By Craig DiLouie, Lighting Controls Association
Published October 2004
High-intensity discharge (HID) lamp dimming has grown in popularity in recent years. Dimming HID lamps can result in energy savings, peak demand reduction and greater flexibility in multi-use spaces.
Dimming reduces energy costs by reducing the input power to the lighting system. It can be used to reduce peak demand and therefore reduce costly utility demand charges that can be a significant component of the total utility cost. And it offers greater flexibility to adapt spaces to different uses.
HID Lamps
HID light sources, ranging from 20W to 2000W in size, can be found in numerous applications, from retail to industrial to public spaces. It is estimated that there are more than 105 million HID lamps in operation in the United States. HID lighting systems consume 12% of all lighting electricity consumed by the commercial sector, 31% in the industrial sector, and 87% in all outdoor stationary applications—an average of 17% of all electricity consumed by all lighting systems in the United States (see Table 1).
Table 1. Facts and estimates concerning HID usage in the U.S. Source: U.S. Lighting Market Characterization: National Lighting Inventory and Energy Consumption Estimate, Navigant Consulting, Inc./U.S. Department of Energy, September 2002.
Figure 1. High pressure sodium & metal halide lamp.
HID lamps are similarly constructed in that they feature an arc tube of stress- and heat-resistant material that contains gases, metals and the electrodes. They are identified via the predominant distinctive metals contained in the arc tube: high-pressure sodium (sodium), mercury (mercury) and metal halide (metallic halides).
The arc tube is housed in a protective glass envelope. When starting voltage is applied to the electrodes from the ballast or ignitor, an arc is formed between them. Electrons in the arc stream collide with atoms of vaporized metals. The result of this action is the emission of light energy. Due to the high pressures of HID lamp operation, these wavelengths are concentrated in the visible light spectrum and therefore do not require a phosphor coating as a filter.
Of the three types of HID lighting, high-pressure sodium and metal halide are the most efficacious and offer the best color, limiting mercurys use. Metal halide offers superior color quality with a bright white light, while most high-pressure sodium offer the greatest efficiency at the expense of color with an orangish light.
Dimming Strategies
Dimming can be employed in HID lighting systems to save energy, and enable the space to adapt to different uses, ambient conditions and time of day.
"HID dimming saves energy and thereby reduces owner operating costs," says A. J. Glaser, President of the Lighting Controls Association and Colorado-based HUNT Dimming. "It also maximizes end-user satisfaction by providing proper light levels."
Save energy: Dimming can be used to save energy during periods when the space is unoccupied but needs to stay lighted for safety and security reasons. Dimming can be achieved either manually via input from a switch or automatically via input from a control device. Automatic dimming can be set to respond to a preset schedule or variable ambient conditions such as occupancy and available daylight.
Occupancy. Dimming is a highly practical control method for saving energy with HID lighting systems to address periods of non-occupancy in spaces that must be constantly lighted.
High pressure sodium lamps can take 3-5 minutes to warm up; they take less than a minute to hot-restrike but dont reach full light for 3-4 minutes. Metal halide lamps take 2-10 minutes to warm up and 12-20 to hot-restrike, while pulse-start metal halide lamps take 1-2 minutes.
Given these characteristics, it is not practical to shut off and restart the lamps based on occupancy if the space must be made usable again quickly. In these situations, the lamps must be operated continuously, resulting in energy waste.
In addition, most lamp manufacturers rate HID lamp life at a minimum of 10 hours per start. Any reduction in burn time per start below this minimum will result in shorter lamp life.
If the lamps are dimmed instead in response to a signal from an occupancy sensor or time-programmable controller indicating the space is unoccupied, significant energy savings can occur during these periods, but the lamps will be