Search

The self-cooling greenhouse is a concept to enable crop cultivation in adversely hot climates. It sacrifices a fraction γ of the incident solar energy to drive a refrigeration system, thus lowering the internal temperature below ambient. Heat is actively rejected to a stream of coolant such as air or water. To maintain availability of sunlight for photosynthesis, γ should be as small as possible. Nonetheless, the laws of thermodynamics dictate a minimum value of γ. Using the approach of endoreversible thermodynamics and the theory of selective blackbody absorbers, we determine ideal minimum values achievable for cases of both thermal and photovoltaic solar collection with and without solar concentration. To achieve an internal temperature 10 °C below that of the incoming coolant, a minimum γ = 0.056 is needed using multicolour absorption at maximum concentration C = 46300 – representing an absolute minimum for either type of solar collection. Without concentration (C = 1) a selective thermal collector permits minimum γ = 0.089 and a single-junction PV solar collector permits minimum γ = 0.15. We discuss briefly implications for development of a real self-cooling greenhouse to approximate the performance of these ideal cases.

Desalting brines from Reverse Osmosis (RO) plants is one of the most promising applications of Membrane Distillation (MD) systems. The development of accurate models to predict MD system performances plays a significant role in the design of this kind of industrial applications. In this paper, a commercial-scale Permeate-Gap Membrane Distillation (PGMD) module was modelled by means of two different approaches: Response Surface Methodology (RSM) and Artificial Neural Networks (ANN). Condenser inlet temperature, evaporator inlet temperature, feed flow rate and feed water salt concentration were selected as inputs of the model, while permeate flux and Specific Thermal Energy Consumption (STEC) were chosen as responses. The prediction abilities of both RSM and ANN models were compared with further experimental data by using the Analysis of Variance (ANOVA) and the Root Mean Squared Error (RMSE). The results show that the ANN model is able to predict in a more precise way the behaviour of the module for the whole range of input variables. Thus, ANN model was used to find the optimal operating conditions, for the module operating at feed water salinity of 70 and 105 g/L, concentrations that can be reached when desalting RO brines.

An experimental characterization of the performance of two multi-channel spiral-wound air-gap membrane distillation modules at commercial-scale was carried out for seawater desalination. The only difference between them was the membrane surface areas: 7.2 m2 and 24 m2 and thus the length of the channels (1.5 and 5 m respectively). The influence of the main operating parameters (evaporator inlet temperature, condenser inlet temperature and feed flow rate) was quantified. It was observed that the trade-off between productivity and energy efficiency was mostly related to the heat recovery inside the module. This was affected by the operation and also by the design of the modules: the longer the channel, the better the thermal efficiency but the worse the productivity. Empirical models were derived for each module. Optimum values of the three operating parameters to simultaneously maximize permeate flux and minimize specific heat consumption were obtained using surface response methodology (RSM). In the longest module it was possible to find operating conditions that maximize the productivity without losing thermal efficiency, but in the shortest module a compromise had to be found between both.

This work presents the evaluation of an innovative system based on vacuum multi-effect membrane distillation modules (V-MEMD) for seawater desalination at pilot scale. This four-effect unit introduces a remarkable modification from previous V-MEMD systems, consisting of the use of the seawater feed flow as cooling in the condenser, rather than a separate circuit. Preheating the feed in the condenser improved heat efficiency (maximum gained output ratio obtained for seawater was 3.2). Maximum distillate fluxes reached 8.5 l h−1 m−2 for hot feed temperature 75 °C and feed flow rate 150 l h−1. Increasing both parameters to raise the productivity was hindered by the inability of the condenser to cope with all the steam generated in previous effects, resulting in overheating and overpressure. Furthermore, a loss of 40% of distillate production was measured due to the increase of seawater cooling temperature by 8 °C along the year. Finally, it was observed that scaling reduced distillate production up to 50%. Acid cleaning successfully removed scaling and restored the performance. Subsequently, the use of an antiscalant as a pre-treatment was sufficient to prevent it.

Membrane distillation (MD) is an emerging thermal-based membrane technology that has the capacity to treat a wide range of feed waters, including high salinity brines and wastewater. Compared with pressure-driven processes, MD has the following advantages: low electrical energy consumption, complete removal of non-volatile solutes and ambient operating pressures. On the other hand, membrane fouling and pore wetting remain a challenge holding back its wider industrial implementation. To manage this issue, effective pretreatment and cleaning strategies are essential, as with other membrane processes. Despite the importance of these aspects for achieving a stable and reliable water treatment process, far too little attention has been paid to them in the MD literature. Evidence of this is that a comprehensive review on both membrane cleaning and pretreatment in MD is still lacking, despite the large and growing number of publications related to the technology. In this work, we first provide an overview of the three main types of fouling present in MD: inorganic, organic and biological fouling. After this we review, in detail, the use of pretreatment protocols in a range of MD applications, highlighting their efficacy in preventing fouling and ensuring stable performance. Next, we review the use of state-of-the-art chemical and physical cleaning strategies and discuss the development of ‘self-cleaning’ membranes that are superhydrophobic or omniphobic. Finally, future research directions are suggested, with the aim of helping researchers working in this field to develop membrane distillation technology towards wider commercial implementation.

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.

In this paper, a commercial spiral wound PGMD module was modeled and optimized for seawater desalination using Response Surface Methodology (RSM). Permeate flux (P flux ) and specific thermal energy consumption (STEC) were the main performance parameters to optimize, while evaporator inlet temperature (T evap ), condenser inlet temperature (T cond ) and feed flow rate (F) were the three operating parameters chosen. Analysis of variance (ANOVA) was used to evaluate statistically the response surface models. According to the study, T evap had the strongest effect on P flux and STEC, increasing the former and decreasing the latter, F increased both responses, and T cond had a weak effect on P flux and practically none on STEC. The models were validated with further experimental data and a good correlation between experimental and predicted values of the responses was obtained for P flux and STEC respectively. An optimization was performed to determine the operating conditions that produce a maximum value of P flux and a minimum value of STEC simultaneously. The result of the multiple responses optimization using desirability function was a maximum P flux of 2.66 l/h·m 2 and a minimum STEC of 255.8 kWh/m 3 .

© 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 present study evaluates the feasibility of applying forward osmosis (FO) process for the pretreatment of feed solution to a Multi Stage Flash (MSF) desalination plant. For the first time, real brine reject and real seawater were used as the draw solution and the feed solution, respectively in the FO process. The FO pretreatment is expected to dilute the brine reject and reduce the concentration of divalent ions, which are responsible for scale formation on the surface of heat exchanger in the MSF evaporator unit. The FO experiments were performed at different draw solution temperatures ranging between 25 and 40 °C, different draw and feed solutions flowrates and different membrane orientations. A maximum average membrane flux of 22.3 L/m2·h was reported at a draw solution temperature of 40 °C and 0.8 and 2.0 LPM flow rate of draw and feed solutions, respectively. The experimental results also revealed the process sensitivity to the feed solution temperature. It was found that the average membrane flux in the FO process operating at 0.8 and 2 LPM draw and feed solution flow rates, respectively was 16.9 L/m2·h at 25 °C brine temperature but increased to 22.3 L/m2·h at 40 °C brine temperature. These membrane fluxes resulted in 3% and 8.5% dilution of the draw solution at 25 °C and 40 °C temperatures, respectively. The average membrane flux in the FO mode was equal to that in the PRO mode at low flow rates but it was lower than that in the PRO mode at high flow rates of the feed and draw solutions. The outcomes of the study are very promising with regard to membrane flux and dilution of draw solution. This research is made possible by NPRP award (NPRP10-0117-170176) from Qatar National Research Fund (QNRF). The statements made herein are solely the responsibility of the authors. In addition, the authors wish to thank Qatar Foundation for the financial support provided to one of the co-authors through a graduate sponsorship research award ( GSRA4-2-0402-17013 ). The authors also wish to thank Qatar Electricity and Water Company (QEWC) for the supply of brine. Scopus

In this work, the salinity of permeate obtained with membrane distillation (MD) in pilot scale systems was analyzed. Experiments were performed with three different spiral-wound commercial modules, one from Solar Spring with 10 m2 surface membrane area and two from Aquastill with 7.2 and 24 m2. Intermittent operation meant that high permeate conductivity was measured in the beginning of each experiment, which was gradually decreasing until reaching a constant value (3&ndash, 143 &micro, S&middot, cm&minus, 1 for seawater feed). The final quality reached did not depend on operating conditions, only the time it took to reach it. This can be because the permeate flux dilutes the minimal feed leak taking place through pinholes in the membranes. Larger feed leak through the membrane was observed when operating in vacuum-enhanced air-gap MD configuration (V-AGMD), which is compatible with this explanation. However, for the increase of feed leak with salinity (up to 1.8 M), a conclusive explanation cannot be given. Pore wetting due to crystallization is discarded because the high permeate quality was recovered after washing with distilled water. More studies at higher salinities and also at membrane level are required to investigate this.

A 72 m3/d pilot low-temperature multi-effect distillation plant located at the Plataforma Solar de Almeria has been experimentally characterized at steady state to study the influence of the variation in certain parameters that control the process (the hot water inlet temperature as external thermal energy source and the last effect vapor temperature) on the distillate production, the thermal consumption, and the thermal energy efficiency of the plant. Results allowed characterizing the increase in the water production and the thermal consumption with the increase in the hot water inlet temperature and with the decrease in the last effect vapor temperature. The performance ratio reached its maximum when the last effect vapor temperature ranged from 25 to 35°C, since the temperature difference between effects was lower. The preliminary characterization of this plant provides useful experimental information for design criteria and for the analysis of control strategies of other large-scale MED plant...

© 2016 Balaban Desalination Publications. All rights reserved. Novel desalination approaches are required to provide both drinking and agricultural water as there is ever increasing stress upon precious freshwater resources. It was our hypothesis that a modified Forward Osmosis (FO) process had the potential for production of irrigation water comprising of appropriate concentrations of fertilizers from a seawater feed. Four agents, KNO3, Na2SO4, CaNO3, and MgCl2, plus 35 g/L seawater were used as the draw and feed solutions of the FO process. Net Driving Pressure in the FO process was manipulated either by increasing the concentration of draw solution (FO process) or by increasing feed pressure (Pressure Assisted FO (PAFO) process). A series of nanofiltration (NF) and reverse osmosis (RO) membranes were used for the regeneration of draw solution. The results suggested that a PAFO process was more energy efficient than simple FO, provided the energy relating to the brine flow from the NF/RO membrane for pressurizing the feed solution of PAFO process was used. Furthermore, this study suggested using a mixture of a primary draw solution, MgCl2, and a secondary draw solution, KNO3, for NO3 supply into the irrigation water was preferable. As such, MgCl2 provided the driving force for fresh water extraction while KNO3 was the source of fertilizer in the irrigation water. Results showed that water quality provided by application of a MgCl2 + KNO3 draw solution was better than that from KNO3 or Ca(NO3)2. The concentrations of NO3 and SO4 in irrigation water were within recommended levels when the diluted draw solution was regenerated by a dual stage low-pressure RO process.

Forward Osmosis (FO) has been proposed as an alternative method for seawater desalination, wherein Reverse Osmosis (RO) membrane technology is used for regeneration of the draw solution. Previous studies have indicated that a standalone Reverse Osmosis unit is more energy efficient than a Forward Osmosis-Reverse Osmosis system and as such it was recommended that an FO-RO system was best employed only for the desalination of high salinity seawaters. This study examined FO-RO applicability in more detail by examining the impact of seawater salinity, impact of an Energy Recovery Device (ERD), and the effect of membrane fouling. For comparison purposes, the performance of the FO process was improved to minimize the impact of concentration polarization and optimize the concentration of draw solution. Model calculations revealed that FO-RO was more energy efficient than RO when no Energy Recovery Device was employed. However, results showed there was no significant difference in the power consumption between the FO-RO system and the RO unit at high seawater salinities, particularly when a high efficiency ERD was installed. Moreover, the FO-RO system required more membrane area than conventional a RO unit which may further compromise the FO-RO desalination cost.

Experiments were conducted with a commercial membrane distillation module, which has an air-gap membrane distillation (AGMD) and spiral-wound configuration. Tests were performed simulating the salinity and temperature of the water using as a case study the Albian aquifer in Algeria. Solar thermal energy from a field of stationary flat-plate solar collectors was supplied to the module. The total productivity of the AGMD system was analyzed for different operational conditions and compared with that from the RO system. The effect of brackish water temperature on the energy consumption was scrutinized, and the energy requirements assessed toward the design of a pilot unit for decentralized autonomous solar desalination. Results showed that for producing a higher volume of distillate, module with less surface area is better. Also, it was found that recovery ratio increases linearly with the temperature difference in both modules. The maximum value was 6% for the highest feed flow rate operated in modu...

A stand-alone small-scale renewable heat powered polygeneration system to provide cold for space cooling, electricity, and seawater desalination was proposed and numerically modelled. The system is based on Permeate/Conductive-gap MD modules (P/CGMD) and an ammonia/water absorption power-refrigeration system driven by heat from an evacuated flat plate solar collector field with a biomass-fired backup boiler. Energy and exergy performance of the system was analysed using climate conditions of a typical location well-endowed with solar irradiation and characterized by a potential shortage of freshwater supply and high cooling demand. In the base-case, the system provided 130 kW of cooling capacity at 11 °C, 6.4 kW of net electrical power, and 41.4 m3/day of desalinated water based on annual average weather conditions of Almeria (Spain). The overall system's resource utilization efficiency and exergy efficiency were estimated at 44.2% and 6.9% respectively. The use of CGMD reduced the required specific membrane surface area by 53% compared to the PGMD. The results of this study show that a thermally coupled absorption refrigeration system and MD process can be implemented (i) to increase the efficiency of the entire thermal energy conversion process and (ii) to cost-effectively utilize the solar thermal installation, particularly in regions where desalination is a necessity.

A screen-printed carbon electrode (SPCE) modified with copper (II) 1-(3-bromobenzyl)-5-methyl-1H-1,2,3-triazole-4-carboxylate [Cu(bmtc)2(H2O)] and was used as an electrochemical sensor of hydrogen peroxide. The material was characterised by crystal X-ray and cyclic voltammetry. The electrochemical behaviour of the modified electrode [Cu(bmtc)2(H2O)]/SPCE was evaluated against bare SPCE for the reduction of 1.0 mM H2O2 in phosphate buffer (pH = 7.0), where the modified electrode responded ~ −0.55 V even though the SPCE did not present response. On the other hand, analytical parameters such as reproducibility (2.7%), repeatability (2.6%), LOD (5.8 × 10−7 mol L−1), selectivity (physiological substances) and stability were evaluated.

The N-methylimidazolium hexachloroantimonate salt Cl6Sb·C4H7N2 (MIMSb), was prepared and fully characterized. In 1H NMR spectrum, the N-H proton shifted to downfield because of the presence of SbCl6− and appears as a triplet at 13.19 ppm. Characterization with IR spectroscopy shows strong absorption band at around 699 cm−1 which is attributed to Sb−Cl stretching. Furthermore, UV–visible analysis at high concentrations in DMSO suggested that MIMSb interacts with DMSO leading to an absorption in visible region at λmax of 426 nm. Electrochemical characterization using cyclic voltammetry demonstrates three redox processes with reduction peaks at 0.69, −0.13 and −0.50 V. Finally, molecular structure of the product was determined by X-ray diffraction analysis. The crystal structure determination was carried out with Mo-Kα X-ray and data measured at 100 K. The title compound crystallizes in monoclinic P21/c space group with unit cell parameters a = 7.1131 (5) A, b = 12.4436 (9) A, c = 14.1658 (11) A, V = 1241.86 (16) A3 and Z = 4. The crystal packing is stabilized by H---Cl interaction. The analysis of intermolecular interactions was realized through the mapping of contact descriptors dnorm, shape-index and the fingerprint reveling that the most significant contribution to the Hirshfeld surface (69.4%) is from H---Cl contacts. Finally, the catalytic activity of MIMSb was probed in the cyclopropanation of styrene with ethyl diazoacetate.

The coupling of Reverse Electrodialysis with Membrane Distillation is a promising option for the conversion of waste heat into electricity. This study evaluates the performances of the integrated system under different operating conditions, employing validated model and correlations. This work provides a detailed description of the behaviour of a real RED-MD heat engine and indicates the set of inlet concentrations, velocities and equipment size which returns the highest cycle exergy efficiency. These operating conditions were selected for the pilot plant developed within the EU-funded project RED Heat to Power. For the first time, a perspective analysis was also included, considering highly performing RED membranes and future MD module. Relevant results indicate that technological improvements may lead to interesting system performance enhancement, up to an exergy efficiency of 16.5%, which is considerably higher than the values reported in literature so far.

Three novel trans-halogeno(4-acetoxyphenyl)bis(triphenylphosphine)palladium(II) complexes (XPd(4-AcOC6H4)(PPh3)2, with X = I (3), Br (4) and Cl (5)) were synthesized and characterized by spectroscopic techniques and by X-ray diffraction analysis. The SEM images of the particles obtained after synthesis and microwave treatment show uniform morphology and particle size lower than 1 μm for the iodo-palladium complex (3). The electrochemical performance of the palladium-based anode materials was evaluated by cyclic voltammetry and galvanostatic cycling. The results show that the nature of the halogen ligand (X = I, Br and Cl) has a high impact on the reaction mechanism during the first discharge and cycling performance. Thus, the three materials IPd(4-AcOC6H4)(PPh3)2 (3), BrPd(4-AcOC6H4)(PPh3)2 (4) and ClPd(4-AcOC6H4)(PPh3)2 (5) deliver high initial irreversible capacities of 1089, 444 and 684 mAh g−1, respectively, during the first discharge at 20 mA g−1 at a voltage window between 0.01 and 3.0 V. In addition, the iodo-palladium-based anode material shows the highest specific capacity and a high capacity retention after 200 cycles at a current density of 50 mA g−1 with an average discharge capacity of about 170 mAh g−1.

Thermal energy storage tanks are habitually combined with solar thermal fields to improve the dispatchability of these facilities. From the dynamical point of view, the start-up phase is relevant since if the storage device is unloaded in terms of energy or widely stratified, the transitory regime can take long time until reaching the operating point. In this paper, an optimal real-time procedure based on a hierarchical controller for improving the start-up phase is proposed. The hierarchical controller is composed of two layers based on a Model Predictive Control (MPC) technique and Proportional Integer Derivative (PID) controllers. Real experimental tests were performed in a pilot facility located at Plataforma Solar de Almeria (Almeria, Spain). In addition, a comparison in simulation with the typical manual procedure and with two techniques proposed previously in the literature for the same plant is provided. The results demonstrate the benefits obtained by using the proposed method; since it reduces the start-up phase in 34 [min] in comparison with the manual operation, and in 26 and 6 [min] with respect to the two previous techniques.

A performance analysis of a salinity gradient heat engine (SGP-HE) is presented for the conversion of low temperature heat into power via a closed-loop Reverse Electrodialysis (RED) coupled with Multi-Effect Distillation (MED). Mathematical models for the RED and MED systems have been purposely developed in order to investigate the performance of both processes and have been then coupled to analyze the efficiency of the overall integrated system. The influence of the main operating conditions (i.e., solutions concentration and velocity) has been quantified, looking at the power density and conversion efficiency of the RED unit, MED Specific Thermal Consumption (STC) and at the overall system exergy efficiency. Results show how the membrane properties (i.e., electrical resistance, permselectivity, water and salt permeability) dramatically affect the performance of the RED process. In particular, the power density achievable using membranes with optimized features (ideal membranes) can be more than three times higher than that obtained with current reference ion exchange membranes. On the other hand, MED STC is strongly influenced by the available waste heat temperature, feed salinity and recovery ratio to be achieved. Lowest values of STC below 25 kWh/m3 can be reached at 100 °C and 27 effects. Increasing the feed salinity also increases the STC, while an increase in the recovery ratio is beneficial for the thermal efficiency of the system. For the integrated system, a more complex influence of operating parameters has been found, leading to the identification of some favorable operating conditions in which exergy efficiency close to 7% (1.4% thermal) can be achieved for the case of current membranes, and up to almost 31% (6.6% thermal) assuming ideal membrane properties.

Membrane distillation is an attractive technology for solar-powered decentralized desalination that has not yet reached commercial breakthrough on a large scale. The main barriers are energy consumption and cost. Since the latter are mostly related to the former, thermal energy efficiency is key to assessing the potential of the different available membrane distillation systems at a commercial scale. As discussed here, existing membrane distillation technologies use mostly flat sheet membranes in plate and frame and spiral-wound modules. Modules based on hollow fibre membranes are also considered, as well as the concept of multi-effect vacuum membrane distillation for improved heat recovery. The heat efficiency of each system is analysed based on available experimental results. Better internal heat recovery and capacity for upscaling are found to be important elements of distinction which make multi-channelled spiral-wound modules working in air-gap configuration stand out currently, with the lowest heat consumption of all large scale modules. Potential for improvement of this and other technologies is also discussed, and an estimation based on the associated costs for solar energy is used for establishing boundary conditions towards the implementation of membrane distillation for solar desalination.

In this paper, an expert-guided kinodynamic RRT algorithm (EGK-RRT) is presented. It aims to consider how a human pilot would navigate a kinodynamic robot. One of the characteristics of this algorithm is the fact that, unlike the original RRT for kinodynamic systems, it generates deterministic control sequences which can be reproduced as long as the sequence of references (sampled states) are known. Here, the performance of the proposed algorithm is tested against the basic RRT, showing that the EGK-RRT greatly improves in terms of execution speed. In addition to this, the influence of using a visibility check and an inertia estimation in order to select the nearest neighbor is also analyzed, demonstrating that a combination of both factors leads to a better overall performance, both in execution speed and in quality of the generated path.

A dual stage pressure retarded osmosis (PRO) process was investigated for power generation using different types of membranes. Polyamide (PA) and cellulose triacetate (CTA) membranes were used in the first and second stage of the PRO process to improve the process performance due to the high water permeability of PA membranes. A comparison between dual stage PA–CTA and CTA–CTA membrane systems were carried out using seawater as a draw solution, while fresh water and wastewater effluent were the feed solution in the first and second stage of the process. The impact of draw solution flow rate on the process performance was evaluated. The performance of first and second stage of the PRO process increased by 11.5 and 28.6%, respectively, when the draw solution flow rate increased by a factor of 2.5. In return, there was a negligible increase in the total specific power consumption of the PRO process. In general, power consumption of the dual stage PRO process was as low as 0.3 kWh/m3. Furthermore, the...

Two-stage forward osmosis (FO)–reverse osmosis (RO)/BWRO and conventional RO/BWRO processes were evaluated for saline water treatment. Three different salts, NaCl, MgCl2 and MgSO4, were used as a draw solution. The performance of FO, RO and BWRO regeneration processes was simulated using predeveloped software. The simulation results showed that the water and solute flux across the FO membrane decreased with increasing the FO recovery rate. The highest water flux was in case of 1.2 mol NaCl draw solution for seawater desalination. The total power consumption for seawater desalination was lower in the RO process than in the FO–RO process. However, as the RO recovery rate increased, the difference in total power consumption between the conventional RO process and 0.65 mol MgCl2 FO–RO processes was insignificant. In case of brackish water desalination, the lowest specific power consumption and permeate TDS were in case of the conventional BWRO process and then followed by 0.32 mol MgSO4, 0.22 mol MgCl...

This work presents the development of a feedback control system for a pilot membrane distillation facility powered with solar energy located at Plataforma Solar de Almeria (PSA), Spain. The control system allows to fix a suitable operating temperature at the inlet of the distillation system, improving the operation quality. Four direct control schemes based on Proportional Integral (PI) controllers and Feedforward (FF) are designed as well as a reference governor which generates temperature references for the heat generation circuit direct control layer. Simulations and experimental tests are shown to demonstrate the effectiveness of the proposed scheme.

This paper presents an optimal operation strategy in terms of thermal efficiency and distillate flux production for a Solar Membrane Distillation (SMD) system. Firstly, a study of the Membrane Distillation (MD) module is presented, revealing the optimal operation strategy. Secondly, a hierarchical control system with two layers is developed and tested. The upper layer consists on a Nonlinear Model Predictive Control (NMPC) scheme which allows us to obtain the maximum temperature at the inlet of the MD module, by optimizing the use of solar energy. The lower layer is composed by a direct control system that is in charge of reaching the setpoint calculated by the upper layer. Simulation results are shown in order to demonstrate the effectiveness of this control approach.

Although Membrane Distillation (MD) has been extensively studied for desalination, it has other applications like removing all kinds of solutes from water and concentrating non-volatile substances. MD offers the possibility of producing a clean stream while concentrating valuable compounds from waste streams towards their recovery, or emerging contaminants and pathogens present in wastewater in order to facilitate their chemical elimination. This paper analyses the elimination of bacterial spores from contaminated water with MD and the role of MD in the subsequent treatment of the concentrate with photo-Fenton process. The experiments were performed at Plataforma Solar de Almeria (PSA) using a plate and frame bench module with a Permeate Gap Membrane Distillation (PGMD) configuration. Tests were done for two different kinds of spores in two different water matrixes: distilled water with 3.5wt% of sea salts contaminated with spores of Bacillus subtilis (B. subtilis) and wastewater after a secondary treatment and still contaminated with Clostridium sp. spores. An analysis of the permeate was performed in all cases to determine its purity, as well as the concentrated stream and its further treatment in order to assess the benefits of using MD. Results showed a permeate free of spores in all the cases, demonstrating the viability of MD to treat biological contaminated wastewater for further use in agriculture. Moreover, the results obtained after treating the concentrate with photo-Fenton showed a shorter treatment time for the reduction of the spore concentration in the water than that when only photo-Fenton was used.

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.

© 2018 Elsevier Ltd The performance of Dual Stage Pressure Retarded Osmosis (DSPRO) was analyzed using a developed computer model. DSPRO process was evaluated on Pressure Retarded Osmosis (PRO) and Forward Osmosis (FO) operating modes for different sodium chloride (NaCl) draw and feed concentrations. Simulation results revealed that the total power generation in the DSPRO process operating on the PRO mode was 2.5–5 times more than that operating on the FO mode. For DSPRO operating on the PRO mode, the higher power generation was in the case of 2 M NaCl-fresh and 32% the contribution of the second stage to the total power generation in the DSPRO. To the contrast, he total power generated in the DSPRO operating on the FO mode was in the following order 5M-0.6M > 5M-0.7M > 2M-0.01 > 2M-0.6 M. Interestingly, single stage process operating on the FO mode performed better than DSPRO process due to the severe concentration polarization effects. The results also showed that power density of the DSPRO reached a maximum amount at a hydraulic pressure less than the average osmotic pressure gradient, Δπ/2, due to the variation of optimum operating pressure of each stage. Moreover, results showed that the effective specific energy in the PRO process was lower than the maximum specific energy. However, the effective specific energy of the DSPRO was larger than that of the single stage PRO due to the rejuvenation of the salinity gradient, emphasizing the high potential of the DSPRO process for power generation.

Power generation by means of Pressure Retarded Osmosis (PRO) has been proposed for harvesting the energy of a salinity gradient. Energy recovery by the PRO process decreases along the membrane module due to depleting of the chemical potential across the membrane and concentration polarization effects. A dual stage PRO (DSPRO) design can be used to rejuvenate the chemical potential difference and reduce the concentration polarization on feed solution. Several design configurations were suggested for the membrane module arrangements in the first and second stage of the PRO process. PRO performance was evaluated for a number of salinity gradients proposed by coupling Dead Sea water or Reverse Osmosis (RO) brine with seawater or wastewater effluent. Maximum specific energy of inlet and outlet feeds was calculated using a developed computer model to identify the amount of recovered and remaining energy. Initially, specific power generation by the PRO process increased by increasing the number of modules of the first stage. Maximum specific energy is calculated along the PRO module to understand the degradation of the maximum specific energy in each module before introducing a second stage PRO process. Adding a second stage PRO process resulted in a sharp increase of the chemical potential difference and the specific energy yield of the process. Between 10% and 13% increase of the specific power generation was achieved by the DSPRO process for the Dead Sea-seawater salinity gradient depending on the dual stage design configuration. For Dead Sea-RO brine, 12–16% increase of the specific power generation was achieved by the dual stage PRO process. For Dead Sea-wastewater and RO brine-wastewater, a neutral and sometimes negative impact occurred when a second stage PRO process was introduced. We concluded that, for a given draw solution concentration, dual stage performs better than the conventional PRO process at high feed salinities, yet requires lower hydraulic pressure.

Solar Membrane Distillation (SMD) is an under-investigation desalination process suitable for developing self-sufficient small scale applications. The use of solar energy considerably reduces the operating costs, however, its intermittent nature requires a non-stationary optimal operation that can be achieved by means of advanced control strategies. In this paper, a hierarchical control system composed by two layers is used for optimizing the operation of a SMD pilot plant, in terms of thermal efficiency, distillate production and cost savings. The upper layer is formed by a Nonlinear Model Predictive Control (NMPC) scheme that allows us to obtain the optimal operation by optimizing the solar energy use. The lower layer includes a direct control system, in charge of attaining the variable references provided by the upper layer. Simulation and experimental tests are included and commented in order to demonstrate the benefits of the developed control system.

Salinity gradient power is proposed as a source of renewable energy when two solutions of different salinity are mixed. In particular, Pressure Retarded Osmosis (PRO) coupled with a Reverse Osmosis process (RO) has been previously suggested for power generation, using RO brine as the draw solution. However, integration of PRO with RO may have further value for increasing the extent of water recovery in a desalination process. Consequently, this study was designed to model the impact of various system parameters to better understand how to design and operate practical PRO-RO units. The impact of feed salinity and recovery rate for the RO process on the concentration of draw solution, feed pressure, and membrane area of the PRO process was evaluated. The PRO system was designed to operate at maximum power density of Δ P = Δ π 2 . Model results showed that the PRO power density generated intensified with increasing seawater salinity and RO recovery rate. For an RO process operating at 52% recovery rate and 35 g/L feed salinity, a maximum power density of 24 W/m2 was achieved using 4.5 M NaCl draw solution. When seawater salinity increased to 45 g/L and the RO recovery rate was 46%, the PRO power density increased to 28 W/m2 using 5 M NaCl draw solution. The PRO system was able to increase the recovery rate of the RO by up to 18% depending on seawater salinity and RO recovery rate. This result suggested a potential advantage of coupling PRO process with RO system to increase the recovery rate of the desalination process and reduce brine discharge.

In desalination, similarly with other industries, the cost of the final product is one of the most important criteria that define the commercial success of a specific technology. Therefore, when new projects are planned or new technologies are proposed, the analysis of the expected costs attracts a lot of attention and is compared to (perceived) costs of state-of-the-art desalination or costs of alternative fresh water supply options. This comparison only makes sense if the cost assessment methodologies are based on the same principles and use common assumptions. This paper assesses: (i) the methodologies used to calculate the water cost; (ii) the boundary conditions and (iii) the input data and assumptions. It has been found that most papers in the literature use suitable equations and boundary conditions. Also certain elements like land costs are ignored, but in most cases this is duly acknowledged and justified. However, the quality of the input data for the hardware costs, the operating costs, and the financial parameters are not always appropriate. Guidance for the methodology, data and assumptions that should be used is provided depending on the purpose for which the cost of the desalinated water is calculated.