Desalination 153 (2002) 9–16

0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved

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 TechnologyCenter, Sharm El Sheikh, Egypt, May 4–6, 2002.

*Corresponding author.

A wind-powered seawater reverse-osmosis system withoutbatteries

Marcos S. Miranda*, David InfieldCenter for Renewable Energy Systems Technology CREST, Loughborough University, LE11 3TU, UK

Tel. +44 (1509) 228144; Fax +44 (1509) 610031; email: M.S.Miranda@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 inisolated inland areas. In such places, electricity supplies are often expensive and unreliable, while the wind resourceis abundant. The system presented here comprises a 2.2 kW wind turbine generator powering a variable-flow Reverseosmosis (RO) desalination unit. It is highly efficient, rugged, built with off-the-shelf components and suitable foruse in remote areas. Operation at variable-flow allows the uncertainty and variability of the wind to be accommodatedwithout need of energy storage. Batteries, which are common in stand-alone systems, are avoided and water productionis dependent on the instantaneous wind speed. A model-based control strategy is used to independently maximizeboth the energy extracted from the wind and the water output of the RO unit. A computer model of the system hasbeen developed based on component models, identified through laboratory testing. Performance predictions arepresented and discussed.

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

1. Introduction

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

of fossil fuels. Increasing awareness of the depletionof current sources has led to a global effort in theresearch and development of renewable energytechnologies, such as wind, solar, tidal and geo-thermal energy.

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

In the context of the utilization of the moreestablished renewable energy sources: the sun(thermal and PV), and the wind, stand-alonedesalination systems have been widely discussed[1–5]. Since desalination is an especially pro-mising application that involves broad fields ofstudy, many different solutions have been proposed.Even if one focuses on one particular renewablesource and a specific desalination method, theremay still be many options available in terms ofthe final system configuration.

One of the critical limiting factors to the widerimplementation of renewable energy driven de-salination systems is the intermittence and unpre-dictability of the renewable source. Two distinctproblems have been identified in an earlier study[5]. The first is that most desalination technologiesare not suitable for operation at variable power,and the second is that lack of continuity of energysupply, or even limited power availability overvariable periods of time may cause the demandnot to be met. A common solution to these problemsis the use of energy storage, which can accumulateenergy surpluses (long-term storage) and/or smoothout shorter-term variations in the supply (short-term storage). On the negative side, the use of suchdevices results in increases to both capital andoperating costs.

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

Table 1Applicability of renewable energy sources to waterdesalination techniques

Wind Solar Tidal Geothermal

RO ME RO MSF

VC MSF VC ME

FS ED FS VC

ED RO ED RO

SD ED

2. Wind power systems

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

The second category is the electricity generationwithin stand-alone systems. In contrast to gridconnected systems, these are built to be used insites where maintenance may be sporadic andtechnical assistance unavailable, thus greaterrobustness is required in their design, incurringhigher capital costs than otherwise. Grid connectedturbines are used as an additional supply, com-plementing the conventional base load powersystem. In stand-alone systems, the wind can oftenbe the sole source of energy and this should befully taken into consideration during the designstage.

Due to the inherently random characteristic ofthe wind, certain key aspects must be attended toin the design process of stand-alone systems.Besides the wind resource potential, the natureof the electricity load needs to be given carefulconsideration, in particular whether disconnectionis acceptable, and if so, for how long.

This consideration relates to the presence andsizing of any storage system that may be includedin the system. Energy storage plays an essentialrole in determining system performance, as well

Source Hanafi [1].

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

as the aforementioned influence on capital andmaintenance costs.

For the design of a small stand-alone system,a key challenge is to find a good compromisebetween reliability and system complexity thatmeets the economical constraints. This is not asimple matter, and it will mostly depend on thetype of load and the local resources.

3. Reverse osmosis systems

Reverse osmosis (RO) is now a well-establishedtechnology for the desalination of water and inparticular seawater. Nevertheless, the use of ROin small stand-alone systems (in the range of a fewm3/d) is still an area of developing technology.Islands and isolated inland areas, where theelectricity supply may be a problem, form an idealapplication for such stand-alone systems. In theseremote places, electricity is often supplied by aweak grid or even generated locally, by means ofdiesel generators.

Despite the many advances in RO membranetechnology over recent years, such as higherrejection rates and improved tolerance to pressurefluctuations, considerable attention still needs tobe given to operational conditions. It is believed thathigher efficiencies can be obtained when operatingat certain flow and pressure conditions.

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

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

membrane mechanical resistance;• 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 consequentmembrane fouling;

Fig. 1. RO membrane operational window.

• Maximum product concentration — if theapplied pressure is less than a determined value,permeate concentration will be too high.

4. Wind-RO configuration possibilities

A classification of the different wind poweredreverse osmosis systems found in the literature hasbeen made.

This was based on some of the points previouslydiscussed: the existence of an alternative electricalsupply (weak grid or diesel generator); the matchingof the available wind energy to the load; and theoperational characteristic of RO membranes.

4.1. Systems with back up (diesel/grid)

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

The main benefit of these systems, as in anyhybrid wind-diesel configuration is the achieve-ment of fuel savings, which may increase thegenerator availability and reduce overall energycosts.

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

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unavailability of the RO system since it cannotbe powered by the wind turbine alone.

4.2. Systems without back up

Systems without an external energy source canbe divided into two categories, with emphasis onthe RO unit operation: systems which run underapproximately constant operating conditions; andthose that experience variable operational con-ditions.

4.2.1. Near constant operating conditions

This first type of operation can be implementedby three different means: on/off switching of theRO units; usage of storage devices; and de-ratingthe wind turbine. In all three cases, an attempt ismade to supply the individual RO modules withapproximately constant power.

4.2.1.1. Storage devices

In this strategy, storage devices are employedto accumulate energy surplus during periods whenthe power generated by the wind turbine is greaterthan the load demand from the desalination unit.This surplus would then be used later when thegenerated power is insufficient to meet the loaddemand.

One common way of storing the surplus energyis by using batteries [3]. In this case, the relationbetween operational pressure, storage sizing andaverage wind speed should be considered in thedesign stage. In addition, capital and maintenancecosts should be carefully assessed. A disadvantageof this approach to the system design is the ratingof the energy storage system, since this can makeit economically unattractive at higher power levelsdue to the sizing of the battery bank.

4.2.1.2. RO unit switching

This strategy is based on the use of a higherpower wind turbine connected multiple smallerRO units. The power control is achieved byswitching the units on and off so as to match the

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

Although, frequent cycling of RO units is notusually recommended, this problem can be over-come by implementing different types of con-figuration. Rahal [4] uses a higher power windturbine operating at near constant speed (generationmanagement) connected to many equally smallerRO units switching on/off (load management). Tosmooth out the fluctuations, short-term energystorage (a flywheel in this instance) is used. Varyingthe pitch angle of the wind turbine blades controlsthe generated power.

Another possibility [5], suitable for smallersystems (with medium/low power turbines of lessthan 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 implementedto absorb any power surplus, keeping the systemvoltage and frequency constant.

4.2.1.3. Wind turbine de-rating

This approach consists of making use of theflat end of a pitch controlled wind turbine powercurve to operate the RO unit at approximatelyconstant power [6]. An implication of this con-figuration is that, since the turbine rated power isonly achieved at high wind speeds, it would haveto be de-rated by changing the settings of the pitchingmechanism. This will cause the generated powerto be flattened at lower wind speeds and conse-quently to have lower values. Therefore, the originalrating of the turbine rotor should be considerablyhigher than the RO unit rated power making thesystem more expensive.

4.2.2. Variable operating conditions

In contrast to systems that operate under constantconditions, another operational strategy is basedon the establishment and imposition of certain

M.S. Miranda, D. Infield / Desalination 153 (2002) 9–16 13

operational limits [6], as previously illustrated inFig. 1. This means that, based on the input powerto the RO unit (flow times pressure), a controlstrategy is determined which imposes a fixedoperating point on the system that lies within theallowed region (i.e. the operational window ofthe RO unit shown in Fig. 1).

By doing this, an attempt is made to operatethe system autonomously over a wider powerrange, without the need to use a back up unit orstorage devices. The overall effect is to reducecapital and operating costs. One aspect that shouldbe emphasized is that very little is known aboutthe consequences of variable operation of ROmembranes. It is recognized that mechanical fatiguecan occur and that lifetime of the RO elementsmay be shortened and performance impaired.

5. Proposed wind-RO system

The proposed system comprises a small (2.2 kW)wind turbine directly supplying a reverse osmosisdesalination unit. The pumping system is composedof two variable speed drives (inverter/inductionmotor) driving a medium and a high-pressure pump,each. The speed of each pump is individuallycontrolled to maximize both wind power captureand product water flow. A Clark pump is used torecover the energy from the brine-stream, makingthe system highly efficient over a wide operatingrange.

A detailed description of the system and itscomponents is presented by CREST, Lough-borough University [6].

5.1. Control strategy

The use of two positive displacement pumpsindirectly enables the control of feed pressure andflow independently. This characteristic makes itpossible to operate the system at any point withinthe operational window shown in Fig. 1, provideda suitable control strategy is employed. This iscritical in maximizing the efficiency of a battery-

less system operating from a variable energy sourcesuch as the wind.

Extensive laboratory testing was carried outso as to develop suitable mathematical models forindividual components over a wide operating range.Based on these individual models, a completesystem model was built to understand and assesssystem behavior as well as predict its perform-ance.

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

6. Performance predictions

The following performance predictions aretaken from a detailed Matlab-Simulink model ofthe entire system. This model is based on extensivein-house component characterization, supplementedby manufacture’s data. The salinity of the seawaterfeed is taken to reflect that of the Red Sea at40,000 ppm, which is isosmotic with 34,300 ppmof pure NaCl. Additionally, the feed temperatureis constant and equal to 25°C.

6.1. Long-term (steady-state) performance

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

The importance of this characteristic relies onthe fact that it can be used in a system siting study.Similarly to a wind turbine power curve, it wouldgive the expected output from the system, giventhe resource in a certain location. A statisticalanalysis can be conducted by applying the windspeed probability (Weibull distribution) for thislocation against the characteristic of Fig. 2. The

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

result is the water production probability expectedfor this location.

Fig. 3 shows the system specific energy as afunction of the wind speed. It presents an overallvalue below 3.5 kWh/m3, being slightly higher atlower wind speeds. Nevertheless, it is almostconstant throughout the operating range. Thistranslates in the fact that water production wateris directly related to the available power.

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

Fig. 3. System specific energy consumption.

6.2. Short-term (dynamic) performance

A dynamic model of the system was developedto verify its performance during normal transientoperation, under turbulent wind. This is parti-cularly important in the determination of the controlstrategy and fine-tuning of the controller para-meters.

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

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

Finally, Fig. 4d presents the system instanta-neous specific energy. This figure is obtained bydividing the instantaneous wind turbine outputpower (in kW) by the product water flow (m3/h).It should be pointed out that desalinated waterhas a higher energetic content than seawater andtherefore represents an energy storage mediumitself.

Such low figures for the specific energy em-phasize the importance of assessing individualcomponent efficiency, in an attempt to minimizesystem overall losses.

The average value of about 3.4 kWh/m3 forthe system specific energy seems remarkably lowwhen compared with the literature standard of 10–15 kWh/m3 achieved in standard (no energy reco-very) systems. Additionally, it should be mentioned

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that energy recovery — using turbines, reverserunning pumps or other devices — is a relativelycommon practice in bigger systems, with ratedoutputs of at least 100 times higher. This is alsodue to the improved efficiencies associated to theincreased sizing of components. Nevertheless,such efficiencies would not be easily achieved insmaller components such as the ones needed toimplement a system of the same size of the oneproposed in this work.

7. Conclusions

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

Fig. 4. Wind speed, product flow, product concentrationand specific energy.

A high-efficiency configuration was proposed,having a Clark pump as the concentrate streamhydraulic energy recovery device. One of the mainobjectives of this analysis was to verify the inte-gration of the several components — which wereindividually tested and modeled — and their per-formance over a wide operational range. Moreimportant still, was to demonstrate the proposedconfiguration as a promising alternative for thedesalination of seawater.

Finally, a more representative validation of thesystem 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, particularlyconsidering the transients present in a system sup-plied by an extremely variable and unpredictablesource, such as the wind.

Acknowledgement

This work was carried out in conjunction withDulas Ltd, Machynlleth, UK and was supportedby ETSU, DTI, UK and CNPq, Department ofScience and Technology, Brazil.

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