In a study recently published by “Physical Review Letters,” a pair of MIT researchers demonstrated that an LED can actually put out more optical power than the electrical power fed into it. Although this might on its face seem to violate the law of thermodynamics, in reality there is a phenomenon taking place which allows the LED to actually use the heat energy created by the LED to add to its photon production. The energy fed into their LED was so small, and the output so meager, they were able to take advantage of a LED’s tendency to become more efficient the lower the current fed into it. Although their demonstration doesn’t say anything important about potentially increasing the efficiency of LEDs used for general illumination, it does demonstrate the extreme efficiency potential with which LEDs operate.
In the real world, commercial grade LED lighting isn’t quite so efficient. In terms of general energy conversion to visible light output, today’s LEDs are capable of reaching up to 80 percent efficiency under perfect conditions, but for the most part average around 60-70 percent. Still, this is a significant improvement over incandescent bulbs which waste up to 90 percent of their energy as heat. LEDs even surpass metal halide for actual energy to light conversion, with metal halide converting about 50 percent of the electrical energy supplied into visible light.
Of course, other factors come into play as well when considering real-world applications. Incandescent, fluorescent and HID light sources, for example, radiate light in all directions. This means that in order to project the maximum amount of their light output onto the targeted area, the fixture housing them must incorporate a reflector in order to redirect much of their output. The problem here, however, is that, even with an effective reflector, a lot of light is lost to fixture absorption, diffusion and spillage. The end result is less light reaching the targeted area, adversely affecting how efficiently the fixture can illuminate a given amount of area.
LEDs, on the other hand, are directional light sources, meaning they only radiate light over a limited portion of their surfaces. The result is less light lost to absorption and more light reaching the targeted area. Because of this, it is possible to use an LED fixture producing less actual lumens to direct the same amount of light onto a target as a more powerful HID or fluorescent. Regardless, the most common way different light sources are compared efficiency wise is to look at their actual lumen per watt ratios, which as we’ll show isn’t a very good way to measure actual efficiency. Outside of the laboratory, the incandescent bulb produces about 14-17 lumens per watt, the metal halide around 65 to 100 lumens per watt, and the LED around 60 to 100 lumens per watt. As we can see, metal halide and LED light sources provide similar lumen output. But this does not mean they will provide the same efficiency performance in real-world applications.
Directing the light
Consider, for example, an LED and metal halide fixture side by side. Each lamp produces 100 lumens per watt, and each produces a total of 1,000 lumens. That would mean we have two very different lamps, each using 10 watts to produce 1,000 lumens. Now, if we had the metal halide bulb suspended naked over a surface, about only ¼ of those 1,000 lumens would be directed toward that surface; the rest would radiate upwards and off to the sides, providing no useful illumination. (Fig B) In order to get that lost light onto the target, we must fit the metal halide with a reflector. Now, with our reflector in place to redirect much of that lost light downwards, we still only see about 60 percent of the lamp’s output directed toward our target surface due to lumen losses from diffusion and absorption. (Fig A) The result is that of the initial 1,000 lumens, only 600 lumens are actually directed downwards toward our targeted surface.
Now if we look at the LED, it produces directional light, typically radiating its output over a 120-150 degree area. This means we can hang our LED fixture over our target surface and get nearly all of the initial 1,000 lumens directed downwards toward it without the need for a reflector or the losses than come with the use of one. (Fig C) So we could, in effect, actually use an LED fixture producing 600 lumens and using only 6 watts, to direct the same amount of light to our target as a 10-watt metal halide producing 1,000 lumens.
Switch and save
In the end, what really counts are the results found through real-world application of LEDs. To that point, we’ll provide a few documented examples showing just how much savings can be had with a switch to LEDs.
- In 2011 the city of Sacramento California replaced all the metal halide lighting within a 180,000-square-foot parking garage with LEDs coupled to occupancy sensors. After a short evaluation period, the city determined that an 88-percent reduction in energy use was achieved, with an annual savings of $34,500 expected as a result.
- Advocate BroMenn Medical Center located in Bloomington-Normal, Ill., replaced their metal halide parking lot lighting with LED fixtures. Their annual savings amount to $33,000, with another $13,000 in savings expected from reduced maintenance costs.
- The Lewis Bear Company in Pensacola, Fla., a regional franchise distribution point for Anheuser Busch, performed a full upgrade of its facilities with LEDs. The result was an annual savings of $54,498.
These are only a few examples of just how effectively LEDs can improve efficiency and reduce costs. In each of these cases, and many more like them, not only was efficiency and reliability improved, but light levels and quality were either maintained or improved over the metal halide systems being replaced.
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