Cooling an LED Array

You've a fan-cooled heatsink already. Is there thermal grease between the module and and the heatsink? If not, that will help greatly. It is doubtful you will need a Peltier cooler for your module in addition to the heatsink.

Do you have specs on your module's recommended junction temperature, package thermal resistance, and so forth? Any specs on your heat sink?

Also read this: http://www.mechatronix-asia.com/LED_heat_sink_calculation_simulation_thermal_design.html

Interesting link...It seems the longevity of the light increases at lower operating temperatures, and the heatsink becomes less critical with the thermacool chip....So I don't have all the specs, I'm just going to ballpark the 85 watts as the heatload....I'm pursuing the unknown! haha

Solar Eagle, I just installed LED lighting in our entire plant. They were locally made. Most of them are low wattage but we 58 330 watt high bays and I have 2 100 watt spot lights. They have nothing more than passive aluminum heat sinks, which work very well. The 100 watt lights can be on all day and the heat sink is barely warm to the touch.

The spot lights are at ground level so today or tomorrow, I'll take the side off of one to see if I can get a picture of how the LED board is attached to the sink.

You will not need thermoelectric cooling.

Here are the pics of my 100 watt LED.

The LED's are mounted to aluminum block that has been milled down to about 1/4".

This pic is harder to see, but the block with the LED's is mounted to the aluminum 1/4" thick case and the 10" x 10" heat sink is mounted directly to the back of the case. Controls and power are below where the LEDS are.

That's a lot of aluminum, no wonder why it runs cool. The 330 watt high bays are similar in construction except much longer and the heat sinks fins are a lot smaller. Probably because those are not enclosed by a piece of lexan like the spot lights are.

I think if you could mount your LED's to an aluminum block say about 4" x 8" 1/4" thick, added 2 older, larger cpu heat sinks, you probably wouldn't need the fans.

I have been led () to believe that LED's use less power per unit of useful light. Does your 100W LED array give more/better light than a 100W halogen lamp? Plus if you need to add cooling at approx 70w then we are looking at comparing to a 150W halogen. I know that there are difficulties measuring the light output of LED luminaires. Comparing illuminance with a Lux meter is said to be not useful. Led output is measured in Lumens because the spread is nowhere like that of an incandescent or fluro. I find that if i simply turn both the LED and the other type of lamp on at the same time and look at the wall they are adjacent to, it is simple to see which outshines which.

I bought a LED strip 5m long for a passage and found it was darker than the single 20W CFL downlight currently fitted. The upshot being that it was more energy efficient to fit 2 CFL's.

I made this 384 LED luminaire and find it underwhelming. 1 x26w fluro is brighter. This is a pic taken with flash.

Jim

Oh it's bright, basically like sunrise when you turn it on....There should be some formula for determining lumens...a lot I think has to do with color, these are 6000k+, and wattage.....that's a great looking room you have....This light is for more utilitarian purposes than mood or decorative lighting, but I will make some effort to make it look like a finished product...still have no idea how it will look in operation other than a lot of the light will be reflected off the ceiling....considering a 3" diameter 8" height white frosted glass cylinder for shade....

I like the shade idea. As a suggestion you could use acrylic and machine part of the inside surface ( like a thread ) to give diffused light emanating from the shade itself. In my luminaire i drilled holes to take the LEDs leaving the rough surface for that same reason. At the time i bought the LEDs they were the brightest and whitest. It sounds like they can make them brighter and whiter now.

Jim

For any LED, the wattage (heat load) to be dissipated can be approximated as equal to the output wattage of the LED. In your case, you are required to dissipate 100W heat load.

When you talk of array, the LEDs used are 'surface mounted diodes' (SMD) and are generally spread over a large surface of aluminium which itself acts as the heat sink with fins at the back. Depending upon the number of LEDs being used and the power of each, you can place them at such a distance apart such that the temp rise at the junction is within the allowable limits.

Thermal management becomes more complex when 'Chip-on-board' (COB) LEDs are used as COBs have concentrated heat flux. In such case, you may have to use heat pipe based thermal system to quickly move away the heat energy from the LED.

In any case, the desired temp rise, measured at LED cathode should be limited to about 30 to 35°C over and above the ambient for constant lumens output from LED and for it's long life.

What I have mentioned are all natural convection systems and avoids unnecessary use of a fan which is susceptible to failures.

A 150W Heat pipe based High-bay solution is shown below.

All the best.

Yes sorry it's a COB type LED....and the light will be facing up, with heatsink on bottom, and contained in a decorative housing...

..the light will be facing up, with heatsink on bottom...

There's an issue here and you are aware of it. The heat sink being at bottom, the thermal rejection will be back on to the LED as hot air will travel upwards. This, I think, has led you to think of Peltier. Well ! as others mentioned, it is true that they are highly inefficient. Moreover, you have to also dissipate heat from the peltier.

I am not sure the coolers suggested would work either because of orientation.

You may have to find ways to quickly spread this heat to other parts of the fitting where it could be rejected.

I'll get back in case something lights up inside my head.

I've decided to go with 154 watt thermocooler chip....and the heatsink has a fan....facing down....so that should be ok

The 100 watt LED is a single chip with 100 LED's on it....I am estimating ~15% efficiency which would leave 85 watts of heat, however the exact reading will have to be taken in actual operation....I already have a heatsink that will work but I want to add a thermalcooler chip, so the LED would be atop the thermochip and then mounted on the heatsink...but the thermalcooler chip is only 58-65 watts cooling, so I'M going to find an 85 watt one....

Then I just need to figure out how to ramp up the thermal chip with the light so the cooling power will match the heat produced.....

Bear in mind that Peltiers are prey to overheating and are 5% efficient. 1 Watt moved = 21 watts into the heat sink. So a Peltier to pump 85 watts will consume about 1700 watts of 12 volt power, leaving your final heat to the air at almost 1780 watts.

I suspect this may overheat the Peltier module = melted and ruined. Start at 5 watts and let it reach steady state and go up. The online cheap peltier guys lie about their capabilities IMHO.

Peltier efficiency search

I believe your figures are considerably off, but I will be testing these components for accuracy and performance....rest assured

http://www.marlow.com/resources/general-faq/14-how-efficient-is-a-thermoelectric-cooler.html

Yes, I saw that, but it seems to me that they are overly optimistic = sales oriented...

Assume about 80-85% of the electricity ends up as heat. Usually these high power LEDs come attached to a heat transfer sink that you need to couple to either a larger quiet passive radiator or a fan powered active dissipator - which will need power for the fan. Most computer CPU heat sinks run from the 3.3, 5 or 12 volts supply - read the label and add the needed power supply. You can also make a convective tube radiator.

A peltier active cooler is a heat mover that is about 20% efficient and is a power hog, not reccommended

Google the topic

You are absolutely right that about 85% of your 100 watt LED will be heat. But a heat sink and fan from a CPU with a 100 watt TDP will be more than enough to keep it cool. I really don't think you need a thermoelectric cooler. IMHO.

NO I probably don't need it, but it makes the project more interesting, and I believe the LED will last longer....I realize that thermochips are not very efficient, but this light will not be run on high much, so it's not really a factor, the energy use overall is small...

Again you are right, I have 2 thermoelectric coolers with heat sinks that I scavenged from a 4 cubic foot refrigerator, with fans on both sides, that I am looking for a use for.

The cooler you keep your LED array, the longer it will last, My LED supplier has put a 100,000 hour life on my 330 watt high bays, with passive cooling, longer than I plan on working there.