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Energy Management
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Daylight Harvesting
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Switching vs. Dimming
Bi-level or multi-level switching using split-ballast schemes provides a selection of light levels—usually 100%, 50% and 0%, or 100%, 66%, 33% and 0%. The advantages of switching include a lower initial cost and simpler design and commissioning. The disadvantages of switching include typically lower energy savings and less flexibility than continuous dimming. In addition, abrupt changes in light levels can be considered irritating by occupants even if they understand the intent of the changes. Continuous dimming of discharge lamps using controllers and compatible dimmable ballasts provides a selection of light levels from 100% to 1%, with a 100% to 5-10% dimming range typically specified and considered sufficient for energy management applications. The advantages of continuous dimming include the highest level of flexibility and user satisfaction, and also often the highest energy savings. Disadvantages include the addition of dimmable ballasts and potential wiring to the initial cost and commissioning. Due to their advantages and disadvantages, switching is often recommended for spaces with non-stationary tasks such as corridors. Continuous dimming is often recommended for spaces where users perform stationary tasks, such as offices.
Manual vs. Automatic
Daylight harvesting control strategies can be established so that the controls can be operated manually by users or automatically using photosensors. Since automatic strategies do not depend on human initiative, they are generally more effective at saving energy. Daylight harvesting using continuous dimming equipment automatically controlled by a photo sensor, in fact, can generate 30-40% savings in lighting energy consumption, significantly reducing operating costs for the owner.
Does daylight harvesting live up to its potential?
The 2004-2005 Dimming Study, co-sponsored by the Lighting Controls Association, explored attitudes in the specification distribution and contractor sales channel by providing and analyzing survey data from architects, lighting designers, engineers, electrical and lighting distributors, and electrical contractors. The 219-page study was based on a survey distributed to 4,317 industry participants with a 6.7% response.
Respondents were shown a list of dimming strategies and equipment types and asked to rate how well they typically meet the respondent’s performance expectations on a scale of 1 to 5, with 1 being “didn’t meet expectations,” 3 being “met expectations,” and 5 being “exceeded expectations.” The result was a series of weighted averages for each statement that are reflective of the average opinion of each respondent group. A rating of 4.0 or higher indicates that the statement, on average, has a high level of agreement by the respondent group.
Daylight harvesting ranked as about meeting expectations among lighting designer and engineer respondents (2.9), about meeting expectations among architect respondents (3.1), and more than meeting expectations among electrical contractor respondents (3.7). However, architect, lighting designer and engineer respondents all ranked daylight harvesting as having been less able to meet their performance expectations than most others in a list of 13 control strategies and technologies.
