a review of membrane processes and renewable energies for desalination

Desalination 245 (2009) 214–231

A review of membrane processes and renewable energiesfor desalination

Catherine CharcossetLaboratoire d’Automatique et de Génie des Procédés, UMR CNRS 5007, UCB Lyon 1, ESCPE Lyon,

43 Bd du 11 Novembre 1918, 69 622 Villeurbanne Cedex, FranceTel. +33 (4) 72 43 18 67; Fax : +33 (4) 72 43 16 99; email: [email protected]

Received 31 March 2008; Accepted 29 June 2008

Abstract

The growing scarcity of freshwater is driving the implementation of desalination on an increasingly large scale.However, the energy required to run desalination plants remains a drawback. The idea of using renewable energysources is fundamentally attractive and many studies have been done in this area. Membrane processes are alsogaining much interest for their scaled-up ability and their economic feasibility. This article provides a state-of-the-art review on membrane processes associated with renewable energies for seawater and brackish water desalination.The membrane processes include reverse osmosis, membrane distillation and electrodialysis. They are coupled withrenewable energies such as solar, wind, wave, and hydrostatic pressure. This article presents the main results in thisfield including principles, plant design and implementation, mathematical models and economic feasibility.

Keywords: Desalination; Membrane process; Renewable energy; Solar energy; Wind energy; Membranedistillation; Reverse osmosis

1. Introduction

Today, about three billion people around theworld have no access to clean drinking water [1].According to the World Water Council, by 2020,the world will be about 17% short of the freshwater needed to sustain the world population.Moreover, about 1.76 billion people live in areasalready facing a high degree of lacking water.The need for fresh water is at the top of the

international agenda of critical problems, at leastas firmly as climate change. As a consequence ofthe growing scarcity of freshwater, the implemen-tation of desalination plants is increasing on alarge scale.

Generally, desalination processes can be cate-gorized into two major types: (1) phase-change/thermal and (2) membrane process separation.Some of the phase-change processes include

doi:10.1016/j.desal.200 .0 08 6. 200011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved.

C. Charcosset / Desalination 245 (2009) 214–231 215

multi-stage flash, multiple effect boiling, vapourcompression, freezing, humidification/dehumidi-fication and solar stills. Membrane based pro-cesses include reverse osmosis (RO), membranedistillation (MD) and electrodialysis (ED) [2]. ROrequires electricity or shaft power to drive thepump that increase the pressure of the salinesolution to that required. The required pressuredepends on the salt concentration of the resourceof saline solution; it is normally around 70 bar forseawater desalination. MD and ED also requireelectricity. The energy required to run desali-nation plants remains a drawback. Therefore, theidea of using renewable energy sources is funda-mentally attractive.

Renewable energy systems offer alternativesolutions to decrease the dependence on fossilfuels. Renewable energy resources (e.g. solar,hydroelectric, biomass, wind, ocean and geo-thermal energy) are inexhaustible and offer manyenvironmental benefits compared to conventionalenergy sources [3–6]. Each type of renewableenergy has its own advantages that make it suitedto certain applications. Almost none of themreleases gaseous or liquid pollutants during ope-ration. In their technological development, therenewable energies range from technologies thatare well established and mature to those that needfurther research and development.

Among the several possible combinations ofdesalination and renewable energy technologies,some are more promising in terms of economicand technological feasibility than others [7–9].Their applicability strongly depends on the localavailability of renewable energy resources andthe quality of water to be desalinated. Moreover,some combinations are better suited for large sizeplants, whereas some others are better suited forsmall-scale applications. The selection of theappropriate renewable technology depends on anumber of factors. These include plant size, feedwater salinity, remoteness, availability of gridelectricity, technical infrastructure, and the typeand potential of the local renewable energy

resource [7,10]. A number of parameters has to beinvestigated before selecting an appropriaterenewable energy source desalination system(i.e., [11–14]). The first is the evaluation of thewater resources. This should be done both interms of quality and quantity (for brackish waterresource). If brackish water is available, then thismay be more attractive as the salinity is normallymuch lower (<10,000 ppm). In inland sites,brackish water may be the only option. On acoastal site seawater is normally available. Theidentification and evaluation of the renewableenergy resources in the area complete the basicsteps to be performed towards the design of arenewable energy source desalination system.

The purpose of this article is to provide astate-of-the-art review on membrane processesassociated with renewable energies for seawaterand brackish water desalination. The membraneprocesses include RO, MD and ED. They arecoupled with renewable energies such as solar,wind, wave, and hydrostatic pressure. This articlepresents the main results in this field includingprinciples, plant design and implementation,mathematical models and economic feasibility.

2. Membrane distillation

2.1. Principles

Membrane distillation (MD) is a thermallydriven membrane process in which a hydrophobicmicroporous membrane separates a hot and coldstream of water (e.g., [15,16]). The hydrophobicnature of the membrane prevents the passage ofliquid water through the pores while allowing thepassage of water vapour (Fig. 1). The temperaturedifference produces a vapour pressure gradientwhich causes water vapour to pass through themembrane and condense on the colder surface.The result is a distillate of very high purity which,unlike in conventional distillation, does not sufferfrom the entrainment of species which are non-volatile. For desalination processes, the seawater

C. Charcosset / Desalination 245 (2009) 214–231216

(a)

(b)

Fig. 1. (a) Principle of the membrane distillation process.(b) Schematic diagram of an example of a MD system[9,25].

passes on the one side of the membrane at anelevated temperature, for example 80°C. At theother side of the membrane, a lower temperature

for example obtained by cooling the condenserfoil to 75°C, creates a water vapour partial pres-sure difference between the two sides of themembrane and allows the evaporation through themembrane. The water vapour condenses on thelow-temperature side and distillate is formed. MDmay be carried out in various modes differing ina way of permeate collection, the mass transfermechanism through the membrane, and thereason for driving force formation [17]. Varioustypes of MD have been known for several years:direct contact, air gap, sweeping gas and vacuum.

2.2. MD and solar energy

Few demonstration projects using solar ther-mal MD have been built. First, Hogan et al. [15]describes a 0.05 m3/d system using 3 m2 of solarcollectors. Their system, which was tested inSydney, consisted of a hollow-fiber membranemodule for MD and a heat recovery exchangerfor reducing capital costs. This solar-poweredMD unit was found to be technically feasible,being compatible with the transient nature of theenergy source.

The cost of solar thermal MD was evaluatedby several authors. Martínez and Florído-Diaz[18] proposed a model of solar thermal MD basedon a dusty-gas model of gas transport throughporous media. Later, Ding et al. [19] proposed amathematical model that can describe the com-ponents of a solar-powered MD pilot plant. Theirresults showed that the proposed solar-poweredMD pilot plant has some unique features, whichdiffer from a similar MD process operated atsteady-state conditions in a laboratory. Theanalysis of the system revealed that heat recoveryvia an external heat exchanger is effective, and aneconomical way to intensify the process.

At the same time, two solar thermal MD unitswere developed and installed in Jordan through aEC-funded project. A compact unit was installedin the northern part of Jordan (Irbid) and wasoperated with brackish water since September


Fig. 2. Schematic diagram of the solar-driven membrane distillation plant in Aqaba, Jordan. Reprinted from Banat et al.[21], copyright 2008, with permission from Elsevier.

2005 [20], while the second one was installed inthe south of Jordan (Aqaba port) and has beenoperated with untreated seawater since February2006 [21]. Each unit consists of flat plate col-lectors, PV panels, spiral air gap membrane distil-lation module(s), and a data acquisition system(Fig. 2). The effect of process parameters such asbrine temperature and salt concentration wereinvestigated [22]. Recently, the same authorsprovided an economic analysis of these solarthermal MD units [23]. Based on their calcula-tions, the estimated cost of potable water pro-duced by the compact unit is 15 $/m3 and 18 $/m3

for water produced by the large unit. The authorspointed out that membrane lifetime and plantlifetime are key factors in determining the waterproduction cost. The cost decreases with increas-ing the membrane and/or plant lifetime. Koschi-kowski et al. [24] discuss the design and develop-ment of a stand-alone MD system powered by5.9 m2 of corrosion-tie, seawater-resistant,thermal collectors. The maximum of distillate

gain during the test period of summer 2002 wasabout 130 l/d under the meteorological conditionsof Freiburg (Germany).

An interesting alternative (Memstill® process)was developed recently by TNO (Netherlands)for desalination of seawater by air gap MDcarried out in a counter current flow configura-tion [25]. Cold seawater flows through acondenser with non-permeable well wettableevaporator in counter current mode. The wall ofthe evaporator consists of a microporous hydro-phobic membrane through which vapour candiffuse and by which liquid water (with dissolvedsalts) is retained. The condenser and the mem-brane can either be tubular or flat sheets withspacers between the sheets. Meindersma et al.[26] compared the energy and investment costs ofthe Memstill® process and RO for seawaterdesalination capacity of 105,000 m3/d for dif-ferent conditions. The heat supply to the MDprocess was generated by cogeneration of heatand electricity, fuel fired or by a waste heat


source. The total fixed costs for MD were shownto be between 0.16–0.17 $/m3, compared to 0.25–0.35 $/m3 for RO.

In traditional MD, hydrophobic porous mem-branes are used. This requires extensive pre-treatment in order to minimize fouling. Akzo(today Membrana) invested significantly in thistechnology, however abandoned the process dueto intrinsic fouling problems [27]. The apolar orsurface-active molecules in the feed adsorb ontothe hydrophobic membrane materials reducingflux and increasing the chance of wetting themembrane pores. This drastically reduces theselectivity of the process due to leakage of themembrane. Recently, Zwijnenberg et al. [27]proposed a new type of membrane material andconfiguration tested using solar thermal energy,similar to air-gap MD. Contrary to normal MD,the described process uses dense pervaporationtype membranes; the process is called solar-driven pervaporation. A tunnel of a transparentfoil is constructed in which the black membranetubes collect the solar radiation (solar dewprocess). The feed water flowing in the inside ofthe tubes heats up to about 70°C and evaporatesat the outside of the membrane after which itcondenses at the cooler tunnel floor. In order totest the performance, the retention of 30 elementswas tested as a function of time and concentrationof the sea. Retentions over 99.99% were mea-sured under steady-state conditions. The flux ofthe system was about 5 l/(m2 d) when normalizedto collector surface and using a day of 9 h. Theauthors sated that this new process allows the useof feed waters like seawater and brackish waterwithout pre-treatment giving constant fluxes intime and producing high quality water in a singlestep.

MD was not associated with renewablesources other than solar energy, to our knowl-edge. A reason is that MD is a thermally drivenprocess; therefore solar energy can be directlyapplied. Another reason may be due to thelimitation of the ME process itself, requiring

extensive pre-treatment in order to minimizefouling. According to the place where the planthas to be implemented and its renewable energiesresources, other configurations could beevaluated, associating ME to wind energy, hybridsolar PV-wind, or wave energy. In this point ofview, studies already performed on RO and wind,hybrid solar PV-wind, or wave energy could beuseful.

3. Reverse osmosis

3.1. Principles

Among the various desalination technologies,reverse osmosis (RO) is one of the most efficientrequiring about 3–10 kWh of electric energy perm3 of fresh water produced from seawater [28].RO is a pressure-driven process that separatestwo solutions with different concentrations acrossa semi-permeable membrane [29]. The rate atwhich fresh water crosses the membrane is pro-portional to the pressure differential that exceedsthe natural osmotic pressure differential. Themembrane itself represents a major pressuredifferential to the flow of fresh water. The majorenergy requirement is for the initial pressurizationof the feed water. For brackish water desalinationthe operating pressures range from 15 to 30 bar,and for seawater desalination from 55 to 70 bar[30]. As fresh water permeates across the mem-brane, the feed water becomes more and moreconcentrated. There is a limit to the amount offresh water that can be recovered from the feedwithout causing fouling. Seawater RO plants haverecoveries from 25 to 45%, while brackish waterRO plants have recovery rates as high as 90%.

RO system major components include mem-brane modules, high-pressure pumps, powerplant, and energy recovery devices as needed.Two major factors controlling the energy require-ments of an RO system are membrane propertiesand salinity of the feed water. Higher watersalinity requires more energy to overcome the


osmotic pressure, where the RO system needsonly mechanical power to raise the pressure offeed water.

Pre-treatment of seawater feeding RO mem-branes is recognised as a key in designing ROdesalination plants [31]. Depending on severalparameters which influence the choice of the pre-treatment like dissolved organic carbon, SDI,turbidity, algae content and their evolution duringthe seasons, and temperature, the pre-treatmentcan comprise different technologies, such as con-ventional pre-treatment (i.e. ballasted sedimen-tation, air flotation, dual-media filtration, mono-media filtration, double stage filtration) oradvanced technologies including membranescoupled with a conventional process [32,33]. Theuse of an adapted pre-treatment minimizes thefouling problems and can provide good protectionof the membranes and a longer lifetime.

3.2. RO and solar energy

The potential use of solar energy for waterdesalination has been studied extensively [34].Solar energy desalination is generally the col-lecting of solar thermal energy that is used fordesalination directly in solar stills, or that isconverted to electricity first and then used ineither thermal or membrane processes for desali-nation. Photovoltaic (PV) powered RO systemshave been implemented e.g. in remote areas of theEgyptian desert [35], rural areas of Jordan [36],and remote communities in Australia (e.g., [37]).The implementation of PV-powered RO systemswas also evaluated in Agrigento in Sicily [38] andin the small village of Ginostra in Sicily [39].

A preliminary design of a solar thermal-drivenRO system was presented by Bowman et al. [40]for the production of 7.6 to 26.5 m3 of desalinatedwater from brackish water with a salinity of5400 ppm in Saudi Arabia. The RO system ismade of two hollow-fiber modules in parallelwith a total capacity of 79.5 m3 at 2757 kPa with

5400 ppm TDS feedwater and a 75% conversionrate. Using both modules in parallel allows forsingle- or dual-module operation, depending onthe available energy. Simulation results for anaverage March day show that the system wouldoperate in solar-only mode for 8.5 h, producing atotal of 28.2 m3 of desalinated water.

Kalogirou [41], Tzen et al. [42] and Bou-guecha et al. [43] analysed the cost of PV–ROdesalination systems, and Al Suleimani and Nair[44] present a detailed cost analysis of a systeminstalled at Heelat ar Rakah camp of Ministry ofWater Resources, Oman. If PV connected to a ROsystem is commercial nowadays, the mainproblem of this technology is reported to be thehigh cost of the PV cells. The distance at whichthe PV energy is competitive with conventionalenergy depends on the plant capacity, on thedistance to the electric grid and on the saltconcentration of the feed [2].

Thomson and Infield [45–47] simulated andimplemented a PV-driven RO with variable flowthat was able to operate without batteries,designed for Eritrea. They performed laboratorytests to validate the model and control of thesystem: 3 m3/d with a PV array of 2.4 kWp. InSaudi Arabia, a PV–RO brackish water desali-nation plant was installed. It was connected to asolar still with 5 m3/d production. The feed waterof the water still was the blowdown of the ROunit (10 m3/d) [48]. A detailed cost analysis wasalso reported. Joyce et al. [49] proposed a smallRO system running on photovoltaic units, forsmall rural sites or during catastrophes wheredrinkable water is not available. This autonomoussystem can be made using commercially availablesmall RO compact units with typical daily pro-duction on the order of 100–500 l and functioningwith pressures as low as 5 bar. Herold et al. [50]reported the installation of a small RO plantsupplied by a PV power supply, which wasinstalled at the island of Gran Canaria. A briefeconomic analysis shows that the water produc-


Fig. 3. Schematic diagram of the PV-powered UF/NF hybrid membrane desalination system. Reprinted from Richards andSchafer [52]. Copyright 2008, with permission from Elsevier.

tion costs are still high (about 16 $/m3). However,the authors stated that it could be lowered infuture.

A solar-powered direct osmosis process wasproposed by Khaydarov and Khaydarov [51]. Theseparation is driven by natural osmosis, whichdoes not require external pumping energy as inthe RO process. The authors sated that thespecific power consumption of the desalinationprocess is then reduced from approximatively5 kWh/m3 for seawater RO to a value of less than1 kWh/m3. A pilot device including solar batterieswith capacity of 500 W for pumping and solarthermal exchangers for recovery of workingsolution was installed in a village in the Aral Searegion (Uzbekistan).

A particularly interesting prototype usingultrafiltration and RO/nanofiltration combinedwith solar energy was developed in Australia forremote communities which have access to eithercontaminated surface or brackish water [37,52–55] (Fig. 3). Membranes were tested with regardsto flux, recovery, retention, power and specificenergy consumption. The systems provide about1000 l of drinking water per (solar) day, thespecific energy consumption (SEC) was below 5W.h/l when operated above 7 bar. The system isautonomous as it requires no other infrastructureother than a water source. In contrast to othersystems, no batteries are used and consequently,

power fluctuates. The system performance wasevaluated against attributes of social sustain-ability such as the unit’s capacity to meet com-munity water needs (both quality and quantity),the human resources available to operate andmaintain the unit and the community response tothe unit [56].

Other systems for brackish water and seawaterdesalination use an organic Rankine cycle (e.g.,[57–59]). Water circulating inside these tubes isheated to a temperature of 77°C and then sent toa heat exchanger where it generates superheated1,1,1,2-tetrafluoroethane (HFC-134a) vapour[57]. The superheated vapour is used in expand-ing devices which generate the mechanicalenergy necessary for the pumps in the RO system,the water circulation loop and the HFC-134aloop. There is no conversion to electrical energy;therefore, the mechanical energy goes directly tothe expanding devices. The system uses the ROfeedwater as the cooling fluid to condensate theHFC-134a vapour. The prototype of this systemhas a surface area of 240 m2 of evacuated tubecollectors. For a RO unit conversion factor of30% and a 3 kWh/m3 consumption, the estimateaverage annual production is estimated by theauthors to be 1450 m3 of desalined water.

Husseiny and Hametser [60] proposed thedesign of a hybrid RO–ED desalination system.The RO system uses the electricity generated by


a solar concentration system and a Rankine cycle,while the ED system uses electricity generated byphotovoltaic panels. The specific aspects of eachof the subsystems make the overall systemflexible.

A very large range of plants has been pro-posed all over the world. A comparison betweenthe different technologies in terms of costs(implementation, use, maintenance), and produc-tion (recovery rate, flux) is, however, difficult.The prices and the material performances dependgreatly on the place of the implementation, aswell as the year when the project was performed.More work will have to be done for a thoroughevaluation of these various technologies.

3.3. RO and wind energy

Since RO is reported to be one of thedesalination processes with the lowest energyrequirements and coastal areas present a highavailability of wind power resources, wind-powered desalination represents a promisingalternative of renewable energy desalination (e.g.,[61–63]). For example, wind-powered RO plantshave been implemented on the islands of theCounty of Split and Dalmatia (Croatia) [64], onthe island Utsira in Norway (ENERCON project)[65,66], and in remote communities in Australia[67].

Feron [68] was among the first to evaluate theeconomic feasibility of a wind-powered RO plantby mathematical modelling analysis under thefollowing assumptions: intermittent, dependingon wind availability and variable feed water pres-sure, depending on the prevailing wind speed.The author concludes that the economic use of awind-powered RO plant may be restricted toareas with high wind speeds and high fuel prices.He also pointed out that wind-powered ROdesalination could become more economicbecause of current developments: decreasing ROplant costs because of the continuing develop-

ment of membrane science, decreasing windturbine costs, and steady or increasing fuel costs.

Cost analysis of a wind-assisted RO systemfor desalinating brackish groundwater in Jordanwas later conducted by Habali and Saleh [69].The high-pressure pump of the system waspowered by either a diesel engine or a wind-energy converter. The analysis was based onmeasured wind speed distribution and powercurves of the wind-energy converter in Jordan.The authors stated that it would cost less todesalinate brackish water with a wind-assistedRO system than with a conventional diesel-powered system.

An analytical study of utilizing wind-poweredunit for RO desalination was conducted byKiranoudis et al. [70]. Generalized design curvesfor process structural and operation variableswere derived. The study indicated that the unitcost of freshwater production by a conventionalRO plant can be reduced up to 20% for regionswith an average wind speed of 5 m/s or higher.Later, García-Rodriguez et al. [61] analysed theinfluence of the main parameters on the cost offresh water: climatic conditions, nominal powerof the wind turbine, salt concentration of seawateror brackish water, design arrangement, operatingconditions, plant capacity, cost of RO modulesand cost of wind turbines.

Recently, Forstmeier et al. [63] demonstratedthat the costs of a wind-powered RO desalinationsystem are in line with what is expected for aconventional desalination system, proving to beparticularly cost-competitive in areas with goodwind resources that have high costs of energy. Inall these studies, results obtained were theoreticaland not verified by experimental data.

At the same time, the implementation ofseveral wind-powered RO desalination systemprototypes has been reported. A small-scale wind-powered RO system was tested by Robinson et al.[67]. Fresh water production by their system was0.5 to 1 m3/d, which is the estimated volume


needed by a typical remote community inAustralia. A pressure vessel to store the feedwaterunder pressure was included. There was no feed-back control mechanism for the system operation,and when the available wind power was low, asmall diesel or portable gasoline pump was used.

A prototype wind-powered RO desalinationsystem was later constructed and tested onCoconut Island off the northern coast of Oahu,Hawaii, for brackish water desalination [71]. Thesystem has four major subsystems: a multivanedwindmill/pump, a flow/pressure stabilizer, a ROmodule, and a control mechanism. These authorsshowed that at an average wind speed of 5 m/s,brackish feedwater at a total dissolved solidsconcentration of 3000 mg/l and at a flow rate of13 l/min could be processed. The average rejec-tion rate and recovery ratio were 97% and 20%,respectively. Energy efficiency equal to 35% wasshown to be comparable to the typical energyefficiency of well-operated multi-vaned wind-mills.

Miranda and Infield [72] developed a systemwith a 2.2 kW wind turbine generator powering avariable-flow RO desalination unit. Operation ata variable flow allows the uncertainty and varia-bility of the wind to be accommodated withoutneed of energy storage. Batteries, which arecommon in stand-alone systems, are avoided andwater production is dependent on the instan-taneous wind speed.

A prototype of a fully autonomous wind-powered desalination system has been installedon the island of Gran Canaria in the CanarianArchipelago [73]. The system consists of a windfarm, made up of two wind turbines and a fly-wheel, which supplies the energy needs of agroup of eight RO modules throughout the com-plete desalination process (from the pumping ofseawater to the storage of the product water), aswell as the energy requirements of the controlsubsystems. The authors concluded that thissystem can be applied to seawater desalination,

both on a small and large scale, in coastal regionswith a scarcity of water for domestic and/oragricultural use but and wind energy resources.

As for RO associated to solar energy, a verylarge range of plants has been proposed all overthe world. A comparison of the different tech-nologies in terms of costs (implementation, use,maintenance) and production (recovery rate, flux)is again difficult, and further studies are neededfor a thorough evaluation.

3.4. RO and hybrid solar PV-wind power

The complementary features of wind and solarresources make the use of hybrid wind–solarsystems to drive a desalination unit a possiblealternative. RO and hybrid solar PV-wind powersystems have been designed and implemented,e.g. in the rural areas of the Sultanate of Oman[74], in Israel [75], in the northern part ofMexico, in a small island on the German coast ofthe North Sea [76], and at the site of BorjCedriaon the southern suburbs of Tunis city [77]. Theimplementation of a plant was evaluated inAgrigento (Sicily) [38].

Two RO desalination plants using a GKSS(Germany) plate module system supplied by a6 kW wind energy converter and a 2.5 kW solargenerator have been designed for remote areas[78]. Two of these prototypes were installed inthe northern part of Mexico and in a small islandon the German coast of the North Sea [76].

A hybrid wind/photovoltaic power unit con-nected to a RO desalination plan was imple-mented on Libya’s coast of the MediterraneanSea [79,80]. The nominal production of the plantwas intended to be 300 m3/d for the supply of avillage with potable water. The facility designwas flexible for the integration of a diesel gene-rator and electrochemical storage.

Mohamed et al. [81,82] presented the designof a stand-alone hybrid wind-PV system to powera seawater RO desalination unit, with energy


recovery using a simplified spreadsheet model. Adaily and monthly simulation and economicanalysis were also performed. The calculatedfresh water production cost was 5.2 €/m3, and therealized energy saving was close to 50% when apressure-exchanger-type energy recovery unitwas considered.

Gilau and Small [1] analysed the cost-effectiveness of sweater RO system using windand solar radiation as renewable energy sources.Using the wind and solar conditions of Eritrea,East Africa, the hourly water production wascomputed with a capacity of 35 m3/d, a specificenergy consumption of about 2.33 kW h/m3,which is a lower value than that achieved in mostof the previous designs.

Recently, a floating island was proposed(DESIRES®: DESalting Island on Renewablemulti-Energy Supply) [83]. The plant included:(a) an artificial, floating island 10–100 km fromthe shore, 0.06–0.65 km2 in size with hexagonalshape, 0.1–1 km in diameter and 20 m deep; (b) acombination of renewable energy sources (eolian,solar, tidal, wave and hydrothermal gradient);(c) the use of a storage reservoir aboard forstabilisation and coping with fluctuations inenergy supply and water demand; (d) the use ofpeak energy supply (during storm events) topump stored fresh water to land and to pressurizethis water to generate hydro-power on land;(e) RO; and (f) technological solutions to reduceenergy consumption and maintenance. The plantwas estimated to produce 5–500 Mm3/year ofhigh-quality fresh water from seawater at a costof 0.88–1.32 $/m3. Although very complete andoriginal, no plant was, to our knowledge,implemented.

3.5. RO and wave energy

Most of the works on wave energy conversionhave focused on electricity production [84]. Anysuch converter could, in principle, be coupled toelectrically-driven desalination plant, either with

or without connection to the local electricity grid.Various concepts have associated wave energyconverter and RO.

The first reported technology (Delbuoy) usedoscillating buoys to drive pistons pumps anchoredto the seabed [85]. These pumps fed seawater tosubmerged RO modules. Mathematical model-ling, wave tanking testing and sea trials in PuertoRico were conducted [84]. The authors stated thatthe Delbuoy system was especially useful in areasthat are remote, have insufficient or unreliablepower supplies or have high power costs. Thereason the project did not continue may be due tothe relatively inefficient means of wave energyconversion. However, it could benefit from sim-plicity and scalability and work as a broadbandabsorber that does not need to be tuned toparticular oceanic conditions [84].

A second technology consisting of a three-section hinged barge was developed in the Shan-non Estuary (Ireland) [86]. The two oscillatingarms of the floating barge are attached symme-trically to a central section, which is inhibitedfrom pitching by an underslung inertial dampingplate. Large forces are therefore developedbetween the arms and the centre section. Theseforces are harnessed by means of pistons,pumping either hydraulic oil, for conversion intoelectrical power, or seawater for feeding ROunits. The author concluded that this system maybe primarily developed to produce potable waterfor remote locations.

Another technology, the oscillating watercolumn device, was installed at Vizhinjam, Indiain 1990 [87]. The device was constructed on aconcrete caisson connected by a pier to the shore.It works on the principle of a column of air beingcompressed and decompressed with the rise andfall of the waves. A turbine extracts energy fromthe air column. The desalination plant can be runusing either the supply from wave power or,during low wave conditions, by electricity boardsupply or a diesel generator to ensure a con-tinuous supply of fresh water. The plant delivers


between 4 and 10 m3/d of freshwater, dependingon the period of operation. The Vizhinjam systemmay be envisaged as a solution for small coastalcommunities.

Sawyer and Maratos [88] propose anotherconcept that uses the water hammer effect togenerate large intermittent pressures, by means ofa valve that opens and shuts at the end of thepipe. The pressure developed depends on thecompressibility of the water and the elasticity ofthe pipe wall. The authors show that it is theo-retically feasible to use the water hammer effectto develop pressures sufficient to drive RO. Thetechnology is very similar to the hydro-ramwidely used to lift irrigation water from rivers,although hydro-rams usually generate lowerpressures than those required for RO. An eco-nomical feasibility study of the concept waspresented and costs were shown to be potentiallyfavourable compared to conventional RO plant.

Very recently, Folley et al. [89] proposed adesalination plant consisting of RO membranestogether with a pressure exchanger-intensifier forenergy recovery. A numerical model of the com-bined wave-power and desalination plant showsthat it is possible to supply the desalination plantwith sea-water directly pressurised by the waveenergy converter, eliminating the cost and energylosses associated with converting the energy intoelectricity and back to pressurised water.

Other projects on water desalination associ-ating RO and wave energy should probably beavailable in the next future, as wave energy isgaining in popularity. A main challenge will beagain the economics of the plants.

3.6. RO and hydrostatic pressure

The potential exploitation of the hydrostaticpressure of seawater at a sufficient operativedepth was considered by several investigatorsfrom the 1960s in view of increasing the energyefficiency of the then developing RO industrialdesalination technology (i.e., [90,91]). More

recently, several configurations were proposedfor fresh water production from seawater usingRO and hydrostatic pressure: submarine, under-ground and ground-based [92,28].

In conventional surface-based industrialdesalination plants applying RO technology, thefreshwater flow at the membrane outlet isapproximatively 20–25% of the inlet seawaterflow, depending on membrane type and charac-teristics. The resulting brine is disposed off thesea. While RO installations generate the requiredpressure with high-pressure pumps, the sub-marine approach uses seawater hydrostaticpressure. The desalinated water, produced atabout atmospheric pressure and collected in asubmarine tank at the same working depth, ispumped to the sea surface. It was shown that thisapproach saves about 50% of the electricity con-sumption with respect to an efficient conventionalRO plant (about 2–2.5 kWh/m3) since only theoutlet desalinated water is pumped instead of theinlet seawater, thus reducing the pumping flowrate by 55–80% [93]. The advantage of thisconfiguration is also to avoid the pre-treatment ofthe inlet seawater, therefore saving costs forchemicals and equipment.

Al-Kharabsheh [94] also proposed a ROdesalination system utilizing hydrostatic pressure.The system consisted of a storage tank, connect-ing pipes, RO module with a moving hollowpiston, filter boxes, seawater storage tank, pump,and valves. The storage tank is to be to be placedat the top of a mountain and connected to the ROmodule that is placed slightly above the sea level.An energy efficiency analysis shows that thesystem energy requirement is 0.85 kWh/m3 offresh water produced from seawater, which ismuch less than that required by conventional ROplants, usually 3–10 kWh/m3 of fresh waterproduced from seawater.

Despite several patents on this technology(e.g., [95–97]), no installation combining RO andhydrostatic pressure has been implemented to ourknowledge. More efforts on these technologies,


especially on their economic and practicalaspects, should appear in the future.

4. Electrodialysis

4.1. Principles

Electrodialysis (ED) has been in commercialuse for desalination of brackish water for the pastthree decades, particularly for small- andmedium-scale processes [98]. The process utilizesan electric field to remove the salt ions in thebrackish water which passes between pairs ofcation-exchange and anion-exchange membranes(Fig. 4). The cations migrate from the brackishwater towards the negative electrode through thecation-exchange membranes which allow onlycations to pass. On the other hand, the anionsmigrate towards the anode through the anion-exchange membranes. In a conventional process,a large number of alternating cation-exchangeand anion-exchange membranes are stackedtogether, separated by flow spacers which areplastic sheets that allow the passage of water. Thestreams in alternating flow spacers are a sequenceof diluted and concentrated water which flow inparallel to each other. To prevent scaling, theprocess utilizes inverters which reverse thepolarity of the electric field about every 20 min.This process is called electrodialysis reversal(EDR).

4.2. ED and solar energy

The use of PV cells with ED is attractive forareas in which solar energy is available through-out the year and has been reported by severalauthors. Lundstorm [99] was the first to present asmall-scale process for water desalination usingsolar-powered ED. Later, Ishimaru [100] studiedthe reliability of an ED system operated byphotovoltaic cells in a remote area of Japan todesalinate feed water with a TDS value of1500 ppm. The 200 m3/d unit was reported to

produce drinking water satisfactory qualityduring the 2-year period of study.

Veza et al. [101,102] tested an ED desali-nation plant to treat brackish water while drivenfrom an off-grid wind energy system, located inGran Canaria Island (Spain). The unit includedpower converters for the membrane stacks andvariable frequency drivers for the feed pumps. Anumber of tests were carried out showing goodflexibility in the same way as a plant connected tothe grid would do.

AlMadani [98] developed an experimentaldevice of ED associated with photovoltaic cells.The stack consisted of 24 cell pairs, arranged infour hydraulic stages and two electrical stages.The influence of process parameters (flowrates,temperature) was studied with aqueous NaClsolutions, as well as natural groundwater ofmedium salinity. Salt removal of 95% for ground-water and 99% for NaCl solutions was obtainedat low product flowrates of 550 dm3/d.

Ortiz et al. [103] developed a mathematicalmodel that allows predicting the functioning of anED system powered by photovoltaic energy. Theapplication of the model allows the design of thesystem: electrodialyser size, number and con-figuration of the PV modules for the desalinationof brackish water, as well as the study of itsbehaviour in different geographical locations. Themodel has been compared successfully to thedesalination of NaCl solutions [104]. The authorsdrew conclusions on the interest of desalinationof brackish water by means of ED powered byPV energy in remote areas where the volume ofdaily treated water required is small (about 1 to10 m3/d).

ED, as MD, was only associated to solarenergy. This may be due to the limitation of theED process itself, with implies higher energyconsumption and more complicated operationsthan RO. According to the location of the plantand its renewable energies resources, other con-figurations could be evaluated, associating ED towind energy, hybrid solar PV-wind, or wave


Fig. 4. Principle of the electrodialysis process [29].

energy. Studies already performed on RO andwind, hybrid solar PV-wind, or wave energycould be useful.

5. Conclusions

Many original solutions for desalination usingmembrane processes and renewable energies havebeen proposed. RO is most often chosen as it isone of the most efficient in terms of energy con-sumption. Some RO plants are particularly suitedfor small communities in remote locations,although others may find large-scale applications.Although a very large amount of work has beenconducted in this field (including plant designand implementation, mathematical models, andeconomic feasibility), only a few are currentlybeing used.

Most of the desalination plants are proposedfor the purpose of providing drinking water tosmall communities, especially remote ones. Many

places all over the world are concerned, includinge.g., the Egyptian desert [35], rural areas ofJordan [36], remote communities in Australia(e.g., [37]), Sicily [38], Ireland [86], and India[87]. This undoubtedly confirms interest devotedto water desalination associated to renewableenergy sources in these parts of the world wheresun and/or wind are particularly abundant. Forthese communities, the economics of the plants(installation, operation) play a major role.Although many works have considered theeconomics of these plants, further evaluationsshould include the recent cost evolution (mem-brane devices, wind turbines and flywheels,photovoltaic arrays, etc.), and an appropriateselection of materials for the places under con-sideration. From this point of view, multidisci-plinary studies including chemical engineering,material engineering and geographic sciencescould provide a more complete picture.

Another crucial aspect is the social integrationof the desalination plants. The plants have to be


properly designed to be used in the communityunder consideration. From this point of view, thestudy of Werner and Schäfer [55] is a very com-plete one. A prototype using ultrafiltration andRO/nanofiltration combined with solar energywas developed in Australia for remote com-munities which have access to either contami-nated surface or brackish water (e.g., [37,51,54]).The system performance was evaluated againstattributes of social sustainability such as theunit’s capacity to meet community water needs(both quality and quantity), the human resourcesavailable to operate and maintain the unit and thecommunity response to the unit [55]. Other workson social aspects of the plants implementationshould be performed in other places all over theworld.

Membrane desalination associated with re-newable energies is undoubtedly valuable in thesetimes where fresh water and fuel resources aredecreasing. The potential applications of mem-brane desalination associated with renewableenergies for both industrial and developingcountries may be a good example of strategies forengineering development: advancing technology,prioritising people [105]. Following Ricoeur[106], Bowen [105] recently recalls a simpleexpression of the attitude that the engineer couldadopt in applying his or her skills: “Here I am,how can I help you?”, which could be appliedhere.

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a review of membrane processes and renewable energies for desalination

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