Greener LEDs

April 7, 2010

Most of us have heard suggestions or exhortations that we should replace our old-fashioned incandescent light bulbs (first developed 130 years ago by Thomas Edison) with more energy-efficient compact fluorescent lamps.  The difference in efficiency is significant; a typical incandescent bulb produces about 13-15 lumens of visible light for every watt of electricity consumed.  Most of the bulb’s output is in the form of heat.  A compact fluorescent lamp [CFL], in comparison, produces about 45-70 lumens per watt, depending on the design.  Despite the efficiency advantage, though, CFLs are not an ideal solution, for several reasons:

  • Although their potential lifetime is an order of magnitude better than comparable incandescent lamps, it can be severely curtailed if the lamp is switched on and off frequently.
  • Some older lamp designs have flicker in the light output at a frequency of about 100-120 Hz (twice the AC line frequency).  Although this is not perceptible, and does not affect most people, there is some evidence that it can have an adverse effect on some sensitive people.
  • Fluorescent lamps contain mercury, which is a potent neurotoxin.  Used lamps must be disposed of properly, and special precautions are recommended if a lamp is broken.  (For example, a vacuum cleaner should not be used to clean up the mess, because it can aerosolize a mixture of phosphor and mercury.)
  • Fluorescent lamps cannot be used with conventional light dimmers.

Light-emitting diodes [LEDs] are in many ways a more attractive light source.  They have very long expected lifetimes, in a range of 35-50 thousand hours.  (By comparison, lifetime for a CFL is 10-15 thousand hours, and an incandescent lamp has an expected life of only 1-2 thousand hours.)  Moreover, they tend to fail gradually, producing less and less light, rather than “burning out” all at once.  Since they are solid-state devices, there is no glass envelope to break, and they do not contain any volatile toxic material. Their light output per watt of electricty used is at least as good as CFLs.  They are already widely used in specialized applications; you almost certainly own several devices that use LEDs as indicator lights.  One of their drawbacks for more general use, though, has been the difficulty of producing what we would see as “white” light.   It has been easy for some time now to produce LEDs that emit red or blue light, but green has been a stumbling block.  And the three primary colors are needed to produce white light.  (There are some current “white” LED devices, but these use a blue or UV LED shining on a phosphor, as used in fluorescent lamps, to get to the right part of the spectrum.  This, unfortunately, significantly decreases energy efficiency.)

According to an article at the PhysOrg site, researchers at the National Renewable Energy Laboratory [NREL] have developed a method of producing LEDs that emit deep green light.   As the article points out, an LED is in some sense a solar cell that runs backwards.  Solar cells convert light energy into electricity; LEDs convert electricity into light.   The lab has substantial experience in designing and fabricating solar cells, and they decided to explore how that might be used in developing better LEDs.  They now feel that they have a recipe for making a very high-quality “white light” LED:

The aim now is to provide a fourth color to make that white light even whiter.

NREL plans to use a slightly deeper red and a lemony green, which would then be combined with a blue and a very deep green made using the gallium nitride based technology.

Using this mix of colors, it should also be possible to vary the spectrum, or color temperature, of the light produced.  (The color temperature of a light source is the temperature of an ideal black-body radiator that would produce the same spectrum as the source.  Higher temperatures correspond to “bluer” light.)  For example, it might be possible, using the same light source(s), to produce “white” light that resembles sunlight (color temperature about 5500 K), or traditional incandescent lights (color temperature typically ~ 3000 K.)

The efficiency benefits of a switch to LED lighting could also be impressive.  A Department of Energy estimate gives the potential annual saving on electricity at $120 billion.  And I wouldn’t have to be hauling out the ladder to replace light bulbs anywhere near as often.

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