When chamfered ceiling is used, the illuminance levels increased in the rear part of the room by 21–36% and decreased by 20–55% in the front. If specular ceiling is used a further increase in illuminance levels occur [39,52]. Changing ceiling surface reflectance type to specular further increased illuminance levels in the back of the room was observed. In addition, the external curved and chamfered exterior light shelves performed better when a curved ceiling is used [54].
- 10
- Dynamic control of mirrored light shelves can offer some benefits since they can redirect sunlight into a specific area on the ceiling. The results suggest that small window-to-floor ratio achieve the largest percentage increase in daylight illuminance in the deeper part of the space. Motorized designs achieved an increase in lighting energy savings in comparison with a static mirror light shelf from 8–22% [42,51,53,62,70].
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Strengths | Weaknesses |
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Opportunities | Threats |
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Kontadakis, A.; Tsangrassoulis, A.; Doulos, L.; Zerefos, S. A Review of Light Shelf Designs for Daylit Environments. Sustainability 2018, 10, 71. https://doi.org/10.3390/su10010071
Kontadakis A, Tsangrassoulis A, Doulos L, Zerefos S. A Review of Light Shelf Designs for Daylit Environments. Sustainability. 2018; 10(1):71. https://doi.org/10.3390/su10010071
Chicago/Turabian StyleKontadakis, Antonis, Aris Tsangrassoulis, Lambros Doulos, and Stelios Zerefos. 2018. "A Review of Light Shelf Designs for Daylit Environments" Sustainability 10, no. 1: 71. https://doi.org/10.3390/su10010071