Hello Kuif
That is a nice sized closet I am sure you can make some magic happen in there. If cost is no object you can run them soft and significantly reduce the heat that needs to be removed, making the grow ventilation very quiet. You could panasonic bath fans that are near silent. If you can scare up some 3000K CXA3070 ABs and run them at 700mA, they are 49% efficient minimum and dissipate 24.5W ea. So that is only 12.25W of heat per COB, relatively easy to handle, especially with active cooling. Cost is about $4/PAR W.
Regarding the copper plate, although it would spread the heat away from the COB very quickly, it does introduce another thermal interface between the copper and aluminum and it is very expensive in terms of man hours to prepare those surfaces correctly, unless it is machined with precision like high performance CPU heatsinks. So I think the best approach is to use a heatsink with a reasonably thick base plate of pure aluminum, all the way to the fins and to make sure you have sufficient surface area to dissipate the heat into the air (30-40cm²/W for active cooling)
That said, if you run the CXA3070 at 700mA and use active cooling, there will be virtually no gains to be had by overbuilding the cooling system beyond the levels we normally recommend. The reason is because modern whites and blue stand up very well to heat (temp droop) and tend to suffer mostly from current droop instead. So it would be most effective to spend your resources running them softer rather than overbuilding the cooling system. For example, when I tested a CXA3070 at 2.1A on a basic solid aluminum 100W CPU cooler with the fan running at 12V, the decrease in light output as it warmed up was only 2%. Based on that test, I expect there to be virtually no decrease in output from ambient to thermally to warm, when running at 700mA and actively cooling. That is awesome for us!
Everything I said above does not apply to the Cree XPE and XPE2 reds, they really suffer from temp droop and not so much from current droop. However, our standard heatsink recommendations are effective at keeping them cool, especially if the stars are flattened.
So if you want to push the envelope to new territory of efficiency, you could simply run even softer than 700mA and use twice as many COBs. At 300mA the 3000K CXA3070 AB bin is 54.9% efficient minimum (Tj 50C), but Tj would be much lower than 50C in this case, so you could reach 56.5% efficiency minimum, 58% typical. That is about 190lm/W of warm white! The downside is the cost, about $9/PAR W, 4X-5X more than some DIYers are paying for 40% efficient lamps. You could push it even further by running them at 270mA. Driver cost would be negligible, plenty of cheap 270, 300 and 350mA drivers available.
