Engineered Systems - November 2008 - (Page 54)

G INTELLIGENT LIGHTING GREEN INTELLIGENT BUILDINGS Lighting Controls For Daylight Spaces Provide Tremendous Benefits BY DANIEL TREVINO aylit areas in a wide variety of buildings offer terrific energy saving opportunities. 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 FUNCTIONALITY Daylighting 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 SPACE Consider 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. 54G ES GREEN INTELLIGENT BUILDINGS November 2008 D 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 COMMISSIONING Photosensors 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

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Engineered Systems - November 2008 Contents Editor’s Note HVAC Challenge Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Show Me The Motor Money The IAQ Top 10 Putting A Damper on Tragedy Special: Lighting Controls Computers & Software Products Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - November 2008

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