Most of us know that solid state lighting design requires effective thermal management. Thermal discussion was the #1 hold up in the early days of LED traffic signals. In fact, for the first 2 years after ITE and NEMA became involved with LED Traffic Signal Standards, they conducted studies of solar heating effects in various geographical locations. But that's history, and obviously LED traffic signals are a huge success.
Traffic signals must be their brightest at the hottest time of day. Since outdoor lighting with LEDs takes place at night, do we assume the Sun is not an issue? This would definitely be a bad assumption. Referring back to the traffic signal studies, it was demonstrated that non-energized traffic signal heads of any exterior color, exposed to the Sun, had internal tempratures exceeding 85°C. This fact must not be overlooked in the design of outdoor solid-state luminaires; after all, they will be in the Sun all day long.
Why? The LED luminaires will not be on during the day, so why be concerned with the solar heat?
It's true that LEDs degrade with excessive junction temperature while driven with current; the LED junction typically will not be degraded without electrical bias, even at storage temperatures.
However, white LEDs of the phosphor conversion type can be degraded when the phosphor is subjected to excessive heat, with or without electrical bias. That would be the case in luminaires that behave as solar collectors.
(Note that previous LED technologies by some manufacturers had epoxy encapsulant that was sensitive to the blue light or Ultra-Violet, and it yellowed over time. Obviously for outdoor applications, no element should yellow from UV exposure because of the required daily exposure to UV from the Sun.)
LEDs rated for 50,000 hours that are stated to have a 12-year service life may give a surprising reduction in lumen maintenance when subjected to 85°C on a daily basis for several years. A nighttime Luminaire is often characterized at 50% duty cycle, i.e. 50,000 hours is stated as a 12-year lifetime of lumen maintenance. However, if that Luminaire is powered at night and collecting solar heat by day, it may actually be a 6-year product.
Phosphor manufacturers have thermal degradation graphs for their product, which may show a permanent shift in CCT as well as conversion efficiency degradation. Some LED manufacturers have taken steps to distance the phosphor coating from the heat-producing junction.
There are SSL Luminaires with heat exchangers designed to address the solar issues, and some Luminaires really are efficient solar collectors. Can you spot the difference?
Thursday, September 4, 2008
Subscribe to:
Post Comments (Atom)
10 comments:
If you've ever put a piece of aluminum in the Sun, you know there is a lot to this. The following I found in a patent application on the art:
....Solar heating of space and liquids is a well known science today, and solar collectors constructed of known materials collect solar energy predictably to the known relationships of collector surface area and coating or finish, collector material, BTU rate, watts and thermal resistance.
The delivery rate of solar energy used in collector calculations is 320 BTU/hour per square foot area of flat surface solar collector perpendicular to the solar rays at sea level. One BTU per hour is equivalent to 0.29 watts of power...
Here's more patent application text with a few key words omitted:
...Measurement of the preferred embodiment heat exchanger in direct Sunlight with an ambient temperature of 43°C (109°F) found the LED Mounting Flange to be 52°C, a rise of 9°C that is considerably lower temperature than anticipated by calculation. Measurements of an equivalent perpendicular-square-inch prior art version without the ____ ____ ____ ____ found the LED mounting surface at 60°C in the same ambient of 43°C, thus a 17°C rise in temperature.
The thermal solution of the present invention is more effective in neutralizing solar heating with ambient air than first anticipated, and end results are illustrated by actual temperature measurement...
Definitely food for thought.
Okay, I'm up for the challenge.
I found these to be definite solar collectors:
GE Area Light also shown to left
LEDtronics Shoebox
A black heat exchanger will improve the thermal radiation by about 20% over a reflective or shiny metalic surface. That's for cooling.
That same black heat exchanger will absorb that much more solar energy in the daytime, i.e. will be hotter than a shiny metalic reflective surface.
Therefore, a black heatsink must get rid of 20% more heat when the Sun is up, i.e. needs 20% more area in the shade. This is not new science, solar heating is an industry all by itself.
How many LED Luminaire designers do you suppose realize the science of thermodynamics suggested here?
Ask them. Some of their answers will surprise you, some will be highly entertaining. Report their answers here so we all may benefit.
Here's a streetlight that sure appears to be a solar collector. What's the concensus?
Hey there interested-in-leds.....
That LEDtronics link seems to be bad.
Is this the one you had in mind
LEDtronics Shoebox ???
It does look pretty hot (in the Sun, that is).
I was told that phosphor will do funny stuff at about 93°C.
But this solar heating also means that a LED luminaire must never turn on during the solar peak or the LEDs will be degraded.
But what about LED traffic lights? They are out there burning all day long. Seems something wrong with that logic.
We measured the temperature rise of the IntenCity SL32 and found it to be 50°C during a 22°C ambient. That is hotter than any other fixture we tested.
Just FYI
Very Interesting Article!
I Buy Lights
Does anyone know of a good mid level Materials Scientist that has experience with phosphor process development that they can recommend for an opportunity with a growing award winning start-up LED Lighting manufacturer?
Post a Comment