Economic evaluation of desalination by small-scale autonomous solar-powered membrane distillation units

Solar-powered desalination is an attractive and viable method for the production of fresh water in remote arid areas. One of the most important factors determining desalination decisions is economics. This paper presents an economic assessment performed to estimate the expected water cost, which is the ultimate measure of the feasibility of the stand-alone system. Based on the calculations, the estimated cost of potable water produced by the compact unit is $15/m³, and $18/m³ for water produced by the large unit. Membrane lifetime and plant lifetime are key factors in determining the water production cost. The cost decreases with increasing the membrane and/or the plant lifetime.     

Exergy analysis of desalination by solar-powered membrane distillation units

There has been an increasing interest in using exergy as a potential tool for analysis and performance evaluation of desalination processes where the optimal use of energy is considered an important issue. Unlike energy, exergy is consumed or destroyed due to irreversibilies in any real process and thus provides deeper insight into process analysis. Exergy analysis method was employed to evaluate the exergy efficiency of the “compact” and “large” solardriven MD desalination units. The exergy efficiency of the compact and large units with reference to the exergy collected by the solar collector was about 0.3% and 0.5% but was 0.01% and 0.05%, respectively, when referenced to the exergy of solar irradiance. The exergy efficiency of the flat plate solar collectors in both units varied diurnally and the maxima was 6.5% ad 3% for the compact and large units, respectively. The highest exergy destruction was found to occur within the membrane distillation module.     

Comparison of solar thermal technologies for applications in seawater desalination

This paper deals with a global analysis of the use of solar energy in seawater distillation under Spanish climatic conditions. Static solar technologies as well as one-axis sun tracking were compared. Different temperature ranges of the thermal energy supply required for a desalination process were considered. At each temperature range, suitable solar collectors were compared in some aspects as: (1) fresh water production from a given desalination plant; (2) attainable fresh water production if a heat pump is coupled to the solar desalination system; (3) area of solar collector required for equivalent energy production. Results showed that direct steam generation (DSG) parabolic troughs are a promising technology for solar-assisted seawater desalination.     

Seawater desalination by direct contact membrane distillation

Membrane fouling still posts as one of the major obstacles in membrane distillation (MD). This is why the MD approach still cannot successfully compete with other conventional seawater desalination methods. In this study, both the NaCl solution and real seawater are used as the feed of MD processes to investigate the differences in permeate flux, product water quality and membrane fouling. The results indicate the accumulation rate of membrane scale can be depressed by reducing the degree of polarization if NaCl solution is used as the feeding fluid, however, this kind of depression effect is not so obvious when real seawater is used as the feeding fluid. An ultrasonic cleaning technique is found to be an effective way to restore the flux rate for these MD processes and extend the life span of MD membrane.     

减压膜蒸馏浓缩盐水溶液的研究现状

减压膜蒸馏技术是一种新型膜分离技术,在许多领域中呈现出明显优势。综述了减压膜蒸馏技术浓缩盐水溶液的研究现状,分析了影响分离性能的相关因素以及该技术在处理浓盐水中的应用效果,指出了减压膜蒸馏技术浓缩盐水溶液的应用前景。     

Analysis of a solar-powered membrane distillation system

Nowadays, in dry and rural areas, solar-powered membrane distillation (SPMD) technology is considered a feasiblemeans for the production of pure water from brackish water. Prior to the design and construction of a SPMD pilot plant, there is a need to predict its performance theoretically by means of a computational simulation program. Unlike previous approaches followed by other investigators to develop a mathematical model that can describe the components of a SPMD pilot plant, the developed mathematical model in this study is based on the fact that the SPMD process by nature is unsteady. The performance of a proposed SPMD pilot plant is then obtained by means of a numerical solution of the model with the aid of a simulation computer program. The results reveal that the proposed SPMD pilot plant has some unique features, which differ from a similar MD process operated at steady-state conditions in a laboratory. The analysis of the system has shown that heat recovery via an external heat exchanger is not only possible, but even effective, and an economical way to intensify the SPMD process. The plant productivity can be improved by increasing the heat-exchanger capacity (KA), decreasing the flow rates of both feed and permeate or otherwise by increasing the effective surface area of the membrane. The achieved enhancements in the SPMD pilot plant productivity are directly related to an improved heat recovery rate in the heat exchanger. However, further analysis reported in this paper shows that the increase in KA and membrane area should be optimized for any planned capacity in the design of a SPMD pilot plant.     

Mathematical modeling of an inclined solar water distillation system

An inclined solar water distillation (ISWD) system, which generates distilled water (i.e., condensate) and hot water at the same time, was modeled and simulated. In the parametric studies, the effects of feed water mass flow rate and solar intensity on the system parameters were investigated. Finally, the system was simulated using actual deviations of solar intensity and environment temperature during a typical summer day in North Cyprus. The system can generate 3.5–5.4 kg (per m² absorber plate area) distilled water during a day (i.e., 7 am till 7 pm). The temperature of the produced hot water reached as high as 60°C, and the average water temperature was about 40°C, which is good enough for domestic use, depending on the type of feed water. The simulation results are in good agreement with the experimental results.     

Validating the performance simulation program “SOLDES” using data from an operating solar desalination plant

A computer program (named SOLDES) was developed to simulate the operation of solar desalination plants which utilize evacuated tube collectors, heat accumulators and multiple-effect distillation (MED) systems. The heat accumulator used is of the thermally stratified type using pure water as the storage fluid. The procedure was written in Fortran language and consists of a main program, 22 sub-programs, two system data files and four meteorological data files. The absorber area of the solar collector field can be varied between 500 m² and 20,000 m²; the storage capacity per unit collector area of the heat accumulator can vary between 0.05 and 1.00m³/m²; the capacity of the evaporator can be varied between 100 m³/d to 2000 m³/d. The heat collecting system uses a bypass circuit to allow the heat collecting fluid (pure water) to recirculate back to the solar collector field when the outlet temperature from the collector field is below a set-point. When the collector outlet temperature rises above the set-point, operation is switched over to the accumulator side. A solar-cell-type controller is used to start and stop the water circulating pump of the collector field. The operation of the MED evaporator is controlled by the state of charge of the heat accumulator by the use of set-point switches which allow the evaporator to start up when the accumulator water temperature is above a set-point and to shut down if the water temperature drops below the set point. In order to validate the SOLDES program, a comparison was made between the predicted results of the program and the actual measured data from a solar plant of similar design features to the simulation program. The selected plant was the one in actual operation in Abu Dhabi, UAE, which has almost identical design features as the simulation program and has been in operation since 1984. The data from the plant collected during 1985 were used to compare the simulation results for the months of January and June. These two months were found to be typical of a winter month (January) and of summer months (June). Except for days when a plant interruption took place, such as a power failure, the agreement between the measured and simulation data appears to be quite good.     

Application of Taguchi method in optimization of desalination by vacuum membrane distillation

This research focuses on desalination via vacuum membrane distillation (VMD). In order to enhance the performance of VMD in desalination and to get more flux, effects of operating parameters on the yield of distillate water were studied. Four parameters at three levels were selected: temperature (35, 45, and 55 °C), vacuum pressure (30, 80, and 130 mbar), flow rate (15, 30, and 60 mL/s) and concentration (50, 100, and 150 g/L). Taguchi method was used to plan a minimum number of experiments. The optimal levels thus determined for the four factors were: temperature 55 °C, vacuum pressure 30 mbar, flow rate 30 mL/s and concentration 50 g/L. The results show that increasing temperature and decreasing vacuum pressure improve permeate flux. However, the permeate flux increases with increasing flow rate initially and then reaches to a maximum value at 30 mL/s and then decreases with increasing the flow rate.     

An experimental study on an inclined solar water distillation system

An inclined solar water distillation system was designed and tested under actual environmental conditions of northern Cyprus. Unlike solar still systems, the feed water falls down on the solar absorber plate, and the system produces fresh water and hot water simultaneously. It was suspected that the longer the flowing water is held on the absorber plate, the greater the rate of evaporation, leading to an increase in the amount of distilled water. Therefore, the system was tested with three variants: bare plate, black-cloth wick, and black-fleece wick. As was expected, the wicks increased the fresh water generation by two or three times of a bare plate. The hot water temperature was good enough for domestic usage.     

The effect of using different designs of solar stills on water distillation

Solar distillation is one of the important methods of utilizing the solar energy for the supply of potable water to small communities where the natural supply of fresh water is inadequate or of poor quality, and where sunshine is abundant. Solar energy utilization in two different types of solar stills is considered, and factors that influence the productivity of solar stills are discussed. The present investigation showed that the productivity of asymmetric greenhouse type still (ASGHT) having mirrors on its inside walls was higher than that of the symmetric greenhouse type still (SGHT) and more efficient. It was found that the distilled water output of the asymmetrical greenhouse type was 20% higher than that of symmetric greenhouse type. Performance characteristics of the two stills showed that the temperature at the water surface is closely related to the incident solar radiation, and the productivity of the stills can be increased with decreasing water depth, and by the addition of dye.     

Simulation of membrane distillation modules for desalination by developing user's model on Aspen Plus platform

This paper presents a simulation study of direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) for desalination. Simulation models are built on Aspen Plus^® platform as user defined unit operations for these two types of modules, respectively. Large scale modules for practical industrial applications are simulated and studied for the effects of design and operation variables, as well as the importance of heat and mass transfers of each phase. For each type of modules with heat recovery design, the response surface method (RSM) is applied to develop the performance-variables quadratic model, followed by the multivariable optimization. Optimal designs can realize separation efficiencies, defined as the ratio of water produced to the feed, of 8.2% and 5.8% for DCMD and AGMD, respectively.     

Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation

The mixture of inorganic salt LiCl and soluble polymer polyethylene glycol (PEG) 1500 as non-solvent additive was introduced to fabricate hydrophobic hollow fiber membrane of polyvinylidene fluoride (PVDF) by phase inversion process, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. Compared with other three membranes from PVDF/DMAc, PVDF/DMAc/LiCl and PVDF/DMAc/PEG 1500 dope solution, it can be observed obviously by scanning electron microscope (SEM) that the membrane spun from PVDF/DMAc/LiCl/PEG 1500 dope had longer finger-like cavities, ultra-thin skins, narrow pore size distribution and porous network sponge-like structure owing to the synergistic effect of LiCl and PEG 1500. Besides, the membrane also exhibited high porosity and good hydrophobicity. During the desalination process of 3.5 wt% sodium chloride solution through direct contact membrane distillation (DCMD), the permeate flux achieved 40.5 kg/m² h and the rejection of NaCl maintained 99.99% with the feed solution at 81.8 °C and the cold distillate water at 20.0 °C, this performance is comparable or even higher than most of the previous reports. Furthermore, a 200 h continuously desalination experiment showed that the membrane had stable permeate flux and solute rejection, indicating that the as-spun PVDF hollow fiber membrane may be of great potential to be utilized in the DCMD process.     

First experimental results of a new hybrid solar/gas multi-effect distillation system: the AQUASOL project

From 2002 to 2006, a combined R&D project named AQUASOL has been carried out at the facilities of the Plataforma Solar de Almería (Spain). Main objective of this project has been the development of a hybrid solargas desalination system based on multi-effect distillation process that meets at the same time the requirements of low-cost, high efficiency and zero discharge.

The final AQUASOL plant, implemented at the Plataforma Solar de Almería for its evaluation under real meteorological conditions, is composed of: (i) a 14-cell forward-feed vertically-stacked MED unit, (ii) a 500m2 stationary CPC (compound parabolic concentrator) solar collector field, (iii) a 24 m3 thermal storage system based on water, (iv) a new advanced prototype of double-effect absorption (LiBr-H2O) heat pump, (v) a smoketube gas boiler to guarantee 24-h operation

This paper shows the first experimental results obtained during the test campaign of the project. The performance ratio reached by the distillation plant in different operational modes is evaluated, as well as the issues related with the operation of the subsystems that compose the AQUASOL desalination system.     

Treatment of waters colored with methylene blue dye by vacuum membrane distillation

Textile industries consume large amounts of water and generate highly dye-contaminated effluents. Textile wastewaters have to be treated in order to be recycled in the process or to meet legislative requirements before being discharged. The objective of the present study was to examine the potential use of the vacuum membrane distillation process for the treatment of dyed solutions. Methylene blue (MB) was used as a model dye. Batch experiments were conducted on dilute MB-water mixtures using a tubular polypropylene membrane module. The concentration of MB dye within the feed reservoir was monitored over time. The impact of operating variables such as feed temperature, flow rate and initial dye concentration was investigated. A mathematical model incorporating temperature and concentration polarization effects was developed and validated on the experimental data.     

Concentration of apple juice using direct contact membrane distillation

The direct contact membrane distillation process (DCMD) has been used to investigate the concentration of apple juice. Results show that at a constant temperature of juice in the hot cell, an increase in the flux permeates of DCMD resulted in reducing the temperature of cooling water in the cold cell. Increasing the temperature of juice in the hot cell reduces influence of the cooling water temperature in the cold cell on the flux permeate of DCMD. The influence of temperature polarization on the effectiveness of DCMD in apple juice concentration has also been detected. The dependence of flux permeates on operating temperature. The concentration of soluble substances in concentrate and hydrodynamic conditions in the experimental equipment has also been studied. In the concentration of apple juice, 50% of solids content was obtained when the permeate flux reached about 9 l/m2·h. Further concentration of juice to 60–65% solids resulted in reduced productivity (3.8–3.0 l/m²·h) and therefore a decrease in the biological value of the concentrate.     

A neural network-based optimizing control system for a seawater-desalination solar-powered membrane distillation unit

Several schemes have been proposed so far for coupling desalination processes with the use of renewable energy. One of their main drawbacks, however, is the nature of the energy source that requires a discontinuous and non-stationary operation, with some control and optimization problems. In the present work, a solar powered membrane distillation system has been used for developing an optimizing control strategy. A neural network (NN) model of the system has been trained and tested using experimental data purposely collected. Afterwards, the NN model has been used for the analysis of the process performance under various operating conditions, namely distillate production versus feed flow rate, solar radiation and cold feed temperature. On this basis, a control system that optimizes the distillate production under variable operating conditions has been developed, implemented and tested.     

膜蒸馏和膜吸收技术现状及发展

膜蒸馏和膜吸收是一种以蒸汽压差为推动力的新型高效的膜分离技术,作者介绍了膜蒸馏和膜吸收技术发展现状,机理及优缺点,并对其在有机废水处理中的应用及今后发展方向进行了论述。     

膜蒸馏海水淡化研究进展及发展趋势

海水淡化是解决我国水资源短缺的重要措施之一。膜蒸馏海水淡化技术可以充分利用太阳能等低品位热源，具有成本低、设备简单、操作容易、能耗低等优点，在海水及苦咸水淡化方面应用前景广阔。     

膜蒸馏脱盐过程膜润湿的研究进展

膜蒸馏（MD）适合在偏远地区小规模制备淡水,有脱盐率高、接近常压操作、可利用太阳能等优势。但MD膜润湿影响了装置的运行稳定性,是制约MD脱盐应用的重要因素之一。本文介绍了MD膜润湿的评价方法,包括测定透水压力、临界膜孔润湿深度及可视化在线监测膜润湿进程等;简述了膜污染、MD操作变量、膜蒸馏形式及不同结垢晶体各因素对膜润湿的影响;从防垢和强化通量、掌握合适的膜清洗周期、物理干预及提升MD膜性能等方面分析了抑制膜润湿的措施;并分析了一些脱盐实验时间较长的MD膜润湿情况;指出具有自清洁性的光催化膜及防垢性能更强的双疏膜和Janus膜,具有抑制MD膜润湿的巨大潜力。本文有助于对脱盐过程中MD膜润湿的预测和有效控制。