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Top lighting Strategies: Large single level floor areas and the top floors of multi-story buildings can benefit from top lighting. The general types of top lighting include skylights, clerestories, monitors, and saw tooth roofs.

Skylights: Horizontal skylights can be an energy problem because they tend to receive maximum solar gain at the peak of the day. The daylight contribution also peaks at midday and falls off severely in the morning and afternoon. There are high performance skylight designs that incorporate reflectors or prismatic lenses that reduce the peak daylight and heat gain while increasing early and late afternoon daylight contributions. Another option is light pipes where a high reflectance duct channels the light from a skylight down to a diffusing lens in the room. These may be advantageous in deep roof constructions.

Clerestory Window: A clerestory window is vertical glazing located high on an interior wall. South-facing clerestories can be effectively shaded from direct sunlight by a properly designed horizontal overhang. In this design the interior north wall can be sloped to better reflect the light down into the room. Use light-colored overhangs and adjacent roof surfaces to improve the reflected component. If exterior shading is not possible, consider interior vertical baffles to better diffuse the light. A south-facing clerestory will produce higher daylight illumination than a north-facing clerestory. East and west facing clerestories have the same problems as east and west windows: difficult shading and potentially high heat gains.

Roof Monitor: A roof monitor consists of a flat roof section raised above the adjacent roof with vertical glazing on all sides. This design often results in excessive glazing area, which results in higher heat losses and gains than a clerestory design. The multiple orientations of the glazing can also create shading problems.

Saw tooth Roof: A saw tooth roof is an old design often seen in industrial buildings. Typically one sloped surface is opaque and the other is glazed. A contemporary saw tooth roof may have solar collectors or photovoltaic cells on the south-facing slope and daylight glazing on the north-facing slope.

Unprotected glazing on the south-facing saw tooth surface may result in high heat gains. In these applications an insulated diffusing panel may be a good choice.

Daylighting Controls: A building designed for daylighting but without an integrated electric lighting system will be a net energy loser because of the increased thermal loads. Only when the electric lighting load is reduced will there be more than offsetting savings in electrical and cooling loads. The benefits from daylighting are maximized when both occupancy and lighting sensors are used to control the electric lighting system.

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Figure 105: Clerestory Window: Here stepped clerestory windows provide daylight for the interior workspace.

 

Occupancy sensors detect when a space is occupied by using passive infrared, ultrasonic, or a combination of the two technologies. Once the heat or movement of the occupant is no longer detected, and after a preset delay time, the sensor will emit a signal to extinguish the lights. Occupancy sensors used alone are good for low or intermittent use areas such as storage rooms, restrooms, and even corridors.

Light level sensors have a photoelectric "eye" that measures the illumination in a room. Threshold on and off values can be set to respond to specific lighting conditions. These sensors can operate on/off switching of various luminaries or lamps within luminaries and they can also operate a continuous dimming system. Continuous dimming system will obviously cost more than switching systems but they have greater user satisfaction because the change in lighting levels is not as noticeable.

Fluorescent lighting systems are the most common daylight control lamp source because of the availability of step switching and dimming systems. HID sources are typically not a good choice for daylight switching because of the extended strike and re-strike times. There are now two-step HID sources available that may be useful in some step switching applications where the "off" mode is not desired during a typical day. A daylighting design will use both occupancy and light sensors. With these two control strategies the lights will come on only when the room is occupied and only if there is insufficient daylight. In most designs a manual over-ride is provided for user convenience.

Design Coordination: When using daylighting, the electrical lighting and interior design require special consideration.

Electric Lighting Design Coordination: The coordination of the electrical lighting system with the daylighting design is critical for the success of the system. The layout and circuiting of the lighting should correspond to the daylight aperture. In a typical side lighting design with windows along one wall it is best to place the luminaries in rows parallel to the window wall and circuited so that the row nearest the windows will be the first to dim or switch off followed by successive rows.

Interior Design Coordination: In order to maintain the designed performance of the daylighting system, the person responsible for interior finishes and furnishing must be aware of the desired reflectance values. Dark interior finishes can compromise an otherwise great daylighting design.

Modeling Daylighting: Physical models are a very effective way to analyze daylighting performance. Even simple models can begin to inform the designer of how daylight will behave in the building. It is important that the daylight apertures be accurately modeled and that the materials used to construct the model have the designed reflectance values. The model can then be tested on the actual site or under artificial sky conditions in a daylighting laboratory. A sundial for 36 degrees north latitude attached to the model base allows the designer to simulate various dates and times of the year. Computer analysis is another method of testing a daylighting solution. Several lighting programs such as Lumen-Micro, Radiance, and Lightscape have daylighting calculations. Typically a three-dimensional digital model is constructed using computer-aided design software that is then imported into the lighting software. The programs then require the operator to define all surface characteristics, sky conditions, location, and date and time. Many of these programs can produce photo-realistic renderings of the proposed design.

Daylighting is simply the use of natural light in a building. It's typically incorporated in a whole-building design to reduce electric lighting loads. There are basically two different types of building products involved in daylighting design:

  • Fenestration: Windows, Doors, and Skylights

  • Lighting Controls.

Your whole-building design will help determine which daylighting products will work best.

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