Lighting Controls For Daylight Spaces Provide Tremendous Benefits
October 29, 2008
Numerous case studies document savings of 30% to more than 40% when daylighting controls are used to reduce electric light usage. At the same time, controls improve occupant comfort by maintaining the desired level of lighting.
For these reasons, daylighting control usage is increasingly being included in both energy codes and guidelines for sustainable design. Furthermore, since peak electrical load patterns tend to parallel periods of the most plentiful daylight, daylighting control is a natural choice for load reduction.
Lighting control functionalityDaylighting controls work by using a photosensor to measure either the daylight contribution or the ambient lighting level. Based on this information and a control algorithm, lights in either one or multiple zones are switched or dimmed to maintain an optimum lighting level. Manual overrides may be included if desired.
Controls that measure only the incoming daylight contribution are called open loop. The term loop refers to a feedback loop, and open loop controls receive no feedback. Controls that detect the combined lighting levels from daylighting and controlled electric sources are called closed loop, and receive constant feedback.
Before deciding to specify open loop or closed loop controls, whether to employ switching or dimming and how many control zones to use, it is important to consider the characteristics of the space and its intended use.
Choose control strategy to suit each spaceConsider how much daylight contribution a space will receive, and whether or not the light level is likely to be consistent. Areas that receive abundant daylight with minimal fluctuations throughout the day, and that experience transient activity, such as lobbies and hallways, are good candidates for switching controls.
Areas with insufficient daylight contribution to replace the electric lighting and areas that must be conducive to occupant concentration, such as offices or classrooms, benefit from dimming.
Lighting fixtures and lamp location should also be noted. Lights that are outside of the normal view of the occupants can be switched without causing a distraction. Lighting that is within view is best dimmed.
Finally, determine whether or not the daylight will be evenly distributed throughout the space. If it will be uneven, such as in a side lit area, the space will require multiple control zones.
Specifying daylighting controls Daylighting control can be achieved using a standalone unit to control a single zone or a multi-channel system that includes a separate photosensor and controller. Either kind of device should use a photodiode that perceives only visible light.
The benefits of standalone devices include simple installation and lower cost. When dimming will be used, closed loop devices with sliding setpoints provide superb control. A setpoint is the desired lighting level, which may change from day to night.
Multi-channel control systems are recommended for larger spaces and spaces with uneven daylight contribution. When dimming, an open-loop proportional algorithm is recommended to provide a good balance between ceiling illuminance and the work plane.
When switching is desired, an on/off, bi-level, or tri-level control algorithm may be used with either a standalone unit or a system.
Control location and commissioningPhotosensors may have a wide or narrow spatial response, or angle of sensitivity. They are typically ceiling or skylight mounted and aimed according to this response and the application. Photosensors for closed loop systems should sample lighting levels that are representative of the controlled space, avoiding direct illumination from a window, skylight or lighting fixture. Photosensors for open loop systems should look toward windows or up into a skylight wells and be positioned so that they do not view the controlled lighting.
If used, manual controls should be placed in view of the controlled area. Occupant satisfaction with daylighting controls has been shown to be the highest when individuals are provided with manual control.
Because daylight coverage is unique in each space, commissioning is required to tune the system to the conditions of the application. Commissioning involves setup procedures, calibration, and the input of control settings once furnishings are in place.
Additional considerationsEnergy codes also play a role in the design process and may dictate the use of dimming or multi-channel devices. As an example, California’s Title 24-2005 not only requires daylighting controls for 50% of the lighting in daylit spaces greater than 250 sq ft, but also requires multi-level controls (daylighting or astronomical time clock) for spaces greater than 2,500 sq ft.
Daylighting controls can be used to achieve significant energy savings in classrooms, lobbies, atriums, private and open offices, retail spaces, warehouses, gyms, restaurants, and other spaces. Evaluating each space helps ensure an optimal system design that is virtually unnoticeable to the building occupants. Lights dim without causing distraction, or switch off when daylight levels increase so that the light level change is not noticeable. GIB