Exploring new methodologies to meet the growing demands for water and energy sources is an important step towards a sustainable future. HDH is a promising technology applied for both water desalination and industrial water treatment However, due to the lower Gain Output Ratio (GOR) of HDH systems, its wide-ranging applications are restricted. This issue can be resolved solely by increasing the GOR of the HDH systems. The main principle of HDH systems considered in this study is the distillation of water under atmospheric condition by an air loop saturated with water vapor and driven by natural convection thus eliminating the need for pumping power. In this work, a numerical investigation is carried out for predicting the performance of a simple HDH cycle taking place in a rectangular enclosure (cavity) of different aspect ratios. The cavity has two horizontal adiabatic walls and two vertical isothermal walls; one of them is heated while the other one is cooled. The vertical hot wallis kept wet by maintaining a thin layer of water flowing downstream and the cavity is assumed long enough to justify the assumption of two-dimensional flow. A computational model is developed for predicting the velocity, temperature and concentration fields within the cavity as well as calculating the rate of water evaporating from the cavity hot side and condensing on its cold side. The model is based on numerical solution of the conservation equations of mass, momentum, energy and species considering the Boussinesq approximation. The model has been validated and the effect of aspect ratio on the performance of the HDH system is investigated. The results show that the aspect ratio of 1.5 produces the maximum heat and mass transfers. In addition, high convection rates tend to maximize the Nusselt number. The values of Nusselt based on latent heat exceed the Nusselt based on sensible heating. (C) 2016 Elsevier B.V. All rights reserved.