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Desalination 249 (2009) 1280–1287
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Desalination
j ourna l homepage: www.e lsev ie r.com/ locate /desa l
Investigation of different operational strategies for the variable operation of a simplereverse osmosis unit
Robert Pohl c,⁎, Martin Kaltschmitt a, Robert Holländer b
a Institute of Environmental Technology and Energy Economics, Hamburg University of Technology (TUHH), Hamburg, Germanyb Environmental Technology and Environmental Management, University of Leipzig, Germanyc Leipziger Institut für Energie GmbH, Torgauer Str. 116, D-04347 Leipzig, Germany
⁎ Corresponding author. Synlift Systems GmbH, GusBerlin, Germany. Tel.: +49 30 467 999 20; fax: +49 30
E-mail addresses: [email protected], R.Poh
0011-9164/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.desal.2009.06.029
a b s t r a c t
a r t i c l e i n f oArticle history:Accepted 28 June 2009Available online 7 October 2009
Keywords:DesalinationReverse osmosisRenewable energyWind powerLoad rangeSpecific energy consumption
Recent studies and projects showed that a combination of a reverse osmosis desalination plant with a windpower supply is technologically feasible if the reverse osmosis plant is operated with fluctuating and inter-mittent loads and thus follow the energy supply characteristic of the wind turbine. On this background thegoal of this paper is to simulate the system behaviour of a simple reverse osmosis plant under changingprocess parameters (e.g. feed pressure, recovery or feed flow). These variations are systematized within so-called operational strategies. Therefore, four different operational strategies are analysed in detail withregard to given restrictions e.g. by the membrane system. For each of these strategies the specific energyconsumption over the total usable load range is computed with the simulated hydraulic characteristics ofeach operational strategy. The analysis of the gathered data shows that a membrane system should beoperated with constant permeate recovery under fluctuating wind power. This operational strategy provideslow specific energy consumption over a broad load range.
tav-Meyer-Allee 25/12, 13355467 999 [email protected] (R. Pohl).
ll rights reserved.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Background
Due to the already high prices for fossil fuel energy and the risk ofstill increasing prices, the possibilities of using renewable sources ofenergy attract more andmore attention. This is also true within appli-cations like in seawater desalination. Besides other possible combina-tions of renewable energy sources and desalination processes, thehighly-developed wind energy utilisation promises to be economicfeasible for seawater desalination based on reverse osmosis systems,since the wind power technology is proven and coastal sites oftenprovide sufficient wind speeds [1]. Therefore water production costsof around 1.40 €/m3 are estimated for good wind sites [2]. Othersources calculated an even lower cost range (e.g. [3,4]).
Currently, wind energy can be used competitively within hybridsystems for desalination together with diesel generators in remoteareas when the fuel costs are higher than 0.50€/l. Grid connectedwind powered desalination seems to be economically feasible at siteswith high wind speeds and high electricity prices [5]. In general, com-pared to exclusive fossil based systems such hybrid systems are be-coming better with increasing wind speeds and/or an increasing shareof wind energy within the overall system [6].
Besides others (e.g. storage facilities for drinking water and/orenergy), the installation of an oversized reverse osmosis system,divided into several independent RO-units (e.g. introduced in [7]) incombination with their variable operation is one possibility forincreasing wind energy penetration. The better the power demandof the reverse osmosis plant could be matched with the unstable andfluctuating wind power the less the necessity to balance the powerflow by expensive energy storage or a back-up system. Besides a lowspecific energy consumption, a broad range for the variable powerconsumption of a single RO-unit is crucial for the practicability of thevariable operation. The broader the total usable load range, the less theoverall RO-capacity needs to be modularised. Thus, the specific instal-lation costs and the frequency of critical operating conditions (e.g.activation/deactivation procedures) for a single RO-unit decrease.
The power consumption of a simple seawater reverse osmosis(SWRO) system, containing only one membrane unit, a pump and anenergy recovery device (ERD) is determined by the pressures andflows at the entrance or outlets of the membrane unit, Fig. 1.
Thus two hydraulic parameters have to be fixed to determinethe power consumption of such a SWRO-system (e.g. feed pressure pfand feed flow qf or permeate flow qp and recovery y). For the hydrau-lic parameters of the SWRO-system specific operational limits resultfrommembrane manufacturers specifications (e.g. maximum qf, maxi-mum pf or minimum concentrate flow qc) and the required permeateconcentration cp. These limits define an “operational window” withineach combination of hydraulic parameters is valid, [8–10]. In orderto systematise the variation of the hydraulic parameters within the
Fig. 1. Schematic representation of a simple seawater reverse osmosis system.
1281R. Pohl et al. / Desalination 249 (2009) 1280–1287
operational window and subsequently the variation of the powerconsumption, the variation of hydraulic parameters could follow a so-called “operational strategy”. Here one process parameter is keptconstant over the entire operational window (e.g. the recovery y),while the remaining parameters could vary.
pf feed water pressure, barqf feed water flow, m3 h−1, m3 d−1
cf feed concentration, mg/l TDSqp permeate water flow, m3 h−1, m3 d−1
pp permeate pressure = p0, barcp permeate concentration, mg/l TDSqc concentrate water flow, m3 h−1, m3 d−1
y permeate recovery, % (qp/qf)T Temperature, °CJ w̄ average system permeate flux, l m−2h−1
qp, i permeate water flow per element, m3 h−1
yi permeate recovery per element, %p0 ambient pressure, barP power consumption, kWSEC specific energy consumption, kWh m−3
ηP efficiency of the high-pressure pump, %ηERD efficiency of the energy recovery, %
In other known investigations dealing with variable operation ofthe SWRO-system, for example the operational strategy is related to aspecial hydraulic system. According to [11,12], a constant recovery yis achieved by using special energy recovery devices. Adding a secondhigh-pressure feed pump, allows an operation with variable recovery.This provides the possibility for optimising the variable operation forlower specific energy consumption [13,14]. In [4] the operational strat-egy is based on maximum water production using a work exchangerfor energy recovery. An electrical coupling of the recovered energyprovides the possibility for adjusting the power consumption in a flex-ible way, e.g. with a linear increase of the recovery with increasingavailable power [15]. Other researchers followed the idea to apply a“membrane friendly operation” with a constant pressure pf [16] ordid not specify the operational strategy in detail [17,18]. In [19] twodifferent operational strategies are mentioned but are not furtherinvestigated. None of the investigations describe, which operationalstrategies are applicable and which strategy is the most suitable one.
1.2. Objective
The objective of this study is to identify such operational strategiesthat allow the best possible use of the fluctuating wind energy supply.Determinant criteria that the strategies should meet are low specificenergy consumption, a broad range of the variable power consump-tion, and small absolute values of the feed pressure alterations. Theinvestigation is carried out by simulating a reverse osmosis system atcertain operating points chosen according to the different operational
strategies. The followed procedure is described in detail and com-prehensively. The investigation of the operational strategies withrespect to the behaviour of the process parameters within the possi-ble load range can help to choose appropriate hydraulic componentsand to design the hydraulic circuit. Thus, it provides a basis for thedevelopment of the control scheme for a SWRO-system that operateswith a fluctuating energy supply, e.g. wind power.
To simplify matters, only the hydraulic parameters are variedwithin a pre-defined simple reverse osmosis system. To highlight theeffect of these hydraulic parameters on the power consumption,pumps and energy recovery devices are considered to operate withconstant efficiencies. And to simplify the overall system for a trans-parent simulation, the power requirements for pre-treatment andauxiliary equipment are neglected.
1.3. Methodology
The different operational strategies are investigated in three steps.
− First, the boundary conditions determining the performance andefficiency of the assumed reverse osmosis system are defined re-garding the specifications of themembranemanufacturer: The feedproperties and the required permeate quality as well as the char-acteristics of the membrane unit. The valid operational window ofthe hydraulic parameters is defined by simulation. For the com-parison of all operational strategies to be simulated, a common setpoint of operation is defined.
− Then, the variations of the process parameters within the opera-tional window are systematized by defining four operational strat-egies. The operation of the reverse osmosis system is simulated forindividual operating points, resulting from each operational strat-egy. Analysing the resulting process parameters against the powerconsumption provides an in-depth insight into the behaviour ofthe process parameters over the range of variable load.
− Finally, the specific energy consumption (SEC) is computed foreach operational strategy and every operating point. The processparameters as well as the specific energy consumption in relationto the load range are analysed. These results are discussed to iden-tify the strategy that best meets the requirements of the variableoperation.
The simulations are carried out with the known reverse osmosisdesign software DOW FilmTec — ROSA 6.1.5 for each individualoperating point.
This software fulfils the accuracy requirements for the purpose ofthe simulations to be carried out. A detailed comparison of the per-meate flow qp and its concentration cp estimated with ROSA, com-pared with measurements [8,13] has shown that only the simulatedpermeate flow is overestimated: Especially in regions with low netdriving pressure, the simulated permeate flow is around 150% of themeasured permeate flow. This overestimation leads to a higher pro-ductivity and a lower permeate concentration, if it is assumed that thesalt passage is modelled realistic.
Nevertheless the software is used here, since the main objective isthe comparison of several simulated cases against each other. Theshortcoming of the software mentioned above is considered duringthe interpretation of the results.
2. Simulation and comparative analysis
2.1. Design and operational window
2.1.1. Design of the SWRO-SystemFollowing the presented methodology, realistic assumptions
for the membrane system and the boundary conditions affecting itsoperation are made, see Table 1. The reverse osmosis system containsa single pass with one stage. Depending on the feed water and the
Table 1Feed water conditions and design of the SWRO-system.
Feed watercf 35,646 mg/l Concentration for standard seawater [20]SDI 3 Silt density index achieved by pre-treatment
using micro or ultra filtration [20]T 25 °C Typical temperature of seawater in regions
where desalination is used
SWRO-systemStructure One pass single stage with four membrane
elements in seriesElements SW30-HR400i, DOW FilmTec™Status Aged system, represented by a fouling factor
of FF=0.85ηP 70 % Total efficiency of the high-pressure pumpηERD 85 % Total efficiency of the energy recovery from
the concentrate flow
1282 R. Pohl et al. / Desalination 249 (2009) 1280–1287
required permeate quality this design is sufficient in most cases.Neither recycle flow nor blending is assumed.
The RO-system contains one pressure vessel with four modulesSW30-HRLE400i in series [21]. These membrane elements areequipped with a new connector technology providing smaller hy-draulic losses and reduced risk for leakages [22]. Applying additionalpressure vessels in parallel will increase the capacity and load linearly.But this does not affect the power consumption and useable load rangein relation to the overall productivity compared to a single pressurevessel.
The hydraulic system of the SWRO-system is simplified to thehigh-pressure feed pump and an energy recovery device (ERD)represented by their efficiencies ηP and ηERD, as shown in Fig. 1.The efficiency for the ERD of ηERD=85% assumed here, representsa compromise between the typically higher values for displace-ment type ERD and the lower efficiencies of hydrodynamic ERD.For a better comparison of the simulation results, the dependenceof these efficiencies on different throughputs is not considered (i.e.the efficiencies are assumed to be constant). Additionally, it isassumed that the throughput of the high pressure pump and theERD could be adjusted independently from each other. This could beachieved by an electrical re-injection of the recovered energy. In [15]a plant design is proposed where the energy recovery turbine turnsan asynchronous generator connected via a rectifier to the DC-busof the variable frequency drive of the high-pressure pump. Control-ling the plant by throttle valves should be avoided. By throttlingthe concentrate flow its inherent pressure energy dissipates andcannot be used to lower the energy consumption of the plant. Thus,SWRO-plants with throttle valves will show higher specific energyconsumptions – especially at low recoveries – than plants with ERD.Therefore, adjusting flow and pressure by throttling is not assumed inthis study.
Table 2Operating points containing the operational window of the RO-system according to the nu
1 2 3 4 5
pf [bar] 30.88 50.07 62.47 63.08 64qf [m3/h] 15.89 15.89 15.89 13.9 11.75y [%] 3.92 18.71 26.77 30.00 34.35qc [m3/h] 15.27 12.91 11.64 9.73 7.71cp [mg/l] 500 126 98 103 111J ̄w [l/(m2h)] 4.19 20.00 28.61 28.05 27.16qp, i [m3/h] 0.21 0.87 1.25 1.25 1.25yi [%] 1 6 8 9 11
2.1.2. Operational windowTo analyse the variable operating conditions the formation of an
operational window based on the imposition of certain operationlimits is a generally accepted approach [8–10,23]. For the investigatedreverse osmosis system the operational window has been identifiedby simulation based on the constrains presented in [20] and given bythe ROSA-software. The calculated limiting operating points are listedin Table 2 and displayed in Fig. 2. The connection of the consecutivepoints represents the valid operational window for the designedSWRO-system graphically.
In Fig. 2 it is shown that between points 1 and 3, the limit is definedby the maximum allowed feed flow of qf=15.89 m3/h for one mem-brane element. A higher feed flow would cause too high mechanicalloads. From points 3 to 6 the operational limit is defined by themaximum allowed permeate flow per element of qp,i=1.25 m3/h andthe limit from points 6 to 8 is defined by the maximum recovery permembrane element of yi=15%. These limits are reached in the first ofthe four elements of thepressure vessel. Thus, themaximumpressure atpoint 6 results at pf=66.5 bar against the maximum allowed pressureof 83 bar for the SW30-HR400i elements. In practice, this differencecould provide a buffer for a required pressure increase caused by colderwater, fouling or membrane compaction. From points 8 to 10 theminimum recommended concentrate flow qc defines the limit. Thisconcentrate flow carries all rejected substances out of the module. Thehigher the fouling potential and the brine concentration, the higher theconcentrate flow should be kept in order to avoid fouling.
The operational limit between points 10 and 1 is given by thepermeate concentration cp. In this study this limit is chosen to cp=500 mg/l TDS according to the standard of the World HealthOrganisation for drinking water. This limit is strongly site dependent,since it is very sensitive to changes in temperature, feed concentrationand solvent rejection of the membrane. The reported overestimation ofthe permeate flow computed with FilmTec's ROSA-program in regionswith low pressure, leads to the conclusion that the concentration limitwill be rather reached at higher pressures than given for points 10, 11,12 and 1 for real systems.
2.1.3. Set pointWithin the operational window each pair of feed pressure and feed
flow is a possible operational point. For a fair and easy comparison ofdifferent operational strategies it is necessary to define one commonset point for all investigated strategies. This set point defines themaximal power consumption for all operational strategies under twoadditional restrictions:
− First, the recovery is chosen toy=30%. This reduces the risk of scalingand fouling and allows the operation of the system with minimaladdition of chemicals like scaling inhibitors or cleaning agents.
− Second, this recovery is achieved by the four elements with anmaximum average system permeate flux of J̄W=20 l/(m2h).
The set point is indicated in Fig. 2 with the crossing of the line of arecovery y=30% and of theflux J̄W=20 l/(m2h) through the operational
mbering in Fig. 2. Bold faced data indicate the limit of the process parameter.
6 7 8 9 10 11 12
66.5 60 50.05 41.42 33.95 31.16 30.848.32 7.01 4.87 4.22 3.67 7.56 9.91
43.93 42.43 39.11 30.00 19.32 8.52 6.414.66 4.04 2.96 2.95 2.96 6.94 9.27
137 163 239 318 500 500 50024.59 20.00 12.82 8.52 4.77 4.35 4.281.25 1.05 0.73 0.48 0.26 0.21 0.2
15 15 15 11 7 3 2
Fig. 2. Operational window of the considered SWRO-system.
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window. This set point is chosen as an arbitrary value and should not bemixedwith the nominal values for reverse osmosis systems operating atconstant load. All set point parameters are summarised in Table 3. Fromthis set point, the relevant power related process parameters for eachoperational strategy are varied until a limit is reached.
2.2. Operational strategies
In order to systematise the variation of the hydraulic parameterswithin the operational window and subsequently the variation of thepower consumption, four different operational strategies are defined.Therefore, one parameter effecting the power consumption remainsconstant, while the others can vary over the entire possible range.
− A constant feed pressure is applied and only the feed flow andthus the recovery vary.
− A constant permeate recovery is applied by changing feedpressure and feed flow in a way that keeps the same ratio of permeateflow against feed flow for each operating point.
− A constant feed flow is applied. The feed pressure and thus therecovery vary.
− A constant concentrate flow is achieved by changing the feedflow and the feed pressure.
Considering the operational window, the limiting operating pointof each strategy is determined by altering the variable processparameter starting from the common set point until the related limitis reached.Within the resulting range of the varied parameters, four orfive operating points are chosen. Fig. 3 illustrates the operating pointsdefined by feed pressure pf and flow qf with the resulting permeaterecovery for each operational strategywithin the operational window.
For all operating points the feed and concentrate pressure (pf, pc),the feed and concentrate flow (qf, qc), the permeate flow and con-
Table 3Parameters of the common set point for all operational strategies.
Feed pressure pf 53.66 barFeed flow qf 9.91 m3/hPermeate recovery y 30.0 %Average system permeate flux J ̄w 20.0 l/(m2h)Permeate flow qp 2.97 m3/hPermeate concentration cp 141 mg/lPower consumption P 12.5 kWSpecific energy consumption SEC 4.2 kWh/m3
centration (qp, cp) and the recovery (y) are simulated. With spread-sheet software the net power consumption P is computed by Eq. (1)for all recorded operating points.
P =pf ·qfηP
−pc·qc·ηERD: ð1Þ
2.2.1. Constant feed pressureThis operational strategy follows the idea to change the power
consumption only by varying the feed flow. Conventional SWRO-plants operate with almost constant pressure. Since it is not yetconcluding investigated if and how a variable feed pressure affects thelong term performance of a RO-membrane, this operational strategy isassumed as “membrane friendly” [16].
Starting from the set point only the feed flow input is reducedcausing an increase of the overall recovery up to y=40% until thelimiting recovery of yi=15% is reached for the first membraneelement. The development of the feed pressure and flow, the per-meate concentration and flow, the concentrate flow and the permeaterecovery over the net power consumption are illustrated in Fig. 4.
Due to the constant feed pressure pf, the net driving pressure(the difference of the applied pressure and the counteracting osmoticfeed pressure) for the water permeation is only affected by the feedconcentration over the membrane. Reducing the feed flow qf andincreasing the recovery towards smaller power consumption, increasesthe concentration polarisation. The resulting increase of the feedconcentration and thus its osmotic pressure at the membrane reducesthe net driving pressure slightly. Therefore the permeate flow cp is onlyreduced by around 33%. In parallel thepermeate concentration increasesonly from 141 mg/l at the set point to 200 mg/l at the lower power limit.
2.2.2. Constant permeate recoveryStarting from the set point, pf and qf are decreased in a way to keep
the recovery at y=30% over thewhole range. The lower limit is definedby the minimum concentrate flow of qc=2.95 m3/h (70.8 m3/d), seeFig. 5.
The feed pressure pf needs to be varied by around 13 bar. Due to thereduction of pf the net driving pressure for the permeation is reducedand thus the permeate concentration increases up to 318 mg/l at thelower power limit. The permeate flow qp is reduced by 57% against theset point. This operational strategy can be realised with special energyrecovery devices where the constant recovery could be achieved with
Fig. 3. Operating points with permeate recovery for each operational strategy.
1284 R. Pohl et al. / Desalination 249 (2009) 1280–1287
the fixed ratio for feed and concentrate flow of the energy recoverydevice (displacement type) [11,12].
2.2.3. Constant feed flowStarting from the set point, here the feed flow qf remains constant
while only the pf is decreased down to around 31 bar, Fig. 6. This limit isgivenby the increasingcp, reaching the500mg/l limit due to the reductionof the net driving pressure. In parallel the qp is reduced by around 78% to
Fig. 4. Variation of the operational parameters with constant feed pressure.
15.4 m3/d.With the decrease of pf the recovery is reduced to y=8%. Sincethe qf is constant, qc increases to 92% of qf with decreasing power.
2.2.4. Constant concentrate flowWith this operational strategy, feed pressure and flow are varied
in a way that the concentrate flow qc remains constant, see Fig. 7. Dueto the significant reduction of the net driving pressure, pf drops toaround 31 bar when cp would exceed 500 mg/l. The permeate flow qp
Fig. 5. Variation of the operational parameters with constant permeate recovery.
Fig. 6. Variation of the operational parameters with constant feed flow.
Fig. 7. Variation of the operational parameters with constant concentrate flow.
1285R. Pohl et al. / Desalination 249 (2009) 1280–1287
is reduced by 78%, too. Similar to the relations of the flows in theconstant feed flow strategy, here the qf is reduced until qc is equal toaround 92% of qf.
This operational strategy is favourable for hydraulic systems usinga work exchanger, pressure exchanger or PX-system as ERD, where avariable concentrate flow could cause a less efficient or unstableoperation of the ERD.
2.3. Comparison of operational strategies
With the known hydraulic parameters and the power consump-tion, the specific energy consumption (SEC) as criteria for the energyefficiency of the RO-unit is computed by Eq. (2).
SEC =Pqp
: ð2Þ
For the first review, in Table 4, the process parameters at the lowerload limit and the range of the SEC are listed for all operationalstrategies. Fig. 8 shows the curves for the SEC over the entire loadrange for each operational strategy.
In the upper load range between 7.9 kW and 12.5 kW, theoperational strategy with constant feed pressure provides the lowestSEC of all strategies. A further advantage is the small increase of cp toaround 200 mg/l. Its narrow range of power consumption (down toonly 63% of the power consumption of set point) and the increasedrisk for scaling due to the rising recovery are disadvantageous.
The constant recovery strategy provides the broadest load rangeand the lowest SEC below a power of P=7.9 kW. Due to the constantrecovery, the amount of feed that needs to be pressurised remains thesame in relation to the produced permeate and therefore decreaseswith decreasing power. The increase of the permeate concentration ishigher compared to the constant pressure strategy, but the limit of500 mg/l is not reached.
A load range comparable to the strategy of constant recoveryprovides the operation with constant concentrate flow. Due to thedecrease of the recovery down to 8%, the energy losses the ERD increasein relation to the total energy input. This leads to a higher SEC fordecreasing power. This strategy shows an optimal SEC at around 84% ofset point power. Nevertheless, the SEC is higher compared to thestrategies with constant pressure and constant recovery, where the SECdrops continuously with decreasing power consumption. Anothersignificant disadvantage is the sharp increase of the permeateconcentration for lower power consumption. First, considering theperformance of the used simulation software ROSA, cp would besignificantly higher in real systems at low power consumption. Second,a higher feed concentration, a lower salt rejection or an increasedwatertemperature would shift the lower limit to higher loads. Both aspectswould narrow the useable load range in real systems.
For the constant feed flow strategy the relations are similar.Additional to the increasing losses of the energy recovery caused by
Table 4Lower limits of process parameters, the power and the specific energy consumption foreach simulated operational strategy.
pf qp y cp qf qc P SEC(bar) (m3/d) (%) (mg/l) (m3/d) (m3/d) (kW) (kW/m3)
Set-point: 53.7 71.3 30.0 141 238 167 12.5 4.22Const. feedpressure
53.7 55.2 40.4 202 137 82 7.9 4.22–3.43
Const.recovery
41.4 30.5 30.0 318 101 71 4.1 4.22–3.20
Const. feedflow
30.8 15.4 6.4 500 238 223 5.7 4.22–8.98
Const.concentrateflow
31.2 15.6 8.5 500 181 167 4.5 4.22–6.84
Fig. 8. Specific energy consumption of the operational strategies over the resulting load range.
1286 R. Pohl et al. / Desalination 249 (2009) 1280–1287
decreasing water recovery here, the losses of the feed pump causes ahigher SEC compared to the strategy of constant concentrate flow.Again, for this operational strategy cp limits the load range with thesame consequences described for the constant concentrate flow.
The analyses display clearly that a broad load range can beobtained only with variable feed pressure. The applied pressure playsa more important role in energy consumption than the flow. Since it isnot yet clearly investigated which negative effects are related with avariable pressure, possible pressure alterations should be minimized.The three strategies operating with variable pressure show differentpressure alterations over their load range. The strategy of constantrecovery would need a pressure decrease by 25% only, compared tothe strategies with constant feed and concentrate flow. Both wouldrequire a pressure decline by around 57%.
3. Conclusions
The aim of this studywas to simulate different operational strategiesfor a simple SWRO-system. The resulting operational window of theassumed SWRO-unit gives an ideaup towhich limits the variation of thedifferent operational parameters is possible. The alterations of theprocess parameters causing a variation of the power consumption aresystematised by the definition of four operational strategies. Theseoperational strategies are investigated with regard to the alteration ofthe process parameters and the specific energy consumption over theuseable load range.
− The simulations have shown that the operational strategy ofconstant feed pressure exhibits the smallest load range.
− The constant feed pressure strategy and constant recovery strategyprovide low SEC over their specific load range.
− The constant recovery and constant concentrate flow strategiesprovide a broad load range.
− The operational strategywith constant feedflowdisplays a high SEC.It provides no advantage in comparison to the other operationalstrategies.
− Concerning energy consumption, load range, pressure alterationand permeate quality, the operational strategy of constantrecovery seems to be the best compromise to operate a SWRO-system with highly fluctuating wind energy. The broad load rangeand the low SEC allow the operation of the SWRO-system evenunder low wind speeds, alternatively to a shutdown of the plant.
Focusing on the pure membrane system the results show generally,which operational strategy is the most suitable under certain con-
siderations, e. g. broadest range of power consumption, lowest specificenergy consumption, and low feed pressure alterations. The obtainedresults can serve as abasis for designinga variable operatedSWRO-plantin a concrete application. For further investigations regarding theconcrete application, the constant efficiencies for pumps and energyrecovery devices will be replaced by typical performance curves. Theeffect on power consumption of the SWRO-plant caused by differentpre-treatment options should be carried out by a sensitivity analysis.
The obtained results should be verified by practical experience orspecial experiments. The potential effects of an unsteady and highlyfluctuating feed pressure on the membrane performance anddeterioration are investigated in detail in a R&D-project at the Institutfür Energetik und Umwelt in Leipzig.
Acknowledgement
This paper presents results of research carried out by a publicsponsored R&D-project. The authors would like to thank the projectexecution organisation EuroNorm GmbH which supports the projectunder registration number InnoWatt-IW051260 financially on behalfof the Federal Ministry of Economics and Technology, Germany.Special thanks go to SYNLIFT Systems GmbH, Berlin for the seriousdiscussion of the investigated problem and achieved results.
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