Modeling-based investigation on the contribution of radial heating into integrated VMD hollow-fiber modules at different scales for autonomous desalination

Abstract

Integrating solar energy with vacuum membrane distillation (VMD) is a promising means to provide fresh water in remote areas. Our previous work on integrated solar VMD modules focused on optimizing flat-sheet integrated modules; this study examines the feasibility of up-scaling VMD modules containing outside/in hollow fibers with an external module surface collecting solar heat (HF-SC-VMD). Such modules have been experimented with at the lab scale; however, additional information is required concerning the parameters governing the productivity in such modules at the industrial scale, notably concerning the impacts and limits induced by radial heating. We contribute to this goal by estimating the temperature profiles and the maximal permeate production achievable for an HF-SC-VMD module at a semi-industrial scale in the case of the more favorable heating conditions provided via solar radiation, where the maximum temperature acceptable for the constituent materials of the module is assumed to be the temperature reached by the feed close to the shell wall (Twall). A two-dimensional shell-side model is developed to describe the coupled radial and longitudinal heat and mass transfers accounting for temperature polarization while considering module design parameters and operating conditions for future optimization. The model properly describes the experimental VMD performances (without radial heating) with and without temperature polarization. The influence of Twall on the permeate flux and temperature profiles is examined to discuss the effect of radial heating. The specified semi-industrial-scale HF-SC-VMD module (2.1 m2) could produce 37.8 L/h of drinkable water (covering the daily needs of ~150 people) if the full module design (including the solar collector) allows Twall to reach 70 °C, confirming the interest of continuing research toward optimizing a full HF-SC-VMD module.