One of the important objectives of thermal systems engineering is to analyze the utilization of thermal energy in an efficient manner. Analysis on entropy generation may give an insight about thermal efficiency of various thermal systems. Entropy generation analysis during natural convection in a porous trapezoidal structures of various inclination angles (phi) with isothermal (case 1) and non-isothermal (case 2) hot bottom wall has been performed. Simulations have been done to obtain solutions in terms of isotherms (0), streamfunction (psi) and entropy generation maps (S-0 and S-psi) for various ranges of modified Darcy number (Da), Prandtl number (Pr) at modified Rayleigh number, Ra 10(6). It is observed that S-0,S-max occurs near bottom corners for case 1 whereas that occurs near middle part of the left and right portions of the bottom wall for case 2. The entropy production due to fluid flow irreversibilities (S-psi,S-max) occurs either at the top wall, or side walls or along the interface between two counter-rotating cells. It is found that, square cavities may be the optimal geometry in comparison to trapezoidal cavities for both cases. Also, trapezoidal cavities with phi = 60 degrees may be the alternative optimal design in comparison with phi = 45 degrees for all Pr fluids. (C) 2013 Elsevier Ltd. All rights reserved.