sustainable fuel cell integrated membrane desalination systems

Presented at the First Oxford and Nottingham Water and Membranes Research Event, 2–4 July 2006, Oxford, UK.

Desalination 227 (2008) 14–33

Sustainable fuel cell integrated membrane desalination systems

Rajindar SinghSiemens Water Technologies Corp., Colorado Springs, CO, USA

Tel. +1 (719) 550-2208; email: [email protected]

Received 22 May 2007; Accepted 25 June 2007

Abstract

According to the United Nations, between two and seven billion people will face water shortages by the year2050. Further, it is estimated that the amount of water available per person will shrink by a third during the next twodecades. Inadequate supply of good-quality water coupled with higher water demand due to rapid population growthand industrialisation in developing countries are among the major reasons for the worsening water situation. Currentshortages of potable water around the world and looming water scarcity especially in the developing countries is thedriving force behind the implementation of membrane technologies for seawater and brackish water desalination.Typical energy consumption in seawater reverse osmosis (RO) plants operating at 40–45% product water recoveryand with energy recovery from the high pressure reject stream currently is about 3–4 kWh/m3. The near-term goalof the industry is to reduce energy consumption to less than 2 kWh/m3 by using a combination of energy efficientRO pumps, more efficient energy recovery devices, high performance low energy RO membranes, hybrid membranesystems, advanced pretreatment technologies and alternate energy integrated membrane systems. The beneficialaspects of using alternate energy systems such as on-site distributed fuel cell systems integrated with membranedesalination units in remote locations are discussed.

Keywords: RO; Fuel cells; Desalination; Hybrid; Membrane; UN

1. Introduction

Scarcity of water resources is often the limit-ing factor for economic and social development.According to the UN Committee on NationalResources, about 80 countries comprising 20% ofworld’s population are suffering from serious

water shortages today, a situation that will getmuch worse by 2025 [1]. Desalination — thermaland membrane — supplies potable water on theorder of only 0.1% of overall fresh water usage.Current total installed seawater desalination plantcapacity in the world is about 17 Mm3/d. Total

doi:10.1016/j.desal.2007.06.0190011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.

R. Singh / Desalination 227 (2008) 14–33 15

desalination plant capacity is projected to in-crease from 40 Mm3/d in 2006 to 96 Mm3/d in thenext 10 years, of which more than half is forecastfor membrane systems [2,3]. Membrane desalina-tion vis-à-vis thermal desalination has grownrapidly in the last decade for several reasons:C Energy consumption for seawater RO

(SWRO) is about one-half of multiple effectdistillation, the most efficient thermal desali-nation process.

C High reliability membrane operation withgreater than 95% on-line factor because oflonger membrane life and effective pretreat-ment. Membrane systems are also compactand modular.

Typical energy consumption in SWRO plantsoperating at 40–45% product water recovery andwith energy recovery from the high pressurereject stream currently is about 3–4 kWh/m3.Efforts are underway to reduce the energy con-sumption of SWRO to less than 2 kWh/m3 inorder to make SWRO desalination competitiveworldwide [4,5]. The theoretical limit is about1.2 kWh/m3 due to several limiting factors includ-ing osmotic pressure, product water recovery andconcentration polarisation.

One option for reducing energy consumptionis to use integrated systems, e.g. a thermaldesalination system coupled with a single-stageSWRO system. The rationale for developing inte-grated energy-water systems is to reduce capitalcost, energy consumption and the cost of desali-nating seawater since 50–60% of a RO systemoperating cost is due to energy consumption.Further, by blending higher total dissolved solids(TDS) RO product water (200–400 ppm) withthermal distillate (25 ppm), water production canbe more economical for potable water production.Cogeneration using dual-purpose plants thatprovide both electricity and waste heat for heat-ing RO feed water is another attractive option.Such integrated membrane systems can providewater when the electricity needs are low [6,7].

Desalination systems powered by conven-tional energy sources, however, can have signi-ficant operating costs resulting from a largevariability in energy costs because of fuel pricevolatility. Because of the scarcity of water inremote areas, high energy consumption andenvironmental pollutant emissions from conven-tional power plants using fossil fuels, renewableenergy based small desalination systems such assolar distillation have been built in the last30 years. Renewable energy sources such as windpower and solar energy have been successfullyintegrated with RO plants of small to mediumcapacity since 2000 [8,9]. From an environmentalpollution point of view, 1 kWh of energy pro-duced by wind generators eliminates the emissionof 0.6 kg of CO2, 1.33 kg of SO2 and 1.67 kg ofNOx from fossil fuel power sources [8]. Thelargest wind-powered membrane desalinationplant was commissioned in Perth, Australia, in2007 [10]. Wave-powered desalination [11] andfuel-cell-powered desalination are among thenewest alternate energy sources investigated[12,13]. The efficacy of integrated fuel cell mem-brane desalination systems for reducing energyconsumption, thereby, reducing the cost ofproducing potable water from brackish water andseawater, especially, in remote locations indeveloping countries, is discussed in this paper.

2. Membrane desalination

Water shortages around the world have pro-vided an impetus for the construction of mediumto very large (0.3 Mm3/d) SWRO desalinationplants since 2000. Some of the recent largeSWRO plants in operation [2,3,5,14] includeAshkelon, Israel (325,000 m3/d); Fujairah, UAE(170,000 m3/d); Perth, Australia (140,000 m3/d);Tuas, Singapore (136,000 m3/d); Carboneras,Spain (120,000 m3/d); Point Lisas, Trini-dad(115,000 m3/d) and Tampa Bay, USA Point(95,000 m3/d). In addition, a 200,000 m3/d

R. Singh / Desalination 227 (2008) 14–3316

SWRO plant is under construction in Hamma,Algeria. Several large SWRO plants are in theworks in USA and Australia including a500,000 m3/d plant near Sydney, Australia [15].

Membrane desalination costs have droppeddue to the development of high rejection and highproductivity membranes (development of achlorine-resistant polyamide membrane, however,remains elusive), improved process controls, andlower energy consumption. High productivityelements consisting of higher surface area anddenser membrane packing yield higher quantitiesof product water. Additional cost savings arepossible with newer RO feed water pretreatmentprocesses for example replacing conventionalpretreatment with membrane filtration (ultra-filtration and microfiltration), developing moreefficient RO pumps and higher efficiency energyrecovery devices, and employing novel plantdesign configurations [4,6,16–21]. For example,it has been demonstrated in bench-scale tests thatmulti-pass/multi-stage arrays encompassing nano-filtration (NF) and RO integrated membrane sys-tems reduce energy consumption [22]. Althoughthe capital costs for membrane filtration are 20–30% higher than conventional pretreatment, itproduces guaranteed higher quality (less fouling)RO feed water including SDI <3, turbidity <0.1NTU and high retention/rejection of micro-organisms (bacteria and algae), thereby ensuringlower operating and maintenance costs.

Brackish water desalination was the firstsuccessful application of RO with the first large-scale plant built in the late 1960s using celluloseacetate (CA) membranes. Nearly all the brackishwater desalination plants are either RO or electro-dialysis membrane based. There are more than1,000 brackish water membrane desalinationplants in USA. The largest brackish water RO(BWRO) plant in the US (Yuma, Arizona) wasbuilt in 1991 and operated in 1992. Although ithas been on “ready reserve” since 1993, it wasoperated again in spring 2007 for 90 days suc-cessfully. The 270,000 m3/d BWRO plant is

designed to treat agricultural drainage water, anddivert the RO product water to the ColoradoRiver to lower the river’s salinity before it flowsto Mexico [23]. The RO plant consists of 9360CA membrane elements as follows: approxi-mately two-thirds of the spiral-wound elementsare Koch (Fluid Systems), 30 cm diameter ×150 cm long with a surface area of 117 m2, andone-third are Hydranautics, 20 cm diameter ×100 cm long with a surface area of 33 m2.

Membrane desalination has become a viabletechnology for several reasons: C A substantial reduction in the cost of desali-

nated seawater from US $1.75/m3 to US<$1.00/m3 in the last 20 years. The cost forBWRO is US $0.25–0.75/m3.

C Substantial reduction in specific energy usagefrom 8 kWh/m3 in 1980 to 3–4 kWh/m3 today.The figure for BWRO is 0.75–1.0 kWh/m3.

C Membrane life of up to 5 years and reliableperformance with minimal supervision.Some of the salient features of SWRO and

BWRO desalination processes that determineplant design, and influence capital and operatingcosts are summarised below [17]. C Feed pressure: 50–70 bar g for SWRO vs. 20–

30 bar g for BWRO.C Feed water TDS: seawater = 32,000–44,000

mg/L; brackish water = 1,000–15,000 mg/L.C Feed water quality: seawater, fairly consistent;

brackish water, highly variable (see Table1).C Feed water treatment: Relatively simple for

SWRO because of low water recovery, highionic strength, low TOC level (<1.0 mg/L)and low concentration of bicarbonate ions (seeFig. 1); relatively complex for BWRO (seeFig. 2).

C Product water recovery: 35–50% in the caseof SWRO vs. 70–85% in the case of BWRO.SWRO permeate flux: 10–12 L/m2.hr vs. 17–20 L/m2.h for BWRO.

C Product water TDS: 300 ppm (33,000 ppmseawater) vs. <80 ppm (2,500 ppm brackishwater). SWRO salt rejection >99.7%.


Fig. 1. A seawater RO plant process flowschematic with product water post treatment.Pretreatment may also include coagulantdosing, dual-media filtration, and/or anti-scalant dosing.

Table 1Composition of representative brackish waters and seawater

Item/Iona WeltonMohawk, AZ

Coalinga,CA

Tularosa,NM

FortMorgan, CO

Rio Grande,TX

OrangeCounty, CA

Seawater

Cations:Calcium 204 129 420 299 163 140 400Magnesium 91.7 89 163 36 51 10 1252Potassium 8 — 2.3 — — 35 —Sodium 804 521 114 216 292 300 10,561Strontium 3.1 — — — — — —Anions:Bicarbonate 433 161 270 334 275 275 140Boron — 2.8 0.14 — 0.23 0.8 4Chloride 1090 262 170 87 492 350 18,980Fluoride — — — — 0.08 0.8 1.4Iron 0.35 1.2 — 0.02 0.03 0.3 —Manganese 0.48 — 0 — 0.03 0.1 —Nitrate — — 10 25 1.5 1.0 1.5Phosphate 0.13 — — — — 1.0 —Sulfate 902 1260 1370 960 336 350 2650Other: — —Silica (SiO2) 32 49 22 — 35 10 —pH 7.95 7.7 7.2 7.4 8.1 8.0 7.2TDS 3628 2478 2410 1880 1611 1400 33,990Alkalinityb 355 133 221 274 226 226 —Hardnessb 850 692 1750 898 616 391 —aIon as mg/L; bIon as CaCO3.


Fig. 2. Brackish water RO plant process flow schematic with various pretreatment options.

18


SWRO product water specifications: TDS<400 mg/L; chloride level <100 mg/L; andWHO standard boron level of <0.5 mg/L.Meeting boron levels is critical in SWROplant design.

C Membrane array design: Usually one-pass,single-stage for SWRO (see Fig. 3) because oflow recovery vs. single-pass, two-stage forBWRO.

C Critical parts material: High corrosion resis-tant duplex stainless steel for SWRO plants.

In SWRO plants large flow rates of concen-trated brine are discharged at high pressure. Thispressure energy recovery from the high-pressurebrine stream is critical to the economic viabilityof the seawater membrane desalination process.One of the first major studies addressing energyrecovery was reported in 1969. Subsequently, amajor feasibility study on energy recovery inSWRO and BWRO plants was done in 1980 [24].Typically, in the case of SWRO plants 25–40%of energy can be recovered depending on the typeof recovery device used [4,5,14]. Currently, themajority of energy recovery devices use PeltonWheel turbines, which transform potential energyin the brine stream to high pressure RO pumpshaft power as shown in Figs. 3 and 4. Theadvantage of the Pelton wheel is the flat effi-ciency curve in a wide range of concentrateflows. The largest Pelton Wheel system in theworld is installed at the Point Lisas plant inTrinidad. These turbines achieve efficiencies of88–90%. A more recent energy recovery type isthe isobaric chamber device or pressure exchan-ger, which facilitates the exchange of energy inthe brine stream directly to the incoming seawateras shown in Fig. 5. Since the pressure exchangersare positive displacement devices (direct energytransfer), high energy recovery efficiencies of94–96% are achievable. The pressure exchangersystem can maintain these high efficiencies inde-pendent of the operating conditions that need tobe adjusted with time to maintain the same

product water recovery. However, there is onedrawback; because of mixing of feed and brine,the RO feed water TDS increases from 4 to 6%[5,21].

3. Membrane performance

The choice of a membrane and membraneelement is usually determined by the compositionof feed water (due to its fouling potential) andproduct water quality. Homogenous asymmetricCA membranes (blends of cellulose acetate andcellulose triacetate) and polyamide (PA) mem-branes are the most commonly used in RO andNF plants. The data in Table 2 show that CAmembranes have a neutral charge, are morehydrophilic, less prone to fouling, and can tole-rate low levels of chlorine. Because of theseproperties, CA membranes are often preferred inwastewater applications where the SDI is high,and in potable water purification where residualchlorine is required for example cellulose tri-acetate hollow fibre membranes are often used inseawater desalination applications when bio-fouling is a potential problem. Polyamide mem-branes have higher rejection and flux, higherdurability than CA membranes but have verylimited tolerance to chlorine. Spiral-wound (SW)membrane elements are the work-horse in ROand NF membrane applications. SW modules arecompact and inexpensive. The flow regime in SWelements is turbulent with superficial velocitiesranging between 10 and 60 cm/s, correspondingto Reynolds (Re) numbers of 100–1300.Although these Re numbers indicate laminarflow, turbulence is due to the mesh spacers in thefeed channel. Standard SW modules are 20 cm.diameter × 100 cm long with a membrane surfacearea of 440 ft2 (30–40 m2). Larger SW modules,40 cm × 150 cm, with a very large surface area(~220 m2) have been developed. The installedcost of an RO plant can be reduced by up to 27%using large diameter elements [25].

Fig. 3. Process and instrumentation diagram for a seawater RO one-pass, single-stage membrane system with an energy recovery turbine and a drawbacktank.

20R. Singh / D

esalination 227 (2008) 14–33


Fig. 4. Process flow diagram of ahydraulic turbine energy recoverysystem.

Fig. 5. Process flow diagram ofa pressure exchanger isobaricchamber energy recovery system.

The flux of a solvent through a membrane isdirectly proportional to the applied pressure [17].For liquids other than pure water, however, thisproportionality does not exist as shown in Fig. 6due to fouling and/or concentration polarisation.Fouling causes a loss in water flux and quality,reduced operating efficiency, lost service time,more frequent cleaning, premature membranereplacement, and higher operating costs. Fouling

is often the result of a strong interaction betweenthe membrane and the components in the feedstream, for example fouling by colloids, silica,iron and biomaterials can be especially severe. Asa rule, a flux reduction that is reversible is due toconcentration polarisation, whereas an irrever-sible flux reduction is due to fouling. Flux drop isusually recoverable following cleaning. Similarly,the precipitation of sparingly soluble salts results


Table 2Characteristics of CA and PA membranes

Property Regenerated CA Polyamidea

Membrane type Homogenous asymmetric Homogenous asymmetric; thin-film compositeSalt rejection, % ~95 > 99 (>99.7% for SWRO)pH range 4–6 1–13Feed pressureb, bar g 15–30 10–20Hot water sanitisable (85EC) No Yesc

Surface charge Neutral AnionicChlorine (oxidants) tolerance, ppm 1–2 <0.1Pre-treatment High Very strict Organics removal Good EffectiveBiological growth Problematic NoFouling tolerance Good FairSurface roughness Smooth RoughMembrane module types dSW, FP, HF, T SW, FP, HFe, Te

aRO and NF membranes only; bNot for brackish and seawater desalination; c TFC membranes only;dSpiral-wound, flat-plate, hollow-fibre, tubular; eNot applicable to TFC membranes.

Table 3Sources of membrane fouling

Substance Extent and/or mechanism

Fe, Mn, Al hydroxides Severe fouling, rapid kineticsMineral saltsa Form mineral scales when their solubilitiy is exceededColloids Electrically charged; SDIb and zeta potential determine foulingMicrobiological Forms a biofilm gel layerProteins Fouling by hydrophobic and charge interactionsPolyelectrolytes Fouling by charge interactionOrganic acids Humic and fulvic acids cause severe foulingOil and grease Hydrophobic membrane foulingSuspended solidsc Cannot exceed 0.5 ppm

aCaCO3, CaSO4, BaSO4, SrSO4; bSilt Density Index; cApplicable to RO and NF.

in scaling, which can be prevented by a well-designed pretreatment system and minimised bydefining operating conditions. Hence, membranesystems are typically operated under constant fluxconditions by increasing feed pressure with time.Substances that cause fouling and scaling aregiven in Table 3 [17] with an approximate break-

down as follows: (a) organics, 48%, (b) silica,20%, (c) iron oxide, 7%, (d) aluminium oxide,6%, (e) calcium sulphate, 3.5%, (f) calciumcarbonate, 1.5%, (g) others, 14%. Fouling controltechniques are given in Table 4. Pretreatmentcosts typically vary between 15 and 30% of theoverall plant cost.


Fig. 6. Membrane flux characteristics of a thin-film composite PA membrane, spiral-wound RO module for various feedwaters.

Table 4Treatment methods for controlling fouling

Foulant Fouling control

General Hydrodynamics/shear, operation below critical flux, chemical cleaningInorganic (scaling) Operate below solubility limit, pretreatment, reduce pH to 4–6 (acid addition), low

recovery, additives (antiscalants); some metals can be oxidized with oxygenOrganics Pretreatment using biological processes, activated carbon, ion exchange (e.g. MIEX),

ozone, enhanced coagulationColloids (<0.5 µm) Pretreatment using coagulation and filtration, MF, UFBiological solids Pretreatment using disinfection (e.g. chlorination/dechlorination), filtration, coagulation,

MF, UF

4. Fuel cell alternate energy

Fuel cells are electrochemical engines thatconvert the available chemical free energy in afuel, usually hydrogen and oxygen, to electricalenergy directly without going through the heat-exchange process. Gaseous fuel is fed continu-ously to the anode where it gets oxidised and the

oxidant — air or oxygen — is fed continuously tothe cathode where it gets reduced. Electro-chemical reactions take place at the electrodes toproduce an electric current in the external circuit.The electrolyte separating the anode from thecathode conducts ions between the electrodescompleting the electric circuit. The overall


Fig. 7. Schematic process flow diagram of a PAFC system.

reaction of the electrochemical process with waterand heat as the only by-products is: H2 + ½O2 =H2O + Heat.

Since the chemical energy is directly trans-formed into electricity, the theoretical efficiencyis not limited by the Carnot cycle as it is in thecase of conventional power plants. In theory, it ispossible to construct a fuel cell of 80% to 90%efficiency. In practice, because of irreversiblelosses (over-potentials), the efficiency of a fuelcell system is 40% to 45% based on the lowerheating value of the fuel. However, efficiencies of80% have been achieved for fuel cell powerplants with cogeneration, i.e. combined heat andpower systems, and hybrid fuel cell-reheat gasturbine cycles have efficiencies approaching70%. Importantly, since fuel cells operate atnearly constant efficiency independent of size,small plants operate nearly as efficiently as largeones [26].

Fuel cells power plants are also environ-mentally benign; emissions of sulfur dioxide

(SO2) and nitrogen oxide (NOx) are nearly 1,000times lower than those of fossil-fuel power plants,and the carbon dioxide (CO2) emissions profile isbetter based on overall plant efficiency. In fact,the overall emissions are so low that fuel cellplants are exempt from air permitting in the SouthCoast and Bay Area Air Quality ManagementDistricts in California — the most stringent limitsin the United States. Similarly, a 200-kW fuel cellsystem running on anaerobic digester gas in NewYork City has negligible emissions; <0.5 ppm ofCO, <0.5 ppm volatile organic compounds,<1 ppm SO2 and <0.4 ppm NOx [27].

There are several types of fuel cells dependingon the electrolyte and operating temperature [26]:C alkaline fuel cell (AFC at 50–90EC); C direct methanol fuel cell (DMFC at 50–

120EC);C polymer electrolyte fuel cell (PEMFC at 80–

120EC);C phosphoric acid fuel cell (PAFC at 180–

210EC);


C molten carbonate fuel cell (MCFC at 600–650EC); and

C solid oxide fuel cell (SOFC at 800–900EC).

Of these, PAFC has been commercialised forstationary applications, and PEMFC is the mostattractive for vehicular systems. Both MCFC andSOFC are best suited for dual heat and powerapplications. The single cell potential for mostfuel cells is 0.6 to 0.8 V (d.c.), e.g. it is 0.65 V ata current density of 160 mA/cm2 at 200EC for aPAFC, and 0.75 V at a current density of430 mA/cm2 at 85EC for a PEMFC [26].

A typical fuel cell power plant consists of afuel processor that reforms the fuel, e.g. naturalgas or methanol to hydrogen, a multi-cell fuel cellstack (5–500 kW), a power conditioner that con-verts fuel cell d.c. power output to a.c. power, anda heat exchanger (for heating during start-up andfor removing heat due to irreversible lossesduring operation). The electrochemical cell stackis thus a small but vital component of a fuel cellpower plant as shown in Fig. 7 [26].

5. Fuel cell RO desalination process integration

Integration of fuel cells with desalination unitshas been investigated in the last few years[12,13]. Specifically, different configurationshave been evaluated in which a fuel cell stackprovides electricity to a RO unit, while stackwaste heat is recovered via a heat exchanger forpreheating seawater feed to a multi-stage flashthermal desalination unit and/or the RO unit.These studies have shown that regardless of thetype of desalination process (membrane or ther-mal), when fuel cells are integrated with desali-nation units, waste heat and power generated bythe fuel cell is efficiently utilised by the desaltingprocess.

A schematic diagram of a fuel cell-RO hybridsystem is shown in Fig. 8 where the fuel cellstack waste heat is used for pre-heating salinefeed water. According to one analysis [12],

energy consumption for desalination is reducedby 8–10% due to lower feed pressure when thefeed water temperature is increased from 20EC to28EC. Higher feed water temperature also resultsin higher productivity (a rule of thumb is thatpermeate flow rate increases 3% per oC rise intemperature as a result of reduced viscosity), e.g.a 100,000 m3/d product water desalination plantoperating at 39% product water recovery at 20EC,and 43% recovery at 28EC. Higher productivity,in turn, means fewer membrane elements toachieve the same product water flow rate, result-ing in reduced capital and operating costs. Thereis, however, a small penalty in terms of higherdissolved ions concentration in product water;product water quality decreases with rise intemperature due to higher osmotic pressure, andbecause solute (ions) flow through the membranehas a higher activation energy than water flow.

One intangible but significant benefit of a fuelcell powered RO desalination plant is the limitedwater required for fuel cell operation compared tomassive quantities of water required for con-ventional power plants (CPP) for system cooling,e.g. a 20-MW fuel cell power plant requires only7–10 m3/h water for the fuel processor (seeFig. 7). Fuel cell plants also reduce or eliminateadverse environmental impact on aquatic eco-systems; for example in the case of CPP inte-grated SWRO systems, power plant thermaldischarge to the oceans is a serious drawback.

The studies so far have been based on hightemperature fuel cells, viz., MCFC and SOFC[12,13]. However, since neither of these fuel cellsis economically viable yet, it is more prudent toinvestigate commercialised fuel cell technologies,specifically PAFC systems. Several hundred200-kW PAFC systems operating on variousfuels — natural gas, propane, butane, methane,pure hydrogen, and methane from anaerobicdigesters at wastewater treatment plants or land-fills — have been installed around the worldduring the last decade [28]. These on-site dis-tributed units generate power for hospitals, hotels,


Fig. 8. Schematic representation of a hybrid system combining fuel cell and RO unit.

schools, military installations, manufacturingplants, municipal facilities and wastewater treat-ment plants. A typical 200-kW PAFC unit gene-rates enough power to supply electricity to nearly150 households, and 226,800 kcal/h of usableheat at 60EC. As independent stand-alone units,therefore, they are ideally suited for supplyingpower to remote communities as well as provid-ing power and waste heat to membrane desali-nation units.

6. Integrated fuel cell–RO desalination systemdesign

Based on preliminary calculations, the abovewaste heat figure of 226,800 kcal/h is adequate toheat 22.5 m3/h of RO plant feed water from 20ECto 30EC. With this information as the startingpoint, engineering design and operation data forseawater and brackish water membrane desali-nation systems were generated for a communityof 150 households that is supplied electricity andwaste heat by a PAFC fuel cell system shown inFig. 8.

6.1. SWRO system design

The SWRO system design was based on thefollowing assumptions:

C feed water flow rate = 45 m3/hC feed water total dissolved solids (TDS) =

34,262 mg/LC feed water temperature = 20EC and 30ECC Polyamide thin-film composite RO membrane

(Hydranautics)C Product water recovery = 35, 40, 45 and 50%C Product water flux constant for each tem-

perature sub-setC Pelton Wheel energy recovery turbine.

The SWRO system was modeled usingHydranautics Membrane Solutions Design Soft-ware, version 8.8 (2004) based on the seawateranalysis in Table 1. Typical feed water treatmentunit operations are shown in Fig. 1. The optimalspiral-wound membrane element was SWC5,which is one of the lowest energy consumptionRO membrane elements for seawater.

The design data are presented in Table 5. In allcases, the SWRO unit was a single-pass, single-stage membrane array (6:0; 7:0; 8:0) similar tothe one shown in Fig. 3. Each pressure vesselcontained six membrane elements per pressurevessel. The product water TDS increased withproduct water recovery due to higher concen-tration polarisation. There was a slight drop inTDS at 45% recovery possibly due to a slightincrease in flux. The increase in product water

Table 5Seawater membrane desalination system design parameters

Case Membranetypea/membrane arrayb

FeedTemp.,EC

Feedpressure,bar g

Reject/brinepressure,bar g

Productflow,m3/h

PWRc

%Avg.flux,L/m2.h

ProductTDSd,mg/l

RO pumppower,kW (1)

Energyrecov.kW (2)

Energyrecov.,%

RO motorenergy,kW(1)-(2)

Specificenergy,kWh/m3

A SWC56:0

20 47.1 44.4 15.9 35 11.9 183 73.6 31.6 42.9 42 2.94

B SWC56:0

30 44.3 43 15.9 35 11.9 250 69.2 30.2 43.6 39 2.73

C SWC57:0

20 49.3 46.7 18.2 40 11.7 199 77 30.6 39.7 46.4 2.83

D SWC5 7:0

30 46.8 45.6 18.2 40 11.7 274 73.1 30.1 41.1 43 2.63

E SWC57:0

20 54 51.1 20.5 45 13.1 190 84.4 30.6 36.2 53.8 2.92

F SWC5 7:0

30 50.9 49.6 20.5 45 13.1 262 79.5 30.1 37.9 49.4 2.68

G SWC58:0

20 57.6 54.7 22.5 50 12.7 213 90 30.1 33.4 59.9 2.96

H SWC5 8:0

30 54.7 53.5 22.5 50 12.7 295 85.5 29.8 34.9 55.7 2.75

Notes: Feed water flow rate = 45 m3/h. Feed water TDS = 34,262 mg/L. Reject/brine osmotic pressure range = 38 (A) – 51 (H) bar.RO pump η = 80%. RO pump motor η = 90%. Energy recovery turbine η = 88%.aHydranautics TFC membrane rating: 30.3 m3/d at 99.8% rejection. Membrane element surface area: 37.16 m2.bOne-stage array. Six spiral-wound modules/vessel.cProduct water recovery.dTotal dissolved solids.

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esalination 227 (2008) 14–33


TDS was nearly 38% when the feed water tem-perature increased from 20EC to 30EC. As dis-cussed earlier, product water quality decreaseswith rise in temperature due to higher osmoticpressure, and because solute (ions) flow throughthe membrane has a higher activation energy thanwater flow.

The dissolved ion concentration varied be-tween 183 mg/L (Case A) and 295 mg/L (Case H)corresponding to membrane solute rejection, R, of99.5% and 99.1%, respectively, where R = [(feedconcentration ! permeate concentration)/feedconcentration].

The feed pressure and net energy requireddropped by 6–7% when the feed water tempera-ture increased from 20EC to 30EC while main-taining the same product water flux. The feedpressure increased with recovery due to higherenergy required to pump water through largerarrays, e.g., the number of pressure vesselsincreased from six at 35% recovery to eight at50% recovery. The specific energy for the ROsystem was calculated using equations given inthe Appendix. The energy numbers varied be-tween 2.63 kWh/m3 and 2.96 kWh/m3. The ROpump, motor and energy recovery turbine effi-ciencies were assumed to be 80%, 90% and 88%,respectively. Finally, the specific energy con-sumed was minimum for Case D when recoverywas 40% at 30EC. The optimum design andoperating point most likely was between 40% and45% recovery.

6.2. SWRO–fuel cell integration

The energy required to heat 45 m3/hr RO feedwater to the RO membrane unit is 453,600 kcal/h.This would require one 400 kW PAFC system ortwo 200 kW PAFC systems. Fuel cells like mem-brane systems are modular and fuel cell stacks ofvarying power rating are easily scaled-up. Typi-cally 10–12% of PAFC power generated isrequired for system parasitic needs, i.e. net fuelcell power available is 352 kW. For an RO

system operating at 40% recovery and 30EC(Case D, Table 5), 43 kW would be required forthe RO high-pressure pump, which is typicallyabout 60% of the total energy usage at a SWROplant. Hence, the total energy usage is 72 kW. Inaddition, up to 12 kW would be required for otherancillary equipment and general communityneeds. Thus, about 268 kW of fuel cell power isavailable for a community of 150 households or1.79 kW per household. At 40% recovery, the ROplant is rated for 18.2 m3/h of desalinated waterwith a TDS of 274 mg/L (Case D). Assuming10% of potable water is reserved for plant main-tenance such as membrane cleaning and 60% isrequired for community needs, the remaining30% would be available for personal householdneeds or about 36 L/h per household should beavailable.

6.3. BWRO system design

The BWRO system was based on the follow-ing assumptions:C feed water flow rate = 45 m3/hC feed water temperature = 20 and 30ECC polyamide thin-film composite RO membrane

(Hydranautics).C product water recovery = 70 and 75%C product water flux constant for each tem-

perature sub-setC no energy recovery device.

The BWRO system was modeled usingHydranautics Membrane Solutions Design Soft-ware version 8.8 (2004) based on brackish wateranalysis in Table 1. Typical feed water treatmentunit operations are shown in Fig. 2. The optimalspiral-wound membrane elements were ESPA2(energy saving polyamide) and CPA2 (standardpressure polyamide).

The design data are presented in Table 6. In allcases, the BWRO unit was a single-pass, two-stage membrane array (6:3 or 6:4). The productwater TDS increased with product water recoverydue to higher concentration polarisation. The


increase in product water TDS was nearly 44%when the feed water temperature increased from20EC to 30EC. The membrane solute rejection, R,varied between 96.9% and 98.3% for lower TDS(1656 mg/L) water, and between 96.8% and97.7% for higher TDS (3674 mg/L) water.

The feed pressure and net energy requireddropped by 11% when the feed water temperatureincreased from 20EC to 30EC at the same productwater flux. The higher gain in pressure drop inthe case of BWRO as compared to SWRO was aresult of lower osmotic pressure in the rejectstream. The specific energy for the RO unit alonevaried between 0.42 kWh/m3 and 0.63 kWh/m3.The RO pump and motor efficiencies wereassumed to be 80% and 90%, respectively.Finally, the specific energy consumed was mini-mum for Case III when recovery was 75% at30EC.

6.4. BWRO–fuel cell integration

As discussed above, energy required to heat45 m3/h RO feed water to the RO membrane unitis 453,600 kcal/h. This would require one400 kW PAFC system. Assuming 12% of fuelcell power generated is required for PAFC systemparasitic needs, net fuel cell power available is352 kW. For an RO system operating at 70%recovery and 30EC (Case II, Table 6), 14 kWwould be required for the RO high-pressurepump. In addition, up to 35 kW could be requiredto power raw feed water and product waterpumps, other ancillary equipment and generalcommunity needs. Thus, about 300 kW of netfuel cell power is available for a community of150 households or 2 kW per household. If the ROpump efficiency is less than assumed in this case(80%), the available power would be lower. At70% recovery, the RO plant is rated for 31.8 m3/hof desalinated water with a TDS of 41 mg/L(Case II). Assuming 10% of potable water isreserved for plant maintenance such as membranecleaning and 60% is required for community

needs, the remaining 30% would be available forpersonal household needs or about 63 L/h perhousehold should be available.

6.5. Post-analysisThe net energy and water supply data gene-

rated assumes that both fuel cell and RO systemsare in operation simultaneously. Since thedemand for electricity and water is never constantin a 24-h period while fuel cells work moreefficiently at constant loads, two different sce-narios are envisaged for fuel cell integrated ROplants, viz. (a) off peak operation, and (b) peakoperation such that the integrated systemalternates between water production and elec-tricity supply to the grid. Thus, during off peakhours when electricity demand is low, fuel cellpower would supply more electricity to the ROunit to produce water continuously and stored,whereas during peak hours when electricitydemand is high, it would be more economical togenerate more electricity with reduced productionof RO purified water.

The product water pH is 5.6, which is typicalof RO plants. Hence, the product water is treatedwith caustic soda and lime for corrosion controlin the distribution system (Fig. 1). In addition,chlorine and ammonia are added to form chloro-amines for disinfection. A drawback tank isrequired to flush the brine channels in membraneelements with RO permeate (Fig. 3). Drawback isthe process of natural osmosis that occurs whenthe system is shutdown.

The fuel cell reformer requires about0.25 m3/h of low TDS water (Fig. 7). This wouldnecessitate polishing a fraction of RO productwater as a slip-stream in a second-pass RO unit.For example, in the case of one SWRO system(Case D, Table 5), the polishing RO unit is asingle-stage 1:0 array consisting of one 10 cm ×100 cm membrane element (HydranauticsESPA2-4040) designed for 50% recovery withfeed water flow rate equal to 0.5 m3/h. The

30R. Singh / D

esalination 227 (2008) 14–33Table 6Brackish water membrane desalination system design parameters

Case Membranetypea/membranearrayb

FeedTDSc

mg/l

Feedtemp.,EC

Feedpressure,bar g

Reject/ brinepressure,bar g

Productflowm3/h

PWRd,%

Avg.flux,L/m2.h

ProductTDS,mg/l

RO pumppower,kW

RO motorenergy,kW

Specificenergy,kWh/m3

I ESPA2 (1)/6:3

1656 20 9 4.9 31.8 70 15.9 28.5 14.1 15.7 0.49

II ESPA2 (1)/6:3

1656 30 8 3.9 31.8 70 15.9 41.3 12.5 13.9 0.44

III ESPA2 (1)/6:4

1656 30 8.1 4.5 34 75 15.3 51.2 12.7 14.1 0.42

IV ESPA2 (1)/6:3

3674 20 11.6 7.7 31.8 70 15.9 90.4 18.1 20.1 0.63

V CPA2 (2)/6:4

3674 20 11.6 8.9 31.8 70 15.6 82.9 18.1 20.1 0.63

VI CPA2 (2)/6:4

3674 30 10.3 7.9 31.8 70 15.6 119 16.1 17.9 0.56

Notes: Feed water flow rate = 45 m3/h. Reject/brine osmotic pressure range = 3.1 (I) – 7 (VI) bar.RO pump η = 80%. RO pump motor η = 90%.

aHydranautics TFC membrane rating: (1) 34.1 m3/d at 99.6% rejection; (2) 37.9 m3/d at 99.7% rejection.bTwo-stage array. Six spiral-wound modules/vessel.cTotal dissolved solids.dProduct water recovery.


polishing RO unit reduces total dissolved ionconcentration from 274 mg/l in feed water(SWRO permeate) to 4.9 mg/L in the permeatecorresponding to 98.2% rejection. The RO pumpmotor is rated for 0.4 kW. A similar polishing ROunit would be required for a BWRO system.Alternately, pure water can be recovered from thecathode vent gas after scrubbing to removephosphoric acid.

7. Discussion

The efficacy of sustainable fuel cell integratedmembrane desalination systems for small com-munities has been demonstrated. The PAFCsystem provides power both to the communityand the desalination plant. In addition, the fuelcell stack low-grade heat is used to raise RO feedwater temperature from 20EC to 30EC, therebyreducing the RO plant energy usage by 7–11%.Reduction in the cost of energy by 10–20% in thenext decade through such several incrementalinnovative design and operation techniqueswould make membrane desalination technologyan important source of potable water around theworld.

It is well documented that membrane pro-ductivity increases with feed water temperaturealbeit at a slight penalty in product water quality[17]. Warmer temperature results in reduced ROfeed pressure and concomitant power costsavings. Increase in permeate flux of up to 60%was reported when feed water temperature wasincreased from 20EC to 40EC possibly due tochanges in membrane morphology and highersolvent flow [29]. However, in the 30–40ECrange, higher membrane permeability is adverselyaffected by increased osmotic pressure and highersalt passage [21]. Co-location of SWRO plantswith conventional power plants (CPP) is a seriousconsideration for utilising power plant waste-heat; the cooling water is used as the SWRO plantfeed water [30]. However, there are several draw-

backs for co-location especially (a) the CPP mustproduce more cooling water than the desalinationplant can process, (b) the RO membranes can getexposed to fouling by iron, copper, or nickel, and(c) the source water must be cooled if thetemperature exceeds 40EC. The benefits of fuelcell integrated membrane desalination plants, bycontrast, include negligible air pollution, minimalwater requirements, and little or no environmentalimpact on aquatic ecosystems resulting frompower plant thermal discharge. Further, on-sitedistributed fuel cell systems not only eliminatethe costly installation of hundreds of miles oftransmission lines but also mitigate extremelyhigh losses of up to 30% incurred during trans-mission in developing countries [1,26]. Conven-tional electricity supply transmission costs must,therefore, be included in the overall cost ofdesalinated water.

The application of fuel cells is most compe-titive in developing countries where electricpower generation is expensive, inadequate andunreliable especially in the case of rapidly grow-ing economies of India and China. Further, thereis either acute shortage of power in rural areas orthey are not electrified. Rural community biogasplants are extensive in China and are becomingpopular in India. Recently, the world’s largestfuel-cell plant using biogas generated from fer-mented organic waste was installed in Germany[31]. PAFC systems operating on reformedbiogas are ideally suited for rural electrification.Supplying potable water to remote communitiesin India from centralised large desalination plantsby pipelines and barges has been suggested [32].An on-site, stand-alone, energy-efficient, dual-purpose fuel cell integrated membrane desali-nation system would be a superior alternative.

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Appendix — Specific energy calculations

C (I) Required pump power = feed water flowrate, m3/h × feed pressure, bar g/(pumpefficiency × 36)

C (II) Turbine recovered energy = reject flowrate, m3/h × reject pressure, barg/(turbine efficiency × 36)

C Power absorbed by motor = (I)–(II)

C Specific energy consumption = powerabsorbed by motor, kW/(motor efficiency ×product flow rate, m3/h)

sustainable fuel cell integrated membrane desalination systems

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