Experimental Validation of a Solar Powered Multistage Flash Desalination Unit with Alternate Storage Tanks

water

Article Experimental Validation of a Solar Powered Multistage Flash Desalination Unit with Alternate Storage Tanks

Mishal Alsehli

Department of Mechanical Engineering, College of Engineering, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia; [email protected]

Abstract: The fossil fuels that power conventional desalination systems cause substantial environ- mental impact. Solar desalination can satisfy critical water needs with only a minimal contribution to global warming. The current work presents an attractive new design suitable for regions with limited water resources and high solar radiation rates. This work is an experimental study of a newly designed, solar-powered, multi-stage ﬂash (MSF) desalination plant. The design could address the need to increase the limited water resources in solar energy-rich areas. The prototype consists of a solar collector, an MSF unit, and a novel dual thermal storage tank design. In this prototype, preheated brine is directly heated by circulation through the solar collector. Two tanks serve the MSF unit; one tank feeds the MSF unit while the other receives the preheated feed water. The two tanks alternate roles every 24 h. The study was conducted in Taif, Saudi Arabia, throughout the month of September 2020. The results of the experiment showed that 1.92 square meters of solar collector area is needed for an average daily production of 19.7 kg of fresh water, at a cost of approximately $0.015 per liter.

Keywords: multi-stage ﬂash; solar desalination; thermal storage; water production

Citation: Alsehli, M. Experimental Validation of a Solar Powered Multistage Flash Desalination Unit 1. Introduction with Alternate Storage Tanks. Water 2021, 13, 2143. https://doi.org/ Resource shortages for both energy and water production are expected to be a crucial 10.3390/w13162143 challenge in the coming decades. Some countries suffer from a lack of water resources due to pollution, while others endure dry weather conditions. In addition, the exponential Academic Editor: Pei Xu increase in the world population drives the demand for fresh water. According to the United Nations (UN), the world population was 2.6 billion in 1950. In 1987, it had doubled Received: 29 June 2021 to 5 billion, followed by an estimated 7.7 billion in 2020. The available data predicts further Accepted: 31 July 2021 rapid exponential growth in the global population, with an expected increase to 9.7 billion Published: 4 August 2021 by 2050 [1]. The basic human right of consuming fresh water is threatened, as almost 30% of the world’s population lacks access to fresh drinking water [2]. Consequently, there Publisher’s Note: MDPI stays neutral is an urgent need for the desalination of salt water to meet the global demand for fresh with regard to jurisdictional claims in water. Today’s enormous desalination plants require a high energy consumption to supply published maps and institutional afﬁl- fresh water to satisfy daily needs, which cannot be met by the available groundwater. iations. Currently, an estimated 15,906 desalination plants operate in over 177 countries around the world [3], mostly in the Middle East, with a total capacity of more than 95 million cubic meters/day. A subset of 4826 of these plants supply more than 47% of the global production of fresh water to populations in need. The high energy consumption of today’s Copyright: © 2021 by the author. enormous desalination plants cannot be met in many locales due to high energy costs or Licensee MDPI, Basel, Switzerland. scarcity of affordable energy. Additionally, the high energy consumption of traditional This article is an open access article desalination plants contributes heavily to global warming, which has the effect of exacer- distributed under the terms and bating the current water shortages and is not sustainable in the long term, emphasizing conditions of the Creative Commons the need to consider alternative approaches. Satisfying the fresh water needs of growing Attribution (CC BY) license (https:// populations will be possible if non-renewable energy sources are replaced with renewable, creativecommons.org/licenses/by/ environmentally friendly systems. 4.0/).

Water 2021, 13, 2143. https://doi.org/10.3390/w13162143 https://www.mdpi.com/journal/water Water 2021, 13, 2143 2 of 18

Desalination processes are generally divided into two types: thermal and membrane desalination. Each type can be further subdivided. For instance, thermal processes can be classified as multi-effect boiling (MEB), multistage flash (MSF), or vapor compression (VC). Membrane processes are divided into the reverse osmosis (RO) and electro dialysis (ED). Furthermore, based on energy supply sources, the desalination processes can be classified into four main categories: thermal, mechanical, electrical, and chemical energies. According to Stewart, 28% of the world’s desalination is performed using thermal process desalination, while membrane process accounts for 72% [4]. In terms of energy consumption, the thermal desalination of seawater is considered to be an energy-intensive process [3]. In a thermal process, the distillation of seawater is attained by using a thermal energy source. Thermal energy may be obtained from nuclear energy, fossil fuel, or a free non-conventional source such as solar thermal energy, geothermal energy, and other energy efficient techniques. Currently, renewable energy only powers about 0.02% of the desalination market [5]. The MSF desalination process is employed by about 21% of the world’s total installed or contracted desalination systems [4]. The MSF process has been improved dramatically in its capacity and thermal performance, and it is used in areas where the fuel is available at a reasonable price. The reason for the popularity of MSF processes is their capacity for high plant availability. They tolerate seawater salinity and are highly reliable. The major disadvantage of MSF is that the process is highly energy-intensive [6]. The total specific energy requirement for MSF has been reported to be as high as 20–28 kWh/m3 [7]. With of all these factors in play, it would seem that the best solution to water shortage issues in the long and short terms lies in the MSF desalination of sea water being integrated with renewable energy sources. The use of solar energy in desalination is one of the most promising applications of renewable energy sources. Solar energy occupies nearly 57% of the renewable energy-based desalination market [8]. There are many schemes of solar integrated MSF systems worldwide. Some of them only use daylight hours, while others have thermal storage and control systems for the continuous production of desalinated water. Rates of desalinated water production based on solar collector areas vary from 10 to 60 L/m2/day [9]. According to recent literature reviews, considerable research has been done to study solar desalination systems using the MSF principle. The following is a summary of some studies: In 1979, Manjarrez carried out a project to prove the feasibility of solar energy as a source of the energy required in the MSF process [10]. In 1979, Ibarra presented a project on solar desalination. The project consisted of three parts: MSF desalination system, solar energy flat collectors, and a system to store energy and run the unit continuously. The project was developed to provide the MSF unit with solar energy by increasing the temperature of a medium fluid, instead of the steam system that is usually used as a thermal energy source [11]. In 1980, Rajvanshi designed an MSF desalination system that uses solar energy. The scheme was designed to produce about 5.25 × 10 m3/yr with 11.52 km2 located in the Thar Desert of India as the collector area [12]. In 1985, Moustafa proposed a solar multistage flash desalination system consisting of a solar line-concentrating collector field with a total area of 4700 m2, a thermal storage tank, and a 12-stage multistage flash desalination subsystem to produce about 300 L/h. The function of the thermal storage subsystem was to supply heat during periods of low radiation and during nighttime [13]. In 1997, Farwati examined a solar multistage flash desalination system consisting of collectors, a storage tank, a heat exchanger, and a multistage flash desalination to produce about 13.1 m3 of distilled water. The function of the heat exchanger was to pass the thermal energy from the medium fluid to brine [14]. In 2001, Lu H et al. presented an MSF system coupled with salinity-gradient solar ponds. The system was installed at El Paso, USA. The study focused on the technical feasibility of the system [15]. Water 2021, 13, 2143 3 of 18

In 2014, Nafey et al. presented a small-capacity top brine temperature (TBT) MSF system. The systems consisted of a flat plate solar collector, a vertical flash unit, and a condenser unit. During the summer, the system’s daily production was 4.2 to 7 kg/day/m2 of fresh water [16]. In 2016, Al-Sehli designed a new system for a multi-stage solar water desalination unit, characterized by continuous operation with a limited solar collector area. The daily production of distilled water reached 53 kg per square meter of the solar collector area [17]. In 2018, Al-Othman et al. designed a novel solar driven MSF desalination plant in the UAE. The system consisted of parabolic trough collectors (PTC), a solar pond, a fire tube boiler, and MSF desalination. The function of the fire tube boiler was to supply heat during periods of low radiation [13]. The total area of PTCs and the solar pond were 3160 m2 and 0.53 km2, respectively. The system’s daily production was 1880 m3 of fresh water [18]. In 2020, Mendez et al. proposed a solar multistage flash (MSF) desalination unit integrated with a solar chimney. The goal of this system was the dual production of electricity and water. The system consisting of a solar chimney, a hydro-storage system, a water turbine, and a thermal desalination unit. The system produces about 8.3 kg/s of fresh water [19]. Previous work shows that the major disadvantage of solar desalination is limited freshwater production. Desalination cannot compete with a conventional MSF desalination process in terms of capacity. While some solar designs operate only during the daytime because they have no energy storage system, other designs use energy storage systems but need a huge area of solar collectors to increase the temperature of the medium fluid up to the design value. To overcome all the previous disadvantages, in this work MSF desalination is coupled with a solar system to provide the required thermal energy. The design has the following advantages and novel aspects, compared to previous work from the literature: • No brine heater or heating steam needed to reach to TBT. • The novel dual tank storage system is used to operate the desalination plant continuously and at full capacity. • There is no medium fluid; sea water is heated and stored directly. • Minimized area of solar field by using the circulating system in the thermal storage tank. The description of the design, experiment setup, and the results will be discussed in the following sections.

2. Materials and Methods 2.1. Solar-Powered MSF Desalination Plant Design A prototype solar desalination plant was built and tested for this study. The average capacity of the model in September 2020 was 19.7 kg per day per collector area of distilled water. The system has a thermal storage system capable of running the desalination plant continually for all day at full load. This design uses solar energy to directly heat the sea water then store it for later use. This means that the current system consists of two separate subsystems: the energy storage unit and the desalination unit. The energy storage is the subsystem that collects and stores the solar energy. The desalination unit is an MSF unit that has a similar conventional design divided into stages; each stage contains a preheated seawater tube, a brine flashing chamber, a demister, and a distillate collector. In a conventional MSF, the feed water is heated in the brine heater by thermal energy obtained from fossil fuels or indirect solar energy, while in the solar MSF–thermal storage tank design, the brine heater is eliminated. Instead, the stages are connected to thermal storage tanks to provide them with hot brine. The structure of this desalination plant can be divided into three blocks: the multistage flash, the solar field, and the thermal storage tanks. The complete layout of the plant is shown in Figure1. Water 2021, 13, x FOR PEER REVIEW 4 of 19

tank design, the brine heater is eliminated. Instead, the stages are connected to thermal storage tanks to provide them with hot brine. Water 2021, 13, 2143 The structure of this desalination plant can be divided into three blocks:4 of 18 the multistage flash, the solar field, and the thermal storage tanks. The complete layout of the plant is shown in Figure 1.

Figure 1. Schematic of the design.Figure 1. schematic of the design.

The solar field consistsThe solar of field a flat consists plate collector of a flat plate that collector converts that solar convert energys solar into energy thermal into thermal energy to be absorbed by the sea water to increase its temperature to TBT. The thermal energy to be absorbed by the sea water to increase its temperature to TBT. The thermal storage system consists of two tanks to store the hot sea water. One will be in service storage system consists of two tanks to store the hot sea water. One will be in service (discharging mode) to provide the MSF unit with hot brine, while the other will be on (discharging mode)hold to (charging provide the mode) MSF to unit store with the sea hot water brine, and while provide the other the MSF will unit be onwith hold hot brine on (charging mode)the to storenext d theay. sea water and provide the MSF unit with hot brine on the next day. In general, the complete storage cycle includes three steps: charging, storing, and In general, thedischarging. complete On storage the first cycle day, tank includes 1 is filled three up steps: with sea charging, water, which storing, is heated and by circu- discharging. Onlating thefirst it through day, tankthe solar 1 is collectors. filled up The with filling sea and water, heating which processes is heated for tank by 1 is called circulating it throughcharging/storing the solar collectors. mode. Tank The 2, which filling was and filled heating up with processes hot water forat TBT tank on 1 the is previous called charging/storingday, provides mode. the Tank desalination 2, which unit was (stages) filled with up with hot brine. hot water Feeding at the TBT stages on the with the hot previous day, providesbrine is thecalled desalination discharging unitmode. (stages) The TBT with is the hot maximum brine. Feedingtemperature the that stages the sea water with the hot brinereaches; is called it is dischargingthe temperature mode. of the The feed TBT water is leaving the maximum the brine temperatureheater in conventional desalination. Its designed value is approximately 70 °C. On the second day, the processes that the sea water reaches; it is the temperature of the feed water leaving the brine heater are switched in such a way that tank 2 is filled with sea water and simultaneously heated in conventional desalination. Its designed value is approximately 70 ◦C. On the second by circulating it through solar collectors. This way, tank 2 is in charging/storing mode and day, the processestank are 1 switchedis in discharging in such mode. a way The thatcirculating tank 2pumps is filled circulate with the sea sea water water and (feed water) simultaneously heatedthrough by the circulating solar field dur it throughing the daytime solar collectors.to absorb the This solar way, energy tank, which 2 is increases in the charging/storingsea mode water and temperature tank 1 is in discharging mode. The circulating pumps circulate the sea water (feed water) through the solar field during the daytime to absorb the solar energy, which increases2.2. MSF the Operational sea water Process temperature. All MSF process steps are depicted in Figure 1 and are described as follows: sea water 2.2. MSF Operationalstarts Process from the last stage, where it enters a bundle of tubes located at the top of the stages All MSF processto heat steps sea arewater depicted up due into the Figure latent1 and heat are from described the vaporization as follows: of the sea flashing water brine. The starts from the lastbrine stage, condenses where itaround enters these a bundle tubes as of a tubes distillate located water at free the of top any of salt. the The stages temperature to heat sea water up due to the latent heat from the vaporization of the flashing brine. The brine condenses around these tubes as a distillate water free of any salt. The temperature of the sea water increases gradually as it moves towards the first stage. In conventional desalination, seawater leaves the first stage and enters the brine heater to increase its temperature to TBT. In this design, the thermal storage system is the heating section, where the seawater enters the storage tank and heats up at the solar collector field. It stays there for a whole day to absorb the heat during daytime, when its temperature increases to the maximum design value, known as the top brine temperature. Simultaneously, the other tank in service provides the stages of desalination with the hot brine. The brine is pumped into the bottom of the first stage where it begins to flash progressively under vacuum along Water 2021, 13, x FOR PEER REVIEW 5 of 19

of the sea water increases gradually as it moves towards the first stage. In conventional desalination, seawater leaves the first stage and enters the brine heater to increase its temperature to TBT. In this design, the thermal storage system is the heating section, where the seawater enters the storage tank and heats up at the solar collector field. It stays there Water 2021, 13, 2143 for a whole day to absorb the heat during daytime, when its temperature increases5 of 18 to the maximum design value, known as the top brine temperature. Simultaneously, the other tank in service provides the stages of desalination with the hot brine. The brine is pumped into the bottom of the first stage where it begins to flash progressively under vacuum each stage. Flashedalong vapor each stage. condenses Flashed on vapor the outer condenses surface on ofthe the outer condenser surface of tubes the condenser at the top tubes at of each stage, andthe carrytop of theeach feed stage water, and insidecarry the for feed heating. water Condensationinside for heating. helps Condensation to improve helps to the heat recovery,improve making the theheat unit recovery, more making efficient. the The unit distillate more efficient. water, The which distillate forms water, the which condensing vaporforms at thethe condensing stages, is collected vapor at inthe a stages, tray connecting is collected allin thea tray stages, connecting and then all the is stages, removed at the lastand stage.then is Eachremoved stage at is the connected last stage. to Each the nextstage via is connected an orifice, to causes the next the via brine an orifice, to flow from onecauses stage the to brine the next to flow due from to the one vacuum. stage to the At next the lastdue stage,to the vacuum the high-salinity,. At the last stage, concentrated brinethe high is discharged.-salinity, concentrated A vacuum brine ejector is discharged. is provided A vacuum to create ejector a vacuum is provided in the to create successive stages,a vacuum allowing in the the successive brine to flashstages, into allowing steam the below brine theto flash boiling into temperaturesteam below the of boiling water. The arrangementtemperature is made of water. to permit The arrangement a gradual decreaseis made to in permit pressure a gradual during decrease successive in pressure stages. Removingduring the non-condensablesuccessive stages. Removing gases is anotherthe non-condensable role of the vacuum gases is another ejector. role of the vacuum ejector. 2.3. Experiment Setup 2.3. Experiment Setup A small-scale solar powered MSF desalination was designed and built. Figure2 is an A small-scale solar powered MSF desalination was designed and built. Figure 2 is an illustration of thisillustration design, of which this design, enables which the study enables of the the study solar of MSF the solar system. MSF Tosystem. understand To understand the system mathematically,the system mathematically, see Appendix seeA .Appendix The model A. consistsThe model of consists three components: of three components: a a flat plate collector,flat storageplate collector, tanks, storage and a multistagetanks, and a flashmultistage desalination flash desalination unit. unit.

Figure 2. A photographFigure of the 2. experimental A photograph system. of the experimental system.

The MSF chamberThe MSF consists chamber of several consists parts of severa whichl parts are which from topare tofrom bottom top to asbottom follows: as follows: heat exchangerh tubes,eat exchanger steam space,tubes, steam demister space, for demister steam purification,for steam purification, a tray for a collectingtray for collecting condensed water,condensed a small orificewater, a at small the bottomorifice at of the the bottom chamber of the to chamber enable to brine enable flow brine through flow through the stages, andthe a brine stages, well. and The a brine chamber well. The is rectangularchamber is rectangular in shape, within shape, dimensions with dimensions 70 cm 70 cm long, 35 cm wide, and 35 cm high. Its walls have a thickness of 2 mm, and are made from long, 35 cm wide, and 35 cm high. Its walls have a thickness of 2 mm, and are made from 316 L low-carbon stainless steel coated with rust-resistant paint to prevent corrosion and 316 L low-carbonreduce stainless heat loss. steel The coated system with includes rust-resistant two tanks. paint The tovolume prevent of each corrosion tank is and 1.30 cubic reduce heat loss. The system includes two tanks. The volume of each tank is 1.30 cubic meters, which is sufficient to feed the MSF unit with feed water for a whole day. A flat-plate solar collector is placed on a frame inclined at an angle of 45◦. Table1 shows the technical

speciﬁcations of the solar collector.

Table 1. Technical speciﬁcation of the ﬂat plate collector.

Aperture Area m2 1.92 Overall height mm 1730 Overall width mm 1170–1215 Depth mm 73 Fluid capacity L 1.6 Zero-loss efﬁciency η0 0.75 Water 2021, 13, 2143 6 of 18

A distillate storage tank is located next to the last stage. The distillate storage tank collects the desalinated water via tubes connecting it with the distillate tray. The different parts of the experiment are connected to the system unit by PVC tubes with 1-inch diameters.

2.4. Experimental Measurements Four parameters were tracked and measured while the experiment was in progress: temperature, solar radiation, wind velocity, and ﬂow rate. The different types of measurements employed are explained below. Temperature Lascar EL USB-TC-LCD thermocouples (Lascar Electronics Co., Ltd., Whiteparish, UK) were used to measure system temperatures with an accuracy of ±1.0 ◦C. Solar Radiation A pyranometer to measure solar radiation was placed outside the collector with calibration accuracy equal to ±10 W/m2 and short-wave sensitivity from 7 to 14 µV/Wm2. Wind Velocity A MagiDeal MS6252B digital anemometer with accuracy of ±3.0% was used to measure the wind speed. Flow Rates DIGITEN G1/”; Flow Water Sensor Meters (Shenzhen Hui Qi Mei Technology Co., Ltd., Shenzhen, Guangdong, China) with accuracy of ±1% were used to measure the ﬂow rate of the feedwater, brine, condensate, and blowdown. Details of the measurement instruments include, type, reading range, and accuracy, as summarized in Table2.

Table 2. Speciﬁcations of the instruments used in the experiments.

Measurement Instrument Type Range Manufacturer Accuracy Water ﬂow meter 12E 10–100 L/min ±1% WAYSEAR Solar power meter TM-207 2000 W/m2 ±10 W/m2 Tenmars Air speed meter GM8901 0–45 m/s ±3.0% Benetech Thermocouple OM-EL −200 to 390 ◦C ±1.0 ◦C Omega

3. Results The experiment described in this paper was performed in the western region of Saudi Arabia. This region is known for its considerable exposure to sunlight year-round, allowing the solar desalination plant to operate 24 h a day for the entirety of a year. Table3 presents the technical specifications used for the experiment. For the purposes of the experiment, the system operated continuously during both September and December of 2020. During December 2020, the system was subjected to the lowest temperatures experienced by the region at any one time, which verified that the storage system would supply the unit with sufficient heat for the duration of the year. Given a surface collection area of 1.92 m2, the system produced an average of 19.7 kg of distilled water per day. The average solar irradiance measured at the surface of the solar collector was 6.3 kWh/m2/day. The highest value of solar field outlet temperature occurs at around 2:00 p.m. when solar radiation is at its highest value. Additionally, the duration of sunlight in a given day during the summer is greater than that of the winter, resulting in greater heat absorption during the summer season. As shown in Figure3, the availability of solar energy in September occurs for a 12 h period between 5:00 a.m. and 5:00 p.m. The red line represents the highest solar radiation day, while the blue represents the lowest solar radiation day. Water 2021, 13, x FOR PEER REVIEW 7 of 19

Table 3. System technical specifications and operation data used for the experiment.

MSF Parameters Unit Values

Number of MSF stages, 푁푠 3 3 푀푝푘 m /day 1.3 Water 2021, 13, 2143 7 of 18 Solar Collector Parameters Values Collector area m2 1.92 Overall height mm 1730 Table 3. System technicalOverall speciﬁcations width and operation data usedmm for the experiment.1170–1215 Depth mm 73 MSFFluid Parameters capacity UnitL Values1.6 Number푍푒푟표 of− MSF푙표푠푠 stages,푒푓푓𝑖푐𝑖푒푛푐푦Ns 휂0 0.753 M 3 푇pk푡ℎ𝑖푐푘푛푒푠푠 m /daym 0.021.3 Solar Collector Parameters Values 푇𝑖푛푠푢푙푎푡𝑖표푛 m 0.05 Collector area m2 1.92 Overall height mm 1730 The highest value of solar field outlet temperature occurs at around 2:00 p.m. when Overall width mm 1170–1215 solar radiation isDepth at its highest value. Additionally, mmthe duration of sunlight in 73 a given day during the summerFluid capacity is greater than that of the winter, L resulting in greater heat 1.6 absorption duringZero the −summerloss ef ficiencyseason. ηAs0 shown in Figure 3, the availability of solar energy 0.75 in Sep- tember occursT forthickness a 12 h period between 5:00 a.m. andm 5:00 p.m. The red line 0.02 represents the highest solarTinsulation radiation day, while the blue representsm the lowest solar radiation 0.05 day.

1200 high solar radiation day 1000 low solar radiation day

) 800 2

600

400

200 Solar Intensity (W/m IntensitySolar 0 1 3 5 7 9 11 13 15 17 19 21 23 Water 2021, 13, x FOR PEER REVIEW 8 of 19 Time (hr)

Figure 3. Hourly variations of solar radiation in September. FigureFigure 4 shows 3. Hourly the variations useful hourly of solar heat radiation absorbed in September. by the brine in the charging tank over Figure4 shows the useful hourly heat absorbed by the brine in the charging tank over two days. The red line represents the absorption during the highest solar radiation day, two days. The red line represents the absorption during the highest solar radiation day, while the blue represents the absorption on the lowest solar radiation day. Their peaks while the blue represents the absorption on the lowest solar radiation day. Their peaks were 1.8 kW and 1.6 kW, respectively. This comparison indicates two different weather were 1.8 kW and 1.6 kW, respectively. This comparison indicates two different weather conditions: a bright sunny summer day and a day with less sun irradiance. conditions: a bright sunny summer day and a day with less sun irradiance.

2000 80 1800 70 1600 60 1400

1200 50

C ° 1000 40

800 TBT Useful heat 30 600 Useful(W) Heat TBT 20 400 200 10 0 0 1 3 5 7 9 11 13 15 17 19 21 23 Day

Figure 4. (Left) Proﬁles of the useful heat transfers from the solar collector to the charging tank for Figurethe highest 4. (Left) (red) Profiles and lowest of the useful (blue) solarheat transfers radiation from days. the (Right) solar collector Charging to tank the charging brine temperature tank for theproﬁles highest for (red) the highest and lowest (red) (blue) and lowest solar (blue)radiation solar days. radiation (Right) days. Charging tank brine temperature profiles for the highest (red) and lowest (blue) solar radiation days.

The variations in mass flow rate and maximum temperature of the brine for each tank were evaluated hourly. The resulting relationships showed a relatively linear relationship in mass flow rate and a non-linear relationship in temperature. Figure 5, below, shows the temperature and the feed mass profiles of the tank for two days. Both the variations in mass flow rate and temperature were evaluated over 24 h periods. The first 24 h show the charging process at high solar radiation levels while the second 24 h show the charging process at low solar radiation levels. In order to make the charging tank temperature reach the designed TBT gradually without exceeding the desired value, the switching time was set to align with the sunset, approximately 8 p.m. By starting the filling process during this period, a large amount of preheated brine enters the storage tank, and no further heating occurs until the following morning. At sunrise, the circulation through the collector starts, which not only helps to achieve the acceptable TBT, but also assists in avoiding thermal losses. Every tank has a minimum level of water at the end of the discharging process. The temperature of the water in the tank at the end of discharging process is the TBT of that day. Once the tank switches from discharging to charging mode, the tank receives the preheated brine at a temperature of about 40 °C. This huge difference between the TBT and the preheated brine temperature causes the temperature in the tank to go from its minimum level to the new TBT value at a gradual rate. This new TBT will be nearly equivalent to the TBT for the previous day, because the amount of the incoming water will be calculated according to sun availability. For this reason, the TBT is almost the same for the two days even with the difference in the amount of the feedwater, as shown in Figure 5.

Water 2021, 13, 2143 8 of 18

The variations in mass flow rate and maximum temperature of the brine for each tank were evaluated hourly. The resulting relationships showed a relatively linear relationship in mass flow rate and a non-linear relationship in temperature. Figure5, below, shows the temperature and the feed mass profiles of the tank for two days. Both the variations in mass flow rate and temperature were evaluated over 24 h periods. The first 24 h show the charging process at high solar radiation levels while the second 24 h show the charging process at low solar radiation levels. In order to make the charging tank temperature reach the designed TBT gradually without exceeding the desired value, the switching time was set to align with the sunset, approximately 8 p.m. By starting the filling process during this period, a large amount of preheated brine enters the storage tank, and no further heating occurs until the following morning. At sunrise, the circulation through the collector starts, which not only helps to achieve the acceptable TBT, but also assists in avoiding thermal losses. Every tank has a minimum level of water at the end of the discharging process. The temperature of the water in the tank at the end of discharging process is the TBT of that day. Once the tank switches from discharging to charging mode, the tank receives the preheated brine at a temperature of about 40 ◦C. This huge difference between the TBT and the preheated brine temperature causes the temperature in the tank to go from its minimum level to the new TBT value at a gradual rate. This new TBT will be nearly equivalent to the TBT for the previous day, because the amount of the incoming water will Water 2021, 13, x FOR PEER REVIEW 9 of 19 be calculated according to sun availability. For this reason, the TBT is almost the same for the two days even with the difference in the amount of the feedwater, as shown in Figure5.

80 140

70 120

60 100 50

C) 80 ° 40

TBT ( TBT 60 30 (kg/hr) Mf 40 20 Mf TBT 10 20

0 0 1 3 5 7High9 solar11 13 radiation15 17 19 21 23 25 27 29 31Low33 solar35 37radiation39 41 43 45 47

Figure 5. Top brine temperature proﬁles and variations of feed water.

FigureFigure6 5.shows Top brine the temperature cumulative profiles distillate and variations production of feed rate water. per hour on 26 September 2020. The total production for this day was 19.7 kg, while the average production rate was Figure 6 shows the cumulative distillate production rate per hour on 26 September approximately2020. The 0.78 total kg/h. production As shown for this in day Figure was6 19.7, the kg, production while the average increase production was linear rate with was time. Thisapproximately indicates that 0.78 the kg/h. MSF As unit shown was in working Figure 6, continuously the production without increase any was interruption, linear with and the productiontime. This indicates rate was that steady the MSF throughout unit was working the day, continuously regardless of wi thethout variation any interrup- in the solar energy.tion, and This the production production rate rate could was steady only bethroughout achieved the by day using, regardless the new of thermal the variation dual tank storagein the system, solar energy. where This the production MSF was rate separated could only from be theachieved variation by using in the the solarnew thermal energy during thedual day, tank resulting storage insystem, a constant where distillatethe MSF was production separated rate from without the variation interference in the fromsolar solar energyenergy variation. during the day, resulting in a constant distillate production rate without interfer- Figureence7 fromserves solar to energy better visualizevariation. the daily changes in mass flow and temperature. This Figure is composed of three bar graphs: one for energies, one for temperatures, and one for mass flows25 over a week-long span from 23–30 September. The first graph shows the daily total useful heat gain; the second shows the corresponding daily temperature change for the feedwater;20 and the third shows the total distillate production. The second and the

5 Accumulative productivity (kg/d) productivityAccumulative 0 1 3 5 7 9 11 13 15 17 19 21 23

Time hrs

Figure 6. Cumulative distillate production rate on 26 September 2020.

Figure 7 serves to better visualize the daily changes in mass flow and temperature. This Figure is composed of three bar graphs: one for energies, one for temperatures, and one for mass flows over a week-long span from 23–30 September. The first graph shows

Water 2021, 13, x FOR PEER REVIEW 9 of 19

80 140

70 120

60 100 50

C) 80 ° 40

TBT ( TBT 60 30 (kg/hr) Mf 40 20 Mf TBT 10 20

0 0 1 3 5 7High9 solar11 13 radiation15 17 19 21 23 25 27 29 31Low33 solar35 37radiation39 41 43 45 47

Figure 5. Top brine temperature profiles and variations of feed water.

Figure 6 shows the cumulative distillate production rate per hour on 26 September 2020. The total production for this day was 19.7 kg, while the average production rate was approximately 0.78 kg/h. As shown in Figure 6, the production increase was linear with time. This indicates that the MSF unit was working continuously without any interrup- Water 2021, 13, 2143 9 of 18 tion, and the production rate was steady throughout the day, regardless of the variation in the solar energy. This production rate could only be achieved by using the new thermal dual tank storage system, where the MSF was separated from the variation in the solar energythird during graphs the start day, a dayresulting after in the a ﬁrstconstant graph, distillate because production the values rate of without the daily interfer- temperature enceand from the solar distillate energy production variation. depended on the useful heat gain on the previous day.

15 Water 2021, 13, x FOR PEER REVIEW 10 of 19

the daily 5total useful heat gain; the second shows the corresponding daily temperature

change for the feedwater; and the third shows the total distillate production. The second Accumulative productivity (kg/d) productivityAccumulative and the third0 graphs start a day after the first graph, because the values of the daily temperature and 1the distillate3 5 production7 9 depend11 13ed on15 the useful17 19heat gain21 on23 the previous day. Time hrs

FigureFigure 6. Cumulative 6. Cumulative distillate distillate production production rate on rate 26 onSeptember 26 September 2020. 2020. (a) Figure15,000 7 serves to better visualize the daily changes in mass flow and temperature. This Figure is composed of three bar graphs: one for energies, one for temperatures, and one for mass flows over a week-long span from 23–30 September. The first graph shows 14,500

14,000

13,500

13,000 Daily average radiation average (W) Daily 12,500 23 24 25 26 27 28 29 30 Days of September

79 (b)

C) 76 ° 73 70 67 64 61

Supply temperature ( temperature Supply 58 55 23 24 25 26 27 28 29 30 31 Days of September

Figure 7. Cont.

Water 2021, 13, x FOR PEER REVIEW 11 of 19

Water 2021, 13, x FOR PEER REVIEW 11 of 19 Water 2021, 13, 2143 10 of 18

5 Daily production (kg/day) production Daily

5 Daily production (kg/day) production Daily 0 0 23 24 25 26 27 28 29 30 31 23 24 25 26 Days 27of September28 29 30 31 Days of September Figure 7. (a) Measured solar radiation, (b) medium fluid supply temperature, and (c) daily distillate Figure 7. (a) Measured solar radiation, (b) medium ﬂuid supply temperature, and (c) daily distillate production. Figureproduction. 7. (a) Measured solar radiation, (b) medium fluid supply temperature, and (c) daily distillate production. FigureFigure8 shows8 shows the the feedwater feedwater and and the the distillate distillate production production during during a week a week of September of Septem- 2020.berFigure 2020. The 8 leftThe showsy left-axis ythe- representsaxis feedwater represents the and distillate the the distillate distillate production production production while while theduring right the a right axisweek representsaxis of Septem- represents the berfeedwater.the 2020. feedwater. The Asleft shown yAs-axis shown represents in the in Figure,the Figure,the thedistillate valuesthe values production of the of feedwaterthe whilefeedwater the were right were within axis within therepresents rangethe range of the100of feedwater. 100 to 117 to 117 kg/day, As kg/day, shown while while in thethe the distillateFigure, distillate the production valuesproduction of the was was feedwater between between 17were 17 and and within 20 20 kg kg the daily. daily. range The The ofvariations 100variations to 117 in kg/day,in the the feedwater feedwater while the and anddistillate the the distillate distillate production production production was between were were according 17according and to20 thekg to solardaily.the solar energy The en- variationsavailability.ergy availability. in the feedwater and the distillate production were according to the solar energy availability. 21.5 120 21.5 21 120 2120.5 115 20.5 20 115 110 2019.5 110 19.5 19 105 1918.5 105 18.5 18 100

Production (kg/d) water Feed 1817.5

Distillate(kg/d) 100

ProductionMf (kg/d) water Feed 17.5 17 95 Distillate(kg/d) Mf 1716.5 95 16.5 16 90 16 24 25 26 27 28 29 30 31 90 24 25 26 Day 27 28 29 30 31 Day Figure 8. Variations of total distillate production rate and feed water. Figure 8. Variations of total distillate production rate and feed water. Figure 8.System Variations Performance of total distillate production rate and feed water. ForSystem further Performance study and to compare alternative methods for desalination, the following criteriaSystemFor were further Performance used: study and to compare alternative methods for desalination, the following criteriaForDistillate further were used: Productionstudy and to and compare Recovery alternative Ratio (RR) methods for desalination, the following criteriaThe Distillatewere variation used: Production in the distillate and Recovery production Ratio is (RR) related to the solar energy that the system absorbs.DistillateThe Thevariation Production daily in distillate the and distillate Recovery production production Ratio per (RR) unit is related area of to solar the solar collector energy is that a very the impor- system tantabsorbs.The factor variation The that daily can in the bedistillate useddistillate inproduction the production performance per is unit related analysis area to of the solar to solar show collector energy the effectiveness is that a very the importantsystem of the absorbs.solarfactor desalination Thethat dailycan be distillate used unit. in Dividing production the performance the per maximum unit analysis area and of to solar minimumshow collector the effectiveness values is a very of the important of distillate the solar 2 factorproductions that can bybe theused solar in the collector performance area (1.92 analysis m ) leads to show to the the value effectiveness range between of the 8.8solar and 10.7 kg/day/m2.

Water 2021, 13, x FOR PEER REVIEW 12 of 19

Water 2021, 13, 2143 desalination unit. Dividing the maximum and minimum values of the distillate produc-11 of 18 tions by the solar collector area (1.92 m2) leads to the value range between 8.8 and 10.7 kg/day/m2. TheThe ratioratio ofof distillatedistillate producedproduced toto thethe feedwaterfeedwater massmass isis calledcalled thethe recoveryrecovery ratio,ratio, shownshown asas a a constant constant value value in in Figure Figure9 .9. Its Its value value is is approximately approximately 18%, 18%, which which means means that that 100100 kg kg of of feedwater feedwater is is required required to to produce produce 18 18 kg kg of of distillate. distillate. This This value value is acceptableis acceptable for for a three-stagea three-stage system. system.

11.5 0.20 0.18 11

0.16 ) 2 10.5 0.14

10 0.12

0.10 RR 9.5 0.08 Distillate 9 RR 0.06

Distillate (kg/d/m Distillate 0.04 8.5 0.02 8 0.00 24 25 26 27 28 29 30 31 Day

Figure 9. Variations of total distillate production rate and recovery ratio (RR). Figure 9. Variations of total distillate production rate and recovery ratio (RR). Performance Ratio (PR) PRPerformanceis the ratio Ratio between (PR) the required power for evaporating the brine to the total solar powerPR is the incident ratio between on the collector.the required Equation power for (1) evaporating was used for the this brine calculation, to the total where solar thepower numerator incident representson the collector. the evaporating Equation (1 power) was used through for this the calculation, three stages, where while the thenu- denominatormerator represents represents the evaporating the incident power solar power through on the collector.three stages, while the denomina- tor represents the incident solar power on the collector. M λ = DIST f g PR 푀퐷퐼푆푇휆푓푔 (1) 푃푅 = Qu (1) 푄푢 Water 2021, 13, x FOR PEER REVIEW Figure 10 shows the daily PR for the experiment. Its range was found to be13 2.9 of to19 3.4, Figure 10 shows the daily PR for the experiment. Its range was found to be 2.9 to 3.4, with an average of 3.2. The performance ratio is sensitive to the TBT, the number of stages, with an average of 3.2. The performance ratio is sensitive to the TBT, the number of stages, and the total collector area. and the total collector area. 3.5 3.4 3.3 3.2 3.1

PR 3.0 2.9 2.8 2.7 2.6 24 25 26 27 28 29 30 31 Days of September

Figure 10. Measured performance ratio for the solar MSF system between 21 and 31 September. Figure 10. Measured performance ratio for the solar MSF system between 21 and 31 September.

The Specific Energy Consumption (SEC) The overall efficiency of the system can be measured based on the energy required for producing distillate. This value is known as specific energy consumption (SEC), which is the total useful heat energy divided by the quantity of produced distillate. Figure 11, below, illustrates the SEC for the MSF system over a period of a week. As shown in the Figure, its values range from 678 to 787 KJ/kg, with an average value of 717 KJ/kg. The SEC depends on the number of stages, the feedwater flow rate, and the total collector area. According to Leblanc, the average specific energy consumption for three stages is about 850 KJ/kg [20].

900 800 700 600 500 400

SEC (kJ/kg) SEC 300 200 100 0 24 25 26 27 28 29 30 31 Days of September

Figure 11. Measured SEC for the solar MSF system between 24th and 31th September.

Economic analysis Use of the solar MSF in desalination assists in the reduction of freshwater production costs. An economic analysis of the MSF cost is considered necessary to highlight both the payback period of the experiment and the value of the produced fresh water. Table 4, below, displays the individual costs that contribute to the required investment necessary

Water 2021, 13, x FOR PEER REVIEW 13 of 19

3.5 3.4 3.3 3.2 3.1

PR 3.0 2.9 2.8 2.7 2.6 24 25 26 27 28 29 30 31 Days of September

Water 2021, 13, 2143 Figure 10. Measured performance ratio for the solar MSF system between 21 and 31 September.12 of 18

The Specific Energy Consumption (SEC) The overall efficiency of the system can be measured based on the energy required The Specific Energy Consumption (SEC) for producing distillate. This value is known as specific energy consumption (SEC), which The overall efficiency of the system can be measured based on the energy required for is the total useful heat energy divided by the quantity of produced distillate. Figure 11, producing distillate. This value is known as specific energy consumption (SEC), which is below, illustrates the SEC for the MSF system over a period of a week. As shown in the the total useful heat energy divided by the quantity of produced distillate. Figure 11, below, Figure, its values range from 678 to 787 KJ/kg, with an average value of 717 KJ/kg. The illustrates the SEC for the MSF system over a period of a week. As shown in the Figure, its SEC depends on the number of stages, the feedwater flow rate, and the total collector area. values range from 678 to 787 KJ/kg, with an average value of 717 KJ/kg. The SEC depends Accordingon the number to Leblanc, of stages, the average the feedwater specific flow energy rate, consumption and the total collectorfor three area.stages According is about to 850Leblanc, KJ/kg [20 the]. average specific energy consumption for three stages is about 850 KJ/kg [20].

900 800 700 600 500 400

SEC (kJ/kg) SEC 300 200 100 0 24 25 26 27 28 29 30 31 Days of September

Figure 11. Measured SEC for the solar MSF system between 24th and 31th September. Figure 11. Measured SEC for the solar MSF system between 24th and 31th September. Economic analysis EconomicUse of theanalysis solar MSF in desalination assists in the reduction of freshwater production costs.Use of An the economic solar MSF analysis in desalination of the MSF assists cost in is the considered reduction necessaryof freshwater to highlight production both costs.the An payback economic period analysis of the of experiment the MSF cost and is the considered value of thenecessary produced to highlight fresh water. both Table the 4, paybackbelow, period displays of thethe individualexperiment costs and that the contributevalue of the to theproduced required fresh investment water. Tab necessaryle 4, below,for the displays MSF to the operate individual successfully. costs that The contribute cost of constructionto the required and investment repair of the necessary MSF unit signiﬁcantly affects the payback period, with a value of $1970. It is assumed that the lifetime of the unit is approximately 20 years when run within its standard operating conditions. The average productivity of the unit was found to be 8.5 L/m2, giving a payback period of 4925 days [21].

Table 4. Investment cost of components of the MSF.

Components of MSF Unit Cost ($) MSF module 300 Solar collector 850 Storage tanks 260 Pumps & valves 220 Pipes 40 Structure 300

Fuel saving In the thermal desalination process, over 350,000 tons of oil are required to produce 13 million tons of freshwater daily [5]. Since the desalted water output is 38 kg/d, the fuel required for desalination is 0.001 tons. This ratio can be used to give a rough estimate for fuel costs saved by the design, which produces 18 kg of freshwater daily, to approximately (0.001 ton/day) (17 barrel/ton) (60 $/barrel) = $1/day, assuming the cost of crude oil to be $60/barrel. Water 2021, 13, 2143 13 of 18

Table5 compares system performance and costs from other reported studies. It justiﬁes the economic and technical viability of the system.

Table 5. Comparison of major parameters.

Collector Capacity Location Model/Exp. TBT (◦C) # of Stages Cost ($/m3) Reference Size (m2) (m3/d) Tianjin, China Experiment NA 78 0.3 1 4.67 [22] Gaza Experiment 5.1 NA 0.145 3 NA [23] Suez, Egypt Model/Exp. 2.39 40–67 0.002–0.017 1 NA [16] Tamilnadu, India Experiment 2 NA 0.0085 1 9 [24] Taif, SA Experiment 1.92 70 0.0197 3 1.5 This work

4. Discussion In conventional desalination, steam temperature is one of the most important parameters impacting the MSF desalination process. The overall plant performance, distilled water production, and the TBT are strongly affected by the heating steam temperature. In this design, the thermal storage tank has the same function as the brine heater, so there is no separate heating steam or brine heater. Thus, the overall plant function depends on the temperature of the thermal storage tank, which represents the TBT. By increasing the thermal storage tank temperature, the distilled product rate increases due to the increased flashing rate. Consequently, more condensate will form, resulting in an increase in plant total production and overall plant performance. Therefore, it is worth discussing the factors that affect the TBT of the solar desalination plant. These are many, including the day of the year, feed flow rate, tank size, collector surface area, and collector efficiency. Day of the year The system was set up to run on day 355, i.e., the 21st of December, the day with lowest average temperatures of the year, to ensure that the storage system provided the unit with the required heat throughout the whole year. The TBT is affected by seasonal variation and location. During daytime, the solar collectors supply the storage tank with solar energy to increase the feed water temperature to the desired TBT. Hence, the TBT is strongly affected by the absorbed solar radiation, which is function of day of the year. The highest value of the solar field outlet temperature occurs around 2 p.m. when solar radiation is at its highest value. Moreover, days in the summer are longer than those in winter, resulting in more efficient absorption of heat in summer. Storage tank Temperature The storage tank temperature is held above the required desalination input temperature value due to losses throughout the day. To ensure minimum loss, the storage tank is insulated. The thermal storage tanks have two modes: charging (store mode/standby mode) and discharging (feed mode/in-service mode). The tank’s mode is switched every 24 h. To avoid scale formation, the solar field outlet temperature and temperature inside the thermal storage tank must be limited to about 120 ◦C. Tank Size The size of thermal storage tank is based on the peak volume of feed water during the year. A greater feed flow rate increases the size of the storage tank. The size of thermal storage tanks is determined based on plant capacity. So, for a plant capacity of 75 kg per hour, the size of one tank should be 1.3 m3. The balance between storage tank size and the total production is critical for cost analysis. Consequently, an increase in the production capacity requires a larger tank size. Solar field area The TBT is affected by the total area of the collector and number of collectors. Increas- ing the number of collectors increases the total absorbing area, which in turn increases the amount of solar heat gained by the feed water and the outlet temperature. Water 2021, 13, 2143 14 of 18

Collector inlet temperature The collector inlet temperature or storage tank temperature is equal to the average temperature of the water feed leaving the first stage during the night. This temperature will increase gradually during a period of daytime collection due to the absorption of heat from the collector. Collector outlet temperature Determining the collector outlet temperature depends on the initial storage tank temperature, the mass flow rate of the water, and the solar field area. The collector outlet average temperature is about 3 ◦C above the collector inlet temperature, as the temperature of the storage tank increases to the maximum value of 70 ◦C. Start-up The model is set to run preform desalination in normal operation mode. In other words, the model increases the thermal storage tank temperature to the TBT from the temperature of the water feed leaving the first stage at about 40 ◦C. On the other hand, during startup operation mode, the thermal storage tank temperature must be increased from the sea water temperature of about 30 ◦C to the TBT. The startup procedure needs a long time (or an auxiliary boiler) to raise the temperature to the required point. Solar field efficiency The solar field efficiency is defined as the ratio between the useful heat gain absorbed by the seawater and the beam solar energy incident on the aperture. The maximum value of the collection efficiency is about 72% when the solar radiation is the highest, and the maximum value of the solar field outlet temperature is around 70 ◦C.

5. Conclusions Conventional desalination processes are fossil fuel-intensive, costly, and environmentally harmful. It is imperative to find alternative methods to produce fresh water. Solar energy is believed to be an effective solution to water scarcity. A new multistage flash desalination with an integrated solar system was built and tested in this work. An experiment prototype was used in this design. The system consisted of three components: a three stage MSF unit, thermal storage tanks, and solar collectors. Two storage tanks were used for solar charging and MSF feeding. Of the two thermal storage tanks, one was in charging mode to store heat and the other was in discharging mode to provide the plant with thermal energy day and night, at full capacity. The roles of these tanks were switched on a daily basis. To get the target TBT, the mass flow rates were adjusted to track seasonal variations in solar power. The experiment work was conducted for a specific location, Taif, Saudi Arabia, using a three stage MSF unit. The results indicated: • Average distillate production was 19.7 kg/day with a solar collector area of 1.92 m2, which can be expressed as 10.3 kg/m2/day. • The TBT was about 70 ◦C • The average daily PR and SEC for the experiment were 3.2 and 717 kJ/kg, respectively. • The cost analysis for the solar MSF plant indicates a per-unit production cost of $0.015/L The present study focuses on experimental work. In the future, the results of the experimental work will be compared with a numerical model. A dynamic mathematical model will be developed and simulated with solar powered three-stage desalination. An optimization simulation should also be performed to determine the best performance, and an economic feasibility study should be performed to show the net benefit of the proposed project.

Funding: This work was ﬁnancially supported by Taif University Researchers Supporting Project number (TURSP-2020/205). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Water 2021, 13, 2143 15 of 18

Data Availability Statement: All the results shown in the manuscript can be requested from the author. Acknowledgments: The author would like to gratefully acknowledge the financial support by Taif University Researchers Supporting Project number (TURSP-2020/205), Taif University, Taif, Saudi Arabia. Conflicts of Interest: The author declares no conflict of interest.

Appendix A MSF process modeling Once-Through MSF (MSF-OT) Model To model the MFS unit, it is easy to illustrate a single-stage flash evaporation. Each stage in the MSF is divided into four control volumes: 1. The flashing brine pool. 2. The distillate. 3. The vapor space. 4. The condenser tubes. The mathematical model for the single flash is simple, and it includes total mass and salt balances, as well as energy balances for the chamber and the condenser. The distillate that condenses at each stage is given by Equation (A1)

" #i−1 Cp∆TB Cp∆TB Di(d) = MFW (d − 1) 1 − (A1) λavg,i λavg,i

where the parameter λavg,i(d) represents the average latent heat for the ith stage of the MSF, which is a function of the temperature. The total distillate produced in a day, MDIST(k) is therefore given by Equation (A2).

( ) = n ( ) MDIST d ∑i=1 Di d (A2) Solar Field Model The model is deﬁned by processing complete design speciﬁcations which include parameters of the sun’s angles and the geometric dimensions of collector. Parameters such as beam radiation, incident angles, heat losses, and dimensions are used to calculate absorbed solar radiation, and are then used in an energy balance equation to give the amount of energy gained in units of watts (Qu). The governing equation is a widely used measure of collector heat gain. Generally known as the “Hottel Whillier-Bliss equation”, it is written as h i Qu = FR A Iτα − UL Tf i − Ta (A3) Tf i − Ta η = F τα − F U R R L I

where Q u Useful energy gain W; η Collector efficiency; FR Collector heat removal factor; I Intensity of solar radiation, W/m2; α Absorption coefficient of plate; τ Transmission coefficient of glazing. Thermal Storage System Model The thermal storage tank system consists of following units: • Two thermal tanks, one in charging mode and the other in the discharging mode. • Circulation pumps. • Pipes. The present model was developed by taking a thermal storage tank as a single unit and then calculating both the mass and energy balances of that unit. This system was assumed to be a fully mixed tank. A single flow comes in from a hot source, and a single Water 2021, 13, x FOR PEER REVIEW 17 of 19

The model is defined by processing complete design specifications which include parameters of the sun’s angles and the geometric dimensions of collector. Parameters such as beam radiation, incident angles, heat losses, and dimensions are used to calculate absorbed solar radiation, and are then used in an energy balance equation to give the amount of energy gained in units of watts (Qu). The governing equation is a widely used measure of collector heat gain. Generally known as the “Hottel Whillier-Bliss equation,” it is written as Water 2021, 13, 2143 16 of 18 푄푢 = 퐹푅퐴[퐼휏훼 − 푈퐿(푇푓𝑖 − 푇푎)] (A3) 푇푓𝑖 − 푇푎 휂 = [퐹 휏훼 − 퐹 푈 ( )] flow stream leaves to a load. The level of푅 the fluid푅 퐿 in the퐼 tank can vary because the entering and leaving flows are not equal. In this scenario, the temperature of the surplus flow where 푄 Useful energy gain W; 휂 Collector efficiency; 퐹 Collector heat removal fac- stream is the푢 temperature of the contents of the tank. Here, the푅 thermal storage tanks are tor; I Intensity of solar radiation, W/m2; α Absorption coefficient of plate; τ Transmission assumed to be perfectly insulated, and heat losses are considered. Figure A1 presents the coefficient of glazing. tank energy balance. Thermal Storage System Model Energy balance The thermal storage tank systemdE consists. of. following. units: = E − E + E (A4) • Two thermal tanks, one in chargingdt modein outand theg other in the discharging mode. • Circulation pumps. E1 = Mchg(h, d)CpTchg(h, d) (A5) • Pipes. The present model was developed by taking a thermal storage tank as a single unit E = Q (h, d) (A6) and then calculating both the mass and2 energyu balances of that unit. This system was assumed to be a fully mixed tank. A single flow comes in from a hot source, and a single flow stream leaves to a load. TheE3 =level(M ofFW the(d) fluid/24) TinFW the(d tank)Cp can vary because the entering(A7) and leaving flows are not equal. In this scenario, the temperature of the surplus flow stream is the temperature of the contentsh of the tank. Here, thei thermal storage tanks are E4 = UA(h, d) Tchg(h, d) − Tamb(h, d) (A8) assumed to be perfectly insulated, and heat losses are considered. Figure A1 presents the tankPerformance energy balance. ratio MDIST λavg,i Energy balance PR = (A9) Qu 푑퐸 = 퐸 ̇ − 퐸 ̇ + 퐸̇ (A4) Recovery ratio 푑푡 푖푛 표푢푡 푔 M RR = DIST (A10) MFW

Figure A1. Energy balance of the storage tank.

Figure A1. Energy balance of the storage tank. Nomenclature

Variable Units Description A m2 Collector Area d Day h h Hour Di(k) kg/h Distillate Produced at each Stage FR Heat Removal Factor I W/m2 intensity of solar radiation MDIST(d) kg/day Distillate feedwater production rate MFW (d) kg/day Feedwater ﬂow rate from MSF Water 2021, 13, 2143 17 of 18

Mchg(h, d) kg Mass of Charming Tank Ns Number of MSF Stages Qu(h, d) W Useful Heat Gain ◦ TA (h, d) C Ambient Temperature ◦ TTBT C Top Brine Temperature ◦ Tchg(h, d) C Charging Tank temperature ◦ Ti C Stage Temperature Tth m Tank wall thickness η0 Zero Efficiency 2 ◦ UL W/m C collector overall heat loss coefficient ◦ UAchg(h, d) J/(h C) Loss coefficient for Charging Tank η Collector Efficiency E J Energy λavg,i J/kg Latent Heat for the MSF Cp kJ/kg·K Specific heat for the brain α Absorption coefficient of plate τ Transmission coefficient of glazing FR Collector heat removal factor ◦ ∆TB C Brine Flow Decreases

References 1. Berenguel-Felices, F.; Lara-Galera, A.; Muñoz-Medina, M.B. Requirements for the Construction of New Desalination Plants into a Framework of Sustainability. Sustainability 2020, 12, 5124. [CrossRef] 2. 1 in 3 People Globally Do Not Have Access to Safe Drinking Water—UNICEF, WHO. 2019. Available online: https://www. who.int/news/item/18-06-2019-1-in-3-people-globally-do-not-have-access-to-safe-drinking-water-unicef-who- (accessed on 15 April 2021). 3. Bundschuh, J.; Kaczmarczyk, M.; Ghaffour, N.; Tomaszewska, B. State-of-the-art of renewable energy sources used in water desalination: Present and future prospects. Desalination 2021, 508, 115035. [CrossRef] 4. Burn, S.; Hoang, M.; Zarzo, D.; Olewniak, F.; Campos, E.; Bolto, B.; Barron, O. Desalination techniques—A review of the opportunities for desalination in agriculture. Desalination 2015, 364, 2–16. [CrossRef] 5. Eltawil, M.A.; Zhengming, Z.; Yuan, L. A review of renewable energy technologies integrated with desalination systems. Renew. Sustain. Energy Rev. 2009, 13, 2245–2262. [CrossRef] 6. Sea Water Desalination System, McMurdo. 1993. Available online: https://nsf.gov/pubs/stis1993/opp93104/opp93104.txt#:~{}: text=Advantagesofamulti-stage,ahighgainoutputratio (accessed on 17 April 2021). 7. Prajapati, M.; Shah, M.; Soni, B.; Parikh, S.; Sircar, A.; Balchandani, S.; Thakore, S.; Tala, M. Geothermal-solar integrated groundwater desalination system: Current status and future perspective. Groundw. Sustain. Dev. 2021, 12, 100506. [CrossRef] 8. Sharon, H.; Reddy, K.S. A review of solar energy driven desalination technologies. Renew. Sustain. Energy Rev. 2015, 41, 1080–1118. [CrossRef] 9. Darawsheh, I.; Islam, M.D.; Banat, F. Experimental characterization of a solar powered MSF desalination process performance. Therm. Sci. Eng. Prog. 2019, 10, 154–162. [CrossRef] 10. Manjarrez, R.; Galván, M. Solar multistage flash evaporation (SMSF) as a solar energy application on desalination processes. Description of one demonstration project. Desalination 1979, 31, 545–554. [CrossRef] 11. Ibarra-Herrera, X. Solar multistage flash evaporation (smsf) as a solar energy application on desalination processes. Desalination 1979, 30, 91. [CrossRef] 12. Rajvanshi, A.K. A scheme for large scale desalination of sea water by solar energy. Sol. Energy 1980, 24, 551–560. [CrossRef] 13. Moustafa, S.M.A.; Jarrar, D.I.; El-Mansy, H.I. Performance of a self-regulating solar multistage flash desalination system. Sol. Energy 1985, 35, 333–340. [CrossRef] 14. Farwati, M.A. Theoretical study of multi-stage flash distillation using solar energy. Energy 1997, 22, 1–5. [CrossRef] 15. Lu, H.C.; Walton, J.; HPSwift, A. Desalination coupled with salinity-gradient solar ponds. Desalination 2001, 136, 13–23. [CrossRef] 16. Nafey, A.S.; Mohamad, M.A.; El-Helaby, S.O.; Sharaf, M.A. Theoretical and experimental study of a small unit for solar desalination using flashing process. Energy Convers. Manag. 2007, 48, 528–538. [CrossRef] 17. Alsehli, M.; Choi, J.-K.; Aljuhan, M. A novel design for a solar powered multistage flash desalination. Sol. Energy 2017, 153, 348–359. [CrossRef] 18. Al-Othman, A.; Tawalbeh, M.; El Haj Assad, M.; Alkayyali, T.; Eisa, A. Novel multi-stage flash (MSF) desalination plant driven by parabolic trough collectors and a solar pond: A simulation study in UAE. Desalination 2018, 443, 237–244. [CrossRef] 19. Méndez, C.; Bicer, Y. Integration of solar chimney with desalination for sustainable water production: A thermodynamic assessment. Case Stud. Therm. Eng. 2020, 21, 100687. [CrossRef] 20. Leblanc, J. Solar Thermal Desalination—A Modelling and Experimental Study; RMIT University: Melbourne, Australia, 2009. Water 2021, 13, 2143 18 of 18

21. Zarzoum, K.; Zhani, K.; Ben Bacha, H.; Koschikowski, J. Experimental parametric study of membrane distillation unit using solar energy. Sol. Energy 2019, 188, 1274–1282. [CrossRef] 22. Jiang, J.; Tian, H.; Cui, M.; Liu, L. Proof-of-concept study of an integrated solar desalination system. Renew. Energy 2009, 34, 2798–2802. [CrossRef] 23. Abu-Jabal, M.S.; Kamiya, I.; Narasaki, Y. Proving test for a solar-powered desalination system in Gaza-Palestine. Desalination 2001, 137, 1–6. [CrossRef] 24. Joseph, J.; Saravanan, R.; Renganarayanan, S. Studies on a single-stage solar desalination system for domestic applications. Desalination 2005, 173, 77–82. [CrossRef]