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The feasibility of pressure-retarded osmosis (PRO) for power generation was evaluated with consideration of the energy inputs and losses in the process. The effects of the concentration polarization, reverse salt diffusion, and external resistance at the membrane porous layer were quantified, for the first time, along the membrane module to determine their contributions to the energy loss in the PRO process. Concentration polarization was responsible for up to 40% of the energy loss during the PRO process. However, increasing the PRO membrane modules from 1 to 4 resulted in a variable increase of the energy output depending on the salinity gradient. The energy requirements for draw and feed solution pretreatment were estimated to be over 38% of the total energy inputs. Results showed that coupling seawater (SW) with river water (RW) was unable to generate sufficient energy to compensate for the energy inputs and losses during the PRO process. With 0.39 kwh/m 3 maximum specific energy in the PRO process, the energy yield of reverse osmosis brine (ROB)-wastewater (WW) salinity gradient was slightly greater than the total energy inputs, although using Dead Sea-SW/ROB salinity gradient was more promising. Overall, the primary current limitation is the lack of suitable PRO membranes that can withstand a high hydraulic pressure.

© 2017 Elsevier Ltd This work proposes an analysis of conventional (single stage) and dual stage Closed-Loop Pressure Retarded Osmosis (CLPRO) for power generation from a salinity gradient resource. Model calculations were performed taking into account the influence of operating parameters such as the draw solution concentration, membrane area, and draw solution pressure on the performance of the CLPRO process. Modeling results showed that the dual stage CLPRO process outperformed the conventional CLPRO process and power generation increased 18% by adding a second stage of PRO membrane. Multi-Effect Distillation (MED) was selected for the regeneration of the draw solution taking advantage of an available source of waste heat energy. The performance of MED process has been assessed by investigating two key parameters: the specific thermal consumption and the specific heat transfer area. The model calculations showed that the power generation by the single and dual stage CLPRO was higher than the electrical power consumption by the MED plant. In the case of the power generation obtained by the dual stage CLPRO, it was 95% higher than the electrical power consumption by the MED plant, proving the possibility of using low-grade heat for producing electricity from a salinity gradient resource.

The evaluation of a novel solar seawater desalination system implemented at the University of Almeria (Spain) is presented. It integrates a solar thermal field based on static collectors and a thermal desalination system based on the vacuum multi-effect membrane distillation technology. The distillation unit has a particular innovation to increase its thermal performance, using a seawater flow to condense the steam and preheat the feed. Experiments were made under different environmental conditions to assess the role of the thermal storage system for minimizing the effect of disturbances in solar radiation. Thermal energy could be delivered at a stable temperature to the distillation module, even with variable solar radiation. A simulation analysis based on a quasi-dynamic model was also performed to evaluate the distillate production profile and the operating time during a typical year considering different temperature setpoints at the inlet of the membrane module (60, 70, and 80 °C). The simulated volume of distilled water generated annually ranged from 41.7 to 70.5 m3, depending on the setpoint. The membrane distillation unit produced water almost uniformly along the year, with an average flux of (5.5 ± 1) l h−1 m−2 at the maximum setpoint, which was proved the most favourable.

The demand of freshwater has surpassed the renewable limit and new water sources are associated with an intensive use of energy. Coincidence between scarcity of water and availability of solar radiation makes solar energy the most suitable option to mitigate the water deficit. This paper analyzes the use of energy for decentralized water production using membrane desalination systems fed with solar energy. An analysis is performed based on experimental results from the most advanced commercial prototypes of different technologies of membrane distillation using various configurations, i.e., air-gap, permeate-gap and vacuum; with flat-plate and spiral-wound membranes. The systems operate with thermal energy, although there is some electrical consumption for pumping and in some cases for sustaining vacuum. The thermal energy requirements per unit volume of water produced are assessed in each case, considering the effect of different operational conditions like the temperature regime and the salinity of the input water.