6.3 Lighting Controls: Occupancy/Vacancy Sensors, Daylight Harvesting, Dimming, and Task Tuning

Upgrading to LED fixtures provides an immediate baseline reduction in lighting power. However, to maximize the return on investment and achieve deep energy savings, energy managers must deploy intelligent lighting controls. Lighting controls attack the "annual operating hours" variable in the energy savings equation, ensuring that light is only delivered precisely when and where it is needed.

Occupancy and Vacancy Sensors

Sensor-based controls automatically manage lighting based on human presence in a space. They are essential for intermittent-use areas such as private offices, conference rooms, restrooms, and storage closets.

Occupancy Sensors

An occupancy sensor automatically turns the lights ON when it detects motion in the space and automatically turns them OFF after the space has been vacant for a predetermined time delay (e.g., 15 minutes). While convenient, occupancy sensors can waste energy if they trigger lights in spaces that receive adequate daylight or when someone enters a room only momentarily.

Vacancy Sensors

A vacancy sensor (also known as a manual-on/automatic-off sensor) requires the user to physically press a wall switch to turn the lights ON. Like an occupancy sensor, it automatically turns the lights OFF after the space becomes vacant. Vacancy sensors generally yield greater energy savings (typically 10% to 30% more) than occupancy sensors because lights are never triggered inadvertently. Many energy codes now mandate vacancy sensors in specific spaces, rather than automatic-on occupancy sensors.

Sensor Technologies

Sensors rely on two primary technologies:

• Passive Infrared (PIR): Detects the movement of heat signatures across a grid. They require a direct line of sight to the occupant and are best suited for smaller, enclosed spaces.
• Ultrasonic: Emits high-frequency sound waves and measures the Doppler shift of the reflected waves caused by movement. They do not require a line of sight, making them ideal for spaces with partitions, like restrooms or open office cubicles.
• Dual-Technology: Combines both PIR and Ultrasonic technologies. The lights typically require both sensors to trigger to turn on (preventing false-ons) but require only one technology to detect motion to keep the lights on (preventing false-offs).

Daylight Harvesting

Daylight harvesting is an automated control strategy that utilizes natural sunlight to offset the amount of electric lighting needed in a space. It is highly effective in perimeter zones with large windows or spaces with skylights.

How It Works

A photosensor (or daylight sensor) measures the amount of light in a designated daylight zone. The sensor communicates with a lighting controller or directly with the LED driver. As the sun rises and natural light floods the space, the electric lights are automatically dimmed to maintain the target illuminance level on the work plane. When clouds roll in or the sun sets, the electric lights brighten.

Control Architectures

• Continuous Dimming: The electric lights smoothly and imperceptibly adjust their output in direct proportion to the available daylight. This provides the best occupant comfort and the highest energy savings.
• Stepped Switching: The lighting system turns off discrete banks of lamps or fixtures in stages (e.g., 100%, 66%, 33%, OFF) as daylight increases. This is a legacy approach often used with older fluorescent systems that could not easily dim.

Daylight harvesting can generate energy savings of 20% to 60% in perimeter zones, making it a critical component of modern sustainable building design.

Dimming and Task Tuning (High-End Trim)

Because LED technology is inherently dimmable, energy managers can implement strategies that were historically difficult or expensive with fluorescent systems.

Task Tuning (High-End Trim)

Lighting systems are frequently over-designed. Engineers may specify a target of 50 foot-candles for an office, but due to conservative assumptions in the Lumen Method (such as high Light Loss Factors to account for future dirt and lumen depreciation), the newly installed system might deliver 75 foot-candles. This "over-lighting" wastes energy.

Task tuning, or high-end trim, involves artificially capping the maximum output of the luminaire at the commissioning stage. The energy manager programs the luminaire to max out at, for example, 80% of its physical capacity. To the occupant, the light level is perfect (delivering exactly the required 50 foot-candles). To the utility bill, the fixture is now operating at a permanently reduced wattage. Task tuning guarantees immediate and persistent energy savings of 10% to 30%, independent of occupancy or daylight.

Personal Dimming Control

Giving occupants the ability to manually dim their local lighting via wall sliders or desktop applications increases satisfaction and generally reduces energy consumption, as people tend to prefer lower light levels than standardized engineering codes dictate.

Networked Lighting Controls (NLC)

Advanced building portfolios are rapidly adopting Networked Lighting Controls (NLC) or Luminaire Level Lighting Controls (LLLC). In these systems, every luminaire is equipped with its own integrated occupancy sensor, photosensor, and wireless communication node.

The Power of NLC

• Granular Zoning: Instead of controlling whole circuits, facility managers can group and re-group luminaires via software, creating tiny control zones without rewiring.
• Layered Strategies: An NLC system can simultaneously execute task tuning, daylight harvesting, and vacancy sensing on the exact same fixture.
• Energy Monitoring: The system tracks and reports real-time energy consumption data, proving the ROI of the retrofit to management.
• Space Utilization Data: Because every fixture has a motion sensor, the lighting network doubles as a dense thermal map of the building, providing valuable data on how conference rooms and desks are actually being utilized.

Estimating Control Savings Percentages

When calculating the total savings of a comprehensive lighting project, energy managers apply estimated savings percentages (Control Factors) to the baseline energy consumption.

These control factors are multiplied by the baseline kWh to determine the post-retrofit energy profile, demonstrating that intelligent controls are just as vital as the light fixtures themselves.