The Dark Side of LED Lightbulbs

Read any article about energy-efficient lighting, and chances are good that the story will condemn incandescent bulbs, praise compact fluorescent bulbs a bit, and then bow to the coming nirvana of LED lightbulbs. Their efficiency is divine, the lighting so pretty! They’ll outlive the pets! And let’s not forget the ability to control them with dimmer switches.

This luminous future may yet happen. But there’s at least one substantial, potentially deal-killing obstacle that must be overcome before these bulbs can illuminate our lives with virtue. Usually when we hear about a sustainable technology that isn’t quite mature, the cause stems from their manufacturers, who haven’t yet benefited from economies of scale, or from the materials needed to make them, which are just a touch too expensive.

Not so for LEDs. The problem lies in a technical mystery, a phenomenon called “droop,” in which the efficiency of the bulb plummets as the current through the device increases. What that means is that at higher wattages, LEDs become inefficient. And that is a big problem.

If they worked in a wider range of conditions, LEDs would be extremely appealing. Though expensive, the promise of a 10- to 30-year life span and their ultra-low power needs over the course of those decades can present an attractive economic case. Currently, LEDs come in at 16 cents per lumen at lightbulb-type brightness, compared with .1 cents per lumen for incandescent bulbs—but their superior efficiencies could offset the cost. [For the record, the key measure for lighting is how many lumens you can squeeze out of a watt. Incandescents average about 15-20 lumens per watt, while LEDs can rake in much more than 100.]

For household-quality lighting, the performance of LED lightbulbs drops down below 100 lumens per watt, which isn’t going to work when one bulb can easily cost more than $50. You need to squeeze out every available lumen in order to entice ordinary consumers to double their groceries bill on behalf of a single lightbulb.

Nobody has yet identified the reason for the efficiency droop, and the world’s leading light researchers have all published academic papers claiming completely different root causes. To identify the problem at its most basic level and in the most sweeping terms, LED droop probably has something to do with the base material off of which these devices are built up. Basically, to build an LED first requires the manufacture of a highly defined crystalline structure—the substrate on which a whole bunch of other layers of semiconductor materials will be stacked to produce light. The fewer the defects in the crystalline structure, the better the LED.

As it stands, green and blue LEDs (which are an essential component in white LED bulbs) can only be grown on a sapphire substrate. This base layer is chock full of defects, unlike the different substrate used for red LEDs. Understanding why these structures can produce light in spite of all the defects, and understanding what’s happening on the atomic level in these unpredictable crystalline layers, may be key to solving droop.

There are other challenges, such as the untested nature of LED performance later in its life, and the problem of the light’s directionality—LED light doesn’t fill a room the way an incandescent does, producing more of a spotlighting effect.

A lighting revolution is indeed already happening, but it’s a bit more muted and halting than some advocates propound. I’m not claiming to be an expert on droop or the mechanics of making light. The point is rather that the claims of sustainability panaceas don’t always take into account the substantial technological hurdles that must first be overcome.

Posted By Sandra Upson

Photo Credit: Tjerk Michael de Graaff via flickr/rubberdreamfeet