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Lighting designers and end-users have long detested the off-color glow and buzzing din of aging high intensity discharge (HID) luminaires. As we look toward new innovations, budget and design constraints prevent the adoption of recent additions to the marketplace due to high introductory price points, lack of awareness and low confidence. In some cases, the best opportunity is to work with an established technology and improve its performance.
Over the years, HID lamps have become the default light source in many indoor applications. In California, alone, there are over 50 million HID fixtures. These fixtures are used during on-peak demand periods in retail stores, warehouses, service-station canopies, gymnasiums, auditoriums and convention halls. During offpeak hours, they are used in the same places along with street, pathway and security lighting. HID sources used in these applications include high pressure sodium (HPS) and metal halide (MH) lamps. These lamps offer high light output, efficient optical control and affordability. While offering many advantages, the magnetic ballast systems used with these lamps are notoriously inefficient, require frequent maintenance and lack recent technological advancements.
In the 1960s, researchers released statements that HID lamps were unstable at high frequencies. As with most advancements aided by the passage of time, the concept was revisited in later years, and in the 1990s several start-ups emerged with solutions that proved past assumptions to be incorrect. Since then, the marketplace has seen a small but steady influx of high-frequency electronic ballasts for HID sources that increase the potential for more widespread use without previous application restrictions. High-frequency electronic ballasts promise longer product life spans, reduced lumen depreciation, reduced color shift and efficient dimming.
While many magnetic ballasts are still in use, change is imminent. The question that remains is, how long will it be until the industry changeover to the emerging technology is complete? Also, growth in emerging LED and induction sectors provide competition for the traditional sources.
BLACK AND PINK
When magnetic ballasts ignite an HID lamp, high voltage forces electrons from one electrode to the other. Once the discharge arc ignites and is stabilized, the lamp goes through an extended warm-up period. The magnetic system is designed to strike a specific voltage value each time it is energized. This value is significantly higher than what most lamps require at the beginning of their life, but is designed for the voltage needed much later in the lamp’s life. The magnetic system does not account for this change in voltage. Lamp damage and a shortened lamp life can occur as a result. The lamp electrodes undergo accelerated erosion due to this ignition process. As the electrodes degrade, particles of tungsten are re-deposited on the arc tube wall as black splatter, which absorbs a portion of the light generated by the lamp and decreases its lumen maintenance.
As a magnetic ballast operates a lamp, the gas composition in the arc stream changes, affecting the color Lighting Research & Education of the light. This is the primary cause of “greenish” or “pinkish” hue associated with magnetic-ballast HID fixtures that have been in use for a period of time.
After the arc has stabilized, the magnetic-ballast system operates the lamp at ~60 Hz. Lower frequency drive results in longer extinguished periods for the arc. The electrodes cool during this time. The significant temperature gradient between ignition and the cooling period stresses the electrodes and contributes to additional erosion.
DIMMING CHALLENGES
 Reducing light levels when spaces are unoccupied is becoming a popular practice for exterior and commercial environments. To date, HID sources with magnetic ballasts have limited possibilities for use in applications where dimming or multi-level switching is preferred for significant energy savings and lengthening the product lifespan. An established approach to dimming is to reduce the voltage. Using this method with magnetic ballasts also accelerates the decay of the electrode. By reducing the voltage and maintaining the same frequency of ~60 Hz, the time period that the arc is extinguished is lengthened. Additional electrode cooling occurs, increasing the thermal gradient and the subsequent damage (Figure 1).
MINIMUM VOLTAGE
The unique electrical characteristics of high-frequency electronic ballasts greatly reduce the tube wall blackening. High-frequency electronic ballasts are programmed with variable arc striking algorithms that ignite the lamp with the minimum voltage necessary. This function reduces the rate of electrode erosion, arc tube wall blackening and gas leaks. This “gentle” arc strike also accelerates the warm-up period to full light output and greatly reduces the hot re-strike period.
 High-frequency electronic ballasts reduce thermal stresses on the electrode during regular operation due to a shortened extinguished period. Many high-frequency electronic ballasts change the operating frequency relative to how the lamp is dimmed, which keeps the extinguished arc time to a minimum. Additionally, highfrequency electronic ballasts protect lamps from voltage fluctuations that contribute to electrode erosion. All of these factors combine for improved lumen maintenance and a longer lamp life (Figure 2).
IMPROVED LIGHTING DESIGN
These ballasts represent a significant opportunity for improved lighting design. To accommodate lumen maintenance over time, spaces are often over-lit when the fixtures are installed. Improvements in sustainable light output and longer lamp life means initial lighting levels can be scaled back to the appropriate level for the space, which results in fewer fixtures used in the lighting design.
Similar to fluorescent technologies, high-frequency electronic HID ballasts typically operate at higher electrical efficiencies (~90-93 percent) than their magnetic counterparts. When combined with a more robust system design, this feature offers a significant increase in reliability and energy savings while also reducing maintenance costs for the facility manager. Reductions in warm-up time and hot re-strike periods add functionality and expand the potential applications for HID technologies. HID lighting system design can now incorporate occupancy sensors, timer-based devices and daylighting controls for additional energy savings based either on reliable dimming functionality or on/off control. High-frequency electronic ballasts can be calibrated to initially function at a lower light level and slowly increase over the life of the lamp to account for lumen maintenance. This capability is a significant benefit when attempting to maintain a specific target light level, without including additional extraneous fixtures which use more energy than necessary.
Many high-frequency electronic HID ballasts offer network controls. These systems utilize either a secondary low-voltage circuit or a power-line carrier system for individual ballast control. This affords the lighting designer and end-user convenient access to the lighting system control topology, regardless of the original wiring circuitry. If facility use changes or if additional features are needed, adjustments can be made from a digital access port. Facility managers can further utilize this network to schedule preventative maintenance. The ballast and network records the electrical characteristics of the HID lamp and utilizes the information to predict failures. With this data, the facility can schedule group relamping more efficiently.
FUTURE OPPORTUNITIES
HID sources are typically affordable and efficient. As with most new technologies, high-frequency electronic HID ballasts do present cost challenges. On average, a high-frequency electronic HID ballast is priced two to three times more than a magnetic ballast. To help soften the sticker shock, electronic HID systems are typically 25 percent more efficient than magnetic HID systems which means to get the same light output an electronic system will use 25 percent less energy and if we apply electronic ballasts to mercury vapor applications which the federal government has outlawed, energy savings can be as high as 50 percent.
Rigorous application-specific testing for high-frequency electronic HID ballasts is needed. The electronics inside any ballast can be sensitive to heat and corrosion. Life testing under specific operating conditions is necessary to determine the product life in harsh environments. Lamp manufacturers currently do not offer extended warranties for lamps used with high-frequency electronic ballasts. More data is needed to understand the effects of high-frequency drive on HID lamp life and lumen maintenance.
The opportunities for the use of HID sources with complex, environment-responsive controls are growing. Without the documentation of how the systems will perform over long periods of time and third-party testing, guarantees should be examined with skepticism. As with emerging alternative sources such as induction and LED, research is needed to determine whether early promises will be upheld.
To date, HID lamps with high-frequency electronic ballasts have reached a fraction of the market. Given their many advantages over magnetic systems, it is not a question of “if” they will be valuable marketplace contenders, but a question of “when.” California Title 20 already requires HID ballasts to have a minimum of 88 percent electrical efficiency, and further discussion regarding controls for outdoor lighting is ongoing. As this market segment expands, lighting designers will have access to lower-priced HID high-frequency ballast systems while reducing energy use, adding functionality and improving the visual environment.
July 09 |
