What size of COB is ideal for cost per watt?

Ok, so I had an idea but I need some knowledge. Four of the new cxm 32 cobs (55$ each) would be about 220$ total and I could be run at like 160W each, toss in two 320W power supplies for a total of 527$ for 640W. 113$ to spare. If you can find a used car rad, cheapo cpu water blocks and an amazon special water pump, you've got a bare bones, water cooled cob setup for very close to 1$ per watt.

I'm just wondering though, will your efficiency drop dramatically if your cob is cool but still being driven hard? It seems like COBs are mediocre at transferring the heat through their thermal interface and into the heat sink. From what I read on the Vero 29 data sheet you'll lose lumen efficiency with increased amperage on a somewhat exponential scale, even at Tc of 25C and then your temperature going from 25C to 85C affects that by an additional 10% loss of lumens per watt. I was scheming and thinking that maybe you could cover the cob surface with epoxy and then put a water cooled acrylic lens on it, or something of that nature. They're supposed to work at very low temperatures, so I'm thinking of ways to get them cold on a budget and prevent condensation.

Hadez411 said:

Ok, so I had an idea but I need some knowledge. Four of the new cxm 32 cobs (55$ each) would be about 220$ total and I could be run at like 160W each, toss in two 320W power supplies for a total of 527$ for 640W. 113$ to spare. If you can find a used car rad, cheapo cpu water blocks and an amazon special water pump, you've got a bare bones, water cooled cob setup for very close to 1$ per watt.

I'm just wondering though, will your efficiency drop dramatically if your cob is cool but still being driven hard? It seems like COBs are mediocre at transferring the heat through their thermal interface and into the heat sink. From what I read on the Vero 29 data sheet you'll lose lumen efficiency with increased amperage on a somewhat exponential scale, even at Tc of 25C and then your temperature going from 25C to 85C affects that by an additional 10% loss of lumens per watt. I was scheming and thinking that maybe you could cover the cob surface with epoxy and then put a water cooled acrylic lens on it, or something of that nature. They're supposed to work at very low temperatures, so I'm thinking of ways to get them cold on a budget and prevent condensation.

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You'd lose so much output water tight sealing something in front of/on top of the LES.

I find it funny there's still so many people that come on here determined to water cool their cobs.
A fat heatsink and some PC fans strapped to it, is more than enough.

This is neat. He uses mineral oil and sticks a pc fan straight in it.

Airwalker16 said:

I find it funny there's still so many people that come on here determined to water cool their cobs.
A fat heatsink and some PC fans strapped to it, is more than enough.

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It is true that the cpu fans will keep it mid way between a Tc of 25C and Tc of 80C, so I assume the best you could achieve is a 5% gain in lumens over the fanned heatsinks if there's only a 10% difference from 25C-85C and you're already halfway. 117$ is enough for 4 big fan sinks too I guess.

"To the man with naught but a hammer, the whole world looks like a nail."

A company called liquid leds has a patent on cooling leds in water, they were doing this about 8 years ago.

Hadez411 said:

It seems like COBs are mediocre at transferring the heat through their thermal interface and into the heat sink.

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What makes you think that?

If you have a princess auto near by check the surplus section. They have a couple different fan cooled heat sinks. I think 5$ per. Not sure if the are enough but look up,to the job.
I tried a DIY heat pipe.1/2" copper 2' long with a copper flat square plate soldered at the bottom. With 10cc H2O as coolant, under vacuume. The water did move the heat up the pipe pretty much instant but the 2 foot pipe didn't have enough surface area to cool enough. A heater core with a fan may work.
Any reason you want to go cob rather than EB? iam running 18 EB in a 4x4 at the moment and I like the no cooling needed.

1212ham said:

What makes you think that?

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I was reading that their surface temp is about 20 degrees higher than the back.

Turpman said:

If you have a princess auto near by check the surplus section. They have a couple different fan cooled heat sinks. I think 5$ per. Not sure if the are enough but look up,to the job.
I tried a DIY heat pipe.1/2" copper 2' long with a copper flat square plate soldered at the bottom. With 10cc H2O as coolant, under vacuume. The water did move the heat up the pipe pretty much instant but the 2 foot pipe didn't have enough surface area to cool enough. A heater core with a fan may work.
Any reason you want to go cob rather than EB? iam running 18 EB in a 4x4 at the moment and I like the no cooling needed.

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The cobs are for in the greenhouse. It has high ceilings and in the spring we need supplemental lighting to get the garden plants started properly. EB strips need to be too close for spray watering the hundreds of plants and their footprint blocks too much incoming light from the sun. Also, sometimes I'd like to do different kinds of growing where a collimated light is better than a blanket of light for depth of penetration.

That is amazing, you did exactly what I want to do. The only difference is I'm going to make little aluminum fins and solder them on to the copper pipe for surface area. I did a rough calculation and most 60W passive heat sinks need about 150 sq. inches of surface area. So I'm thinking 3"x3" fins with 3/4" hole should be around 8 sq. inches, so we'll say about 16 fins, That's awesome that yours worked. Did you try making a shorter one? 2 feet is kinda huge. I was hoping to do 3/4" copper pipe and maybe 10-12" max. I had also thought of putting in a copper wye and having two heat pipe off-shoots to cut the height in half.

Is there a specific ratio of water to air that you need in the pipe? I can't find much info on doing a copper heat pipe from scratch and nothing about any limits to the height and width ratio.

I see yes blocking natural light is no good.
I did make a shorter one to begin with, not much surface area. You could probly go quite short if you had fins on it.
What I did was solder a 1/8 short copper pipe on to the bigger one to use as a vacuum port. Fill, apply vacuum and pinch it off with a side cutter on a couple places, remove vac and immediately solder. Can't help you with the amount of liquid needed but not much h2o expands 2000 times. You also may want to use something that doesn't freeze if it's outside. Acetone works as well. You may be able to source some used baseboard heat exchangers for your project they are usuallaly a pipe with fins. Would be good starting material.

To answer hight width ratio. I don't think it matters with an evacuated system. Hot gas pretty much instantly reaches everywhere.

Airwalker16 said:

I find it funny there's still so many people that come on here determined to water cool their cobs.
A fat heatsink and some PC fans strapped to it, is more than enough.

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With water cooling you can reuse the heat better. Lots of people have trouble keeping the temperature up in their grow room.

Turpman said:

To answer hight width ratio. I don't think it matters with an evacuated system. Hot gas pretty much instantly reaches everywhere.

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I have a vacuum pack sealer with an auxiliary line. Will this suffice? The other option is to boil the water and solder it together while still steaming.
I'm not sure if more vacuum is better or if even a little is enough to make water boil above room temp. I'm sure there's a formula for boiling temperature that includes atmospheric pressure, but alas I'm no physicist and don't have pressure measuring gauges etc.

wietefras said:

With water cooling you can reuse the heat better. Lots of people have trouble keeping the temperature up in their grow room.

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True, I should run the hot return lines under the planter pots first and keep the soil warm.

Hadez411 said:

I have a vacuum pack sealer with an auxiliary line. Will this suffice? The other option is to boil the water and solder it together while still steaming.
I'm not sure if more vacuum is better or if even a little is enough to make water boil above room temp. I'm sure there's a formula for boiling temperature that includes atmospheric pressure, but alas I'm no physicist and don't have pressure measuring gauges etc.

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Not sure what they get down to. I used an older AC evacuator. Looks like a fridge pump basically.
Just try it and see what the boiling point is once pulled down.