Daylight Harvesting

Switching versus dimming

Bilevel or multilevel switching using split-ballast schemes provides a selection of light levels—usually 100 percent, 50 percent and 0 percent or 100 percent, 66 percent, 33 percent and 0 percent. 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 percent to 1 percent, with a 100 percent to 5 percent to 10 percent 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 versus 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 percent to 40 percent savings in lighting energy consumption, significantly reducing operating costs for the owner.

Does daylight harvesting live up to its potential?

The 2004-05 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 percent 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 one to five, with one being didn’t meet expectations, three being met expectations, and five 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 four 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.

"Design analysis often shows day lighting control to be one of the most promising energy conservation strategies for commercial buildings; consequently, day lighting controls are more frequently installed," says David Eijadi, Fellow of the American Institute of Architects, Principal at The Weidt Group, which provides energy design assistance, including day lighting as an energy conservation strategy, to architects and engineers, consulting on more than 150 projects annually. "Because energy codes may eventually mandate the use of day lighting controls, it seems prudent to look for object lessons for success and failure from the set of early adopters."