Full-scale validated Air Gap Membrane Distillation (AGMD) model without calibration parameters

Air gap membrane distillation (AGMD) is one of the most widely discussed membrane distillation configurations at the moment and has been regarded as more thermally efficient than direct contact membrane distillation (DCMD), due to the insulation properties of the air gap. Several AGMD models are available in the literature. However, most of the models developed to date are either missing validation or are only validated at lab-scale. A major hurdle in modelling membrane distillation is the lack of information about the condensation that is occurring inside the gap. Often, major parameters such as the average condensate thickness are taken from semi-empirical formulas or are simply estimated based on educated guesses. Moreover, some studies had shown that at certain conditions the gap can be completely flooded with condensate, which raises the question whether the module can be modelled as air gap altogether. In this study, a previously developed and thoroughly validated DCMD model is extended by adding the air gap compartment. In this way the model only needs to be adjusted for the gap-related parameters. A simple technique is demonstrated for observing the condensation in real time, which also allows to experimentally obtain the value of the average condensate thickness parameter and the flooding of the gap. The model is subsequently thoroughly and simultaneously validated with experimental data from two commercially available modules with areas of 7.2 and 24 m(2), showing an excellent fit to the experimental data. Moreover, this work shows a direct comparison between AGMD and DCMD in terms of flux and thermal efficiency at full-scale using modules with identical geometries from the same manufacturer