Hi mate,
as always: it depends.
Here is my opinion:
1. First thing is the size of your radiator. If the piece will be of the size of 2 midsized cpu radiators (so lets say 750g of aluminium with some copper core) then it is absolutely no problem. I think I would even try with a 400-500g piece (or even slightly lower but I don`t want be billed for your COB LED). While operating the radiator should be warm, even slightly hot will do, but your led life will be considerably shortened.
2. Of course the design (optimized for passive convection) and material of the radiator matters - you of course prefer copper (silver, grafen and diamond coating are welcome too
)
3. Placing of the radiator (unblocked convection and fresh air)
4. THE LED. 30w quality led? Or a non labeled cheapo? Best of the best could achieve lets say 16w light output and 14w heat. Worst will do 8w light and 22w heat (numbers out of my head just for general reference).
5. Led driving: of course the current settings will change the values from point 4. If you drive on very low current then the efficiency will rise, but you will have to use more leds to achieve the same light output.
6. Even led mounting is important. Radiators efficiency rises for higher temps (delta between radiator temp and ambient temp).
Screwed led would transfer heat to the radiator more effectively, allowing to cool down the precious led core better and transfering the heat to the radiator (making the radiator delta temp bigger). Glued or screwed without thermal paste will let cook the led in its sausage, leaving it hot inside and not allowing to transfer the heat to the radiator.
I hope I could help.
