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They are not going to work as well as an active fan (or H2O, if appropriate). They operate in a more or less enclosed environment, and even if they have sufficient surface are to radiate significant amount of heat, where is that heat going to go? A fan pushes the air out of the case while drawing in new air, but you are going to have to depend on good ole thermodynamics to move that hot air with a passive cooler. It's quiet, I'll give you that, but passive coolers can even result in performance penalties due to CPU heat issues. Here is a review of a pretty good passive cooler, and it still has this weakness.