Photoelectric control: the effectiveness of techniques to reduce switching frequency

Photoelectric switching has the potential to save energy by turning off lighting when daylight illuminances are adequate. However one problem with this control type is frequent switching, annoying occupants. This paper examines three types of control algorithm which can reduce the number of switching operations. Under differential switching, the lighting is switched on if the illuminance falls below a critical value Eon, but not switched off again until the illuminance reaches a substantially higher value Eoff. Another option is the use of a time delay. The final control mode is solar reset where the lighting can only be switched off at certain set times of day. Operation of the various controls was simulated using model room illuminance data recorded every minute. The analysis reveals that solar reset gives the fewest switching operations but at the cost of increased energy consumption. A time delay, where the daylight illuminance has to exceed a target value for a set time before switching off can occur, gives the fewest switch offs for a given lighting energy use. However differential switching ensures that switching off occurs at a favourable adaptation level. The paper outlines ways to predict lighting energy use under the different control strategies.