What is Daylight Harvesting?
 

Forget harvesting pumpkins. This fall, the California Lighting Technology Center at the University of California Davis is going to be working on a better way of harvesting daylight.

"Daylight harvesting" is a term for a process that involves installing photosensors on a ceiling to determine how much daylight is in a room and dimming or brightening the electrical lights in response.

Photosensors in general aren’t new. They’ve been in use in outdoor applications for at least 30 years, and if you've ever seen streetlights slowly start to glow as dusk falls, you’ve already seen photosensors in action.

But photosensor technology for indoor spaces is a little more complicated.

Daniel Trevino, a daylighting project line manager at WattStopper, a company that makes indoor photosensors, explains that photosenors that are used indoors have to adjust to light levels more precisely than ones used outside so humans don’t perceive a change in light levels.

That’s tricky, Trevino says, because sometimes the sensors pick up ultraviolet or infrared light instead of just the light that’s visible to the human eye. And sometimes the light that hits the ceiling doesn’t correspond to the light that’s hitting the table in the center of the room where someone is sitting.

“Daylight harvesting technology has been out there for a long time… but for one or another reason people have complained over time because it’s hard to change the state of the light while you’re in the space,” Trevino says. “When you’re working on your computer and the lights change, [people don’t like that].”

Although the technology has improved greatly over time, Trevino says problems like these have kept daylight harvesting technology from becoming more widely accepted.

That’s why WattStopper and the California Lighting Technology Center are developing a new project to make photosensors more reliable.

This November, researchers will install not one, but two photosensors on the ceiling of a Wal-Mart store. When both photosensors show the same reading, the electric lights will get brighter or dimmer.

“You can fool one photosensor easily by doing certain things, but it’s going to be hard to fool two photosensors,” Trevino explained.

If it’s successful, Trevino says the improved daylight harvesting technology could be adopted by big box stores all over the country, which are all looking for ways to save on their energy costs.

But daylight harvesting for residential applications could still be a few years off, Trevino says.

Right now, there are about a dozen companies in the United States that manufacture photosensors and specialize in daylight harvesting technology, and all of them make products that are geared toward commercial spaces, such as offices and classrooms, which need to be lit all day, whether by the sun or by electricity.

So far, manufacturers have ignored the residential lighting market, where it’s assumed that if someone is home, the best way for them to save energy is to install an occupancy sensor instead, or simply turn the lights off when they leave the room.

Still, Trevino says daylight harvesting is a growing technology, and he predicts that as the technology improves, photosensors will be installed in homes, as well.

“I have high expectations that one day photosensors will be used in the residential market,” he says.

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