Marine Current Energy Converters to Power a Reverse Osmosis Desalination Plant

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Article Marine Current Energy Converters to Power a Reverse Osmosis Desalination Plant

Jennifer Leijon * , Johan Forslund , Karin Thomas and Cecilia Boström Division of Electricity, Uppsala University, 751 05 Uppsala, Sweden; [email protected] (J.F.); [email protected] (K.T.); [email protected] (C.B.) * Correspondence: [email protected]; Tel.: +46-18-471-5800

Received: 5 October 2018; Accepted: 22 October 2018; Published: 24 October 2018

Abstract: Some countries are facing issues on freshwater and electricity production, which can be addressed with the use of renewable energy powered desalination systems. In the following study, a reverse osmosis desalination plant powered by marine current energy converters is suggested. The marine current energy converters are designed at Uppsala University in Sweden, speciﬁcally for utilizing low water speeds (1–2 m/s). Estimations on freshwater production for such a system, in South Africa, facing the Indian Ocean was presented and discussed. It is concluded that the desalination plant cannot by itself supply freshwater for a population all the time, due to periods of too low water speeds (<1 m/s), but for 75% of the time. By using ten marine current energy converters, each with a nominal power rating of 7.5 kW, combined with a reverse osmosis desalination plant and water storage capacity of 2800 m3, it is possible to cover the basic freshwater demand of 5000 people. More studies on the hydrokinetic resource of the Western Indian Ocean, system cost, technology development, environmental and social aspects are necessary for more accurate results.

Keywords: marine current- tidal energy converters; hydro-kinetic energy; desalination; freshwater production; renewable energy; western Indian Ocean

1. Introduction In recent years, there has been an evident need to include more renewable energy sources (RES) in the electricity production globally. Different parts of the world, e.g., Sub-Saharan Africa, face combined problems with lack of electricity, freshwater and food (water-energy-food nexus [1]), and there are incitements to solve these problems simultaneously, in a collaborative and holistic way. Desalination systems are today used to remove the salt from salty water (most commonly seawater [2]). The produced freshwater can be used for e.g., agriculture, industry or in a household. However, desalination consumes a lot of energy, which is sometimes supplied by fossil fuel based power [3]. The negative sustainability aspects of this has enhanced research and commercialization of desalination plants powered by RES [4]. So far, a lot of the focus has been drawn towards desalination powered by solar [5] or wind, but other different RES has been studied [6]. Considering these huge correlated issues regarding water, energy and food, and including the fact that 70% of our globe consists of water: the option to use ocean energy to power a desalination process of seawater is interesting [7–9]. The topic of this paper is utilization of marine current energy converters (hydro-kinetic energy) for powering desalination plants [10]. One of the drawbacks with RES for electricity production is the intermittency or unpredictability of the source. However, tides (generated by forces between the moon, sun and earth) are very predictable, and not too dependent on local weather conditions, and many ocean- or river currents are continuously ﬂowing, i.e., have a relatively low variability in comparison to for example solar and wind. The development of tidal energy converters (TECs) and marine current energy converters

Energies 2018, 11, 2880; doi:10.3390/en11112880 www.mdpi.com/journal/energies Energies 2018, 11, x FOR PEER REVIEW 2 of 13

Energies 2018, 11, 2880 2 of 13 example solar and wind. The development of tidal energy converters (TECs) and marine current energy converters (MCCs) are still at an early stage, but the opportunities of utilizing tides or marine (MCCs)currents arehave still gained at an interest. early stage, For example, but the opportunities one recent review of utilizing paper discusses tides or marinethe economic currents aspects have ofgained TECs interest. [11] and For another example, presents one recent the reviewchalleng paperes with discusses increased the economictidal energy aspects using of a TECs PESTEL [11] (political,and another economic, presents thesocial, challenges technological, with increased environmental tidal energy and using legal) a PESTELanalysis (political, [12]. Other economic, more generalsocial, technological, reviews on the environmental marine current and or legal) tidal analysis energy converters [12]. Other are more given general in e.g., reviews [13,14]. on the marine currentThe or aim tidal of energy this paper converters is to are initiate given inthe e.g., work [13 ,14with]. marine current powered desalination, investigatingThe aim production of this paper of isfreshwater to initiate and/or the work electricity with marinelocally, currentusing marine powered current desalination, energy converters.investigating Specifically, production a brief of freshwater case study and/or for the electricitywestern Indian locally, Ocean using (WIO) marine is presented current energy for a locationconverters. outside Specifically, eastern South a brief Africa, case study focusing for on the the western marine Indian current Ocean energy (WIO) technology is presented developed for a atlocation Uppsala outside University eastern in South Sweden, Africa, in combination focusing on thewith marine a reverse current osmosis energy (RO) technology desalination developed plant. Thisat Uppsala type of University off-grid system, in Sweden, including in combination MCCs connected with a to reverse an RO osmosis desalination (RO) plant, desalination may also plant. be Thisapplicable type ofin off-gridthe event system, of natural including disasters, MCCs where connected there is toan anurgent RO desalinationneed to set up plant, the basic may societal also be structuresapplicable with in the electricity event of naturalfor medi disasters,cal devices, where freshwater there is supply an urgent etc. need to set up the basic societal structures with electricity for medical devices, freshwater supply etc. 1.1. Uppsala Marine Current Energy Converter 1.1. Uppsala Marine Current Energy Converter The MCC at Uppsala University produces power from the motion of streaming water (i.e., from a hydro-kineticThe MCC at energy Uppsala source) University using produces a vertical power axis fromfixed-pitch the motion turbine of streaming directly waterconnected (i.e., fromto a permanenta hydro-kinetic magnet energy generator, source) with using similari a verticalties to axis wind fixed-pitch energy converters. turbine directly The Nd connected2Fe14B magnet to a permanentbased synchronous magnet generator, generator with similaritieshas been todeveloped wind energy at converters. the Division The Nd 2ofFe 14ElectricityB magnet (http://www.teknik.uu.se/electricitybased synchronous generator has been/research-areas/marine-currents/ developed at the Division of Electricity(accessed (onhttp://www.teknik. 31 July 2018)) at Uppsalauu.se/electricity/research-areas/marine-currents/ University, and used in offshore experiments. (accessed A sketch on 31of Julythe device 2018)) is at shown Uppsala in University,Figure 1a. Theand usedmarine in offshorecurrent device experiments. has five A 3.5 sketch m long of the vertic deviceal blades is shown and in a Figureradius1 a.of The3 m, marine to give current a total 2 cross-sectionaldevice has five 3.5area m of long 21 m vertical2. The bladesMCC was and first a radius deployed of 3 m, at to the give experimental a total cross-sectional site in Söderfors area of 21in the m . Theriver MCC Dal was(Dalälven, first deployed depth: at7 them), experimentalSweden, in 2013 sitein [15] Söderfors and the in thesame river device Dal (Dalälven,is still used depth: for experiments7 m), Sweden, at inthe 2013 site. [ 15The] and device the is same installed device down is stillstream used of for a experimentshydropower atplant, the site.so the The water device flow is isinstalled regulated. downstream Figure 1b of shows a hydropower a photo of plant, the sodeployment the water flowfrom is[15]. regulated. The nominal Figure 1speedb shows and a power photo outputof the deployment of the generator from is [15 15]. rpm The nominaland 7.5 kW, speed respec andtively. power It output is beyond of the the generator scope of is this 15 rpmpaper and to study7.5 kW, the respectively. magnetic circuit It is beyond of the MCC, the scope but ofthe this rare paper-earth to magnets study the could magnetic be changed circuit to of i.e., the ferrite MCC, permanentbut the rare-earth magnets magnets in order could to beimprove changed the to i.e.,sustai ferritenability permanent of the magnetssystem, as in orderdiscussed to improve for wave the powersustainability in [16]. ofThe the added system, extra as weight discussed of using for wave ferrite power magnets in [ 16instead]. The of added Nd2Fe extra14B magnets weight is of not using an issueferrite for magnets the converter instead as of it Nd is placed2Fe14B on magnets the bottom is not of an the issue river for orthe sea, converter but may introduce as it is placed difficulties on the relatedbottom to of the deployment. river or sea, but may introduce difficulties related to the deployment.

(a) (b)

Figure 1. (a(a) )The The marine marine current current energy energy converter converter develo developedped at Uppsala at Uppsala University University in Sweden; in Sweden; (b) (Deploymentb) Deployment of the of thedevice device in Söderfors, in Söderfors, Sweden, Sweden, 7 March 7 March 2013 2013 (Figure (Figure 4 in 4 [15]). in [15 ]).

The large Uppsala MCC generator with many magnetic poles (here almost 2 m in diameter [17] and 112 poles [18]) generates power from lower water ﬂow velocities than most TECs and MCCs.

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In fact, in contrast to many other hydro-kinetic energy converters, this device is built to be efficient at water speeds lower than those regarded to low by most other marine current technologies, 2 m/s, with a rated water velocity of 1.4 m/s [18]. Vertical axis turbines (VATs) are not as common as horizontal axis TECs. However, the VAT enables power generation from streaming water from all incoming directions (omnidirectional). Moreover, the simplicity of a direct driven system is an advantage due to reduced need of maintenance related to the pitch of the blades, yawing system and gearbox that for wind power turbines consisted of up to 42% of the down time according to [19]. The Uppsala marine current energy converter can produce power in water speeds ranging from about 1 m/s up to almost 2 m/s. The power coefficient curve of the turbine has been experimentally derived in [20] to have a peak efficiency of 0.26 when run at optimal tip-speed-ratio (TSR), i.e., the ratio between water speed and speed of the blades. If the water speed is lower than 1.0 m/s, the turbine cannot produce a net positive hydrodynamic torque, and if the water speed is higher than 2 m/s, the loads on the struts and blades may be too high, for the specific design. At higher water speeds than 2 m/s, the turbine can be operated at a point where the power capture and rotational speed is lower, to ensure the safety of the device. To measure the flow of the water in the river and how it is affected by the marine current turbine, Acoustic Doppler Current Profilers (ADCPs) are used. The MCC system can be controlled in order to improve its average power output, although aspects of more complicated or costly electrical systems, increasing maintenance demands, can be considered [21]. The visibility of the marine current device is rather low, only leading to vorticities on the water surface. Marine current energy converters lead to an environmental impact, and environmental concerns for the specific location should be addressed before installation. Coastal communities often rely on fishery etc., but the marine current device proposed is put on the seabed, a couple of meters below the sea surface, allowing for ships to pass over the energy converters while generating electricity.

Power from a Vertical Axis Turbine The power available in streaming water, P [W], depends on the velocity of the streaming water, according to the equation: 1 P = Aρv3 (1) 2 where A [m2] is the projected cross section of the vertical axis turbine rotor, ρ [kg/m3] is the density of water and v [m/s] is the water velocity [m/s]. The hydrodynamic efficiency of the turbine is described by the power coefficient, CP, which is the relation between the power absorbed by the turbine and the power available in the undisturbed water flow, written as

P C = t (2) P P for the power output, Pt [W], from the turbine. The power absorbed by the turbine depends on the ratio of the speed of the tip of the blade to the water speed, the TSR, calculated as

ωR TSR = (3) v for a turbine angular velocity, ω [rad/s], and a radius of the turbine, R [m]. For the MCC used in Söderfors, Sweden, as stated earlier, the maximum power coefﬁcient CP is 0.26, generated at a tip speed ratio of 3.1.

1.2. Reverse Osmosis Desalination Seawater has typically total dissolved solids (TDS) of at least 35,000 mg/L. In contrast, good drinkable water is about 600 mg/L [22]. The daily basic human freshwater need for drinking and hygiene is at least 20 L per person [23]. The energy consumption of desalination varies with chosen Energies 2018, 11, 2880 4 of 13 desalination process, salinity and temperature of inlet water, requirements on product water etc. Today, membrane desalination techniques (such as RO) are more common than thermal desalination. RO is the most common desalination technology, with a total cumulative contracted capacity in the world of about 99.8 million m3/day [2]. Seawater reverse osmosis (SWRO) for production of drinkable water requires about 2.5–4 kWh/m3 [3,24]. Currently, the cost of SWRO desalinated water varies with different parameters, such as location [25], and desalination plants can be of different sizes and for different purposes. In [5], solar powered desalination for three different sizes of freshwater (and electricity) demands was investigated: small demand of less than 100 m3/day, intermediate demand of 100 to 25,000 m3/day, and high demand of 25,000 m3/day and 10 MW. However, very large desalination plants produce hundred thousand m3 freshwater each day (e.g., the contracted SWRO-plant Shoaiba 4 in Saudi Arabia, 400,000 m3/day) [2]. Considering reverse osmosis desalination, the intermittency of RES may decrease the lifetime of the RO membranes and have negative effects on the overall performance of the desalination system [26]. Safe storage and distribution of freshwater is an important question, but out of the scope of this paper. As several regions rely to a large extent on desalinated water or other water related systems, the event of accidents involving a shutdown of desalination plants or damage of parts of the water system, or possible natural catastrophes disrupting the water access etc., highlights the necessity of e.g., water storage [27,28]. Water storage systems can also be beneﬁcial for desalination systems during periods where the water production is lower, due to e.g., RES intermittency, or to balance up periods of higher water demands from the consumers.

1.3. Freshwater Demand and Marine Currents by the WIO Some of the countries facing the WIO have very limited access to safely managed drinking water and sanitation services. According to [29], the percentage of the population with access to at least a basic freshwater source in 2015 was 40% in Somalia, 58% in Kenya, 50% in Tanzania, 47% in Mozambique and 51% in Madagascar; in contrast, 85% of the South African population had access to a basic freshwater source. The freshwater situation for parts of Kenya, Comors Islands and Tanzania, facing the WIO, is reflected in [30], discussing e.g., that there may be a disparity of access to drinkable water within the same region and that many households in the studied areas used very little water with respect to the basic human needs. Few studies have discussed the potential of using marine renewable energy sources in the WIO, especially by the coastline of some of the poorest African countries, and there are not many published research articles on the hydro-kinetic energy resource there. The ocean waves in the WIO outside the Kenyan region Kilifi was analyzed in [31] for wave powered desalination purposes. The tidal- and marine current resource for parts of the WIO was briefly analyzed in [32], including investigations of parts of Kenya, Tanzania, Mozambique and Madagascar facing the WIO. It was concluded e.g., that tidal currents from 1.5 m/s to 4.5 m/s were determined at some meters depth for the different sites, and that marine currents occurred differently during two monsoon seasons, with a mean speed of 0.5 m/s to 1.2 m/s for the different regions, and a maximum water speed of 1.9 m/s [32]. In [33,34], the speed of the Agulhas Current, as it passes some coastal regions in eastern South Africa (Cape Morgan, north and south East London, Fish River and Port Edward), was determined using ADCPs, and the potential of power generation from the current was discussed.

2. Case Study: Western Indian Ocean The following case study discusses a marine current powered RO desalination plant in South Africa, facing the WIO. The study aims to provide rough a step-by-step method to estimate the capability of a certain site to supply a population with freshwater. The method will not include any cost considerations since the technology used for the study is not commercially launched yet. An investigation will be made of the resource, turbine placement, array conﬁguration and a proposed electrical system for desalination purposes. In the following, the data on current speed etc. presented Energies 2018, 11, 2880 5 of 13 in [33,34] will be used for a case study on marine current powered desalination by East London in

SouthEnergies Africa, 2018, 11 shown, x FOR PEER in Figure REVIEW2. 5 of 13 Energies 2018, 11, x FOR PEER REVIEW 5 of 13

FigureFigure 2. 2.East East London London in in South SouthAfrica, Africa, mapmap from G Googleoogle Maps Maps (www.google.com/maps (www.google.com/maps (accessed (accessed on on Figure 2. East London in South Africa, map from Google Maps (www.google.com/maps (accessed on 2121 August August 2018)). 2018)). 21 August 2018)). 2.1.2.1. Estimating Estimating Water Water Speed Speed and and Determining Determining TurbineTurbine Placement 2.1. Estimating Water Speed and Determining Turbine Placement TheThe ADCP ADCP data data from from [ 34[34],], acquired acquired duringduring one year year from from August August 2009 2009 in in the the north north part part of ofEast East The ADCP data from [34], acquired during one year from August 2009 in the north part of East LondonLondon in in South South Africa, Africa, gives gives averageaverage water speed va valueslues at at the the depths depths of of about about 3030 to to50 50m mat atthe the London in South Africa, gives average water speed values at the depths of about 30 to 50 m at the site;site; this this is shownis shown in Figurein Figure3. The 3. redThe andred yellowand yellow dashed dashed lines correspondlines correspond to the to lowest the lowest and highest and site; this is shown in Figure 3. The red and yellow dashed lines correspond to the lowest and highest limits of water speed the turbine can convert to electrical energy. If the water speed would limitshighest of water limits speed of water the turbinespeed the can turbine convert can to convert electrical to energy.electrical If energy. the water If the speed water would speed exceed would the exceed the highest limit for the turbine (not applicable for this site), a control system can be highestexceed limit the forhighest the turbine limit for (not the applicable turbine (not for thisapplic site),able a controlfor this systemsite), a cancontrol be implemented system can be in a implemented in a similar way that is currently done for wind power, to reduce the power captured similarimplemented way that in is a currently similar way done that for is windcurrently power, done to for reduce wind thepower, power to reduce captured the power by the captured turbine by by the turbine by changing the operating point of the turbine. changingby the turbine the operating by changing point the of theoperating turbine. point of the turbine.

Figure 3. One year of data (starting from August) on the marine current water speeds [m/s] for FigureFigure 3. One3. One year year of dataof data (starting (starting from from August) August) on theon marinethe marine current current water water speeds speeds [m/s] [m/s] for northfor north East London, South Africa, is shown, divided in weeks, presented previously in [34]. The north East London, South Africa, is shown, divided in weeks, presented previously in [34]. The Eastmaximum London, and South minimum Africa, values is shown, for the divided Uppsala in weeks,marine presentedcurrent conv previouslyerter are highlighted, in [34]. The showing maximum maximum and minimum values for the Uppsala marine current converter are highlighted, showing anda velocity minimum band values where for power the Uppsala can be produced. marine current converter are highlighted, showing a velocity banda velocity where powerband where can be power produced. can be produced. Furthermore, Reference [34] investigates how the vertical water speed profile varies with Furthermore, Reference [34] investigates how the vertical water speed profile varies with distance from shore and water depth, and their estimated results are shown in Table 1, with a base distance from shore and water depth, and their estimated results are shown in Table 1, with a base

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Furthermore, Reference [34] investigates how the vertical water speed profile varies with distance Energies 2018, 11, x FOR PEER REVIEW 6 of 13 from shore and water depth, and their estimated results are shown in Table1, with a base value of 1.0; no values are given where it is an assumed seabed. As can be seen in Table1, the water speed value of 1.0; no values are given where it is an assumed seabed. As can be seen in Table 1, the water decreases closer to the shore and also the deeper it gets; this can be used to determine the most suitable speed decreases closer to the shore and also the deeper it gets; this can be used to determine the place for the energy converter depending on its rating and desired output. Here, we assume that we most suitable place for the energy converter depending on its rating and desired output. Here, we can utilize the water speeds described in with the speed profile in Table1: 0–50 m deep and 51–100 m assume that we can utilize the water speeds described in with the speed profile in Table 1: 0–50 m from the coastline, and that this corresponds to the water speed profile in Figure3. However, to utilize deep and 51–100 m from the coastline, and that this corresponds to the water speed profile in the free flowing water at a water depth higher up from the bottom, the MCC may have to be put on a Figure 3. However, to utilize the free flowing water at a water depth higher up from the bottom, the structure or to include a tower between the generator and the turbine. One could also consider using MCC may have to be put on a structure or to include a tower between the generator and the turbine. one device with a taller turbine, instead of several smaller turbines, that would reach the desired water One could also consider using one device with a taller turbine, instead of several smaller turbines, depth. Since the focus of this study is the possibilities of supplying freshwater using the marine current that would reach the desired water depth. Since the focus of this study is the possibilities of resource, and not the costs relating to installation, an exact design choice of the converter turbine is supplying freshwater using the marine current resource, and not the costs relating to installation, an outside the scope of the paper. exact design choice of the converter turbine is outside the scope of the paper.

Table 1. The vertical water speed proﬁle, presented in [34]. Table 1. The vertical water speed profile, presented in [34]. 0–50 m from Coastline 51–100 m from Coastline 101–200 m from Coastline 0–50 m from Coastline 51–100 m from Coastline 101–200 m from Coastline 0–500–50 m m depth depth 0.90.9 2.0 2.0 3.2 3.2 50–10050–100 m m depth depth -- 1.0 1.0 1.6 1.6 100–200 m depth - - 0.8 100–200 m depth - - 0.8

To increase thethe totaltotal powerpower output,output, oneone cancan installinstall multiplemultiple marinemarine currentcurrent converters.converters. Each turbine willwill produceproduce aa wakewake that that interacts interacts with with the the other other turbines turbines and and their their wakes, wakes, meaning meaning that that if the if thedevices devices are placedare placed close close to each to other,each other, they will they operate will operate at different at different power capturepower capture points. Thepoints. incident The waterincident angle water will angle affect will the affect power the capture power forcapture each turbine.for each Numericalturbine. Numerical simulations simulations of the effect of the of effectspacing of (distancespacing (distance between turbinebetween centers) turbine betweencenters) turbinesbetween andturbines incident and water incident angle water are presentedangle are presentedin [35]. The in range[35]. The of incident range of angles incident present angles for present the speciﬁc for sitethe specific chosen shouldsite chosen be investigated, should be investigated,to determine whatto determine device spacing what device is appropriate. spacing is Figure appropriate.4 shows Figure the incident 4 shows angles the incident of the streaming angles of waterthe streaming presented water in [34 presented] for the South in [34] African for the site, South but African here plotted site, but in relation here plotted to the in starting relation position to the startingangle. For position most partangle. of theFor year,most thepart angles of the varyyear, between the angles− 40vary and between +40 degrees, −40 and and +40 for degrees, a few short and fordurations a few short during durations the year, during the angles the year, reach the− angles80 degrees. reach According−80 degrees. to According Figure2 in to [ 35Figure], the 2 average in [35], thepower average coefﬁcient power reduces coefficient maximum reduces 2% maximum with a spacing 2% with of 6 a turbine spacing diameters of 6 turbine and diameters an incident and angle an incidentof 80%. angle of 80%.

Figure 4. IncidentIncident anglesangles of of the the streaming streaming water water over over the the measured measured weeks, weeks, at north at north East London,East London, South Africa,South Africa, presented presented in [34]. in [34].

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2.2. Suggested System of Marine Current Driven Desalination In the following, it is assumed that a small coastal community or facility (e.g., school or hospital) is in need of freshwater. A SWRO plant, with the assumed energy demand of 4 kWh/ m3 [3,24], willEnergies be driven2018, 11, off-gridx FOR PEER by REVIEW an array of ten MCCs to produce drinkable water. In this rough estimation,7 of 13 the marine current resource measured for East London, South Africa, will be used, as presented inpresented [34]. The in structure [34]. The of structure the analysis of the made analysis in this made paper in isthis similar paper to is previoussimilar to analysis previous regarding analysis waveregarding powered wave desalination powered outsidedesalination Kenya, outside presented Kenya, in [31 presented,36]. The designin [31,36]. and parametersThe design of theand low-speedparameters generator of the low-speed at the marine generator current at power the ma grouprine atcurrent Uppsala power University group willat Uppsala be used inUniversity the case study.will be The used turbine in the is connectedcase study. to aThe permanent turbine magnetis connected synchronous to a permanent generator magnet (PMSG). synchronous The design ofgenerator the generator (PMSG). makes The itdesign efficient of atthe the generator low rotational makes speedsit efficient [18 ].at Thethe efficiencylow rotational of the speeds generator [18]. isThe in theefficiency experiments of the generator in [18] found is in tothe be experiments at least 80% in for [18] all found rotational to be speedsat least and80% givesfor all an rotational output line-to-linespeeds and voltage gives ofan 150 output VRMS atline-to-line nominal operation. voltage of Since 150 the V generatorRMS at nominal is a variable-speed operation. Since machine, the thegenerator electrical is frequencya variable-speed and the machine, output voltage the electric the generatoral frequency produces and the needs output to bevoltage converted the generator to fit the requirementproduces needs of the to desalinationbe converted plant. to fit the requirement of the desalination plant. TheThe proposedproposed topology for the the electrical electrical system system ca cann be be seen seen in inFigure Figure 5.5 The. The final final system system will willhowever however include include ten MCCs ten MCCs and andwater water storage storage system. system. The output The output of each of MCC, each MCC,3-phase 3-phase AC, is AC,connected is connected to one to joint one jointDC DCbus buslink link using using rectifiers. rectifiers. Then Then the the DC DC voltage voltage is is boosted forfor thethe transmissiontransmission cable,cable, andand finally,finally, an an inverter inverter is is included included before before the the RO RO plant. plant. It It is is assumed assumed thatthat thethe energyenergy converters converters are are located located with with a distancea distance of fiveof five turbine turbine diameters diameters from from each other,each other, as to notas to affect not eachaffect other each significantly, other significantly, about 1 about km out 1 km from out the from shore. the shore.

FigureFigure 5.5.Rough Rough overviewoverview ofof thethe proposedproposed MCC,MCC, withwith thethe exceptionexception thatthat thetheﬁnal finalproposed proposedsystem system includesincludes 10 10 MCCs MCCs and and water water storage, storage, powered powered desalination desalination system. system.

TheThe electricalelectrical outputoutput ofof thethe MCCMCC willwill bebe simplifiedsimplified toto dependdepend onon thethe powerpower inin thethe undisturbedundisturbed water,water, describeddescribed inin EquationEquation (1),(1), multipliedmultiplied byby the the number number of of MCCs, MCCs,N MCC,, andand constantsconstants thatthat representrepresent thethe efficiencyefficiency ofof thethe system,system, assumingassuming thatthat thethe turbineturbine operatesoperates atat optimaloptimal tip-speed-ratio.tip-speed-ratio. ExperimentalExperimental resultsresults ofof thethe MCCMCC coupledcoupled toto aa passivepassive rectifierrectifier areare presentedpresented inin [21[21],], concludingconcluding thethe efficiencyefficiency of of the the system system was was 19%. 19%. The boostThe boost converter conv anderter inverter and inverter are assumed are assumed to have ato total have efficiency a total ofefficiency 90%. The of losses 90%. inThe the losses transmission in the transmission line will depend line onwill the depend chosen on voltage the chosen leveland voltage the resistance level and ofthe the resistance cable. Power of the lossescable. ofPower about losses 3% are of abou expectedt 3% forare aexpected 1 km cable for a delivering 1 km cable 50 delivering kW at 400 50 V DCkW. Theat 400 difference VDC. The in difference incoming in water incoming direction water results directio in reducedn results power in reduced by about power 2% (asby about described 2% (as in Sectiondescribed 2.2 ).in The Section full simplified 2.2). The expressionfull simplified for the expression power [W] for delivered the power from [W] the chosendelivered MCC from to thethe ROchosen plant MCC is: to the RO plant is: 1 3 3 P = N ·0.19·0.9·0.97·0.98· Aρv ≈ N ·1706·v . (4) RO =MCC∙ 0.19 ∙ 0.9 ∙ 0.97 ∙ 0.98 ∙ MCC ∙ 1706 ∙ . (4) 2 There are several assumptions made to estimate how many people the MCC powered RO plant can sustain per week. The power reaching the RO plant is calculated using Equation (4) and multiplied with 168 h/week to receive the energy from the system in Wh/week. The operation of ten converter units is assumed. Also, it is assumed that the desalination plant needs 4 kWh/m3 and a basic daily personal freshwater need of 20 L/day, resulting in 0.14 m3/week and person. An expression for the estimated amount of people that will be supplied with freshwater each week [people/week] as a function of water speed and number of MCCs is:

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There are several assumptions made to estimate how many people the MCC powered RO plant can sustain per week. The power reaching the RO plant is calculated using Equation (4) and multiplied with 168 h/week to receive the energy from the system in Wh/week. The operation of ten converter units is assumed. Also, it is assumed that the desalination plant needs 4 kWh/m3 and a basic daily personal freshwater need of 20 L/day, resulting in 0.14 m3/week and person. An expression for the estimated amount of people that will be supplied with freshwater each week [people/week] as a function of water speed and number of MCCs is:

Energies 2018, 11, x FOR PEERN REVIEW·1706 ·v 3 ·168 8 of 13 MCC estimated = 511.8·N ·v 3 (5) 4000 ·0.14 MCC estimated ∙ 1706 ∙ ∙ 168 = 511.8 ∙ ∙ (5) To limit the effect of variable freshwater4000 ∙ 0.14 production that is a result of the varying water speeds, water storageTo limit tanks the areeffect considered. of variable Thefreshwater purpose production of the tanks that is a to result guarantee of the varying a certain water population speeds, size freshwaterwater storage availability tanks throughoutare considered. the The entire purpose year. of It the is assumed tanks is to that guarantee the water a certain storage population units fulﬁlls requirementssize freshwater to store availability drinkable throughout freshwater the for entire at least year. one It year.is assumed that the water storage units fulfills requirements to store drinkable freshwater for at least one year. 3. Results 3. Results Figure6 presents the total power delivered by the ten MCCs to the desalination plant and the people suppliedFigure 6 with presents freshwater the total (assuming power delivered a very by basic thedaily ten MCCs demand to the of desalination 20 L/person) plant for theand differentthe weekspeople at the supplied South African with freshwater site (north (assuming East London), a very withoutbasic daily a waterdemand storage of 20 system.L/person) The for RO theplant different weeks at the South African site (north East London), without a water storage system. The can produce freshwater 75% of the time, and it is shown that the power output ﬂuctuations range from RO plant can produce freshwater 75% of the time, and it is shown that the power output fluctuations 0 kW to a maximum of 82 kW, equivalent to nearly 25 thousand people. At the lowest operating point range from 0 kW to a maximum of 82 kW, equivalent to nearly 25 thousand people. At the lowest of theoperating MCC, 1 point m/s, of the the RO MCC, plant 1 m/s, can the deliver RO pl freshwaterant can deliver for freshwater 5000 people. for 5000 people.

FigureFigure 6. The 6. estimatedThe estimated power power output output of theof the MCC MCC and and the th numbere number of of people people that that each each week week that that could fulﬁllcould their fulfill basic their freshwater basic freshwater demand demand from a ROfrom plant a RO powered plant powered by ﬁve by MCCs, five MCCs, without without water water storage, in thestorage, WIO. in the WIO.

To increaseTo increase the the number number of of peoplepeople supplied supplied by bythe theplant, plant, water water storage storage capacity capacity is included. is included.The goal is to reach an intermediate level of supply, about 100 m3/day, to supply 5000 people each week The goal is to reach an intermediate level of supply, about 100 m3/day, to supply 5000 people each with very basic water demand. The priority of the system is to (1) supply a minimum amount of week with very basic water demand. The priority of the system is to (1) supply a minimum amount of people with water, and to (2) fill up the tank. If the water speed is equal to, or above, the lowest peoplespeed with limit, water, it will and supply to (2) ﬁlltheup freshwater the tank. needed If the waterand the speed rest is is stored equal in to, the or above,tank until the the lowest water speed limit,storage it will supplyis full. When the freshwater the storage needed is full, and all thethe rest freshwater is stored produced in the tank will until be supplied the water to storage the is full. Whenpopulation. the storage The water is full, storage all the will freshwater only be used produced to supply will the be population supplied towhen the population.the water speed The is water storagetoowill low. only The bewater used storage to supply is assumed the population to be empt wheny when the the water system speed is connected. is too low. Figure The water7 shows storage is assumedthat an tointermediate be empty level when of the supply, system 5000 is pe connected.ople/week, Figurecan be7 achieved shows thatusing an the intermediate proposed MCC level of supply,powered 5000 people/week, RO system including can be a achievedstorage capacity using the(one proposed or several MCCtanks) powered of 2800 m RO3. system including a storage capacityTo give a (one broader or several view of tanks) how big of a 2800 population m3. a certain water storage size can guarantee, the fraction of time a certain tank size can supply a population is plotted in Figure 8. The relationship between tank size and fraction of time the population is supplied is close to linear all the way until it reaches 100%. Naturally, the slope of the curve depends on the population size and the size of the tank, the bigger the tank is in relation, the more it affects the capability to supply freshwater.

Energies 2018, 11, 2880 9 of 13

Energies 2018, 11, x FOR PEER REVIEW 9 of 13

Energies 2018, 11, x FOR PEER REVIEW 9 of 13 Energies 2018, 11, x FOR PEER REVIEW 9 of 13

Figure 7. NumberFigure 7. Number of people of people supplied supplied with with freshwater freshwater ov overer one one year year using using a water a storage water tank storage of tank of 2800 m3. 2800 m3.

To give a broader view of how big a population a certain water storage size can guarantee, the fraction of time a certain tank size can supply a population is plotted in Figure8. The relationship between tank size and fraction of time the population is supplied is close to linear all the way until it reaches 100%. Naturally, the slope of the curve depends on the population size and the size of the tank, Figure 7. Number of people supplied with freshwater over one year using a water storage tank of the bigger the tank3 is in relation, the more it affects the capability to supply freshwater. Figure2800 m 7. . Number of people supplied with freshwater over one year using a water storage tank of 2800 m3.

Figure 8. The fraction of time, in percent, when 1000–10,000 people could be supplied with freshwater depending on water storage size in m3.

Since the population size the system can supply is directly dependent on the size of the water storage, it is interesting to look at the tank size versus population size it can supply, which is summarized in Figure 9. The biggest population size it can supply is 5430 people using at tank capacity of 3100 m3. At this point, all the available freshwater produced during the year is used. It can also be seen in Figure 6 that at the time of connection of the system, around 10,000 people can be Figuresupplied 8. TheFigure fractionwith 8. Thefreshwater. offraction time, of in time, percent, in percent, when 1000–10,000when 1000–10,000 people people could could be suppliedbe supplied with with freshwater 3 freshwater depending on water storage3 size in m . dependingFigure on water8. The storagefraction sizeof time, in m in. percent, when 1000–10,000 people could be supplied with freshwaterSince the depending population on size water the storage system size can in supply m3. is directly dependent on the size of the water Sincestorage, the population it is interesting size to the look system at the cantank supplysize versus is directlypopulation dependent size it can supply, on the which size ofis the water storage, itsummarized isSince interesting the populationin Figure to look9. size The atthebiggest thesystem tankpopulation can sizesupply size versus is itdirectly can population supply dependent is 5430 sizeon people the it size canusing of supply, theat tankwater which is summarizedstorage,capacity in it Figure ofis 3100interesting 9m.3. TheAt thisto biggest lookpoint, at all populationthe the tank available size sizeversusfreshwat it population caner produced supply size during is it 5430can the supply, peopleyear is used.which using It is at tank capacitysummarized ofcan 3100 alsom be3 .seenin At Figure in this Figure point,9. The6 that allbiggest at the the availabletimepopulation of connection freshwater size it of can the supply producedsystem, isaround 5430 during 10,000people the people using year canat is tankbe used. It can capacitysupplied of with 3100 freshwater. m3. At this point, all the available freshwater produced during the year is used. It also be seencan also in be Figure seen in6 thatFigure at 6 thethat at time the time of connection of connection of of thethe system, system, around around 10,000 10,000 people people can be can be suppliedsupplied with freshwater. with freshwater.

Figure 9. Population size the system can supply as a function of water storage size.

FigureFigure 9. Population 9. Population size size the the system system can can supply as as a function a function of water of water storage storage size. size.

Energies 2018, 11, 2880 10 of 13

4. Discussion As can be seen in Figure6, no power will be produced from the marine current energy system during some weeks yearly at this site, due to too low marine currents. It produces power 75% of the time. The analysis was based on data with variations over weeks, but for full understanding of how to implement such a system, the daily (hourly) variation of the marine current resource should be considered. That at least 5000 people could benefit each week from the proposed system is a very hopeful initial result, shown in Figure7, suggesting that it may be interesting to continue the research on marine current powered desalination. To include water storage tanks, one large or several smaller, as shown in Figures7 and8, is preferable to maintain a reliable freshwater source. The analyzed system is scalable and can be adapted for the specific site. As the RO desalination process relies on a steady power level, to not lower the performance of the RO membranes or change the freshwater production from a variation in applied pressure, and due to the same location of the salty water and the energy source, the use of almost constantly streaming water to power the desalination system may in this case be better suited for RO than other RES. Moreover, the need and cost of energy storage may decrease when using a less intermittent source, but may still be necessary. Figures6 and7 show variations in power and water production. The issue of short term electric power fluctuations could be addressed using energy storage, such as batteries, capacitors or flywheels. The lack of electricity production during longer periods of too low (or too high) water current speeds, can be complemented using other RES, such as wind, solar or wave power, in a hybrid system. For a more in-depth analysis, the weather at the site, highlighting periods of drought or rain, as well as other water sources available and their local importance could be analyzed and used to adapt the system in size. As noted in Section 1.3, the investigated site, East London in South Africa, is not the location with the most severe need of freshwater and electricity of the regions facing the WIO. But this location was chosen due to the available data and its proximity to coastal regions with a higher need of freshwater supply. Societies with a strong economy may be more likely to have an investigated coastline than societies with weak economy. If we are serious in the attempt of helping countries with unreliable electric grids or low availability of freshwater sources to develop water and renewable energy systems, more attention on the resource estimation of these locations are needed. There is a gap in the resource analysis of the marine currents (as well as tidal currents and ocean waves etc.) outside the coast of some of the poor countries facing the WIO, such as Somalia, Kenya and Mozambique, in need of freshwater and electricity. This complicates the estimation of opportunities to produce electricity or freshwater with the use of marine renewable energy sources here. Therefore, more research on the marine resources, especially presented in publications easily understood for the designers of energy conversion systems, will be of great interest. In the following, we have only considered the WIO, but there may also be e.g., rivers suitable for hydro-kinetic energy conversion. Some of the largest rivers in Sub-Saharan Africa are already used for hydropower. Possibly, marine current energy converters could be installed downstream in rivers already utilizing hydropower, as was done for the Uppsala University Söderfors project, taking advantage of the water discharge from the hydropower plant. Also, as the Uppsala marine current energy converter is designed and rated for relatively low water flow velocities, there are several regions, perhaps previously neglected in the discussion on marine current- or tidal energy conversion due to low water speeds, that may be suitable for marine current powered desalination. The development of tidal or marine current energy devices is at an early stage, and more research is needed before deployment and implementation of full systems, especially for remote regions and for desalination purposes. The cost of the discussed system and produced water, as well as installation and maintenance, is still not known. Also, there is a necessity of investigating the ecological aspects of the project before continuing. For example, the desalination process creates a salty solute, the concentrate, which has to be managed to not damage the local marine area. Other areas to investigate are the social aspects of the system, distribution of the water and its specific usage etc. In short, building up new Energies 2018, 11, 2880 11 of 13 collaborative projects between the water and energy sectors with a sustainability focus necessarily includes several aspects, possibly enhancing the complexity, but also the profits, from the final system.

5. Conclusions The following work included an initial rough description of marine current energy converter systems as drivers of desalination processes. A method of how to estimate and analyze the key components of a MCC powered RO plant and storage system was presented. The method was applied to a case study of a South African coastal region by the western Indian Ocean. It is evident that without a water storage tank, the plant cannot continuously supply a population with freshwater since the speed of the water currents will be too low, during several weeks of the year, for the MCC to extract electrical power. Without a tank, the plant consisting of ten MCCs can supply between 0 and 25 thousand people per week. To mitigate the intermittency, a water storage tank was considered. With a capacity of 2800 m3, the tank could improve the system to be able to provide basic freshwater continuously for 5000 people per week, an intermediate level of supply in this context. The impact of water storage size to supply a certain population size has been presented. With a water storage capacity of 2800 m3 connected to the system, at least 5000 people could provide basic freshwater. However, this study is not enough for preparing deployment and installation of such a system and there are many future research questions regarding e.g., system sustainability before producing freshwater (and electricity) using marine currents in remote coastal regions. The conclusion of this study is that more research on the local marine current resource by the WIO, as well as the energy conversion- and desalination system, and more collaboration over research areas would be beneﬁcial for enhancing RES powered water systems globally.

Author Contributions: All authors have collaboratively contributed to the manuscript. Most of the work regarding paper writing and calculation was conducted by J.L. and J.F. while C.B. and K.T. supervised the work. Funding: This project was financially supported by the Swedish Research Council, grant number 2015-03126, STandUP for Energy and J. Gust. Richert foundation. Acknowledgments: In this section you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments). Conflicts of Interest: The authors declare no conflict of interest.

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