The better shape for heating depends on the specific requirements of the application. For example, in cooking, a concave shape might be preferable for concentrating heat on the food, while in space heating, a convex shape might be better for distributing heat evenly throughout a room. Additionally, the material properties of the heating surface and the method of heat transfer (e.g., conduction, convection, radiation) will also influence its effectiveness.
Many thanks for your help. I need also the reference that explain this point. The appliation is heating the water by solar in convex and concave shape.