Reducing Lighting Energy Consumption
By Marlon Hathaway, PE, LEED AP, RCDD
Building owners frequently say they want a Leadership in Energy and Environmental Design (LEED) level building at no additional expense. This represents many challenges. In the discipline of electrical engineering, this poses the question, “What LEED-conscious lighting decisions can be made without adding costs?” This can be accomplished in a multitude of ways, but the three biggest and best methods begin in design: limit watts per square foot, turn off lighting when not needed, and take advantage of natural light.
Energy usage, or watts per square foot, starts with an appropriate lighting design. This includes choosing a suitable luminaire (fixture), determining the required lighting level, and modeling the area/space to be illuminated. To choose an appropriate luminaire we need to know the ceiling or mounting method, the ceiling height, and surface reflectances. This is the time when working with local lighting representatives will help, as they have the greatest knowledge of available products. Next is determining the lighting level to be obtained. We can find this information in the latest edition of The Lighting Handbook from the Illuminating Engineering Society (IES). Then we need to model the space with appropriate software to determine fixture spacing for a maximum to minimum ratio. When the best lighting design is determined, the design is compared to the maximum allowable watts per square foot for the space. Typical spaces can be well illuminated at 80% of the American Society of Heating, Refrigerating and Air-Conditioning Engineer (ASHRAE) Standard 90.1 mandated watts per square foot.
Once we are confident with the lighting design, we turn our attention to lighting control. Remember the constant reminders to shut the lights off when leaving a room in order to conserve energy? Now control devices do it for us. The three types of on/off lighting control include occupancy sensors, vacancy sensors, and timed-off centralized systems. Occupancy and vacancy controls can be standalone zone control or networked systems. Standalone devices generally consist of wall or ceiling mounted units using several technologies to sense occupant presence. If an occupant is not detected, both occupancy and vacancy sensors will run for a preset time and then shut off the lights. The difference between these two types of controls is an occupancy sensor will turn the lights back on (if left on) when the occupant returns; the vacancy sensor requires manual input. The last type of automatic control, the timed-off centralized control system, is simply programmed to shut off zones at a predetermined time. Over-ride switches are used to turn the lighting back on if the occupant remains in the space. The lights then stay on for a predetermined amount of time before turning back off. This is a very simple explanation of a modern on/off lighting control system. There are systems which use a combination of all of the above in both centralized and distributed forms.
With the addition of photoelectric sensors or daylight sensors and dimming LED drivers, controls can do more than turn lighting systems on and off. If the space has natural daylight through windows or skylights, the artificial lighting system can be dimmed to a predetermined level. Most LED luminaires come with a dimming driver which uses a signal to determine lighting intensity. If a signal is not present, the luminaire illuminates to 100%. A daylight sensor placed in the space can send an analog signal to each luminaire to dim in the presence of natural light. On/off control remains unaffected and all the previous methods of control remain in place. While daylight control is not as popular in Northeast Ohio as in other areas of the country, it is a viable option for spaces with southern elevations and many window surfaces. Daylight sensors could also be used with shade control systems for very sunny days when natural light may be too intense in a space.
Lighting control and design account for a small percentage of the overall LEED points system, but represent a feasible option for the owner to curtail overall energy consumption. Modern lighting systems and guidelines have reduced energy use to a fraction of what was needed just 10 years ago. Through the use of good design practices, an electrical engineer can conserve energy and save the building owner money.