Daylight Design of Buildings by Baker Nick;Steemers Koen;

Author:Baker, Nick;Steemers, Koen;
Language: eng
Format: epub
Publisher: Taylor & Francis Group

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Photoelectric switching with time delay. Two different types of time delay are possible:

(a)

a switching-linked time delay, where switching off cannot occur until at least n minutes after the last switch on, where n is a preset delay;

(b)

a daylight-linked time delay (Figure 8.6), where switching off cannot occur until the daylight illuminance has exceeded the target value Es for n minutes.

No delay in switching on is recommended, because that could lead to illuminances falling well below desired levels. Of the two different control strategies, a daylight-linked time delay (waiting for the daylight illuminance to exceed the target value for n minutes) gives significantly less switching. Even quite a short time delay (15 minutes or so) gives a significant reduction in switching. The number of switch-offs declines with increasing time delay, but for very long time delays (an hour or more) the returns diminish. The energy consumption for a switch with time delay will be greater than for an instantaneous on/off switch. This is because there will be times when daylight exceeds the switching illuminance but the lighting is still left on because the time delay has not elapsed.

Suppose the photoelectric control switches at an illuminance Es and has a time delay of n minutes. The lighting energy consumption will be the same as that for a photoelectric switch without time delay, switching at a higher illuminance Emod. Analysis gives the following functions for Emod.

For switching-linked time delay:

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