Apparatus comprises: a panel (100) having first and second main faces (101, 102); and a sealed system internal within the panel and comprising plural passages (103) each extending from a first manifold cavity (107) at a first end of the panel to a second manifold cavity (107) at a second end of the panel and containing a fluid in both gas and liquid states, wherein each of the passages includes one or more protruding features (122, 123, 124) on a side of the passages that is closer to the first main face.