Quote Originally Posted by sideshowmel
I agree with the passive cooling. I've tried both, and the fan based one made me worry constantly that it would fail. Must be worse in the mass manufactured ones where you can't get at it to clean the dust off so easily.

I don't necessarily agree with your point about emitter spacing. A sufficiently thick copper baseplate on the heatsink will dissipate kilowatts in an area the size of your palm as long as there are enough fins. Aluminium probably isn't conductive enough to do that, but I've had 50w of chips in a square inch without perceptible dimming (side by side with unused module from the same batch) after two seasons.
We are definitely not talking about fans here, maybe in other designs however Nuventix coolers can take on water and continue running 100,000 hours, passive cooling would mean there would be simply radiating and sitting heat in one area, over long run times (theoretically speaking) it wouldn't be a good idea.
If we were to use passive cooling we would be talking about Copper Foams however to make that viable, the grow lights would cost somewhere in the $2500-3000 range. Fins alone would not satisfy our internal standard of < 50C peak temperatures at the thermal slug.