Abstract
Distillation across an air gap is implemented in several novel small-scale desalination technologies such as air gap distillation and multi-effect solar stills. These systems achieve better energy efficiency than simple solar stills by recovering the enthalpy of condensation towards feed preheating or further evaporation in subsequent effects. The air gap thickness between the evaporating and condensing fluids is a critical design variable that strongly affects overall performance. A thinner gap results in lower resistance to vapor diffusion and therefore helps attain a higher rate of pure water production and lower specific energy consumption, for the same hot and cold stream temperatures. In this manuscript, we experimentally evaluate methods to achieve a gap size lower than 5~mm, which is the thinnest gap size reported in the literature. While a superhydrophobic surface modification is usually applied to enhance condensation heat transfer (through dropwise condensation), here, a superhydrophillic surface modification is proposed. This ensures film condensation and prevents the growth of large condensate drops that can bridge the thin gap and come in contact with the saline feed, thereby getting contaminated with salt. Attaching a thin mesh to the condensation surface helps realize the same objective at lower cost and complexity. A 3~mm air gap is experimentally demonstrated using these modifications, resulting in a gained output ratio of close to 1 even with a small (304~cm2) evaporation surface area.