Wind Powered Water Desalination Youssef Dahioui

*, Khalid Loudiyi#

"'School o/Science and Engineering

Al Akhawayn University in Ifrane

/frane, Morocco

ly.dahioui@aui.ma 2k.loudiyi@aui.ma

Abstract- Renewable energy, and more specifically, wind

energy powered desalination, has been going through an

upwards trend, especially during the last decade. Still, there is a

domain that has not been much researched 111; hence, this paper

tries to address wind powered independent desalination systems.

With its lower energy consumption and portability, Reverse

Osmosis (RO) method has been chosen as the desalination

technology that will be integrated with wind energy. In this work

a MA TLAB simulation is used to find out the effects of

fluctuating wind energy on a system that is designed to operate

under steady conditions. The results show that varying electrical

power leads to extreme fluctuation in feed water pressure,

beyond the operational range of the RO membranes. Still, this

does not draw a cross on the wind and desalination combo; in

fact, several methods exist to diminish the pressure variation,

including wind turbine de-rating, or use of pressure stabilizers.

K�words-Desalination, wind energy, reverse osmosis,

MA TLAB, simulation

I. INTRODUCTION

Without water, nothing alive on this planet would have existed. 75% of the surface of the earth is covered by water; nevertheless, only 3% of that water is available as fresh water, and only 13% of those 3% are directly available for drinking and any other domestic, industrial or agricultural uses [2]. Just a few decades ago, fresh water was viewed as an eternal, renewable and easily accessed resource; however, nowadays, water shortage has become a serious issue that may be the main cause of conflicts in the near future. One of the most widely used indicators for defining water stress is the Falkenmark indicator, illustrated in Table I and defined as the entire annual water available for human use [3].

TABLE I WATER SHORTAGE CATEGORIES

Index (m3 per capita) Category

>1700 No stress 1000 - 17000 Stress 500 - 1000 Scarcity < 500 Absolute Scarcity

To tackle this water shortage issue, water desalination has represented for years, an effective, yet an energy consuming method [4]. Thus, and taking into account the current energy world market, with increasing fossil fuel prices, renewable energy powered desalination is taking an interesting trend.

II. DESALINATION TECHNOLOGIES

A. Thermal Technologies

1) Multi-stage Flash Distillation: Multi-stage Flash (MSF) is the most used thermal desalination technology, worldwide; it represents about 50% of the installed capacity [5]. Basically, this process is about evaporating feed water in a group of chambers, each having a lower pressure than the previous one. When getting into one stage, water flashes, or evaporates instantaneously due to the low pressure implying a lower evaporation point.

2) Multi-Effect Distillation: The Multi-Effect Distillation (ME) was the first process used for desalination of seawater [6]. A quantity of water is heated up till becoming vapor then goes through a heat exchanger. Feed water that is going to be desalinated is sprayed in the heat exchanger condensing vapor flowing through it. Latent heat released due to condensation causes some of the feed water to evaporate and flows to another heat exchanger for the process to take place again until a significant quantity of condensate water has been collected [6].

3) Vapor Compression: Vapor compression (VC) process is a more recent method for water desalination; still, it is based on a simple principle. As shown in Fig.l, it is usually composed of three parts, a compressor, a heat exchanger and an evaporator. Some of the vapor produced within the evaporator is sent to the compressor, increasing its pressure, and its temperature as well. The superheated vapor that leaves the compressor, gets into the heat exchanger submerged within the feed water, and causes its evaporation [7].

� --=====::== Compressor L�I �

- {} . r l�W'·� ===:===i[:;;:;:::::�� -I. -= Feed Waler

/=,-esh Water

Fig. 1 Vapor compression cycle

B. Membranes Technologies

978-1-4673-6374-7/13/$31.00 ©2013 IEEE

1) Electrodialysis: Electrodialysis (ED) is the oldest desalination membrane-based technology and has been used all around the world for more than 40 years. More than 10 million m3 of water are produced on a daily basis using this technology [8]. From Fig.2, it is a process in which ions are attracted to their respective electrically charged electrode through ion-selective semi-permeable membranes. Positively and negatively charged dissolved salts in the aqueous solution are attracted to the electrode with the opposite charge, and membranes, that allow either cations or anions to pass through, are installed in an alternative way, thus creating concentrated and purified streams.

Concentrate Oiruate Concentrate

� + � � +

0 0 0 0 0 0 cathode

0 0 0 0

Feed Water

Fig. 2 Electro dialysis process

2) Reverse Osmosis: The movement of solvent molecules from one space with low solute concentration to another space through a semi-permeable membrane with a higher solute concentration is what is called osmosis, observed in 1748 [9]. What causes this movement is the difference in chemical potential between the two solutions, affected in its turn by three different factors: Salt concentration, the higher the concentration the lower the chemical potential; temperature, the higher the temperature, the higher the chemical potential; pressure, the higher the pressure the higher the chemical potential This movement, or osmosis process, would carry on until equilibrium in chemical potential between the two sides is reached; this is the "osmosis equilibrium."

Therefore, in order to desalinate water, it is the reverse process that has to be done, called "reverse osmosis." As pointed up in Fig.3, some pressure has to be applied at the beginning to initiate the flow from the saline side to the fresh side; this is the "osmotic pressure."

- r T:U Osmotic PresS(Jre

Fresh Water Saline Water'

D' Semi-permeable Membrane

Fig. 3 Reverse osmosis process

III. W[ND POWERED DESALlNAT[ON

One problem that has been facing water desalination is its significant energy needs. Consequently, to incorporate renewable energy and more specifically wind energy that can be considered nowadays as a mature technology, there are two main aspects or factors that should be taken into consideration:

• Specific energy consumption (Table II) or ability to produce as much water as possible from the available energy during any period

• Operability under variable conditions and this is what actually keeps the thermal technologies away; they usually require a long start-up time and significant energy waste could result from frequent stops.

TABLE II

SPECIFIC ENERGY PER DESALINATION TECHNOLOGY [15]

Desalination Technology Specific

Energy

(kWh/m3)

Multi-stage Flash 6-9 Multiple Effect 10 - 14.5 Vapor Compression 7 - 15 Electrodialysis 0.7 - 2.2 Reverse Osmosis 3 - 13

Other factors could be taken into account as well such as ease of maintenance and portability. At the end, RO seems to be the technology with the highest potential.

In the literature review we have gone through [10], [11], [12] very few implemented projects dealt with this problematic. Amongst these, a prototype system using mechanical energy transmission between the wind turbine and the water pump. This type of desalination using mechanical energy instead of electrical faces several difficulties related to more frequent failures due to the use of mechanical bearings, and most importantly, the end result was not really satisfying, since the water product quality was not high enough to be drinkable even though it still could be used for irrigation. Another prototype implemented in Coconut Island, Hawaii uses electrical energy of the wind turbine, and a feedback system that enables the control of water flow, thus stabilizes its pressure. [t was able to achieve a cost of $5.4 per m3 [10].

A. Wind & RO

Reverse Osmosis has been continuously improved over the years making it one of the most energy efficient, and representing more than half of the new desalination plants that are being installed every year. This is mainly due to the development of the main part of a RO system, its membranes. Lots of efforts have been put into improving their

performance, their resistance to pressure fluctuations. Eventually, to optimize their performance, they have to undergo a specific flow rate and pressure, and this is the main challenge with fluctuating wind energy [13], [14], [7].

Thus, as demonstrated in Fig.4, that was made arbitrarily, when designing the system, there are several conditions that have to be respected to maximize the lifetime of the membranes and avoid any significant deterioration:

• Maximum feed Pressure (membrane resistance) • Maximum brine flow rate (membrane resistance) • Minimum brine flow rate (fouling problem) • Maximum product concentration (depends of osmotic

and applied pressure)

Pressure

Minimum

Brine Flow

Rate

Maximu m

Al lowe d

Co n c entrati on

Maxim um Pressure

RO membrane

O p erational

Region Maxi mum

Brin e Flow

Rate

Flow Rat·e

Fig. 4 RO membrane operating region under specific conditions

1) System with Backup: Basically, an additional energy source (diesel generator, grid connection) will be used besides wind energy. This will compensate for electricity coming from the wind generator during low or no wind. This represents an easy solution for the wind fluctuations; however, this makes our system completely dependent on the additional source. [n the case of energy shortage, or power cuts, the system will simply stop operating.

2) Independent System (No Backup): To keep the system operating at nearly steady conditions and without any backup generators, there are three main ways:

• Storage: either electrical storage with batteries or water storage, by pumping water into a tanl<- when surplus of electrical energy is produced

• Switching ON/OFF modules: this requires the availability of several independent reverse osmosis systems all connected to the same wind energy generator. Systems will be turned on or off depending on the available energy [11], [\5].

• Wind Turbine Power limitation: this consists in applying a pitch mechanism that would put a boundary on the power produced from the wind turbine generator, thus, limiting the power fluctuation [11], [15].

In the case of variable conditions, no major efforts are taken into stabilizing the electrical power, flow rate or pressure of the feed water going into the RO membranes. Since, these filters have been designed to operate optimally

under specific conditions; any extreme variation is expected to cause mechanical fatigue and impact significantly the lifetime of the membranes [11], [[6].

B. MATLAB Model

Since, there has not been much major research regarding RO membranes operating under variable conditions, we designed a MA TLAB model simulation (Fig.S) in order to investigate the effects of fluctuating wind energy on the overall system.

1) Wind Turbine

Funct i on

Consta nt

Fig. 5 Wind turbine MATLAB model

The wind turbine model has been designed in a way to gather wind speed data over a fixed period of time from a spreadsheet file.

A saturation model was added, to limit the wind speed input, assuming that a pitch mechanism would starts operating when wind speed reaches 12 ms-1. In order to compute the electrical power, we used the relation shown: P ='h p A v3 Cp

Where the different parameters in the equation are:

• Cp: Power coefficient (given a value of 0.4)

(1)

• A: rotor area (computed for a turbine with 5.2m diameter)

• p: air density (l.225 kgm-3) • v: wind speed (taken from an excel file)

2) Whole System

Fig. 6 Wind Powered Desalination System Model

This model (Fig.6) calculates, depending on the power coming from the wind turbine generator, the feed water flow rate and its pressure. The main purpose of this example is to

illustrate some of the effects of variable wind conditions on our desalination system.

For the flow rate calculation we have used [17]:

With P representing the electrical power ( in k W), H the head (given a value of 1m), g the gravitational acceleration (9.81 ms-\ and 11 the pump efficiency.

For the pressure calculation we used [18]:

QF being the feed flow rate calculated in the previous equation, R the recovery rate (given a value of 0.5), p the membrane permeability (given a value of 0.2464m3/m2h/bar), Am the membrane area (given a value of 1 m2), and n the osmotic pressure

As it can be noticed in the graph resulting from the simulation (Fig.7), wind speed variation leads to significant pressure changes. Thus, from this simulation, a simple wind powered desalination RO system may be regarded as not feasible.

Fig. 7 Wind speed impact on pressure and flow rate

p, • -\\lndSpOIQd

rIawR:tc:

The remedy for this situation could be de-rating the wind turbine. Thus, by using the same model, but flattening the electrical power coming from the wind turbine generator we get the following results of de-rating shown in Fig.8.

"" ,

Fig. 8 De-rating impact on pressure variation

-Pr.uW'_ - ..... nllSp.ad

'1ooI!'.,.

By reducing the maximum power produced by the wind turbine, we simply diminish the pressure fluctuation; therefore, we may reduce it down to an acceptable level that would not result in mechanical damage.

Although this may be seen as an effective and easy solution, it at the same time represents a waste of potential electrical energy, due to pitching, or in other terms, waste of investment.

C. Parts a/the system

1) Variable Speed Drive: With an energy source as intermittent as wind energy, use of variable speed pumps is a necessity. This variable speed drive is controlled by the wind turbine generator and includes: rectifier, DC link capacitor and variable frequency inverter (Fig.9). This later component will adjust the speed and torque of the pump depending on the available power. This should allow the pump to continue operating smoothly without sudden changes under variable wind conditions.

Power Grid Rectifier In veTter f------j DC Unk

Fig. 9 Variable Speed Drive

2) Pressure Stabilizer: Since, pressure seems to be the most critical issue when it comes to wind powered desalination with its irreversible effects on the membranes, it is extremely vital for it to be dealt with. The simplest way that comes to mind, in order to stabilize the pressure, would be storage, but since, a minimum of pressure is required to be

able to produce fresh water through the reverse osmosis system, a pressurized water tank would be needed [10].

From that point, a choice can be made on whether to have a continuous or non-continuous operating system. In the non­continuous case feed water is stored and it is taped when needed. On the other hand, the continuous system uses solenoid valves (figure 10) controlled by pressure sensors. The number of valves that open will depend on the pressure within the tank [19]. The higher the pressure, the more valves will open, to keep it within the acceptable operating range for the membranes.

Pressure Sensors Feed Water

Valves

Fig.lO Pressure-Controlled Valves

3) Energy Recovery Device: One positive aspect of the reverse osmosis process is that there is a very small drop pressure across the RO vessels; in other words, the concentrate stream keeps most of the initial pressure of the feed water. Therefore, energy recovery devices have been designed to make use of this significant amount of pressurized brine that would, otherwise, be wasted through direct discharge [20], [21]. That pressure will be transferred to some of the feed water (Fig. I I), hence increasing the pressure within the pressurized tanl<- faster than in typical conditions, allowing for more water production.

High PresSlJre Feedwoter � ••••••• � High Pressure Brine

Low Pressure Feedworer ---- '--_____ L..----" � Low Pressure Bn·ne

Fig. 11 Energy Recovery Device

Most ERDs incorporate positive displacement technology and can be highly efficient going up to 96% of efficiency.

D. Cost Analysis

For the last part, the cost per unit of water produced will be estimated.

Starting with the capital cost, the purchase of a 5.1 kW wind turbine used for this analysis, a variable speed pump, a small RO system, will cost around $13 700. According to Energy Recovery Inc (ERI), ERDs cost around 4% of the capital cost, which amounts to $548 [22].

Concerning, the operational costs, besides maintenance, an annual change of RO membranes is assumed; a yearly amount of $300 each was assumed. Thus, for a lifetime of about 20 years and an interest rate of 5%, the net present value of the operational costs would be $4522.

Summing both the capital and operational costs would give us, as a final total cost, a value of $18770.

With an average wind speed of 4m.s·1 leading to a daily average of around 3m3 of water produced, during the 20 years lifetime of the project, the total water production would be 21 900 m3.

Therefore, this wind powered desalination system will be producing fresh water at a cost of 7.35 MAD/m3•

IV. CONCLUSIONS

By using a low wind speed, and a low water production, the cost of water produced does not go beyond 7.35 MAD which is a highly satisfying cost, especially when compared with other countries, where the cost can go up to 30 MAD/m3, in some of the Greek Islands, for example, where powered desalinations systems with high wind integration have been implemented [2].

Water Cost ($) per m3 Vs. Wind Speed (m.s·l)

1 0,8

§ 0,6 t; 8 0,4

0,2 o

4 6 8 Wind Speed (m.s·l)

Fig. 12 Water production cost per wind speed

10 12

Eventually, production cost could be much lower in windy regions, and on higher implementation scale (figure 12). Therefore, it is safe to say that independent wind powered desalination systems will have a bright future in the coming years. For the Moroccan context, we believe that our proposed system will be suitable for implementation in regions where there's urgent need for water (either industrial or sanitary); and thus all the coastal southern Saharan region. In these regions all required conditions are present: important wind resource (the Sahara trade winds), raw material (sea and brackish water), in addition to a permanent need for fresh water.

However, it is important to mention that this system stays economically attractive only for low saline water desalination. Thus, we propose for future work conducting additional experimentation and research to effectively optimize the system and make it economically attractive for all desalination cases.

REFERENCES

[1] c.Generaal, "Wind Driven Reverse Osmosis Desalination for Small Scale Stand-Alone Applications," M.S. thesis, Faculty of Aerospace Engineering, Delft University of Technology, Delft, Netherlands, 2011.

[2] E.Spang, "The potential for wind-powered desalination in water scarce countries," M.S. thesis, Department of Law and Diplomacy, The Fletcher School, Tufts University, Medford, Massachussets, 2006.

[3] Matlock. (2011). A Review of Water Scarcity Indices and Methodologies. The sustainability Consortium. [Online]. Available: http://www.sustainabilityconsortium.org.

[4] A. Swift, K.Rainwater, 1. Chapman, D. Noll, A. Jackson, B. Ewing, L.Song, G. Ganes an, R. Marshall, V. Doon, P. Nash. (2009). Wind Power and Water Desalination Technology Integration. Desalination and Water Purification Research and Development Program Report No. 146. [Online]. Available: http://www.usbr.govlresearch/AWT/reportpdfs/reportI46.pdf.

[5] E.Miller. (2003). Review of Water Resources and Desalination Technologies. [Online]. Available: http://prod.sandia.gov.

[6] Tata. (2009). Dessalement de I'eau de mer : bilan des demieres avancees technologiques ; bilan economique ; analyse critique en fonction des

contextes. [Online]. Available http://www.agroparistech.fr/IMG/pdfITATA-DUCRU_sr_final.pdf.

[7] Department of Agriculture, Fisheries & Forestry - Australia. (2002). Introduction to Desalination Technologies in Australia. Summary Report. [Online]. Available: http://www.environment.gov.au/water/publications/urban/pubs/desalinat ion-summary. pdf.

[8] H. Strathmann. (nd). Assessment of Electrodialysis Water Desalination

Process Costs. [Online]. Available: http://gwri-ic. technion. ac. i I/pdf/l DS/82. pdf.

[9] Williams. (2003). A brief Review of Reverse Osmosis Membrane Technology. [Online]. Available:

http://www.eetcorp.com/heepm/RO_ReviewE.pdf. [10] C.K. Liu. (2009). Wind-Powered Reverse Osmosis Water Desalination

for Pacific Islands and Remote Coastal Communities. Desalination and Water Purification Research and Development Program Report No. 128. [Online]. Available: http://www. usbr.gov/researchl A WT Ireportpdfs/report128. pdf.

[II] M.Marcos, "Small-Scale Wind-Powered Seawater Desalination Without Batteries," PhD. dissertation, Univ. of Loughborough, Loughborough, United Kingdom, 2003.

[12] E. Rabinovitch. (2008). Drinking with the wind. [Online]. Available: http://www.citg.tudelft.nl.

[13] O. Galal. (2011). Solar Desalination. [Online]. Available http://dii­eumena.com.

[14] 1. Kaufler. (2012). Seawater Desalination (RO) as a Wind/Solar Powered Industrial Process - Technical and Economic Specifics -[Online]. Available: http://events.exicon-intl.com.

[15] M.Marcos, D.Infield. (2002). A wind-powered seawater reverse osmosis system without batteries. [Online]. Available:

http://www.spectrawatermakers.com/landbased/med ialwi nd-powered _ro _ nobatteries. pdf

[16] M. Goosen, H. Mahmoudi, N. Ghaffour, S. Sablani. (nd). Application of Renewable Energies for Water Desalination. [Online]. Available: http://cdn.intechweb.orglpdfs/13 7 55. pdf.

[17] GRUNDFOS. (nd). The centrifugal Pump. Grundfos Research and

Technology. [Online]. Available:

http://www.grundfos.comlThe_Centrifugal]ump.pdf. [18] Assimacopoulos. (nd). A tool for the design of desalination plants

powered by renewable energies. [Online]. Available: http://environ.chemeng.ntua.gr.

[19] C.K. Liu, 1. Park, M. Reef: Q. Gang. (2002). Experiments of a prototype wind-driven reverse osmosis desalination system with feedback control. [Online]. Available: https:llwiki.duke.edu.

[20] 1. Kaufler. (2006). Wind & Solar Powered Seawater Desalination. [Online]. Available: http://events.exicon-intl.com.

[21] E.Kondili, K.Kaldellis. (nd). Wind Energy Based Desalination Process

and Plants. [Online]. Available: http://ikaros. teipir.gr/mechengiOPSI Archimedes/WRECX _wind _ desal_ Kondili.pdf.

[22] Energy Recovery Inc. (2011). The Economics of Downtime. [Online]. Available: http://www.energyrecovery.com/whitepaper -.Jldfs

International Journal of Green Energy, 4: 471–481, 2007Copyright © Taylor & Francis Group, LLCISSN: 1543-5075 print / 1543-5083 onlineDOI: 10.1080/15435070701583060

471

A WIND-POWERED SYSTEM FOR WATER DESALINATION

Eyad S. HrayshatTafila Technical University, Tafila, Jordan

A wind-powered reverse osmosis desalination system is proposed in order to assess thepotential of the development of water desalination in Jordan. A simulation model for theprediction of the power delivered for a given value of wind speed is adopted. Based on theaverage wind speed data and salinity of the feed water, the amount of water that can be pro-duced at eight different sites is calculated. According to the annual amount of water pro-duced, the selected sites can be divided into three different categories. The first one, whichincludes Hofa and RasMuneef, is considered to be “adequate” for wind-powered reverseosmosis desalination. Its annual amount of water output forms about 57% of all water pro-duced at all the eight sites combined. The second category, which includes Safawy,Twaneh, and Tafila, is considered to be “promising”. Its water output adds up to about 30%of all water produced at all sites. The third category, which includes Jurf AlDaraweesh,Aqaba, and Shoubak, is considered to be “poor”. Only about 13% of the water producedfrom all sites combined can be obtained from these three sites.

Keywords: Reverse Osmosis; Wind; Desalination; Jordan

INTRODUCTION

The expanding population and the climatic and topographical conditions of Jordanhave exerted enormous pressure on the limited water resources and created a severewater supply–demand imbalance where the renewable water resources are among thelowest in the world, and are declining with time. Resources are already seriously limitedand are far below the water poverty line of 1000 m3/capita/year. Available water fromexisting renewable sources is projected to fall from 160 m3/capita/year in 2002 to 90m3/capita/year by the year 2025 (Malkawi, 2003). The supply-demand imbalance has influ-enced the quality of water resources where over-extraction from groundwater aquifersexploited the aquifers at more than double their sustainable yield in the average.

Desalination of brackish or sea water now represents a consolidated system toresolve the water emergency. The main drawback of this solution, however, remains thehigh energy consumption. Considering their limited availability in Jordan, high cost and,above all, the negative environmental impacts caused by their use, it is imperative tosearch for new alternative sources to supplement or substitute for conventional fuels. Inview of the aforementioned problems, considering renewable energy resources such solar

Address correspondence to Eyad S. Hrayshat, Tafila Technical University, P.O. Box 66, Tafila 66110,Jordan. E-mail: ehrayshat@yahoo.com

472 HRAYSHAT

and wind energies seems very attractive, especially for remote areas with no electricitygrid and which would be expensive to connect. Fortunately, Jordan is blessed withabundant solar and wind energy sources (Hrayshat, 2002; Hrayshat and Al-Soud, 2004;Hrayshat, 2005).

The use of solar energy for desalination plants in Jordan has been investigated bymany researchers (Al-Rawajfeh et al., 2003; Al-Rawajfeh et al., 2004; Mohsen and Jaber,2001; Abdallaha et al., 2005). However, proper exploitation of wind energy as a source ofpower for desalination plants in Jordan has not yet been investigated to the extent thatsignificant results and/or design methods could be obtained.

The objective of this paper is to assess the potential of wind-powered desalination asa viable alternate water source for eight selected Jordanian sites. A wind-powered reverseosmosis (RO) desalination system is proposed, and a simulation model is utilized for theevaluation of the produced water amount based on the average wind speed data and salinityof the feed water (TDS of 3000, 5000, 7000, and 10000 mg/L).

WIND DATA

The wind speed data used in this paper was measured and recorded at eight stations— distributed all over the country — of the Jordanian Meteorological Department, at tenmeters above ground level, between 1990 and 2001. These stations are: Hofa, RasMuneef, Safawy, Twaneh, Tafila, Jurf AlDaraweesh, Aqaba, and Shoubak.

The data has been averaged over the twelve years. Each data is recorded every fiveminutes and then averaged on an hourly basis and stored as hourly values. Monthly fileswere obtained for each year, with the data recorded in four columns: month, day, hour,and hourly mean wind speed. The hourly mean wind speed is the average of the twelvepieces of data corresponding to the twelve periods of five minutes that make up each hourof original data.

BRACKISH WATER IN JORDAN

In Jordan, two main sources are available to be desalted: the Aqaba Gulf and thebrackish deep groundwater in some basins. Preliminary studies showed that by the year2010, more than 2 × 107 m3 of brackish water could be developed in central Jordan. Thisfigure may reach 7 × 107 m3 by the year 2040 (Jaber and Mohsen, 2001). According to thewater quality analysis conducted by the Japanese International Cooperation Agency(JICA) on brackish water in Jordan, the total dissolved solids results (TDS) were in therange of 5,000–10,000 ppm (JICA, 1995). The salinity of water is around 5000–10,000 mg/Las TDS with water temperatures of 32–36°C. NaCI is the main component of salt in water(3000–6000 rag/L), besides the cations of Ca, Mg and the anions of SO4 and HCO3 thatare considered to be scaling substances existing in relatively high concentrations, and thecalculated total hardness is in the range of 1500–3000 mg/L as CaCO3. The Fe concentra-tion is 5–15 mg/L. As for SiO2, which is a fouling substance for membranes, itsconcentration is in the range of 10–20 mg/L.

THE WIND-POWERED RO DESALINATION SYSTEM

The wind-powered RO desalination system consists of the membrane separationsection, which is fed via a high pressure reciprocating pump (pressurizing the feed

WIND-POWERED SYSTEM FOR WATER DESALINATION 473

stream up to the desirable pressure levels), and which is properly connected to a hydro-turbine, for the recovery of energy by the brine stream leaving the process. The perme-ate stream leaving the membranes constitutes the lean product of the system. The highpressure pump operates by means of a three-phase motor which is supplied by electricalpower. In this case, electrical power is available by the wind turbine. The technical dataof the utilized wind turbine is furnished in table 1. An advanced control system isrequired for the regulation of the power source. The schematic diagram of the system isshown in Figure 1.

The volume of a cylinder of air approaching a rotating wind turbine is given by thefollowing equation (Fanchi, 2004):

The mass of the cylinder of air equals:

Table 1 Technical data of the utilized wind turbine.

Cut in speed (m/s)

Cut out speed(m/s)

rated speed(m/s)

Rated power(kW)

Rotor diameter (m)

CP

2.5 17 5.6 6 4 0.36

Figure 1 Schematic diagram of the wind-powered reverse osmosis desalination system.

Turbine

Feed

Membrane

Brine

Wind Turbine

Motor

Pump

F B

DProduct

M

Δ Δn = A L (1)

Δ Δ Δm A La a= =r n r (2)

474 HRAYSHAT

Assuming that the cylinder of air is moving with speed v directly at the turbine, thenthe kinetic energy of the moving air will be:

The length of the cylinder of air that reaches the wind turbine in a time interval Δt is:

Substituting eq. (4) in eq. (3) gives:

The rate of air arrival is the wind power, which equals:

The area A is the surface area of the circle formed by the rotating tip of the rotorblade, which equals:

Using eq. (7), eq. (6) can be rewritten in the following form:

Eq. (8) shows that wind power is proportional to the square of the radius of the fancreated by the rotating rotor blade.

The power in the wind is converted to mechanical power with an efficiency (coeffi-cient of performance) Cp, which is transmitted to the generator through a mechanicaltransmission with efficiency ηm and which is converted to electricity with an efficiencyηg. The electrical power output is then:

Using the aforementioned model, the power delivered for a given value of windspeed is predicted. The relationship between energy consumption and water salinity forthe RO system is shown in Figure 2. With brackish water of 2000 mg/L and 5000 mg/LTDS, the amount of energy required is 1.1 kWh/m3 and 1.6 kWh/m3, respectively (JICA,1995).

RESULTS AND DISCUSSION

Figure 3 exhibits the monthly average of the measured wind speeds between 1990and 2001 for all the selected sites. Ras Muneef and Hofa have the most potential amongthe selected sites in terms of their wind speed values.

Δ Δ ΔKE m A La= =0 5 0 52 2. .n r n (3)

Δ ΔL t= n (4)

Δ ΔKE A ta= 0 5 3. r n (5)

pKE

tAw a= =Δ

Δ0 5 3. r n (6)

A = pR2 (7)

pw a= p r n2

R2 3 (8)

pe m g w m g a= =C C Rp ph h rph h r n

232 (9)

WIND-POWERED SYSTEM FOR WATER DESALINATION 475

Using the proposed mathematical model, the power delivered for a given value ofwind speed is predicted. Then the relationship between energy consumption and watersalinity for the RO system (see figure 2) is used for the calculation of the amount of water,produced by the system.

Figures 4–11 show the daily water production during a 1-year cycle at the selectedsites for different values of TDS (3000, 5000, 7000, and 10000 mg/L). As depicted in figures 4and 5, Hofa is the most potential site for wind powered RO desalination, followed by Ras

Figure 2 Energy consumption of the reverse osmosis desalination system as a function of the total dissolvedsolids content.

0

0.5

1

1.5

2

2.5

0 2000 4000 6000 8000 10000

TDS [mg/L]

SPE

CIF

IC E

NE

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Figure 3 Monthly average wind speeds for all the selected sites.

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476 HRAYSHAT

Muneef. With a TDS of 7000 mg/L (the actual brackish water salinity at these sites is inthe range of 5000 to 10000 mg/L), the amount of water that can be produced at these sitesduring the month of July are 4.8 and 3.1 m3/ day respectively.

Figure 12 exhibits the annual amount of water, which can be produced at all theeight selected sites for different values of TDS. Based on the obtained results, the selectedsites can be divided into three different categories: the first one, which includes Hofa and

Figure 4 The daily water production during a 1-year cycle at Hofa as a function of the total dissolved solidscontent.

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Figure 5 The daily water production during a 1-year cycle at Ras Muneef as a function of the total dissolvedsolids content.

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WIND-POWERED SYSTEM FOR WATER DESALINATION 477

Ras Muneef, is considered to be “adequate” for wind-powered RO desalination. Theannual amount of water produced at these two sites forms about 57% of all water pro-duced at all the eight sites combined. The second category is considered to be “promis-ing”. It includes Safawy, Twaneh, and Tafila. Their water output adds up to about 30% ofall water, produced at all sites. The third category, which includes Jurf AlDaraweesh,

Figure 6 The daily water production during a 1-year cycle at Safawy as a function of the total dissolved solidscontent.

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Figure 7 The daily water production during a 1-year cycle at Twaneh as a function of the total dissolved solidscontent.

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Aqaba, and Shoubak is considered to be “poor”, because only about 13 % of the waterproduced from all sites combined can be obtained from these three sites. It is obvious thatwind powered RO desalination at these sites is not a good option. Therefore, other alterna-tives for water desalination should be taken into account.

CONCLUSIONS

In terms of their potential for wind-powered RO desalination, some of the selectedsites were considered to be “adequate”. They include Hofa and Ras Muneef. Hofa is

Figure 8 The daily water production during a 1-year cycle at Tafila as a function of the total dissolved solidscontent.

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Figure 9 The daily water production during a 1-year cycle at Jurf AlDaraweesh as a function of the totaldissolved solids content.

0

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WIND-POWERED SYSTEM FOR WATER DESALINATION 479

considered to be the best among all of the selected sites for wind-powered RO desalina-tion. Other sites were considered to be “promising”. They include Safawy, Twaneh, andTafila. The rest of the sites studied, namely Jurf AlDaraweesh, Aqaba, and Shoubak areconsidered to be “poor” for wind-powered desalination. Therefore, other alternatives forwater desalination should be taken into account at these sites.

Figure 10 The daily water production during a 1-year cycle at Aqaba as a function of the total dissolved solidscontent.

0

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Figure 11 The daily water production during a 1-year cycle at Shoubak as a function of the total dissolved solidscontent.

0

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NOMENCLATURE

A The cross-sectional areaCP Power coefficientPw Wind powerR Radius of the rotor bladev Wind speed (m/s)ΔKE The kinetic energy of the moving airΔL The length of the cylinder of airΔm The mass of the cylinder of airΔt Time intervalΔV Volume of a cylinder of air

Greek lettersρa Air density (kg/m3)

REFERENCES

Abdallaha, S., Abu-Hilal, M., Mohsen, M.S. (2005). Performance of a photovoltaic-powered reverseosmosis system under local climatic conditions. Desalination 183: 95–104.

Al-Rawajfeh, A., Glade, H., Ulrich, J. (2003). CO2 release in multiple effect distillers controled bymass transfer with chemical reaction. Desalination 156: 109–123.

Figure 12 Annual water production for all the selected sites as a function of the total dissolved solids content.

0

250

500

750

1000

1250

1500

1750

2000

Hof

a

Ras

Mun

eef

Safa

wy

Tw

aneh

Taf

ila

Jurf

Ald

araw

wes

h

Aqa

ba

Shou

bak

AN

NU

AL

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CT

ION

[m

3 ]3000 mg/L 5000 mg/L 7000 mg/L 10000 mg/L

WIND-POWERED SYSTEM FOR WATER DESALINATION 481

Al-Rawajfeh, A., Glade, H., Qiblawey, H., Ulrich, J. (2004). Simulation of CO2 release in multiple-effect distillers. Desalination 166: 41–52.

Fanchi, J. (2004). Energy: technology and directions for the future. London: Elsevier AcademicPress.

Hrayshat, E. (2002). Wind energy in Jordan: current status and future potential. Proc. World Renew-able Energy Congress-VII. Germany.

Hrayshat, E. (2005). Wind availability and its potential for electricity generation in Tafila/Jordan.Renewable and Sustainable Energy Reviews 9: 111–117.

Hrayshat, E., Al-Soud, M. (2004). Solar energy in Jordan: current state and prospects. Renewableand Sustainable Energy Reviews 8: 193–200.

Jaber, J.O., Mohsen, M.S. (2001). Evaluation of non-conventional water resources supply in Jordan.Desalination 136: 83–92.

JICA. (1995). Final report on brackish ground water desalination in Jordan. Amman, Jordan.Malkawi, S.H. (2003). Wastewater Management and Reuse in Jordan. Proc. First Regional Water

Reuse Conference. Jordan.Mohsen, M.S., Jaber, J.O. (2001). A photovoltaic-powered system for water desalination. Desalina-

tion 138: 129–136.

Desalination 277 (2011) 274–280

Contents lists available at ScienceDirect

Desalination

j ourna l homepage: www.e lsev ie r.com/ locate /desa l

Wind energy technologies integrated with desalination systems:Review and state-of-the-art

Qingfen Ma a,⁎, Hui Lu b

a Department of Mechanical Engineering, Hainan University, Danzhou, 571737, Chinab Institute of Environment and Plant Protection, Chinese Academy of Tropical Agriculture Sciences, Danzhou, 571737, China

⁎ Corresponding author. Tel.: +86 898 31132006.E-mail address: mqf0920@gmail.com (Q. Ma).

0011-9164/$ – see front matter. Crown Copyright © 20doi:10.1016/j.desal.2011.04.041

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 January 2011Received in revised form 11 April 2011Accepted 14 April 2011Available online 6 May 2011

Keywords:Renewable energyWind energySeawater and brackish waterDesalination

Throughout the world, desalination is intensively used as a means to reduce current or future water scarcity,especially for the coastal areas. However, the dramatic increase in desalinated water supply will create a seriesof problems, themost significant of which are those related to energy consumption and environment impacts.Renewable energy provides an energy security and environmental friendly option simultaneously whendecreasing global reserves of fossil fuels threatens the long-term sustainability of global economy. Thus, theintegration of renewable resources in desalination and water purification is becoming increasingly attractive.In this paper an attempt has been made to present a review, in brief, work of the highlights that have beenachieved during the recent years worldwide and the state-of-the-art for most important efforts in the field ofdesalination by wind energy, which is one of the most common form of renewable energies. The wind energytransform patterns, modeling and experimental studies of various wind energy powered desalination plant,and the prototypes established worldwide are majorly discussed. Moreover, two important technologicalproblems in wind utilization are discussed, and the present or potential countermeasures for the intermittentcharacteristic and direct utilization of wind energy are presented.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Water and energy are two inseparable items that govern our livesand promote civilization. The social and economic health of themodern world depends on sustainable supply of both energy andwater. As of today, about three billion people have no access to asecure source of fresh water and about 1.76 billion people live in areasalready facing a high degree of lacking water [1]. Meanwhile, with theincrease of population, industrial and agricultural activities, availablewater resources has been excessively exploited and severely polluted.The need for fresh water is at the top of the international agenda ofcritical problems, at least as firmly as climate change.

Because of the growing scarcity of freshwater, a trend to intensifieduse of desalination as a means to reduce current or future waterscarcity can be observed. Seawater or brackish water can be desaltedand supplied in large quantity, but this will create a severer series ofproblems, the most significant of which are those related to energyconsumption and environment impacts. If desalination is accom-plished by conventional technology, it will require the burning ofsubstantial quantities of fossil fuels, which will aggravate the energycrisis worldwide and environment pollution. The particular environ-mental impact in desalination system is not well known yet, many

11 Published by Elsevier B.V. All rig

environmental studies related with desalination technologies arebeing conducted [2].

Renewable energy provides a variable and environmental friendlyoption and national energy security at a time when decreasing globalreserves of fossil fuels threatens the long-term sustainability of globaleconomy. The integration of renewable resources in desalination andwater purification is becoming increasingly attractive. However, atpresent, total worldwide renewable desalination installations amountto capacities is less than 1% of that of conventional fossil fueldesalination plants [3]. This is due mainly to the high capital andmaintenance costs required by renewable energy, making thesedesalination plants noncompetitive with conventional fuel desalina-tion plants. However, the cost of renewable energy systems has beensignificantly reduced during the last decades. Therefore, futurereductions as well as the rise of fossil fuel prices could make possiblethe competitiveness of seawater or brackishwater desalination drivenby renewable energies.

Solar thermal and photovoltaic (PV) systems, wind power,biomass, oceanic, geothermal and nuclear energy etc. are the basickinds of renewable energy used nowadays. Among above renewableenergies, wind energy has been maturely used for power productionand wind turbines are commercially available on a wide range ofnominal power. The electrical or mechanical power generated by awind turbine can be used to drive desalination plants. The windpowered desalination systems are one of the most frequentrenewable desalination plants, especially for coastal areas presenting

hts reserved.

275Q. Ma, H. Lu / Desalination 277 (2011) 274–280

a high availability of wind energy resources [4]. In addition, accordingto some authors, among the various renewable energy resources, theintegration of desalination with wind energy had the least impact onthe environment, with an important environmental impact reductionof 75% [5]. Thus, in high wind-potential areas where desalination isalso required, wind energy is the preferred energy source option.

In this paper, the status and development of coupling wind energysystems with desalination units are reviewed. The following discus-sion concentrates on the main wind-driven desalination models andexperiments as well as prototypes and implementations, aiming totrace the development process and the problems arising, and light uptheir perspective characteristics and trends.

2. Matching wind energies with desalination units

2.1. Desalination technologies

Currently available desalination technologies can be mainly catego-rized into two groups:

(1) Thermal desalination (phase change process) that involvesheating the feed (seawater, brackish water or other impairedwater) to “boiling point” at the operating pressure to produce“steam”, and condensing the steam in a condenser unit toproduce freshwater. Thermal desalination process includesmulti-stage flash (MSF), multi-effect distillation (MED), me-chanical/thermal vapor compression (MVC/TVC), membranedistillation (MD) and solar distillation (SD).

(2) Membrane desalination (non-phase change process) thatinvolves separation of dissolved salts from the feed waters bymechanical or chemical/electrical means using a membranebarrier between the feed (seawater or brackish water) andproduct (potable water). In the membrane desalination, thereverse osmosis (RO) and electro-dialysis (ED) are technolo-gies used frequently.

The dominant desalination processes are MSF and RO—44% and42% of world wide capacity, respectively. The MSF represents morethan 93% of the thermal process production, while RO process morethan 88% of membrane process production [4].

2.2. The coupling interface between wind energy and desalination unit

Wind energy and desalination plants are two different technolo-gies, which can be coupled in various ways. The interface between thewind energy system and the desalination system is met at the place/subsystemwhere the energy generated bywind energy is promoted to

Fig. 1. Existing interfaces between wi

the desalination plant. Considering that the energy requirements fordesalination continues to be a highly influential factor in system costs,the integration of renewable energy systems with desalination seemsto be a natural and strategic coupling of technologies. Currently, windenergy can power desalination plants directly or indirectly throughfour types of energy media: electricity, thermal energy, gravitationalpotential energy and kinematical power (shaft power). Fig. 1 showsthe existing interfaces between wind energy and desalination unitwhich can be found in the following discussions.

Electricity is the most commonly used energy form as the interfacebetween wind energy and desalination process. After having changedinto electricity, the energy from wind plant can be employed to drivedesalination processes such as green house, RO, ED andMVC [6–9]. Thewind plant can be on or off the grid. For the intermittent characteristicof wind power, usually backup facilities like battery, water tank,flywheel systemmight be integrated into the system to store or releaseenergy when the wind speed exceeds or cannot achieve the requiredvalue.

The technique of direct conversion from wind energy to thermalenergy has been studied for room heating and hot-water supplysystems because the efficiency of direct wind-thermal conversion ishigher than that of wind-electricity conversion and their structuresare simpler. Nakatake and Tanaka proposed a newly designed,maritime lifesaving small distiller. The wind energy was directlyconverted to frictional thermal energy to heat the distiller. Theproposed distiller could be driven by wind only and was predicted toproduce 1.5 kg/d ormorewhen a 6 m/swind blew steadily all day on asunny or cloudy day [10].

To reduce the energy loss caused by the wind-electricityconversion, gravitational energy has also been used as the interfacebetween wind energy and desalination process. Fadigas and Diasdesigned an alternative configuration to conventional RO desalinationsystems by incorporating the use of gravitational potential energy,without using either electricity or fossil fuels. The gravitationalpotential energy, presented by water stored in a reservoir above acertain height, was converted by wind energy from windmills (orwind turbines) [11].

Besides, interesting experimental research about directly couplingthe kinematical power fromwind turbines and a desalination unit hasbeen carried out. Projects AERODESA I and AERODESA II of CanaryIslands Technological Institute included the direct coupling of windenergy and RO unit by means of shaft power [4]. In Coconut Island offthe northern coast of Oahu, Hawaii, a brackish water desalinationwind-powered RO plant was established. The system was drivendirectly by the shaft power of a windmill using a high pressure pump.The water production rate can be maintained at 13 l/min for wind

nd energy and desalination unit.

Fig. 2. Breakdown of renewable energy powered desalination system technologiesimplemented worldwide [19].

276 Q. Ma, H. Lu / Desalination 277 (2011) 274–280

speed of 5 m/s [12]. Witte et al. proposedWindDeSalter® Technologyand analyzed its feasibility by calculation. The core of this technologywas using the substantial part of the available kinematical energyfrom aWEC (wind energy converter) directly to drive the compressorof an MVC plant or the high-pressure pump of a RO plant. The WECwas also integrated with all necessary functional elements such asseawater reservoir, filtering installation, pump units, desalinationunits, compressor, heat exchanger and drinking water storage tank[13].

3. Wind+desalination units

3.1. Wind+RO

Reverse osmosis (RO) is a pressure-driven process that separatestwo solutions with different concentrations across a semi-permeablemembrane [14]. RO system major components include membranemodules, high-pressure pumps, power plant, and energy recoverydevices as needed. RO is one of the most efficient desalinationtechnologies, requiring about 3–10 kWh of electric energy per m3 offreshwater produced from seawater [15]. Since RO is the desalinationprocesswith the lowest energy requirements and coastal areas presenta high availability ofwindpower resources, according to someauthors,wind powered RO plants appear to be one of the most promisingalternatives of renewable energy desalination [16–18]. Fig. 2 [19]shows a breakdown of renewable energy powered desalinationsystem technologies implemented worldwide. The most commonwindpoweredwater treatment systems in the pastwereRO andwind-powered RO systems made up approximately 19% of total RESdesalination facilities, second only to photovoltaic-powered RO units(32%).

3.1.1. Feasible and economic evaluationSeveral simulation studies have been done to discuss the feasibility

of wind powered RO technologies, based on various models withdifferent emphases. Feronwas among thefirst to evaluate the economicfeasibility of a wind-powered RO plant by mathematical modelinganalysis under some assumptions. The author concluded that theeconomic use of a wind-powered RO plant might be restricted to areaswith high wind speeds and fuel prices. However, it could become moreeconomic because of current developments such as decreasing RO plantcosts and wind turbine cost, and steady or increasing fuel costs [20].

Later, Habali and Saleh conducted a cost analysis of a wind-assistedRO system for desalinating brackish groundwater in Jordan. Theauthors stated that it would cost less to desalinate brackishwater with

a wind-assisted RO system than with a conventional diesel-poweredsystem [21]. Kiranoudis et al. performed a detailed analysis of a wind-powered RO plant. Not only different wind turbines and membraneswere analyzed, but also seawater and brackish water feed wereconsidered.Moreover, generalized design curves for process structuraland operational parameters were derived [22].

Voivontas et al. developed a method to evaluate the potentialmarket for RES (renewable energy resources) powered desalinationsystems. The results showed that Aegean Islands were the most aridareas in Greece, with abundant wind energy. Thus most wind-powered RO desalination plants could operate economically in mostof the arid Aegean Islands [23]. They also explored a computer-aideddesign tool as a means to compare the alternative options on the basisof economic indicators, combining technologies that guarantee thedesalination energy needs. Using the model, the effects of criticaldesign parameters on the water selling price were analyzed for awind-powered RO plant [24].

Garcýa-Rodriguez et al. analyzed the influence of the mainparameters on the cost of fresh water: climatic conditions, nominalpower of the wind turbine, salt concentration of seawater or brackishwater, design arrangement, operating conditions, plant capacity, cost ofRO modules and cost of wind turbines [16]. Romero-Ternero et al. [25]quantified the unit cost of fresh water generated from representativewind-powered seawater RO system as well as the exergy efficiency ofthe process by means of thermoeconomy. The unit cost of freshwaterwas determined exclusively by considering a wind-powered desalina-tion system. The exergoeconomic analysis showed that thewind-drivenseawater RO desalinationwas cost-effective for the representativewindpower site with medium plant capacity and the unit cost of freshwaterwas 76 c€/m3 [25].

In the past five years, with the rapid development of both windenergy and RO technologies and the increasing aggravation of theconventional energy crisis, feasibility and economic analyses of windpowered RO plant appear to be more important to assist the design,site selection, water production, and cost/price estimation etc. Koklasand Papathanassiou [26] proposed a logistic model to provide insightin the component selection criteria of an autonomous wind-driven ROplant. The simulation of the system operation was performedemploying a variety of different configurations with respect to thesize of its main components (wind turbine, RO plant and batteries).For each case, the annual water production was calculated, aneconomic assessment performed and the water production costestimated [26].

Forstmeier et al. [18] developed physics-based system models toconfirm the technical feasibility of using wind as the power source fordesalination, including both RO and MVC units. The results showedthat the costs were in line with what was expected for a conventionaldesalination system, proving to be particularly cost-competitive inareas with good wind resources that had high costs of energy. Thuswind-powered desalination could be competitive with other desali-nation systems, providing safe and clean drinking water efficiently inan environmentally responsible manner [18]. An integrated modelincorporated in the REDDES software for the use of renewableenergies (wind, solar) in the desalination of seawater was developedby Koroneos et al. [27]. The desalination technologies (mainly RO andMVC) were coupled with RES power systems to produce potablewater at the lower possible cost. The results indicated that waterproduction costs of an RES-desalination configuration dependedheavily on the available RES potential. The greater the RES potentialthe smaller the energy production cost from the RES unit and thussmaller water production cost from the desalination unit.

Spyrou and Anagnostopoulos [28] proposed a RO desalination unitpowered by wind and solar electricity production systems and by apumped storage unit. A specific computer algorithmwas developed tosimulate the entire plant operation and perform economic evaluationof the investment. Design optimization studies of the plant for various

277Q. Ma, H. Lu / Desalination 277 (2011) 274–280

objectives were conducted, like the minimization of fresh waterproduction cost or the maximization of water need satisfaction [28].

Koutroulis and Kolokotsa [29] presented a methodology for theoptimal sizing of PV modules and wind-generator powered ROsystems. Among a list of commercially available system devices, theoptimal number and type of units were determined so that the20-year round total system cost wasminimized, while simultaneouslythe consumer's water demand was completely covered. The corre-sponding optimal sizing results indicated that the total cost of the ROsystem was highly affected by the operational characteristics of thedevices comprising the system [29].

Bourouni et al. [30] proposed a new model based on the GeneticAlgorithms allowing the generation of several individuals (possiblesolutions) for coupling small RO unit to RES to minimize the totalwater cost. A particular interest was focused on the hybrid systems(PV/WIND/Batteries/RO), and a case of PV/RO unit, installed since2007 in Ksar GhilŠne village of southern Tunisia was studied [30].

3.1.2. Prototypes and installationsThe prototypes of wind-powered RO desalination system have

been reported in many regions of the world and a range ofexperiments has been conducted with various concentrations. Sofar, most of these installations, either connected to a utility network oroperating in a stand-alone mode, have been installed in Europe.

In France, as early as 1982, a small system was set at Ile du Planier.It was a 4 kW turbine coupled RO desalination unit with productionrate of 0.5 m3/h. The system also was designed to operate via batteries[31]. Another case where wind energy and ROwas combined is that ofthe Island of Drence in 1990. The wind turbine, rated at 10 kW, wasused to drive a seawater RO unit [32]. A very interesting experiencewas gained at a test facility in Lastours, where a 5 kW wind turbineprovided energy to a number of batteries (1500 Ah, 24 V) and via aninverter to an RO unit with a nominal power of 1.8 kW.

In Spain, the desalination leader of Europe [33], a pilot wind-powered RO plant was installed at Canary Island, in 1984. It wasconnected to the grid as auxiliary energy when the wind power wasnot enough for plant operation, with production rate of 200 m3/d andenergy consumption of 5 kWh/m3 [34]. In 1993, a 56 m3/d hybriddiesel-wind-RO plant providing fresh water and electricity for localpeople and a battery-less wind-RO plant started operating at Pájara,Fuerteventura Island. The system consists of two diesel engines and awind turbine of 225 kW. The Canary Islands Technological Institutedeveloped the concept AEROGEDESA based on the long-termexperience accumulated at Canary Islands, referring to a compact,stand-alone wind-RO system with capacities between 5 and 50 m3/d[35]. A wind/RO system without energy storage was developed andtested within the JOULE Program in 2001 by the University of LasPalmas. The RO unit had a capacity of 43–113 m3/h, and theW/G had anominal power of 30 kW [17]. Recently, experience of a seawater ROplant with capacities of 5000 m3/d in Gran Canaria, Canary Islands hasbeen reported. The power produced by the wind generators wasvariable throughout the year; at times excess power was sold to theconventional power network in place, and sometimes the RO plantconsumed supplementary power from the network grid [36].

Additionally, a 500 l/h seawater RO unit driven by a 2.5 kW windgenerator without batteries was developed and tested by the Centrefor Renewable Energy Systems Technology (CREST) UK. The systemoperated at variable flow, enabling it to make efficient use of thenaturally varying wind resource, without need of batteries [7].Excellent work on wind/RO systems has been done by ITC withinseveral projects such as AERODESA, SDAWES and AEROGEDESA [37].A great job on the combination of wind/RO has also been done byENERCON, the German wind turbine manufacturer. ENERCON pro-vides modular and energy-efficient RO desalination systems driven bywind turbines (grid-connected or standalone systems) for brackishand seawater desalination. Market-available desalination units from

ENERCON range from 175 to 1400 m3/d for seawater desalination and350 to 2500 m3/d for brackish water desalination. These unitscombine with other system components, such as synchronousmachines, flywheels, batteries and diesel generator, supply andstore energy and water precisely according to demand [38].

Other wind-driven RO systems in Europe are as follows:

• A RO system driven by a wind power plant, in Island of the CountySplit and Dalmatia [39];

• Island of Suderoog (North Sea), with 6–9 m3/d [40];• Island of Helgoland, Germany, with 2×480 m3/d [40];• Island of St. Nicolas, West France, hybrid wind-diesel [40];• Island of Drenec, France, with wind energy converter of 10 kW [40];• Ile du Planier, France Pacific Islands,with production rate of 0.5 m3/h[40].

Except Europe, engineers of other regions of the world have alsomade efforts to install and test the wind-RO desalination units. In1986, the installation of a RO plant in the Middle East began. It was a25 m3/d plant connected to a hybrid wind-diesel system [41]. When asecond RO plant of 168 m3/d was commissioned by the WaterAuthority of Western Australia, at Denham in Shark Bay, in 1991, thepower requirements exceeded the diesel grid's capacity and asupplementary 30 kW West wind turbine was installed to powerthe plants. Being grid connected, the plant imported power whenrequired and exported power back to the grid when excess powerwasgenerated [42]. In Coconut Island off the northern coast of Oahu,Hawaii, a brackish water desalination wind-powered RO plant wasinstalled. The systemwas using directly the shaft power production ofa windmill with the high pressure pump and RO. In particular aconstant fresh water production of 13 l/min can be maintained forwind speed of 5 m/s [12].

3.2. Wind+MVC

Although mechanic vapor compression (MVC) consumes moreenergy than RO, it presents fewer problems due to the fluctuations ofthe energy resource than RO. MVC systems are more suitable forremote areas since they are more robust, and they need fewer skilledworkers and fewer chemicals than RO systems. In addition, they neednomembrane replacement and offer a better quality product than RO.In case of contaminated waters, the distillation ensures the absence ofmicroorganisms in the product.

Few applications have been implemented using wind energy todrive a mechanical vapor compression unit. A pilot plant was installedin 1991 at Borkum Island (Germany), where a wind turbine with anominal power of 45 kW was coupled to a 48 m3/d MVC evaporator,with a 36 kW compressor [43]. The experience was followed in 1995by another larger plant at the Rügen Island, in Baltic Sea, with acapacity of 360 m3/d and wind energy production capacity of 300 kW[44].

Additionally, a 50 m3/d wind MVC plant was installed by InstitutoTecnologico de Canarias (ITC) in Gran Canaria, Spain, within the SeaDesalination Autonomous Wind Energy System (SDAWES) project[45]. The wind farm was composed of two 230 kW wind turbines, a1500 rpm flywheel coupled to a 100 kVA synchronous machine, anisolation transformer located in a specific building, and a 7.5 kWuninterruptible power supply located in the control dome. A detailedanalysis of the influence of the main parameters of wind poweredMVC systems was performed by Karameldin et al [46]. The studyindicated that the operating evaporator temperature and temperaturedifference recommended were 50 °C and 3 °C respectively. Under theaverage prevailing wind speed in these areas, the system pro-ductivities were 203, 398 and 938 m3/d when the wind turbinediameters were 20, 28 and 43 m respectively [46].

278 Q. Ma, H. Lu / Desalination 277 (2011) 274–280

3.3. Wind+ED

Finally, ED process is interesting for brackish water desalinationsince it is able to adapt to changes of available wind power and it ismore suitable for remote areas than RO. Modeling and experimentalresults of on-grid tests of installed such system at the ITC, GranCanaria, Spain was presented by Veza et al. The main goal of thisproject was to test and identify themost suitable desalination systemsfor connection to the medium off-grid wind farm. The capacity rangeof this plant was 192–72 m3/d [47]. Later, they developed anoperational envelope for the electrodialysis reversal unit, off-grid,i.e., only coupled to the wind farm. The desalination unit showed goodflexibility, adapting smoothly to variations in wind power, even whensudden drops or rises occurred [48].

4. Challenges and emerging/potential countermeasures to theutilization of wind energy in desalination process

4.1. Intermittent characteristic and emerging countermeasures

Since the intermittent characteristic of wind energy, the desalina-tion system driven by completely wind energy is affected by powervariations and interruptions. The power variations, however, have anadverse effect on the performance and component life of certaindesalination equipment. Hence, back-up systems might be integratedinto the system to reduce the effect. Meanwhile, there is also anothersolution, to integrate wind energy with other energy source, eitherconventional or renewable energy, such as solar PV or thermal, diesel[49], etc.

4.1.1. Integration wind energy with other energy sourceThe complementary features of wind and solar resources make the

use of hybrid wind-solar systems to drive a desalination unit apossible alternative. Solar energy desalination is generally thecollecting of solar thermal energy that is used for desalination directlyin solar stills, or that is converted to electricity by photovoltaic (PV)process first and then used in either thermal of membrane processesfor desalinations [50,51].

4.1.1.1. Wind/PV hybrid system. As early as 1979, Petersen et al.reported two RO-desalination plants with the GKSS-Research Centre(Germany) plate module system supplied by a 6 kW wind energyconverter and a 2.5 kW solar generator for remote areas [52]. Later,they reported another two such prototypes which were installed inthe Northern part of Mexico (Concepción del Oro) and in a smallisland at the German coast of the North Sea (Soderoog) [53]. TheCadarache Centre (France) designed another unit that was installed in1980 at Borj Cedra (Tunisia). The system consisted of a 0.1 m3/dcompact solar distiller, a 0.25 m3/h RO plant and an ED plant for 4 g/lbrackish water. The energetic system consisted of a photovoltaic fieldof 4 kW peak and two wind turbines [31].

Test results of a PV/wind powered brackish water RO plantinstalled in Israel were reported by Weiner et al [54]. Its productionwas 3 m3/d and expected life-span was 15 years. Two-day batterystorage and a diesel generator were built to serve as the back-up of thesystem. The test results showed that the optimum ratio between thepower of a desalination unit and PV/wind peak power was of theorder of 30–50% [54].

Kershman et al. presented a hybrid wind/PV powered ROdesalination plant implemented on Libya's coast of the MediterraneanSea. The nominal production of the plant was intended to be 300 m3/dto supply a village with potable water. While the expected nominalpower load for the operation of the RO desalination system was70 kW (net power after recovery), the solar PV system was designedfor 50 kW, and the WEC for 200 kW nominal outputs. The facility

design was flexible for the integration of a diesel generator andelectrochemical storage [55,56].

Except for the existing desalination plants, some novel ideas oranalyses for wind/PV hybrid systems have also been made. Mohamedet al. developed a simplifiedmethod for sizing and simulating a hybridwind-PV powered RO desalination unit based on a techno-economicanalysis. The water production cost calculated (5.21€/m3) was verypromising compared to the water transportation cost by tankers insome Greek islands that can reach 6–12€/m3, far below transporta-tion cost of 20€/m3 in Algeria [57,58]. Gilau and Small analyzed thecost-effectiveness of a stand alone small-scale renewable energy-powered seawater reverse osmosis (SWRO) system for developingcountries by a new methodology and an energy optimization model.Applying the model, using the wind and solar radiation conditions forEritrea, East Africa, they computed that for a two-stage SWRO systemwith a capacity of 35 m3/day, the specific energy consumption wasabout 2.33 kW h/m3, which was a lower value than that achieved inmost of the previous designs [59].

4.1.1.2. Wind/solar thermal hybrid system. Recently, wind energy hasbeen attached with the solar still to power the desalination processtogether with solar energy. Mohamed and Zhao [60] designed,fabricated and evaluated a new hybrid desalination system thatconstituted of wind turbine and inclined solar water distillationintegrated with main solar still. A small wind turbine was used tooperate a rotating shaft fitted in the main solar still to break boundarylayer of the basin water surface. The system can produce distilled andhot water. It was estimated that the electricity annual savings was192.22 RMB/kWh/m2, and the quality of distilled water as well as hotremaining water was good enough for domestic usage [60].

Moreover, wind and solar thermal can also be combined togetherto drive the thermal desalination process. Fernández-López et al. [61]analyzed an integrated desalination scheme consisting of twosequential systems: a MED plant and a MVC system based onevaporator equipment. The MED stage was driven by thermal solarcollector, whereas the energy consumption of MVC was fuelled bywind-powered turbines. The final products were dry salt and freshwater, with the desalted water production of 100 m3/h and the priceof 0.59€/m3 [61].

4.1.1.3. Wind/multi-renewable energy hybrid system. Regarding toother renewable sources combined with wind energy, an interestingdesign—a floating island was proposed by Stuyfzand and Kappelhof[62], although no such plant was implemented [63]. The plant was anartificial, floating island 10–100 km from the shore, 0.06–0.65 km2 insize with hexagonal shape, 0.1–1 km in diameter and 20 m deep. ROunit was driven by a combination of renewable energy sourcesincluding wind, solar, tidal, wave and hydrothermal gradient. Astorage reservoir aboard was used for stabilization and coping withfluctuations in energy supply and water demand. The plant wasestimated to produce high-quality freshwater of 5–500 Mm3/year at acost of 0.88–1.32€/m3.

4.1.2. Exploitation of wind-suited desalination unitThe disconnect relationship between the variable power production

of wind and the need for consistent energy input for most desalinationsystems is important to improve the efficiency of wind-powereddesalination units. If desalination units could be designed to respondeffectively to variable energy input, i.e., the desalination units wouldoperate at variable capacities based on the available wind, there is noneed of backup energy storage or integrationwith other energy sources.A company based in Germany, ENERCON GmbH, has addressed thisspecific limitation of RO technology. As a company focused primarily onwind energy products, ENERCON's operations have expanded to includedesalination technologies. They have developed an RO technology thatinvolves a piston system used for energy recovery that also enables

Table 1Basic information about some wind-driven desalination applications.

Plant location Water type Desalination unit, capacity W/T Nominal power Commissioning year Unit water cost;energy consumption

Ile du Planier, France SW/BW RO, 0.5 m3/h 4.0 kW 1982Canary Island, Spain SW RO, 200 m3/d 42 kW 1984 5.0 kWh/m3

Island of Drence, France SW RO, 10 kW 1990Pájara, Fuerteventura Island,Spain

SW RO, 56 m3/d 225 kW 1991

Denham, Shark Bay, Australia BW RO, 130 m3/d 30 kW 1991 12.5 cent $/kWhBorkum Island, Germany SW MVC, 48 m3/d 45 kW 1991Rügen Island, Germany SW MVC, 360 m3/d 360 kW 1995 2.1 kWh/m3

Gran Canaria, Spain SW RO, 200 m3/d 460 kW 1999 RO, 7.5 kWh/m3

MVC, 50 m3/d MVC, 14.4 kWh/m3

ED, 72–192 m3/d ED, 2.4 kWh/m3

Coconut Island, Hawaii BW RO, 2.7 l/min 1.2 kW 1999Canary Island, Spain SW RO, 5000 m3/d 2.64 MW 2002 19.865 cent €/m3; 2.9 kWh/m3

CREST, UK SW RO, 500 l/h 2.5 kW 2004 2.6$/m3

ENERCON, Germany SW/BW RO, 175–1400 m3/d (SW);350–2500 m3/d (BW)

200 kW 2006 2–2.25 kWh/m3

Note: SW: Seawater; BW: Brackish water.

279Q. Ma, H. Lu / Desalination 277 (2011) 274–280

variable levels of energy input. ENERCONplants have nofixed operatingpoint and the water production can range from max. 12.5% to 100% ofthe nominal capacity by adjusting the piston speed according todemand. This has two main advantages: firstly, operation is possiblewith a fluctuating energy supply, and secondly, output can be adjustedflexibly to water demand without shutting down the plant [38].

4.2. The potential of direct utilization of wind energy

The technologies have been developed to some extent to integratewind energy directly or indirectly to the desalination process such asRO, MVC, ED and solar still. Although RO is the major desalinationprocess connected to wind power, it is not always the appropriateone. For the remote areas which are short of fresh water but abundantof wind energy and seawater, more robust, easily operated windpowered desalination system should be explored. Consideringenvironment protection, the system needs to discharge fewerchemicals and offer high quality fresh water but not the properwater from RO unit. Thus, in the long term, thermal desalination unitsdirectly powered by wind power are more attractive due to their littleimpact on environment, high quality of treated water and energysaving feature.

So far, MVC is the major thermal desalination process integratedwith wind. However, due to the higher operation temperature(compared to the seawater temperature), additional heat source isalways necessary. In the single wind powered desalination systems,the transformation of wind energy to heat must reduce the energyutilizing efficiency. Therefore, if the wind powered desalination unitcan operate at lower temperature, eliminating the transformationfrom wind energy to heat, the desalination cost and energy utilizingefficiency might be improved. From this point of view, two potentialdevelopments of wind-desalination system are suggested by authorsas follows:

• The humidification–dehumidification process presents severalattractive features which are proper for the wind power, includingmodest level of technology employed, simplicity of design,relatively high efficiency compared to other thermal processes,and, most of all, the ability to combine with low temperaturerenewable energy source. Thus, it is of great importance to workfurther on problems related to the coupling interface of wind energyand desalination unit, design and optimization of operational andstructural parameters, and cost evaluation systems as well.

• Vacuum distillation might be another alternative as the desalinationunit powered by wind energy. In the process of vacuum distillation,the phase changes at low temperature, which makes the extra

thermal source unnecessary. In addition, the wind power can drivethe equipments such as vacuum pumps and compressor directly,avoiding the energy loss caused by energy conversion process. Thus,this kind of system might increase the usage efficiency of windenergy, and further research should focus on system design,parameter optimization and feasibility analysis, etc.

5. Conclusions

The use of wind energy for desalination appears nowadays as areasonable and technically mature option towards the emerging andstressing energy and water problems. In spite of intensive researchworldwide, the actual penetration of wind-powered desalinationinstallations is still low. During the recent past, there has been a ratherintense attempt to develop effective small or medium scaledesalination plants, mainly powered by wind energy. The technolo-gies have been developed to some extent to integrate wind energydirectly or indirectly to the desalination process such as RO, MVC, EDand solar still. So far, RO is the major desalination process connectedto wind power and MVC is the major thermal desalination processpowered by wind. The basic information of major desalinationprototypes or plants mentioned in this paper is concluded inTable 1. Through these activities, considerable experience has beengained.

Similar to other renewable energies, overcoming the intermittentcharacteristic and improving the energy utilizing efficiency of windenergy are two important technological problems in the present orfuture research. At present, the solutions for the intermittentcharacteristic of wind energy are mainly integrating the wind energywith other kind of energy and designing flexible desalination unit tofit the variation of wind. As to the improvement of the energy utilizingefficiency of wind energy, the authors has pointed out two potentialkinds of wind-powered desalination units, possibly utilizing the windenergy directly with less energy loss.

Acknowledgement

The financial supports by the National Natural Science Foundationof China (51009044), Scientific Research Fund of Hainan ProvincialEducation Department (Hjkj2011-05) and Start-up Fund Project ofHainan University (kyqd1106) are gratefully acknowledged.

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DESALINATION

ELSEVIER Desalination 153 (2002) !%I6

A wind-powered seawater reverse-osmosis system without batteries

Marcos S. Miranda*, David Infield

Center for Renewable Energy Systems Technology CREST, Loughborough University LEll3TW, UK Tel. + 44 (I J09) 22il44; Fax +44 (ISO@ 610031; email: MS.&iranda@lboro.ac.uk

Received 20 April 2002; accepted 30 April 2002

Abstract

The development of small-scale stand-alone desalination systems is important to communities on islands and in isolated inland areas, In such places, electricity supplies are often expensive and unreliable, while the wind resource is abundant. The system presented here comprises a 2.2 kW wind turbine generator powering a variable-flow Reverse osmosis (RO) desalination unit. It is highly efficient, rugged, built with off-the-shelf components and suitable for use in remote areas. Operation at variable-flow allows the uncertainty and variability of the wind to be accommodated without need of energy storage. Batteries, which are common in stand-alone systems, are avoided and water production is dependent on the instantaneous wind speed. A model-based control strategy is used to independently maximize both the energy extracted from the wind and the water output of the RO unit. A computer model of the system has been developed based on component models, identified through laboratory testing. Performance predictions are presented and discussed.

Keywork: Wind power; Reverse osmosis; Energy recovery; Seawater desalination; Renewable energy

1. Introduction of fossil fuels. Increasing awareness ofthe depletion of current sources has led to a global effort in the research and development of renewable energy technologies, such as wind, solar, tidal and geo- thermal energy.

There is no need to dwell on the importance of energy in the daily life of modern society and that its availability relies mostly on the existence

*Corresponding author.

Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, May 4-6, 2002.

00 l l-9164/02/$- See front matter CQ 2002 Elsevier Science B.V. All rights reserved PII: SOOIl-9164(02)01088-3

10 M.S. Miranda, D. Infield/Desalination I53 (2002) 9-16

This motivation for using renewable energy is even greater if stand-alone desalination applica- tions are considered. This is because the energy required for the process is particularly expensive in the remote areas where fresh water is required. Renewable energy sources can provide a reliable energy supply alternative for water desalination. Initial cost and resource availability are the most significant limitations.

In the context of the utilization of the more established renewable energy sources: the sun (thermal and PV), and the wind, stand-alone desalination systems have been widely discussed [l-5]. Since desalination is an especially pro- mising application that involves broad fields of study, many different solutions have been proposed. Even if one focuses on one particular renewable source and a specific desalination method, there may still be many options available in terms of the final system configuration.

One of the critical limiting factors to the wider implementation of renewable energy driven de- salination systems is the intermittence and unpre- dictability of the renewable source. Two distinct problems have been identified in an earlier study [5]. The first is that most desalination technologies are not suitable for operation at variable power, and the second is that lack of continuity of energy supply, or even limited power availability over variable periods of time may cause the demand not to be met. A common solution to these problems is the use of energy storage, which can accumulate energy surpluses (long-term storage) and/or smooth out shorter-term variations in the supply (short- term storage). On the negative side, the use of such devices results in increases to both capital and operating costs.

A summary of the technically possible com- binations of renewable energy sources and desali- nation techniques is given in Table 1. Wind energy is a most attractive source in the short-term since the technology is well developed and relatively cheap. It will be the subject of the work presented here.

Table 1 Applicability of renewable energy sources to water desalination techniques

Wind Solar

RO ME vc MSF FS ED ED RO

SD

Source Hanafi [ 11.

Tidal

RO VC FS ED

Geothermal

MSF ME vc RO ED

2. Wind power systems

The use of wind energy for electricity generation can be divided into two main application areas. The first and foremost of these is the commercial generation of bulk electricity through grid- connected systems.

The second category is the electricity generation within stand-alone systems. In contrast to grid connected systems, these are built to be used in sites where maintenance may be sporadic and technical assistance unavailable, thus greater robustness is required in their design, incurring higher capital costs than otherwise. Grid connected turbines are used as an additional supply, com- plementing the conventional base load power system. In stand-alone systems, the wind can often be the sole source of energy and this should be fully taken into consideration during the design stage.

Due to the inherently random characteristic of the wind, certain key aspects must be attended to in the design process of stand-alone systems. Besides the wind resource potential, the nature of the electricity load needs to be given careful consideration, in particular whether disconnection is acceptable, and if so, for how long.

This consideration relates to the presence and sizing of any storage system that may be included in the system. Energy storage plays an essential role in determining system performance, as well

M.S. Miranda, D. Infield / Desalination 153 (2002) 9-16 11

as the aforementioned influence on capital and maintenance costs.

For the design of a small stand-alone system, a key challenge is to find a good compromise between reliability and system complexity that meets the economical constraints. This is not a simple matter, and it will mostly depend on the type of load and the local resources.

3. Reverse osmosis systems

Reverse osmosis (RO) is now a well-established technology for the desalination of water and in particular seawater. Nevertheless, the use of RO in small stand-alone systems (in the range of a few m3/d) is still an area of developing technology. Islands and isolated inland areas, where the electricity supply may be a problem, form an ideal application for such stand-alone systems. In these remote places, electricity is often supplied by a weak grid or even generated locally, by means of diesel generators.

Despite the many advances in RO membrane technology over recent years, such as higher rejection rates and improved tolerance to pressure fluctuations, considerable attention still needs to be given to operational conditions. It is believed that higher efficiencies can be obtained when operating at certain flow and pressure conditions.

An important contribution dealing with variable flow and pressure operation of RO membranes, which looks at the feasibility of wind-powered systems, is that of Feron [2].

In this work, an operational window is estab- lished (Fig. 1) determining the operational parameter variation to which a membrane can be safely sub- mitted. The four limits that define this window are: l Maximum feed pressure - determined by the

membrane mechanical resistance; l Maximum brine flow rate - should not be

exceeded to avoid membrane deterioration; . Minimum brine flow rate - it should be ob-

served to avoid precipitation and consequent membrane fouling;

I ” 0 (2 (limin) r

Fig. 1. RO membrane operational window.

. Maximum product concentration - if the applied pressure is less than a determined value, permeate concentration will be too high.

4. Wind-R0 configuration possibilities

A classification of the different wind powered reverse osmosis systems found in the literature has been made.

This was based on some of the points previously discussed: the existence of an alternative electrical supply (weak grid or diesel generator); the matching of the available wind energy to the load; and the operational characteristic of RO membranes.

4.1. Systems with back up (diesel/grid)

In these systems, an additional energy source is provided (a diesel-powered generator or even the local grid) so that the power supplied to the RO is constant. The back-up generation com- plements the power generated from the wind turbine to match the RO unit power consumption.

The main benefit of these systems, as in any hybrid wind-diesel configuration is the achieve- ment of fuel savings, which may increase the generator availability and reduce overall energy costs.

On the other hand, problems such as fuel shortages, diesel generator maintenance, inter- ruptions or power cuts in the supply, may lead to

12 MS. Miranda, D. Infield/Desalination 153 (2002) 9-16

unavailability of the RO system since it cannot be powered by the wind turbine alone.

4.2. Systems without back up

Systems without an external energy source can be divided into two categories, with emphasis on the RO unit operation: systems which run under approximately constant operating conditions; and those that experience variable operational con- ditions.

4.2.1. Near constant operating conditions

This first type of operation can be implemented by three different means: on/off switching of the RO units; usage of storage devices; and de-rating the wind turbine. In all three cases, an attempt is made to supply the individual RO modules with approximately constant power.

4.2.1. I. Storage devices

In this strategy, storage devices are employed to accumulate energy surplus during periods when the power generated by the wind turbine is greater than the load demand from the desalination unit. This surplus would then be used later when the generated power is insufficient to tneet the load demand.

One common way of storing the surplus energy is by using batteries [3]. In this case, the relation between operational pressure, storage sizing and average wind speed should be considered in the design stage. In addition, capital and maintenance costs should be carefully assessed. A disadvantage of this approach to the system design is the rating of the energy storage system, since this can make it economically unattractive at higher power levels due to the sizing of the battery bank.

4.2.1.2. RO unit switching

This strategy is based on the use of a higher power wind turbine connected multiple smaller RO units. The power control is achieved by switching the units on and off so as to match the

demand to the total power generated instanta- neously by the turbine. There is no limitation concerning the system Rower rating, and this ap- proach is feasible up to power levels of hundreds of kilowatts.

Although, frequent cycling of RO units is not usually recommended, this problem can be over- come by implementing different types of con- figuration. Rahal [4] uses a higher power wind turbine operating at near constant speed Cgenemtion management) connected to many equally smaller RO units switching on/off (load management). To smooth out the fluctuations, short-term energy storage (a flywheel in this instant) is used. Varying the pitch angle of the wind turbine blades controls the generated power.

Another possibiiity [5], suitable for smal1er systems (with medium/low power turbines of less than 50 kW rated), is the switching of few (two/ three) desalination units with distinct power levels. Additionally, some auxiliary loads (such as pump- ing/heating and dump loads) can be implemented to absorb any power surplus, keeping the system voltage and frequency constant.

4.2.1.3. Wind turbine de-rating

This approach consists of making use of the flat end of a pitch controlled wind turbine power curve to operate the RO unit at approximately constant power [6}. An implication of this con- figuration is that, since the turbine rated power is only achieved at high wind speeds, it would have to be de-rated by changing the settings of the pitching mechanism. This will cause the generated power to be flattened at lower wind speeds and conse- quently to have lower values. Therefore, the original rating of the turbine rotor should be considerably higher than the RO unit rated power making the system more expensive.

4.2.2. Variable operating conditions

In contrast to systems that operate under constant conditions, another operational strategy is based on the establishment and imposition of certain

MS. Miranda, D. Infield/Desalination 153 (2002) 9-16 13

operational limits [63, as previously illustrated in Fig. 1. This means that, based on the input power to the RO unit (flow times pressure), a control strategy is determined which .imposes a fixed operating point on the system that lies within the allowed region (i.e. the operational window of the RO unit shown in Fig. 1).

less system operating from a variable energy source such as the wind.

By doing this, an attempt is made to operate the system autonomously over a wider power range, without the need to use a back up unit or storage devices. The overall effect is to reduce capital and operating costs. One aspect that should be emphasized is that very little is known about the consequences of variable operation of RO membranes. It is recognized that mechanical fatigue can occur and that lifetime of the RO elements may be shortened and performance impaired.

Extensive laboratory testing was carried out so as to develop suitable mathematical models for individual components over a wide operating range. Based on these individual models, a complete system model was built to understand and assess system behavior as well as predict its perform- ance.

Based on these analyses, optimal performance trajectories were determined. These are the opera- ting conditions that maximize, at the same time, wind power conversion for a given wind speed and desalinated water production with the avail- able power. The control strategy makes the system follow these trajectories by controlling the speed of the positive displacement pumps.

5. Proposed wind-R0 system 6. Performance predictions

The proposed system comprises a small (2.2 kW) wind turbine directly supplying a reverse osmosis desalination unit. The pumping system is composed of two variable speed drives (inverter/induction motor) driving a medium and a high-pressure pump, each. The speed of each pump is individually controlled to maximize both wind power capture and product water flow. A Clark pump is used to recover the energy from the brine-stream, making the system highly efficient over a wide operating range.

The following performance predictions are taken from a detailed Matlab-Simulink model of the entire system. This model is based on extensive in-house component characterization, supplemented by manufacture’s data. The salinity of the seawater feed is taken to reflect that of the Red Sea at 40,000 ppm, which is isosmotic with 34,300 ppm of pure NaCl. Additionally, the feed temperature is constant and equal to 25°C.

6.1. Long-term (steady-state) performance A detailed description of the system and its

components is presented by CREST, Lough- borough University [6].

5. I. Control strategy

The first analysis carried out aimed at estab- lishing the input-output characteristic ofthe system, i.e. the relationship between wind-speed and fresh water production, as depicted in Fig. 2.

The use of two positive displacement pumps indirectly enables the control of feed pressure and flow independently. This characteristic makes it possible to operate the system at any point within the operational window shown in Fig. 1, provided a suitable control strategy is employed. This is critical in maximizing the efftciency of a battery-

The importance of this characteristic relies on the fact that it can be used in a system siting study. Similarly to a wind turbine power curve, it would give the expected output from the system, given the resource in a certain location. A statistical analysis can be conducted by applying the wind speed probability (Weibull distribution) for this location against the characteristic of Fig. 2. The

14 M.S. Miranda, D. Infield / Desalination 153 (2002) 9-16

4-

3.

2.

1 4 6 a 10 12 14 16

0-W

Fig. 2. Product water flow vs. wind speed.

4

n-

3 r.

3

6 a 10 12 14

PW

16

Fig. 3. System specific energy consumption.

result is the water production probability expected for this location.

Fig. 3 shows the system specific energy as a function of the wind speed. It presents an overall value below 3.5 kWh/m3, being slightly higher at lower wind speeds. Nevertheless, it is almost constant throughout the operating range. This translates in the fact that water production water is directly related to the available power.

6.2. Short-term (dynamic) performance

A dynamic model of the system was developed to verify its performance during normal transient operation, under turbulent wind. This is parti- cularly important in the determination ofthe control strategy and fine-tuning of the controller para- meters.

The following figures show the behavior of the system for a particularly high wind speed data series. The time-series used is 5 min long, with an average wind speed of 8.3 m/s.

Fig. 4a shows the wind speed time series used as an input to the system. The desalinated product water flow is plotted in Fig. 4b. Its average value is 8.5 m3/d. The dependence between both curves is obvious. Contrary to the product water flow, which is proportional to the net driving pressure, the salt passage through the membranes is mostly a function of the difference in concentration between the feed and the product streams (appro- ximately constant). Therefore, the product water concentration (Fig. 4c) will be inversely propor- tional to the flow. The concentration shown is resultant of the mixing of the different product concentrations of the four modules used with an average value of almost 300 ppm.

Finally, Fig. 4d presents the system instanta- neous specific energy. This figure is obtained by dividing the instantaneous wind turbine output power (in kW) by the product water flow (m3/h). It should be pointed out that desalinated water has a higher energetic content than seawater and therefore represents an energy storage medium itself.

Such low figures for the specific energy em- phasize the importance of assessing individual component efficiency, in an attempt to minimize system overall losses.

The average value of about 3.4 kWh/m3 for the system specific energy seems remarkably low when compared with the literature standard of 1 O- 15 kWh/m3 achieved in standard (no energy reco- very) systems. Additionally, it should be mentioned

M.S. Miranda, D. Infield / Desalination 153 (2002) 9-16 15

A high-efficiency configuration was proposed, having a Clark pump as the concentrate stream hydraulic energy recovery device. One of the main objectives of this analysis was to verify the inte- gration of the several components -which were individually tested and modeled - and their per- formance over a wide operational range. More important still, was to demonstrate the proposed configuration as a promising alternative for the desalination of seawater.

Finally, a more representative validation of the system will be only possible after prototype testing, which will follow in the next phase of the project. Prototype testing will provide a deeper under- standing of the system performance, particularly considering the transients present in a system sup- plied by an extremely variable and unpredictable source, such as the wind.

0 50 100 160 200 250 300

(9

Fig. 4. Wind speed, product flow, product concentration and specific energy.

that energy recovery - using turbines, reverse running pumps or other devices - is a relatively common practice in bigger systems, with rated outputs of at least 100 times higher. This is also due to the improved efficiencies associated to the increased sizing of components. Nevertheless, such efficiencies would not be easily achieved in smaller components such as the ones needed to implement a system of the same size of the one proposed in this work.

7. Conclusions

This work presented the analysis of a small wind-powered reverse osmosis desalination system.

Acknowledgement

This work was carried out in conjunction with Dulas Ltd, Machynlleth, UK and was supported by ETSU, DTI, UK and CNPq, Department of Science and Technology, Brazil.

References

111

[21

I.31

[41

151

A. Hanafi, Desalination using renewable energy sources, Desalination, 97 (1994) 339-352. S. Alawaji, M.S. Smiai, S. Rafique and B. Stafford, PV-powered water pumping and desalination plant for remote areas in Saudi Arabia, Applied Energy, 52(2-3) (1995) 283-289. H. Ehmann, A. Wobben and M. Cendagorta, PRO- DESAL - development and pilot operation of the first wind powered reverse osmosis seawater desalina- tion plant, Mediterranean Conference on RES for Water Production, Santori, Greece, 1 O-l 2 June 1996, pp. 84-87. C.T. Kiranoudis, N.G Voros and Z.B. Maroulis, Wind energy exploitation for reverse osmosis desalination plants, Desalination, 109 (1997) 195-209. CRES, Greece, Desalination Guide Using Renewable Energies, THERMIE - DG XVII, European Commis- sion Report, 1998.

161 P. Feron, Use of windpower in autonomous reverse

16 MS. Miranda, D. Infield / Desalination 153 (2002) 9- I6

osmosis seawater desalination, Wind Engineering, 9(3) (1985) 180-199.

[7] D.G Infield, Performance analysis of a small wind powered reverse osmosis plant, Solar Energy, 61(6) (1997)415-421.

[8] Z. Rahal and D.G Infield, Wind powered stand alone desalination, EWEC’97, Oct. 1997, pp. 802-806.

[9] AS. Neris, GB. Giannakopoulos and N.A. Vovos,

Autonomous wind turbine supplying a reverse osmosis desalination unit, Wind Engineering, 19(6) ( 1995) 325-346.

[lo] M. Thomson, M.S. Miranda and D.G Meld, A small- scale seawater reverse-osmosis system with excellent energy efficiency over a wide operating range, Euromed, 2002, Desalination Strategies in South Mediterranean Countries.

E L S E V I E R Desalination 171 (2004) 257-265

DESALINATION

www.elsevier.com/locate/desal

Preliminary experimental study of a small reverse osmosis wind-powered desalination plant

F a b r i z i o M o r e n o , A l v a r o P i n i l l a * Department of Mechanical Engineering, Universidad de los Andes, Cra 1 4, No. 184, 10 Bogota, Colombia

Tel. +57 (1) 332-4322; Fax: +57 (1) 332-4323; emaih apinilla@uniandes.edu.co

Received 15 April 2004; accepted 4 June 2004

Abstract

The paper describes the work carried out in the development of a small wind-powered desalination plant. An alternative control system was studied to serve as a direct interphase between a reverse osmosis desalination plant and a small wind energy conversion system. The main purpose was to reduce or eliminate the need for an energy storage system (usually, a battery bank). In order to achieve this objective, an experimental prototype of a desalination plant and a wind generator simulator were developed. The systems were evaluated under laboratory-controlled conditions and subjected to field trials. The experimental plant desalinates highly saline seawater (35,000 mg/L) at a rate of approximately 0.4 m3/d. This amount of potable water is sufficient to supply the basic water demands in a small community in an isolated location. The paper also describes the identification of technical problems associated with operating a desalination plant with an intermittent source of energy (wind).

Keywords: Wind energy; Simulation; Reverse osmosis; Seawater desalination

1. Introduction

Reverse osmosis (RO) is a process used to desalinate salt water. The process has the advan- tage that it requires low energy consumption compared to other desalination processes. Feron and Smulders o f Eindhoven Technical University (Netherlands) found in the 1980s that RO has the lowest energy consumption amongst most methods of desalination [ 1 ].

*Corresponding author.

In general, desalination processes are required in coastal or isolated locations where there are no water sources other than water from the sea, or the water source are in deep wells that produce salty water. A convenient alternative is the supply of potable water through independent and self- sufficient solutions such as that offered by thJis combination o f technologies (a RO plant and the wind energy conversion system).

Depending upon the concentration o f salts dissolved in water, the pressure required for the

0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved

doi: 10.1016/j.desal.2004.06.191

258 F. Moreno, A. Pinilla /Desalination 171 (2004) 257-265

RO process varies from 1.4 to 8.3 MPa (200- 1,200 psi). There is a practical relationship that allows the calculation of the minimum required osmotic pressure: at least 7 kPa (1 psi) is required for every 100 mg/L of salts dissolved in the water; therefore, if the seawater has 35,000 mg/L of salts, at least 2.45 MPa (350 psi) are needed for the desalination process to take place [2].

Desalination plants usually run on fossil fuel (diesel or motor gasoline) generators or they are connected to the local electricity network. Some- times energy is generated by renewable energies (solar or wind) which use energy storage systems (battery banks). The battery bank is expensive in RO plants for this type of application. Conse- quently, if a control system can be devised to allow the direct operation of the plant, initial costs will fall substantially, and the system will be simpler and more feasible to use.

The RO desalination plant is composed of five systems (Fig. 1): (1) a pre-treatment system, whose function is to reduce substances harmful to the RO membrane; (2) the desalination system, composed of a high-pressure pump and the RO membrane; (3) the power supply system, whose function is to generate the power required, (4) a control system, i.e., the interphase between the power supply and the desalination system; and (5) the post-treatment system to make the water of optimum quality for human consumption.

2. Pretreatment systems

The function of a pre-treatment system is to eliminate agents that will block the membrane. Most of these pretreatment systems are complex, however, and what is needed is a simple and effective form ofpre-treatment which requires no electricity for operation (a passive system).

A granular medium slow pre-filter was chosen [3] due to its simplicity in construction, operation and low associated costs. The first two stages of the pre-filter are anthracite (amorphous fragile

coal) beds and river sand. They allow suspended colloid particles to filter out by deposition on the grains of the filter medium. The final stage of the pre-filter uses granulated activated coal, which is used to free the filtered water of oxidizer agents and bacteria that might form a biological block.

The results of tests on this type of pre-filter show that it is effective for removing agents harmful to the RO membrane. The design para- meters of the pre-filter are determined by the type of medium, size and depth of the filtration beds, the surface area of the filters, the static pressure head available to act as the driving force and the method of operation of the pre-filter including cleaning.

The performance of this type of pre-filter is determined by the head loss through the filter and the resulting quality of the water. The pre-filter should be changed each time the maximum known head loss is reached or each time the quality of the pre-filtered water indicates that this should be done. The change ofpre-filter is easy, and it demands no more than removing the blocked-up filtration media and adding new filtra- tion beds [4].

3. Pump-RO membrane system

This system is composed of a commercial RO membrane and a high-pressure pump. The mem- brane was chosen, beating in mind, two main constraints: it must be designed to desalinate sea- water and it must have the lowest possible feed flow rate. The membrane selected was the SW30- 2521 (Dow Chemicals FILMTEC TM Division), with a maximum flow rate of 0.37 L/s and a max- imum operating pressure of 6.9 MPa (1,000 psi).

The pump used was a Hydra-Cell M-03-E (Warner Engineering) positive displacement pis- ton pump; its main features are a maximum flow of 0.14 L/s and 8.2 MPa maximum operating pressure. The pump is made of stainless steel (essential for operation in salt water).

F. Moreno, A. Pinilla / Desalination 171 (2004) 257-265 259

t Salt water . . . . . . . . . .................................................. 1. Pre-Treatment

System Pure water

~ Electrical Energy.~.~.-..--m,m

nk

Desalination Tank 2. Desafination System Prefilter

=n=l~ ! . . . . . . . . . . . . . . . . . Pu 'rn- . . . . . . . . . . . . . . . . . . . . . . . -..~i ~J_. ~ Concentrate

....................................... ~ . , i , . ~ ' + ~ . _ , ' , _ 1 7 ~ e : ~ l : ~ = " pe~;::e ,,

-t I -

r a I . . . . . . . . . . . . . . . . . -~"

i i Post-treatmen " - - : i I I Pe~mn~a te i

L . . . . . . . . . . . . . . . . . I H 4. Control System

3. Energy supply system

Windgenerator

Fig. 1. Simplified scheme of a RO desalination plant and a wind energy supply system.

The variables that affect the performance of the membrane are: • feeder concentration (Ca) - - This is assumed

to be constant since the plant is designed for desalination of seawater with concentration of 35,000 mg/L;

• operating pressure - - this affects the quality of the water varying the permeate concen- tration (Cp) and permeate flow rate (Qp)];

• feeder flow rate, Qa (affects Qp and Cp).

The reverse osmosis system application (ROSA) simulation program, supplied by the manu- facturer, was used to determine the magnitude of the effect of these operating factors on the variables of membrane output and predicted performance.

The results of the ROSA simulation deter- mined the operating limits of the membrane (Fig. 2). The left limit for the minimum concen-

trate flow rate was suggested by the manufacturer (0.066 L/s). The right limit for the maximum feeder flow rate was 0.37 L/s. These two limits are caused by ionization and polarization of the RO membrane. The upper limit depends on the maximum concentration of salt permitted in water for human consumption (1,000 my/L), as estab- lished by the World Health Organization. Based on these results, two operating parameters for the desalination plant were established: an operating pressure above 3.4 MPa in order to obtain a water quality with less than 1,000 mg/L and a minimum feeder flowrate of 0.066 L/s for the membrane to perform correctly.

4. Strategies for electricity supply

Two alternative sources of supply were evaluated for the RO membrane-pump combina.-

260 F. Moreno, A. Pinilla / Desalination 171 (2004) 257-265

E ¢= 0

8 e,

8

E ¢1.

Lef t L imi t Opt im um Ope ra t i ng Zone Rght limit . . . . . . . . . . . . . . . . . . . . . . . . . . .

14oo

t200 _ _ . Upper limit

1000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X

800 . 3,4 MPa (500psi)

,= ~ , _ ......... t 4,1 IvPa ~600psi/

I ~ I m m ~ m ~ ~ m ! m

60O

400

200

0

0.00

5,5 t, Pa (soopsi)

T ' . . . . . . . . . .

0.05 ~ 0.10 0.15 0.20 0.25 0.30 0.35 ~ 0,40 0.45 0,50

Feed Flowrate [ L / s ]

Fig. 2. Determination of desalination limits obtained by reverse osmosis system application (ROSA) simulation.

tion. One is by connecting the system to an external AC supply allowing variations in voltage and frequency for the excitation of a conventional electric motors. The other was to operate the system from a DC source, using an AC-DC rectifier.

4.1. Alternating current electricity supply

This alternative was evaluated using a theo- retical model, which was subsequently supported by laboratory tests. Some results are provided to show that it is a viable alternative.

A study was made of the performance of a permanent magnet synchronous generator by theoretical analysis [5] and experimentation. The analysis defined the behaviour of the electrical characteristics of the generator such as voltage, frequency and power in relation to changes in the rotational speed of the electricity generator.

Tests were made on three-phase motors under

variable electrical conditions. Three 3-phase 220 V AC/60Hz motors of different power (0.67 kW, 0.9 kW and 1.1 kW) were tested. They were connected directly to the output of a small commercial three-phase 900 W wind generator in the wind simulator bench. The tests for the motors connected directly to the electrical gene- rator suggested that the motor could work under variable conditions but would not supply the torque required by the load due to the demand for voltage. Two alternatives were proposed to solve this problem: a three-phase transformer could be used at the output of the wind generator or the wind generator should generate at a high voltage.

It is worth noting that the performance of the generator depends on the type of electrical load to which it is connected. Since the motors used were squirrel's cage induction motors, the load was inductive, and this produces a lag of voltage in relation to current. This means a fall in the voltage supplied by the generator [6].

F. Moreno, A. Pinilla / Desalination 171 (2004) 257-265 261I

The results obtained by using transformers allowed the conclusion that transformers can be used to increase generator output voltage, and thus to increase the torque capacity that the motor can deliver. The characteristics of the wind gene- rator used, however (a wind generator used to charge batteries, low voltage, high current), do not provide an acceptable solution. Also, since the load is inductive, the global power factor of the plant is further reduced.

Nonetheless, the use of high-voltage wind generators shows that they can operate a desali- nation plant simply and efficiently since they eliminate the need for additional elements that consume power such as rectifiers, transformers and battery banks.

,4.2. Direct current electricity supply

The evaluation of this scenario was made with a theoretical model of a DC motor. A permanent magnet DC motor was selected and it copes with the power requirements of the system that met the electrical restrictions. A forced commutation rectifier was used, given the advantages that this type of device has with small generators [5].

The theoretical analysis of the motor was supported by the operating data supplied by the manufacturer. The model shows a reduction in speed with an increase in motor torque, but this is :not significant at high speeds. The performance of speed as the DC current is increased over the armature, for different voltages, shows a minor dependence on changes in current, but a high dependence on variations of voltage.

In conclusion, the advantages of this scenario are the easy speed control and the torque-speed characteristics, flexibility in operating conditions with changing voltage and current and the response to changes in power, without the need or an electronic control system. However, size and cost are greater than that of an equivalent AC motor, and a rectifier is required.

4.3. Modelling and theoretical simulation o f the AC alternative

With the power characteristics of a commer- cial 1.5 kW wind generator with a rotor diameter of 3 m, a theoretical curve relating wind rotor speed to wind speed was developed [7,8].

In accordance with the theory of three-phase motors and tests run on them, a motor speed as a function of wind generator output was calculated. Since the pump is a positive displacement device, the speed determines the feeder flowrate regard- less of working pressure.

Table 1 Theoretical data of plant performance at a given wincl speed profile

Time, s Wind speed, Theoretical potable m/s water flowrate,

dm3/s

30 8.83 9.49 60 7.55 9.03 90 8.11 9.26 120 7.48 8.99 150 8.16 9.28 180 8.04 9.24 210 7.66 9.08 240 8.56 9.42 270 8.51 9.40 300 7.04 8.75 330 8.79 9.48 360 8.71 9.46 390 7.86 9.17 420 7.05 8.76 450 7.67 9.08 480 7.97 9.21 510 7.33 8.92 540 8.98 9.53 570 8.88 9.51 600 7.10 8.79 630 8.12 9.27

11.9

~ W i n d Speed (m/s) + Theoretical Potable Water Flow rate

10.9

o ~. 9.9

=

8.9

7.9

6.9

F. Moreno, A. Pinilla / Desalination t 71 (2004) 25 7-265 262

12.9 9,60

9.40

E 9,20 ~'~

9.00 E

0 8.80 "~

8.60 ~=

®

8.40

8.20

30 90 150 210 270 330 390 450 510 570 630

Time [s]

Fig. 3. Theoretical water flowrate produced by the desalination unit operating under varying wind speed profiles.

The theoretical model can determine the amount of potable water produced over time, considering fluctuating wind speeds (Fig. 3). This theoretical simulation assumes a salt water concentration of 35,000 mg/L and a resulting permeate with less than 1,000 mg/L, at a constant system feed pressure of 3.4 MPa. The figure shows that for a wind speed variation from 7 m/s to 9 rn/s (typical wind speed variation at the site of the future location of the plant), the theoretical potable water flowrate varies from 8.8 dm3/s to 9.5 dm3/s (it is almost constant for variations in wind speed). Table 1 summarizes the theoretical data.

5. Laboratory testing of the prototype desali- nation plant

An experimental prototype was constructed (Fig. 4) and the plant was then subjected to a

number of laboratory tests. The pro-treatment system is comprised of an intake tank (1) connected to two pro-filters (2) arranged in paral- lel with a flute (3). The pro-filtered water (pro- filtrate) is carried down the pro-filter discharge flute (4) to a pro-filtrate storage tank (5). Finally, the pro-filtrate is feed into the desalination system (6).

The desalination system is bolted to a steel structure. A three-phase 0.9 kW motor (7), the positive displacement pump (8), the RO mem- brane (9) and the control and measurement panel (10) are fitted to the structure. The control panel is composed of two pressure gauges, one on the suction side of the pump and the other on the output side, with two flow-meters, one measuring the flow rate of permeated water and the other measures the flow rate of rejected water.

The photograph also illustrates the wind gene- rator simulator assembly (11) composed of the

Desalination inlet Tank (6)

F. Moreno, A. Pinilla / Desalination 171 (2004) 257-265

'Inlet )

263

Fee

Motol

Pump

Instrl contn

Fig. 4. Photograph of the experimental prototype of the RO wind-powered desalina- tion plant.

1200

~1000 .E ~= 8o0

6oo 8 i 400

~oo

~ SirnLdation • Expenmenlal

+

3,00 3.50 4,00 4.50 5+00 5,50 6.00 Feed Pressure [MPa]

Fig. 5. Validation of operation of the prototype and ROSA simulation (feed flow rate with 35,000 mg/L of salts).

generator supplying AC current to the desali- nation plant. The prime mover of the simulator is connected to a frequency controller that allows variations in speed in accordance with the wind- speed variations simulated by a computer through a PLC.

The desalination unit was subjected to tests in the laboratory at a constant electricity supply

using a solution of salt water (close to 35,000 mg/L) and tea in a proportion of 1:3. In order to validate the plant 's performance, the ROSA simulated results were compared with the experimental laboratory tests.

In Fig. 5 the experimental laboratory results show an adequate desalination plant output with regard to feed pressure variation, securing a per- meate concentration of less 1,000 mg/L of salts dissolved. Table 2 shows a comparison of ROSA simulation and the experimental laboratory results on the desalination plant with respect to recovery and salt rejection rates. It is worth noting the close agreement in the results, both simulated and experimental, as well as, a good salt rejection and recovery rates not only when compared by numerical values but in the general tendency.

This experimental prototype will permit more conclusive information on the overall perf- ormance of the desalination plant when tested in field trials matched to a wind generator, as the next step of the project.

264 F. Moreno, A. Pinilla / Desalination 171 (2004) 25 7-265

Table 2 Experimental laboratory data of the desalination unit compared with simulation results

Feed pressure, Feed Permeate Recovery, Permeate Salt MPa flowrate, L/s flowrate, L/s % concentration, mg/L rejection, %

ROSA simulation (feed concentration: 35,000 mg/L): 3.4 0.19 0.006 3.0 771 97.8 4.1 0.19 0.010 5.3 462 98.7 4.8 0.19 0.014 7.7 339 99.0 5.5 0.19 0.018 9.7 271 99.2

Laboratory experimentation (feed concentration: 35,725 mg/L): 3.6 0.133 0.003 2.5 935 97.3 4.0 0.133 0.005 3.8 707 98.0 4.5 0.133 0.008 6.3 409 98.8 5.0 0.133 0.013 10.0 308 99.1 5.5 0.133 0.017 12.5 211 99.4

6. Conclusions and recommendations

This phase of the project explored the possi- bility of operating a small desalination plant without using any form of electronic controls. The most appropriate alternative for operating a desalination plant is the high-voltage wind gene- rator, connected directly to the motor. It allows variations of voltage and frequency in the generation of electricity to excite conventional electrical motors. This is a simple and efficient way of operating a plant since it eliminates addi- tional power-consuming devices such as recti- fiers, transformers and battery banks.

Although the DC electricity supply allows control of speed and torque-speed characteristics, and it offers flexibility in operating characteristics with variations in voltage and current, the system is larger and more expensive than the AC equi- valent. The DC supply also needs a rectifier. Therefore, this option provides a solution but does not meet the requirement of simplicity that the project intends to provide.

The immediate phase of the project will allow more representative validation of the system after

testing the prototype under variable electrical conditions. The testing of the prototype will give a deeper understanding o f the performance of the system, particularly considering wind speed variations.

Further work will be carried out to perform a broader set of experiments on the pre-treatment system, an in-depth study of the effect of temp- erature on the performance of the RO membrane and the analysis of the most appropriate post- treatment system.

Acknowledgements

The authors wish to express their thanks to Instituto Colombiano para el Desarrollo de la Ciencia y la Tecnologia (BID- COLCIENCIAS) and the Colombian commercial company Acquaire Ltda., for their cooperation and funding of this research project. The authors would also like to thank the company Severn Trent - Universal Aqua for its kind donation of the RO membrane.

F. Moreno, A. Pinilla / Desalination 171 (2004) 257-265 265

References

[ 1] P. Smulders and P. Feron, Seawater desalination and wind energy, Commission of the European Com- munities, Hamburg, 1984.

[2] A. Pinilla and F. Moreno, Desarrollo de un sistema de control para los procesos de desalinizaci6n de agua y refrigeraci6n, con base en plantas de osmosis inversa y refrigeradores, operados con sistemas de energia no conveneional, Informe T6cnico No 1, COLCIENCIAS, Bogota, 2003 (in Spanish).

[3] J.C. Botero, Estudio de un sistema de desalinizaci6n de agua marina por osmosis inversa, Proyecto de Grado, Departamento de Ingenierla Mechnica. Universidad de Los Andes, Bogota, Colombia, 2002 (in Spanish).

[4] G. Tchobanoglous and E. Schroeder, Water Quality, Addison-Wesley, New York, 1985.

[5] A. Grauers, Design of direct-driven permanent- magnet generators for wind turbines, Chalmers University of Technology, G6teborg, Sweden, 1996.

[6] B. Guru and H. Hiziroglu, Electric Machinery and Transformers, 2nd ed., Saunders College, 1995.

[7] S. Espinosa, Disefio y construcci6n de un control de velocidad del eje de un motor, para simular las condiciones de incidencia de viento en el rotor de un molino, Proyecto de Grado, Departamento de Inge- nieria Mec~inica. Universidad de Los Andes, Bogota, Colombia, 1998 (in Spanish).

[8] A. Zapata, Caracterizaci6n de un generador de imanes permanentes, Proyecto de Grado, Departa- mento de Ingenieria Mec6xtica, Universidad de Los Andes, Bogota, Colombia, 2001 (in Spanish).

lable at ScienceDirect

Energy 36 (2011) 4372e4384

Contents lists avai

Energy

journal homepage: www.elsevier .com/locate/energy

Assessment of a stand-alone gradual capacity reverse osmosis desalination plantto adapt to wind power availability: A case study

Baltasar Peñate a,*, Fernando Castellano a, Alejandro Bello a, Lourdes García-Rodríguez b,1

aWater and Renewable Energies Departments, Canary Islands Institute of Technology (ITC), Playa de Pozo Izquierdo s/n. 35119 Santa Lucía e Las Palmas (Spain)bDepartamento de Ingeniería Energética, Universidad de Sevilla, ETSI, Camino de Los Descubrimientos, s/n. 41092-Sevilla (Spain)

a r t i c l e i n f o

Article history:Received 28 September 2010Received in revised form25 March 2011Accepted 3 April 2011Available online 30 April 2011

Keywords:Wind energyReverse osmosis desalinationMedium scaleGradual capacity

* Corresponding author. Tel.: þ34 928727511; fax:E-mail addresses: baltasarp@itccanarias.org (B.

(L. García-Rodríguez).1 Tel.: þ34 954487231; fax: þ34 954487233.

0360-5442/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.energy.2011.04.005

a b s t r a c t

Desalination driven by renewable energies is an interesting technology in isolated coastal areas. Itsfeasibility and reliability are guaranteed by innumerable designs implemented and experiences carriedout, mainly focused on small capacity systems. However, only mature and efficient technologies aresuitable for medium or large scale desalination. In the case of seawater desalination, wind-poweredreverse osmosis is the most efficient, mature and cost-effective technology. This paper assesses themost suitable design for seawater reverse osmosis desalination driven by off-grid wind energy systems.A high innovative design based on gradual capacity with nominal production of 1000 m3/d is comparedto a conventional fixed capacity desalination plant. Due to the intermittent wind resource, the gradualcapacity desalination plant is able to fit the available energy and maximize the annual water production.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The Instituto Tecnológico de Canarias (ITC) e a public researchcompany of the Canary Islands (Spain) e coordinated a pioneeringproject called SDAWES (Seawater Desalination with an Autono-mous Wind System) [1]. It was the first initiative intended to testa SeaWater Reverse Osmosis (SWRO) system (8 � 25 m3/d) feddirectly with wind power, alongside other technologies (electro-dialysis and mechanical vapour compression). The eight ReverseOsmosis (RO) modules were connected or disconnected dependingon the available wind power resources, thus resulting in a gradualproduction capacity system. Subiela et al. [1] analysed the lessonslearnt of this experience and concluded that SWRO technology wasthe best of all the desalination technologies tested using windenergy. Nevertheless, an improved design of a gradual productioncapacity RO system should be conceptually and experimentallyanalysed.

This paper deals with a comparative analysis of two differentwind-powered reverse osmosis plants for seawater desalination,based on the experience gained by the ITC within SDAWES project.

þ34 928727590.Peñate), lourdesg@esi.us.es

All rights reserved.

The two plants analysed in this paper are driven by an off-grid windenergy system:

� The first system is an energy efficient design of a SWRO plantwith 1000 m3/d of nominal production capacity. The plantcould operate within a restrictive range of power consumption,only when wind resource is enough to supply all the energyrequired by the RO plant.

� The second system is an energy efficient design of a 1000 m3/dgradual production capacity SWRO plant. This system consists ofthree RO racks, two of them of 400 m3/d capacity and the otherone of 200 m3/d. The three racks are able to operate indepen-dently, being connected or disconnected depending on theamount of energy available.

The selected capacity for the analysis can be considered asmedium capacity within the framework of renewable energy-powered desalination. However, analyses of SWRO powered bywind energy have been normally focused only on small scaledesalination [2,3]. García-Rodríguez [4] reviews desalinationtechnologies driven by wind power. Romero-Ternero et al. [5]analyzes different parameters for a medium capacity SWROdriven by wind energy in the Canary Islands. The product costranges obtained prove that this technology is the most cost-effective compared to other systems based on renewable energy-driven desalination [6].

Fig. 1. Seawater reverse osmosis desalination plant driven by wind energy design process.

B. Peñate et al. / Energy 36 (2011) 4372e4384 4373

The connection of a wind system to a SWRO plant may bepossible either, through electrical or mechanical coupling. Previousexperiences in mechanical coupling [7e10] have not obtainedsatisfactory results for medium capacity systems. In this case, thehigh-pressure pump of the RO systems is connected directly toa Wind Turbine (WT) through a shaft linkage or by a hydraulicsystem.

Therefore, wind energy conversion into electricity is the onlyfeasible option to operate a medium or large scale desalinationplant. Product cost and the influence of design and operationalparameters are analysed. In addition, there are some pilot schemeson a small scale that demonstrate the technical feasibility of suchsystems for medium and large capacity.

Several experiences of RO desalination powered by WTs havebeen developed. Within JOULE Programme, the OPRODES projectcombined a variable 43e113 m3/h RO unit with a 30 kW nominalpower wind generator in 2001. This experience was coordinated by

Fig. 2. Basic scheme of the wind energy system connecte

the University of Las Palmas de Gran Canaria [10]. Moreover, Cartaet al. [11] describes an initiative madewithin VALOREN Programmeof the European Commission in the isolated village of Puerto de laCruze at the southern of the island of Fuerteventura. Awind/dieselRO system fully covers the energy and potable water requirementsin stand-alone conditions. The power system consisted of a 225 kWWT e two 160 kVA diesel engines with flywheel and synchronousgenerator of 75 kVA each, to produce electricity for village lighting,fish preserving in refrigerated chambers, sanitation and a 56 m3/dSWRO plant. Each one of the two diesel sets is mechanically con-nected through an electromagnetic clutch to the flywheel. Eachflywheel can deliver half of the envisaged peak power demand e

100 kW e during 30 s approximately, without frequency decreasesbelow the minimum level allowed (48 Hz).

In general, all the experiences require a studied design andpower control system to ensure optimized operation and stabilityin the system functioning. In general, the use of wind energy to

d to the seawater reverse osmosis desalination plant.

Fig. 3. Diagram of a gradual capacity seawater reverse osmosis plant.

Table 21000 m3/d fixed capacity seawater reverse osmosis rack nominal characteristics.

RO rack characteristicsa,b,c

No. pressure vessels 11Pressure vessel configuration 2 SW30HRLE-400i þ

5 SW30ULE-400iTotal No. elements 77Total active membrane area, Am (m2) 2861.32Average flux, qvp/Am (l/(m2$h)) 14.54

B. Peñate et al. / Energy 36 (2011) 4372e43844374

supply a stand-alone SWRO system, adapted to medium or largewater demands, entails taking into account a number of technicalconsiderations:

� Use of synchronous wind turbines with power control: thisgenerator is able to adapt to load variations quickly andgenerate more energy according to needs. The latest WTdevelopments for medium and high power propose an energytopology production that combines a synchronous generator ofpermanent magnets with a bi-directional power converter (fullconverter). This topology allows controlling theWT power flowwith the electrical grid connected.

� Short-term (flywheel) and mid-term (batteries) appropriatedimensioned of storage system. First one is useful for primaryregulation, providing active and reactive power control and

Table 1Main design parameters of 1000 m3/d fixed capacity seawater reverse osmosisplant.a

Main streams Volumetricflow (m3/d)

Pressure(MPa)

Pipe dimension(mm)

Feed water intake (A) 2500 0.25 160Product water (I) 998 0.07 100Brine water (H) 1502 0.10 160High-pressure pump outlet (C) 1000 5.77 101 (4-inch)K-200 inlet (G) 1496 5.57 127 (5-inch)Booster pump inlet (D) 1494 5.51 101 (4-inch)Booster pump outlet (E) 1494 5.77 101 (4-inch)RO rack inlet (F) 2494 5.74 127 (5-inch)

a Recovery rate, 40%; fouling factor, 0.85; energy recovery device: differentialpressure high-pressure side ¼ 60 kPa, differential pressure low-pressureside ¼ 30 kPa; leakages, 0.4%.

frequency stability to the system. Second one warranties timeof continuous operation.

� RO plants connected to an off-grid wind farm have got toconsider a charge control system. This system must allowabsorb or disconnect the power consumed in operation.

No. ff ERD K-200 required 2Energy recovery efficiency, hERD 0.98K-200 energy savings, kW 104.0High-pressure pump electrical

power required, kW70.2

Booster pump electrical powerrequired, kW

6.4

Intake pump electrical powerrequired, kW

35.4

RO process specific energyconsumption, kWh/m3

1.93

Total specific energy consumption,kWh/m3

2.78

a Recovery rate, 40%; fouling factor, 0.85.b Hydraulic performance of high-pressure pump, 0.92; BOoster Pump (BOP)

performance, 0.77; intake seawater pump performance, 0.74; variable frequencydrives performance, 0.98 and motor efficiencies, 0.95.

c Isobaric energy recovery device (ERD) was designed according to the followingcriteria: 30 kPa ERD low-pressure site differential; 60 kPa ERD high-pressure sidedifferential; 0.4% lubrication flow e leakages.

Table 3Volumetric flows of 1000 m3/d gradual capacity seawater reverse osmosis plant.a

Main streams Volumetric flow (m3/d) Pressure(MPa)

TrainNo. 1

TrainsNo. 1 þ 2

TrainsNo. 1 þ 2 þ 3

Feed water intake (A) 571 1714 2857 0.25Product water (I) 199 599 997 0.07Brine water (H) 372 1113 1854 0.10High-pressure pump

outlet (C)200 600 1000 5.66

K-200 inlet (G) 370 1111 1852 5.38Booster pump inlet (D) 370 1110 1850 5.32Booster pump outlet (E) 370 1110 1850 5.66RO rack inlet (F) 570 1710 2850 5.63

a Recovery rate, 35%; fouling factor, 0.85; energy recovery device: differentialpressure high-pressure side ¼ 60 kPa; differential pressure low-pressureside ¼ 30 kPa; leakages, 0.4%.

Table 5Minimum operation points of 1000 m3/d gradual capacity seawater reverse osmosisrack characteristics.

RO rack characteristicsa,b,c

TrainNo.1

TrainsNo. 1 þ 2

TrainsNo. 1 þ 2 þ 3

Product volumetric flow, m3/d 139 429 719Average flux, qvp/Am (l/(m2$h)) 11.11 11.45 11.51Energy recovery efficiency, hERD 0.98 0.97 0.96K-200 energy savings, kW 26.1 81.1 136.1High-pressure pump electrical

power required, kW9.2 28.5 47.8

Booster pump electrical powerrequired, kW

1.0 3.2 5.4

Intake pump electrical powerrequired, kW

5.6 17.4 29.1

RO process specific energyconsumption, kWh/m3

1.76 1.77

Total specific energy consumption 2.73 2.74 2.75

a Recovery rate, 35%; fouling factor, 0.85.b Hydraulic performance of high-pressure pump, 0.92; BOoster Pump (BOP)

performance, 0.77; intake seawater pump performance, 0.74; variable frequencydrives performance, 0.98 and motor efficiencies, 0.95.

c Isobaric energy recovery device (ERD) was designed according to the followingcriteria: 30 kPa ERD low-pressure site differential; 60 kPa ERD high-pressure sidedifferential; 0.4% lubrication flow e leakages.

B. Peñate et al. / Energy 36 (2011) 4372e4384 4375

� Water storage, preferably in height, to meet the water demandin long temporary spaces of lack of wind resource or seriousbreakdowns of the system.

According to the expertise of the ITC, for designing an optimizedstand e alone RO-WIND system the following points should beconsidered:

� Variable frequency drives for pumps to achieve slight flowvariations must be included in the process. It reaches slightchanges in power consumption. In this case, it should beassessed the effects of harmonic generation over the grid [1]and analysed the installation of devices to minimize thiseffect over the stand-alone grid.

� Design a control system and gradual operation mode. Thisconnection requires a variable capacity to modify quickly thewater production in the case of significant variations of thewind resource. This gradual production regulation modeallows adapting the energy consumption progressively. Suchsystems require and adequate balance between the generated

Table 41000 m3/d gradual capacity seawater reverse osmosis rack nominal characteristics.

RO rack characteristicsa,b,c

TrainNo. 1

TrainsNo. 1 þ 2

TrainsNo. 1 þ 2 þ 3

No. pressure vessels 2 6 10Pressure vessel configuration 2 SW30HRLE-400i þ 5 SW30ULE-400iNo. elements 14 42 70Total active membrane area, Am (m2) 520.84 1561.32 2601.8Average flux, qvp/Am (l/(m2$h)) 15.94 15.98 15.96No. K-200 needed 1 1 2Energy recovery efficiency, hERD 0.98 0.97 0.97K-200 energy savings, kW 41.3 123.9 206.5High-pressure pump electrical

power required, kW14.5 43.5 72.5

Booster pump electrical powerrequired, kW

2.1 6.3 10.5

Intake pump electrical powerrequired, kW

8.1 24.3 40.5

RO process specific energyconsumption, kWh/m3

1.99

Total specific energy consumption 2.96 2,97 2,97

a Recovery rate, 35%; fouling factor, 0.85.b Hydraulic performance of high-pressure pump, 0.92; BOoster Pump (BOP)

performance, 0.77; intake seawater pump performance, 0.74; variable frequencydrives performance, 0.98 and motor efficiencies, 0.95.

c Isobaric energy recovery device (ERD) was designed according to the followingcriteria: 30 kPa ERD low-pressure site differential; 60 kPa ERD high-pressure sidedifferential; 0.4% lubrication flow e leakages.

power, the energy consumed in the RO plant and the availableenergy stored.

� Variable power system operation will generate frequent andsignificant fluctuations in the main operation parameters e

flow and pressure basically. It is recommended to select highquality materials, especially for high-pressure water pipelinesand to assess carefully the choice of the RO membranes.

2. Work hypothesis and design procedure

The systems proposed are stand-alone desalination plants. Inboth of them, a SWRO plant driven by a wind farm, able to adapt itsproduction to the available power. Loads in this case are referred toSWRO system. There are two major subsystems:

1. No. 1: The Wind Energy Conversion Subsystem (WECS) iscomposed by the following items:� WTs that generate electricity,� Energy Storage System (ESS), which could be either,mechanical (flywheel) or electrical (batteries) or both,depending on the necessity of short-terms or medium/long-terms storage and control requirements,

� Electrical connection between WT and the RO subsystem,� Master power electronic equipment, which controls themain parameters of the grid (voltage, frequency and reactive

Table 6Summary of energy requirement in a 1000 m3/d fixed capacity seawater reverseosmosis design.

Energy requirements 1000 m3/d fixed capacity

Nominal operationpoint

Minimaloperation point

Product water volumetric flow, m3/d 998 748Recovery rate, R 0.40 0.34Electrical power required, kW 112 82.8RO process specific energy

consumption, kWh/m31.93 1.73

Total specific energy consumption 2.78 2.73

Table 7Summary of energy requirement in a 1000 m3/d gradual capacity seawater reverse osmosis design.

Energy requirements 1000 m3/d gradual capacity

Train No.1 Trains No. 1 þ 2 Trains No. 1 þ 2 þ 3

Nominaloperation point

Minimal operationpoint

Nominaloperation point

Minimal operationpoint

Nominaloperation point

Minimal operationpoint

Product water volumetric flow, m3/d 199 139 599 429 997 719Recovery rate, R 0.35 0.35 0.35 0.35 0.35 0.35Electrical power required, kW 24.7 15.8 74.1 49.1 123.5 82.3RO process specific energy consumption, kWh/m3 1.99 1.76 1.99 1.77 1.99 1.77

B. Peñate et al. / Energy 36 (2011) 4372e43844376

power) and manages active power flow between WT,storage system and RO subsystem,

� Control system: it regulates the RO modules start-up andstop, and the charge/discharge regulation of the ESS throughthe master power electronic equipment.

2. No. 2: The SWRO plant uses the electricity generated by theWECS subsystem.

The calculation procedure is summarized in Fig. 1. The designbegins with the dimensioning of the SWRO plant and the calcula-tion of its energy consumption. These data determine the size of theenergy system. The power of theWT depends on the wind resourceavailable at the chosen location. The ESS capacity depends on theWT power and the energy consumption of the selected RO plant.Energy production, equivalent hours, capacity factor and annualwater production are calculated for the WT model selected. Threedifferentmodels ofWTswere preselected in order to identify whichis the model with best adaptability to the system requirements. Asmentioned before, it is advisable to use an ESS that smoothesfluctuations in wind resource. The choice of the type of ESS and itsmaximum capacity is a key factor in the system design but it is notconsidered in this work, due to the specific solution, whetherbatteries, flywheel or both, will depend on the characteristics of theWT and the RO power response.

Fig. 2 shows the system as a whole, showing the WECS andSWRO plant subsystems main flows and their equipments.

Table 8Monthly wind speed average and typical deviation at Pozo Izquierdo (Gran Canariaisland) at 10 m height.

Month Speed average, m/s Typical deviation

Jan 5.86 3.34Feb 7.09 3.71March 5.08 2.93Apr 5.6 3.67May 8.44 4.03Jun 8.82 4.06July 12.56 3.31Aug 10.22 3.13Sept 9.2 4.39Oct 5.06 3.14Nov 5.86 2.88Dec 4.47 3.11Annual 7.36 m/s

3. Seawater reverse osmosis desalination plant

Technical and design characteristics of the desalination unitshould allow to achieve the highest energy efficiency and highquality of the product water (permeate). Two SWRO plant designpossibilities are considered and compared below.

� Fixed Capacity (FC) SWRO plant: one train of 1000m3/d. The FCsystem is able to operate only when the wind resource isenough to supply all the energy required by the RO plant in itslow-energy operation point.

� Gradual Capacity (GC): Three RO racks with productioncapacities of 200 m3/day (Rack No. 1) and 2 � 400 m3/day(Racks No. 2 and 3), respectively. The GC system is able tooperate with production gradually adapted to the availablewind power. According to that it is obtained the previous rackcapacities. It most adequate to prevent a low capacity rackinstead of three identical ones with the aim to produce watereven with low wind power periods.

According to this design the following treatment line raises:

� Seawater intake through beach-wells: in the case of GC casewill be necessary to install two intake pumps for supplying thefeed water demanded depending on the capacity required.

� RO pre-treatment: Antiscalant dosing and filtration with sandand cartridge filters.

� RO process: Each RO train (one in FC case and three in the GCdesign) consists of a High-Pressure Pump (HPP), EnergyRecovery Device (ERD), BOoster Pump (BOP) and RO rack. Thelatter is integrated by several pressure vessels with sevenmembrane elements each. The design of each pressure vesselconsists in seven-element hybrid membrane interstage design(2 SW30HRLE-400i þ 5 SW30ULE-400i membrane elements).Each of the pumps is equipped with a Variable FrequencyDriver (VFD) which allows slightly adapting the flows to thewind power available.

� Brine discharge: consisting of a brine pipe drain by directdrainage to sea.

� Clean-in-place system: Chemical cleaning pump and tank.

The general description of each RO train is similar to the desa-lination unit shown in Fig. 2. Fig. 3 shows the proposed design forthe GC system, with three RO trains and different pumps and ERDs.The isobaric ERDs transfer the energy from the membrane rejectstream directly to the membrane feed stream [12]. In the case of GCan ERD which permits recovery and flux variations is required. Thedegree towhich centrifugal ERD performance varies as a function ofrecovery and membrane flux changes depending upon the char-acteristics of a particular device and must be considered in theSWRO design process. In this GC design the selection of a flexibleERD is important. Isobaric ERDs must allow varying membrane fluxand recovery independently. If the flow rate of the booster pump isset with a variable frequency drive to be equal to the flow rate of theHPP, the systemwill operate at 50% recovery rate. If the flow rate ofthe booster pump is increased to double the flow rate of the HPP,the system will operate at 33% recovery rate. These operationconditions do not change significantly the HPP flow rate or thepermeate flow rate [13]. In this sense, the RO Kinetic� ERD isproposed.

Monthly wind speed average (10 m)

0

2

4

6

8

10

12

14

Jan Feb March Apr May Jun Jul Aug Sept Oct Nov Dic

(m/s)

Wind speed

Annual wind speed av.7,36 m/s

Fig. 4. Monthly wind speed average at Pozo Izquierdo e Gran Canaria island (3500 equivalent hours/years) at 10 m height.

B. Peñate et al. / Energy 36 (2011) 4372e4384 4377

Therefore, the GC design allows operating with, one, two orthree racks depending on the available wind power. In turn,installed ERD should be flexible and able to be quickly adapted tothe flow and recovery rate desired. Each RO train should work atvariable recovery rate, flow and pressure, within ranges defined bythe membranes and installed equipments. Besides, the FC designcan only work until limited minimum recovery rate that will bedefined by the lower limit of feed flow to the RO train.

For designing the RO trains, the leading membrane manufac-turer Filmtec was selected. All simulations were gauged usingFilmtec RO design software Rosa v.7.01 [14]. The SWRO trainsdesign parameters are the following:

� Seawater characteristic: 38,170mg/l TDS (east Atlantic seawaterbeach-well), temperature: tsw ¼ 20 �C, pH 7.2, density:rsw ¼ 1.03 kg/l, kinematic viscosity: nsw ¼ 1.25$10�6 m2/s

� Brine density: rb ¼ 1.05 kg/l, brine kinematic viscosity: nb ¼1.80$10�6 m2/s.

� Fouling factor: 0.85.� Seawater intake pumps: submerged centrifugal pump;hydraulic performance: hIP ¼ 0.74.

� Seawater pressure required (from intake to high-pressurepump inlet): pIN ¼ 850 kPa.

� Recovery rate: variable R ¼ 30e45%.� HPP: axial piston pump; hydraulic performance, hHPP ¼ 0.92.� BOP: centrifugal pump; hydraulic performance, hBOP ¼ 0.77.� Variable frequency drives performance: hVFD ¼ 0.98.

Table 9Monthly and annual power generation, capacity factor and equivalent hours of three wi

Wind generator FUHRLANDER FL100 (100 kW) VESTAS V27 (

Month MWh Equiv. hours Cap. Factor MWh EJan 23.80 238.00 0.32 45.59Feb 31.02 310.20 0.46 58.90March 23.74 237.41 0.32 45.79Apr 22.49 224.90 0.31 42.97May 58.05 580.53 0.78 109.31Jun 46.54 465.40 0.65 87.48July 71.27 712.70 0.96 133.84Aug 58.77 587.70 0.79 110.12Sept 48.00 480.00 0.67 89.86Oct 25.00 250.01 0.34 47.98Nov 22.65 226.50 0.31 43.59Dec 14.76 147.60 0.20 28.38Annual 446.10 4,460.96 0.51 843.80 3

a 3500 equivalent hours/years; 7.36 m/s annual wind speed average; Rough length: z

� Motor performance: hVFD ¼ 0.92.� ERD: K-200 model [15]; brine volumetric flow rate, 15e30 m3/hr; low-pressure site differential, pLP ¼ 30 kPa; high-pressureside differential, pHP ¼ 60 kPa, lubrication flow e leakagerate, 0.4%.

Based on aforementioned data, Tables 1 and 2 give the nominalcharacteristics of the FC RO plant. Table 1 shows the nominaloperation point data of the plant. The nomenclature of the massand energy streams is related to Fig. 3. A 40% recovery rate is ob-tained and the high pressure at nominal operation is 5.74 MPa. TheRO train configuration consists of 11 pressure vessels (203 mm (8-inch) diameter and 6.9 MPa (1000 psi) nominal working pressure).The pressure vessels will house 77 SWROmembranese see Table 2.Very low specific energy consumption is achieved for the desali-nation process. Simulations get the minimum operation point forFC design, in which is able to work respecting the product waterquality parameters, minimum working pressure and the designflow of hydraulic pipes installed. In this case, the minimum pointcorresponds to a production of 748 m3/d and R ¼ 34%. The reduc-tion of the energy consumption is up to 26% of the required energyin comparison with the nominal operation point.

The following Tables 3e5, show the design parameters of theSWRO plant with a GC design. Tables 3 and 4 show the RO char-acteristics, the number of pressure vessels in operation is variable.It increases from 2 to 10 depending on the wind power availability,corresponding to a number of membranes from 14 to 70. Each of

nd turbines. Pozo Izquierdo e Gran Canaria Island location.a

225 kW) ENERCON E32 (300 kW)

quiv. hours Cap. Factor MWh Equiv. hours Cap. Factor202.62 0.27 68.56 228.54 0.31261.78 0.39 88.30 294.33 0.44203.51 0.27 70.25 234.16 0.31190.98 0.27 64.55 215.16 0.30485.81 0.65 159.92 533.08 0.72388.80 0.54 127.61 425.37 0.59594.84 0.80 185.88 619.59 0.83489.42 0.66 160.18 533.94 0.72399.38 0.55 128.05 426.84 0.59213.23 0.29 73.48 244.92 0.33193.73 0.27 66.83 222.78 0.31126.13 0.17 43.22 144.06 0.19,750.24 0.43 1,236.83 4,122.77 0.47

0 ¼ 0.0024; wind turbine reliability ¼ 0.9.

Table 10Annual simulation results for three wind turbines coupled to a 1000 m3/d fixedcapacity seawater reverse osmosis design.

Annual simulation

Results FL100 V27 E32

Total water produced, m3 172,896 230,318 250,865Annual average product

volumetric flow, m3/d473.69 631.01 687.30

Total water not-produced, m3 191,457 134,035 113,488Total plant operation hours, h 4487 5688 6135Annual operation rate, % 51% 65% 70%Total plant stop hours, h 4273 3072 2625Energy produced by wind

turbine (kWh)468,674 886,511 1,299,430

Energy consumed by thedesalination plant (kWh)

467,161 620,901 676,008

Excess energy (kWh) 1512 265,610 623,421Productivity ratio, qvp/PWT 0.37 0.26 0.19

Table 11Annual simulation results for three wind turbines coupled to a 1000 m3/d gradualcapacity seawater reverse osmosis design.

Annual simulation

Results FL100 V27 E32

Total water produced, m3 159,166 223,090 245,540Annual average product

volumetric flow, m3/d436.07 611.21 672.71

Total water not-produced, m3 204,822 140,899 118,448Total plant operation hours, h 6464 6844 7062Annual operation rate, % 74% 78% 81%Total plant stop hours, h 2296 1916 1698Energy produced by wind

turbine (kWh)468,674 886,511 1,299,430

Energy consumed by thedesalination plant (kWh)

468,599 660,850 728,179

Excess energy (kWh) 74 225,661 571,250Productivity ratio, qvp/PWT 0.34 0.25 0.19

B. Peñate et al. / Energy 36 (2011) 4372e43844378

the 3 possible design points could reduce the energy required byvarying the pressure and feed flow. In addition, the minimumoperation points of each rack are given in Table 4.

4. Wind energy conversion subsystem

From above section the loads of the wind energy system areknown. A summary of the minimum and nominal energy requiredby the RO plants is shown in Tables 6 and 7 for both, FC and GCdesigns, respectively.

Once the energy consumption is defined, the next step is thedesign of the WECS. The WT power will be defined depending onwind resource and consumption range of the RO plant. Three WTswith a nominal power around the calculated power range of thedesalination plant are selected for the comparative analysis. In thiscase three wind generators of 100 kW, 225 kW and 300 kW ofnominal power are considered. All simulations were developedusing the ITC software CE2000. This software has been designedwithin a Regional Program for the wind resources management ofthe Canary Islands. The theoretical fundaments used for thiscalculation proceeding is shown in the Appendix A.

Fig. 5. Annual wind energy produced and water obtained with a Vestas 27 W

To design the WECS the following parameters are used:

� Wind speed, v (m/s): The location at Pozo Izquierdo (GranCanaria, Canary Islands) for the whole systemwas chosen. Thewind speed for amodel year is represented in Table 8 and Fig. 4.

� Rough length: z0 ¼ 0.0024 m.� Air density: rai ¼ 1.204 kg/m3 (at 20 �C).� Air dynamic viscosity: maid ¼ 1.80$10�5 (kg/m$s)� Air cinematic viscosity: maic ¼ 1.50$10�5 (m2/s)� Relative air energy: 244.2W/m2 (at rai¼ 1.225 kg/m3 and 15 �C).� WT reliability ¼ 0.9� WT 1: FUHRLANDER FL100 (100 kW) [16]� WT 2: VESTAS V27 (250 kW) [16]� WT 3: ENERCON E32 (300 kW) [16]� ESS: previous consideration have to be considered and revisedwith further calculation:

BMinimum total ESS considered: 3 h (estimation for thenormal operation of the RO plant and maintain tasks).BTotal useful ESS capacity: 1200 AhBTotal ESS performance: 0.7

T coupled to a 1000 m3/d fixed capacity seawater reverse osmosis design.

Fig. 6. Annual wind energy produced and water obtained with a Vestas 27 wind turbine coupled to a 1000 m3/d gradual capacity seawater reverse osmosis design.

B. Peñate et al. / Energy 36 (2011) 4372e4384 4379

The values of energy produced, equivalent-hours and capacityfactor for each WT over one year are represented in Table 9 (equa-tions in the Appendix A). The capacity factor is the relationshipbetween the real energy produced by a wind generator and theenergy theoretically produced during a defined period. Equivalent-hours are the number of hours that aWTwouldhave been operatingto the nominal power to produce the same amount of energy overthe same period of time, i.e. the capacity factor expressed in hours.

Values given in Table 9 determine how the WT fits to theproposed site. A proper capacity factor value must exceed 0.4,including aspects of the WT availability. The FL100 wind generator

W ind energy produced vs. W

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Da y

Ave

rag

e h

ou

rly

ener

gy

(kW

)

Water pr

Fig. 7. Wind energy produced and water obtained with a Vestas 27 wind turbine coupled

proposed exhibits the best capacity factor, however annual energyproduction is somewhat less than the energy provided by the othertwo proposed WTs.

The energetic and hourly water production calculations areperformed after obtaining the specificWT data for the selected site.From those, the monthly and annual calculations are carried out.The objective is to identify which combinations offer the bestsolution to obtain the highest water production and operation hourrates. For this purpose the operation range of both desalinationplants is divided in different power required steps. Two steps forthe FC plant and five steps in the case of GC plant. The energy

ater obtained (February)

16 17 18 19 20 21 22 23 24 25 26 27 28

s

0

10

20

30

40

50

60

70

80

90

100

Pro

du

ct v

olu

met

ric

flo

w

(m3/

h)

o duced (m3/h) Wind machine

to a 1000 m3/d fixed capacity seawater reverse osmosis design (February simulation).

W ind energy produced vs. W ater obtained (February)

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Da ys

Ave

rag

e h

ou

rly

ener

gy

(kW

)

0

10

20

30

40

50

60

70

80

90

100

Pro

du

ct v

olu

met

ric

flo

w

(m3/

h)

Water pro duced (m3/h) Wind machine

Fig. 8. Wind energy produced and water obtained with a Vestas 27 wind turbine coupled to a 1000 m3/d gradual capacity seawater reverse osmosis design (February simulation).

B. Peñate et al. / Energy 36 (2011) 4372e43844380

balance of the whole system is realised every hour followinga sequence calculation:

PowerdemandedSWROnðkWÞ ¼

Step1Step2Step.Step.Step5

�kW/m3=h

�aPowerStoragen�1ðKWÞa

�PowerStoragen�2

Powerexcessn�1 ðkWÞ�a

�PowerWindn�1

PoweravailableSWROn�1 ðkWÞ�

Tables 10 and 11 summarize the resulted of this sequencesimulation for the two RO plant designs. Results obtained for the FCsystem (Table 10) shows that the three wind generators cover morethan 50% of the maximum output capacity of the plant. However,

W ind energy produced vs

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1

Da ys

Ave

rag

e h

ou

rly

ener

gy

(kW

)

Fig. 9. Wind energy produced and water obtained with a Vestas 27 wind turbine coupled

taking into account the investment that involves the desalinationplant, the option of FL100with 51% of annual operation rate could be

dismissed. Regarding the E32/300 kWWT, the values of productionand coverage are higher than those presented by the V27/225 kWWT. However, the cost of the increase in water production andhigher excess of energy are considered insufficient to justify the

. W ater obtained (Ju ly)

8 19 20 21 22 23 24 25 26 27 28 29 30 310

10

20

30

40

50

60

70

80

90

100

Pro

du

ct v

olu

met

ric

flo

w

(m3/

h)

Water pro duced (m3/h) Wind machine

to a 1000 m3/d gradual capacity seawater reverse osmosis design (July simulation).

Energy balance (February)

-150

-125

-100

-75

-50

-25

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Da ys

Ave

rag

e h

ou

rly

ener

gy

(kW

)

0

125

250

375

500

625

750

875

1.000

1.125

1.250

1.375

1.500

1.625

1.750

1.875

2.000

Bat

tery

cap

acit

y (A

h)

Wind machine

SWRO plant

B alance (kW)

B attery cap. (A h)

Fig. 10. Energy balance of a Vestas 27 wind turbine coupled to a 1000 m3/d fixed capacity seawater reverse osmosis design (February simulation).

B. Peñate et al. / Energy 36 (2011) 4372e4384 4381

increase in the investment cost. For that reason, theV27/225kWWTcould be considered like the optimum WT to install.

Besides that, Table 11 shows results of GC system analysis. FCplant allows the production of a greater amount of water per year in

Energy balance

-150

-125

-100

-75

-50

-25

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Da y

Ave

rag

e h

ou

rly

ener

gy

(kW

)

Fig. 11. Energy balance of a Vestas 27 wind turbine coupled to a 1000 m3/d

comparison with GC design, but the desalination unit operates lessnumber of hours along the year. Consequently, the GC designproduces between 2 and 8%water production fewer than FC design.However, the annual operation rates are higher with the GC design

(February)

16 17 18 19 20 21 22 23 24 25 26 27 28

s

0

125

250

375

500

625

750

875

1.000

1.125

1.250

1.375

1.500

1.625

1.750

1.875

2.000

Bat

tery

cap

acit

y (A

h)

Wind machine

SWRO plant

B alance (kW)

B attery cap. (A h)

gradual capacity seawater reverse osmosis design (February simulation).

E nergy balance (July)

-150

-125

-100

-75

-50

-25

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Da ys

Ave

rag

e h

ou

rly

ener

gy

(kW

)

0

125

250

375

500

625

750

875

1.000

1.125

1.250

1.375

1.500

1.625

1.750

1.875

2.000

Bat

tery

cap

acit

y (A

h)

Wind machine

SWRO plant

B alance (kW)

B attery cap. (A h)

Fig. 12. Energy balance of a Vestas 27 wind turbine coupled to a 1000 m3/d fixed capacity seawater reverse osmosis design (July simulation).

B. Peñate et al. / Energy 36 (2011) 4372e43844382

and the excess energy is lower. It could be considered as anadvantage in order to reduce investment costs due to the highestproduction-excess energy relation obtained.

The following graphs (Figs. 5 and 6) represent the annualevolution of the energy produced by the WT V27/225 kW andwater production. It can be observed for both RO plant designs thatin months with many calm days the operation of the plant is moreintermittent. On the other hand, in days with excellent windresource the plant is operated for entire days at nominal operationpoint. These periods show temporary spaces with surplus energy.

Non-operation period are not related to the large power of theWT installed, but hours of calm of the site. For this reason, theseperiods do not reach overall percentages of more than 80% of

E nergy balanc

-150-125

-100-75-50

-250

2550

75100125

150175200

225250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Da ys

Ave

rag

e h

ou

rly

ener

gy

(kW

)

Wind machine

SWRO plant

B alance (kW)B attery cap. (A h)

Fig. 13. Energy balance of a Vestas 27 wind turbine coupled to a 1000 m3/

operating hours of the plants and these are determined to the windfrequency distribution of the site selection. Besides that, the GCdesign presents a number of operating hours higher than FC design,but lower at the nominal operation RO.

Figs. 7e10 are related to the same systems, they present thebehaviour of the system as a whole. The two SWRO plant designsare represented in specific months, operated by using the V27/225 kWWT. Firstly, Figs. 7 and 8 depict the operation of the systemin a low wind resource month (February) for FC and GC plants,respectively. Secondly, the corresponding operation withina month of high wind resource (July) is given in Figs. 9 and 10.

Considering Figs. 7 and 8, the GC design is able to operate morethan 80% of the month, although it produces a 3.3% less water than

e (Ju ly)

18 19 20 21 22 23 24 25 26 27 28 29 30 31

0125

250375500

6257508751.000

1.1251.2501.375

1.5001.6251.750

1.8752.000

Bat

tery

cap

acit

y (A

h)

d gradual capacity seawater reverse osmosis design (July simulation).

B. Peñate et al. / Energy 36 (2011) 4372e4384 4383

the FC design. The FC design produces 18488.3 m3 in 456 h ofoperation. It represents an average production of 660.3 m3/d,opposite to 638.6 m3/d of GC design.

Besides that, regarding results obtained for July (Fig. 9), thedifferences between both designs are minor. The FC design is ableto work a total of 715 h while the GC design achieves 97.7% of themonth. The average daily productions are similar, 957.4 m3/d forthe FC design, and 954 m3/d to the GC design, i.e. both designs areworking at the nominal point without any interruption over 96% ofthe month. It should be taken into account that months with highand constant wind resources generate a surplus energy that definesthe choice of the appropriate WT.

Finally, Figs. 10e13 depict the energy balances from the repre-sentative months previously considered, February and July.

These graphs showhowthe variations in the energy produced byV27WTcause batteries loading and unloading in periods between 1and 5 h. This transfer of energy allows that the system couldmanagethe response times of the RO plant operation in processes of energydeficit. In the case of GC design the system is kept running evenduring hours of low or null wind resource. These graphs show howthe design of the SWRO plant with the ability to operate at variableflow and pressure is able to adapt quickly the energy required to theenergy available for any raised designs. Besides that, in periods ofwind resource shortage (Figs. 12 and 13) the ESS plays his role asregulator of the system. Even, the ESS allows for adapting the ROoperation point e from nominal to minimum operation point e

without needing to stopworking. In this sense, the GC designwouldincrease the useful life of the ESS by permitting the consumption ofless battery life cycles. As said above, during the high wind resourcemonths the two layouts behave in the same way.

5. Summary of results

This section focuses on the energy efficient design and compar-ison of a high innovation 1000 m3/d gradual and fixed capacitySWRO plants powered by an off-grid wind energy system. In the firstcase the desalination plant is more versatile because it is able tooperate racks independently or jointly, depending on the amount ofenergy available. In the second case, the plant could only operatewithin a more restrictive range of power consumption when windresource is enough to supply all the energy required by the RO plant.

Three commercial WTs close to the calculated RO plant powerand energy ranges are selected for the comparative analysis (100,225 and 300 kW). The simulation is performed with wind data ofPozo Izquierdo location (Gran Canaria Island).

Fixed capacity plant allows the production of a greater amount ofwater per year in comparisonwith gradual capacity design, althoughthe desalination unit does not operate more hours during the year.The gradual capacity design produce between 2 and 8% water lessthan fixed capacity design. However, the annual operation rates arehigherwith the gradual capacity plant and the excess energy is lower.Both designs reach rates above 95% production hours in months ofhighwind resource. Besides that, inmonthswith lowwind resources,the fixed capacity design produces more water compared with thegradual capacity design. In addition, gradual capacity systems shouldbe designed by using synchronous WTs with power control, capableto quickly adapt to fluctuations in demand by adjustment in powerproduction. Besides, a good dimensioning of the ESS (flywheel,batteries) that allows storing or transferring the energy during thetransitional periods, as well as storing the minimum amount ofenergy consumed for the boot process.

In the case of an off-grid system the power control component ismore important than the priority of producing the maximumvolumeofwater. This is because the power control technologyof theenergy generation systems still requires the contribution of demand

control systems for good tuning between variable renewable energypower generation and the power demand. Given this approach,gradual capacity designs are a major contribution to the design andoptimization of desalination powered by renewable energy sources.Currently, the development of wind generation technology allowshaving generation systems with primary power control andsupplementary services thatmake possible coupling to desalinationtechnology without power control, i.e. fixed capacity RO plant.

Besides, this contribution allows to achieve reductions in theESS capacity (batteries, where appropriate). With an adequateestimation of the wind resource (using prediction systems) and thecontrol of the desalination plant power demand, the requirementsof ESS can be minimised. Only the need of providing power inunintended electrical cuts and maintain electrical parameters onthe margins of quality assurance of supply to the load.

The power available from the renewable energy resource definethe performance of the system, due to the improvements in the ROsystem are normally associated with reductions in the windgeneration system performance. This means that it should bestudied the fixed capacity desalination plant production (andperformance) enhancements regarding the variable capacity. Inparallel, the losses in the performance of the energy generation andthe need to increase the ESS should be thoroughly studied.

For a system similar to that designed in this paper, but con-nected to a weak electrical grid, whose target is to supply water toa site with minimal impact on the grid, the priority criteria shouldbe water production and the whole system performance. In thiscase, it would be necessary to simulate different variable capacityoperation RO plant points and check that the variable performanceof the system from the optimal one means a capacity reduction.

However, the development of an adequate synchronous WTpower control means that the power loss to the maximum that canbe generated, despite the improvements that it entails for thecontrol of the combination SWROeWECS. This loss must bedetermined in each case due to the dependence of the performanceof the WT and the performance of the electronic devices installed.

6. Conclusions

Desalinationdrivenbyrenewable energies is anattractive chancefor freshwater production in arid regions. Onlymature and efficienttechnologies are suitable for medium to large scale desalination.

The design of two possible combinations useful for renewableenergy-powered SeaWater Reverse Osmosis desalination plant(SWRO) are addressed in this paper. An energy efficient design andcomparison of a high innovative 1000 m3/d Gradual Capacity (GC)and Fixed Capacity (FC) SWRO plants powered by an off-grid windenergy system is performed.

In the first case, the plant is more versatile because it is able tooperate trains independently or jointly, depending on the amount ofenergy available. In the second case, the plant can operate withinamore restrictive range of power consumptionwhenwind resourceis enough to supply all the energy required by the RO plant.

FC plant allows the production of a greater amount of water peryear in comparison with gradual capacity design, but the desali-nation unit does not work more hours during the year. The GCdesign produces between 2 and 8% water production fewer than FCdesign. However, the annual operation rates are higher with the GCplant and the excess energy is lower. Both designs reach rates above95% production hours in months of high wind resource. In monthswith low wind resource, the FC design produces more watercompared with the GC design.

In the case of an off-grid system, the power control componentbecomes a priority on producing the maximum volume of water.The power control technology of the energy renewable generation

Table A1 (continued).

Energy generated bya wind turbine (W)

Mean power generated:P ¼ RN

0 PðvÞ$f ðvÞdvP(V) ¼ wind generator characteristic curve(two specific parts of the curve):

PðvÞ ¼ Piþ1 � Piviþ1 � vi

ðv� viÞ þ Pi

f(v) ¼ wind speed distribution in the siteWeibull distribution law:

P ¼ PN�1i¼1

R viþ1v

�KC

vC

K�1

$e�

vC

K�

�Piþ1 � Piviþ1 � vi

ðv� viÞ þ Pi

#dv

B. Peñate et al. / Energy 36 (2011) 4372e43844384

systems still requires the contribution of demand control systemsfor good tuning between variable renewable energy power gener-ation and the power demand. So that, the GC is a major recom-mendation for the design and optimization of the SWROdesalination powered by wind energy.

In general, the designs proposed require of an itemised study ofthe annual localwater needs and thewindavailabilitywith the aim toensure the optimized operation and stability in the functioningof thesystem.Theuseofwindenergy to supplya stand-aloneSWROsystem,adapted to medium or large water demands, obliges to take intoaccount the water storage as the key energy storage system. Thissolution warranties to meet the water demand in periods of lack ofwind resource or serious breakdowns of the system.

Energy generated: Mean powermultiply by the period.

Energy generatedby a wind farm (W)

Total energy produced by several windgenerators connected in an specific area:

ET ¼�Pi¼NAdif

i¼1PJ¼NGdif

j¼1 ½NGjEj4estji��½3f �

NAdif ¼ No. of different areas (i) envolvedin the wind farmNGdif ¼ No. of wind turbines (model j)Ej ¼ Energy produced by a windturbine j and period t

Acknowledgements

The authors wish to thank the Spanish PROFIT Programme fortheir financial assistance with DEREDES project e Desarrollo de ladesalación con energías renovables (FIT-310200-2006/2007-175)and the European Commission for their financial assistance withthe POWERSOL project e Mechanical power generation based onsolar heat engines e (FP6-INCO2004-MPC3-032344) within theInternational Cooperation Activities Programme.

Ej ¼ Pj$t3f ¼ wind turbine reliability4estji ¼ Correction factor of the windturbines j in each area i.

Appendix A. Theoretical fundaments used for dimensioningthe wind energy systems.

Table A1Summary of equations.

Probability densityfunction of the generalWeibull distribution

f ðv;K;CÞ ¼�KC

��v

C

�K�1

$e

h�ðv

CÞKi

K ¼ shape parameterC ¼ scale parameter, m/sn ¼ wind speed, m/s

Vertical wind variation(Mikhail & Justus) vh ¼ va

�HhHa

�a

a ¼ vertical exponent.

:a ¼ 1

ln�Hg

H0

�� 0:0881

1� 0:0881ln Ha

10

ln�va6

�

Hg ¼ geometric mean elevation betweentwo vertical elevations (a, h);Hg¼(Ha$Hh)1/2, mH0 ¼ Rough length, mva ¼ mean wind speed (m/s) at elevation Ha

vh ¼ mean wind speed (m/s) at elevation Hh.

Wind energy (W)Pi ¼ 1

2rV2

i AVi ¼ 12rAV3

i

Vi ¼ wind speed at i time (m/s)A¼ area, m2

r ¼ air density (kg/m3)Wind power density (W/m2):

PiA

¼ 12rV3

i ðW=m2Þ

Wind power at different intervals:

Et1�t2 ¼ R t2t1

12rV3

i dt

V3 ¼ following Weibull law

< V3 >¼ RN0 v3f ðvÞdv ¼ c3G

�1þ 3

K

�

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