Imagine a half dome, sitting a few feet above a campfire. The dome captures smoke, and combusts it with a fresh burst of oxygen, then radiates the heat to those nearby.
Campfires are already fairly efficient, but they still produce a large amount of smoke – wasted heat, unburnt fuel. Collecting it to a single point, heated by the fire below, and adding oxygen to complete the combustion seems like it could be a nice idea.