Recently, I saw a news report that some city was converting all its streetlights to LEDs. (I hadn’t any idea that they made them, but that’s besides the point.) The reason was that they’d save energy and money with them.
Will they really save energy with them over low pressure sodium lights? Or even over mercury-vapor? LEDs consume little energy, but to get lots of light, you need lots of LEDs and the energy consumption adds up. And these were LEDs were white, which are the most energy-consumptive. I was under the impression that for area lighting, LEDs consume more energy than gas-discharge lighting for a given level of illumination. Is my understanding incorrect?
I don’t know about the power/illumination question, but the major saving will be in maintenance - LEDs will last 4-5 times longer than any other sort of bulb, and there is a huge cost in maintaining street lighting, due to insurance and danger payments etc, not to mention disposal of blown bulbs containing toxic and dangerous materials. Moving to LEDs will save so much money that any additional power costs are minimal.
Here in Minneapolis, the city has been replacing the bulbs in traffic signals with LED ones for some time. (At first, it was only the red and yellow lights – they couldn’t get green LED’s bright enough. But now that seems to have been fixed.)
Traffic lights are small, directional, and don’t need to be that bright – just bright enough to be seen, not to actually light up anything. But streetlights are expected to light up a whole section of the street, and everything in that area. I don’t think current LED’s would be suitable for that. Even in homes, LED’s are currently used mostly in directional lights, like desk lamps, reading lights, etc.
According to Wikipedia you are correct. I had been under the mistaken impression that LED’s were the most efficient lighting technology around. It says that the luminous efficacy of current technology LED’s are 3.8%–10.2%, prototype LED’s are up to 22%, and low pressure sodium are 27%.
One of the advantages of LEDs over some other forms of lighting is that they run on fairly low voltages - this means that for some applications - probably not so much main lighting for roads, but certainly other things like illumination of traffic signs, low-level ambient lighting in bus shelters, illumination of footpaths, etc - they can be used in a self-contained/standalone, solar-powered, low-maintenance installation.
LEDs are perfectly capable of lighting up an area. LED flashlights, with a single diode, are 20-30% brighter than an ordinary bulb, and I can use mine to light up an entire dark room. And these streetlights will have arrays of them.
In a recent article about LED lighting I read that the trend in LEDs will soon have them being the most efficient practical lighting as well as the longest lasting. I read also that laser diodes can be even more efficient and may well some day become the standard illumination source. The same article mentioned a golf-ball-sized glass globe filled with something - sodium and inert gasses, maybe - which glows brilliantly when you focus microwaves on it, with much much higher efficiency. The article considered that these things just might become the dominant illuminants, if supplying the microwaves got much easier for some reason.
Street lights, here in Ann Arbor. From Ann Arbor Becomes an LED City (PDF), available on the linked page,
I’ve seen them, and they’re kind of ugly, but not because they’re LEDs. The lights they are putting them in have big, 16 inch diameter glass (or plastic) spheres. The LED lights end up with a bunch of polka dots on the bottom half of the sphere. On the plus side, since they’re not lighting the top of the sphere, less light will be thrown uselessly up to the sky.
Thanks to Zenbeam for reporting what city was converting to LED streetlights. Ann Arbor was the city I’d heard about.
Interesting. And a very good thing. Fluorescents are good, but they have the environmental drawback of having a little bit of mercury in them. The amount is a fraction of a gram per tube (either straight or compact), but that adds up when they make billions of them. Eventually there will be lots of mercury released when the tubes are thrown out or broken.
That’s the sulfur lamp. They have two disadvantages. First, they produce a lot of RF interference and second, they produce lots of light, far more than you want for home use. AIUI, they can’t be scaled down.
The first problem can be ameliorated (but not eliminated) by shielding. The second means they are restricted to use in places like stadiums or factories with light pipes to distribute the light to where it’s desired. I suppose they could do the same for street lighting.
It’s a new product, so they’re somewhat vague on it…
From what I read in a technical journal, it’s very similar to current Metal-halide HID lamps, except it’s excited externally. The efficiency is supposed to be as good or better than HID, which is to say, very good. Lifetime is greater than 50,000 hours.
They are already being designed into projection TVs, so I suppose it’s not just hype…
I heard a report the other day that was discussing an unexpected problem with LED traffic lights. Apparently the heat generated is so low that snow and ice can build up on them and where older lights would cause it to melt quickly the LED lights remain hidden.
That shouldn’t be a problem for LED streetlights. White LEDs are less efficient than colored LEDs, and streetlights need to be much brighter than traffic lights.
In fact, proper cooling of the LED is a major engineering hurdle for bright LED lights. Partly because LEDs are easily damaged by overheating, but also because they do put out a lot of heat. My 4-watt LED bicycle headlight gets almost too hot to touch, and it came with a warning that said it may not work at full brightness without an air flow to keep it cool. (I think a thermal protection circuit switches it to a low-power setting if it gets too hot.)