TD Ameritrade’s new tower, which opened for business last year, encompasses 530,000 square feet and capacity for approximately 1,800 associates. It makes use of 5,000 lighting control sensors to harvest daylight. Other sustainability features include wind turbines to power parking structure lighting; glass “frit” lamination between exterior glass panels to filter sunlight and reduce heat inside the building; rain water harvesting to flush toilets and irrigate landscaping; and solar thermal arrays to heat hot water and preheat boiler water.

Daylight harvesting systems use daylight to offset the amount of electric lighting needed in a building. Daylight harvesting is achieved by integrating lighting control systems that can dim or switch electric lighting on and off in response to daylight availability. (Dimming control systems require dimming ballasts.) If a dimming system is well-calibrated, tenants and occupants probably won’t notice changes in electric lighting levels. If the system only turns electric lights on or off, tenants and occupants will likely notice changes in lighting levels.

Daylight harvesting systems use a photosensor that detects light levels and signals a controller to adjust electric lighting based on daylight in the space. Two types of systems exist: open-loop and closed-loop. With an open-loop system, a photosensor – positioned either on an exterior wall or roof, or inside facing a window or skylight – detects daylight only. The photosensor in a closed-loop system detects light from both daylight and electric lighting systems.

A setpoint is defined to indicate required light levels, and the daylight harvesting system then adjusts electric light output in response to changes in available daylight.

Daylighting harvesting systems work best when combined with full-building lighting control systems that also involve automated shading, occupancy sensing, and personal dimming. Advanced daylight systems may also include localized, manual override controls so users can make task lighting adjustments.

David Johnson, regional sales manager for Leviton Lighting & Energy Solutions, points out that proper commissioning is crucial to daylight harvesting performance. A formal process should be followed to ensure that desired light levels are reached. “First we need to understand what the project space is utilized for,” he explains. “Second, we calculate the required light levels for those spaces so tasks and jobs can be done with proper light. The third step is to commission a daylighting system to meet that requirement, and the final step is to make sure that the transition from artificial light to natural light is seamless to the user.”

Johnson has seen daylight harvesting systems produce significant energy cost savings for high-rise buildings. Some daylight harvesting system providers offer the system itself along with commissioning and a solution for measuring and verifying savings from the daylight harvesting system. “Installing a system is half of daylight harvesting,” says Johnson. “Being able to prove and document exactly how much energy is being saved is critical.”

Daylight Harvesting Controls in Action
For the 12-story TD Ameritrade Tower in Omaha, LEED Platinum rating was the goal – and daylight harvesting helped the building get there. The facility’s daylight harvesting system makes use of a combination of schedule-based controls (relay panels), occupancy controls, and daylight harvesting controls. This three-tiered approach helped achieve energy reductions of approximately 48 percent when compared to another building of similar size and age.

LED task lights with infrared sensors are located at each TD Ameritrade Tower workstation; the task lights turn off when an employee leaves the workstation or if enough daylight reaches the space.

More than 5,000 lighting control sensors are used at TD Ameritrade Tower to harvest daylight effectively.

New in Daylight Harvesting:
Researchers at the California Lighting Technology Center at the University of California, Davis have developed a new daylight harvesting approach called the Simplified Daylight Harvesting system (SDH), which provides automatic and continuous calibration.

The system uses photosensor readings to set lights to on, off, or intermediate levels, but also gives users the ability to adjust the on and off setpoints to meet their own preferences. The SDH consists of a photosensor to measure light levels, relays to switch the states of the electric lights, a controller that determines when to change lighting states, and an optional occupancy sensor.

Differences in photosensor signals are automatically calculated every time the lights are switched, and serve to calibrate and manage the system’s response. This can account for changes in furniture layout and reflectance of interior surfaces, and also helps the system adapt to decreasing levels of electric light as lamps age.

Because of the automatic calibration capabilities and the ability for users to adjust setpoints, the system doesn’t require calibration by a technician.