The Lighting You Don't See – and Don't Pay For

Recently a colleague and I had the opportunity to get a tour of Lutron’s lighting showroom in Washington, D.C. I was surprised and impressed by how well the lighting controls react to varying ambient conditions and ultimately help reduce lighting loads by 20% to 60%. The strategies I saw there could be used by many buildings in the U.S.

Of course, the showroom has daylight harvesting sensors that dictate the lumen output of the ceiling fixtures. However, it also has window shades, occupancy sensors, scheduling software, and integration with security card access controls. The key is that the lighting controls seamlessly coordinate these building components.

In a nutshell, the lighting system creates “scenes” for different building operating modes (presentation, classroom, office, conference, etc.) and even “scenes” for specific occupants during different times of the day. For example, each area can be customized to particular occupants’ needs.

As with many existing buildings, the operating schedule of the showroom office can be programmed with startup and shut down times. However, during hours when the showroom is closed, the system also “listens” for occupants to arrive. When an occupant swipes a card to gain access to the showroom, the system goes to work. Preprogrammed with the angle of the sun, it adjusts the window shades, then dims the lighting to compensate for available daylight. It performs these actions only in the area in which the occupant works. If the occupant deviates from his/her usual location (going to the kitchen, bathroom, etc.) the system detects that movement via occupancy sensors and adjusts accordingly. All the information can be tracked and analyzed for savings computations and further optimization. The 3,100-square-foot office we toured, which has a south-facing exposure, had a measured lighting energy savings of 20.1 MWh during the past twelve months, according to Lutron.

Despite such performance, what most surprised me during my visit was how little I could notice the effect of dimming. Our tour guide asked my colleague and me to observe the ceiling lights and watch for a change. Over a 3-second period when the lights were dimmed 20% we noticed nothing. Now I am biased (believing that I know something about lighting), but I am telling you that over that 3-second dimming period, my eyes could not detect the change (even on the second attempt).

This result can be leveraged to many existing facilities. It is well known that when utilizing daylight harvesting, you want the dimming function to be slow and delayed so that occupants don’t notice the light intensity going up/down when clouds quickly obscure the sun. However, being able to “get away” with a 20% savings (even without any daylight harvesting controls) does offer new opportunities. For example, consider that most U.S. buildings are designed to be “over-lit” to account for lumen depreciation and general light loss over time. In these buildings a dimming ballast may offer some extra savings during the first year by “right sizing” the lighting output.

Below are some basic calculations for a dimming ballast with a daylight harvesting option. Assume a 2-lamp, T-8 fixture with 3,500 hours/year and 10 cents per kWh. Also assume that an ordinary ballast costs $20 and a dimming ballast costs $50. If the dimming ballast saves 40% during its first full year, the incremental payback would be:

                        =($30)/(0.064kWatts/fixture)(.4)(3500 hours/year)($.10/kWh)

                        = 3.35 years.