That's what Apple used to do with their Macbooks. Heat pipes into the metal frame which conducts it over a larger surface area creating natural convections whic

That's what Apple used to do with their Macbooks. Heat pipes into the metal frame which conducts it over a larger surface area creating natural convections which carry the heat into the air. The real downside is that mass plays a substantial role. Meaning thicker metal, larger cases, are better than thinner/smaller. Whereas a fan can accomplish similar results for less cost and space (but not passively). If people don't think passive can work they need to check out Apple's recent Mac Pro that passively cools two Radeon Pro 580Xs. There's no limit, except cost. Personally I still feel a combination of heat pipe, into heat sink, with a small progressive fan is the ultimate. If it is large enough the fan should stay off except under heavy load.