The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. Number of geometries of roughness elements has been investigated on the heat transfer and friction characteristics of solar air heater ducts. In this paper an attempt has been made to review on element geometries used as artificial roughness in solar air heaters in order to improve the heat transfer capability of solar air heater ducts. The correlations developed for heat transfer and friction factor in roughened ducts of solar air heaters by various investigators have been reviewed and presented. (C) 2007 Elsevier Ltd. All rights reserved.