Biggest Barrier to LED lighting

Okay, there are several barriers, and it could be easily argued that there are more serious ones than this. I believe the biggest problem with LED lights will be shortened life to to overheating in standard fixtures (I believe short CFL life is most often caused by overheating, and I'm not convinced that Energy Star or other standards address this). Most fixtures that have been designed for edison-base incandescent bulbs are not designed to remove the heat from the fixture or bulb. Instead, the fixture is designed to withstand the heat (ever see a label on a light fixture that requires 105 C rated wiring?). Some of the worst offenders, as far as keeping the heat in, are recessed lights that are rated for insulation contact. This means that the fiberglass (or blown-in) insulation can completely cover the light fixture inside the ceiling. We did some testing with Illumra wireless relays in the ceiling box and found that even with a 40 or 60 watt incandescent bulb, the wiring box could reach 80 or 90 degrees C.

Okay, enough background, why is this such a big deal for LED lights? Put simply, like any electronic device, LEDs do not like to be hot. It shortens their life a lot. For the physics behind this (if you care) the Arrhenius equation gives you some idea - doubling the temperature doesn't just cut it's life in half, it cuts it much shorter. You see, most light fixtures have been designed to take light from a (fairly focused) point source, the filament in the bulb, and diffuse it evenly over a space. Putting lots of LEDs at one point concentrates the heat, making it harder to keep the LEDs cool. Spreading out the LEDs is a good solution, but then the fixture isn't designed to take light from many points and distribute it.

I've seen a number of LED lights in a fluorescent tube form factor, but with most (or all?), the LEDs radiate only out one side of the bulb, defeating the reflectors in the fixture, preventing the light from shining where it should. In an office environment, you would probably get very uneven lighting with this solution (until they start mounting the LEDs 360 degrees around the tube).

The real solution is to design the fixture from the start with LEDs in mind. This will require a "ballast" (really a power supply) that provides a constant current to run the LED string, along with either failed LED bypass or failed LED indication (if each LED is removable). I suggest that, if a standard small form plug-in LED module included a small parallel LED with a large value resistor, then when that LED fails (open) the small cheap LED would illuminate to show which module(s) need to be replaced. Otherwise we'll be in the same situation as with a string of holiday string lights, when one bulb is slightly loose, the whole string fails to light. The whole string should be driven with a constant current (for most even illumination). Typical drive currents are 350 mA, and if the correct number are arranged in series, the voltage of the string could be set not far below the voltage provided by a rectified AC line voltage, which will help with efficiency (using a switching constant-current power supply with AC line power factor correction is even better - see my earlier posts about that).

More to the point of overheating, spreading the LEDs around a large surface helps solve the problem. Imagine a standard office ceiling fixture with 4 T8 fluorescent tubes. Remove the tubes, then cover the surface of the internal reflector with LEDs. The heat is spread out, the whole string can be in series, and when one cartridge fails, the small red LED shows you which one to replace. This solves most (if not all) of the issues with LED lights. Old fixtures could even be retrofitted with this solution without requiring complete replacement of the fixture. With a more expensive "ballast," dimming could be supported for daylighting or load shedding. As I've mentioned before, however, phase-cut dimming is not a good idea. Either powerline-based data communication or wireless control (insert shameless plug for Illumra products here-remember I work for them :-) is the only good option for dimming the lights. The extra cost (in the bulb/ballast/fixture) to support phase cut dimming (just so you can use your old "standard" wall box dimmer) is not worth it.

Two unrelated notes:

One, I've been adding new features recently to the firmware in many of the Illumra dimmers and load controllers. From day one, we've always tried to select default operating modes that are the most convenient for the users of the products. At the same time, while they save energy as much as possible, we don't want their operation to be intrusive. For example, if a receiver is set up to turn off the lights when the space has been unoccupied for a while, for a time, after the lights turn off, if occupancy is detected, they turn back on. However, California Title 24 (which includes regulations updated in 2005 for how these devices must operate) doesn't allow automatic turn-on when you enter a room. We're making Title 24 compliance the standard operating mode for our receivers, even though it's potentially more intrusive (meaning you must turn on the lights manually when you first enter the room). It's been a tough decision, but since an automatic-on mode is available, we'll still support modes that require less intervention. Perhaps if you are ever irritated by a light that could turn on automatically, but doesn't, you can remember that you may be saving a little more energy (and money) that way.

Two, while I won't be there personally, if you're in New York next week, drop by our booth at the Lightfair tradeshow. Illumra products will be shown in one part of the EnOcean Alliance booth, as well as integrated into a number of products under various brand names throughout the show.