Energy efficiency of membrane distillation up to high salinity: Evaluating critical system size and optimal membrane thickness

Abstract

This study presents a comprehensive analytical framework to design efficient single-stage membrane distillation (MD) systems for the desalination of feed streams up to high salinity. MD performance is quantified in terms of energy efficiency (represented as a gained output ratio, or GOR) and vapor flux, both of which together affect the specific cost of pure water production. Irrespective of the feed salinity, permeate or conductive gap MD (P/CGMD) performs better than direct contact MD (DCMD) when the heat transfer resistance of the gap (in P/CGMD) is lower than that of the external heat exchanger in DCMD. Air gap MD's (AGMD) better performance relative to the other configurations at high salinity and large system area can be explained in terms of its thicker 'effective membrane', which includes the air-gap region. CGMD and DCMD employing a thick membrane are also resilient to high salinity, similar to AGMD, while not being susceptible to the gap flooding that can harm AGMD's performance. A method is described to simultaneously determine the cost-optimal membrane thickness and system size as a function of the ratio of specific costs of heat energy and module area. At low salinity and small system size, GOR rises and flux declines with an increase in membrane area. For salty feed solutions, there exists a critical system size beyond which GOR also begins to decline. Since both GOR and flux are lower, no economic rationale favors operation above this critical size, irrespective of the costs of thermal energy and system area. A closed-form analytical expression for this critical system area is derived as a function of the feed salinity and two dimensionless ratios of heat transfer resistances within the MD module.Keywords: membrane distillation, high salinity, energy efficiency, system size, optimal membrane thickness * Corresponding author: lienhard@mit.

The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies

“…For example, permeate-gap MD (PGMD) and conductive-gap MD (CGMD), were recently proposed as novel process designs, which, like AGMD, incorporate heat recovery along the membrane module, but with higher energy efficiency. 131,132 We expect that further system design and optimization can guide the development of PGMD and CGMD based STD systems with effective heat recovery.…”

Comprehensive condensation flow regimes in air gap membrane distillation: Visualization and energy efficiency

“…This comparison of flux versus GOR for PGMD and AGMD is shown in Figure 6 and Figure 7, where these models are described in other work [23]. The performance of MD can be compared in GOR (energy efficiency) and permeate flux axes, which are inversely linked to the energy costs and membrane area costs respectively [23]. As seen in Figure 6, at high salinity, AGMD significantly outperforms PGMD.…”

“…The overall efficiency is best indicated by GOR [67], while the capital expenses it competes with can be described by flux, which is linked to membrane area and thus system size. This comparison of flux versus GOR for PGMD and AGMD is shown in Figure 6 and Figure 7, where these models are described in other work [23]. The performance of MD can be compared in GOR (energy efficiency) and permeate flux axes, which are inversely linked to the energy costs and membrane area costs respectively [23].…”

“…MD designs resemble counter-current heat exchangers with a liquid-repelling membrane and channel on the hot fluid side [10][11][12], where heat transfer across the membrane ideally occurs mostly by evaporation and condensation of pure water [13][14][15][16][17]. Of the MD configurations developed, air gap MD (AGMD) is one of the most common and has been shown to have the greatest potential for high thermal efficiency under more saline conditions [6,[18][19][20][21][22][23][24][25].…”

Entropy Generation Minimization for Energy-Efficient Desalination

“…Membrane distillation is in many ways similar to HDH desalination, in that water is vaporized from a warm saline stream and condensed in a counterflow, heat recuperation arrangement [63][64][65]. However, in MD systems, the saline stream is separated from the fresh cooler stream by a hydrophobic, porous membrane through which vapor alone can pass.…”