The technology I speak of is from Juno Lighting Group. I had a lunch and learn on this stuff and it's pretty cool. There are testing reports from every mfg that will indicate what the usable life of the LED is (LM-79) which is calculated in hours. Juno's "Lumen Depreciation Indicator" counts to 50,000 hours on their fixtures, then turns on a little indicator under the trim when the usable life is approaching. When it reaches the end of life, the can will actually turn off. Then, the user can press and hold the button to reset for an additional 5,000 hours up to 2 or 3 times... to buy a little bit of time. (don't quote me on the numbers there, but the theory is correct) I just sell the stuff!
Also, when it comes to handling the problem of Lumen depreciation, Lithonia Lighting's LED Troffer's have an option that actively manages the lumen output over the life of the fixtures so as to maintain constant lumen output over the usable life of the fixture.
This is how it works: When you wire up the fixture for the first time, the system drops the output and therefore wattage down to 80% of output. (Design Lumens) Then over the life of the fixture the system will bump up power consumption to always maintain Design Lumens. The user won't notice any lumen depreciation over time.
Now, as for how you would know you've reached end of life? Not an option on this particular fixture. As far as how this relates to LED Street Lighting, The LED are just a semi-conductor. The technology is there. Just need to move it to that platform.
I've been told power companies drive around looking for lights out, maybe that's not true. But waiting for someone to call and tell you that a light is out isn't very pro-active either.
The technology is out there to count the number of hours a fixture has run, then turn it off when it reaches its (non-catastraphic) end-of-life.
Here's where LED Street lighting can get REAL interesting when it comes to impacting the bottom dollar. The technology is also to a point where the streetlights talk to eachother and a central computer over a WAN. Then, when one fails, it will create a work order telling city/utility/property manager where the problem is, and what the problem is. No longer do workers have to drive around looking for street lights that aren't working at 10pm. Pretty interesting stuff.
tekochip's comment about melting snow is right on the money. Although streetlights, being down facing, may not be as serious, I've seen the problem with stop lights being covered with snow which is something I had never seen before.
My other question about LED's relates to the advantage of not having to change them as often. Unfortunately, my experience with LED based technology is that they don't "burn out" as more common lights do. What is going to happen if the streetlights get dimmer and dimmer? When is it going to be a dangerous situation and what will the trigger be for getting someone up there to change it?
While the LED lights certainly do save a lot of power and energy, they have the potential to save a lot of money in maintenance costs as well. Hopefully the market for street lighting is a bit less cost sensitive, allowing the use of better quality and higher reliability components, so that the support electronics will outlast the LED portion of the assembly. I would certainly like to never need to climb up and change another sodium vapor lamp bulb again.
Good question, Mydesign. Cree claims that its LED models can serve as direct replacements, in terms of performance, for a 70W high pressure sodium vapor lamp, a 150W high pressure sodium vapor lamp, and even a 200W high pressure sodium vapor lamp, but we have no quantitative data to support that claim.
Another issue is from the Astronomy community. Light pollution is an ever increasing problem. The spectrum from low pressure sodium lights can be filtered out. Can the white light from the LEDs be filtered or will we never see the stars again?
Charles, I agree that LEDs are cost effective and energy saving light sources, which are best suited for in-house and office purposes. But is it that much powerful for replacing the sodium vapor lamps in streets? If that’s the case, a considerable amount of energy can be saved.
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