Page 192 - Solar Power in Building Design The Engineer's Complete Design Resource

162 ENERGY CONSERVATION

Automatic daylight controls Daylighting controls consist of photosensors
that compare actual illumination levels with a reference illumination level and
gradually reduce the electric lighting until the reference level has been reached.
These controls are also deployed for power adjustment factor (PAF) lighting cred-
its in the day-lit areas adjacent to windows. It is also possible to reduce the general
lighting power of the controlled area by separate control of multiple lamps or by
step dimming.
Stepped dimming with a time delay prevents cycling of the lights, which is typically
implemented by a time delay of 3 minutes or less before electric lighting is reduced or
is increased.
Light control in daylight is accomplished by use of photodiode sensors. Note that
this requirement cannot be met with devices that use photoconductive cells. In gen-
eral, stepped switching control devices are designed to indicate the status of lights in
controlled zones by an indicator.

Interior photosensor device Daylighting control systems in general use photo-
sensor devices that measure the amount of light at a reference location. The photo-
sensor provides light level illumination information to the controller, which in turn
enables it to increase or decrease the area electric light level.
Photosensor devices must, as previously stated, be certified by the CEC. Devices
having mechanical slide covers or other means that allow for adjusting or disabling of
the photosensor are not permitted or certified.

Multilevel astronomical time switch controls Areas with skylights that
permit daylight into a building area are required to be calculated by the prescriptive
calculation method and to be controlled by mandatory automatic controls that must be
installed to reduce electric lighting when sufficient daylight is available. Multilevel
astronomical time switch controls or automatic multilevel daylight controls specially
designed for general lighting control must meet the mandatory requirements for auto-
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matic controls when the particular zone has an area greater than 2500 ft .
The purpose of astronomical time switch controls is to turn off lights where
sufficient daylight is available. Astronomical timers accomplish this requirement by
keeping track of the time since sunrise and amount of time remaining before sunset.
As a basic requirement, the control program must accommodate multilevel two-step
control for each zone programmed to provide independently scheduled activation and
deactivation of the lights at different times.
In the event of overly cloudy or overly bright days the astronomical timers are
required to have manual override capability. Usually, the override switches in a
zone are configured so that lights will revert to the off position within 2 hours,
unless the time switch schedule is programmed to keep the lights on.
To comply with the power consumption regulation requirements, light control is not
allowed to be greater than 35 percent of the total lighting load at the time of minimum
light output. Device compliance also mandates that devices be designed to display the
date and time, sunrise and sunset times, and switching times for each step of control.