review

Arun Subramani a,*, MohammJoseph G. Jacangelo a,b

aMWH Americas Inc., 618 Michillinda Avenub ore, M

2.1. Enhanced system design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19082.2. High efficiency pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19102.3. Energy recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19102.4. Advanced membrane materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1910

2.4.1. Nanocomposite membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1911

* Corresponding author. Tel.: 1 626 568 6002; fax: 1 626 568 6015.mani).

Avai lab le a t www.sc iencedi rec t .com

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 0E-mail address: Arun.Subramani@us.mwhglobal.com (A. SubraContents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19082. Minimization of energy usage for desalination processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1908Energy recovery

Water sources

energy efficient design combined with high efficiency pumping and energy recovery devices

have proven effective in full-scale applications. Integration of advanced membrane

materials and innovative technologies for desalination show promise but lack long-term

operational data. Implementation of renewable energy resources depends upon geography-

specific abundance, a feasible means of handling renewable energy power intermittency,

and solving technological and economic scale-up and permitting issues.

2011 Elsevier Ltd. All rights reserved.The Johns Hopkins University, Baltim

a r t i c l e i n f o

Article history:

Received 25 October 2010

Received in revised form

27 December 2010

Accepted 31 December 2010

Available online 9 January 2011

Keywords:

Reverse osmosis

Nanotechnology

Renewable energy0043-1354/$ e see front matter 2011 Elsevdoi:10.1016/j.watres.2010.12.032ad Badruzzaman a, Joan Oppenheimer a,

e, Arcadia, CA 91007, USA

D 21205, USA

a b s t r a c t

Energy is a significant cost in the economics of desalinating waters, but water scarcity is

driving the rapid expansion in global installed capacity of desalination facilities. Conven-

tional fossil fuels have been utilized as their main energy source, but recent concerns over

greenhouse gas (GHG) emissions have promoted global development and implementation of

energyminimization strategies and cleaner energy supplies. In this paper, a comprehensive

review of energy minimization strategies for membrane-based desalination processes and

utilization of lower GHG emission renewable energy resources is presented. The review

covers the utilization of energy efficient design, high efficiency pumping, energy recovery

devices, advanced membrane materials (nanocomposite, nanotube, and biomimetic),

innovative technologies (forward osmosis, ion concentration polarization, and capacitive

deionization), and renewable energy resources (solar, wind, and geothermal). Utilization ofEnergy minimization strategieutilization for desalination: AReview

s and renewable energy

journa l homepage : www.e lsev ie r . com/ loca te /wat resier Ltd. All rights reserved.

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sys

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or membrane-based technologies based on the separation

2. Minimization of energy usage fordesalination processes

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 01908oped since the 1960s (Loeb and Sourirajan, 1963) and

currently surpass thermal processes in new plant installa-

tions (Greenlee et al., 2009). Membrane-based desalination

technologies are favored over thermal-based desalination in

regions where the cost of energy for steam production is high

(NREL, 2006). The energy requirements for seawater desali-

nation using thermal-based technologies are on the order of

7e14 kWh/m3when compared to 2e6 kWh/m3 formembrane-

based technologies (Veerapaneni et al., 2007; Semiat, 2008;

Anderson et al., 2010). The energy requirements are lower

for RO, but energy consumption still remains the major

operational cost component due to the high pressure pumps

required to feed water to the RO process. These pumps are

responsible for more than 40% of the total energy costs

(Service, 2006; Souari and Hassairi, 2007). Reducing energy

consumption is, therefore, critical for lowering the cost of

desalination and addressing environmental concerns about

GHG emissions from the continued use of conventional fossil

Factors influential inminimizing energy usage in desalination

processes using RO membranes can be classified according to

enhanced system design, high efficiency pumping, energy

recovery, advanced membrane materials, and innovative

technologies. Each of these factors is described in more detail

below.

2.1. Enhanced system design

The design and configuration of membrane units have

a significant effect on the performance and economics of an RO

plant (Wilf and Bartels, 2005). In the past, membrane units for

seawater were usually configured as two stages with six

elements per pressure vessel. The two-stage system resulted in

a high feed and concentrate flow,which reduced concentration

polarization at the expense of a greater feed pressure needed to

compensate for the increasedpressuredropacross theROtrain.process adopted (Greenlee et al., 2009). Although thermal

desalination has remained the primary technology of choice

in the Middle East, membrane processes have rapidly devel-to be targeted for seawater reverse osmosis (RO) (GWI, 2010).

Desalination processes are broadly categorized as thermal

nation followed by a discussion on the utilization of renew-

able energy resources to reduce GHG emissions.2.4.2. Nanotube membranes . . . . . . . . . . . . . . . . .

2.4.3. Biomimetic membranes . . . . . . . . . . . . . . . .

2.5. Innovative technologies . . . . . . . . . . . . . . . . . . . . . . .

2.5.1. Forward osmosis . . . . . . . . . . . . . . . . . . . . . .

2.5.2. Ion concentration polarization . . . . . . . . . .

2.5.3. Capacitive deionization . . . . . . . . . . . . . . . .

3. Renewable energy utilization . . . . . . . . . . . . . . . . . . . . . . . .

3.1. Solar energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1.1. Solar thermal processes . . . . . . . . . . . . . . . .

3.1.2. Solar electromechanical process . . . . . . . .

3.2. Wind energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3. Geothermal energy . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.4. Hybrid systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.5. Design and implementation of renewable energy

4. Research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

In response to increasing water demand, many municipalities

and water suppliers are considering more energy intensive

seawater desalination to supplement inadequate freshwater

sources (GWI, 2010). Desalination has been successfully

implemented to provide additional water to communities

experiencing shortages by applying processes developed over

the last 40 years (Gleick, 2006). It is estimated that the capital

expenditures fornewdesalinationplantswill exceed$17 billion

by the year 2016 out of whichmore than $13 billion is expectedfuels as the primary energy source for seawater desalination

plants. A large number of energy minimization approachesand renewable energy alternatives are rapidly being devel-

oped, investigated and implemented around the globe

(Charcosset, 2009). Thus, providing an updated, comprehen-

sive review of sustainable design and operational strategies to

reduce energy usage and GHG emissions is warranted. This

paper critically reviews in a holistic manner the latest devel-

opments and technologies for reducing energy consumption

by reverse osmosis desalination processes and addresses

strategies for integrating renewable energy as a source of

alternative clean energy supply. The paper is organized by

a discussion about energy minimization strategies for desali-. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1911

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1912

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1912

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1912

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1913

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1913

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1913

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1913

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1913

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1915

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1915

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1916

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1916tems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1916. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1917. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1917. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1917. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1917Design efforts to reduce power consumption resulted in the use

of single-stage configurations for high salinity feed water

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 0 1909applications, and in some cases, the use of seven (or) eight

elements per pressure vessel (Wilf andBartels, 2005; Petry et al.,

2007).Thepressuredropreduction inusingasingle-stage rather

than a two-stage systemwas reported to result in a 2.5% lower

power requirement (Wilf and Bartels, 2005).

More recently, further reduction in RO desalination cost

has been shown to occur from optimal process configuration

and control schemes. Theoretical cost minimization frame-

work have been developed and experimentally implemented

using a controller to quantify the effect of energy cost with

respect to membrane cost, brine management cost, energy

recovery, and feed salinity fluctuation (Zhu et al., 2009b, 2010).

A control system utilizing real-time sensor data and user-

defined permeate flow requirements has been implemented

to compute in real-time the energy-optimal set-points for

controlling concentrate valve position and feed flow rate

(Bartman et al., 2009, 2010). Implementation of the control

system demonstrated the ability to achieve energy-optimal

operation of the RO system close to the theoretically predicted

energy consumption curves.

When stringent water quality requirements mandate the

use ofmulti-pass RO (Sauvet-Goichon, 2007), the overall power

consumption of the RO system can be lowered if a portion of

the first pass permeate is pumped to the second pass (Zhu

et al., 2009). Since the permeate produced from the front-end

elements is lower in salinity than thepermeateproducedat the

back-end elements, lower feed pressure is required for the

second passwhen the front-end permeate is utilized as feed to

the second pass. In a multi-pass system, the lowest energy

consumption is obtained when membranes with the highest

salt rejection is used in the first-pass (Zhu et al., 2009a). In

another study, various mixing operations between feed,

concentrate, and permeate streams were evaluated to assess

their potential on energy usage (Zhu et al., 2010a). It was

determined that various mixing approaches may provide

certain operational or system design advantages but they do

not provide an advantage from an energy usage perspective.

A novel designmodification to reduce pressure drop across

membrane elements is the use of a pressure vessel with

a center port design (van Paassen et al., 2005). In this inno-

vative configuration, feed water enters the pressure vessel

through two feed ports on each end of the pressure vessel in

the first stage. The concentrate is collected through a middle

port and flows to a similar port on the pressure vessels in the

second stage. Thus, the flow path is reduced by half and

although the membrane unit has eight elements per pressure

vessel, the flow path length is reduced to four elements per

stage, creating a lower pressure drop that lowers the feed

pressure. A 15% reduction in the feed pressure has been

reported using the center port design when compared to

a conventional side port design (Wilf and Hudkins, 2010). The

disadvantage of the center port design is the potential for

scaling due to excessive concentration polarization. Thus,

pilot testing and long-term operational data are recom-

mended before considering implementation of the center port

design in order to determine the influence of water quality

variations on feed water recovery.

Reduction in energy consumption for RO systems treatinghigh salinity feedwater has also been achieved by using a two-

stage hybrid system with concentrate staging (Veerapaneniet al., 2005). The first stage consists of high rejection brackish

water membrane elements (or) high permeability seawater

membrane elements. The second stage consists of standard

seawater elements. Using a two-stage system with brackish

(or) low-pressure seawatermembranes in thefirst stage lowers

feedpressure requirementsdue to lowermembrane resistance

(Veerapaneni et al., 2007) Asmost of the permeate is produced

in the first stage with the high permeability membranes, the

pressure of only a small fraction of the remaining flow is

boosted, resulting in significant energy savings. A two-pass

nanofiltration (NF) membrane system also substantially

reduces the energy consumption (Long, 2008). The power

consumption of a two-pass NF process was estimated to be

2.06 kWh/m3 compared to 2.32 kWh/m3 for a two-stage hybrid

brackish and seawater membrane system (Long, 2008). A 5%

and 12% reduction in energy consumptionwas obtainedwhen

using a hybrid brackish/seawater or two-pass NF element

system, respectively (Long, 2008).

Energy consumption is also reduced by minimizing the

pressure drop across membrane elements (Macedonio and

Drioli, 2010). An approach by which to reduce the axial pres-

sure drop in membrane elements involves the use of a novel

feed spacer design that reduces the hydraulic pressure drop

in the RO elements (Subramani et al., 2006; Guillen and Hoek,

2009). The feed spacer pattern used in most spiral wound

membrane elements causes a variation in the flow path of the

feedwater resulting in a higher axial pressure drop than flow in

anopen channel (Guillen andHoek, 2009). Although feed spacer

geometry was found to have a marginal impact on mass trans-

fer, thinner spacer filaments spreadapart substantially reduced

hydraulic pressure losses. In addition, certain non-circular

spacer filament shapes produced lower hydraulic losses when

compared to conventional circular spacer filament shapes

(Guillen and Hoek, 2009). Although various feed spacer geome-

tries have been shown to reduce hydraulic pressure loss in RO

elements, actual data from pilot-scale and full-scale operation

are still minimal since spiral wound elements with novel feed

spacer configurations are not readily available. Commerciali-

zationof feedspacers that reduce theaxial pressuredrop across

membrane elements could potentially reduce the feed pressure

requirements during RO seawater desalination.

A plant design approach for improving the economics of

desalination and at the same time reduce the impact on

environment due to brine discharge is the co-location of

membrane desalination plants with existing coastal power

generation stations (Voutchkov, 2004). In this approach,

overall desalination power demand and associated costs of

water production are reduced as a result of the use of warmer

source water. The cooling water discharged from the

condensers in a power plant is 5e15 Cwarmer that the sourceocean water. When this water is used by the RO plant, 5e8%

lower feed pressure is required to desalinate the water when

compared to desalination of colder source ocean water. This

approach also has the advantage of sharing a common intake

facility. In the Middle East, RO and thermal-based technolo-

gies are combined to provide a hybrid design (Cardona and

Piacentino, 2004). Such hybrid designs not only result in

capital savings by sharing a common intake and outfallfacility but also have a 40e50% increase in water production

related to pre-heating of feed water to the RO plant.

2.2. High efficiency pumping

With respect to pumping, energy is predominantly consumed

from operation of primary feed pumps, second pass feed

pumps (as required), pretreatment pumps, product water

transfer pumps, chemical feed pumps, and water distribution

pumps. The distribution of power usage in a two-stage

seawater RO system is shown in Fig. 1. More than 80% of the

power is required for the operation of the primary feed pumps

(Wilf and Bartels, 2005). Although the flow and head of

a pumping systemare determined by the design specifications

without thenecessity to throttlehighpressurepumpsor energy

recovery devices, a variable frequency drive is often incorpo-

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 01910rated into the electric motor unit that drives the high pressure

pump (Torre, 2008). All of the above mentioned pumping

methods have been demonstrated to significantly improve

efficiency and reduce energy requirements at full scale.

2.3. Energy recovery

Energy consumption for RO desalination processes is reduced

by using energy recovery devices (ERD) that have been shown

to recover energy from the RO concentrate (Andrews and

Laker, 2001; Wang et al., 2004). Before the concentrate stream

is sent for disposal, pressure from the stream is recovered by

passing it through an ERD. The fraction of power recovered

dependson the typeandefficiencyof the equipmentused.Two

broad classes exist for ERDs (Wang et al., 2004). Class I devices

use hydraulic power to cause a positive displacement within

the recovery device, and the hydraulic energy is directly

transferred in one step (Greenlee et al., 2009). Class II devices

use the hydraulic energy of the RO concentrate in a two-stepof the RO system, the selection and operation of pumps and

other elements of a pumping system play an important role in

reducing overall energy usage in the plant.

To achieve the highest possible pumping efficiency, several

procedures are performed including: (1) verifying energy effi-

cient operation of the pumping system, (2) utilizing a premium

efficiency motor, and (3) utilizing a variable frequency drive

(Manth et al., 2003). To achieve an energy efficient operation,

a pumps speed must fall within a specified range for optimal

efficiency or the best efficiency point (Veerapaneni et al., 2007).

The use of high speed and high flow pumps at lower total

dynamic head provides the optimal speed needed for highest

efficiency. To accommodate the variability of feed pressure

with time (due to salinity and temperature fluctuations)Fig. 1 e Distribution of power usage in a two-stage

seawater RO system (Wilf and Bartels, 2005).process that first converts the energy to centrifugal mechan-

ical energy and then back to hydraulic energy. Most of the

seawater desalination plants in operation today use a Class I

type of ERD (Greenlee et al., 2009). When an ERD is used,

a fraction of the feed must bypass the primary high-pressure

pump and a booster pump is used to account for pressure

losses in the RO membrane modules, piping, and ERD

(Greenlee et al., 2009). Thepressure orwork exchanger (PWE) is

aClass I typeof ERD.Thepeltonwheel, reverse running turbine

pump, and turbo charger are examples of a Class II type of ERD.

Efficiency greater than 95% can be achieved using a Class

I type of ERD (Greenlee et al., 2009). The PWE transfers the

hydraulic energy of the pressurized RO concentrate stream to

the RO feed water stream (Avlonitis et al., 2003; Stover, 2007).

PWE systems can be categorized as two types: those that

provide a physical barrier (piston) between the RO concentrate

stream and feed side of the system, such as a Dual Work

Exchanger Energy Recovery (DWEER), and those without

a physical barrier such as a Pressure Exchanger (PX) (Cameron

and Clemente, 2008; Mirza, 2008). In the case of a DWEER, the

system is based on moving pistons in cylinders which is well

suited for a wide range of water viscosities and densities, but

results in a large foot print (Mirza, 2008). A PX device has

higher efficiency since no transformational losses occur in the

device, but individual PXs have limited flow rates and

a higher capacity must be achieved by arranging several

devices in series. PX devices are also associated with very high

noise levels requiring a sound abatement enclosure (Mirza,

2008). Another disadvantage of a PX device is the degree of

mixing that occurs between the feed water and concentrate

stream. A feed salinity increase of 1.5%e3.0% caused by such

mixing will increase the required feed pressure for the RO

system (Wang et al., 2004, 2005).

Class 2Centrifugal ERDs (such as the peltonwheel and turbo

charger) are limited in capacity and are usually optimized for

narrow flow and pressure operating conditions (Stover, 2004,

2007). The turbo charger is typically used in smaller capacity

RO installations (Oklejas et al., 2005). The reverse running

turbine pump is not suitable for a low flow range due to poor

efficiency (Mirza, 2008). The efficiency of commercial pelton

wheels can reach 90% (Stover, 2007). The overall efficiency of

the mechanically coupled reverse running turbine pump is in

the 75%e85% range. For the submersible generator type, the

overall efficiency is in the 62%e75% range (Mirza, 2008). The

efficiency of the turbo charger ranges from 55% to 60%.

2.4. Advanced membrane materials

Significant improvements in the salt rejection capacity and

permeability of RO membranes for treating high salinity feed

waters have been achieved in recent years. In 1980, seawater

RO systemsconsumedmore than 26 kWh/m3. Today, seawater

ROsystemsconsumeonaverageonly3.4kWh/m3 (Changet al.,

2008). The minimum theoretical energy use (50% recovery) is

about 1.08 kWh/m3 for seawater desalination (Voutchkov,

2010). Thus, there are further avenues for improving the

permeability of RO membranes using novel membrane mate-

rials such that the energy consumption is minimized. But, thenew generation membranes must provide at least double the

permeability of current generation RO membranes. This is

based on a recent approach to determine the minimization of

energy costs by improvingmembrane permeability (Zhu et al.,

2009c). Adimensionless factorwasused to reflect the impact of

feed water osmotic pressure, salt rejection requirement,

membrane permeability, and purchase price of electrical

energy and membrane module. It was estimated that unless

the permeability of the RO membrane is doubled and the

capital cost of pressure vessels directly impacted by a lower

membrane area requirement, further improvements in

seawater RO membrane permeability is less likely to signifi-

cantly reduce the cost of desalination. New generation RO

membrane which show promise in providing more than

double the permeability of currently available RO membranes

are discussed below. New generation RO membranes offer

reduced feed pressure requirements while maintaining rejec-

tion. Todays high productivity membrane elements are

designed with two features that include more fresh water per

membrane element and higher surface area and denser

membrane packing (Voutchkov, 2007). New generation RO

membranes can be broadly classified as nanocomposite,

nanotube, and biomimeticmembranes. A comparison of these

operational data are still unavailable. Chemical stability of the

incorporated zeolite nanoparticles within the polyamide

matrix also needs to be demonstrated. If the nanocomposite

membranes are not chemically compatible in a wide pH range,

their applicability would be limited. Also, rejection of the

nanocomposite membrane has been reported only for sodium

chloride, magnesium sulfate, and polyethylene glycol (Jeong

et al., 2007). Rejection of specific constituents, such as boron,

in seawater by ROmembranes has become a concern recently

due to stringent discharge limits (Greenlee et al., 2009). The use

of nanocomposite membranes for seawater desalination

would require thatmore rejectiondata for specific constituents

in seawater be available.

2.4.2. Nanotube membranesTheuse of carbonnanotubeshavealso been shown to consume

lower energy than conventional seawater RO desalination

systems (Truskett, 2003; Holt and Park, 2006; Sholl and Johnson,

2006;Corry, 2008; Jia etal., 2010).The transport ofwaterand ions

occurs through membranes formed using carbon nanotubes

ranging in diameter from 6 to 11 A. Membranes incorporating

sm

tio

ate

0).

rgy

ate

009

on

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 0 1911advanced membranes is described in Table 1.

2.4.1. Nanocomposite membranesThin film nanocomposite RO membranes are made by

combining zeolite nanoparticles dispersedwithin a traditional

polymide thin film (Jeong et al., 2007; Hoek and Ghosh, 2009).

The zeolite nanoparticles are dispersed in one or more of the

monomer solutions used to create the membrane by an inter-

facial polymerization process. Incorporation of zeolite nano-

particles in thepolymermatrix of seawater ROmembraneshas

enhanced flux to more than double that of a commercial pro-

duct with 99.7% salt rejection. Utilization of nanocomposite-

based RO membranes can result in 20% lower energy

consumption (NanoH2O, 2010). AlthoughROmembranesusing

zeolite nanoparticles have been reported to show substan-

tial reduction in the feed pressure requirement, long-term

Table 1 e Comparison of advanced material based reverse o

Membrane type Principle Energy consump

Nanocomposite Zeolite nanoparticles

incorporated in

polyamide matrix

creating enhanced

transport of water

molecules.

20% lower energy

consumption than

conventional seaw

RO (NanoH2O, 201

Nanotube Transport of water

molecules through

structured carbon

and boron nitride

nanotubes.

30e50% lower ene

consumption than

conventional seaw

RO (Hilder et al., 2

Biomimetic Aquaporins used to

regulate transport of

water molecules.

Energy consumpti

is not known.carbon nanotubes are promising candidates for water desali-

nation using RO since the size and uniformity of the tubes

should achieve the desired salt rejection. A ten-fold perme-

ability increase is expected using a carbon nanotube RO

membrane which should result in a 30e50% savings in energy

usage. Simulations have shown that boron nitride nanotubes

have superior water flow properties to carbon nanotubes and

they can achieve 100%salt rejection (Hilder et al., 2009). Theuse

of a nanotube radius of 4.14 A can also be used to functionalize

the membrane to become cation-selective. When a nanotube

radius of 5.52 A is used, themembrane can be functionalized to

become anion-selective (Hilder et al., 2009). Similar to the

nanocomposite membrane, long-term operational data, scale-

up information, chemical compatibility specifications and

tensile strength of the nanotube-based RO membrane is lack-

ing. Information available on the rejection property and water

osis membranes.

n Advantages Drawbacks

r

More than double

the flux of currently

available seawater

RO membranes

(NanoH2O, 2010).

Chemical compatibility

and structural

stability is not known.

Rejection of specific

contaminants is not known.

Long-term operational

data not available.

r

).

Ten e fold higher

flux than currently

available seawater

RO membranes

(Hilder et al., 2009).

Only modeling results available.

Rejection of specific contaminants

is not known.

Hundred times

permeable than

currently available

seawater RO

membranes

Inability to withstand high

operating pressures.

Rejection of specific contaminants

is not known.

Long-term operational(AquaZ, 2010). data not available.

flux of the nanotube membrane is solely based on modeling

results and a significant amount of research still needs to be

conducted to determine if these nanotubes can be polymerized

and casted to achieve the results demonstrated through

modeling.

2.4.3. Biomimetic membranesNew developments have also occurred with the use of biomi-

metic membranes for desalination (Bowen, 2006). Biomimetic

membranes are designed to mimic the highly selective trans-

portofwateracrosscellmembranes.Naturalproteinsknownas

aquaporins are used to regulate the flow of water providing

increased permeability and high solute rejection. Aquaporins

act as water channels which selectively allow water molecules

to pass through while the transport of ions is restricted by an

electrostatic tuning mechanism of the channel interior. This

results in only water molecules being transported through the

aquaporin channels and charged ions being rejected (Sui et al.,

2001; Gong et al., 2007). Aquaporin membranes are considered

to be a hundred times more permeable than commercial RO

biomimetic membranes that needs to be overcome is their

inability to withstand high operating pressures.

technologies is described in Table 2. The technologies are dis-

cussed below.

2.5.1. Forward osmosisIn the forward osmosis process, instead of using hydraulic

pressure similar to a conventional RO desalination process,

a concentrated draw solution is used to generate high osmo-

tic pressure, which pulls water across a semipermeable

membrane from the feed solution (McCutcheon et al., 2005;

Chou et al., 2010). The draw solutes are then separated from

the diluted draw solution to recycle the solutes and to produce

clean product water. A mixture of ammonia and carbon

dioxide gas has been used as the predominant draw solution

(McCutcheon et al., 2006). Other draw solutions utilized are salt

solutions and magnetic nanoparticles (Yang et al., 2009).

Forward osmosis is a combination of membrane and thermal

processes. Specific energy consumption of less than 0.25 kWh/

m3 has been reported for the membrane portion of the process

(Cath et al., 2006; McGinnis and Elimelech, 2007). In a recent

study, a combination of forward osmosis and reverse osmosis

ies

yptio

m3

6; M

200

m e

m3

)

05)

ptio

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 019122.5. Innovative technologies

Innovative technologies utilizing the principles of separation

technology with membranes, osmosis and electric fields have

been introduced in recent years. These technologies have

the potential to offer a substantial reduction in energy consu-

mption for desalination. A comparison of these innovative

Table 2 e Comparison of innovative desalination technolog

Technology Principle Energconsum

Forward osmosis Utilizes natural osmosis to

dilute seawater feed

stream using a draw

solution with higher

osmotic pressure than

the seawater feed.

0.25e0.84 kWh/

(Cath et al., 200

and Elimelech,

Ion concentration

polarization

Nanofluidics in

combination with

ion concentration

polarization utilized

to desalinate seawater.

3.5 kWh/m3 (Ki

al., 2010).

Capacitive

deionization

Ions electrosorbed

by polarization of

electrode (carbon aerogels)

by a direct current

1.37e1.67 kWh/

(brackish water

(Welgemoed, 20

Energy consummembranes with anticipated specific power consumption

savings of 70% of specific power consumption (AquaZ, 2010).

Highly permeable and selective membranes based on the

incorporation of the functional water channel protein Aqua-

porin Z into a novel triblock copolymer has been shown to have

significantly higher water transport than existing RO

membranes (Kumar et al., 2007). A particular difficulty withpower source. of seawater is notwas found to produce a higher flux than the forward osmosis

process alone, thus reducing the specific energy consumption

(Choi et al., 2009). But, the results were valid only for certain

operating conditions and effect of membrane material on

energy efficiency of the process is unknown. The regeneration

of the draw solution requires significant amount of energy and

unless waste heat is available for regeneration of the draw

solution, the forward osmosis process is not considered more

efficient than an RO process (McGinnis et al., 2007; Semiat,

2008). The process also has the advantage of a lower fouling

propensity than RO as a result of the absence of hydraulic

pressure and application of novel thin film composite

membranes (Mi and Elimelech, 2010). A particular drawback of

the forward osmosis process is the utilization of an appropriate

membrane to reduce internal concentration polarization and

increase efficiency (McGinnis and Elimelech, 2007). Although

the forward osmosis process shows promise in terms of

better performance with respect to fouling and scaling on the

with reverse osmosis.

nAdvantages Drawbacks

cGinnis

7)

Lower energy consumption

than RO (McGinnis and

Elimelech, 2007). Lower

fouling potential than

RO due to absence of

transmembrane pressure

(Mi and Elimelech, 2010).

More applicable than

RO only when waste

heat source is

available (McGinnis

and Elimelech, 2007).

Full-scale operation

data is not available.

t Lower energy consumption

than RO. Absence of

membranes and applied

pressure (Kim et al., 2010).

Process suited for

small and medium e

scale systems. Full-

scale operational

data is not available.

.

n

Lower energy consumption

than RO for brackish

water treatment (Oren, 2010).

Absence of membrane

Low feed water

recovery (Oren, 2010).

Full-scale operational

data not available.known. and applied pressure.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 0 1913membrane surface, the rejection capability of the membranes

for specific contaminants such as boron, which is a design

limiting factor for seawater desalination, is unknown. Studies

have been conducted to improve the membrane property used

for forward osmosis (Yang et al., 2009) but long-term opera-

tional data for the forward osmosis process is unavailable and

only bench-scale results have been published.

2.5.2. Ion concentration polarizationIon concentration polarization has been utilized to desalinate

seawater using an energy efficient process (Kim et al., 2010). In

this process, micro and nanofluidics in combination with ion

concentration polarization are used to desalinate seawater.

Ion concentration polarization is a fundamental transport

mechanism that occurs when an ionic current is passed

through an ion-selective membrane. But, in the newly devel-

oped process, no membranes are utilized. An electrical

potential is used to create a repulsion zone that acts as

a membrane in separating charged ions, bacteria, viruses, and

microbes from seawater flowing through a 500 mm 100 mmmicrochannel. Water flows through the microchannel

tangential to a nanochannel where the voltage is applied. The

resulting force creates a repulsion zone and the stream splits

into two smaller channels at a nanojunction. The two streams

created are the treated water and concentrate. More than 99%

salt rejection and 50% recovery has been reported using this

process (Kim et al., 2010). The ion concentration polarization

processhasbeen reported to consumeapproximately 3.5 kWh/

m3, which is comparable to seawater desalination using RO

membranes (Kim et al., 2010). The advantage of ion concen-

tration is that the process is fouling free since membranes are

not used. The process is best suited for small to medium scale

systemswith thepossibility of battery-poweredoperation. The

process is not efficient in the removal of neutrally charged

organics and needs to be combined with other processes to

achieve treatment goals. Results from the ion concentration

polarization are not available for large-scale systems and only

modeling and bench-scale results have been reported.

2.5.3. Capacitive deionizationCapacitive deionization technology is not a recent develop-

ment, but several challenges with the identification of an

optimum material for the manufacturing of the associated

electrodeshavedelayed commercialization (Farmar et al., 1997;

Dermentzis and Ouzounis, 2008; Lee et al., 2009). This tech-

nology was developed as a non-polluting, energy efficient and

cost effective alternative to desalination technologies such as

RO (Welgemoed, 2005). In this technology, a saline solution

flows throughanunrestricted capacitor typemodule consisting

of numerous pairs of high-surface area electrodes. The elec-

trode material, typically a carbon aerogel, contains a high

specific surface area (400e1100 m2 per g) and a very low elec-

trical resistivity. Anions and cations in solution are electro-

sorbed by the electric field upon polarization of each electrode

pair by direct current power sources. For desalination of

brackish water, energy consumption of 1.37e1.67 kWh/m3 has

been reportedusing this technology (Welgemoed, 2005). Energy

consumption for high salinity waters (such as seawater) is notreadily available in the literature. The main drawback with

capacitive deionization is that the feed water recovery3.1.1.1. Solar stills. Asolar still is a simple device that isused toconvert saline water into drinking water (Qiblawey and Banat,

2008). Solar stills are used for direct solar desalination which is

mainly suited for small production systems and regions where

the freshwater demand is less than 200 m3 per day (Rodriguez,achievable is very low (Oren, 2008). Capacitive deionization has

primarily been used for brackish water desalination but with

improvements in electrodematerial and better process control

strategies, the technology holds promise for seawater desali-

nation (Oren, 2008; Anderson et al., 2010).

3. Renewable energy utilization

Renewable energy resources are the best energy supply option

for stand-alone desalination systems in remote regions where

energy supply from an electricity grid is not readily accessible

(Schafer et al., 2007; Gude et al., 2010). In urban regions,

renewable energy may provide treatment cost reductions due

to the implementation of a diversified portfolio of energy

sources and reduces GHG emissions. For an RO system used to

desalinate seawater with traditional fossil fuel based energy

source, CO2 emissions of 1.78 kg/m3 of desaltedwater andNOx

emissions of 4.05 g/m3 of desalted water have been reported

(Raluy et al., 2005). When RO was operated with electricity

generated from wind or solar energy, GHG emissions were

substantially lower. For RO integrated with wind energy

resource, CO2 emissions were 0.1 kg/m3 and NOx emissions

were 0.4 g/m3 (Raluy et al., 2005). For RO integrated with solar

photovoltaic energy resource, CO2 emissions were 0.6e0.9 kg/

m3 and NOx emissions were 1.8e2.1 g/m3 (Raluy et al., 2005).

Thus, an important avenue for reducing GHG emissions is the

utilization of renewable energy sources in place of fossil fuels

(NAS, 2010). A comparison of renewable energy resources is

shown in Table 3 and is discussed in detail below.

3.1. Solar energy

Solar energy is one of themost promising sources of renewable

energy. The quantity of solar energy received by earth is

a function of the season, with the highest quantity of incoming

solar energy received during the summer months (Kiehl and

Trenberth, 1997). Desalination using solar energy can be cate-

gorized as thermal and electromechanical processes. Thermal

processes use solar thermal energywhereas electromechanical

processes use photovoltaic cells.

3.1.1. Solar thermal processesSolar thermal desalination processes are characterized as

either direct or indirect processes (Qiblawey and Banat, 2008).

Direct processes consist of all parts integrated into one system

whereas indirect processes refer to heat coming from a sepa-

rate solar collecting device such as solar collectors or solar

ponds. The predominant solar thermal processes that are

integratedwith or used as desalination systems are solar stills

and solar ponds. Utilization of solar energy for desalination is

described in detail below.2002). The solar still consists of a transparent cover (glass or

plastic) which encloses saline water. The principle of operation

is evaporationandcondensation.Thesolar still cover traps solar

energywithin the enclosure. The trapped solar energy heats the

salinewater causingevaporationandcondensationon the inner

surface of the sloped transparent cover. As the saline water is

heated, its vapor pressure increases. The resultant water vapor

is condensed on the underside of the roof cover and runs down

into troughs, which collect the distillate. The distillate obtained

is of high quality with most salts, organic and inorganic

components removed. The solar still requires frequent flushing

toprevent salt precipitation.Designproblemsencounteredwith

solar stills are brine depth, vapor tightness of the enclosure,

distillate leakage, methods of thermal insulation, and cover

slope,shape, andmaterial (EiblingandTalbert, 1971). Inpractice,

heat losses will occur through a still. Currently available state-

of-the-art single-effect solar stills have an efficiency of approx-

imately 30e40% (Rodriguez, 2002). The solar still of the basin

type is the oldest method and improvements in its design have

been made to increase its efficiency (Naim et al., 2003). Modifi-

cations using passive methods include basin stills, wick stills,

diffusion stills, stills integrated with greenhouses, and other

configurations (Ahsan et al., 2010; Tabrizi et al., 2010;Murugavel

and Srithar, 2011).

Amajor drawbackwith the solar still is the energy loss in the

form of latent heat of condensation. In order to solve this

capability of air increases with temperature. Fresh water is

produced by condensing out the water vapor, which results in

dehumidification of the air. A significant advantage of the HD

technology is that it provides a means for low pressure, low

temperature desalination that operates off of waste heat and is

potentially very cost competitive (Parekh et al., 2004). Since the

heat transfer coefficient of the condensing vapor from air

is much lower than for pure water, the heat transfer area

needed is enormously high and a disadvantage for the process

(Semiat, 2008).

3.1.1.2. Solar ponds. Solar ponds utilize a saline gradient toreduce the heat loss that normally occurs when the less dense

water heated by the sun rises to the surface of a pond and

loses energy to the atmosphere by convection and radiation

(Kalagirou, 2005). The objective of the solar pond is to utilize

a saline gradient to combat the thermal gradient and create

a stagnantand insulatingzone in theupperpartof thepondthat

traps the hot fluid in the lower section of the pond. New tech-

nologies combining solar thermal energy and desalinationhave

been shown to utilize up to 80% less energy than conventional

desalination technologies (Saltworks, 2010). In this thermo-

ionic desalination technology, the energy consumption is

reduced by harnessing low temperature heat to overcome the

sa

roce

tion

t, 2

l us

y, 2

uipm

ant

ated

OE,

esig

le e

hiev

ion

po

ed f

ele

t al

us

, the

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 01914problem, a humidification-dehumidification (HD) principle has

been developed (Mathioulakis et al., 2007). Solar desalination

based on the HD principle results in an increase in the overall

efficiency of the desalination plant and is therefore considered

a promising technique for small capacity, solar-driven desali-

nation plants (Mathioulakis et al., 2007). The basic principle of

the HD process is the evaporation of high salinity water and

condensation of water vapor from the humid air at ambient

pressure. The HD process is based on the fact that air can be

mixed with significant quantities of vapor. The vapor carrying

Table 3 e Comparison of renewable energy resources for de

Renewableenergyresource

Application

Solar Solar still: Direct conversion of

saline to potable water.

Solar pond: Utilization of

salinity gradient to store

heat and produce steam

for electricity generation.

Concentrated solar power:

Hot fluid used in turbine

generator for producing

electricity.

Photovoltaic cell: Conversion

of sunlight directly into

electricity to power

RO desalination.

Simple p

construc

and Bana

Beneficia

(Qiblawe

Same eq

power pl

concentr

plants (D

Hybrid d

renewab

easily ac

desalinat

electrical

Wind Wind turbine: Wind energy

used to generate electricity

to power RO desalination.

Well suit

requiring

(Eltawil e

Geothermal Geothermal steam to

generate electricity to

Continuo

resourcepower RO desalination. necessary (Benergy barrier for desalination. Salt water is evaporated to

produceaconcentratedsalt solution.Theconcentratedgradient

energy from the concentrated salt solution is then used to

operateadesalinationsystem.Usingdesalinationbrine for solar

ponds not only provides a preferable alternative to environ-

mental disposal, but also a convenient and inexpensive source

of solar pond salinity.

3.1.1.3. Concentrated solar power. A new study has estimatedthat25%of theworldselectricity couldcome fromconcentrated

lination.

Advantages Disadvantages

ss. Inexpensive material of

can be utilized (Qiblawey

008).

e of desalination brine

008).

ent used in conventional

s can be used for

solar power

2010).

ns with other (wind)

nergy sources are

able. Well suited for

plants requiring

wer (Eltawil et al., 2009).

Energy loss in the form of

latent heat of condensation

(Mathioulakis et al., 2007).

Large land area requirement

(Kalagirou, 2005).

Capital cost intensive. Output is

intermittent (Trieb et al., 2009).

Large land area requirement.

Capital cost intensive. Output

is intermittent (Kalagirou, 2005).

or desalination plants

ctrical power

., 2009).

Output is intermittent. Resource

is location dependant and

unpredictable (Kalagirou, 2005).

power output, predictable

rmal storage is not

Resource is limited to certain

locations (Kalagirou, 2005).arbier, 2002; EGEC, 2010).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 0 1915solar power (CSP) by the year 2050 (Trieb et al., 2009). Most

commercial CSP facilities use a system of curved mirrors to

collect the suns energy to heat a fluid flowing through tubes.

The hot fluid is then used to boilwater in a conventional steam-

turbine generator to produce electricity. Concentrating solar

power typically uses a Dish/Sterling system. Other methods to

concentrate solar energy utilize a parabolic trough, solar tower,

or linear Fresnel (Trieb et al., 2009). Large mirror fields concen-

trate the sunlight to produce high temperature steam for power

generation that can be used for seawater desalination. Part of

the harvested solar thermal energy is used during the day and

conventional electricity is used during the night for continuous

operation.

3.1.2. Solar electromechanical processDesalination using an electromechanical process involves the

application of photovoltaic (PV) cells. The PV process converts

sunlight directly into electricity. A PV cell consists of two or

more thin layers of semiconducting material (mostly silicon).

When the semiconducting material is exposed to sunlight,

electrical charges are generated and are conducted away by

metal contacts as direct current (DC). The PV sector has been

growing at 20% per annum or more for several years and is

now a multi-billion dollar business in Europe (Infield, 2009).

A total of around 5.95 GW of capacity has been installed

worldwide.

Photovoltaic cells are either monocrystalline silicon cells,

muticrystalline silicon cells, or amorphous cells. Mono-

crystalline cells are made of very pure monocrystalline silicon

whereas multicrystalline cells are produced using numerous

grains of monocrystalline cells (Kalagirou, 2005). The PV panel

is the principle building block of a PV system and any number

of panels can be connected together to give the desired elec-

trical output. The choice of photovoltaic active material has

important effects on system design and performance. Both

the composition and atomic structure of the cell is an

important consideration for assessing performance. Materials

other than silicon which are used for photovoltaic cells are

silver, cadmium telluride, and copper indium selenide

(Kalagirou, 2005; Jacobson and Delucchi, 2009). The advantage

of using cadmium-based and copper-based material for PV

cells is that they are manufactured by relatively inexpensive

industrial processes when compared to crystalline silicon

technologies (Kalagirou, 2005). Limited supplies of tellurium

and indium could reduce the prospects for some types of thin-

film solar cells and large-scale production could be restricted

by the availability of silver (Jacobson and Delucchi, 2009).

The most popular combination of PV cells is with RO. PV is

considered a proper solution for small applications in sunny

areas (less than 50 m3/d) (Mathioulakis et al., 2007). The

feasibility of PV-powered RO for desalination at remote sites is

a proven combination. Since both technologies are fairly

mature, PV-powered RO requires minimum maintenance

(Bayod-Rujula and Martinez-Gracia, 2009). Stand-alone PV

systems are used in areas that are not easily accessible to

electricity. A stand-alone system is independent of the elec-

tricity grid, with the energy produced being stored in batteries

(Bayod-Rujula and Martinez-Gracia, 2009). Typically, a stand-alone PV-powered RO system will consist of a module,

batteries, and charge controller (Bouguecha et al., 2005;Kalagirou, 2005). When electrical loads require an alter-

nating voltage, an inverter is used to transform direct current

into alternating current. The batteries allow operation at

constant flow and pressure. They are sized to stabilize the

power supply to the RO unit on a daily basis, as well as to

account for fluctuations in solar energy and water demand.

Battery less PV-powered RO systems have been tested before

(Thomson and Infield, 2003) but certain disadvantages such as

longer operation in stand by mode needs to be overcome. It is

also common practice to connect PV systems to the local

electricity grid. During the day, the energy generated from the

PV systems is used directly from the grid and power is

utilized from the electricity grid at night. Thus, the grid acts as

an energy storage system.

3.2. Wind energy

Wind has re-emerged as one of the most important and fast-

est growing sustainable energy resources since wind turbines

were first commercialized in the 1970s (Garca-Rodrguez,

2002; Ackermann and Soder, 2002). Wind turbines are

mature technologies and are commercially available. Wind-

powered desalination represents one of the most promising

renewable energy options for desalination, especially for

coastal areas with high availability of wind energy resources

(Zejli et al., 2004; Forstmeier et al., 2007).

After solar energy, wind energy is the most widely used

renewable energy source for small capacity desalination

plants (Kalagirou, 2005). The two common approaches for

wind-powered desalination systems include connecting both

the wind turbines and the desalination system to a grid, or

direct coupling of the wind turbines with the desalination

system (Ackermann and Soder, 2002). The latter option is

likely to be a stand-alone system at remote locations which

have no electricity grid. In this case, the desalination system

may be affected by power variations and interruptions caused

by the power source (wind). Hence, the stand-alone wind

desalination systems are often hybrid systems, combined

with another type of renewable energy source (for instance

solar), or a back-up system such as batteries or diesel gener-

ators (Mathioulakis et al., 2007). For stand-alone wind energy-

driven desalination units, the reported cost of fresh water

produced ranged from $1.35 per m3 to $6.7 per m3 when

compared to RO cost of about $1.0 per m3 (Karagiannis and

Soldatos, 2008; Mezher et al., 2011). The primary concern

with the use of wind energy for desalination is that wind

speed is highly variable. Wind speed varies both geographi-

cally and temporally and varies over a multitude of temporal

and spatial time scales (NREL, 2006). In terms of using a wind

turbine to generate power for desalination, this variation is

amplified by the fact that the available energy in the wind

varies as the cube of the wind speed (NREL, 2006). Thus, the

choice of location of the wind farm is critical for the exploi-

tation of wind resources for power generation to ensure

superior economic performance.

The theoretical maximum aerodynamic conversion effi-

ciency from wind to mechanical power for wind turbines is

59% (Kalagirou, 2005). The need to economize on blade coststends to lead to the construction of slender bladed, fast

running wind turbines with peak efficiencies close to 45%.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 01916Strategies for improving wind turbine performance include

the utilization of lighter turbine blades and improving power

storage systems (Spang, 2006; Thumthae and Chitsomboon,

2009). Wind turbines with blades that are approximately 40%

lighter than standard turbine blades could reduce capital costs

by up to 20e25% (Fairley, 2002). Lighter wind turbine design

with the blades placed on hinges allows flexibility for reduced

drag during high winds (Spang, 2006; Ameku et al., 2008).

Utilization of two blades per turbine rather than three blades

leads to lighter turbine design, but, the three-blade wind

turbines have a lower noise level than two-blade wind

turbines and the rotor moment of inertia is easier to handle.

Wind generators with vertical axis turbines convert wind

energy into electrical energy at a greater efficiency than

horizontal axis turbines and result in approximately 5%

increase in energy production and significantly reduce the

investment cost per kWh (Spang, 2006). With respect to the

materials available for constructing wind turbines, enough

concrete and steel exist and both these commodities are fully

recyclable (Jacobson and Delucchi, 2009). Increased produc-

tion of wind turbines in the future could be slowed due to the

availability of rare-earth metals such as neodymium, but low

cost sources available in China could be utilized to compen-

sate for a shortage of essential metals for manufacturing wind

turbines (Jacobson and Delucchi, 2009).

3.3. Geothermal energy

Geothermal energy sources are classified in terms of the

measured temperature as low (150 C). Geothermal energy is usuallyextracted with ground heat exchangers (Kalagirou, 2005). The

primaryadvantageof geothermalenergycompared to solar and

wind, is that it is both continuous and predictable; as such,

thermal storage is unnecessary (Barbier, 2002). Geothermal

energy is being evaluated for desalination in Queensland,

Australia (QueenslandGeothermal EnergyCenterof Excellence,

2010). This energy center has estimated that a geothermal plant

in the 1000e100,000m3/d capacity can easily provide the entire

fresh water needs for an outback city at the cost of around

$0.73e1.46/m3. The first installation of geothermal energy-

powered desalination plants was reported by the Bureau of

Reclamation of the United States Department of the Interior in

the 1970s (Awerbuch et al., 1976). Geothermal energy has also

been investigated for desalination recently on the Baja Cal-

ifornia peninsula (Hiriart, 2008).

Geothermal energy presents a mature technology which

can be used to provide energy for desalination systems. High

temperature geothermal fluids generate electricity to power

RO plants and are used directly as shaft power for mechan-

ically driven desalination. The main advantage of using

geothermal energy for desalination is that it is a stable and

reliable heat supply 24 h a day, 365 days a year. Also, desali-

nation using geothermal energy source is environmentally

friendly with no emission of greenhouse gases (EGEC, 2010).

3.4. Hybrid systemsCombinations of wind and solar energy have been used for

driving desalination systems (Koutroulis and Kolokotsa, 2010;Karellas et al., 2011). The purpose of using hybrid wind-solar

systems for desalination is based on the fact that in certain

locations, wind and solar time profiles do not coincide

(Mathioulakis et al., 2007). The complementary features ofwind

and solar resources make use of hybrid wind-solar systems to

drive desalinations systems (Charcosset, 2009). Hybrid wind-

solar PV systems have been implemented in the Sultanate of

Oman, Israel, Mexico, Germany, and Italy (Petersen et al., 1981;

Al Malki et al., 1998; Weiner et al., 2001; Pretner and Iannelli,

2002). Two RO desalination plants supplied by a 6 kW wind

energy converter and a 2.5 kW solar generator have been

designed for remote areas (Petersen et al., 1981). Stand-alone

systems for seawater desalination using hybrid wind-PV

system have also been designed (Mohamed and Papadakis,

2004). Using wind and solar conditions in Eritrea, East Africa,

thehourlywaterproductionwasdetermined to be 35m3/dwith

a specific energy consumption of about 2.33 kWh/m3 (Gilau and

Small, 2008). Although several studies have been performed

using hybrid renewable energy desalination systems, none of

them represent large-scale applications.

3.5. Design and implementation of renewable energysystems

Renewable energy desalination systems need to be designed

using an iterative approach (Voivontas et al., 2001). The first

step of the approach involves the definition of a list of alter-

native technologies that satisfy the water demand. A second

step focuses on a detailed design analysis of each candidate

option made to determine the plant capacity, the structure of

the power unit and the operational characteristics. The final

step involves a financial analysis of the investment associated

with the selected renewableenergy-desalinationcombination.

The most challenging issue associated with the imple-

mentation of renewable energy-desalination technology is the

optimum matching of the intermittent renewable energy

power output with the steady energy demand of the desalina-

tion process. Power supply management and demand-side

management are considered as the two options available to

address this problem (Voivontas et al., 2001). In the first case, an

appropriately controlled hybrid renewable energy resourceunit

that is capable of providing a steady energy output is used. This

unit is sized at the nominal power demand of the desalination

process. In the demand-side management option, the desali-

nation process only operates when the energy output of the

renewable energy resource unit is able to cover the energy

demand.

Other options available to address the issue of intermittent

renewable energy power output are different types of energy

storage such as electro-mechanical, virtual (through process

modification), and grid energy (Kalogirou, 1997). Compressed

air energy storage plants have also been used when energy

produced from a wind turbine exceeds grid load capacity

(BINE, 2010). For limited periods, the compressed air stores

cover the short-term reserve requirement, which are needed

due to the unpredictable forecasts of wind power feeding the

grid. In this case, wind turbines do not have to deactivate in

the event of a grid overload, and if there is excess supply ofelectrical energy, the storage technology refines base-load

electricity, converting it to peak-load electricity (BINE, 2010).

but their application for desalination has been limited. The

energy. The selection of appropriate renewable energy

resources depends on factors such as plant size, feed water

Ameku, K., Nagai, B.M., Roy, J.N., 2008. Design of a 3 kW windturbine generator with think airfoil blades. Experimental

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 9 0 7e1 9 2 0 1917reasons for their limited application are technology, cost, and

availability. Although desalination technologies are mature,

technologies for the storage of renewable energy are not

completelymature and avenues for design improvements still

exist. Prices for renewable energy technologies are decreasing

but the capital costs still prohibit their commercialization at

a large-scale. Renewable energy is also not sufficiently avail-

able in certain locales and for regions with adequate supplies,

a lack of adequate storage strategies has sometimes impeded

development due to supply intermittency.

4. Research needs

Minimization of the energy required for seawater RO desali-

nation through utilization of efficient system design, high effi-

ciency pumping, and energy recovery devices has been studied

extensively andnear optimal performance characteristics have

already been tested and achieved. Further design improve-

ments in these categories will only provide marginal reduction

in energy consumption further. Research avenues that show

the most promise for reducing energy usage lie in the devel-

opment and testing of advanced membrane materials which

canenhancetheperformanceof themembrane, in termsofflux

and rejection, and reduce feed pressure requirements. The

development of nanocomposite, nanotube, and biomimetic

membranes show promise but much more data are necessary

in order to validate the application of these membranes under

normaloperationandchemical cleaningconditions. Innovative

technologies, such as forward osmosis, require amore efficient

recovery of the draw solution and methods to reduce internal

concentrationpolarization, inorganic scaling, and foulingof the

membrane. Similarly, the application of ion concentration

polarization and capacitive deionization technologies requiresThe selection of the appropriate renewable energy

resource depends on several factors including plant size, feed

water salinity, remoteness, availability of grid electricity,

technical infrastructure, and the type and potential of the

local renewable energy resource and storage options. In

addition, socio-economic factors and policy need to be

considered as a driver for renewable energy resource imple-

mentation. The applicability of renewable energy resources

for desalination strongly depends on the local availability of

renewable energy and the quality of water required after

treatment. In addition, some combinations of resources are

better suited for large size plants, whereas some others are

better suited for small scale applications. Other important

factors that need to be considered are the capital cost of the

equipment and the land area required for the equipment

installation. When considering resource availability, solar

thermal energy and photovoltaics are considered to be a better

choice over wind and geothermal energy which are location-

dependant. When considering the continuity and predict-

ability of power output, geothermal energy is themost reliable

resource as the output is intermittent and less predictable for

solar thermal, photovoltaic, and wind energy.

Renewable energy resources provide various advantagesfurther study in order to enhance feed water recovery to the

point where this technology might be economically feasible.Thermal and Fluid Science 32 (8), 1723e1730.Anderson, M.A., Cudero, A.L., Palma, J., 2010. Capacitive

deionization as an electrochemical means of saving energyand delivering clean water. Comparison to presentdesalination practices: will it compete? Electrochemica Acta55, 3845e3856.

Andrews, W.T., Laker, D.S., 2001. A twelve-year history of largescale application of work exchanger energy recoverytechnology. Desalination 138 (1e3), 201e206.Acknowledgements

The authors would like to thank the WateReuse Research

Foundation (WRRF) and the California Energy Commission

(CEC) for project funding (Project # WRRF-08 13).

r e f e r e n c e s

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