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Is Energy Savings a Turnoff for Metal Halides?

Source: Venture Lighting International

Many environments that use metal halide lighting leave the lights turned on for many hours at a time. This makes sense when there is consistent foot traffic in the area. In some instances, though, the occupancy fluctuates on a cyclical basis throughout the day with vacancies for some extended periods of time. The lights are on, but no one is home. The question then becomes, "What loss of lamp life would occur if metal halide lighting is turned on for a few hours (say, 3 to 6 hours), and then turned off for a few hours?" To answer this question, you must examine how rated life is determined for high intensity discharge (HID) lighting.

According to the Lighting Research Center [1], the rated life of HID lighting is based on a much longer period of operating "on" time than fluorescent lighting (typically 11 hours versus 3 hours). Keeping the metal halide lamp on longer than the rating period extends the life and turning the lamp off sooner than the rating period shortens the life. So, turning off a metal halide lamp sooner than 11 hours will shorten its rated life. The question, again, is how much?

Venture Lighting, a leading manufacturer of metal halide lighting products, states that its metal halide's 20,000-hour rated life is based on a 10 hour "on" cycle [2]. As a rule-of-thumb, decreasing the burn time below that figure decreases lamp life, but not linearly. For instance, decreasing the burn time to 5 hours (50% rated "on" time) would decrease lamp life 25% (down to 15,000 hours), and decreasing burn time to 2.5 hours would decrease lamp life by 50% (down to 10,000 hours). This is summarized in the following table.

Affect of Burn Time on Rated Life
"On" Time (hrs, continuous)	Expected Life (hrs)
10	20,000
5	15,000
2.5	10,000

This begs another question, "What is the break even point between keeping the lights running and the cost to replace the lamp?" Assume an average electric rate of $0.08/kWh as an operating cost to run a 400-watt metal halide lamp. Consider the following three scenarios:

• Scenario A—The lamp burns continuously 10 hours per day with a rated life of 20,000 hours.
• Scenario B—The lamp burns continuously 5 hours per day with a lamp life of 15,000 hours.
• Scenario C—The lamp burns continuously 2.5 hours twice per day with a lamp life of 10,000 hours.

A 400-watt metal halide lamp should cost about $16 to purchase with an additional $5 for labor to change the lamp. Now, compare the annual cost of ownership for all three scenarios including the cost to operate the lamps and the amortized replacement cost per year.

• Scenario A—To run a 400-watt lamp 10 hours per day continuously for 365 days, the annual energy consumption would be 1,460 kWh (10 hr/day x 365 day/yr x 0.4 kW) at an annual cost of around $117. The lamp would last roughly 2,000 days (20,000/10) or 5.5 years. The amortized annual replacement cost would be $3.80 ($21/5.5 yrs).
• Scenario B—To run the lamp only five hours per day, the annual energy consumption would be 730 kWh at an annual cost of around $58. The lamp would last about 3,000 days or eight years (15,000/5).
• Scenario C—To run the lamp 2.5 hours twice a day, the lamp would also last 2,000 days or 5.5 years (10,000/5).

A summary of these costs is shown in the following table.

Annual Cost of Ownership for a 400-Watt Metal Halide Lamp
Scenario	Burn Time (hrs)	"On" Time per Day (hrs)	Annual Operating Hours	Annual Energy Consumption (kWh)	Annual Energy Cost	Lamp Life (yrs)	Annual Amortized Replacement Cost	Annual Cost of Ownership
A	10	10	3,650	1,460	$117	5.5	$3.80	$120.80
B	5	5	1,825	730	$58	8	$2.63	$60.63
C	2.5	5	1,825	730	$58	5.5	$3.80	$61.80

The energy savings from turning off the lamps more than offsets the loss of lamp life. It always pays to turn off metal halide lamps when they are not needed. Another option is to dim the lights which does not degrade lamp life if the dimming level is kept above 60%. Energy consumption is decreased proportionally during dimming, but more energy will be saved by turning off the lamp.

Power Draw when Turning On

For HID lighting, the energy consumption during start-up is not nominally greater than during operation. The lamp ballast regulates the power to the lamp during both start-up and operation. Any high voltage pulse used for starting is so short in duration that it has no effect on energy consumption. Therefore, there is no break-even point. You will save energy anytime the HID lamp is off. According to Venture Lighting, though there may be a slightly larger current draw during start-up, the voltage is lower than nominal and total power (watts). Power ramps up over a few minutes after start-up. Therefore, there is no larger power draw during start-up than during continuous operation.

When a metal halide lamp extinguishes, the ballast immediately attempts to restrike the lamp. For pulse-start ballasts, it applies 2,000 volts to 5,000 volts every half-cycle until the lamp restrikes. This happens automatically and does not typically require any manual intervention. If the lamp extinguished due to loss of power from a circuit-breaker trip and a few minutes pass before power is restored, the cool-down time would simply shorten the total restrike time. This time depends on whether the metal halide (MH) has the older probe start or the newer pulse start ballast.

Phase	MH Probe	MH Pulse
Warm Up	2-15 minutes	1-4 minutes
Restrike	5-20 minutes	2-8 minutes

Sources

[1] Mid-wattage Metal Halide Lamps. 2005. Rensselaer Polytechnic Institute Lighting Research Center. National Lighting Product Information Program (NLPIP). Lighting Answers. Volume 7 Issue 1.

[2] Conversations with Venture Lighting Technical Support personnel in 2009