Daylight Harvesting is the in industry term for adaptive lighting technologies that take advantage of daylight hours to offset the amount of electricity needed to effectively light a given space. Harvesting systems allow lighting levels to be varied based on activity levels or the time of day, and there’s currently a strong push by exterior lighting decision-makers to make better use of these cost-saving technologies. Municipalities stand to gain much in the way of reduced energy costs and light pollution by simply dimming exterior lighting during those periods when traffic use is low, and many jurisdictions are choosing to adopt such recommended measures for their exterior lighting codes and standards.
Adaptive Technology in Action
Automatic time-of-day controls and photocell sensors automatically turn on outdoor lights at dusk, and off again at dawn, and ensure that unnecessary lighting isn’t accidentally left on during the day, or at other times when it’s not required. Most adaptive lighting systems highly recommend the use of photo sensors specifically for exterior lighting because they’re capable of simply turning lights on when natural light levels fall below a certain level, whether at night or on cloudy or rainy days, and back off when ambient light increases. Photocells have an advantage over some automatic timers because they don’t need to be reset after a power failure and they don’t need to be constantly reprogrammed to keep them in line with seasonal light fluctuations. Typically, a photocell sensor device is mounted directly onto a light fixture to detect changing outdoor light levels.The photo cellis simply a light sensitive resistor that exhibits a higher resistance when it’s dark out and a lower one when it’s light. When the photocell’s resistance is low during the day, current can flow easily through the sensor to the internal circuitry that functions as a switch to keep the artificial light turned off. At night, however, when resistance is higher, the flow of current is diminished and the outdoor light turns on.
Savings in Your Pocket
Turning unnecessary outdoor light sources off during daytime hours can drastically reduce energy costs. Photocells have been proven to work well in almost any climate, as long as they’re kept clean to prevent them from perceiving light levels as lower than they actually are, and turning lights on unnecessarily as a result. Because potential savings are very dependent on the usage of the physical space being harvested for light, as well as on the number and quality of daylight hours the space is exposed to in a given year, studies have estimated a wide savings range of anywhere from 20-60%.
There is a cost associated with the initial investment in photocell technology, as well as with its maintenance, and the simplest way to determine the payback on this financial outlay is to divide the cost by the annual energy savings it could provide. This will give some idea of the number of years that would be required to pay off the upfront costs, and to start actually seeing the difference in your pocket.