In desalination applications where energy is costly, spiral-wound modules are currently in the best position for market implementation, while V-MEMD manages to demonstrate its full potential and hollow-fibre modules achieve effective energy recovery in pilot scale. In the case of V-MEMD, the challenge is increasing the number of effects without a negative impact on the electrical consumption. In the case of HF, upscaling the modules with internal heat recovery or finding viable means of external heat recovery are the challenges.

Potential for improvement in the case of spiral-wound modules lays on semi-batch operation, which has been shown to decrease energy consumption by 10%.51 Regarding the optimal configuration, at pilot scale AGMD modules can reach higher flux and efficiency than DCMD,52,53,54 which, furthermore, as in the case of VMD, need external means for heat recovery. The influence of membranes has not been considered in the discussion of heat efficiency in spiral-wound modules. However, the effect of membrane thickness has been demonstrated less important in pilot than on lab-scale.55

The beneficial effect of feed water de-aeration (elimination of dissolved air to reduce the presence of air in the membrane pores) has been demonstrated for spiral-wound modules working in AGMD and PGMD.56 A further step can be the use of vacuum-enhanced configurations, where a slight suction is applied in the condensation channel. This is not enough to induce evaporation by decreasing the pressure below that of liquid saturation, but typically just to evacuate the non-condensable gases from the pores for enhancing the vapour flow. This can increase the heat efficiency up to 30% according to simulations validated at lab-scale.9,54 Large scale demonstration is currently ongoing at Plataforma Solar de Almería (Spain) with Aquastill modules in vacuum-enhanced AGMD configuration with minimum electrical consumption. Given the experimental figures obtained with the module of 24 m2 MSA, the projection is that its operation in v-AGMD could reach a GOR 10, which would be outstanding for a module producing about 24 l/h.

The scarce implementation of MD technology hampers the performance of a proper comparative economic analysis. Moreover, there are no standards for the calculation of desalination water costs,57 which complicates the comparison even more. Existing studies are mostly based on simulations validated at lab-scale than in results from pilot plant experiments.9,50,54,58 In an evaluation of stand-alone solar powered MD systems, Saffarini et al.58 concluded that solar energy costs were 70% of the total costs of the systems, and Schwantes et al.54 also found similar percentages. Thus, the cost of solar thermal energy can set up boundaries for the economic viability of solar MD. It was found that GOR had to be 8 for solar thermal heat costs to be less than 3 $/m3 for a small size installation.59 Based on that work, Table 2 indicates an estimation of the costs of solar thermal energy for a MD system of GOR 8 sized to reach a total production of 10 m3/day operating during 8 h per day solely with solar heat and no backup. The levelised cost of water obtained with photovoltaic reverse osmosis (PV-RO) installations using batteries for continuous operation can be estimated with better reliability, since the technology is more developed than MD. Thus, considering the values of 5.4–5.8 $/m3 determined for an off-grid 250 m3/day plant,61 solar MD can only compete with PV-RO for seawater desalination in regions with high radiation and temperature (such as Abu Dhabi), if the GOR is at least 8 and if the investment costs of the MD plant (plus operational costs excluding heat) are less than 1–1.2 $/m3. For brine concentration, however, the operational limitations of RO open a clear niche for MD.