An experimental investigation has been carried out to study the heat transfer coefficient by using 90 degrees broken transverse ribs on absorber plate of a solar air heater; the roughened wall being heated while the remaining three walls are insulated. The roughened wall has roughness with pitch (P), ranging from 10-30 mm, height of the rib of 1.5 mm and duct aspect ratio of 8. The air flow rate corresponds to Reynolds number between 3000-12,000. The heat transfer results have been compared with those for smooth ducts under similar flow and thermal boundary condition to, determine the thermal efficiency of solar air heater. (c) 2004 Elsevier Ltd. All rights reserved.