applied sciences

Review A Review of the Water Desalination Technologies

Domenico Curto * , Vincenzo Franzitta and Andrea Guercio

Department of Engineering, University of Palermo, 90128 Palermo, Italy; [email protected] (V.F.); [email protected] (A.G.) * Correspondence: [email protected]

Abstract: Desalination is commonly adopted nowadays to overcome the freshwater scarcity in some areas of the world if brackish water or salt water is available. Different kinds of technologies have been proposed in the last century. In this paper, the state of the mainstream solutions is reported, showing the current commercial technologies like reverse osmosis (RO), Multi-Stages Flash desalination (MSF) and Multi-Effect Distillation (MED), and the new frontiers of the research with the aim of exploiting renewable sources such as wind, solar and biomass energy. In these cases, seawater treatment plants are the same as traditional ones, with the only difference being that they use a renewable energy source. Thus, classifications are firstly introduced, considering the working principles, the main energy input required for the treatment, and the potential for coupling with renewable energy sources. Each technology is described in detail, showing how the process works and reporting some data on the state of development. Finally, a statistical analysis is given concerning the spread of the various technologies across the world and which of them are most exploited. In this section, an important energy and exergy analysis is also addressed to quantify energy losses.

Keywords: desalination; state of art; RO; MED; MSF; new development; renewable energy integration



1. Introduction
 To overcome the ever increasing freshwater demand due to population growth and Citation: Curto, D.; Franzitta, V.; welfare concerns, the first desalination plants were installed in the late 1950s [1–3]. Guercio, A. A Review of the Water The first technologies were thermally driven due to the low cost of fossil fuels (less Desalination Technologies. Appl. Sci. than $3 for one barrel of oil). As the cost of energy has progressively increased, two lines of 2021, 11, 670. https://doi.org/ research have been pursed in order to minimize the total cost of water treatment [1,4]: 10.3390/app11020670 • Increase the energy efficiency of commercial technologies • Investigation and proposal of new solutions Received: 10 December 2020 Accepted: 6 January 2021 For example, in the first case the number of stages in Multi-Stage Flash (MSF) distilla- Published: 12 January 2021 tion has progressively increased from 8–12 to 20 [5,6]. As regards the proposal of innovative solutions, the introduction of semipermeable membranes represents a radical change in the Publisher’s Note: MDPI stays neu- desalination sector. In fact, nowadays Reverse Osmosis (RO) is the most used technology tral with regard to jurisdictional clai- for this purpose [7]. ms in published maps and institutio- The term “desalination” refers to the technological process used to extract freshwater nal affiliations. from brackish or saltwater. Seawater is often the raw water source used to supply this process. Historically, the idea behind the desalination process was introduced by the Royal Navy (the United Kingdom’s naval warfare force) at the end of the 18th century with the Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. purpose of increasing navigational autonomy without storing more water on the ships [8]. This article is an open access article Since in that period ships were equipped with steam engines, the first desalination distributed under the terms and con- technology was single flash distillation, which was improved in the following years into ditions of the Creative Commons At- the more efficient MSF. tribution (CC BY) license (https:// The first type of desalination unit was realized by the G. and J. Weir in 1885 in Glasgow creativecommons.org/licenses/by/ (Scotland) [9]. This company, which later became known as Weir Westgarth, practically 4.0/). had a monopoly as a desalination unit builder until World War II.

Appl. Sci. 2021, 11, 670. https://doi.org/10.3390/app11020670 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 670 2 of 36

In the following years, desalination plants had been installed around the world for civil purposes. In 1907, a Dutch company installed the first desalination plant in the Arabian Gulf countries in the city of Jeddah [10]. By the order of King Abdulaziz Al Saud, the same plant was replaced in 1928, with two units produced by Weir Westgarth being installed (with a total installed capacity of 135 m3/day) [10]. In 1953 other desalination plants were installed in Qatar and Kuwait. In detail, 5 units were installed in Qatar, with a total capacity equal to 682 m3/day, and 10 units in Kuwait, with a total capacity of 4545.5 m3/day. In 1955 another 10 units were installed in Shuwaikh (Kuwait), which were the same size [10]. From this moment on, desalination plants expanded around the world, with the birth of many companies such as Krupp in Germany, Westinghouse in the USA, and SIR (Società Italiana Resine) in Italy [8]. The other mainstream technique is RO, based on semipermeable membranes. Histori- cally, the phenomenon of osmosis was observed for the first time in 1748 by Jean-Antoine Nollet, without any application for about two centuries [11]. In the USA the first studies were started by the researchers Sidney Loeb and Srinivasa Sourirajan in 1956 at the University of California and the University of Florida, respectively. The first membrane was realized in 1959 whereas the first pilot plant was installed in 1965, with a capacity of 19 m3/day [8]. An improvement of this technique was obtained by the development of asymmetric membranes that show a different porosity moving from a face to the other one, allowing a greater water flux through them [11]. The slow diffusion of RO was initially due to the high electricity consumption re- quired to produce freshwater in comparison with other techniques, and the limited life of semipermeable membranes [8]. The first applications were related to brackish water, due to its lower osmotic pressure in comparison with seawater. The first desalination plant, based on RO, for a municipality was realized in 1977 in the USA, with an installed capacity of 11,350 m3/day. In the same area, in 1985 another big desalination plant was realized, having an installed capacity equal to 56,800 m3/day [11]. Great technological progress occurred in the process of reverse osmosis thanks to the increase in membrane lifetimes and the adoption of energy recovery devices to reduce the energy requirements for the process. So, nowadays RO is applied to seawater and is economically competitive with the other technologies. This improvement was achieved during the 1990s thanks to the introduction of an energy recovery system based on the introduction of hydro turbines or similar systems before returning the brackish water to the sea [8]. Currently, RO is the most widespread technology for desalination, followed by MSF and Multi-Effect Distillation (MED). According to the statistics, in the last year the total installed capacity was based essentially on three technologies: RO (68.7%), MSF (17.6%), MED (6.9%). The other technologies had a marginal role (6.8%) [12]. Desalination plants are installed around the world but are mainly concentrated in Middle East and North Africa (47.5% of global capacity). The main raw water source is represented by seawater (70.5% of global capacity) [12]. Several technologies are currently under investigation, with the goal being to reduce the energy demand for freshwater production.

Authors’ Contribution As described above, the desalination sector is quite variegated and subject to continu- ous research and development. Different technologies have been developed during more than a century. Since new solutions are currently under development, the authors want to outline a detailed review on the state of the art of desalination, showing the working principles and reporting some information on the diffusion of each technology. The review also includes the new frontiers of research, like for example, technologies based on water freezing or directly run by solar radiation. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 36

review also includes the new frontiers of research, like for example, technologies based Appl. Sci. 2021, 11, 670 on water freezing or directly run by solar radiation. 3 of 36 Since energy saving and the adoption of renewable energy sources are mainstream topics, all reported technologies are described in detail to clarify the main energy input that is required to run the process. In this way, each desalination technology can be easily coupledSince with energy renewable saving energy and the sources. adoption of renewable energy sources are mainstream topics,The all paper reported is structured technologies in the are following described way: in Section detail to 2 clarifyreports the the main state energyof the art input of desalinatingthat is required technologies, to run the classifying process. In thisthem way, in the each first desalination part and consequently technology can reporting be easily thecoupled detailed with description renewable of energy each solution sources. in the following paragraphs. Section 3 reports The paper is structured in the following way: Section2 reports the state of the art of some technical statistics, among which are the specific energy demand, the installed desalinating technologies, classifying them in the first part and consequently reporting global capacity and the development status. Finally, the conclusions are reported in Sec- the detailed description of each solution in the following paragraphs. Section3 reports tion 4. some technical statistics, among which are the specific energy demand, the installed global capacity and the development status. Finally, the conclusions are reported in Section4. 2. The State of the Art 2. TheToday, State desalination of the Art can be realized using several technologies. In general, a desali- nationToday, plant includes desalination different can be processes realized to using obtain several freshwater, technologies. among In which general, the adesalina- desalina- tiontion unit plant is the includes most differentenergy expensive processes comp to obtainonent. freshwater, A desalination among plant which normally the desalination includes [6]:unit is the most energy expensive component. A desalination plant normally includes [6]: • • Intake,Intake, composed composed by by pumps pumps and and pipes pipes to to take take water water from from the the source source (sea (sea or or brackish brackish water)water) • • Pre-treatment,Pre-treatment, consisting consisting of of the the filtration filtration of of raw raw water water to to remove remove solid solid components components andand the the addition addition of ofchemical chemical substances substances to reduce to reduce the salt’s the salt’s precipitation precipitation and the and cor- the rosioncorrosion inside inside the desalination the desalination unit unit •• Desalination,Desalination, where where freshwater freshwater is is extracted extracted from from saltwater saltwater •• Post-treatment,Post-treatment, to to correct correct pH pH by by adding adding selected selected salts salts to to meet meet the the requirements requirements of of the the finalfinal uses. uses. AsAs introduced introduced before, before, the the desalination process representsrepresents thethe most most energy energy consuming consum- ingwater water treatment. treatment. For For this this reason, reason, this this topic topic is quite is quite widely widely assessed assessed in literature.in literature. BeforeBefore analyzing analyzing the the specific specific solutions, solutions, a a classification classification is is required. required. Alkaisi Alkaisi suggested suggested threethree main main categories categories [13]: [13]: Evaporation Evaporation and and Condensation, Condensation, Filtration Filtration and and Crystallization. Crystallization. TheThe following following Figure Figure 11 showsshows an an upgrade upgrade of of the the classification classification proposed proposed by by Alkaisi, Alkaisi, integratingintegrating the the new new technologies technologies currently currently under under investigation. investigation.

FigureFigure 1. 1. TheThe classification classification of of desalination desalination technologies technologies by by working working principle. principle.

Evaporation and Condensation technologies are the first desalination techniques to be historically introduced and used for civil freshwater production. The idea is to supply thermal energy to seawater, producing a vapor, and then condensate it. This energy can be generated by using the heat from a thermal process (for example, waste heat or Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 36

Evaporation and Condensation technologies are the first desalination techniques to Appl. Sci. 2021, 11, 670 be historically introduced and used for civil freshwater production. The idea is to supply4 of 36 thermal energy to seawater, producing a vapor, and then condensate it. This energy can be generated by using the heat from a thermal process (for example, waste heat or fuel combustion),fuel combustion), or through or through a mechanical a mechanical process. process. In the In first the case, first case,the most the mostcommon common tech- nologiestechnologies are areMED, MED, MSF, MSF, Thermal Thermal Vapor Vapor Compression Compression (TVC) (TVC) and and Membrane Membrane Distillation (MD). Currently other approaches are under investigation,investigation, and among these we can findfind few newnew solutionssolutions suppliedsupplied by by solar solar radiation: radiation: Solar Solar Still Still Distillation Distillation (SSD), (SSD), Solar Solar Chimney Chim- (SC)ney (SC) and and Humidification-Dehumidification Humidification-Dehumidification (HDH) (HDH) desalination. desalination. Regarding the mechanicalmechanical processesprocesses usedused to to produce produce freshwater freshwater through through the the evapora- evapo- rationtion and and condensation condensation of of seawater, seawater, the the main ma techniquein technique isMechanical is Mechanical Vapor Vapor Compression Compres- (MVC)sion (MVC) [14,15 [14,15].]. In case ofof filtrationfiltration technologies, technologies, all all solutions solutions are are essentially essentially based based on on a semipermeable a semiperme- ablemembrane, membrane, i.e., ai.e., layer a layer that that shows shows a different a different mode mode of crossing of crossing behavior behavior according according to the to thesizes sizes or nature or nature of molecules. of molecules. The only The exceptiononly exception is Ion-exchange is Ion-exchange resins (IXR),resins where (IXR), natural where naturalor artificial or artificial materials materials are used are to used capture to captur the dissolvede the dissolved ions in ions a chemical in a chemical way [16 way]. [16]. In this context RO isis thethe mostmost usedused technologytechnology forfor desalination.desalination. TheThe ElectrodialysisElectrodialysis (ED) and and Ion Ion Exchange Exchange Resin Resin (IXR) (IXR) are are used used to toproduce produce water water with with a very a very limited limited concen- con- trationcentration of salts. of salts. Other Other techniques, techniques, as Forward as Forward Osmosis Osmosis (FO), (FO), Nano Nano Filtration Filtration (NF) (NF) and andCa- pacitiveCapacitive Deionization Deionization (CDI) (CDI) are are in the in the development development stage stage [17]. [17 ]. Finally, the CrystallizationCrystallization category comprises techniques that extract freshwaterfreshwater producing iceice asas intermediateintermediate product. product. As As example, example, the the main main techniques techniques are are Secondary Secondary Re- Refrigerantfrigerant Freezing Freezing (SRF), (SRF), Hydration Hydration (HY) (HY) and and Vacuum Vacuum Freezing Freezing (VF) desalination.(VF) desalination. All these All theseapproaches approaches are under are under investigation investigation [18]. A [18] timeline. A timeline showing showing the evolution the evolution of desalination of desal- inationis shown is belowshown in below Figure in2 .Figure 2.

Figure 2. The timeline of desalination technologies.

Another usefuluseful classification classification can can be be realized realized by consideringby considering the kind the ofkind energy of mainlyenergy mainlyrequired required to run theto run process, the process, as shown as shown in Figure in3 Figure. This 3. aspect This isaspect important is important for select for selectrenewable renewable energy energy sources sources to supply to supply the desalination the desalination process process [19]. In detail,[19]. In four detail, kinds four of kindsenergy of are energy considered are considered here: here: • Thermal energy • Mechanical energy • Electrical energy • Chemical energy The first first category could be supplied by sola solarr thermal or geothermal energy sources. It comprises comprises the the following following technologies: technologies: MSF, MSF, MED, MED, TVC, TVC, MD, MD, SC, SC,HDH HDH and andSSD. SSD. In par- In particular,ticular, the thelast last three three approaches approaches are aredesign designeded to directly to directly exploit exploit solar solar radiation radiation [20]. [20 ]. The group of technologies requiring a Mechanical Energy as input comprises MVC, RO, NF, SRF, HY. All these techniques are characterized by the presence of pumps and RO, NF, SRF, HY. All these techniques are characterized by the presence of pumps and compressors, which require a major part of the total energy demand for the process. compressors, which require a major part of the total energy demand for the process. The last two categories have limited examples. Electrodialysis and Capacitive Deion- The last two categories have limited examples. Electrodialysis and Capacitive Deion- ization desalination require the generation of an electric field between two electrodes, ization desalination require the generation of an electric field between two electrodes, sep- separated by an anion membrane and a cation membrane (selective membranes that allow arated by an anion membrane and a cation membrane (selective membranes that allow positive and negative ions to cross, respectively). In this case, electricity is the only way to positive and negative ions to cross, respectively). In this case, electricity is the only way to supply the process. supply the process. As regards the Ions Exchange Resin, the working principle is the chemical replacement of positive and negative ions. In the case of FO, a solute replacement occurs to extract water from a saline solution. It is important to underline that mechanical energy and electricity can be easily converted in both directions, with high efficiency. For this reason, the technologies that require a mechanical energy input, through pumps or compressors, can be easily supplied Appl. Sci. 2021, 11, 670 5 of 36

by electricity by using common electrical motors. Similarly, mechanical energy can be Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 36 converted by alternators into electricity to supply the desalination processes that require electrical energy as input.

FigureFigure 3.3. TheThe classificationclassification ofof desalinationdesalination technologiestechnologies byby mainmain energyenergy input.input.

ThermalAs regards energy the Ions is a differentExchange case, Resin, because the working it can be principle easily obtained is the chemical from electricity, replace- throughment of positive the Joule and Effect negative or heat ions. pumps. In the The case conversion of FO, a solute from thermalreplacement energy occurs into mechani-to extract calwater or electricalfrom a saline energy solution. is obtained by using thermal machines or plants, affected by a low energyIt is efficiency important in comparisonto underline with that themechanical previous energy cases, for and thermodynamic electricity can andbe easily technical con- reasons.verted in both directions, with high efficiency. For this reason, the technologies that re- quireIt a ismechanical important energy to underline input, through that thermal pumps sources or compressors, can be successfully can be easily adopted supplied to produceby electricity electricity, by using in specificcommon conditions. electrical motors. For example, Similarly, in the mechanical case of a energy high temperature can be con- geothermalverted by alternators source, a powerinto electricity plant can to besupply realized the [desalination21]. processes that require elec- tricalThus, energy in as order input. to supply the desalination process with renewable energy sources, it is convenientThermal toenergy distinguish is a different the energy case, sources because that it can can be be easily used obtained to produce from electricity electricity, (or mechanicalthrough theenergy) Joule Effect from or those heat producingpumps. The thermal conversion energy. from thermal energy into mechan- With this goal, the renewable energy sources can be sorted in the following categories, ical or electrical energy is obtained by using thermal machines or plants, affected by a low according to the usual energy output that can be produced: energy efficiency in comparison with the previous cases, for thermodynamic and technical •reasons.Electricity producers, such as wind, hydro, tidal and wave. • ThermalIt is important and electrical to underline energy that producers, thermal sources such as solar,can be geothermal successfully and adopted biomass. to Thepro- duceenergy electricity, output in isspecific usually conditions. selected according For example, to the in features the case of of the a localhigh energytemperature resource. ge- othermalCombining source, the a power technologies plant can for be renewable realized [21]. energy sources utilization and the desali- nationThus, solutions, in order Figure to supply4 is obtained. the desalination process with renewable energy sources, it is convenientIn the following to distinguish paragraphs, the energy each source technologys that iscan analyzed be used into produce detail, describing electricity the (or entiremechanical process, energy) and reporting from those the producing state of the thermal art. energy. With this goal, the renewable energy sources can be sorted in the following catego- ries, according to the usual energy output that can be produced: • Electricity producers, such as wind, hydro, tidal and wave. • Thermal and electrical energy producers, such as solar, geothermal and biomass. The energy output is usually selected according to the features of the local energy re- source. Combining the technologies for renewable energy sources utilization and the desali- nation solutions, Figure 4 is obtained. Appl.Appl. Sci.Sci.2021 2021,,11 11,, 670 x FOR PEER REVIEW 66 of 3636

FigureFigure 4.4. PossiblePossible couplingcoupling betweenbetween desalinationdesalination technologies technologies and and renewable renewable energy energy sources sources (RES). (RES). 2.1. Multi-Effect Distillation (MED) TheIn the first following MED plant paragraphs, was realized each in technology Kuwait in is the analyzed 1950s,using in detail, a triple-effect describing sub- the mergedentire process, tube evaporator. and reporting Despite the state being of thethe firstart. technology for desalination to be intro- duced, MED did not spread because it is particularly affected by the scaling problem on the pipes2.1. Multi-Effect in comparison Distillation with other (MED) thermally supplied desalination technologies [22]. How- ever,The from first the 1980sMED severalplant was research realized projects in Kuwait have been in the realized 1950s, about using MED, a triple-effect investigating sub- lower temperatures to reduce the scaling issues and the corrosion of the pipes [23]. MED is merged tube evaporator. Despite being the first technology for desalination to be intro- also currently used in food industries to extract juice from sugarcane, and to produce salts duced, MED did not spread because it is particularly affected by the scaling problem on from seawater [24]. the pipes in comparison with other thermally supplied desalination technologies [22]. MED units can be arranged in several configurations, taking into consideration the However, from the 1980s several research projects have been realized about MED, inves- shape of the heat exchangers or the brine flow direction regarding the vapor direction. The tigating lower temperatures to reduce the scaling issues and the corrosion of the pipes effects can be assembled in one line or in two parallel lines, working at different pressures [23]. MED is also currently used in food industries to extract juice from sugarcane, and to in order to optimize the heat recovery [22,25]. According to the Top Brine Temperature produce salts from seawater [24]. (TBT), MED can be classified as Low Temperature (below 90 ◦C) or High Temperature (over MED units can be arranged in several configurations, taking into consideration the 90 ◦C). shape of the heat exchangers or the brine flow direction regarding the vapor direction. To reduce the energy costs, steam is usually spilled from a steam turbine inside a The effects can be assembled in one line or in two parallel lines, working at different pres- power plant or recovered from a waste energy source in industrial processes [23]. As thesures primary in order steam to optimize is not in the direct heat contactrecovery with [22,25]. saline According water, the to the condensate Top Brine inside Temper- the evaporatorature (TBT), is MED normally can be recycled classified to the as Low boiler Temperature for reuse [23 (below]. 90 °C) or High Tempera- ture The(over maximal 90 °C). temperature of brine is limited to 120 ◦C by the calcium sulphate scaling, whileTo the reduce minimal the is energy related costs, to the steam available is usually temperature spilled in from the wave. a steam Figure turbine5 shows inside the a diagrampower plant of a MEDor recovered plant with from horizontal a waste energy tubes [ 23source,26,27 in]. industrial processes [23]. As the primaryIn general, steam is this not plant in direct is composed contact with of asaline steam water, supply, the several condensate effects, inside heat the recovery evapo- exchangers,rator is normally a condenser recycled and to athe venting boiler systemfor reuse [22 [23].]. InThe detail, maximal the saline temperature water can of brine be split is limited into two to lines, 120 °C in by order the tocalcium recover sulphate the thermal scal- energying, while of freshwaterthe minimal and is related brineproduced to the avai bylable the temperature system, as depictedin the wave. in Figure Figure5 .5 Aftershows thisthe diagram step, saline of a water MED is plant used with as cooling horizontal fluid tubes for the [23,26,27]. condenser, then preheated by using the heat recovery exchangers that are supplied by the steam produced in each effect chamber [5]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 36 Appl. Sci. 2021, 11, 670 7 of 36

Figure 5. A schemaFigure of a Multi-Effect 5. A schema Distillation of a Multi-Effect (MED) Distillationunit. (MED) unit.

In general, thisThe plant preheated is composed water of is a sprayed steam supply, in the first several chamber effects, on heat the evaporatorrecovery surface, pro- exchangers, a condenserducing a thinand filma venting to promote system rapid [22]. boiling and evaporation thanks to the low pressure In detail, theinside saline the water chamber can andbe split the externalinto two thermal lines, in energy order to supply recover [23 ].the thermal energy of freshwaterThe and vapor brine produced by inside the system, this chamber as depicted is transferred in Figure by5. After pipes this to the following step, saline waterchamber. is used As as the cooling pressure fluid inside for the the condenser, second chamber then preheated is lower than by using the first the one, the boiling heat recovery exchangerstemperature that is alsoare supplied lower. In by this the way, steam it is produced possible toin condensateeach effect chamber the vapor produced in [5]. the first chamber inside the pipes and at the same time produce other vapor inside the The preheatedsecond water chamber is sprayed [28]. in the first chamber on the evaporator surface, pro- ducing a thin film toThis promote process rapid is repeated boiling in and the evaporation subsequent chambersthanks to inthe the low same pressure way, using the steam inside the chambergenerated and the in external the previous thermal flash energy chamber supply to produce [23]. more vapor at lower pressure. In the The vaporlast produced chamber, inside the vaporthis chamber is finally is condensed transferred inside by pipes the condenser,to the following cooled by the saline chamber. As thefeedwater. pressure inside the second chamber is lower than the first one, the boiling temperature is also Thelower. brine In this produced way, it in is the possible previous to condensate chambers is the usually vapor transferred produced in inside the subse- the first chamberquent inside chambers, the pipes in orderand at to th forcee same the time extraction produce of moreother freshwater,vapor inside thanks the to the lower second chamberpressure [28]. inside them [23]. This process is repeatedThe pressure in the inside subsequent the chambers chambers is kept in the below same the way, atmospheric using the conditionssteam by using a dedicated vacuum system. The energy efficiency of MED units depends on the number of generated in the previous flash chamber to produce more vapor at lower pressure. In the effects, normally ranging between 4 and 21 [24]. last chamber, the vapor is finally condensed inside the condenser, cooled by the saline MED units are used to produce freshwater with a flowrate ranging from 2000 to feedwater. 20,000 m3/day. To improve the energy efficiency, MED can be coupled with a Thermal or The brine produced in the previous chambers is usually transferred inside the sub- Mechanical Vapor Compression unit [28]. The biggest desalination plants are concentrated sequent chambers, in order to force the extraction of more freshwater, thanks to the lower in China and Middle East [29]. pressure inside them [23]. The pressure2.2. inside Multi-Stages the chambers Flash (MSF) is kept below the atmospheric conditions by using a dedicated vacuum system. The energy efficiency of MED units depends on the number The first MSF plant was realized in the 1950s in Scotland and after few years it became of effects, normally ranging between 4 and 21 [24]. the most used desalination technology [24]. MSF is also commonly used on ships and along MED units are used to produce freshwater with a flowrate ranging from 2000 to the coastline in several parts of the world, like USA, Middle East and Korea, to produce 3 20,000 m /day. freshwaterTo improve from the energy seawater efficiency, [30]. MED can be coupled with a Thermal or Mechanical Vapor TheCompression MSF process unit shows [28]. The some biggest similitudes desalination with MED, plants previously are concen- described. In fact, trated in Chinaan and initial Middle heat East supply [29]. is also required, using steam spilled from a power plant, and the decreasing pressure is used to force the vapor production [5]. Electricity is required to run 2.2. Multi-Stagesthe Flash several (MSF) pumps distributed along the desalination plant [31]. The first MSF plantFigure was6 shows realized the in diagramthe 1950s ofin aScotland once-through and after MSF few unit years [ 23 it ,became26,27]. The plant can the most used bedesalination conceptually technology divided in[24]. two MSF sections: is also the commonly first is the used brine on heater ships section,and where the along the coastlinefeedwater in several receives parts heatof the from world, an externallike USA, supply, Middle and East the and latter Korea, one to is pro- the heat recovery duce freshwatersection, from seawater where the [30]. thermal energy is recovered to preheat the feedwater [6]. The MSF process shows some similitudes with MED, previously described. In fact, an initial heat supply is also required, using steam spilled from a power plant, and the Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 36

decreasing pressure is used to force the vapor production [5]. Electricity is required to run the several pumps distributed along the desalination plant [31]. Figure 6 shows the diagram of a once-through MSF unit [23,26,27]. The plant can be conceptually divided in two sections: the first is the brine heater section, where the feed- Appl. Sci. 2021, 11, 670 8 of 36 water receives heat from an external supply, and the latter one is the heat recovery section, where the thermal energy is recovered to preheat the feedwater [6].

Figure 6. Figure 6. A schema of a Multi-StagesA schema of Flash a Multi-Stages (MSF) desalination Flash (MSF) unit. desalination unit. Focusing the attention on the picture reported above, the saline feedwater is firstly Focusing the attention on the picture reported above, the saline feedwater is firstly used as cooling water for the condenser and then as a raw source to produce freshwater. used as cooling water for the condenser and then as a raw source to produce freshwater. The saline water increases its temperature progressively, flowing inside the pipes and The saline water increases its temperature progressively, flowing inside the pipes forming the heat exchangers inside the flash stages. To start the process, the saline water and forming the heat exchangers inside the flash stages. To start the process, the saline is heated inside the brine heater by using steam usually spilled from a power plant. This water is heatedsteam inside condenses the brine heater inside by the using brine steam heater usually (outside spilled the tube from bundle), a power so plant. it can be reused in This steam condensesthe steam inside power the plant brine [heater24]. (outside the tube bundle), so it can be reused in the steam power Asplant the [24]. saline feedwater flows inside pipes in the brine heater and in the flash stages, As the salinethe maintenancefeedwater flows operations inside pipes to remove in the thebrine scaling heater are and simpler in the thanflash instages, MED [23]. For this the maintenancereason, operations MSF is to the remove most widespread the scaling thermally are simpler driven than desalination in MED [23]. technology, For this representing reason, MSF is17.6% the most of the widespread total installed thermally desalination driven desalination capacity in the technology, world [12 ].represent- ing 17.6% of the totalAfter installed the initial desalination heating, capacity saline water in the is world laminated [12]. inside the first flash stage. The After the initialvapor heating, produced saline inside water the chamberis laminated condenses inside the thanks first flash to the stage. heat removalThe va- by the saline por produced insidefeedwater the chamber inside the condenses heat exchanger. thanks to The the brine heat collectedremoval by in thethe lowersaline partfeed- of the chamber water inside theis laminatedheat exchanger. in the The subsequent brine co chambers,llected in the where lower the part internal of the pressure chamber is reducedis linearly laminated in thefrom subsequent the first stage chambers, to the terminalwhere the one internal [30]. The pressure vacuum is isreduced obtained linearly by the utilization of from the first stagesteam to ejectors the terminal supplied one by [30]. high The pressure vacuum steam is obtained (as shown by in the Figure utilization6), or by of using vacuum steam ejectors pumpssupplied [6 ].by high pressure steam (as shown in Figure 6), or by using vac- uum pumps [6]. Thanks to the pressure drop, the introduction of heated saline water produces the Thanks to“flashing the pressure effect”, drop, for the which introduction the saline waterof heated boils saline rapidly water inside produces the chamber, the producing “flashing effect”,vapor for [which24]. the saline water boils rapidly inside the chamber, producing vapor [24]. To maximize the energy efficiency of the system, MSF units are typically composed of To maximizeseveral the flashenergy stages, efficiency with theof the total system, number MSF ranging units fromare typically 15 to 25 stagescomposed (greater values are of several flashrelated stages, to with bigger the MSFtotal number plants). Thisranging technology from 15 to is able25 stages to satisfy (greater a freshwater values demand of are related to biggerabout 4000MSF toplants). 57,000 This m3/day, technology requiring is able heating to satisfy at 90 ◦aC–110 freshwater◦C[23 demand]. of about 4000 to 57,000In more m3/day, recent requiring plants, aheating few changes at 90 °C–110 have been °C [23]. introduced. Instead of the condenser, In more recenta heat plants, rejection a few section changes isinserted, have been which introduced. is composed Instead of of two the orcondenser, three flash stages (see a heat rejectionFigure section7). Inis inserted, detail, seawater which is usedcomposed as cooling of two fluid or inthree this flash section. stages After (see this step, a part Figure 7). In detail,of seawater seawater is rejectedis used as and cooling the other fluid partin this is mixedsection. with After a partthis step, of brine a part extracted by the of seawater is rejectedlast flash and stage. the other This saltpart solution is mixed is with used a part in the of mainbrine sectionextracted of by the the desalination last unit, in flash stage. Thisthe salt same solution way as is shown used in in the the previousmain section diagram. of the This desalination technique unit, is applied in the to increase the same way as shownenergy in efficiency the previous of big diagram. desalination This plants, technique composed is applied of 19–40 to increase flash stages the and 2–3 heat energy efficiencyrejection of big stagesdesalination [6,31]. plants, composed of 19–40 flash stages and 2–3 heat rejection stages [6,31].In the last two decades, the reliability of the system has been improved thanks to scal- ing control (adding substances to limit the phenomenon), the introduction of automation and control systems and the choice of better materials for the realization of the desalination units [23]. Similarly to MED, MSF desalination has spread where the thermal energy cost is low, like in Saudi Arabia, the United Arab Emirates and Kuwait [24]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 36

Appl. Sci. 2021, 11Appl., x FOR Sci. PEER REVIEW11 9 of 36 2021, , 670 9 of 36

Figure 7. A schema of a more efficient MSF desalination unit.

In the last two decades, the reliability of the system has been improved thanks to scaling control (adding substances to limit the phenomenon), the introduction of automa- tion and control systems and the choice of better materials for the realization of the desal- ination units [23]. Similarly to MED, MSF desalination has spread where the thermal energy cost is low, Figurelike in 7. Saudi A schema Arabia,Figure of a themore 7. AUnited schemaefficient Arab ofMSF a moreEmirates desalination efficient and MSFunit. Kuwait desalination [24]. unit.

2.3. VaporIn the Compression last2.3. two Vapor decades, (VC) Compression the reliability (VC) of the system has been improved thanks to scalingVapor control Compression (addingVapor substances Compressionis a common to limit technique is a th commone phenomenon), used technique in the desalination the used introduction in the sector, desalination of automa-based on sector, based on tionthe liquid–vaporand controlthe systems liquid–vaporphase transition. and the phase choice transition. of better materials for the realization of the desal- inationTo units clarify [23]. the Toprocess, clarify Figure the process, 8 is considered, Figure8 is considered,reporting the reporting case of thea Mechanical case of a Mechanical Va- Vapor por CompressionSimilarly Compressionto MED, (MVC) MSF unit. (MVC)desalination unit. has spread where the thermal energy cost is low, like in Saudi Arabia, the United Arab Emirates and Kuwait [24].

2.3. Vapor Compression (VC) Vapor Compression is a common technique used in the desalination sector, based on the liquid–vapor phase transition. To clarify the process, Figure 8 is considered, reporting the case of a Mechanical Va- por Compression (MVC) unit.

Figure Figure8. A schema 8. A schemaof a simple of a simpleMechanical Mechanical Vapor Compression Vapor Compression desalination desalination unit. unit.

A vapor compressorA vapor is compressor used to extract is used vapor to extract produced vapor inside produced the chamber. inside the Due chamber. to Due to the the compression,compression, the vapor theincreases vapor increasesits temperature its temperature and pressure. and pressure.By raising By the raising temper- the temperature ature and usingand a usingheat exchanger, a heat exchanger, the pressurized the pressurized vapor can vapor transfer can transfer heat to heatthe saline to the saline water water inside theinside chamber the chamber and produce and produce vapor. vapor. To minimize Tothe minimizeenergy consumption the energy consumptionof the process, of a the heat process, recovery a heat exchanger recovery is exchanger is used to transfer heat from the brine discharge and the condensed feedwater to the saline Figureused to 8. transferA schema heat of a fromsimple the Mechanical brine discharge Vapor Compression and the condensed desalination feedwater unit. to the saline feedwater [31].feedwater [31]. AAfter vapor the compressor preheating,After the is the preheating,used saline to extractwater the isvapor saline mixed waterproduced with is a mixedbrine inside recirculation with the achamber. brine recirculationflow. Due This to flow. This solution is sprayed externally on the main heat exchanger inside the desalination unit. thesolution compression, is sprayed the externally vapor increases on the itsmain temperature heat exchanger and pressure. inside the By desalination raising the temper- unit. MVC essentially requires electricity to run the process; therefore, a small stand-alone ature and using a heat exchanger, the pressurized vapor can transfer heat to the saline desalination unit can be realized to satisfy a freshwater demand ranging from 100 to water inside the chamber and produce vapor. 3000 m3/day. To minimize the energy consumption of the process, a heat recovery exchanger is The same approach is adopted in the Thermal Vapor Compression (TVC) unit, reported used to transfer heat from the brine discharge and the condensed feedwater to the saline in Figure9. feedwater [31]. After the preheating, the saline water is mixed with a brine recirculation flow. This solution is sprayed externally on the main heat exchanger inside the desalination unit. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 36 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 36

MVC essentially requires electricity to run the process; therefore, a small stand-alone MVC essentially requires electricity to run the process; therefore, a small stand-alone desalination unit can be realized to satisfy a freshwater demand ranging from 100 to 3000 desalination unit can be realized to satisfy a freshwater demand ranging from 100 to 3000 m3/day. m3/day. The same approach is adopted in the Thermal Vapor Compression (TVC) unit, re- Appl. Sci. 2021, 11, 670 The same approach is adopted in the Thermal Vapor Compression (TVC) unit, re- 10 of 36 ported in Figure 9. ported in Figure 9.

Figure 9. A schema of a simple Thermal Vapor Compression (TVC) desalination unit. FigureFigure 9. A schema 9. A schema of a simple of a simple Thermal Thermal Vapor Vapor Compression Compression (TVC) (TVC) desalination desalination unit. unit. The only significant difference is related to the method used to increase the vapor The only significantThe only difference significant is related difference to the is relatedmethod to used the methodto increase used the to vapor increase the vapor pressure. In TVC a thermal compressor is adopted, which is supplied by high pressure pressure. In TVCpressure. a thermal In TVC compressor a thermal is compressor adopted, which is adopted, is supplied which by is high supplied pressure by high pressure steam, normally spilled from a power plant. steam, normallysteam, spilled normally from a spilled power fromplant. a power plant. TVC requires thermal and electrical energy. The first one is used for the thermal com- TVC requires TVCthermal requires and electrical thermal energy. and electrical The first energy. one is used The for first the one thermal is used com- for the thermal pression, the latter one for the circulation pumps. TVC is sometimes assembled with MED pression, the lattercompression, one for the the circulation latter one pumps. for the circulation TVC is sometimes pumps. assembled TVC is sometimes with MED assembled with units, realizing a hybrid system called a MED-TVC desalination plant, as shown in Figure units, realizingMED a hybrid units, system realizing called a hybrid a MED-TVC system desalination called a MED-TVC plant, as desalination shown in Figure plant, as shown in 10. 10. Figure 10.

Figure 10. A schema of a TVCFigure and 10. MEDA schema unit. of a TVC and MED unit. Figure 10. A schema of a TVC and MED unit. The comparisonThe between comparison the picture between repo therted picture above reported and the above MED anddiagram the MED shows diagram a shows a The comparison between the picture reported above and the MED diagram shows a few differences.few The differences. steam supply The steamis used supply to produce is used the to vacuum produce inside the vacuum the condenser inside the condenser few differences. The steam supply is used to produce the vacuum inside the condenser and the last effectand of the MED last effectunit. The of MED contaminat unit. Theed contaminatedsteam is condensed steam inside is condensed the first inside effect the first effect and the last effectand of added MED to unit. the The freshwater contaminat outputed steam [32]. Thisis condensed configuration inside is the used first to effect satisfy significant and added to the freshwater output [32]. This configuration is used to satisfy significant and added to freshwaterthe freshwater demand, output between [32]. This 10,000 configuration and 30,000 is m 3used/day to [6 satisfy]. significant freshwater demand, between 10,000 and 30,000 m3/day [6]. freshwater demand, between 10,000 and 30,000 m3/day [6].

2.4. Reverse Osmosis (RO) RO is a desalination technology based on semipermeable membranes, which are specific layers allowing the passage only to selected molecules. In nature, if two solutions with different concentrations of solutes are separated by a semipermeable membrane, the solvent flows spontaneously from the more diluted solution to the more concentrated one, in order to balance the energy potential of both solutions, as shown in Figure 11 (see case a). This flow can be progressively reduced if an increasing external pressure gradient ∆p is applied to the semipermeable membrane (see case b) [33]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 36

2.4. Reverse Osmosis (RO) RO is a desalination technology based on semipermeable membranes, which are spe- cific layers allowing the passage only to selected molecules. In nature, if two solutions with different concentrations of solutes are separated by a semipermeable membrane, the solvent flows spontaneously from the more diluted solution to the more concentrated one, in order to balance the energy potential of both solutions, as shown in Figure 11 (see case a). This flow can be progressively reduced if an increasing external pressure gradient ∆𝑝 is applied to the semipermeable membrane (see case b) [33]. The exact value able to stop the solvent flow is defined as Osmotic Pressure ∆𝑝 Appl. Sci. 2021, 11, 670 (see case c). If the external pressure gradient is greater than the osmotic pressure, the11 sol- of 36 vent flow is inverted, so the solvent can be extracted from the concentrated solution (see case d) [33].

FigureFigure 11. 11. AA depiction depiction of of the the phenomenon phenomenon of of osmosis, osmosis, according according to to the the external external pressure pressure applied to tothe the two two sides sides of of the the membrane. membrane. Case Case (a ).(a). Forward Forward osmosis. osmosis. Case Case (b) (b) Retarded Retarded osmosis. osmosis. Case Case (c) Zero(c) Zeroflow. flow. Case Case (d) Reverse (d) Reverse osmosis. osmosis.

ForThe each exact solution value able the toabsolute stop the osmotic solvent pressure flow is defined𝑝 can as be Osmotic defined Pressure according∆p osmto van’t(see caseHoff’s c). equation If the external [33,34] (In pressure chemistry, gradient the amount is greater ι[c] than is called the osmotic“normality”, pressure, indicat- the ingsolvent the number flow is inverted,of equivalents so the in solvent a unitary can vo belume. extracted The fromequivalent the concentrated represents solutionthe amount (see ofcase a specific d) [33]. substance able to react with (or supply) one mole of hydrogen (H+) in an acid- base reactionFor each or solution react with the absolute(or supply) osmotic a mole pressure of electronsposm can in a be redox defined reaction): according to van’t Hoff’s equation [33,34] (In chemistry, the amount ι[c] is called “normality”, indicating the number of equivalents in a unitary volume.𝑝 =𝜄 The𝑐𝑅𝜏 equivalent represents the amount of(1) a + assumingspecific substance the following able tonotation: react with (or supply) one mole of hydrogen (H ) in an acid-base reaction or react with (or supply) a mole of electrons in a redox reaction): • 𝜄 is the dimensionless van’t Hoff index (also called the number of osmotically active particles), given by the relation 𝜄=1+𝜖𝜈−1, where 𝜖 is the degree of dissocia- posm = ι[c]Rτ (1) tion representing the ratio of how many original solute molecules are dissociated, assumingand 𝜈 the is the following number notation: of ions formed by the molecule dissociation (the stoichiometric • coefficientι is the dimensionless of dissociation van’t reaction). Hoff index As (alsoan example, called the in numberthe case ofof osmoticallysodium chloride active 𝜖1 𝜈=2 𝜄=2 (NaCl),particles), given, by the, consequently relation ι = 1 + e(. ν − 1), where e is the degree of dissociation representing the ratio of how many original solute molecules are dissociated, and ν is the number of ions formed by the molecule dissociation (the stoichiometric coefficient of dissociation reaction). As an example, in the case of sodium chloride (NaCl), e ≈ 1, ν = 2, consequently ι = 2. • [c] is the molar concentration of the solute. • R is the ideal gas constant equal to 8.31441 J K−1mol−1. • τ is the absolute temperature of the solution. As the salt concentration is negligible in freshwater and consequently its osmotic pressure (the minimal pressure required to stop the solvent flow) is equal to the os- motic pressure of saline water. For seawater, the salt concentration ranges between 0.51 and 0.68 mol L−1 [33]. Thus, considering an environmental temperature equal to 25 ◦C, the osmotic pressure according to van’t Hoff’s equation ranges between 25 and Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 36

Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 36

• 𝑐 is the molar concentration of the solute. •• 𝑅𝑐 is is the the ideal molar gas concentration constant equal of tothe 8.31441 solute. J Kmol. •• 𝜏𝑅 is is the the absolute ideal gas temperature constant equal of the to 8.31441solution. J Kmol. • As𝜏 is the the salt absolute concentration temperature is negligible of the solution. in freshwater and consequently its osmotic pressureAs the(the salt minimal concentration pressure isrequired negligible to stop in freshwater the solvent and flow) consequently is equal to theits osmotic pressure (theof minimalsaline water. pressure For required seawater to ,stop the the salt solvent concentration flow) is equal ranges to the between osmotic Appl. Sci. 2021, 11, 670 12 of 36 0.51pressure and 0.68of molsaline L [33].water. Thus, For considering seawater, anthe enviro salt nmentalconcentration temperature ranges equal between to 25 °C,0.51 the and osmotic 0.68 mol pressure L [33]. according Thus, considering to van’t Hoff’s an enviro equationnmental ranges temperature between 25 equaland 33 to bar. 25 Greater°C, the osmotic values canpressure be measured, according as to in van’t the extremeHoff’s equation case of rangesDead Sea, between where 25 the and osmotic 33 bar. 33pressureGreater bar. Greatervalues is equal can values to be290 measured, canbar be[33]. measured, as in the as extreme in the extreme case of Dead case ofSea, Dead where Sea, the where osmotic the osmoticpressureFigure pressure is equal12 visualizes isto equal290 bar the to [33]. 290phenomenon bar [33]. of osmosis: until the external pressure gradient is lowerFigureFigure than 1212 the visualizesvisualizes osmotic thethepressure, phenomenonphenomenon the solvent ofof osmosis:osmo flowssis: from untiluntil the thethe more externalexternal diluted pressurepressure solution gradientgradient to the ismoreis lowerlower concentrated thanthan thethe osmoticosmotic one. In pressure,pressure, this working thethe solventsolvent region, flowsflows it is fromfromalso thepossiblethe moremore to diluteddiluted extract solutionsolution energy to tofrom thethe morethemore mixing concentratedconcentrated of two solutions one.one. In thisthishaving workingworking different region,region, concentrations. itit isis alsoalso possiblepossible This approach toto extractextract can energyenergy be used fromfrom in thethe mixingestuarymixing of ofof twotwo the solutionssolutionsrivers. Indeed, havinghaving the differentdifferent exploitation concentrations.concentrations. of the saline ThisThis gradient approachapproach energy cancan bebe source usedused ininis thecurrentlythe estuaryestuary under ofof thethe development rivers.rivers. Indeed,Indeed, [35]. thethe exploitationexploitation ofof thethe salinesaline gradientgradient energyenergy sourcesource isis currentlycurrently underunder developmentdevelopment [[35].35].

Figure 12. Solvent flow as a function of the external pressure gradient. FigureFigure 12.12. SolventSolvent flowflow asas aa functionfunction ofof thethe externalexternal pressurepressure gradient.gradient. The case without an external pressure gradient is identified as “Forward Osmosis”, The case without an external pressure gradient is identified as “Forward Osmosis”, a conditionThe case which without is used an external in the other pressure osmosis grad ientdesalination is identified technique, as “Forward analyzed Osmosis”, in the a condition which is used in the other osmosis desalination technique, analyzed in the followinga condition subsection. which is used in the other osmosis desalination technique, analyzed in the following subsection. followingBy applying subsection. an external pressure gradient greater than the osmotic pressure, fresh- By applying an external pressure gradient greater than the osmotic pressure, freshwa- By applying an external pressure gradient greater than the osmotic pressure, fresh- terwater is extracted is extracted from from saltwater. saltwater. For desalinationFor desalination purposes, purposes, an external an external pressure pressure between be- water is extracted from saltwater. For desalination purposes, an external pressure be- 15tween and 15 25 and bar 25 is bar normally is normally applied applied for brackish for brackish water, water, and betweenand between 54 and 54 and 80 bar 80 bar for tween 15 and 25 bar is normally applied for brackish water, and between 54 and 80 bar seawaterfor seawater [23, 36[23,36].]. for seawater [23,36]. Thus, accordingaccording toto the the values values reported reported above, above, the the RO RO essentially essentially requires requires electrical electrical (or mechanical)(or mechanical)Thus, according energy energy to to run tothe therun values pumps the pumps reported to significantly to abovsignificantlye, increasethe RO increase essentially the seawater the seawaterrequires pressure electricalpressure before thebefore(or semipermeablemechanical) the semipermeable energy membrane. to membrane.run the A simple pumps A diagramsimple to significantly diagram of a RO of desalinationincrease a RO desalination the unitseawater is depicted unit pressure is de- in Figurepictedbefore in13the [Figure37 semipermeable]. 13 [37]. membrane. A simple diagram of a RO desalination unit is de- picted in Figure 13 [37].

Figure 13. A diagram of a simple Reverse Osmosis (RO) desalination unit.unit. Figure 13. A diagram of a simple Reverse Osmosis (RO) desalination unit. Seawater, after a pre-treatment to remove solid particles, is pressurized by a High Pressure Pump (HPP) in order to supply the RO desalination unit [38]. In desalination applications, the Recovery Ratio (RR) is defined as the ratio between the freshwater flow and the saline feedwater flow [12].

Q f RR = (2) Qs

Considering the working conditions, RR assumes values between 35% and 50%, so practically only half (or less) of the seawater flow becomes freshwater and the remaining Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 36

Seawater, after a pre-treatment to remove solid particles, is pressurized by a High Pressure Pump (HPP) in order to supply the RO desalination unit [38]. In desalination applications, the Recovery Ratio (RR) is defined as the ratio between the freshwater flow and the saline feedwater flow [12].

𝑄 𝑅𝑅 = (2) 𝑄 Appl. Sci. 2021, 11, 670 Considering the working conditions, RR assumes values between 35% and 50%,13 of so 36 practically only half (or less) of the seawater flow becomes freshwater and the remaining part is expelled as brine [39]. To increase the freshwater extraction, the pressure before the partsemipermeable is expelled asmembrane brine [39 ].should To increase be increased, the freshwater but there extraction, are several the technical pressure constraints, before the semipermeableessentially related membrane to the mechanical should be increased,resistance butof the there membrane are several [40]. technical As semipermeable constraints, essentiallymembranes related are not to perfect, the mechanical a limited resistanceamount of of salts the membranecan be found [40 in]. the As semipermeablefreshwater out- membranesput [23]. are not perfect, a limited amount of salts can be found in the freshwater outputThe [23 brine]. flow has a high energy potential, as its pressure is practically the same as the salineThe brine input flow water. has aIn high fact, energy the pressure potential, drop as itsinside pressure the brine is practically circuit is the about same 2–3 as thebar saline[37]. input water. In fact, the pressure drop inside the brine circuit is about 2–3 bar [37]. To reducereduce thethe totaltotal energyenergy consumptionconsumption forfor desalination,desalination, manymany studiesstudies havehave beenbeen realized sincesince the the 1970s. 1970s. In In addition addition to theto th improvemente improvement of membrane of membrane properties, properties, the main the goalmain was goal energy was energy recovery recovery from the from brine the flow brine by flow the introductionby the introduction of an Energy of an RecoveringEnergy Re- Devicecovering (ERD). Device The (ERD). solutions The soluti can beons classified can be classified as [41]: as [41]: • Centrifugal device • Isobaric devicedevice In thethe firstfirst case,case, twotwo technologiestechnologies havehave beenbeen proposed.proposed. TheThe first,first, introducedintroduced inin thethe 1980s, is the the installation installation of of a ahydro hydro turbine turbine (Pelton) (Pelton) to recover to recover the theenergy energy of the of brine the brine flow flowand transfer and transfer it to the it tomain the HPP. main To HPP. complete To complete the energy the demand energy demand of the pump, of the an pump, electrical an electricalmotor is used. motor The is used.pressure The increase pressure is increaserealized isby realized the main by HPP the to main the HPPentire to saline the entire feed- salinewater feedwaterflow. This flow.solution This is solutiondepicted is in depicted Figure 14 in [37]. Figure 14[37].

Figure 14. A schema of a RO unit with a Pelton Turbine.

Pelton turbines turbines are are normally normally used used in in hydr hydropoweropower to toexploit exploit high high heads, heads, between between 300 300m and m and1000 1000 m. Of m. course, Of course, only onlya part a partof the of av theailable available energy energy of the of brine the brineflow (about flow (about 70%) 70%)is transferred is transferred to the tosaline the feedwater saline feedwater due to the due double to the energy double conversion energy conversion (from the (from fluid theto the fluid mechanical to the mechanical shaft and then shaft to andthe fluid) then to[42]. the At fluid) the end [42 of]. the At 1980s, the end another of the centrif- 1980s, Appl. Sci. 2021, 11, x FOR PEER REVIEW 14 of 36 anotherugal ERD centrifugal solution was ERD introduced, solution was represented introduced, by represented the turbocharger by the ERD. turbocharger The working ERD. Theprinciples working of this principles system of are this shown system in areFigure shown 15. in Figure 15.

Figure 15. A schema of a RO unit with a Turbocharger.

InIn detail,detail, thethe salinesaline waterwater isis pressurizedpressurized inin twotwo steps:steps: thethe firstfirst oneone isis entrustedentrusted toto anan HPP, drivendriven byby anan electricalelectrical motor,motor, andand thethe latterlatter oneone is realized by a turbocharger device, whichwhich isis composedcomposed ofof aa hydrohydro turbineturbine andand aa pump,pump, directlydirectly coupled.coupled. ThisThis solutionsolution showsshows a greatergreater energyenergy efficiencyefficiency inin comparisoncomparison toto thethe previousprevious one,one, asas thethe rotaryrotary speedspeed ofof thethe turbochargerturbocharger cancan bebe modulatedmodulated independentlyindependently byby thethe HPPHPP [[43].43]. To improve the energy efficiency even in part-load, limiting the adoption of passive regulating systems, a Dual Turbine System has been proposed. Considering the turbocharger system, shown in Figure 15, a part of the brine flow is spilled to run a Pelton turbine and reduce the power load of the electrical motor in the HPP. A schema of this solution is reported in Figure 16 [37].

Figure 16. A schema of a RO unit with a Dual Turbine System.

The last centrifugal solution is represented by the HEMI (Hydraulic Energy Manage- ment Integration) proposed by FEDCO (Fluid Equipment Development Company) [44]. In detail, the HEMI solution consists of the adoption of an electrical motor in the turbo- charger system, modulating the final pressure of the saline feedwater before the semiper- meable membrane. In Figure 17, the HEMI solution is depicted.

Figure 17. A schema of a RO unit with a Hydraulic Energy Management Integration (HEMI) Sys- tem. Appl. Sci. 2021, 11, x FOR PEER REVIEW 14 of 36 Appl. Sci. 2021, 11, x FOR PEER REVIEW 14 of 36

Figure 15. A schema of a RO unit with a Turbocharger.

In detail, the saline water is pressurized in twotwo steps:steps: thethe firstfirst oneone isis entrustedentrusted toto anan HPP, driven by an electrical motor, and the latter one is realized by a turbocharger device, Appl. Sci. 2021, 11, 670 HPP, driven by an electrical motor, and the latter one is realized by a turbocharger device,14 of 36 which is composed of a hydro turbine and a pump, directly coupled. This solution shows a greater energy efficiency in comparison to the previous one, as the rotary speed of the turbochargerturbocharger cancan bebe modulatedmodulated independentlyindependently byby thethe HPPHPP [43].[43]. ToTo improveimprove thethe energyenergy efficiencyefficiency eveneven inin part-load,part-load, limitinglimiting thethe adoptionadoption ofof passivepassive regulatingregulating systems,systems, aa DualDual TurbineTurbineine System SystemSystemhas hashasbeen beenbeen proposed. proposed.proposed. ConsideringConsidering thethe turbochargerturbocharger system,system, shownshown inin FigureFigure 1515,15,, aaa partpartpart ofofof thethethe brinebrinebrine flowflowflow isisis spilledspilled toto runrun a a Pelton Pelton turbine turbine and and reduce reduce the the power power load load of the of electricalthe electrical motor motor in the in HPP. the A schema of this solution is reported in Figure 16[37]. HPP. A schema of this solution is reported in Figure 16 [37].

Figure 16. A schema of a RO unit with a Dual Turbine System. Figure 16. A schema of a RO unit with a Dual Turbine System. The last centrifugal solution is represented by the HEMI (Hydraulic Energy Manage- The last centrifugal solution is represented by the HEMI (Hydraulic Energy Manage- ment Integration) proposed by FEDCO (Fluid Equipment Development Company) [44]. In ment Integration) proposed by FEDCO (Fluidid EquipmentEquipment DevelopmentDevelopment Company)Company) [44].[44]. detail, the HEMI solution consists of the adoption of an electrical motor in the turbocharger In detail, the HEMI solution consists of the adoption of an electrical motor in the turbo- system,charger modulatingsystem, modulating the final the pressure final pressure of the saline of the feedwater saline feedwater before thebefore semipermeable the semiper- membrane.charger system, In Figure modulating 17, the the HEMI final solution pressure is depicted.of the saline feedwater before the semiper- meable membrane. In Figure 17, the HEMI solution is depicted.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 15 of 36

The Isobaric Devices are recent solutions used to transfer energy from the brine flow FigureFigure 17.17. AA schemaschema ofof aa RORO unitunit with with a a Hydraulic Hydraulic Energy Energy Management Management Integration Integration (HEMI) (HEMI) System. Sys- tem.tem.to the saline feedwater flow without intermediate energy conversions. The idea is de- pictedThe in IsobaricFigure 18. Devices are recent solutions used to transfer energy from the brine flow to theThe saline saline feedwater feedwater flow is without divided intermediate in two flows: energy the first conversions. one is pressurized The idea by is depictedthe main inHPP, Figure driven 18. by an electrical motor, and the latter one is pressurized by the ERD.

FigureFigure 18.18. AA schemaschema ofof aa RORO unitunit withwith aa PressurePressure Exchanger.Exchanger.

The HPP treats a part of the total saline feedwater flow, so this component is smaller in comparison with the previous cases, considering the same freshwater flow. At the same time, this kind of ERD shows higher energy efficiency, reducing the total energy expendi- ture in the desalination process. A solution is the Rotary Pressure Exchanger (RPX), depicted in Figure 19. Inside the device, a ceramic matrix is taken into rotation by the brine flow, which enters with a tan- gential speed component [45]. The channels inside the rotative matrix (part C in the pic- ture) are alternatively connected to the High-Pressure Brine and the High-Pressure Feed- water pipes or to the Low-Pressure Feedwater and the Low-Pressure Brine pipes. In the first case, the channel, previously filled by feedwater, is now refilled by brine, pushing the feedwater inside the High-Pressure Feedwater pipe to nearly the same high pressure as the brine. In fact, a very limited pressure drop is measured due to the rotational motion of the channel matrix. Similarly, in the second case, the saline water is pushed by the feed- water, refilling the channel with feedwater at low pressure [37]. The process is practically continuous because of the high rotary speed of the matrix and the number of internal channels.

Figure 19. The working principles of a Rotary Pressure Exchanger.

The same idea is applied in another way by the Dual Work Exchanger Energy Recov- ery (DWEER™), proposed by Calder AG (now Flowserve Corporation) [37]. The DWEER system is presented in Figure 20. In detail, the device is composed of two cylinders, with two commanded valves (LinX® valve) to control the brine flow, and four automatic valves for the saline feedwater flow [46]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 15 of 36

The Isobaric Devices are recent solutions used to transfer energy from the brine flow to the saline feedwater flow without intermediate energy conversions. The idea is de- picted in Figure 18. The saline feedwater is divided in two flows: the first one is pressurized by the main HPP, driven by an electrical motor, and the latter one is pressurized by the ERD.

Appl. Sci. 2021, 11, 670 15 of 36

Figure 18. A schema ofThe a RO saline unit feedwaterwith a Pressure is divided Exchanger. in two flows: the first one is pressurized by the main HPP, driven by an electrical motor, and the latter one is pressurized by the ERD. The HPP treatsThe a part HPP of treatsthe total a part saline of thefeedwater total saline flow, feedwater so this component flow, so this is smaller component is smaller in comparison within comparison the previous with cases, the considering previous cases, the same considering freshwater the flow. same At freshwater the same flow. At the time, this kind sameof ERD time, shows this higher kind energy of ERD efficiency, shows higher reducing energy the efficiency, total energy reducing expendi- the total energy ture in the desalinationexpenditure process. in the desalination process. A solution is theA Rotary solution Pressure is the Rotary Exchanger Pressure (RPX), Exchanger depicted in (RPX), Figure depicted 19. Inside in Figurethe 19. Inside device, a ceramicthe matrix device, is ataken ceramic into matrix rotation is by taken the intobrine rotation flow, which by the enters brine with flow, a tan- which enters with gential speed componenta tangential [45]. speed The component channels insi [45de]. the The rotative channels matrix inside (part the C rotative in the pic- matrix (part C in ture) are alternativelythe picture) connected are alternatively to the High connected-Pressure toBrine the and High-Pressure the High-Pressure Brine and Feed- the High-Pressure water pipes or Feedwaterto the Low-Pressure pipes or toFeedwate the Low-Pressurer and the Low-Pressure Feedwater and Brine the pipes. Low-Pressure In the Brine pipes. first case, the channel,In the firstpreviously case, the filled channel, by feedwater, previously is now filled refilled by feedwater, by brine, pushing is now the refilled by brine, feedwater insidepushing the High-Pressure the feedwater Feedwater inside the pi High-Pressurepe to nearly the Feedwater same high pipe pressure to nearly as the same high the brine. In fact,pressure a very as limited the brine. pressure In fact, dr aop very is measured limited pressure due to drop the rotational is measured motion due to the rotational of the channel matrix.motion Similarly, of the channel in the matrix. secondSimilarly, case, the saline in the water second is case,pushed the by saline the feed- water is pushed by water, refilling thethe feedwater,channel with refilling feedwater the channelat low pressure with feedwater [37]. The at process low pressure is practically [37]. The process is continuous becausepractically of the continuous high rotary because speed ofof thethe highmatrix rotary and speedthe number of the matrixof internal and the number of channels. internal channels.

Figure 19. The workingFigure 19.principlesThe working of a Rotary principles Pressure of a RotaryExchanger. Pressure Exchanger.

The same idea isThe applied same ideain another is applied way in by another the Dual way Work by the Exchanger Dual Work Energy Exchanger Recov- Energy Recovery ery (DWEER™),(DWEER proposed™), by proposed Calder AG by (now Calder Flowserve AG (now Corporation) Flowserve Corporation)[37]. The DWEER [37]. The DWEER system is presentedsystem in isFigure presented 20. In in detail, Figure the 20 device. In detail, is composed the device of is two composed cylinders, of twowith cylinders, with Appl. Sci. 2021, 11, x FOR PEER REVIEW ® 16 of 36 two commandedtwo valves commanded (LinX® valve) valves to (LinX controlvalve) the brine to control flow, and the brinefour automatic flow, and fourvalves automatic valves for the saline feedwaterfor the saline flow feedwater [46]. flow [46].

FigureFigure 20.20. TheThe workingworking principlesprinciples ofof aa Piston-typePiston-type WorkWork Exchanger.Exchanger.

According to the position of the LinX valve, the brine flow from the desalination unit can be introduced in one piston or in the other one. As the high pressure of the input water, the automatic valve (on the left side of the picture) is opened, transferring the saline feedwater to the booster pump. During this step, the piston is filled with brine water. Changing the position of the LinX valve, now the brine side of the cylinder is connected to the brine discharge pipe. As the water from the feedwater pump has a greater pressure than the brine discharge, the automatic valve is open, refilling the piston with saline water. The system is composed of two cylinders, which allows one to be filled with saline water (low pressure) and the other with brine (high pressure) in alternation [37].

2.5. Forward Osmosis (FO) As introduced in the previous section, Forward Osmosis refers to the natural process via which solvent flows from a more diluted solution to a concentrated one if they are put in contact and separated by a semipermeable membrane. It is interesting to observe that two solutions with different solutes have the same osmotic pressure if they have the same equivalent concentration and temperature, as in- troduced by the van’t Hoff equation (see the Reverse Osmosis paragraph). Therefore, it is possible to extract freshwater by using a solution of glucose more con- centrated than the saline water. This approach is applied in the “hydration bags”, which are an emergency kit equipped with a semipermeable membrane, which contain sugar inside. The bag is used to produce an ingestible draw solution in case of emergency, if a water source is available (rivers, seas, puddles, ponds), thereby avoiding contamination from pathogens or toxins [47]. FO can also be used in a continuative process. In 2005, a research team from the Yale University proposed the utilization of ammonia carbon dioxide as thermolytic draw so- lute [48]. The extraction of freshwater from seawater produces the dilution of the ammo- nia carbon dioxide. These components can be easily recovered using a low temperature distillation. In this way, the main energy input is represented by thermal energy. Renew- able energy sources could be exploited, such as solar and geothermal energy sources. However, this technical solution is not applicable for drinking purposes because of the presence of traces of ammonia in the freshwater [49]. In fact, according to the World Health Organization the maximal value of ammonia in freshwater for drinking purposes should be lower than 1.5 mg/L, but this condition cannot be achieved by this technology [48]. Several researchers are investigating alternative draw solutions and better perform- ing membranes, in order to use this technique in big desalination plants [28]. In order to Appl. Sci. 2021, 11, 670 16 of 36

According to the position of the LinX valve, the brine flow from the desalination unit can be introduced in one piston or in the other one. As the high pressure of the input water, the automatic valve (on the left side of the picture) is opened, transferring the saline feedwater to the booster pump. During this step, the piston is filled with brine water. Changing the position of the LinX valve, now the brine side of the cylinder is connected to the brine discharge pipe. As the water from the feedwater pump has a greater pressure than the brine discharge, the automatic valve is open, refilling the piston with saline water. The system is composed of two cylinders, which allows one to be filled with saline water (low pressure) and the other with brine (high pressure) in alternation [37].

2.5. Forward Osmosis (FO) As introduced in the previous section, Forward Osmosis refers to the natural process via which solvent flows from a more diluted solution to a concentrated one if they are put in contact and separated by a semipermeable membrane. It is interesting to observe that two solutions with different solutes have the same osmotic pressure if they have the same equivalent concentration and temperature, as introduced by the van’t Hoff equation (see the Reverse Osmosis paragraph). Therefore, it is possible to extract freshwater by using a solution of glucose more concentrated than the saline water. This approach is applied in the “hydration bags”, which are an emergency kit equipped with a semipermeable membrane, which contain sugar inside. The bag is used to produce an ingestible draw solution in case of emergency, if a water source is available (rivers, seas, puddles, ponds), thereby avoiding contamination from pathogens or toxins [47]. FO can also be used in a continuative process. In 2005, a research team from the Yale University proposed the utilization of ammonia carbon dioxide as thermolytic draw solute [48]. The extraction of freshwater from seawater produces the dilution of the ammo- nia carbon dioxide. These components can be easily recovered using a low temperature distillation. In this way, the main energy input is represented by thermal energy. Renewable energy sources could be exploited, such as solar and geothermal energy sources. However, this technical solution is not applicable for drinking purposes because of the presence of traces of ammonia in the freshwater [49]. In fact, according to the World Health Organization the maximal value of ammonia in freshwater for drinking purposes should be lower than 1.5 mg/L, but this condition cannot be achieved by this technology [48]. Several researchers are investigating alternative draw solutions and better performing membranes, in order to use this technique in big desalination plants [28]. In order to minimize the energy expended to separate the draw solution, different techniques are under investigation, such as temperature or pH variation, the adoption of electro-magnetic field or light [50]. A possible solution has been proposed by Trevi Systems. The layout of the plant is reported in Figure 21[51]. In detail, after a preliminary saltwater filtration is introduced into the Forward Osmo- sis unit. As the solution on the other side of semipermeable membrane is more concentrated, freshwater is extracted from saline water, diluting the raw solution. Thanks to an external thermal supply, the diluted solution can be separated into two flows: the concentrated raw solution, which is sent to the FO unit, and the freshwater flow, which is further filtered before the storage [51]. This technology is in development phase. Appl. Sci. 2021, 11, x FOR PEER REVIEW 17 of 36

minimize the energy expended to separate the draw solution, different techniques are un- der investigation, such as temperature or pH variation, the adoption of electro-magnetic field or light [50]. Appl. Sci. 2021, 11, x FOR PEER REVIEW A possible solution has been proposed by Trevi Systems. The layout of the plant17 of is36 reported in Figure 21 [51].

minimize the energy expended to separate the draw solution, different techniques are un- der investigation, such as temperature or pH variation, the adoption of electro-magnetic field or light [50]. Appl. Sci. 2021, 11, 670 17 of 36 A possible solution has been proposed by Trevi Systems. The layout of the plant is reported in Figure 21 [51].

Figure 21. A schema of a Forward Osmosis desalination unit.

In detail, after a preliminary saltwater filtration is introduced into the Forward Os- mosis unit. As the solution on the other side of semipermeable membrane is more concen- trated, freshwater is extracted from saline water, diluting the raw solution. Thanks to an external thermal supply, the diluted solution can be separated into two flows: the concen-

trated raw solution, which is sent to the FO unit, and the freshwater flow, which is further filteredFigure 21. before A schema theFigure storage of a Forward 21. [51].A schema This Osmosis technology of a Forwarddesalination is Osmosis in unit. development desalination phase. unit.

2.6. NanofiltrationIn detail, 2.6.after (NF) Nanofiltration a preliminary (NF) saltwater filtration is introduced into the Forward Os- mosis unit. As the solution on the other side of semipermeable membrane is more concen- Nanofiltration isNanofiltration a membrane filtration is a membrane process filtration used to remove process dissolved used to remove ions or or- dissolved ions or trated, freshwater is extracted from saline water, diluting the raw solution. Thanks to an ganic matter toorganic produce matter soft water, to produce i.e., water soft with water, a i.e.,limited water number with aof limited ions that number are re- of ions that are external thermal supply, the diluted solution2+ can2+ be separated into two flows: the concen- sponsible for scalingresponsible (Ca2+, Mg for2+ scaling…). This (Ca technique, Mg is... conceptually). This technique similar to is RO. conceptually The main similar to RO. trated raw solution, which is sent to the FO unit, and the freshwater flow, which is further difference is theThe action main used difference to remove is the the action ions from used the to removesaltwater, the as ions shown from in the Figure saltwater, 22. as shown in filtered before Figurethe storage 22. [51]. This technology is in development phase.

2.6. Nanofiltration (NF) Nanofiltration is a membrane filtration process used to remove dissolved ions or or- ganic matter to produce soft water, i.e., water with a limited number of ions that are re- sponsible for scaling (Ca2+, Mg2+…). This technique is conceptually similar to RO. The main difference is the action used to remove the ions from the saltwater, as shown in Figure 22.

Figure 22. The Figureworking 22. principlesThe working and principlesschema of anda nanofiltration schema of a unit. nanofiltration unit.

NF is used in severalNF is usedapplications in several such applications as water and such wastewater, as water and pharmaceutical wastewater, pharmaceutical and and food processingfood [52]. processing The applications [52]. The for applications the desalination for the of desalinationseawater are of limited, seawater since are limited, since these semipermeablethese semipermeable membranes are membranes more porous, are more allowing porous, the allowing passage the of passagesome dis- of some dissolved solved solids [23].solids [23]. As shown in Figure 23, filtration technologies are classified according to the size of Figure 22. The workingthe particles principles and and molecules schema thatof a nanofiltration are stopped byunit. the membrane [36]. The prefix “Nano” is related to the pore sizes, ranging from 1 to 10 nanometers, so NF is usedsmaller in several than applications other filtration such techniques as water and (microfiltration wastewater, pharmaceutical and ultrafiltration) and but larger than food processingin [52]. the RO. The As applications a consequence, for th thise desalination technology removesof seawater mostly are divalentlimited, since ions (e.g., Ca2+ and these semipermeableMg2+), withmembranes an efficiency are more of between porous, 90%allowing and 98%. the passage The removal of some of monovalent dis- ions is solved solids [23].limited (between 60% and 85%) [2]. As the soft water produced by the NF process has a greater ion concentration than RO, a lower pressure gradient must be applied to the semipermeable membrane (between 34 and 48 bar) [2]. As NF requires a lower energy demand than RO, this solution is under investigation for seawater desalination, introducing a dual-stage unit [46,52]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 18 of 36

Appl. Sci. 2021, 11, 670 As shown in Figure 23, filtration technologies are classified according to the 18size of 36of the particles and molecules that are stopped by the membrane [36].

FigureFigure 23.23. FiltrationFiltration technologiestechnologies byby requiredrequired gradientgradient pressurepressure andand porousporous size.size. ReproducedReproduced withwith ElsevierElsevier LicenseLicense permissionpermission n.n. 49658212330434965821233043 [[36].36].

2.7. ElectrodialysisThe prefix “Nano” (ED) is related to the pore sizes, ranging from 1 to 10 nanometers, so smallerElectrodialysis than other filtration (ED) is an techniques electrochemical (microfiltration desalination and process.ultrafiltration) This technology but larger usesthan ain combination the RO. As a ofconsequence, semipermeable this technolo membranesgy removes and the mostly generation divalent of anions electric (e.g., Ca field2+ and to Appl. Sci. 2021, 11, x FOR PEER REVIEW 2+ 19 of 36 removeMg ), with the dissolvedan efficiency ions of from between the solution 90% an [d53 98%.]. The The working removal principles of monovalent are shown ions inis Figurelimited 24 (between[23,24]. 60% and 85%) [2]. As the soft water produced by the NF process has a greater ion concentration than RO, a lower pressure gradient must be applied to the semipermeable membrane (between 34 and 48 bar) [2]. As NF requires a lower energy demand than RO, this solution is under investigation for seawater desalination, introducing a dual-stage unit [46,52].

2.7. Electrodialysis (ED) Electrodialysis (ED) is an electrochemical desalination process. This technology uses a combination of semipermeable membranes and the generation of an electric field to re- move the dissolved ions from the solution [53]. The working principles are shown in Fig- ure 24 [23,24]. The electric field is generated by two electrodes, supplied in direct current voltage. Each ion has an electric charge (positive or negative). Due to the electric field, each ion is affected by an electric force directly proportional to the amplitude of the electric field and the value of the ion charge. The cations (positive ions, as Na+, Ca2+) are attracted by the anode, while the anions

(negative ions, as Cl−, HCO3−, CO32−) are attracted by the cathode [53]. Figure 24. TheFigure working 24. The principles working of principlesan Electrodialysis of an Electrodialysis desalination desalinationunit. unit.

Anionic and cationicThe electric semipermeable field is generated membrane bys two are electrodes,alternatively supplied installed in in direct the re- current voltage. gion between theEach two ion electrodes. has an electric The first charge one (positive allows the or negative).flow to the Due anions, to the the electric latter to field, each ion is the cations [23].affected In this byway, an the electric migration force directly of ions proportionalgenerated by to the the electric amplitude field ofis theselec- electric field and tively stopped theby the value semipermeable of the ion charge. membranes. As an example, during its motion to the anode (on the left inThe the cations picture) (positive a positive ions, ion as can Na+ cross, Ca2+ the) are cationic attracted membrane by the anode, but not while the anions − − 2− the anionic one.(negative Thus, the ions, electric as Cl field, HCO causes3 , COthe3 motion) are attractedof positive by charges the cathode to the [53 right]. side, while the anionicAnionic membrane and cationic stops the semipermeable motion to the membraneselectrode. In are the alternatively same way the installed in the negative ions migrateregion between to the cathode the two but electrodes. are stopped The firstby the one cationic allows themembranes. flow to the As anions, a the latter result, the ionsto are the confined cations inside [23]. the In this brine way, channels, the migration removing of ions ions from generated the freshwater by the electric field is channels (see Figureselectively 24) [24]. stopped by the semipermeable membranes. As an example, during its motion The first EDto theunits anode were (on commercially the left in the introduced picture) a in positive the early ion 1970s. can cross As a the solution cationic to membrane but avoid the deposition of salts on membranes, at regular intervals the polarity of electrodes is inverted for a few minutes, changing the motion of the ions inside the unit. The feed- water channels work temporarily as brine channels and vice versa [23]. This technology is currently used to produce freshwater from brackish water (salinity up to 2000 ppm) [10].

2.8. Capacitive Deionization (CDI) Like electrodialysis reversal, in Capacitive Deionization (CDI) an electric field is pro- duced between two carbon electrodes supplied with direct current voltage. As a conse- quence of the electric field, the dissolved ions are absorbed into the carbon micropores of the electrodes [54]. To regenerate them, a reverse voltage is applied, releasing ions from the electrodes to the saltwater. The co-ion adsorption phenomenon limits the efficiency of this technology, i.e., the adsorption of ions by electrodes having the same surface charge. To improve the energy efficiency, an anion exchange membrane and a cation exchange membrane could be installed on the electrodes, as shown in Figure 25 [54]. This solution is called Membrane Capacitive Deionization (MCDI) [55]. According to the literature, this technology requires a lower energy consumption for the desalination of brackish water and lower maintenance operations than the electrodi- alysis reversal units [56]. Despite these advantages, CDI is a process under investigation, as it is a recent introduction in the context of saltwater desalination [54,57]. Appl. Sci. 2021, 11, 670 19 of 36

not the anionic one. Thus, the electric field causes the motion of positive charges to the right side, while the anionic membrane stops the motion to the electrode. In the same way the negative ions migrate to the cathode but are stopped by the cationic membranes. As a result, the ions are confined inside the brine channels, removing ions from the freshwater channels (see Figure 24)[24]. The first ED units were commercially introduced in the early 1970s. As a solution to avoid the deposition of salts on membranes, at regular intervals the polarity of electrodes is inverted for a few minutes, changing the motion of the ions inside the unit. The feedwater channels work temporarily as brine channels and vice versa [23]. This technology is currently used to produce freshwater from brackish water (salinity up to 2000 ppm) [10].

2.8. Capacitive Deionization (CDI) Like electrodialysis reversal, in Capacitive Deionization (CDI) an electric field is produced between two carbon electrodes supplied with direct current voltage. As a consequence of the electric field, the dissolved ions are absorbed into the carbon micropores of the electrodes [54]. To regenerate them, a reverse voltage is applied, releasing ions from the electrodes to the saltwater. The co-ion adsorption phenomenon limits the efficiency of this technology, i.e., the adsorption of ions by electrodes having the same surface charge. Appl. Sci. 2021, 11, x FOR PEER REVIEW To improve the energy efficiency, an anion exchange membrane and20 aof cation36 exchange membrane could be installed on the electrodes, as shown in Figure 25[ 54]. This solution is called Membrane Capacitive Deionization (MCDI) [55].

Figure 25. TheFigure working 25. principlesThe working of a principlesCapacitive of Deionization a Capacitive unit. Deionization unit.

2.9. Hydration (HY)According to the literature, this technology requires a lower energy consumption for the desalination of brackish water and lower maintenance operations than the electrodialy- Desalination by Hydration (HY) is based on the production of gas hydrates, which sis reversal units [56]. Despite these advantages, CDI is a process under investigation, as it are crystalline solids composed of water (host) and gas (guest) molecules like nitrogen, is a recent introduction in the context of saltwater desalination [54,57]. carbon dioxide and methane [58,59]. The dissociation of 1 m3 of hydrates can produce 0.8 m3 of water and2.9. 164 Hydration m3 of gas (HY) in standard conditions [18]. As the generationDesalination of hydrates by Hydrationrequires less (HY) severe is based thermodynamic on the production conditions of gas (T hydrates, < 20 which are °C and P > 30crystalline bar) than solidsother composedphase transiti of wateron desalination (host) and gastechni (guest)ques, molecules the idea behind like nitrogen, carbon HY desalinationdioxide is the and production methane of [58 hydrates,59]. The and dissociation then the ofseparation 1 m3 of hydrates into the cancompo- produce 0.8 m3 of nents (gas andwater water). and A 164schema m3 of of gas a Hydration in standard plant conditions is reported [18]. in Figure 26 [18]. As the generation of hydrates requires less severe thermodynamic conditions (T < 20 ◦C and P > 30 bar) than other phase transition desalination techniques, the idea behind HY desalination is the production of hydrates and then the separation into the components (gas and water). A schema of a Hydration plant is reported in Figure 26[18]. In detail, after a preliminary refrigeration seawater is mixed inside a reactor with a mixture of propane and carbon dioxide. This mixture is transferred into the crystallizer, where hydrates are formed thanks to the low temperature and high pressure. The hydrate slurry is separated from the brine and transferred into a decomposer. Thanks to the heat

Figure 26. A schema of a Hydration desalination plant.

In detail, after a preliminary refrigeration seawater is mixed inside a reactor with a mixture of propane and carbon dioxide. This mixture is transferred into the crystallizer, where hydrates are formed thanks to the low temperature and high pressure. The hydrate slurry is separated from the brine and transferred into a decomposer. Thanks to the heat supply, hydrates are converted into freshwater and gas. The last one is recovered to be reused to produce hydrates [18]. This technology should require a lower energy expenditure in comparison with MSF and RO, however no commercial plants are available, as capital costs are high [18].

Appl. Sci. 2021, 11, x FOR PEER REVIEW 20 of 36

Figure 25. The working principles of a Capacitive Deionization unit.

2.9. Hydration (HY)

Appl. Sci. 2021, 11, 670 Desalination by Hydration (HY) is based on the production of gas hydrates, which 20 of 36 are crystalline solids composed of water (host) and gas (guest) molecules like nitrogen, carbon dioxide and methane [58,59]. The dissociation of 1 m3 of hydrates can produce 0.8 m3 of water and 164 m3 of gas in standard conditions [18]. As the generationsupply, hydrates of hydrates are converted requires le intoss severe freshwater thermodynamic and gas. The conditions last one (T is recovered< 20 to be °C and P > 30reused bar) tothan produce other hydratesphase transiti [18]. on desalination techniques, the idea behind HY desalination Thisis the technology production should of hydrates require and a lower then energy the separation expenditure into in the comparison compo- with MSF nents (gas andand water). RO, however A schema no of commercial a Hydration plants plant are is available,reported in as Figure capital 26 costs [18]. are high [18].

Appl. Sci. 2021, 11, x FOR PEER REVIEW 21 of 36

Figure 26. A schema of a Hydration desalination plant. Figure 26. A schema of a Hydration desalination plant. 2.10. Secondary2.10. Refrigerant Secondary Freezing Refrigerant (SRF) Freezing (SRF) In detail, after a preliminary refrigeration seawater is mixed inside a reactor with a Secondary Refrigerant Freezing (SRF) is a desalination process based on the liquid mixtureSecondary of propane Refrigerant and carbon Freezing dioxide. (SRF) This is mixturea desalination is transferred process into based the on crystallizer, the liquid solid phase transition [60]. As the ice formed contains a limited quantity of salts, this wheresolid phase hydrates transition are formed [60]. Asthanks the ice to theformed low temperaturecontains a limited and high quantity pressure. of salts, The this hydrate tech- technique can be used to produce freshwater from seawater. A refrigerant is used to freeze slurrynique iscan separated be used tofrom produce the brine freshwater and transferred from seawater. into a Adecomposer. refrigerant isThanks used to to freeze the heat the the saline water. The main problem is the removal of the ice produced in the process [28]. supply,saline water. hydrates The aremain converted problem into is the freshwater removal ofand the gas. ice Theproduced last one in theis recovered process [28]. to be A A solution proposed by Lin et al. suggests the utilization of low temperatures available for reusedsolution to proposedproduce hydrates by Lin et [18]. al. suggests the utilization of low temperatures available for the regasificationthe regasification of LNG (Liquid of LNG Natural (Liquid Gas) Natural to freeze Gas) seawater to freeze and seawater obtain andice [60]. obtain ice [60]. This technologyAnother should solution require is a depictedlower energy in Figure expenditure 27[ 28]. in Thiscomparison system with is composed MSF of two and RO,Another however solution no commercial is depicted plants in Figure are 27available, [28]. This as system capital iscosts composed are high of [18]. two cham- bers, a reversingchambers, heat pump a reversing and solenoid heat pump valves. and The solenoid unit works valves. by alternatively The unit works produc- by alternatively producing ice and freshwater in the tank on the left (L) or in the other one on the right (R). ing ice and freshwater in the tank on the left (L) or in the other one on the right (R).

Figure 27.Figure A schema 27. A of schema a Secondary of a Secondary Refrigerant Refrigerant Freezing Freezing desalination desalination unit. unit.

In the first case seawater is introduced in tank L. The heat pump is used to transfer heat from this chamber to the other one. As a result of the heat transfer, saline water is converted in a slurry of ice and brine inside tank L, while in tank R ice already formed in a previous step is melted. Stopping the process, brine is drained out by opening the valve in the bottom of the tank L while tank R is refilled with saline water. Reverting the heat pump, inside the tank L ice is melted thanks to the heat supply, producing freshwater. In the meantime, inside the tank R further ice is produced, and the cycle is repeated. This desalination technique is currently under development [28].

2.11. Membrane Distillation (MD) Membrane Distillation (MD) is a desalting process based on hydrophobic mem- branes. These kinds of membranes can be crossed by water as vapor molecules [61]. MD is theoretically able to reject all non-volatile solutes (like salts). The main draw- back of the MD process is the large amount of energy that is consumed during the liquid– vapor phase change process and the incomplete recovery of the latent heat. For these rea- sons, the MD process is energy-inefficient if used as a standalone system [62]. However, this technology works at lower temperatures than other thermal-driven phase transition technologies (MSF, MED). Similarly, the required pressure is lower than other technology based on membranes (RO). MD can be run by using low-grade waste heat as an energy source while operating at a low pressure and shows a negligible sensitivity to varying feed salinity. As an alter- native, solar radiation can be used to supply MD units [28]. Appl. Sci. 2021, 11, 670 21 of 36

In the first case seawater is introduced in tank L. The heat pump is used to transfer heat from this chamber to the other one. As a result of the heat transfer, saline water is converted in a slurry of ice and brine inside tank L, while in tank R ice already formed in a previous step is melted. Stopping the process, brine is drained out by opening the valve in the bottom of the tank L while tank R is refilled with saline water. Reverting the heat pump, inside the tank L ice is melted thanks to the heat supply, producing freshwater. In the meantime, inside the tank R further ice is produced, and the cycle is repeated. This desalination technique is currently under development [28].

2.11. Membrane Distillation (MD) Membrane Distillation (MD) is a desalting process based on hydrophobic membranes. These kinds of membranes can be crossed by water as vapor molecules [61]. MD is theoretically able to reject all non-volatile solutes (like salts). The main drawback of the MD process is the large amount of energy that is consumed during the liquid–vapor phase change process and the incomplete recovery of the latent heat. For these reasons, the MD process is energy-inefficient if used as a standalone system [62]. However, this technology works at lower temperatures than other thermal-driven phase transition technologies (MSF, MED). Similarly, the required pressure is lower than other technology based on membranes (RO). Appl. Sci. 2021, 11, x FOR PEER REVIEW 22 of 36 MD can be run by using low-grade waste heat as an energy source while operating at a low pressure and shows a negligible sensitivity to varying feed salinity. As an alternative, solar radiation can be used to supply MD units [28]. MD units can beMD assembled units can in be four assembled configurations, in four configurations,as shown in Figure as shown 28 [62,63]. in Figure The 28[ 62,63]. The simplest one issimplest the Direct one Contact is the DirectMembrane Contact Distillation Membrane (DCMD), Distillation where (DCMD), two solutions where two solutions are in direct contactare in with direct the contact hydrophobic with the membrane. hydrophobic Due membrane. to the difference Due to of the pressure difference of pressure between the twobetween solutions, the the two vapor solutions, produced the vaporon the producedhot solution on surface the hot can solution cross the surface can cross membrane, goingthe inside membrane, the cold going solution. inside This the technology cold solution. is commonly This technology used in desalina- is commonly used in tion and concentrationdesalination processes and concentration of aqueous solutions processes in of the aqueous food industry solutions [64–66]. in the food industry [64–66].

Figure 28. TheFigure possible 28. The configurations possible configurations of a Membrane of a MembraneDistillation Distillation(MD) unit. (MD) unit.

In Air Gap MembraneIn Air Gap Distillation Membrane (AGMD) Distillation a layer (AGMD) of stagnant a layer air ofis added stagnant between air is added between the membrane theand membrane the cold solution. and the coldThe solution.reason is Thethe reasonreduction is the of reductionthe thermal of theenergy thermal energy ex- expenditure, aspenditure, the air gap as increases the air gap the increases thermal resistance the thermal between resistance the between two fluids. the How- two fluids. However, ever, the addingthe of addingthe air ofgap the obstructs air gap obstructsthe mass theflow mass through flow the through membrane. the membrane. This tech- This technology nology can be usedcan be in useddesalination in desalination applicatio applicationn or to remove or to remove volatile volatile compounds compounds from an from an aqueous aqueous solutionsolution [63,67]. [63 ,67]. Another technology is the Sweeping Gas Membrane Distillation (SGMD), where the hydrophobic membrane separates the hot solution from a sweep gas. Like the AGMD, the thermal efficiency is higher than the DCMD. The mass transfer is promoted by the sweep gas since it is not stationary. As a disadvantage, a large condenser is required. This tech- nique can be used to remove volatile compounds from aqueous solutions [68]. The last technique is the Vacuum Membrane Distillation (VMD), where the sweep gas (or the air gap) is replaced by the vacuum produced by a pump [69]. The condensation is externally realized by the distillation unit. This solution has a high thermal efficiency and is used to separate volatile aqueous solutions [70]. A common problem of all membrane distillation technologies is due to fouling and wetting phenomena, which are in part controlled by using different strategies like pre- treatments [71], surface modification [72], increasing flow rate to promote turbulence [73], and periodical hydraulic and chemical cleanings [74–76].

2.12. Ion-Exchange Resin (IXR) The term “ion-exchange resin” (IXR) indicates a variety of organic compounds, which have been chemically treated to react with the ions of a solution, capturing ions from the solution and releasing other ions from the resin into the solution. In the past, zeolites were used, i.e., minerals having this characteristic. Ion-exchange resins are used in industrial and domestic applications like soft water production, sugar purification, and the extraction of precious elements, such as gold, sil- ver, and uranium from mineral ores. IXR can be classified according to functional group [77]: Appl. Sci. 2021, 11, 670 22 of 36

Another technology is the Sweeping Gas Membrane Distillation (SGMD), where the hydrophobic membrane separates the hot solution from a sweep gas. Like the AGMD, the thermal efficiency is higher than the DCMD. The mass transfer is promoted by the sweep gas since it is not stationary. As a disadvantage, a large condenser is required. This technique can be used to remove volatile compounds from aqueous solutions [68]. The last technique is the Vacuum Membrane Distillation (VMD), where the sweep gas (or the air gap) is replaced by the vacuum produced by a pump [69]. The condensation is externally realized by the distillation unit. This solution has a high thermal efficiency and is used to separate volatile aqueous solutions [70]. A common problem of all membrane distillation technologies is due to fouling and wetting phenomena, which are in part controlled by using different strategies like pretreat- ments [71], surface modification [72], increasing flow rate to promote turbulence [73], and periodical hydraulic and chemical cleanings [74–76].

2.12. Ion-Exchange Resin (IXR) The term “ion-exchange resin” (IXR) indicates a variety of organic compounds, which have been chemically treated to react with the ions of a solution, capturing ions from the solution and releasing other ions from the resin into the solution. In the past, zeolites were used, i.e., minerals having this characteristic. Ion-exchange resins are used in industrial and domestic applications like soft water Appl. Sci. 2021, 11, x FOR PEER REVIEW production, sugar purification, and the extraction of precious elements,23 of such 36 as gold, silver,

and uranium from mineral ores. IXR can be classified according to functional group [77]: • Strongly acid,• realizedStrongly with acid, sulfonic realized acid with groups sulfonic acid groups • Strongly basic,• Stronglybased on basic, quaternary based onamino quaternary groups amino groups • Weakly acidic,• Weaklyrealized acidic, with carboxylic realized with acid carboxylic groups acid groups • Weakly basic,• basedWeakly on basic, primary, based seco onndary, primary, or tertiary secondary, amino or tertiary groups. amino groups. The acid resinsThe (also acid called resins cation (also resins) called are cation designed resins) to are capture designed positive to capture ions (Ca positive2+, ions (Ca2+, Na+, Mg2+, K+, MnNa2++, MgFe3+2+…),K and+, Mn release2+, Fe H3++ ...ions.) andConsequently, release H+ theions. hardness Consequently, of the water the hardness of the is reduced, andwater the acidity is reduced, is increased, and the since acidity the ispH increased, is increased since by the thepH greater is increased concen- by the greater tration of H+ ions.concentration The basic resins of H +(alsoions. called The basicanion resins resins) (also are utilized called anion to capture resins) negative are utilized to capture − 2− 2− − 2− − ions such as Cl−negative, NO32−, SO ions42−, such SiO2 as−, CO Cl 32,− NOand3 release, SO4 OH,− SiO ions.2 , CO3 and release OH ions. This technologyThis was technology developed was at the developed end of the at 1960s the end [78]. of theA classical 1960s [78 arrangement]. A classical arrangement of a desalinationof plant a desalination based on plantIXR is based reported on IXR in Figure is reported 29. in Figure 29.

Figure 29. A schemaFigure of an 29. Ion-ExchangeA schema of anResin Ion-Exchange desalination Resin plant. desalination plant.

The saline waterThe flows saline firstly water through flows firstly the throughweakly acid the weakly resin and acid the resin strongly and the acid strongly acid resin. resin. After thisAfter step, this the step, water’s the water’sacidity is acidity increased. is increased. A degasser A degasser is required, is required, as bicar- as bicarbonates + bonates inside insidethe water the waterreact reactwith withH+ ions, H ions,producing producing carbon carbon dioxide. dioxide. After After this this step, step, water flows water flows throughthrough the the weakly weakly basic basic resin resin and and the thestrongly strongly basic basic resin, resin, reducing reducing the thewa- water’s acidity. ter’s acidity. An amphoteric resin (a mix of acid and basic) is normally added to complete the removal of ions. During the normal process, resins are progressively saturated by the ion exchange. Thence, a regeneration is periodically required to restore the resins. The regeneration uses acid solutions (H2SO4 and HCl) for the acid resins and basic solutions (NaOH and NH4OH) for the basic resins.

2.13. Solar Still Distillation (SSD) Solar Still Distillation (SSD) can be realized by using a blackened tank containing saline, water and air [28,79]. The device is covered with inclined glass. In this way the solar radiation enters the system, increasing the temperature and facilitating the evapora- tion of freshwater. The internal humidity condenses on the surface of the glass, as this part has the lower temperature. The condensate is collected, obtaining freshwater [80]. A pos- sible solution is depicted in Figure 30 [23,24]. The condensate is characterized by being high quality, with a daily production about 2–3 l./m2. Consequently, this system can be used only in small applications. Appl. Sci. 2021, 11, 670 23 of 36

An amphoteric resin (a mix of acid and basic) is normally added to complete the removal of ions. During the normal process, resins are progressively saturated by the ion exchange. Thence, a regeneration is periodically required to restore the resins. The regeneration uses acid solutions (H2SO4 and HCl) for the acid resins and basic solutions (NaOH and NH4OH) for the basic resins.

2.13. Solar Still Distillation (SSD) Solar Still Distillation (SSD) can be realized by using a blackened tank containing saline, water and air [28,79]. The device is covered with inclined glass. In this way the solar radiation enters the system, increasing the temperature and facilitating the evaporation of freshwater. The internal humidity condenses on the surface of the glass, as this part has the lower temperature. The condensate is collected, obtaining freshwater [80]. A possible Appl. Sci. 2021, 11, x FOR PEER REVIEW 24 of 36 solution is depicted in Figure 30[ 23,24]. The condensate is characterized by being high Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 24 of 36 quality, with a daily production about 2–3 l./m . Consequently, this system can be used only in small applications.

Figure 30. A Solar Still Distillation unit. Figure 30. A Solar Still Distillation unit. 2.14. Solar Chimney (SC) 2.14. Solar Chimney (SC) A Solar Solar Chimney Chimney (SC) (SC) desalination desalination unit unit can can be assembled be assembled in the in way the shown way shown in Figure in Figure31 [28].A 31 SolarA [large28 ].Chimney Asolar large collector (SC) solar desalination collector is used isto used convertunit can to convert the be assembledsolar the radiation solar in radiationthe into way the shown into kinetic the in energy kineticFigure energyof31 air, [28]. thanks ofA air,large thanksto solarthe shape to collector the of shape the is chimney, used of the to chimney, convert which whichisthe realized solar is realizedradiation in a transparent in into a transparent the materialkinetic material energy (glass (glassorof plastic).air, orthanks plastic). The to air the The flow shape air inside flow of the insidethe chimney, system the system can whic beh can used is realized be to used produce in to a produce transparent electricity electricity ifmaterial a small if a (glass smallwind windturbineor plastic). turbine is installed The is air installed flow [81]. insideThe [81 ].solar the The systemcollector solar collector can is composedbe used is composed to produceof several of electricity several small SSD small if aunits; small SSD in units;wind this inwayturbine this the way issolar installed the source solar [81]. sourceis also The used issolar also to collector usedproduce to produceis freshwater. composed freshwater. ofThis several technology This small technology SSD is under units; is investi- underin this investigationgationway the [28]. solar [source28]. is also used to produce freshwater. This technology is under investi- gation [28].

Figure 31. A Solar Chimney desalination unit. Figure 31. A Solar Chimney desalination unit. 2.15. Humidification–Dehumidification (HDH) 2.15. Humidification–Dehumidification (HDH) The Humidification Dehumidification (HDH) system is a recent carrier gas based thermalThe desalination Humidification technique. Dehumidification In detail, fr eshwater(HDH) system can be isobtained a recent by carrier condensing gas based the airthermal humidity. desalination The essential technique. components In detail, are fr eshwaterthe humidifier can be and obtained the dehumidifier. by condensing Lawal the etair al. humidity. proposed The two essential possible components solutions, adding are the a humidifier heat pump and to improvethe dehumidifier. the energy Lawal effi- ciencyet al. proposed [82]. two possible solutions, adding a heat pump to improve the energy effi- ciencyBoth [82]. solutions are equipped with three different circuits: air, water and freon. The last oneBoth is solutionsconfined areinside equipped the pipes with and three the diffmainerent components circuits: air, of thewater heat and pump freon. (com- The pressor,last one condenser,is confined lamination inside the valvepipes andand evaporthe mainator). components The air is recirculatedof the heat pumpby a fan (com- in a closepressor, loop, condenser, going through lamination two chambers, valve and where evapor theator). humidification The air is recirculated and the dehumidifica- by a fan in a tionclose occur. loop, goingOnly thethrough water two circuit chambers, is open, where as saline the humidification water is the input, and the while dehumidifica- brine and freshwatertion occur. areOnly the the outputs water [82]. circuit is open, as saline water is the input, while brine and freshwaterAs shown are thein the outputs schema [82]. reported in Figure 32, saline feedwater is firstly refrigerated by theAs condenser shown in ofthe the schema heat pump. reported The in cold Figure saline 32, watersaline isfeedwater used to promote is firstly therefrigerated conden- sationby the ofcondenser air humidity of the inside heat thepump. dehumidifi The coldcation saline chamber water is (on used the to right), promote where the freshwa- conden- tersation is produced. of air humidity During inside this theprocess, dehumidifi the salinecation water chamber temperature (on the right), increases. where After freshwa- that, ter is produced. During this process, the saline water temperature increases. After that, Appl. Sci. 2021, 11, 670 24 of 36

2.15. Humidification–Dehumidification (HDH) The Humidification Dehumidification (HDH) system is a recent carrier gas based thermal desalination technique. In detail, freshwater can be obtained by condensing the air humidity. The essential components are the humidifier and the dehumidifier. Lawal et al. proposed two possible solutions, adding a heat pump to improve the energy efficiency [82]. Both solutions are equipped with three different circuits: air, water and freon. The last one is confined inside the pipes and the main components of the heat pump (compressor, condenser, lamination valve and evaporator). The air is recirculated by a fan in a close loop, going through two chambers, where the humidification and the dehumidification occur. Only the water circuit is open, as saline water is the input, while brine and freshwater are Appl. Sci. 2021, 11, x FOR PEER REVIEW 25 of 36 the outputs [82]. As shown in the schema reported in Figure 32, saline feedwater is firstly refrigerated by the condenser of the heat pump. The cold saline water is used to promote the condensation saline feedwaterof air is humiditynebulized insideinside thethe dehumidificationhumidification chamber chamber (on (on the the left), right), promoting wherefreshwater is the evaporationproduced. of feedwater. During this process, the saline water temperature increases. After that, saline In the solutionfeedwater reported is nebulized in Figure inside 32, the the thermal humidification supply from chamber the condenser (on the is left), trans- promoting the ferred to the salineevaporation water after of feedwater. the dehumidifier unit.

Figure 32.FigureA Humidification–Dehumidification 32. A Humidification–Dehumidification (HDH) unit (HDH) using unit a heat using pump a heat with pump a water with refrigerated a water re- condenser. frigerated condenser. In the solution reported in Figure 32, the thermal supply from the condenser is The alternativetransferred solution to the proposed saline water by the after same the authors dehumidifier is reported unit. in Figure 33, where the thermal supplyThe from alternative the condenser solution is proposedtransferred by to the the same air coming authors from is reported the humidi- in Figure 33, where fier unit [82]. theInstead thermal of the supply forced from air circulation, the condenser a natural is transferred air circulation to the air system coming has from been the humidifier proposed, whereunit the [82 ].thermal Instead supply of the is forced produced air circulation, by a solar panel a natural [28,83,84]. air circulation In any case, system has been HDH desalinationproposed, is a technology where the thermalunder investigation. supply is produced by a solar panel [28,83,84]. In any case, HDH desalination is a technology under investigation.

Figure 33. HDH unit using a heat pump with an air refrigerated condenser.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 25 of 36

saline feedwater is nebulized inside the humidification chamber (on the left), promoting the evaporation of feedwater. In the solution reported in Figure 32, the thermal supply from the condenser is trans- ferred to the saline water after the dehumidifier unit.

Figure 32. A Humidification–Dehumidification (HDH) unit using a heat pump with a water re- frigerated condenser.

The alternative solution proposed by the same authors is reported in Figure 33, where the thermal supply from the condenser is transferred to the air coming from the humidi- Appl. Sci. 2021, 11, 670fier unit [82]. Instead of the forced air circulation, a natural air circulation system has been 25 of 36 proposed, where the thermal supply is produced by a solar panel [28,83,84]. In any case, HDH desalination is a technology under investigation.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 26 of 36

3. Statistics on Desalination As introduced in the previous section, several technologies have been proposed for desalination. The research is currently investigating new solutions to reduce energy con- sumption and improve environmental sustainability, considering renewables as an en- ergy source to supply the process. Many technologies described above are under investigation, so limited data are avail- Figureable. Considering33.Figure HDH unit 33. HDH theusing commercial unit a heat using pump a technologies heat with pump an air with refrigerated fo anr desalination, air refrigerated condenser. the condenser. trends of the installed capacity and number of plants are reported in Figure 34 [12]. As introduced above, ther- mally driven3. desalination Statistics on plants Desalination (MSF and MED) dominated this sector until the 1990s [22,36]. As the freshwaterAs introduced demand in theis still previous growing, section, new plants several are technologies being installed. have MSF been proposed continues tofor grow desalination. linearly, while The researchthe installed is currently capacity investigating of RO is increasing new solutions very quickly to reduce energy [85]. consumption and improve environmental sustainability, considering renewables as an In [12],energy Jones et. source al. reported to supply some the process.statistics on the current big desalination plants installed around Manythe world, technologies using data described from the aboveGlobal are Water under Intelligence investigation, (GWI) so [86]. limited In data are detail, GWI available.is a database Considering containing the information commercial on technologiesthe installed desalination for desalination, plants, the in- trends of the cluding theirinstalled status, operational capacity and years, number installed of plants capacity, are reported geographic in Figure position, 34[ technol-12]. As introduced ogy, and waterabove, sources. thermally driven desalination plants (MSF and MED) dominated this sector until Consideringthe 1990s the [desalination22,36]. As the technology, freshwater demandthe global is stillinstalled growing, capacity new plants (95.37 are × 10 being6 installed. m3/day) is composedMSF continues of RO to(68.7%), grow MSF linearly, (17.6%) while MED the (6.9%) installed NF capacity(3.4%) ED of (2.4%), RO is increasingand very other (1.0%)quickly [12]. [85].

Figure 34. TheFigure trend of34. installed The trend capacity of installed and operative capacity desalinationand operative plants. desalination Reproduced plants. with Reproduced Elsevier License with permission n. 4965830049121Elsevier [12]. License permission n. 4965830049121 [12].

Figure 35 reports graphically the worldwide distribution of desalination plants, con- sidering size, technology and the water source used to produce freshwater [12]. Most of desalination plants are installed along the coastline to adopt the sea as a wa- ter source. Indeed, 60.8% of world’s installed capacity uses the sea as a water source, whereas brackish water is utilized in 20.6% of cases [12]. Appl. Sci. 2021, 11, 670 26 of 36

In [12], Jones et al. reported some statistics on the current big desalination plants installed around the world, using data from the Global Water Intelligence (GWI) [86]. In detail, GWI is a database containing information on the installed desalination plants, including their status, operational years, installed capacity, geographic position, technology, and water sources. Considering the desalination technology, the global installed capacity (95.37 × 106 m3/day) is composed of RO (68.7%), MSF (17.6%) MED (6.9%) NF (3.4%) ED (2.4%), and other (1.0%) [12]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 27 of 36 Figure 35 reports graphically the worldwide distribution of desalination plants, con- sidering size, technology and the water source used to produce freshwater [12].

Figure 35. A map of desalination plants around the world, by sizesize and technology. Reproduced with Elsevier License permission n. 4965830049121 [12]. [12].

TheMost Middle of desalination East and plantsNorth are Africa installed are currently along the equipped coastline towith adopt 4826 the plants, sea as with a water an installedsource. Indeed, capacity 60.8% equal of to world’s 45.32 ×installed 106 m3/day capacity (47.5% uses of the the world’s sea as ainstalled water source, capacity). whereas The remainingbrackish water desalination is utilized plants in 20.6% are ofdistributed cases [12]. in the following way: 3505 plants in East Asia Theand Middlethe Pacific East (18.4% and North of worldwide Africa are currentlyinstalled equippedcapacity), with2341 4826 in North plants, America with an (11.9%),installed 2337 capacity in Western equal toEurope 45.32 (9.2%),× 106 m 13733/day in Latin (47.5% America of the world’s and the installed Caribbean capacity). (5.7%), 655The in remaining Southern desalinationAsia (3.1%), 566 plants in Eastern are distributed Europe and in theCentral following Asia (2.4), way: and 3505 303 plants in Sub- in SaharanEast Asia Africa and the (1.9%) Pacific [12]. (18.4% The main of worldwide sectors are installed industry capacity), (7757 plants 2341 with in North a capacity America of 28.8(11.9%), × 106 2337 m3/day) in Western and municipality Europe (9.2%), (6055 1373 plants, in Latin 59.39 America× 106 m3/day). and the Caribbean (5.7%), 655 inTable Southern 1 shows Asia statistics (3.1%), on 566 the in state Eastern of the Europe art of operative and Central desalination Asia (2.4), plants and 303[5,12– in 14,31,87].Sub-Saharan According Africa (1.9%) to these [12 ].data, The RO main is sectorsthe most are adaptable industry (7757solution plants to exploit with a capacitysea and 6 3 6 3 brackishof 28.8 × water,10 m with/day) an and average municipality capacity (6055ranging plants, from 59.39 1000× (or10 lower)m /day). to 320,000 m3/day. As forTable the energy1 shows supply, statistics RO exclusively on the staterequires of electricity the art of(or mechanical operative desalinationenergy) to run theplants pumps[5,12 along–14,31 ,the87] .water According circuits. to This these feature data, ROsimplifies is the the most potential adaptable coupling solution with to renewableexploit sea energy and brackish sources, water, as demonstrated with an average by capacityseveral studies ranging [19,36,87,88]. from 1000 (or The lower) water to 3 cost320,000 is lower m /day. than As other for thedesalination energy supply, technologies, RO exclusively explaining requires its quick electricity diffusion (or mechani- around thecal energy)world. to run the pumps along the water circuits. This feature simplifies the potential coupling with renewable energy sources, as demonstrated by several studies [19,36,87,88]. TableThe water1. The state cost isof lowerthe art thanof commercial other desalination desalination technologies, technologies. explaining its quick diffusion around the world. Average Energy Consumption Recovery Water Quality Water Cost Technology Capacity Input Electrical Thermal Ratio [ppm] [$/m3] [103 m3/day] [kWh/m3] [kJ/kg] MED 0.6–30 SW 0.25 10 1.5–2.5 230–390 0.52–1.5 TVC 10–35 SW 0.25 10 1.5–2.5 145–390 0.87–0.95 MSF 50–70 SW 0.22 10 4–6 190–390 0.56–1.75 MVC 0.1–3 SW 10 6–12 no 2.0–2.6 SWRO 1–320 SW 0.42 400–500 3–6 no 0.45–1.72 BWRO Up to 98 BW 0.65 200–500 1.5–2.5 no 0.26–1.33 ED Up to 145 BW 0.9 150–500 2.64–5.5 no 0.6–1.05 Note: SW is the abbreviation of Seawater; BW brackish water.

To improve the energy efficiency of desalination technologies, exergy analysis has been recently proposed in several researches [89–94]. Introducing the Second Thermodynamic Law, exergy represents the maximal available energy related to a system, Appl. Sci. 2021, 11, 670 27 of 36

Table 1. The state of the art of commercial desalination technologies.

Energy Consumption Recovery Technology Average Capacity Input Water Quality Water Cost [103 m3/day] Ratio [ppm] Electrical Thermal [$/m3] [kWh/m3] [kJ/kg] MED 0.6–30 SW 0.25 10 1.5–2.5 230–390 0.52–1.5 TVC 10–35 SW 0.25 10 1.5–2.5 145–390 0.87–0.95 MSF 50–70 SW 0.22 10 4–6 190–390 0.56–1.75 MVC 0.1–3 SW 10 6–12 no 2.0–2.6 SWRO 1–320 SW 0.42 400–500 3–6 no 0.45–1.72 BWRO Up to 98 BW 0.65 200–500 1.5–2.5 no 0.26–1.33 ED Up to 145 BW 0.9 150–500 2.64–5.5 no 0.6–1.05 Note: SW is the abbreviation of Seawater; BW brackish water.

To improve the energy efficiency of desalination technologies, exergy analysis has been recently proposed in several researches [89–94]. Introducing the Second Thermodynamic Law, exergy represents the maximal available energy related to a system, obtainable if re- versible processes are only used to achieve equilibrium conditions within the environment (dead state). On the contrary, exergy also represents the minimal energy required by a system to achieve a desired condition, starting from the environmental state, and adopting only reversible processes. In this optic, the efficiency analysis performed according to the Second Thermody- namic Law is a successful tool for suggesting solutions to improve the energy efficiency of the existing desalination technologies. A detailed exergy analysis is reported in [89], where each component is firstly modelled and then analyzed. In the case of desalination plants requiring a mechanical energy input (or electric- min ity), the exergy efficiency is evaluated by Equation (3), defining Wleast as the least work of separation required for an infinitesimal recovery ratio (RR, defined in the previous Equation (2)). Wmin Wmin Wmin η = least = least = least (3) ex W min TDS RDS sep Wleast + τ0Sgen Wleast + τ0Sgen

Wsep is the work required by the real desalination plant, which can be expressed as TDS the sum of the least work, plus the total entropy generation Sgen (referred to as the Total Dead State), multiplied by the absolute environmental temperature τ0. min According to the definition of Wleast, this value is achievable if all processes are reversible and the freshwater production is infinitesimal. As a real process is designed to produce a significant amount of freshwater, the least work demand is introduced Wleast. min This term has a greater value than Wleast, as it includes the entropy generation associated RDS with the condition RR > 0. The remaining entropy generation Sgen (related to the Restricted Dead State) is associated only to the irreversibility of the adopted technologies (finite thermal differences, pressure drops, etc.). The trend of Wleast is reported in Figure 36, as a function of the recovery ratio and the min salinity concentration of freshwater and feed water. As introduced before, Wleast is equal to Wleast if the recovery ratio is equal to zero. In the case of seawater (salinity 35 g/kg), zero ◦ min salinity in produced freshwater and environmental temperature fixed to 25 C, Wleast is equal to 2.71 kJ/kg (of freshwater) [89,95]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 28 of 36

obtainable if reversible processes are only used to achieve equilibrium conditions within the environment (dead state). On the contrary, exergy also represents the minimal energy required by a system to achieve a desired condition, starting from the environmental state, and adopting only reversible processes. In this optic, the efficiency analysis performed according to the Second Thermodynamic Law is a successful tool for suggesting solutions to improve the energy efficiency of the existing desalination technologies. A detailed exergy analysis is reported in [89], where each component is firstly modelled and then analyzed. In the case of desalination plants requiring a mechanical energy input (or electricity), the exergy efficiency is evaluated by Equation (3), defining 𝑊 as the least work of separation required for an infinitesimal recovery ratio (RR, defined in the previous Equation (2)). 𝑊 𝑊 𝑊 𝜂 = = = (3) 𝑊 𝑊 +𝜏𝑆 𝑊 +𝜏𝑆

𝑊 is the work required by the real desalination plant, which can be expressed as the sum of the least work, plus the total entropy generation 𝑆 (referred to as the Total Dead State), multiplied by the absolute environmental temperature 𝜏. According to the definition of 𝑊 , this value is achievable if all processes are reversible and the freshwater production is infinitesimal. As a real process is designed to produce a significant amount of freshwater, the least work demand is introduced 𝑊. This term has a greater value than 𝑊, as it includes the entropy generation associated with the condition RR > 0. The remaining entropy generation 𝑆 (related to the Restricted Dead State) is associated only to the irreversibility of the adopted technologies (finite thermal differences, pressure drops, etc.). The trend of 𝑊 is reported in Figure 36, as a function of the recovery ratio and the salinity concentration of freshwater and feed water. As introduced before, 𝑊 is Appl. Sci. 2021, 11, 670 equal to 𝑊 if the recovery ratio is equal to zero. In the case of seawater (salinity28 of 35 36 g/kg), zero salinity in produced freshwater and environmental temperature fixed to 25 °C, 𝑊 is equal to 2.71 kJ/kg (of freshwater) [89,95].

Figure 36. Least work as function of recovery ratioratio andand salinitysalinity ofof feedwaterfeedwater andand freshwater.freshwater. Repro- ducedReproduced from [ 95from] (CC [95] BY-NC-SA (CC BY-NC-SA 3.0). 3.0).

In the casecase ofof desalinationdesalination plants supplied by thermal energy, EquationEquation (5)(5) cancan bebe min adopted, where Q𝑄least is is the the least least thermal thermal energyenergy requiredrequired withwith anan infinitesimalinfinitesimal recoveryrecovery ratio. This term depends also on the temperature of the thermal source τH, introducing the   Carnot factor 1 − τ0 , which physically represents the ratio of heat that can be converted τH into exergy, using a reversible thermal machine.

 −1 min min τ0 Qleast = Wleast 1 − (4) τH

So, the exergy efficiency can be evaluated by Equation (5), defining Qsep as the thermal requirement for the real plant.

Qmin Qmin Qmin η = least = least = least (5) ex Q  −1  −1 sep Qmin + τ STDS 1 − τ0 Q + τ SRDS 1 − τ0 least 0 gen τH least 0 gen τH

Similarly, the term Qleast is introduced, representing the least thermal input require- ment, assuming significant freshwater production (RR > 0). Mistry et al. analyzed the energy performances of developed desalination technolo- gies, obtaining the results reported in Figure 37 (based on data reported in [89]). The graph reports the exergy efficiency according to Equation (3) for technologies requiring mechani- cal (or electrical) energy and Equation (5) in case of thermally supplied technologies. The authors considered a seawater salinity equal to 4.2% in case of MED and MSF units, and 3.5% (common seawater) in the other cases. The graph emphasizes the high exergy efficiency of RO and the low value of DC-MD; but the result is contrary if entropy generation is considered as indicator [89]. In the case of MED units, the high entropy generation is essentially related to the finite temperature differences between fluids. In fact, considering a simple unit with 6 effects, Mistry et al. demonstrated that entropy generation is essentially related to the effects (56.5%), the feed heaters (12.3%) and the condenser (21.8%). Appl. Sci. 2021, 11, x FOR PEER REVIEW 29 of 36

ratio. This term depends also on the temperature of the thermal source 𝜏, introducing the Carnot factor 1− , which physically represents the ratio of heat that can be converted into exergy, using a reversible thermal machine. 𝜏 𝑄 =𝑊 1 − (4) 𝜏

So, the exergy efficiency can be evaluated by Equation (5), defining 𝑄 as the thermal requirement for the real plant.

𝑄 𝑄 𝑄 𝜂 = = = 𝑄 𝜏 𝜏 (5) 𝑄 +𝜏𝑆 1− 𝑄 +𝜏𝑆 1− 𝜏 𝜏

Similarly, the term 𝑄 is introduced, representing the least thermal input requirement, assuming significant freshwater production (RR > 0). Mistry et al. analyzed the energy performances of developed desalination technologies, obtaining the results reported in Figure 37 (based on data reported in [89]). The graph reports the exergy efficiency according to Equation (3) for technologies requiring mechanical (or electrical) energy and Equation (5) in case of thermally supplied technologies. The authors considered a seawater salinity equal to 4.2% in case of MED and MSF units, and 3.5% (common seawater) in the other cases. The graph emphasizes the high exergy efficiency of RO and the low value of DC-MD; but the result is contrary if entropy generation is considered as indicator [89]. In the case of MED units, the high entropy generation is essentially related to the Appl. Sci. 2021, 11, 670 finite temperature differences between fluids. In fact, considering a simple unit with29 of 366 effects, Mistry et al. demonstrated that entropy generation is essentially related to the effects (56.5%), the feed heaters (12.3%) and the condenser (21.8%).

50% 925.4 1000

Efficiency 40% 800 Entropy generation 31.9% 30% 600

423.0 20% 370.0 400

Exergy efficiency [%] efficiency Exergy 196.0 8.5%

10% 200 [J/(kg-K)] generated Entropy 5.9% 98.0 2.9% 2.4% 1.0% 19.4 0% 0 MED MSF DC-MD MVC RO HDH

Figure 37. Exergy analysis of several desalination technologies. Figure 37. Exergy analysis of several desalination technologies. It is significant to underline that a significant amount of energy is irreversibly wasted as theIt is produced significant brine to underline and freshwater that a signific have aant higher amount temperature of energy thanis irreversibly the environmen- wasted astal the conditions. produced Duebrine to and the freshwater limited temperature have a higher differences temperature between than the environmental fluids and the conditions.environments, Due the to entropy the limited generation temperature is only the differences 6.2% [89]. between the fluids and the environments,MSF unit arethe alsoentropy affected genera bytion the thermalis only the gradient 6.2% [89]. between fluids. It is evaluated that the entropyMSF unit is are mainly also affected generated by by the feed thermal heaters gradient (73.9%) between and brine fluids. heater It is (12.5%),evaluated where that thethe entropy temperature is mainly difference generated is higher. by feed The heat temperatureers (73.9%) disequilibrium and brine heater between (12.5%), produced where freshwater and brine affects also MSF, causing an entropy generation equal to 10% [89]. The only solution to improve the exergy efficiency of thermally driven desalination processes is the increasing of the sizes or the number of heat exchangers in order to reduce the temperature difference between primary and secondary fluids. However, this approach is limited by the fact that increasing the sizes of the heat exchangers raises the initial investment of the plant that which may not be counterbalanced by the economic value of the energy saving [90]. The direct contact membrane distillation is a very energy consuming process. As shown in Figure 37, the exergy efficiency is equal to 1%, with an entropy generation equal to 925.4 J/(kg-K). The entropy is essentially generated by four items: pressure drop through the module (34.5%), temperature difference in the heater (26.3%), temperature disequilib- rium between feedwater, freshwater and brine (22.9%) and temperature difference inside the regenerator (16.3%). In the case of MVC units, the entropy is essentially generated by the finite temperature difference inside the evaporator-condenser (57.2%), the irreversibility of the mechanical compressor (28.1%) and the temperature difference in the regenerator (10.9%). To improve the exergy efficiency, the main solution is the reduction of the temperature difference between the condensing steam and the evaporating brine inside the evaporator-compressor. Therefore, the irreversibility from the heat transfer and the mechanical compressor are both reduced, thanks to the reduction of the compression work. On the other side, the cost for the desalination unit is increased [14]. According to data above reported, the RO unit represents the most efficient technology for desalination, if the energy recovery system is adopted. The exergy efficiency is equal to 31.9% in the case reported by Mistry et al. [89]. The main irreversibility is associated to the pressure drop between the two sides of the semipermeable membrane (54.8%), the irreversibility of the main high-pressure pump (20%) and the chemical disequilibrium (15.9%). To improve further the exergy efficiency of the system, a solution is splitting the desalination process in two (or more) steps. As the osmotic pressure is function of the saline concentration of inlet and outlet fluid, in the first step a lower pressure increase is Appl. Sci. 2021, 11, 670 30 of 36

required, with a significant reduction of the pumping work. In the following step, a lower saline water is treated, achieving in the end the same final pressure in comparison with the case of a traditional RO unit [96]. Finally, the HDH unit is essentially affected by the finite temperature difference be- tween fluids. In particular, entropy is generated by the dehumidifier (53.6%), the humidifier (13.2%), the heater (17.3%) and the temperature disequilibrium of produced fluids with respect to the environmental condition (15.9%) [89]. It is important to underline that this technology is currently under development, so the exergy efficiency could be increased in the next future [82].

4. Conclusions As shown, the seawater desalination sector has many chemical and physical methods capable to obtain the freshwater. Thermally driven solutions (MED, MSF) are historically the first techniques adopted, whereas today the technologies based on membranes (mainly RO) are quickly spreading worldwide. This makes the sector flexible according to the needs of the site. Table2 summarizes the advantages and disadvantages of the systems described below.

Table 2. Advantages and disadvantages of the main desalination technologies.

Technology Advantages Drawbacks Status Ref. - High water quality MED - Low energy Scaling on the pipes Commercial [5,22–25,27–29] consumption - High energy demand - Maintenance operations - Huge investment to remove the scaling are - Corrosion problem MSF simpler than in MED Commercial [5,6,12,23,24,27,30,31] - Slow start up - High water quality - The entire plant is - High rated capacity stopped for maintenance - High water quality MVC - Low energy Low production capacity Commercial [31,32] consumption - Only electrical demand - Lower water quality - Low investments - High costs for - Couplable with many RO membranes and Commercial [23,33,35,36,38–41,43–46] renewable energy sources chemicals - Modular structure of - Subject to biofouling plant No drinking purpose Special application (ammonia carbon FO Low thermal energy (hydration bags) [28,47–50] dioxide) in industrial In development production In development at the Produces soft water (a NF Low energy demand dual-stage unit for [2,23,36,46,52] diluted saline solution) seawater - High purity of - Only for brackish water freshwater (up to 2000 ppm) ED - Energy consumption Commercial [10,23,24,53] - Bacterial contaminants proportional to salt not removed by system concentration Potentially more efficient Only for brackish water CDI In development [54–57] than ED (up to 2000 ppm) Appl. Sci. 2021, 11, 670 31 of 36

Table 2. Cont.

Technology Advantages Drawbacks Status Ref. Potentially more efficient HY than commercial High costs In development [18,58,59] technologies Potentially more efficient SRF than commercial Ice removal In development [28,60] technologies - No applied pressure - Fouling of membrane Commercial for food MD - Low operating - Pretreatments are [28,61–76] application temperature required Electricity required only Commercial for IXR Only for brackish water [77,78] for pump water demineralized water - Run by solar radiation Usable only for small Commercial for small SSD - Realizable with poor [23,24,28,79,80] applications application materials Huge extensions are SC Run by solar radiation In development [28,81] required Optimization of - Low operating thermodynamic cycle HDH temperature and flow rates In development [28,82–84] - Simple operation Three circuits: air, water, freon

The many technologies on the market guarantee products of varying quality and are distinguished by efficiency, but the RO unit represents the best available technology for desalination, according to the previous statistics, thanks to the reduced cost of the water production. Furthermore, the possibility to adopt electrical energy as input would simplify the coupling with renewable energy sources, thanks to the adoption of commercial technologies like photovoltaic panels and wind turbines [97,98]. In this way, it is possible to install small desalination units on minor islands, hopefully supplied by renewable energy sources, in order to satisfy freshwater demand in a sustainable way [99–101].

Author Contributions: Conceptualization, D.C., V.F. and A.G.; formal analysis, V.F.; investigation, D.C.; data curation, D.C. and A.G.; writing—original draft preparation, D.C.; writing—review and editing, D.C. and A.G.; supervision, V.F. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No external data. Conflicts of Interest: The authors declare no conflict of interest. Nomenclature

AGMD Air Gap Membrane Distillation [c] Molar concentration of solute Ca2+ Calcium ion CDI Capacitive Deionization Cl− Chlorine ion 2− CO3 Carbonate Ion DCMD Direct Contact Membrane Distillation DWEER™ Dual Work Exchanger Energy Recovery ∆p Gradient pressure e Degree of dissociation Appl. Sci. 2021, 11, 670 32 of 36

ED Electrodialysis ERD Energy Recovering Device ηex Exergy efficiency Fe3+ Iron ion FEDCO Fluid Equipment Development Company FHP Freshwater High-Pressure FO Forward Osmosis GWI Global Water Intelligence H+ Hydrogen ion HCl Chloridric acid − HCO3 Hydrogen carbonate ion HDH Humidification-Dehumidification HEMI Hydraulic Energy Management Integration HPP High pressure pump H2SO4 Sulfuric acid HY Hydration ι van’t Hoff index IXR Ion-exchange resins J Joule K Kelvin K+ Potassium ion l liter m3 Cubic meter m2 Square meter MCDI Membrane Capacitive Deionization MD Membrane Distillation MED Multi-Effect Distillation mg milligram Mg2+ Magnesium ion Mn2+ Manganese ion mol mole MSF Multi-Stages Flash MVC Mechanical Vapor Compression ν Number of ions formed Na+ Sodium ion NaCl Sodium chloride NaOH Sodium hydroxide NF Nano Filtration NH4OH Ammonium hydroxide 2− NO3 Nitrate OH− Hydroxide ion posm Osmotic pressure ppm Part per million Q f Freshwater flow min Qleast Thermal energy required Qs Saline feedwater flow Qsep Thermal requirement for the real plant R Ideal gas constant RO Reverse osmosis RPX Rotary Pressure Exchanger RR Recovery ratio SC Solar Chimney SGMD Sweeping Gas Membrane Distillation SIR Società Italiana Resine SRF Secondary Refrigerant Freezing 2− SO4 Sulphate ion SSD Solar Still Distillation TDS Sgen Entropy generation dead state RDS Sgen Entropy generation restricted dead state τ Absolute temperature τh Temperature of the thermal source τ0 Environmental temperature TBT Top Brine Temperature TVC Thermal Vapor Compression VF Vacuum Freezing VMD Vacuum Membrane Distillation min Wleast Least work of separation Wsep Work required by the real desalination plant Appl. Sci. 2021, 11, 670 33 of 36

References 1. Lior, N. Advances in Water Desalination; Lior, N., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012; ISBN 9781118347737. 2. Wetterau, G.; Moch, I.; Frenkel, V.; Huehmer, R.; Hunt, H.; Kiefer, K.; Pankratz, T.; Sethi, S.; Silverman, G.; Trussel, S.; et al. Desalination of Seawater—M61, 1st ed.; American Water Works Association: Denver, CO, USA, 2011; ISBN 978-1-58321-833-4. 3. Al-Othman, A.; Darwish, N.N.; Qasim, M.; Tawalbeh, M.; Darwish, N.A.; Hilal, N. Nuclear desalination: A state-of-the-art review. Desalination 2019, 457, 39–61. [CrossRef] 4. Alnajdi, O.; Wu, Y.; Calautit, J.K. Toward a sustainable decentralizedwater supply: Review of adsorption desorption desalination (ADD) and current technologies: Saudi Arabia (SA) as a case study. Water 2020, 12, 1111. [CrossRef] 5. Cipollina, A.; Micale, G.; Rizzuti, L. A critical assessment of desalination operations in Sicily. Desalination 2005, 182, 1–12. [CrossRef] 6. El-Ghonemy, A.M.K. Performance test of a sea water multi-stage flash distillation plant: Case study. Alexandria Eng. J. 2018, 57, 2401–2413. [CrossRef] 7. Lee, K.P.; Arnot, T.C.; Mattia, D. A review of reverse osmosis membrane materials for desalination-Development to date and future potential. J. Membr. Sci. 2011, 370, 1–22. [CrossRef] 8. Rognoni, M. La Dissalazione Dell’acqua di Mare. Descrizione, Analisi e Valutazione Delle Principali Tecnologie, 1st ed.; Dario Flaccovio Editore s.r.l.: Palermo, Italy, 2010; ISBN 9788857900308. 9. Weir, W.V. The Weir Group: The History of a Scottish Engineering Legend; Profile Books: London, UK, 2008; ISBN 978-1861978868. 10. Al-Mutawa, A.M.; Al Murbati, W.M.; Al Ruwaili, N.A.; Al Oraif, A.S.; Al Oraif, A.; Al Arafati, A.; Nasrullah, A.; Al Bahow, M.R.; Al Anzi, S.M.; Rashidi, M.; et al. Desalination in the GCC. The History, the Present & the Future; GCC: Canberra, Australia, 2014. 11. Amio Water Treatment Ltd. AMIO Water Technology. Available online: http://www.amiowater.com/technology (accessed on 3 January 2021). 12. Jones, E.; Qadir, M.; van Vliet, M.T.H.; Smakhtin, V.; Kang, S.M. The state of desalination and brine production: A global outlook. Sci. Total Environ. 2019, 657, 1343–1356. [CrossRef] 13. Alkaisi, A.; Mossad, R.; Sharifian-Barforoush, A. A Review of the Water Desalination Systems Integrated with Renewable Energy. Energy Procedia 2017, 110, 268–274. [CrossRef] 14. Zimerman, Z. Development of large capacity high efficiency mechanical vapor compression (MVC) units. Desalination 1994, 96, 51–58. [CrossRef] 15. Bahar, R.; Hawlader, M.N.A.; Woei, L.S. Performance evaluation of a mechanical vapor compression desalination system. Desalination 2004, 166, 123–127. [CrossRef] 16. Xu, T. Ion exchange membranes: State of their development and perspective. J. Membr. Sci. 2005, 263, 1–29. [CrossRef] 17. Talaeipour, M.; Nouri, J.; Hassani, A.H.; Mahvi, A.H. An investigation of desalination by nanofiltration, reverse osmosis and integrated (hybrid NF/RO) membranes employed in brackish water treatment. J. Environ. Health Sci. Eng. 2017, 15, 1–9. [CrossRef][PubMed] 18. Sangwai, J.S.; Patel, R.S.; Mekala, P.; Mech, D.; Busch, M. Desalination of seawater using gas hydrate technology—Current status and future direction. In Proceedings of the 18th International Conference on Hydraulics, Water Resources, Coastal and Environmental Engineering, HYDRO 2013 International, Madras, India, 4–6 December 2013; pp. 434–440. 19. Ma, Q.; Lu, H. Wind energy technologies integrated with desalination systems: Review and state-of-the-art. Desalination 2011, 277, 274–280. [CrossRef] 20. Ullah, I.; Rasul, M.G. Recent developments in solar thermal desalination technologies: A review. Energies 2019, 12, 119. [CrossRef] 21. Goosen, M.; Mahmoudi, H.; Ghaffour, N. Water Desalination using geothermal energy. Energies 2010, 3, 1423–1442. [CrossRef] 22. Al-Shammiri, M.; Safar, M. Multi-effect distillation plants: State of the art. Desalination 1999, 126, 45–59. [CrossRef] 23. Buros, O.K. The ABCs of Desalting; International Desalination Association: Topsfield, MA, USA, 2000. 24. Shatat, M.; Riffat, S.B. Water desalination technologies utilizing conventional and renewable energy sources. Int. J. Low-Carbon Technol. 2014, 9, 1–19. [CrossRef] 25. Blagin, E.V.; Shimanov, A.A.; Gorshkalev, A.A. Determination of the Criteria for Comparative Analysis of Desalination Plant. IOP Conf. Ser. Earth Environ. Sci. 2019, 264.[CrossRef] 26. Sethi, S.; Wetterau, G. Seawater Desalination Overview. In Desalination of Seawater—Manual of Water Supply Practices, M61; American Water Works Association (AWWA): Denver, CO, USA, 2011; p. 13. ISBN 978-1-58321-833-4. 27. Shatat, M.; Riffat, S.; Ghabayen, S. State of Art Water Desalination Technologies using Conventional and Sustainable Energy Sources. In The 4th International Engineering Conference–Towards Engineering of the 21st Century; IEC: Geneve, Switzerland, 2012; pp. 1–16. 28. Sharon, H.; Reddy, K.S. A review of solar energy driven desalination technologies. Renew. Sustain. Energy Rev. 2015, 41, 1080–1118. [CrossRef] 29. Karagiannis, I.C.; Soldatos, P.G. Water desalination cost literature: Review and assessment. Desalination 2008, 223, 448–456. [CrossRef] 30. Tenno, R.; Nguyen, P. Multistage Flash Evaporator Control in PDE Representation. IFAC-PapersOnLine 2016, 49, 70–75. [CrossRef] 31. Al-Karaghouli, A.; Kazmerski, L.L. Energy consumption and water production cost of conventional and renewable-energy- powered desalination processes. Renew. Sustain. Energy Rev. 2013, 24, 343–356. [CrossRef] Appl. Sci. 2021, 11, 670 34 of 36

32. Research Office Legislative Council Secretariat. Seawater Desalination Technologies; Research Office Legislative Council Secretariat: Tsukuba, Japan, 2015. 33. Helfer, F.; Lemckert, C.; Anissimov, Y.G. Osmotic power with Pressure Retarded Osmosis: Theory, performance and trends—A review. J. Membr. Sci. 2014, 453, 337–358. [CrossRef] 34. Silvestroni, P. Fondamenti di Chimica; CEA, Ed.; Libreria Eredi Virgilio Veschi: Rome, Italy, 1996; ISBN 8840809988. 35. Han, G.; Zhang, S.; Li, X.; Chung, T.S. High performance thin film composite pressure retarded osmosis (PRO) membranes for renewable salinity-gradient energy generation. J. Membr. Sci. 2013, 440, 108–121. [CrossRef] 36. Fritzmann, C.; Löwenberg, J.; Wintgens, T.; Melin, T. State-of-the-art of reverse osmosis desalination. Desalination 2007, 216, 1–76. [CrossRef] 37. Airoldi, D.; Bertani, D.; Garofalo, E.; Guastella, S.; Lembo, E.; Sandroni, C.; Giudici, F. Scenari di Sviluppo Delle FER Nelle Isole Minori Italiane Non Interconnesse e Analisi di Casi Studio; Report of RSE S.p.A. (Ricerca sul Sistema Energetico): Rome, Italy, 2017. 38. Henthorne, L.; Boysen, B. State-of-the-art of reverse osmosis desalination pretreatment. Desalination 2015, 356, 129–139. [CrossRef] 39. Alhathal Alanezi, A.; Altaee, A.; Sharif, A.O. The effect of energy recovery device and feed flow rate on the energy efficiency of reverse osmosis process. Chem. Eng. Res. Des. 2020, 158, 12–23. [CrossRef] 40. Ruiz-García, A.; Melián-Martel, N.; Nuez, I. Short review on predicting fouling in RO desalination. Membranes 2017, 7, 62. [CrossRef] 41. Martin, J.; Eisberg, D. Brackish Water Desalination—Energy and Cost Considerations. Int. Desalin. Assoc. World Congr. Desalin. Water Reuse 2007, 22, 99–131. 42. Gebel, J.; Yüce, S. An Engineer’s Guide to Desalination; VGB PowerTech Service GmbH: Essen, Germany, 2008; ISBN 978-3-86875- 000-3. 43. Guirguis, M.J. Energy Recovery Devices in Seawater Reverse Osmosis Desalination Plants with Emphasis on Efficiency and Economical Analysis of Isobaric Versus Centrifugal Devices; University of South Florida: Tampa, FL, USA, 2011. 44. Oklejas, E.; Hunt, J. Integrated pressure and flow control in SWRO with a HEMI turbo booster. Desalin. Water Treat. 2011, 31, 88–94. [CrossRef] 45. Huang, B.; Pu, K.; Wu, P.; Wu, D.; Leng, J. Design, selection and application of energy recovery device in seawater desalination: A review. Energies 2020, 13, 4150. [CrossRef] 46. Mirza, S. Reduction of energy consumption in process plants using nanofiltration and reverse osmosis. Desalination 2008, 224, 132–142. [CrossRef] 47. Kessler, J.O.; Moody, C.D. Drinking water from sea water by forward osmosis. Desalination 1976, 18, 297–306. [CrossRef] 48. McCutcheon, J.R.; McGinnis, R.L.; Elimelech, M. A novel ammonia-carbon dioxide forward (direct) osmosis desalination process. Desalination 2005, 174, 1–11. [CrossRef] 49. Chekli, L.; Phuntsho, S.; Kim, J.E.; Kim, J.; Choi, J.Y.; Choi, J.S.; Kim, S.; Kim, J.H.; Hong, S.; Sohn, J.; et al. A comprehensive review of hybrid forward osmosis systems: Performance, applications and future prospects. J. Membr. Sci. 2016, 497, 430–449. [CrossRef] 50. Wang, Y.N.; Goh, K.; Li, X.; Setiawan, L.; Wang, R. Membranes and processes for forward osmosis-based desalination: Recent advances and future prospects. Desalination 2018, 434, 81–99. [CrossRef] 51. Trevi Systems Making Water Shouldn’t Waste Water. Available online: https://www.trevisystems.com/technology- (accessed on 3 January 2021). 52. Mohammad, A.W.; Teow, Y.H.; Ang, W.L.; Chung, Y.T.; Oatley-Radcliffe, D.L.; Hilal, N. Nanofiltration membranes review: Recent advances and future prospects. Desalination 2015, 356, 226–254. [CrossRef] 53. Greiter, M.; Novalin, S.; Wendland, M.; Kulbe, K.; Fischer, J. Electrodialysis versus ion exchange: Comparison of the cumulative energy demand by means of two applications. J. Membr. Sci. 2004, 233, 11–19. [CrossRef] 54. Hassanvand, A.; Wei, K.; Talebi, S.; Chen, G.; Kentish, S. The Role of Ion Exchange Membranes in Membrane Capacitive Deionisation. Membranes 2017, 7, 54. [CrossRef] 55. Li, H.; Zou, L. Ion-exchange membrane capacitive deionization: A new strategy for brackish water desalination. Desalination 2011, 275, 62–66. [CrossRef] 56. Kim, N.; Lee, J.; Kim, S.; Hong, S.P.; Lee, C.; Yoon, J.; Kim, C. Short review of multichannel membrane capacitive deionization: Principle, current status, and future prospect. Appl. Sci. 2020, 10, 683. [CrossRef] 57. Porada, S.; Zhao, R.; van der Wal, A.; Presser, V.; Biesheuvel, P.M. Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58, 1388–1442. [CrossRef] 58. Truong-Lam, H.S.; Kim, S.; Seo, S.D.; Jeon, C.; Lee, J.D. Water purifying by gas hydrate: Potential applications to desalination and wastewater treatments. Chem. Eng. Trans. 2020, 78, 67–72. 59. Lv, Q.; Li, X.; Li, G. Seawater desalination by hydrate formation and pellet production process. Energy Procedia 2019, 158, 5144–5148. [CrossRef] 60. Lin, W.; Huang, M.; Gu, A. A seawater freeze desalination prototype system utilizing LNG cold energy. Int. J. Hydrogen Energy 2017, 42, 18691–18698. [CrossRef] 61. Kujawa, J.; Cerneaux, S.; Kujawski, W.; Knozowska, K. Hydrophobic ceramic membranes for water desalination. Appl. Sci. 2017, 7, 402. [CrossRef] 62. Chew, N.G.P.; Zhao, S.; Wang, R. Recent advances in membrane development for treating surfactant- and oil-containing feed streams via membrane distillation. Adv. Colloid Interface Sci. 2019, 273, 102022. [CrossRef][PubMed] Appl. Sci. 2021, 11, 670 35 of 36

63. Alkhudhiri, A.; Darwish, N.; Hilal, N. Membrane distillation: A comprehensive review. Desalination 2012, 287, 2–18. [CrossRef] 64. Gunko, S.; Verbych, S.; Bryk, M.; Hilal, N. Concentration of apple juice using direct contact membrane distillation. Desalination 2006, 190, 117–124. [CrossRef] 65. Hsu, S.T.; Cheng, K.T.; Chiou, J.S. Seawater desalination by direct contact membrane distillation. Desalination 2002, 143, 279–287. [CrossRef] 66. Upadhyaya, L.; Nene, S. Membrane Distillation and Osmotic Membrane Distillation in Downstream Processing. Curr. Biochem. Eng. 2015, 2, 102–110. [CrossRef] 67. Banat, F.A.; Simandl, J. Membrane distillation for dilute ethanol. J. Membr. Sci. 1999, 163, 333–348. [CrossRef] 68. Khayet, M.; Cojocaru, C.; Baroudi, A. Modeling and optimization of sweeping gas membrane distillation. Desalination 2012, 287, 159–166. [CrossRef] 69. Ko, C.C.; Chen, C.H.; Chen, Y.R.; Wu, Y.H.; Lu, S.C.; Hu, F.C.; Li, C.L.; Tung, K.L. Increasing the performance of vacuum membrane distillation using micro-structured hydrophobic aluminum hollow fiber membranes. Appl. Sci. 2017, 7, 357. [CrossRef] 70. Abu-Zeid, M.A.E.R.; Zhang, Y.; Dong, H.; Zhang, L.; Chen, H.L.; Hou, L. A comprehensive review of vacuum membrane distillation technique. Desalination 2015, 356, 1–14. [CrossRef] 71. Cho, H.; Choi, Y.; Lee, S. Effect of pretreatment and operating conditions on the performance of membrane distillation for the treatment of shale gas wastewater. Desalination 2018, 437, 195–209. [CrossRef] 72. Chew, N.G.P.; Zhao, S.; Malde, C.; Wang, R. Polyvinylidene fluoride membrane modification via oxidant-induced dopamine polymerization for sustainable direct-contact membrane distillation. J. Membr. Sci. 2018, 563, 31–42. [CrossRef] 73. Chen, G.; Yang, X.; Wang, R.; Fane, A.G. Performance enhancement and scaling control with gas bubbling in direct contact membrane distillation. Desalination 2013, 308, 47–55. [CrossRef] 74. Laqbaqbi, M.; Sanmartino, J.A.; Khayet, M.; García-Payo, C.; Chaouch, M. Fouling in membrane distillation, osmotic distillation and osmotic membrane distillation. Appl. Sci. 2017, 7, 334. [CrossRef] 75. Peng, Y.; Ge, J.; Li, Z.; Wang, S. Effects of anti-scaling and cleaning chemicals on membrane scale in direct contact membrane distillation process for RO brine concentrate. Sep. Purif. Technol. 2015, 154, 22–26. [CrossRef] 76. Guillen-Burrieza, E.; Ruiz-Aguirre, A.; Zaragoza, G.; Arafat, H.A. Membrane fouling and cleaning in long term plant-scale membrane distillation operations. J. Membr. Sci. 2014, 468, 360–372. [CrossRef] 77. Guyer, P. An Introduction to Ion Exchange Techniques for Water Desalination; CreateSpace: Scots Valley, CA, USA, 2012. 78. Tavani, L. Desalination Process by Ion Exchange. U.S. Patent No. 3,618,589, 9 November 1971. 79. Lindblom, J. Solar Thermal Technologies for Seawater Desalination: State of the art. Renew. Energy Syst. 2010, 1–17. 80. Marimuthu, T.A.; Atnaw, S.M.; Mardarveran, P.A.; Yi, S.S.; Usop, M.A.B.; Gapar, M.K.B.; Rusdan, S.A.B.; Ramli, R.B.M.; Sulaiman, S.A. Design and development of solar desalination plant. MATEC Web Conf. 2017, 131, 2004. [CrossRef] 81. Maia, C.B.; Silva, F.V.M.; Oliveira, V.L.C.; Kazmerski, L.L. An overview of the use of solar chimneys for desalination. Sol. Energy 2019, 183, 83–95. [CrossRef] 82. Lawal, D.; Antar, M.; Khalifa, A.; Zubair, S.; Al-Sulaiman, F. Humidification-dehumidification desalination system operated by a heat pump. Energy Convers. Manag. 2018, 161, 128–140. [CrossRef] 83. Gorjian, S.; Hashjin, T.T.; Ghobadian, B. Seawater Desalination using Solar Thermal Technologies: State of the art. In Proceedings of the 10th International Conference on Sustainable Energy Technologies, Istanbul, Turkey, 4–7 September 2011; pp. 4–7. 84. Müller-Holst, H.; Engelhardt, M.; Schölkopf, W. Small-scale thermal seawater desalination simulation and optimization of system design. Desalination 1999, 122, 255–262. [CrossRef] 85. Qasim, M.; Badrelzaman, M.; Darwish, N.N.; Darwish, N.A.; Hilal, N. Reverse osmosis desalination: A state-of-the-art review. Desalination 2019, 459, 59–104. [CrossRef] 86. Global Water Intelligence DesalData. Available online: https://www.desaldata.com (accessed on 3 January 2021). 87. Al-Karaghouli, A.; Kazmerski, L.L. Comparisons of Technical and Economic Performance of the Main Desalination Processes with and without Renewable Energy Coupling. In World Renewable Energy Forum; ASES: Boulder, CO, USA, 2012. 88. Lindemann, J.H. Wind and solar powered seawater desalination. Applied solutions for the Mediterranean, the Middle East and the Gulf countries. Desalination 2004, 168, 73–80. [CrossRef] 89. Mistry, K.H.; McGovern, R.K.; Thiel, G.P.; Summers, E.K.; Zubair, S.M.; Lienhard, J.H. Entropy generation analysis of desalination technologies. Entropy 2011, 13, 1829–1864. [CrossRef] 90. Kahraman, N.; Cengel, Y.A. Exergy analysis of a MSF distillation plant. Energy Convers. Manag. 2005, 46, 2625–2636. [CrossRef] 91. Cerci, Y. The minimum work requirement for distillation processes. Exergy Int. J. 2002, 2, 15–23. [CrossRef] 92. Piacentino, A. Application of advanced thermodynamics, thermoeconomics and exergy costing to a Multiple Effect Distillation plant: In-depth analysis of cost formation process. Desalination 2015, 371, 88–103. [CrossRef] 93. Cerci, Y. Exergy analysis of a reverse osmosis desalination plant in California. Desalination 2002, 142, 257–266. [CrossRef] 94. Khorshidi, J.; Pour, N.S.; Zarei, T. Exergy Analysis and Optimization of Multi-effect Distillation with Thermal Vapor Compression System of Bandar Abbas Thermal Power Plant Using Genetic Algorithm. Iran. J. Sci. Technol. Trans. Mech. Eng. 2019, 43, 13–24. [CrossRef] 95. Mistry, K.H.; Lienhard, J.H. Generalized least energy of separation for desalination and other chemical separation processes. Entropy 2013, 15, 2046–2080. [CrossRef] Appl. Sci. 2021, 11, 670 36 of 36

96. Ang, W.L.; Mohammad, A.W.; Hilal, N.; Leo, C.P. A review on the applicability of integrated/hybrid membrane processes in water treatment and desalination plants. Desalination 2015, 363, 2–18. [CrossRef] 97. Kasaeian, A.; Rajaee, F.; Yan, W.-M. Osmotic desalination by solar energy: A critical review. Renew. Energy 2019, 134, 1473–1490. [CrossRef] 98. Liu, J.; Mei, C.; Wang, H.; Shao, W.; Xiang, C. Powering an island system by renewable energy—A feasibility analysis in the Maldives. Appl. Energy 2018, 227, 18–27. [CrossRef] 99. García-Rodríguez, L. Renewable energy applications in desalination: State of the art. Sol. Energy 2003, 75, 381–393. [CrossRef] 100. Ciulla, G.; Franzitta, V.; Lo Brano, V.; Viola, A.; Trapanese, M. Mini Wind Plant to Power Telecommunication Systems: A Case Study in Sicily. Adv. Mater. Res. 2012, 622–623, 1078–1083. [CrossRef] 101. Franzitta, V.; Viola, A.; Trapanese, M. Description of Hysteresis in Lithium Battery by Classical Preisach Model. Adv. Mater. Res. 2012, 622–623, 1099–1103. [CrossRef]