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August 30, 2009

In Pursuit of Efficient Lighting

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What will leave Edison to the history books? As a technologist I am often asked what single change would bring about a more stable energy infrastructure - it’s not quite that simple.  Our infrastructure has evolved over the past several hundred years into the distributed, fairly reliable source of electrical and chemical energy that we now enjoy.  To pose this question is like asking what single change could be made in a human body to allow us to live longer - again, not so simple.  If you improve one area, you possibly degrade another. 

This brings up some controversy over moving to electric vehicles in an effort to reduce green house gases and remove the dependency on foreign oil.  If you could simply convert all carbon fuel based vehicles to electric, suddenly the entire electrical grid would be overwhelmed by the charging requirements.  In addition it would create a need for potentially hundreds of new power plants - many of these burning coal or natural gas and producing green house gases!  Not a simple solution...

But possibly, there is a single thing that could make a significant difference in improving our energy consumption - at least for now.  I have mentioned this before in several blogs, but it is fundamental in how modern humans live.  It is lighting - the artificial light that allows us to see when the sun goes down.  I cannot imagine a world without artificial light sources.  However, I periodically fly from coast to coast on a "red-eye" flight and as I look down from 25,000 feet I am constantly amazed on the amount of power being fed to tens of thousands of street lamps - all lit brightly regardless of who might be there.  I even pick out the lone 500 watt mercury vapor lamp on some mountain top location and wonder why it’s there...

According to the U.S. DoE Energy Information Administration (EIA), in 2007 the U.S. used roughly 526 billion kilowatt-hours of electricity for lighting (both commercial and residential). In the following year, a typical nuclear power plant produced roughly 12.4 billion kilowatt-hours, so for the U.S. the lighting needs alone require roughly the equivalent of over 42 nuclear power plants. In addition, the world population is growing requiring more energy.  This means the rate of increase of consumption in itself is increasing.

You cannot simply stop using power, but you can be more efficient with what you have.  As it turns out, Light Emitting Diodes or LEDs have been on the fast track to replace both incandescent and florescent bulbs.  LEDs today are already more efficient than incandescent bulbs, and closing fast on Florescent designs. One problem (among several) that is slowing adoption is in the luminous intensity of an LED. 

The problem stems from the way photons are created within the band-gap of the diode structure.  As electrons cross the band-gap (a forbidden energy level), they transition from a higher energy state to a lower one.  In most diodes, this transition is non-radiative (no light) and is simply converted to heat.  If the band-gap energy is high enough, a photon is created.  This is the basic operating principle of LEDs.  However, most of the photons are caught in wave modes within the semiconductor material and do not add to the light emission - only additional heat as they recombine within the material.

Well, over the last several years some very clever people at MIT started looking at regularly spaced nano-structures that act as waveguides to tunnel those lost photons out of the depths of the LED material.  These are called Photonic Crystals and have driven the luminous intensity and efficacy of LEDs to new highs.  They formed a company around the technology called Luminus to manufacture these ultra-bright LEDs.  This innovation may very well be the first step in realizing a solid-state lighting future.

Now there are still problems with inefficiency due to a phenomenon called Stokes Shift (found in White LEDs using phosphors), thermal conduction requirements (no IR emission as in incandescent bulbs), higher cost plus the addition of electronics required to power and monitor these devices.  However, simply improving the efficiency of every light bulb by 50% in the U.S would immediately remove 30 plus coal burning power plants from operation.  Now that’s significant. Till next time...

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Dave

I've been looking for LED replacement bulbs, but they're pretty expensive.

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