renewable energy driven desalination systems modelling

16
Renewable energy driven desalination systems modelling C. Koroneos * , A. Dompros, G. Roumbas Laboratory of Heat Transfer and Environmental Engineering, Aristotle University of Thessaloniki, P.O. Box 483, 541 24 Thessaloniki, Greece Received 10 January 2005; accepted 6 July 2005 Available online 19 September 2005 Abstract Renewable energy sources for powering desalination processes is a very promising option especially in remote and arid regions where the use of conventional energy is costly or unavailable. Renewable energy driven desalination systems have been extensively discussed as an innovative approach to desalinate water economically and in an environmentally friendly manner. The stochastic nature of renewable energy sources (RES) which results in the use of expensive energy storage systems usually limits the penetration of RES to the power generation system of a region. Desalination systems can utilise in a more economically efficient way the avail- able RES potential. The energy produced is consumed for potable water production which can be stored economically for a large period of time before consumption. An integrated model for the use of renewable energies (wind, solar) in the desalination of sea- water has been developed in the context of REDDES project. In this work, a model is developed where desalination technologies are coupled with RES power systems to produce potable water at the lower possible cost. The presented model is incorporated in the REDDES software. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Renewable energy systems; Water desalination systems; Energy efficiency; Water security 1. Introduction The limited market penetration of renewable sources of energy (RES) can be attributed to a large number of constraints, including problems related to financing, reg- ulation, technical issues, lack of information, education and training. The stochastic nature of RES which results in the use of expensive energy storage systems usually limits the penetration of RES to the power generation system of a region. The produced energy varies in time as wind speed or solar radiance varies and the power has to be consumed directly or else it will be lost. RES penetration in the desalination industry does not face the same barriers as in the case of RES for electricity power production. In the case of RESedesalination coupling the energy is consumed directly for water pro- duction, the water can be stored cheaply in large quan- tities and for long periods [1]. Several desalination processes have been developed but not all of them are reliable and in commercial use. The most important processes are split into two main categories [1]: Thermal (or distillation) processes: Multi-Stage Flash Distillation (MSF), the Multi-Effect Distilla- tion (MED), the Thermal Vapour Compression (TVC) and the Mechanical Vapour Compression (MVC) processes. Membrane processes: Reverse Osmosis (RO) and Electrodialysis (ED) processes. ED is confined to de- salination of brackish water while RO can be used for both, brackish and seawater desalination. * Corresponding author. Tel.: C30 231 0995968; fax: C30 231 0996012. E-mail address: [email protected] (C. Koroneos). 0959-6526/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2005.07.017 Journal of Cleaner Production 15 (2007) 449e464 www.elsevier.com/locate/jclepro

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Page 1: Renewable energy driven desalination systems modelling

Journal of Cleaner Production 15 (2007) 449e464

www.elsevier.com/locate/jclepro

Renewable energy driven desalination systems modelling

C. Koroneos*, A. Dompros, G. Roumbas

Laboratory of Heat Transfer and Environmental Engineering, Aristotle University of Thessaloniki,

P.O. Box 483, 541 24 Thessaloniki, Greece

Received 10 January 2005; accepted 6 July 2005

Available online 19 September 2005

Abstract

Renewable energy sources for powering desalination processes is a very promising option especially in remote and arid regionswhere the use of conventional energy is costly or unavailable. Renewable energy driven desalination systems have been extensively

discussed as an innovative approach to desalinate water economically and in an environmentally friendly manner. The stochasticnature of renewable energy sources (RES) which results in the use of expensive energy storage systems usually limits the penetrationof RES to the power generation system of a region. Desalination systems can utilise in a more economically efficient way the avail-

able RES potential. The energy produced is consumed for potable water production which can be stored economically for a largeperiod of time before consumption. An integrated model for the use of renewable energies (wind, solar) in the desalination of sea-water has been developed in the context of REDDES project. In this work, a model is developed where desalination technologies are

coupled with RES power systems to produce potable water at the lower possible cost. The presented model is incorporated in theREDDES software.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Renewable energy systems; Water desalination systems; Energy efficiency; Water security

1. Introduction

The limited market penetration of renewable sourcesof energy (RES) can be attributed to a large number ofconstraints, including problems related to financing, reg-ulation, technical issues, lack of information, educationand training. The stochastic nature of RES which resultsin the use of expensive energy storage systems usuallylimits the penetration of RES to the power generationsystem of a region. The produced energy varies in timeas wind speed or solar radiance varies and the powerhas to be consumed directly or else it will be lost. RESpenetration in the desalination industry does not facethe same barriers as in the case of RES for electricity

* Corresponding author. Tel.: C30 231 0995968; fax: C30 231

0996012.

E-mail address: [email protected] (C. Koroneos).

0959-6526/$ - see front matter � 2005 Elsevier Ltd. All rights reserved

doi:10.1016/j.jclepro.2005.07.017

power production. In the case of RESedesalinationcoupling the energy is consumed directly for water pro-duction, the water can be stored cheaply in large quan-tities and for long periods [1].

Several desalination processes have been developedbut not all of them are reliable and in commercial use.The most important processes are split into two maincategories [1]:

� Thermal (or distillation) processes: Multi-StageFlash Distillation (MSF), the Multi-Effect Distilla-tion (MED), the Thermal Vapour Compression(TVC) and the Mechanical Vapour Compression(MVC) processes.� Membrane processes: Reverse Osmosis (RO) andElectrodialysis (ED) processes. ED is confined to de-salination of brackish water while RO can be usedfor both, brackish and seawater desalination.

.

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450 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

The selection of a process usually is based on severalparameters, such as site conditions, local circumstances,energy availability, etc. The ‘‘best’’ desalination systemfor a particular application will be the system that reli-ably produces water of the expected quality and quantityat reasonable cost.

The feasible RESedesalination technology combina-tions are very clearly demonstrated in Fig. 1. The differ-ent power forms derived from RES are coupled withthe equivalent desalination technology. The RESedesalination coupling schemes under examination couldbe divided in two categories:

1. RESedesalination coupling schemes that require theRES unit and the desalination unit to be located inthe same area. Such couplings are:a. Wind-shafteMechanical Vapour Compression

(MVC) couplingb. Solar thermal-heateThermal Vapour conversion

(TVC)c. Solar thermal-heateMulti-Stage Flash Distilla-

tion (MSF)d. Solar thermal-heateMulti-Effect Distillation

(MED)e. Solar thermal-heateDistillationf. Geothermal-heateThermal Vapour conversion(TVC)

g. Geothermal-heateMulti-Stage Flash Distillation(MSF)

h. Geothermal-heateMulti-Effect Distillation (MED)2. RESedesalination coupling schemes that do not

require the RES unit and the desalination unit tobe located in the same area. Such couplings are:a. Wind-electricityeMechanical Vapour Compres-

sion (MVC) couplingb. Wind-electricityeReverse Osmosis (RO)c. Solar PV-electricityeReverse Osmosis (RO)d. Solar PV-electricityeMechanical Vapour Com-

pression (MVC) couplinge. Geothermal-electricityeMechanical VapourCom-

pression (MVC) couplingf. Geothermal-electricityeReverse Osmosis (RO)

A short assessment of each technically feasible appli-cation can be made:

� Thermal-distillation processes. Thermal distillationplants such as MED, MSF or TVC can utilise solarthermal energy or geothermal energy. Thermal pro-cesses require a high-energy input (due to the energyrequired for change of phase) and also auxiliary elec-tricity for pumping needs. Solar thermal systems areheavily dependent on solar radiation and weatherconditions and they need large heat accumulators.Generally, solar thermal distillation application are‘‘solar assisted’’ rather than stand-alone. These unitsare suitable when low enthalpy energy is also avail-able. These systems are not considered suitable for

RES

Geothermal

Solar

Wind PV system Solar-

Thermal

HeatShaft Electricity

RO MEDMSFTVCMVC ED

Fig. 1. Coupling of RES with desalination processes.

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451C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

small scale and remote areas. Geothermal systemsare ideal for thermal-distillation processes but arelimited to areas where geothermal fields exist.� Solar PVeRO. The electricity form PV systems canbe used to drive high-pressure pumps in RO desali-nation plants. The main advantage of PVedesalina-tion systems is their ability to develop small sizedesalination plants. The energy production unit con-sists of a number of photovoltaic modules, whichconvert solar radiation into direct electric current(DC). DC/AC inverters have to be used becauseRO uses alternating current (AC) for the pumps.Energy storage (batteries) is required for PV outputpower smoothing or for sustaining system operationwhen insufficient solar energy is available.� WindeRO/MVC. Wind energy can be coupled withRO and MVC processes for the desalination of wa-ter. RO and MVC require electrical or mechanicalenergy as primary energy input, which can be pro-vided from a single wind turbine or a wind farm.The selection between the technologies depends onthe feed water quality and the required product waterquality. MVC, as all distillation processes producewater with very low salinity (below 20 ppm totaldissolved solids). Membrane processes (RO) pro-duce water with higher salinity (500 ppm TDS).Because of the variability of the wind speed, it isdifficult to predict the energy output. Appropriatepower control and conditioning systems are re-quired in order to match the ratio of the power in-put to the desalination load.

Because of the spatial distribution of the RES, RESedesalination coupling schemes that do not require theRES unit and the desalination unit to be located in thesame physical area are of special interest. In the presentpaper, wind energy and solar PV coupled with RO andMVC are examined. The models were developed in theframework of the Renewable Energy Driven DESalina-tion (REDDES) project which was funded by the EU [2].

2. Renewable energy sources

2.1. Modelling of solar photovoltaic energy (PV)

Many types of semiconductor materials can convertlight into electrical power by means of the photovoltaic(PV) effect, but only a few of them have been used assolar cells to date. The commercial market for PV cellstoday is dominated by crystalline silicon (Si). Howeverother materials, notably amorphous silicon (a-Si), cad-mium telluride (CdTe), copper indium diselenide(CuInSe2, CIS), and gallium arsenide (GaAS), are alsoavailable and substantial investments are being madein their development [3].

When light is absorbed by a solar cell, electrons are re-leased and move according to the internal electric poten-tial so that when a load is connected across the contactsan electric current flows. The voltage across a solar cell isprimarily dependent on the design and the materials ofthe cell, whilst the electrical current depends primarilyon the incident solar irradiance and the cell area. Theoutput from a typical solar cell, which is exposed to thesun, increases from zero at sunrise to a maximum at mid-day and then falls again to zero at dusk. The ratio of elec-trical power produced by a solar cell to the incident solarirradiance is known as the PV cell efficiency.

Groups of cells are mounted together on a glass plateand wired in series to form a PVmodule typically around0.5 m2 in size. Groups of modules can be connected to-gether electrically to form a PV array. PV arrays canbe mounted on a fixed structure or on a sun-trackingstructure to maximize the incident solar radiation.

The power production capacity of a PV array isexpressed in Watt-peak (Wp) units. A PV cell of 1 Wpproduces 1 W of electrical energy when exposed to solarirradiance of 1000 Wm�2 at a cell temperature of 25 �C.

Most of the PV systems use monocrystalline or poly-crystalline silicon cells. The most advance commercialcrystalline silicon modules have efficiencies more than15% but typically efficiencies range from 11 to 13%.

Other PV system components, such as cabling,batteries, battery charge controllers, inverters, etc. arecommonly known as ‘‘Balance of System’’ (BOS) com-ponents. These components provide the necessary in-terface with the grid or a specific application such ashousehold loads, telecommunication equipment, etc.

The energy produced by solar cells depends on the so-lar radiation of geographical specified region, the area ofthe PV cells and their efficiency. The produced energy iscalculated by the equation [3]:

PPVZhPVAPVGZhmphT;coeffGAPV ð1Þ

where hPV is the overall efficiency, hmp is the maximumpower point efficiency of the PV module, hT,coeff is thetotal efficiency of dirt (hd), inverter losses (hinv) and los-ses due to connections (hcon) (hT,coeff Z hdhinvhcon), APV

is the PV area (m2), and G is the average solar radiation(W/m2).

The maximum power point efficiency of the PV mod-ule hmp decreases with increasing cell temperature andEq. (2) is used to take this into account [3]:

hmpZhmpSOCCmP;mpðTC�TSOCÞ ð2Þ

where hmpSOC is the module efficiency under standardoperating conditions (SOC), mP,mp is the temperature co-efficient of efficiency at the maximum power point (it hasnegative value) ( �C�1), TC is the actual cell temperature

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452 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

( �C) and TSOC is the temperature under standard oper-ating conditions 25 �C.

The module efficiency at standard operating condi-tions is given by Eq. (3) [3]:

hmpSOCZImpSOCVmpSOC

APVCGSOC

ð3Þ

where ImpSOC is the current at the maximum powerpoint of the module under standard operating condi-tions (A), VmpSOC is the voltage at the maximum powerpoint of the module at standard operating conditions,APVC is the module area (m2) and GSOC Z 1000 Wm�2

is the solar insolation under standard operating condi-tions. ImpSOC, VmpSOC, APVC in Eq. (3) are the PV mod-ule specific and are provided by the manufacturer.

The temperature coefficient mP,mp of the efficiency atthe maximum power point is approximated as follows:

mP;mpZhmpSOC

mVOC

VmpSOC

ð4Þ

where mVOC is the voltage temperature coefficient(V �C�1) and is also provided by the manufacturer.

The working temperature of the module/cell TC is es-timated by the method of Dufie and Beckman, which isbased on the energy balance on the cell surface [3]:

TCZTaCGta

UL

1� nCta

ð5Þ

where G is the incident solar radiation (Wm�2), UL isthe heat loss coefficient from the cell to the surroundings(including losses by convection and radiation from thetop and bottom of the cell, and losses by conductionthrough the mounting framework) (Wm�2 �C�1), Ta isthe ambient temperature ( �C), t is the transmittanceof the cover over the cells, a is the fraction of the radi-ation incident that is absorbed on the surface of thecells, nC is the efficiency of the module in converting ra-diation into electrical power.

The efficiency of the module nC is approximated bytaking it equal to hmpSOC. The product ta is consideredconstant equal to 0.9 (ta Z 0.9). The UL is calculated bythe equation [3]:

ta

UL

ZTC;NOST �Ta;NOST

GT;NOST

ZTC;NOST� 20

800ð6Þ

where TC,NOST is the nominal operating temperature,defined as the cell temperature that is reached whenthe cell is mounted in a normal way at a solar radiationof GT,NOST Z 800 Wm�2, wind speed of 1 m s�1 andambient temperature of Ta,NOST Z 20 �C and at no load

operation. For the purposes of the model, TC,NOST istaken to be constant equal to 46 �C.

If thd is the average hours of daylight in a region (h/day) and G the average incident solar radiation (W/m2),then the PV cell power production is given by theequation:

PPVMZhmphT;coeffGAPVMthd!10�3 ð7Þ

where APVM is the module area and PPVM is the averagedaily PV module energy production (kWh/day). InEq. (7), the PV modules are considered tilted at opti-mum angle for maximum performance.

The required energy production PPV (kWh/day) isgiven by the equation:

PPVZPDES

thd=24Cð1� thd=24ÞaBCHaBDCH

ð8Þ

where PDES is the energy required by the desalinationunit (kWh/day), aBCH is the charging efficiency of thebattery, and aBDCH is the discharging efficiency of thebattery.

The total number of modules needed (NPVM) and thetotal PV area (APV) are calculated by the followingequations:

NPVMZ1

thd=24Cð1� thd=24ÞaBCHaBDCH

PDES

PPVM

ð9Þ

and

APVZNPVMAPVC ð10Þ

The total hardware cost of a PV plant depends on thesurface area or the peak power of the PV modules andthe storage capacity of the batteries [4]:

CPVtotalZ�CPVMCCSUPCCpcond

�PPV;peakCCB;storageSBPV

CCDC=ACPload ð11Þ

where:

1. CPVM is the cost of PV modules (V/Wp) and is takenequal to 5 V/Wp [4];

2. CSUP is the support cost (V/Wp), and is taken equalto 1 V/Wp [4];

3. Cpcond is the power conditioning cost (V/Wp), and istaken equal to 0.5 V/Wp;

4. CB,storage is the battery storage cost (V/kWh), and istaken equal to 170 V/kWh;

5. CDC/AC is the DC/AC converter in (V/W). The costof the inverter is given by the equation [4,5]:

CDC=ACZ1099!10�3!P�069load ð12Þ

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453C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

6. PPV,peak is the peak power output of the PV modules(Wp) and is given by the equation:

PPV;peakZhmpAPV!1000 ð13Þ

7. Pload is the power consumption of the desalinationunit (kW) and is equal to PDES/24;

8. SPV is the storage capacity of the batteries in kWhand is given by the equation:

SPVZPPVð1� thdÞaBCH ð14Þ

Operating and maintenance (O&M) costs are gen-erally low, because there are no moving parts. O&Mcosts represent an annual share going from 1.5 to 2.5%of the total capital cost [3]. In this work the figure of2.5% per year is used, or

CPVO&MZ0:025!CPVtotal ð15Þ

A simple calculation of the cost of the electricity pro-duced by a PV plant may be obtained by using theexpression:

CPV kWhZðCPVtotalCCRÞCCPVO&MC

CB;storageSPVnB

PPV!365ð16Þ

where CPVkWh is energy production cost (V/year), nB isthe lifetime of batteries, CCR is the capital cost recoveryfactor and is given by the equation:

CCRZrð1CrÞn

ð1CrÞn� 1ð17Þ

where r is the discount rate and n is the number of yearsof useful life of the plant.

The electricity production cost depends (Fig. 2) onthe average solar radiation and the average daylighthours of the geographic site under examination.

2.2. Modelling of wind energy

Wind turbines extract the kinetic energy of the windby transferring the momentum of the air passingthrough the wind turbine rotor, into the rotor blades.The rotor blades are aerofoils that act in a similar wayas the wings act on the aircraft. The modern wind tur-bine is a very efficient device in concentrating the energyof the airflow into a single rotating shaft. The power inthe shaft can then be utilised in any way.

The wind turbine rotor can be set on either a horizon-tal or a vertical shaft. In a horizontal shafted turbine,the rotor must be orientated towards the wind, and thisis achieved with either electrical or hydraulic yaw drives.The horizontal axis wind turbine (or HAWT) dominatesthroughout the world and there is no significant contri-bution from vertical axis machines (VAWTs) in Europe.This is because the HAWT has proven more cost effec-tive than the VAWT.

The system cost effectiveness has improved by a factorof 3 over the last 10 years. Reliability is also very highwith the machines available for generation for upwardsof 96% of the time. This technology is at a stage where itcan deliver large-scale implementation reliably, and ata price approaching that of a conventional generationplants [6,7].

The relation between power production and instantwind speed at certain atmospheric conditions is giveby the equation [7]:

PWTðUÞZ1

2rAIRCDðUÞ

pDWT2

4U3 ð18Þ

0.24 9

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

14 19 24

Solar Radiation (kWh/m2day)

EU

RO

/kW

h pr

oduc

ed

6 hours/day daylight

8 hours/day daylight

12 hours/day daylight

Fig. 2. Energy production costs of an AEG:PQ 40/50 PV module as a function of the average solar radiation and daylight hours, r Z 7% and

n Z 15 years.

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454 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

where rAIR is the air density at a given temperature (kg/m3), U is the wind speed (m/s), DWT is the blade diam-eter of the wind turbine (m), and CD(U ) is the efficiencyof the wind turbine that depends also on wind speed.

The produced energy from a wind turbine depends onthe ‘power curve’ of the wind turbine, which is suppliedby the manufacturer. The ‘power curve’ relates the windspeed and the power generated by the wind turbine.Typical power curves of commercial turbines are pre-sented in Fig. 3.

The time variability of wind speed is usually modelledby the K-Weibull distribution. The probability of occur-rence of a specific wind speed is estimated using Eq. (19)[8e12].

pðUÞZk

C

�U

C

�k�1exp

���U

C

�k�ð19Þ

where k is the shape factor and C is the scale parameter.The scale parameter of the Weibull distribution can

be estimated using Eq. (19#).

CZUAVG

G�1C1

k

� ð19 0Þ

where UAVG is the average wind speed (m/s) and G is theGamma function.

Consequently, the only necessary inputs for model-ling the wind speed in a specific geographic region arethe mean annual wind speed and the parameter shape k.Sites with small shape factor k have greater wind var-iability (Fig. 4), and thus the power production froma wind farm would vary accordingly. Thus in sites withlarge k factor, a wind turbine would provide steadierpower.

Usually the mean annual wind speed is measured ata height of 10 m. The wind speed at rotor height canbe estimated from the wind speed at 10 m using Eq. (20).

UAVGZUref

�H

Href

�rð20Þ

where Uref is the average wind speed at height Href and ris an empirical factor.

The annual energy production of the wind turbineEWT in kWh/year is estimated using Eq. (21).

EWTZ8760!

Z UCUTOUT

UCUTIN

pðUÞPðUÞ dU ð21Þ

And in a wind farm with NWT of wind turbines:

ENWTZNWTEWT ð22Þ

The annual output of a Bonus 300/33.4 Mk III windturbine based on Eq. (21) for various average windspeeds and shape factors is shown in Fig. 5. The greaterthe average wind speed of a site the greater is the energyproduction. At the same time, the greater the shape fac-tor the greater is the energy production from the windturbine.

The total capital cost (V) of a wind farm is given bythe following equation.

TCCWTZ

�929:2C2435:6!exp

��PWTR

33:4

��

!PWTRð1CfWTÞNWT ð23Þ

where the PWTR is the rated power of the wind turbine inkW and fWT expresses installation costs. Eq. (23) wasdeveloped for the Greek market [13].

0

100

200

300

400

500

600

700

800

0 10 15 20 25 30

Wind speed (m/s)

Pow

er (

kW)

"Nordex N27/150"

"Vestas V39 600/39"

"NEG Micon 750/44 50 Hz"

"Bonus 300/33.4 Mk III"

5

Fig. 3. Typical wind turbine ‘power curves’.

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455C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

00 5

0.02

0.04

0.06

0.08

0.1

0.12

0.14

10 15 20 25 30 35

Wind Speed (m/s)

Pro

babi

lity

dens

ity k=1.5 mean U=7.5m/s

k=2.5 mean U=7.5m/s

k=1.5 mean U=10.5m/s

k=2.5 mean U=10.5m/s

Fig. 4. Weibull distribution.

Installation costs vary considerably ( fWT w 30e60%) and depend on the number of wind turbines,and the remoteness of the area. Because the presentstudy is referring to remote areas, a high constant valueis assumed 50%.

The operational and maintenance costs are given bythe equation [13]:

CWTO&MZ0:03!TCCWT ð24Þ

A simple calculation of the cost of the electricity pro-duced by a wind farm plant may be obtained by usingthe expression:

CWTkWhZðTCCWT!CCRÞCCWTO&M

EWT

ð25Þ

where CWTkWh is energy production cost (V/kWh), CCR

is the capital cost recovery factor and is given by theequation:

CCRZrð1CrÞn

ð1CrÞn� 1ð26Þ

where r is discount rate and n is the number of years ofuseful life of the plant.

The energy production cost depends strongly on thewind characteristics of the site under examination. Theproduction cost is decreasing as the site is windiest(greater UAVG) and the greater the shape factor k, forthe same wind turbine. Fig. 6 presents the annual costsof a wind farm as a function of average wind speedand shape factor for discount rate r Z 7% and of usefullife of the plant n Z 15 years.

8000007 8 9 10 11 12

1000000

1200000

1400000

1600000

1800000

2000000

Average wind speed (m/s)

Ene

rgy

kWh/

year

k=1.5

k=1.8

k=2

k=2.5

Fig. 5. Energy production of a Bonus 300/33.4 Mk III wind turbine.

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456 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

0.037 8 9 10 11 12

0.035

0.04

0.045

0.05

0.055

0.06

0.065

0.07

Average wind speed (m/s)

Ene

rgy

cost

EU

RO

/yea

r

k=1.5

k=1.8

k=2

k=2.5

Fig. 6. Energy production costs of a Bonus 300/33.4 Mk III wind turbine as function of average wind speed and shape factor, r Z 7% and n Z 15

years.

Desalination plants operate with constant power in-put PDES but the wind turbine power output varies somaximum annual wind energy that the desalinationplant can absorb is estimated by:

EWT�DESZ8760!NWT

Z UDES

UCUTIN

pðUÞPðUÞ dU

CEDES

Z UCUTOUT

UDES

pðUÞ dðUÞ ð27Þ

where EDES is the annual energy requirements of the de-salination plant (kWh/year) and UDES is the intercept ofthe ‘power curve’ of the turbine with the power input re-quirements of the desalination plant: PDES Z P(UDES).

The auxiliary energy is considered to be provided bythe grid in order to cover the energy demand during lowwind speed. The annual energy flows from auxiliary en-ergy sources are estimated by Eq. (28).

EAux�DESZEDES �EWT�DES ð28Þ

The excess power that is not used by the desalinationunit is sold to the grid (in the case of grid-connectedplants) or dumped in the case of stand-alone plants.The wind energy sold to the grid is given by Eq. (29).

EWTSOLDZEWT �EWT�DES ð29Þ

3. Modelling desalination technologies

3.1. Mechanical Vapour Compression (MVC)distillation

The Mechanical Vapour Compression (MVC) distil-lation process is generally used for small and medium

scale seawater desalting units. The heat for evaporationcomes from the compression of vapour rather thanthe direct exchange of heat from steam produced inthe boiler. The mechanical compressor is usually electri-cally driven, allowing the sole use of electrical power toproduce water by distillation (Fig. 7). The compressorcreates a vacuum in the vessel and then compresses thevapor taken from the vessel and condenses it in a tubebundle, also in the same vessel. Seawater is sprayed onthe outside of the heated tube bundle where it boilsand partially evaporates, producing more vapours [2].

The plants, that use this process, are generally de-signed to take advantage of the principle of reducingthe boiling point temperature by reducing the pressure.MVC units have been built in a variety of configurationsto promote the exchange of heat to evaporate the seawa-ter. Extra care is required with the control of the brinelevel in the evaporator and the proper maintenance ofthe compressor. Operation at low temperatures mini-mizes the formation of scaling and corrosion of materi-als. MVC units are usually built in the 20e2000 m3/day(0.005e0.5 mgd) range [2].

Capital and energy costs are significant factors in thedetermination of the total water production cost. Theenergy demand is mainly required to drive the vapourcompressor motor. The operation and maintenance ofthe vapour compressor motor may be half of the totaloperating cost. The energy requirements of VC plantsare between 8 and 12 kWh/m3 [2].

The main equipment for the MVC desalination pro-cess is the evaporator, the heat exchanger and the com-pressor. The feed water is preheated in a heat exchangeror a series of heat exchangers by the hot discharge ofthe brine and the distillate. The distilled water producedby the condensation leaves the plant through the pre-heaters as product water. Plant layout and necessaryinput parameters are presented in Fig. 7 [14e16]. The

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457C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

Brine Discharge

Brine

Seawater Feed

Product Water

Vapor Compresor

Vapor

Brine Discharge

Bri

ne R

ecir

cula

tion

Preh

eate

d Fe

ed

Seawater & Brine

CompressedVapor

Work in

Preheated Feed

1

2

PW

QPW

QPW

QPW

QPW

3

7

5QF

QBQB

8

QF 4

6

Fig. 7. Typical flow diagram of Vapor Compression (VC) distillation plant.

energy requirements of the compressor are estimated byEq. (30):

PMVCZQPWðh2� h1Þ ð30Þ

The energy balance for the evaporator is:

QPWh2CQFh4ZQPWh1CQPWh6CQBh8 ð31Þ

The energy balance for the heat exchanger is:

QFðh5� h4ÞZQPWðh3� h6ÞCQBðh7� h8Þ ð32Þ

where hi is the specific enthalpy of the i stream at tem-perature Ti (kJ/kg), QPW is the produced desalinated wa-ter (kg/day), QF is the seawater feed (kg/day), QB is therejected brine (kg/day), and PW is the energy require-ment (kJ/day).

The recovery ratio of the process is given by:

RZQPW

QF

ð33Þ

From Eqs. (31) and (33), the energy balance for theevaporator can be written as:

Rh2Ch4ZRh1CRh6Cð1�RÞh8 ð34Þ

From Eqs. (32) and (33), the energy balance for heat ex-changer can be written as:

h5� h4ZRðh3� h6ÞCð1�RÞðh7 � h8Þ ð35Þ

Using Eqs. (34) and (35):

h2 � h1ZRh6� h5CRðh3� h6ÞCð1�RÞh7

Rð36Þ

Then Eq. (30) can be written as:

PMVCZQPW

Rh3� h5Cð1�RÞh7R

ð37Þ

The total capital cost (TCCRO) including the site de-velopment and indirect costs is given by the equation:

TCCMVCZ2500!QPW!24!365!3600 ð38Þ

The operational and maintenance costs (O&M, CO&M)are given by the equation:

CMVCO&MZCLABOURCCMAINTCCCHEMCCENRG ð39Þ

where:

CLABOURZ0:2!QPW!3600!365!24 (V/year), thelabour costCMAINTZ0:08!QPW!3600!365!24 (V/year), themaintenance costCCHEMZ0:15!QW!3600!365!24 (V/year), thechemicals costCENRG Z cost of energy (V/year)

A simple calculation of the cost of water producedby an MVC plant may be obtained by using theexpression:

CMVCZðTCCMVC!CCRÞCCMVCO&M

QPW

ð40Þ

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458 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

where CMVC is the water production cost (V/m3), CCR isthe capital cost recovery factor and is given by theequation:

CCRZrð1CrÞn

ð1CrÞn� 1ð41Þ

where r is discount rate and n is the number of years ofuseful life of the plant.

3.2. Reverse Osmosis (RO)

Reverse Osmosis (RO) is a membrane separation pro-cess in which the water from a pressurized saline solu-tion is separated from the solutes (the dissolvedmaterial) by flowing through a membrane. No heatingor phase change is necessary for this separation. Themajor energy required is for pressurizing the feed water.

In practice, the saline feed water is pumped intoa closed vessel where it is pressurized against the mem-brane. As a portion of the water passes through themembrane, the remaining feed water increases in saltcontent. At the same time, a portion of this feed wateris discharged without passing through the membrane.The amount of the feed water discharged to waste in thisbrine stream varies from 20 to 70% of the feed flow, de-pending on the salt content of the feed water [2].

A typical RO system is made up of the following ba-sic components (Figs. 8 and 11):

� Pre-treatment: Feed water pre-treatment is impor-tant in RO because the feed water must pass throughvery narrow passages during the process. Therefore,suspended solids must be removed and the waterpre-treated so that salt precipitation or microorgan-ism growth does not occur on the membranes (bio-fouling). Usually, the pre-treatment consists ofsterilization, fine filtration and the addition of acidor other chemicals to inhibit precipitation.

� High-pressure pump: The high-pressure pump sup-plies the pressure needed to enable the water to passthrough the membrane and have the salts rejected.This pressure ranges from 17 to 27 bar for brackishwater and from 54 to 80 bar for seawater.� Membrane modules: The membrane assembly con-sists of a pressure vessel and a membrane that per-mits the feed water to be pressurized against themembrane. The membrane must be able to with-stand the drop of the entire pressure across it. Thesemi-permeable membranes are fragile and vary intheir ability to pass fresh water and reject the pas-sage of salts. No membrane is perfect in its abilityto reject salts, so a small amount of salts passthrough the membrane and appears in the productwater. The concentrated reject stream (brine)emerges from the membrane modules at high pres-sure. In large plants the reject brine pressure energyis recovered by a turbine, recovering from 20 up to40% of the consumed energy. There are two typesof RO membranes that are being used commercially.These are the spiral wound (SW) membranes and thehollow fiber (HF) membranes, both are used forbrackish and seawater desalination. The choice be-tween the two is based on factors such as cost, feedwater quality and product water capacity.� Post-treatment: It consists of sterilization, stabiliza-tion, mineral enrichment and pH adjustment of theproduct water.� Energy recovery system: A system where a portion ofthe pressure energy of the brine is recovered.

Due to the operation of the RO process in ambienttemperature, corrosion and scaling problems are dimin-ished in comparison with distillation processes. How-ever, effective pre-treatment of the feed water is requiredto minimize fouling, scaling and membrane degradation.Generally a seawater RO plant has low capital costand significant maintenance cost due to the high cost

Brine Discharge

Seawater Feed

Wor

k in

Product Water

Membrane

High Pressure Pump

Energy Recovery System

Wor

k in

Pre-treatment

Post-treatment

Water

Brine

Fig. 8. Typical flow diagram of the RO unit.

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459C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

of membrane replacement. The major energy require-ment for RO desalination is for pressuring the feedwater. The energy requirements of a seawater SW-ROplant is around 5 kWh/m3 for large units with energyrecovery, while for small units it is around 15 kWh/m3.

The application of a pressure larger than the osmoticpressure of a saline solution against a semi-permeablemembrane has as a result the passage of pure waterthrough the membrane. The basic equation that de-scribes the RO process is [17e19]:

JWZQW

AM

ZK 0WðDP�DP�DPDROPÞ

d

ZKWðDP�DP�DPDROPÞ ð42Þ

where JW is the flux of permeate water (m/s), Qw is theflow rate of water through the membrane (m3 s�1), KW

is the specific permeability of water through the mem-brane (m2 h�1 Pa�1), AM is the surface of the membrane(m2), d is the thickness of membrane (m), DP is the os-motic pressure difference between feed water on the sur-face of the membrane and product water (Pa), DP is thepressure difference between feed and product water (Pa),and DPDROP is the pressure drop across the module inthe feed channel (Pa).

The permeability of water through the membrane(KW) depends on the material (polymer) that the mem-brane is constructed of, the temperature and the opera-tional time of the membrane. The temperature affectsconsiderably the water flow rate through the membrane.An increase in temperature of 1 �C results to an increaseof about 3% of water flow rate. As it can be seen fromEq. (42), in order to achieve the desirable flow rate ofproduct water it is necessary to apply pressure to thefeed water above that of the osmotic pressure of the feedwater. RO systems work with feed compression 2e3times greater than the osmotic pressure value. Seawaterosmotic pressure is calculated using the equation:

DPZð0:6955C0:0025!TÞ!108

rSW

ðCM�CPÞ ð43Þ

where T is the temperature in �C, CM is the salt concen-tration on the membrane surface (kg/m3), CP is the saltconcentration in the produced water (kg/m3), and rSWis the density of the seawater (kg/m3) given by theequation:

rSWZ498:4!mðTÞC�248;400!mðTÞ2

C752:4!mðTÞ!CSW

1=2ð44Þ

where mðTÞZ1:0069� 2:757!10�4!T and CSW is theseawater salt concentration (kg/m3).

The average concentration CM on the membranessurface is given by the equation:

ðCM�CPÞðCB�CPÞ

Zexp

�JWk

�ð45Þ

where JW is the flux of water through the membrane(m/s); CB, CM, CP are the concentrations of salts ofthe brine, on the membrane and product water(kg/m3), respectively; and k is the mass transfer coeffi-cient through the membrane (m/s).

The mass transfer coefficient through the membrane,k, for spiral wound modules is given by the relation:

ShZkdh

DB

Z0:065!Re0:875Sc0:25 ð46Þ

where Sh is the Sherwood number, DB is the diffusivityof brine entering the feed channel (m2/s), dh is the hy-draulic diameter of the channel (m), Re is the Reynoldsnumber inside the feed channel, and Sc is the Schmidtnumber.

The diffusivity DB of the brine is given by therelation:

DBZ6:725!10�6!exp

�0:1546!10�3

!CB �2513

273:15CT

�ð47Þ

The Reynolds number inside the feed channel is givenby the equation:

ReZrSWdhUB

hSW

ð48Þ

where hSW is the viscosity of the seawater (Pa s).

hSWZ1:234!10�6!exp

�0:0212!CBC

1965

273:15CT

ð49Þ

The velocity UB of the brine in the feed channel ofa spiral wound module is given by the equation:

UBZQB

AC

ZQB

WMdSP3SPð50Þ

whereWM is the width of the feed channel of the module(m), dSP is the channel’s spacer thickens, eSP is the spacerporosity, and QB is the brine flow rate.

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460 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

The Schmidt number of brine is given by the relation:

ScZhSW

rSW!DB

ð51Þ

The pressure losses in the feed channel DPDROP forspiral wound modules is given by the followingequation:

DPDROPZ6:23!Re�0:3rSWU

2B

2

Lm

dhð52Þ

where UB is the velocity of brine entering the feed chan-nel (m/s), Lm is the length of the feed channel (m), dh isthe hydraulic diameter of the channel (m), and Re is theReynolds number inside the feed channel.

Due to the fact that no membrane is perfect and theconcentration difference between the brine and the prod-uct water, some small amount of salts pass through themembrane. This is a mass transport phenomenon andcan be described by the following equation:

JSZQS

AM

ZKSðCM�CPÞ ð53Þ

where JS is the flux of salts through the membrane (kg/sm2); QS is the flow rate of salts through the membrane(kg/s); CM, CP are the concentrations of salts of themembrane and product water (kg/m3); AM is the areaof the membrane (m2); and KS is the mass transfer coef-ficient of salts through the membrane (m2/s).

It is clear that the KW must be as large as possible andKS as small as possible in order to achieve the smallestresistance to water permeation through the membraneand the greater resistance to salts permeation.

Two important factors for the membrane are the saltpermeation and the salt rejection, which are defined bythe following equation:

Salt rejectionZ1� salt permeationZ1�CP=CF ð54Þ

Also the intrinsic salt permeation and the intrinsicsalt rejection (RIN) are important parameters, whichare defined by the following equation:

Table 1

Rejection of ions from an RO membrane [1]

Ion Salt rejection (%) Ion Salt rejection (%)

NH4C 92 Nitrates 85

NaC 95 Chlorates 95

KC 95 Fluorides 95

MgC 97 Sulfates 97

SrC 97 Phosphates 99

Ca2C 98 Acid carbonates 95

RINZ1� intrinsic salt permeationZ1�CP=CM ð55Þ

The salt rejection is an important characteristic of themembrane and is different for different ions (Table 1).Intrinsic salt rejection (RIN) is an important operationalcharacteristic of the membrane module and is consid-ered to be independent of the driving pressure in themodule.

Another important factor is the recovery ratio (RW),which is defined as the ratio between the flow rates ofproduct and feed water. RO systems in the case of sea-water feed are designed for recovery ratios from 20 to35%.

Given a specific membrane spiral wound element thegeometrical characteristics AM, dh, dSP, eSP, WM, LM

and the operational constants KW, KS, RIN can be deter-mined by the manufacturer. Then Eqs. (9)e(20) can beused for the calculation of the recovery ratio of the ele-ment as a function of the applied pressure DP and thebrine characteristics (CB, QB, T ) entering the element.In Fig. 9 the effect of the salinity of the feed on mem-brane module performance (driving pressure vs recoveryratio) is shown for a typical membrane module. There isa linear relation between driving pressure and recoveryratio as stated by Eq. (42) and the feed water salinity af-fects the offset of the curve as it changes the osmoticpressure of the feed. The feed flow rate affects the slopeand the offset of the driving pressureerecovery ratiocurve (Fig. 10). The increase in feed flow rate increasesthe slope because the recovery ratio is inversely propor-tional to the feed flow rate. The offset of the curve in-creases with the increase in feed flow rate because thehigher the flow rate the higher is the pressure drop insidethe element (factor DPDROP, Eq. (42)).

In an RO plant the membrane assembly consists ofa number of pressure vessels (NPV) and a number ofmembrane elements (NM) in a row inside the pressurevessel that permits the feed water to be pressurizedagainst the membrane (Fig. 11). Inside the pressure ves-sel the brine exiting of one element is the feed for thenext one and so on.

The overall driving pressure required DPF, and thusthe energy requirements depend on the production rateQW, the seawater feed (CSW, T ) characteristics, thenumber of pressure vessels NPRV, the number NM ofmembrane modules in a pressure vessel and the recoveryratio (RW).

The energy requirements of an RO unit given the DPF

can be calculated by the equation:

PROZ8:76

�QWDPF

RWeHPP

� ð1�RWÞRW

QWDPBReREC

�ð56Þ

where eHPP is the efficiency of the high-pressure pump,eREC is the efficiency of the energy recovery system,

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461C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

4.00E+06

4.50E+06

5.00E+06

5.50E+06

6.00E+06

6.50E+06

7.00E+06

7.50E+06

8.00E+06

8.50E+06

0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14

RO Membrane Module Recovery Ratio

Dri

ving

Pre

ssur

e (P

a)

CFEED=35 kg/m3,Feed=300 m3/day

CFEED=40 kg/m3,Feed=300 m3/day

CFEED=50 kg/m3,Feed=300 m3/day

CFEED=60 kg/m3,Feed=300 m3/day

CFEED=70 kg/m3,Feed=300 m3/day

Fig. 9. Driving pressureerecovery ratio (R) of a membrane module at 300 m3/day saline feed rate at various feed salinities (CFEED).

DPBR is the pressure of the rejected brine (Pa), and PRO

is the energy requirement (kWh/year).An example of the implementation of Eq. (56) is

shown in Fig. 12. As it can be seen that increasing therecovery ratio increases the energy requirements. In-creasing the number of membrane modules per pressurevessel decreases the energy requirements for the same re-covery ratio. However, increasing the membrane mod-ules above 14 the decrease is not significant.

For a given seawater feed (CSW, T ) and a requiredwater production QW the parameters: the number ofpressure vessels NPV, the number NM of membranemodules in a pressure vessel, the driving pressure ofthe feed DPF and the recovery ratio (RW) can be opti-mised in order to minimize water cost.

The total equipment cost (TECRO) of an RO unitconsists of the cost of membranes, the cost of pressurevessels, the cost of high-pressure pumps, the cost of en-ergy recovery system, the cost of pre-treatment andpumping unit:

EQCROZCMENMNPRVCCPRVNPRVCCHPPCCREC

CCPRETR ð57Þ

where CME is the cost of membrane element (V), CPRV isthe cost of pressure vessel (V), CHPP is the cost of high-pressure pump HPP (V), CREC is the cost of the energyrecovery system (V), and CPRETR is the cost of pre-treat-ment and intake system (V).

4.00E+06

4.50E+06

5.00E+06

5.50E+06

6.00E+06

6.50E+06

7.00E+06

7.50E+06

8.00E+06

8.50E+06

0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14

RO Membrane Module Recovery Ratio

Dri

ving

Pre

ssur

e (P

a)

CFEED=35 kg/m3,Feed=400 m3/ day

CFEED=60 kg/m3,Feed=400 m3/ day

CFEED=35 kg/m3,Feed=300 m3/ day

CFEED=60 kg/m3,Feed=300 m3/ day

Fig. 10. Driving pressureerecovery ratio (R) of a membrane module at 300 and 400 m3/day saline feed rate at various feed salinities (CFEED).

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462 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

Feed seawater

Product Water

Brine

High PressurePump

Membrane Element

NM Membrane Elementsper Pressure Vessel

NPV Pressure Vessels

Fig. 11. Typical RO plant, the membrane assembly.

The cost equations for HPP and energy recovery sys-tems are [20]:

CHPPZaHPP

�QWDPF

RWeHPP

�bHPP

ð58Þ

CRECZaREC

�ð1�RWÞ

RW

QWDPBReREC

�bREC

ð59Þ

The cost of pre-treatment and intake system given by theequation:

CPRETRZaPRETR

�QW!24!3600

RW

�bPRETR

ð60Þ

The total capital cost (TCCRO) including the site de-velopment and indirect costs is given by equation:

TCCROZ1:411!EQCRO ð61Þ

The operational and maintenance costs (O&M, CO&M)are given by the equation [21e23]:

CROO&MZCLABOURCCMAINTCCCHEMCCMEMREP

CCENRG ð62Þ

where:

CLABOURZ0:2!QW!3600!365!24 (V/year), thelabour costCMAINTZ0:05!QW!3600!365!24 (V/year), themaintenance cost

Feed = 1000 m3/day Feef Salinity 35 kg/m3

2.E+050

3.E+05

4.E+05

5.E+05

6.E+05

7.E+05

8.E+05

9.E+05

1.E+06

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Recovery Ratio

Ene

rgy

requ

irem

ents

(kW

h/yr

)

Nm = 4

Nm = 6

Nm = 8

Nm = 10

Nm = 12

Nm = 16

Nm = 18

Nm = 20

Fig. 12. Energy requirements of an RO plant with energy recovery system vs recovery ratio and number of membrane modules per pressure vessel

(NM). Number of pressure vessels Z 3.

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463C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464

1.49 1.50

3.67

2.77

0

0.5

1

1.5

2

2.5

3

3.5

4

EU

RO

/m3

RO-WIND MVC-WIND MVC-PV RO-PV

Fig. 13. Water production cost.

CCHEMZ0:03!QW!3600!365!24 (V/year), thechemicals costCMEMREPZNMNPRVCME=3 (V/year), membrane ele-ment replacement costCENRG Z cost of energy (V/year)

A simple calculation of the cost of water produced byan RO plant may be obtained by using the expression:

CROZðTCCRO!CCRÞCCROO&M

QW

ð63Þ

where CRO is the water production cost (V/m3), CCR isthe capital cost recovery factor and is given by theequation:

CCRZrð1CrÞn

ð1CrÞn� 1ð64Þ

where r is the discount rate and n is the number of yearsof useful life of the plant.

4. Results and discussion

When several alternative RESedesalination schemesare compared for a specific case, the final decision con-cerning the most prominent combination should bebased, on criteria such as:

- commercial maturity of technology (an appropriateway to validate this is by examining the performanceof similar existing applications);

- availability of local support (installers, technicians,machine shops, etc.);

- simplicity of operation and maintenance of thesystem.

The above factors, in conjunction with available tech-nical information (feed water quality, output water re-quirements (quality and quantity) as well as the typeof RES available) provide a starting point for the engi-neer and the decision maker.

The equations presented can be used easily to esti-mate the energy requirements, the size of the RES unitrequired (Wind or PV) and water production cost de-pending on the RESedesalination configuration. InFig. 13 the water production costs of RESedesalinationunits with production capacity of 500 m3/day is pre-sented as calculated by the model equations. The inputsrequired are wind characteristics (k Z 1.5 and UAVG Z7.5 m/s), solar radiation characteristics (thd Z 8.3 h/day,Ta Z 20 �C G Z 5 kWh/day/m2), the RO membranecharacteristics, the wind turbine characteristics (powercurve) and the solar PV characteristics. For the caseexamined the ROeWIND and MVCeWIND optionsare comparable while the MVCePV option is the mostexpensive solution.

Water production costs of an RESedesalination con-figuration depend heavily on the available RES poten-tial. The greater the RES potential the smaller is theenergy production cost from the RES unit and thussmaller water production costs from the desalinationunit.

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