The rectangular panel is within the most common geometries in phase change materials (PCM). Nevertheless, there is a lack of knowledge regarding how the arrangement (vertical or horizontal) and thickness affect heat flux and phase change duration. Such knowledge would be very helpful both in the design of PCM heat exchangers. This paper studies the behavior of the RT60 paraffin, Pr approximate to 330, phase change temperature between 53 and 61 degrees C; and the fatty acid Palmitic Acid, Pr approximate to 110, phase change temperature of 65 degrees C. Parametric studies analyzing the influence of: temperature of the walls (5 x 10(5) < Ra <= 2 x 10(6)), phase change processes (melting and solidification), panel position and PCM thickness (aspect ratio1/20 and 3/20) are performed. The flow behavior is analyzed over time using velocity plots and volumetric liquid fraction contours. The formation of Benard cells and their evolution is described. Free convection dependence of dimensionless string function and Rayleigh number is discussed. Free convection during melting for horizontal panels becomes very important and the mean heat fluxes increase up to twofold compared to vertical panels. For solidification, however, conduction becomes more relevant. The importance of both mechanisms is highlighted by calculating heat transfer rates. (C) 2016 Elsevier Ltd. All rights reserved.