Conceptually, daylight harvesting is rather simple: Increase the amount of natural light entering a building, then use photo sensors to detect the amount of light coming in through the windows in any given part of a building to automatically adjust the facility’s electrical lighting, window shades and HVAC system accordingly.
Put into practice, it’s a bit more complicated than it initially sounds because there are so many influencing factors.
“It’s a very complicated equation because you’re trying to balance so much,” says Brennen Matthews, the North American specifications sales manager for lighting technology company Lutron Electronics.
The physical orientation of a building relative to the sun’s location is a huge factor that changes hour-to-hour, day-to-day and season-to-season as the sun changes its daily track through the sky, Matthews says. Say, for instance, a building has an east-facing façade. As the sun rises in the morning, shades on the eastern portion of the building might be lowered to 70 percent, then rise as the sun ascends; as it fades again in the afternoon, the shades on the western side of the building will lower accordingly.
“As the sun tracks throughout the day, I’m actually having a relationship between open and closed (levels) on my shades,” says Gary Meshberg, the president of the Lighting Controls Association and the director of sales at Encelium Technologies.
Of course, the lights in the different parts of the building will also change throughout the day, as will the output from the HVAC unit, and the timing and fluctuation of light, shade and HVAC levels will also change as the sun’s height and angle changes with the seasons.
Natural light levels are lower farther into the interior of a building’s space, and closed office spaces along exterior walls sap all daylight from subsequent interior spaces. Yet even open