Thermal Conductivity of Polyvinylidene Fluoride Membranes for Direct Contact Membrane Distillation

Direct contact membrane distillation (DCMD) is a promising desalination technology for locations where low-cost energy (e.g., waste heat or geothermal energy) is readily available. Modeling of the DCMD process facilitates the optimization of membrane properties and module design for system performance. Since DCMD is a purely thermal process, membrane thermal conductivity, which greatly influences the conductive heat loss through membranes, plays an important role for DCMD modeling. Most researchers adopted the isostrain model when estimating the thermal conductivity of polyvinylidene fluoride (PVDF)-based membranes during modeling and very few actual measurements have been reported in literature to confirm the validity of these estimations. In this study, the transient plane source (TPS) method was used to measure the thermal conductivities of PVDF-based DCMD membranes and the observed values were compared to commonly used models, such as isostress and isostrain. Measurements showed the membrane thermal conductivity decreased with the increase of porosity at 23.5 degrees C. Contrary to the practices reported in literature, the isostrain model showed a discrepancy of more than 200% from the TPS measurements whereas the isostress model provided the closest predictions. Overestimation of 49-80% from the isostress model was attributed to the Knudsen effect. Once the effect was incorporated, the Knudsen-corrected isostress model predicted the membrane thermal conductivity at 23.5 degrees C accurately. Increasing temperature led to the departure between the Knudsen-corrected isostress model and the TPS measurements. Contribution of radiation conduction, which increases with temperature cubed and is not included in the isostress model, might have resulted in the deviation.