Daylighting systems can be interesting to : improve visual comfort in protecting from glare and direct insolation ; improve thermal comfort in protecting from solar gains. But also to : redirect daylight to under-lit zones; improve uniformity in the room.
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improve visual comfort in protecting from glare and direct insolation;
improve thermal comfort in protecting from solar gains.
But also to :
redirect daylight to under-lit zones;
improve uniformity in the room.
To this end, there are two main groups of daylighting systems :
1. Daylighting systems with shadings
This kind of systems protects from:
Solar gains and overheating
But also, sometimes, it can:
Occultates a room (for projections, …);
Ensures privacy of a room.
External shading systems are always more efficient than internal ones because they reduce solar heat gains in blocking radiation before it passes through the window.
2. Daylighting systems without shadings
These daylight systems are designed to redirect daylight to under-lit zones, away from the window.
There are different types of solar protections: protections linked to the environment, architectural elements, added protections and protective glass windows.
1.1. Type of protections : position
The protection can be external or internal, but can also be integrated in the glazing.
Internal blinds is the most common way to control sun penetration.
The first aim of curtains is generally to ensure the privacy of a room but can also be interesting to protect from glare.
Roller blinds are a vertical version of curtains. They are composed of a cloth that is spread open in front of a window (Figure a). The protection is completely removable. The blind can be dropped or raised, partially or totally, as a function of needs in terms of solar contributions.
Venetian blinds can create intimacy and protect from glare but can, in a same time, let daylight penetrate in the room.
Venetian blinds are composed of slats that are mobile due to a system of cables or chains (Figure b). Protection depends on how the slats are oriented. Solar protection which can be modulated as a function of needs is the main property of blinds with orientable strips. This adaptation can be made by both withdrawing and inclining the slats.
Integrated blinds offer a better protection against solar gains than internal blinds.
They are situated between the two glasses of a double glazing
Solar control glazing
Glazing can be heat absorbing or heat reflecting. Generally, they reduce heat gains but also penetrating light and modify the color of light.
External shades are the most efficient daylighting systems against solar gains. However, if they are permanent, they reduce daylight. But if they are not permanent, they are really useful because they do not reduce light availability in winter and are good glare and solar gains protection in summer. External shades which are automated according to sky conditions can irritate people if they are continuously adjusted. Moreover this kind of control is expensive and a maintenance is necessary.
Overhang (horizontal / vertical)
Can be solid or made of cloth (Figure c).
External louvers (horizontal / vertical)
Louvers are formed by a series of exterior strips, fixed or mobile. (Figure d).
Awnings, made of flexible and adjustable elements, opaque or diffusing, shade the glass window while keeping a view to the exterior possible (Figure e).
Projected Italian blinds combine the properties of vertical rolling protections and horizontal protections (Figure f). They make it possible to conserve a certain natural lighting contribution.
The sun screen is made up of slats positioned on a chassis (Figure g).Its effectiveness depends on how its slats are oriented as well as the spaces between them.
Roller blinds can also be external. They are composed of a cloth that is spread open in front of a window. The protection is completely removable. The blind can be dropped or raised, partially or totally, as a function of needs in terms of solar contributions.
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The protection canbepermanent, fixed or mobile.
Glazing can be a kind of permanent protection. Its degree of protection is constant over the year.
Fixed protections can be integrated into the architecture of the building (wood overhang,…). This kind of protection is not mobile, but the sun evolving in the sky, the protection does not have the same effect over the year and over the day. So the degree of protection should vary over the year to be more effective against overheating during summer than in winter.
Mobile protections are best adapted to the needs (glare protections, overheating protection or on the contrary benefit of solar gains) but their disadvantages are their control and their maintenance. The automation of the system offers advantages and disadvantages (generally automation brings a better adaptation but can become unbearable for the user).
1.3. Type of protections : transparency
Protections can have, in addition, a full (loggia, protecting glazing, architectural element, …) or an openwork design (venetian blinds, jalousie, curtains, …) which will influence the quantity of light penetrating in the room.
There are various ways help daylight to penetrate deep into a building. During the pre-design stage of the architectural project atria and courtyards could be for example considered.
When the architecture of the building is already fixed, some daylighting systems can be integrated or added to the building:
A light shelf is an architectural element that allows daylight to penetrate deeply into a building.
A solar duct is a system that serves to transmit direct solar light to the very heart of a building, in rooms that can not normally benefit from natural light.
Reflective window sills
The louvres are made of two glazing leaves with a multi-layer film inside. The external face of the micro-perforated film is white, in order to reflect the daylight. The louvres inside colour is black, so as to allow the view towards outside. The louvres are organised in frames of 2m high and 3.3m width. Each frame contains 6 superimposed elements. The frames are organised by sectors, each of them including 3 frames that are interdependent for the louvres rotation command. The louvre control is a function of the sun position (date and time); the louvre location on the façade (orientation et height) and information collected by the meteorological sensors located on the roof (wind, horizontal illuminance, rain and temperature).
The louvres are tilted according to these parameters and make function either of shading (from 0° to 60°) or of reflectors, in order to increase the luminous penetration (110°), when there is no direct sun. For the shading function, the louvres are oriented perpendicularly to the sun. As the rotating movement is only possible by 10° step, the louvre will be perpendicular to the sun rays, with 5° accuracy. Under cloudy sky, the daylight factor values are strongly dependent on the louvres slope. When the louvre slope is equal to 110°, the daylight factor increases by 33% compared to the value obtained with a 90° slope and by 100% compared to the value obtained with a 0° slope. The luminous direct transmittance of the louvre is about 70%. This value is constant as the louvre stays perpendicular to the sun.
David Rennie and ForoutanParand, Environmental design guide for naturally ventilated and daylit office, BRE, 1998.