Techno–Enviro–Economic Feasibility Assessment of Family-Scale Solar Still (F-SSS) Desalination Plant in Central American and Caribbean Sites for Sustainable Clean Water Supply

Abstract

The viability of the family-scale solar still (F-SSS) desalination plant in nine low- and middle-income Central American and Caribbean sites, with improper water treatment facilities and supply networks, has been analyzed and reported in detail. The sizing of the desalination plant was done based on the still’s performance, clean water requirement and solar radiation potential. The still’s performance was estimated using an experimentally validated thermodynamic model. Annual desalinated water productivity per still was about 979.0 L (highest) and 836.0 L (lowest) in Port-au-Prince and Belize City, respectively. The lowest and highest potable water production price was observed in Havana (19.75 to 20.22 USD/m³) and Port-au-Prince (59.23 to 60.62 USD/m³) due to their low and high local interest rates, respectively. The decarbonization potential of the F-SSS desalination plant with a 25-year lifetime ranged between 37 and 641 tons of CO₂ emission. The specific CO₂ generated was found to be the least and highest in San Salvador (4.24 to 4.34 g/L of desalinated water) and Port-au-Price (13.70 to 14.04 g/L of desalinated water), respectively. The energy, finance payback time and sustainability index of the F-SSS desalination plant ranged between 0.59 and 0.67 years, 1.2 and 18.0 months, and 1.03 and 1.04, respectively. The performance, economic and environmental aspects revealed positive signs on the applicability of the F-SSS desalination plant in Central American and Caribbean sites for reliable and sustainable clean water supply. However, this process can be ratified if the concerned governments implement a reasonable subsidy, as is the case with other renewable energy systems.

1. Introduction

Clean water is essential for a healthy lifestyle, improved human capital and economic productivity [1]. However, the reduced availability of fresh water is one of the alarming issues that has been circling the world in recent decades. A rapid rise in the human population, increased water withdrawals and improper water management, along with global climate change and contamination of the available fresh water are the main factors contributing to this paradigm [2,3]. Annually, around 0.8 to 1.8 million lives can be saved globally if proper access to safe water, sanitation and hygiene are made accessible to all humans [4]. These drastic scenarios necessitate the need for innovative, cheap and efficient water treatment techniques to make clean water accessible to all. Moreover, the provision of clean water at the point of need is valuable as it reduces the burden on women and children and improves their lifestyle.

Desalination is one such technique which produces potable water from saline and contaminated water sources, at the point of need [5,6]. Desalination can be achieved either by employing pressure-driven semi-permeable membranes or by utilizing thermal energy-driven evaporation systems [7]. The membrane process consumes less energy and has a higher recovery rate than the thermal process. However, they constantly contribute to environmental damage due to their highly concentrated brine disposal [8]. Conventional desalination technologies like electrodialysis (ED), reverse osmosis (RO), multi-stage flash (MSF), multi-effect distillation (MED) and vapor compression (VC) techniques are very promising in addressing water scarcity. These technologies occupy a huge share in the global desalination market [9]. The global installed desalination capacity has raised from thirty-five million m³/d in 2005 to ninety-five million m³/d in 2018 [8]. More than 40.0% of the global population resides within 100 km from the coast and the majority of these regions ironically suffer from water scarcity [10].

Water scarcity in locations that are within 100 km from the coast can be tackled effectively by sea water desalination [11]. At least 10,000 m³ of oil per year is consumed to produce 1000 m³/d of desalinated water, which indirectly depicts the associated huge greenhouse gas emissions [7]. Moreover, nearly, 70% of the global carbon emissions are from coastal regions which house approximately 50% of the global population [12]. The combining of renewable energies, namely solar, geothermal, wind and ocean energy, with the desalination systems has recurringly proven to be the best option for sustainable desalination even in remote areas [5,13]. This integration helps in achieving United Nations Sustainable Development Goals (SDGs), namely, SDG 6 (clean water and sanitation), SDG 7 (affordable and clean energy) and SDG 13 (climate action) [14,15]. Among renewables, solar energy has been widely incorporated with desalination [16,17]. Moreover, small- and medium-scale solar thermal desalination systems are expected to overcome the present drawbacks and become commercially viable in the near future [18,19].

The solar water desalination systems can be either direct (solar energy collection and desalination take place within the same system), or indirect (solar energy collection and desalination occur in separate systems) types [20]. The desalinated water production rate of direct and indirect desalination systems ranges between 2.0 and 3.0 L/m²d and 5.0 and 12.0 L/m²d, respectively [21]. The solar still has also been more effective in the de-fluoridation of drinking water [22]. Despite their low water production rate, direct solar desalination systems have proved to be more suitable for small-scale applications [20]. The solar still is a well-known and extensively investigated direct solar desalination system [23]. Different configurations of the passive solar still include a single-slope basin solar still [24], double slope basin solar still [25], stepped basin solar still [26], stairway solar still [27], conical solar still [28], pyramid solar still [29], hemispherical solar still [30], spherical solar still [31], tubular solar still [32], vertical solar still [33], rhombus solar still [34] and inclined solar still [35]. Among these configurations, single-slope basin and inclined solar stills are easy to construct. The inclined solar still uses a wick which may clog and require regular replacement [36]. Hence, the single-slope solar still seems more compatible for application in remote and rural regions with less skilled manpower. Integrating solar stills with solar collectors and solar photovoltaic-powered electric heaters are effective in enhancing their desalinated water productivity [37,38,39]. However, the high initial cost, pumping requirements and need for additional maintenance may limit the application of active solar stills in underprivileged regions.

Large-scale solar still desalination plants were deployed successfully for potable water production in Australia, Chile, Greece, India, Spain, Tunisia, the USA, the USSR, and the West Indies during the 19th and 20th centuries [40,41,42,43,44]. A solar still-based desalination plant of 10,000 m² area was identified to provide potable water at a cheaper rate than a solar PV-powered RO plant in California, USA [45]. A three m³/d capacity solar still desalination plant is expected to produce desalinated water at a price less than traditional water supply tariffs in various locations of Somalia [46]. Environmental impacts associated with decentralized desalination systems are significantly lower than centralized desalination systems [47]. Low- and middle-income Central American and Caribbean countries which have vast coastlines, suffer from economic water scarcity [7,48] and are projected to have medium to extremely high water stress by 2040 [19]. Only a few scientific publications are available from the Central American and Caribbean countries in the area of desalination compared to Asia, North America and Australia [10]. Low- and middle-income Central American and Caribbean countries fall in the tropical zone and have 4.4 to 6.0 kWh/m²d of abundant solar irradiance potential [49]. Hence, the utilization of solar energy could be a suitable option for sea water desalination in those regions [19,50].

In this research work, an effort has been made to identify the viability of a family-scale solar still (F-SSS) desalination plant in low- and middle-income Central American and Caribbean countries. An assessment for these sites is not available in any of the existing peer-reviewed literature to the authors’ knowledge and this gap has been fulfilled in this work. The materials and methods associated with the experimental basin solar still’s experimentation, modeling and behavior (performance, characterization, and distillate water quality) are detailed in Section 2. Section 3 presents the details of sites considered for F-SSS desalination plant deployment, including the sizing procedure, economics (price per m³ of desalinated water and finance payback time) and environmental aspects (specific CO₂ emission, sustainability index and decarbonization potential). Major inferences and scope for further investigations have been briefed in Section 4. The methodology formulated for this research work is outlined in Figure 1. The results of the work seem encouraging and justify the potential of sustainable solar desalination in addressing water scarcity in an affordable way. Moreover, the work will be useful to researchers, policy makers and renewable energy advocates who are enthusiastic in addressing water scarcity in a sustainable way.

2. Performance Investigation on Basin Solar Still

The details of the experimental basin solar still are presented in Section 2.1. The experimental procedure and the thermodynamic modelling of the still are briefed in Section 2.2 and Section 2.3, respectively. The weather conditions at the experimental site during the experimental days are described in Section 2.4. The discussion on the instantaneous desalinated water production rate, temperature of still components, daily performance, characterization and water quality analysis are presented in Section 2.5, Section 2.6, Section 2.7, Section 2.8 and Section 2.9, respectively.

2.1. Basin Solar Still—System Description

The conventional simple basin solar still used in this investigation is fabricated using a 1.0 mm thick stainless-steel sheet and is photographically presented in Figure 2 and Figure 3. The still is provided with a V-trough of dimensions 0.055 m wide and 0.040 m deep at the lower end of its basin. The actual aperture dimension of the still is about 0.88 × 0.87 m. An inlet pipe is introduced from one of the side walls of the still which is about 0.10 m from the bottom of the still through which feed sea water is poured with the aid of a funnel. The aperture of the still is covered by a transparent 0.004 m thick tempered glass cover of dimensions 0.935 × 0.910 m. A rubber gasket of 0.02 m width and 0.003 m thickness is installed between the glass cover and the solar still contact area to provide a cushioning effect and proper seating for the glass cover. Further, the edges are wrapped with packing tape to prevent vapor leakage.

The outer sides of the still were covered with 7.5 mm thick foam. Further, the outer bottom side of the still was layered with an expanded polyethylene (EPE) foam of 45.0 mm thickness to reduce heat loss from the system to the ambient air. The basin inner surfaces were coated with a layer of matte black paint to maximize solar radiation absorption for effective basin water heating. The heated basin water evaporates and the generated vapors condense over the glass cover’s inner surface which then roll down the glass cover inclination. The condensate is directed towards the V-trough by a glass strip of 0.875 m length which is pasted at a distance of 0.03 m from the bottom edge of the glass cover. The solar still setup is placed facing due south over a mild steel stand of 0.355 m height.

2.2. Experimental Procedure

The experimental field study on the developed basin solar still was conducted for a span of 4 days from 26 to 29 May 2024 under the climatic conditions of Visakhapatnam (17.7° N, 83.2° E) Andhra Pradesh, India, to evaluate its performance. The system was installed facing due south over the terrace of the Tech-Horizon Building, Indian Institute of Petroleum and Energy (IIPE) Visakhapatnam. The still was loaded on the night before the experimental start date with 20.0 L of feed sea water collected from the Ramakrishna Beach, Visakhapatnam. The basin solar still in this investigation was operated in non-continuous mode, i.e., the still was loaded with feed water only once, and no makeup water was added until the end of the investigation [51]. The study was performed from 9:00 a.m. to 18:00 p.m. each day, during which the thermocouple, anemometer and pyranometer readings along with the desalinated water collection rate were measured every 15 min. The nocturnal desalinated water production on each experimental day was also measured. The qualities of feed sea water and desalinated water were also measured and reported. The equipment used for measurements along with their purpose, accuracy and error percentage are shown in

Table 1. List of equipment utilized for experimentation.

S. No.	Equipment	Quantifying Purpose	Measuring Range	Accuracy	Maximum Error
1.	Kipp and Zonen SMP 10 Class A Pyranometer (Make: OTT Hydromet B.V., Delft, The Netherlands)	Tilted global solar insolation	0.0 to 2000.0 W/m2	±1.0 W/m2	±5.88%
2.	HTC AVM-03 Handheld Digital Anemometer (Make: HTC instrument, Mumbai, India)	Wind speed and ambient temperature	0.0 to 30.00 m/s; −20.0 °C to +60.0 °C	±0.7 m/s; ±1.0 °C	±3.02%
3.	K-Type Thermocouple	Glass cover, basin water and basin temperature	0.0 to 1250.0 °C	±1.5 °C	±4.13%
4.	Measuring Jar	Desalinated water output	0.0 to 100.0 mL	±0.5 mL	±6.25%
			0.0 to 1000.0 mL	±5.0 mL	±1.25%
			0.0 to 2000.0 mL	±10.0 mL	±1.16%
5.	Portable Multi-Parameter Water Tester	pH	0.0 to 14.0	±0.03	±0.60%
		Electrical conductivity	0.0 to 19,990 µS/cm	±2.0% of reading	±3.0%
		Salinity	0.0 to 19,990 ppm	±2.0% of reading	±6.0%
		Total dissolved solids	0.0 to 9990 ppm	±2.0% of reading	±6.0%

. The K-type thermocouple readings and thermophile pyranometer readings were recorded on a desktop computer with the aid of Keysight Datalogger (Model: DAQ970A; Make: Keysight Technologies, Santa Rosa, CA, USA) and “Smart Explorer” software (Version 1.5), respectively. The glass cover, basin water and basin liner temperatures were measured at three different points on each of them, and the average value was used for presentation and analysis.

2.3. Thermodynamic Modeling of Conventional Basin Solar Still

The basin solar still used in this experimental work was simulated by a developed thermodynamic model to predict its distillate yield and thermal and exergy efficiency. The energy budget for the solar still is pictorially represented in Figure 4. The major assumptions considered for its thermodynamic modeling are as follows:

• No vapors escape the still and it is leak free [52];
• Thermal gradients across the thickness of the basin water and glass cover are negligible [52];
• Feed water salinity and depth are nearly constant on the simulation day [53];
• There is no salt concentration in the obtained distillate [54];
• The basin liner loses heat energy by conduction to the ambient air via insulation [55];
• The basin water loses heat energy by convection, radiation and evaporation to the glass cover [55].

The basin liner’s energy budget is as follows:

$$\stackrel{\begin{array}{c}\mathrm{Rate} \mathrm{of} \mathrm{change} \mathrm{of} \\ \mathrm{basin} \mathrm{liner} \\ \mathrm{temperature}\end{array}}{\overbrace{m_{bl}{c}_{bl}{\displaystyle \frac{d{T}_{bl}}{dt}}}}=\stackrel{\begin{array}{c}\mathrm{Incident} \mathrm{solar} \mathrm{radiation}\\ \mathrm{absorbed} \mathrm{by} \mathrm{basin} \mathrm{liner}\end{array}}{\overbrace{I_t{\tau }_{gc}{\tau }_w{\alpha }_{bl}{A}_{bl}}}-\stackrel{\begin{array}{c}\mathrm{Convection} \mathrm{heat} \mathrm{transfer} \mathrm{from}\\ \mathrm{basin} \mathrm{liner} \mathrm{to} \mathrm{basin} \mathrm{water}\end{array}}{\overbrace{h_{c\left(bl-bw\right)}{A}_{bl}\left(T_{bl}-T_{bw}\right)}}-\stackrel{\begin{array}{c}\mathrm{Heat} \mathrm{loss} \mathrm{from} \mathrm{basin} \mathrm{liner} \\ \mathrm{to} \mathrm{ambient} \mathrm{via} \mathrm{insulation}\end{array}}{\overbrace{{\displaystyle \frac{\left(T_{bl}-T_a\right)}{\left(\frac{L_{ins}}{K_{ins}{A}_{bl}}+\frac{1}{h_{ca}{A}_{bl}}\right)}}}}$$

(1)

The basin water’s energy budget is as follows:

$$\begin{array}{c}\stackrel{\begin{array}{c}\mathrm{Rate} \mathrm{of} \mathrm{change} \mathrm{of} \\ \mathrm{basin} \mathrm{water} \mathrm{temperature}\end{array}}{\overbrace{m_{bw}{c}_{bw}{\displaystyle \frac{d{T}_{bw}}{dt}}}}=\stackrel{\begin{array}{c}\mathrm{Incident} \mathrm{solar} \mathrm{radiation} \\ \mathrm{absorbed} \mathrm{by} \mathrm{basin} \mathrm{water}\end{array}}{\overbrace{I_t{\tau }_{gc}{\alpha }_{bw}{A}_{bw}}}+\stackrel{\begin{array}{c}\mathrm{Convection} \mathrm{heat} \mathrm{transfer} \\ \mathrm{from} \mathrm{basin} \mathrm{liner} \mathrm{to} \mathrm{basin} \mathrm{water}\end{array}}{\overbrace{h_{c\left(bl-bw\right)}{A}_{bl}\left(T_{bl}-T_{bw}\right)}}-\stackrel{\begin{array}{c}\mathrm{Convection} \mathrm{heat} \mathrm{transfer} \mathrm{from} \\ \mathrm{basin} \mathrm{water} \mathrm{to} \mathrm{basin} \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{h_{c\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}\\ -\stackrel{\begin{array}{c}\mathrm{Radiation} \mathrm{heat} \mathrm{transfer} \mathrm{from} \\ \mathrm{basin} \mathrm{water} \mathrm{to} \mathrm{basin} \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{h_{r\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}-\stackrel{\begin{array}{c}\mathrm{Evaporation} \mathrm{heat} \mathrm{transfer} \mathrm{from} \\ \mathrm{basin} \mathrm{water} \mathrm{to} \mathrm{basin} \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{h_{e\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}\end{array}$$

(2)

The glass cover’s energy budget is as follows:

$$\begin{array}{cc}\stackrel{\begin{array}{c}\mathrm{Rate} \mathrm{of} \mathrm{change} \mathrm{of} \mathrm{basin} \\ \mathrm{glass} \mathrm{cover} \mathrm{temperature}\end{array}}{\overbrace{m_{gc}{c}_{gc}{\displaystyle \frac{d{T}_{gc}}{dt}}}}& =\stackrel{\begin{array}{c}\mathrm{Incident} \mathrm{solar} \mathrm{radiation} \\ \mathrm{absorbed} \mathrm{by} \mathrm{basin} \\ \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{I_t{\alpha }_{gc}{A}_{gc}}}+\stackrel{\begin{array}{c}\mathrm{Convection} \mathrm{heat} \mathrm{transfer} \mathrm{from}\\ \mathrm{basin} \mathrm{water} \mathrm{to} \mathrm{basin} \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{h_{c\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}+\stackrel{\begin{array}{c}\mathrm{Radiation} \mathrm{heat} \mathrm{transfer} \mathrm{from} \\ \mathrm{basin} \mathrm{water} \mathrm{to} \mathrm{basin} \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{h_{r\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}\hfill \\ & +\stackrel{\begin{array}{c}\mathrm{Evaporation} \mathrm{heat} \mathrm{transfer} \mathrm{from} \\ \mathrm{basin} \mathrm{water} \mathrm{to} \mathrm{basin} \mathrm{glass} \mathrm{cover}\end{array}}{\overbrace{h_{e\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}-\stackrel{\begin{array}{c}\mathrm{Convection} \mathrm{heat} \mathrm{loss} \mathrm{from} \\ \mathrm{basin} \mathrm{glass} \mathrm{cover} \mathrm{to} \mathrm{ambient}\end{array}}{\overbrace{h_{ca}{A}_{gc}\left(T_{gc}-T_a\right)}}\hfill \\ & -\stackrel{\begin{array}{c}\mathrm{Radiation} \mathrm{heat} \mathrm{loss} \mathrm{from} \\ \mathrm{basin} \mathrm{glass} \mathrm{cover} \mathrm{to} \mathrm{the} \mathrm{sky}\end{array}}{\overbrace{h_{r\left(gc-sky\right)}{A}_{gc}\left(T_{gc}-T_{sky}\right)}}\hfill \end{array}$$

(3)

The convective heat transfer coefficient between the basin liner and basin water was estimated by Equation (4) [56],

$$h_{c\left(bl-bw\right)}={\displaystyle \frac{Nu\times K_{bw}}{L_c}}$$

(4)

The Nusselt number was estimated using the following correlations shown in Equation (5) [56],

$$\begin{array}{l}Nu=1.0 \mathrm{for} (\mathrm{Gr}<{10}^5)\\ \hspace{1em} =0.50{\left(Gr\times {\mathrm{Pr}}_{bw}\right)}^{0.25}\mathrm{for} \left({10}^5 < \mathrm{Gr} < {2.0\times 10}^7\right)\\ \hspace{1em} =0.15{\left(Gr\times {\mathrm{Pr}}_{bw}\right)}^{0.33}\mathrm{for} \left(\mathrm{Gr} > {2.0\times 10}^7\right)\end{array}$$

(5)

The Grashoff number was estimated by Equation (6) [56],

$$Gr={\displaystyle \frac{g\times \beta \times \left(T_{bl}-T_{bw}\right)\times {\left(L_c\right)}^3}{{\left(\frac{{\mu }_{bw}}{{\rho }_{bw}}\right)}^2}}$$

(6)

The characteristic length of the basin liner was estimated by Equation (7) [53],

$$L_c={\displaystyle \frac{L\times Br}{2.0\left(L+Br\right)}}$$

(7)

The convection heat transfer coefficient between the basin water and basin glass cover was evaluated by Equation (8) [52],

$$h_{c(bw-gc)}=0.884\times {\left(\left(T_{bw}-T_{gc}\right)+{\displaystyle \frac{\left(P_{bw}-P_{gc}\right)\times \left(T_{bw}+273.15\right)}{\left[\left(\frac{M_a\times P_t}{M_a-M_v}\right)-P_{bw}\right]}}\right)}^{{\displaystyle \frac{1}{3}}}$$

(8)

The evaporation heat transfer coefficient between the basin water and basin glass cover was evaluated by Equation (9) [52],

$$h_{e\left(bw-gc\right)}=0.01623\times h_{c\left(bw-gc\right)}\times {\displaystyle \frac{\left(P_{bw}-P_{gc}\right)}{\left(T_{bw}-T_{gc}\right)}}$$

(9)

The radiation heat transfer coefficient between the basin water and basin glass cover was evaluated by Equation (10) [52],

$$h_{r\left(bw-gc\right)}={\displaystyle \frac{\sigma \times \left[{\left(T_{bw}+273.15\right)}^2+{\left(T_{gc}+273.15\right)}^2\right]\times \left(T_{bw}+T_{gc}+546.30\right)}{\left(\frac{1}{{\epsilon }_{bw}}\right)+\left(\frac{1}{{\epsilon }_{gc}}\right)-1}}$$

(10)

The partial pressures of water vapor at the basin water and glass cover temperatures were estimated by Equations (11) and (12), respectively [57,58],

$$P_{bw}={\displaystyle \frac{\exp \left(25.317-\frac{5144.0}{\left(T_{bw}+273.15\right)}\right)}{1.0+\left(0.57357\times \left(\frac{S_g}{1000.0-S_g}\right)\right)}}$$

(11)

$$P_{gc}=\exp \left(25.317-{\displaystyle \frac{5144.0}{\left(T_{gc}+273.15\right)}}\right)$$

(12)

The ambient convection heat transfer coefficient was estimated by Equation (13) [59],

$$\begin{array}{cc}{h}_{ca}& =2.7+\left(3.8\times V\right)\mathrm{if} \mathrm{V} < 5.0 \mathrm{m}/\mathrm{s}\hfill \\ & =6.15\times {\left(V\right)}^{0.8}\mathrm{if} \mathrm{V} > 5.0 \mathrm{m}/\mathrm{s}\hfill \end{array}$$

(13)

The radiation heat transfer coefficient between the basin glass cover and the sky was estimated by Equation (14) [59],

$$h_{r\left(gc-sky\right)}=\sigma \times {\epsilon }_{gc}\times \left({\left(T_{gc}+273.15\right)}^2+{\left(T_{sky}\right)}^2\right)\times \left(T_{gc}+T_{sky}+273.15\right)$$

(14)

The temperature of the sky (in Kelvin) was estimated by the relation shown in Equation (15) [59],

$$T_{sky}=0.0552\times {\left(T_a+273.15\right)}^{1.5}$$

(15)

The instantaneous distillate yield was estimated by the ratio of evaporative heat transfer to the latent heat of evaporation shown in Equation (15) [58].

$$\stackrel{•}{m_d}={\displaystyle \frac{h_{e\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}{h_{fg}}}$$

(16)

The temperature-dependent latent heat of evaporation of basin water was estimated by Equation (17) [60],

$$\begin{array}{cc}{h}_{fg}& =2493500\times \left(\begin{array}{l}1.0-\left(0.00094779\times T_{bw}\right)+\left(0.0000001312\times {\left(T_{bw}\right)}^2\right)\\ -\left(4.7974\times {10}^{-9}\times {\left(T_{bw}\right)}^3\right)\end{array}\right) \mathrm{if}\left(T_{bw}\le 70 °\mathrm{C}\right)\hfill \\ & =3161500\times \left(1.0-\left(0.0007616\times T_{bw}\right)\right)\mathrm{if}\left(T_{bw}>70 °\mathrm{C}\right)\hfill \end{array}$$

(17)

The properties of humid air within the still were estimated by the correlations presented in [60]. The cumulative distillate yield per day is the summation of instantaneous distillate yield from the start to the end of experimentation on a day. Initial brine salinity on Day 2, Day 3 and Day 4 was estimated by the mass and concentration balance equations shown in Equations (18) and (19) [56]

$$M_f=M_b+M_d$$

(18)

$$M_f{X}_f=M_b{X}_b+M_d{X}_d$$

(19)

For simplicity, the distillate’s salt concentration was assumed to be nil while estimating the brine salinity.

The water transmissivity with respect to its depth (in m) was estimated by Equation (20) [61],

$${\tau }_w=0.38-\left(0.08\times \ln \left(d_w\right)\right)$$

(20)

The daily thermal and exergy efficiency of the solar still was estimated by Equations (21) and (22), respectively [62],

$${\eta }_{th}={\displaystyle \frac{{\displaystyle \sum h_{e\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)}}{{{\displaystyle \sum I_{t}A}}_a}}\times 100$$

(21)

$${\eta }_{ex}={\displaystyle \frac{{\displaystyle \sum \left(h_{e\left(bw-gc\right)}{A}_{bw}\left(T_{bw}-T_{gc}\right)\right)\left(1-\frac{T_a+273.15}{T_{bw}+273.15}\right)}}{{\displaystyle \sum I_t{A}_a\left(1+\left(\frac{1}{3}{\left[\frac{T_a+273.15}{6000.0}\right]}^4\right)-\left(\frac{4}{3}\left(\frac{T_a+273.15}{6000.0}\right)\right)\right)}}}\times 100$$

(22)

The discretization of the differential equations (Equations (1)–(3)) were done using the finite difference technique. The discretized equations were solved by an iteration method using the developed FORTRAN code. The essential input data, namely hourly solar radiation intensity, wind speed and ambient temperature, were fed as a .DAT file to solve the FORTRAN code. The temperatures of the components were initially guessed to predict the heat transfer coefficient, heat transfer rate, distillate yield and solar still performance. If the guessed temperatures matched with the new estimated temperatures, the output data were stored and the predicted new temperature values were updated as initial values for the next time step. If the difference between the guessed and predicted temperatures deviated beyond the tolerance limit, the predicted temperatures were updated as initial values for the current time step and the iteration was repeated. The time step size and convergence criteria used for solving the equations were 0.1 s and 0.001, respectively. Parameters used for simulating the performance of the basin solar still are tabulated in

Table 2. Parameters used for simulating the basin solar still.

Acceleration due to gravity	9.81 m/s2	Effective aperture area of the still	~0.77 m2
Absorptivity of basin liner [58]	0.95	Evaporation surface area	~0.84 m2
Absorptivity of glass	0.05	Length of the glass cover	0.91 m
Absorptivity of water [63]	0.05	Length of the still	0.92 m
Breadth of the still	0.91 m	Molecular weight of air	28.96 kg/kmol
Breadth of the glass cover	0.935 m	Molecular weight of water	18.0 kg/kmol
Density of stainless-steel basin liner plate	7900.0 kg/m3	Specific heat capacity of stainless-steel basin liner plate	477.0 J/kg-K
Density of tempered glass cover [56]	2500.0 kg/m3	Specific heat capacity of tempered glass cover	670.0 J/kg-K
Emissivity of glass [64]	0.90	Stefan–Boltzmann constant	5.67 × 10−8 W/m2K4
Emissivity of water [52,64]	0.90	Thickness of basin liner plate	0.001 m
Thermal conductivity of EPE foam	0.30 W/m-K	Thickness of EPE foam	0.045 m
Thickness of glass cover	0.004 m	Temperature of the Sun [54]	6000.0 K
Transmissivity of glass [54]	0.87	Universal gas constant	8314.0 J/kmol-K

. The simulation results were validated against the experimental results. The validated FORTRAN code was used for simulating the behavior of the solar still at any other desired locations.

2.4. Ambient Conditions in Experimental Site

The instantaneous ambient temperature and wind speed at the experimental site (Visakhapatnam, India) on experimental days are shown in Figure 5. Low ambient temperatures were observed mostly on late evening hours of experimental days. The daily average ambient temperatures through Day 1 to Day 4, were 36.2, 35.7, 36.4 and 35.6 °C, respectively. The measured wind speed data was highly fluctuating throughout the investigation period. The average wind speed was 1.41, 2.02, 1.94 and 2.72 m/s through Day 1 to Day 4, respectively. The global tilted solar radiation intensity variations observed on the experimental days are shown in Figure 6. On Day 1, the solar radiation intensity values were observed to drop from the beginning of the experiment. On the other three days, the solar radiation profile was nearly sinusoidal and smooth. The maximum solar radiation intensity was 566.0, 695.0, 745.0 and 887.0 W/m² through Day 1 to Day 4, respectively. The daily average intensity on Day 1, Day 2, Day 3 and Day 4 was about 279.0, 396.0, 510.0 and 583.0 W/m², respectively. The cumulative tilted global solar radiation intensity through Day 1 to Day 4, was 9.28, 13.17, 16.98 and 19.41 MJ/m²d, respectively.

2.5. Desalinated Water Production Rate

The instantaneous and cumulative diurnal desalinated water production of the basin solar still on experimental days are shown in Figure 6. Despite a relatively high solar radiation intensity during morning hours, the desalinated water output from the still on Day 1 was nearly nil up to 11:30 a.m. The desalinated water started to drop out of the condensate collection tube continuously after 11:30 a.m., only after reaching a certain level in the condensate collection trough. This observation could be linked to the combined effect of basin water heat capacity and the relatively high volume of the installed condensate collection trough. Due to cloudy conditions after 11:30 a.m., the cumulative diurnal desalinated water production on Day 1 was only about 0.56 L. On Day 2, the dripping of desalinated water from the condensate collection tube was observed after 9:15 a.m. On Day 2, Day 3 and Day 4, the time delay between the incident solar radiation intensity and the start of desalinated water dripping could solely be associated with basin water heat capacity. The maximum and average desalinated water production rates of the solar still were about 40.0 and 15.1 mL/15 min, 70.0 and 35.3 mL/15 min, 100.0 and 46.1 mL/15 min, and 100.0 and 57.5 mL/15 min through Day 1 to Day 4, respectively. Basin water depth on each experimental day and the estimated brine salinity along with the comparison of experimental and predicted diurnal desalinated water yield are tabulated in

Table 3. Basin water salinity and depth on experimental days along with deviation between actual and predicted desalinated water yield of the solar still.

S. No.	Experimental Day	Salinity	Basin Water Depth	Diurnal Desalinated Water Production
				Experimental	Simulation	Deviation
		(ppm)	(m)	(L/d)	(L/d)	(%)
1.	Day 1	34,200	0.0238	0.560	0.887	+58.39
2.	Day 2	35,458	0.0230	1.310	1.319	+0.68
3.	Day 3	38,405	0.0212	1.710	1.827	+6.84
4.	Day 4	43,100	0.0189	2.130	2.128	−0.09

. The deviation in predicted and experimental distillate yield was about +58.39% on Day 1. This could be associated with the large volume of distillate first accumulating in the condensate trough followed by distillate dripping later. The volume thus accumulated in the condensate trough becomes unaccounted for in all experimental values. The distillate yield predicted by the simulation was closer to the observed experimental distillate yield values (only −0.09 to +6.68% deviation) on Days 2, 3 and 4. This deviation could be linked to the limitation of the model in replicating the exact experimental operating conditions. The hourly and cumulative desalinated water yield predicted by the developed model is presented in Table S1 of the Supplementary Materials. The predicted results suggested that the proposed model is capable of estimating the performance of the basin solar still at reasonable accuracy.

2.6. Basin Solar Still Components Temperature

The instantaneous temperature variations of the basin, basin water and glass cover during the experimental periods are shown in Figure 7. During the early hours of the first experimental day, it could be noticed that the glass cover temperature shot up due to its low heat capacity in comparison to basin and basin water. However, from 9:30 a.m., the glass cover temperature was constantly lower than that of the basin and basin water temperature due to the combined convection and radiation heat losses to the ambient air. The continuous higher temperatures of the basin and basin water were due to the greenhouse effect provided by the glass cover, in addition to the solar radiation absorption by the basin and convective heat transfer from basin to basin water, respectively. The daily average temperature differences between the basin and basin water were only about 0.28 °C, 0.35 °C, 0.37 °C and 0.38 °C through Day 1 to Day 4, respectively. The daily average temperature differences between the basin water and glass cover were about 4.3 °C, 6.1 °C, 6.7 °C and 7.7 °C through Day 1 to Day 4, respectively. The daily average basin temperature was about 48.3 °C, 52.5 °C, 57.4 °C and 59.0 °C through Day 1 to Day 4, respectively. The increase in the temperature of the components from Day 1 to Day 4 was due to the combined effect of increased daily solar radiation intensity and reduced water depths day by day.

2.7. Daily Performance of Basin Solar Still

The daily performance of the basin solar still on experimental days is shown in Figure 8. In this investigation, the feed water was not replaced daily and thereby the basin water salinity was raised significantly under non-continuous mode operation. The increased desalinated water production from Day 1 to Day 4, in spite of the rising salinity, was due to the increased cumulative daily solar radiation intensity and continuous reduction in water depth over the experimental days. The reduced water depth indicates reduced heat capacity which favors improved evaporation rather than heat energy storage. The nocturnal desalinated water production was 18.8, 10.8, 10.4 and 7.3% of the total daily desalinated water production through Day 1 to Day 4, respectively. This observation indicates that relatively high basin water depth and low solar radiation intensity favors heat storage rather than immediate intense evaporation. This stored heat energy in basin water along with low ambient temperatures during late evening hours caused evaporation and improved condensation, respectively, leading to nocturnal desalinated water productivity. The thermal and exergy efficiencies of the investigated basin solar still were around 23.6 and 0.93, 34.7 and 1.88, 35.1 and 2.36 and 36.4 and 2.71%, respectively, through Day 1 to Day 4. The very low exergy efficiency on Day 1 was due to low desalinated water production and a high daily average ambient temperature of 36.2 °C, which decreased the Carnot efficiency factor used for exergy output estimation.

2.8. Basin Solar Still Characterization

The instantaneous thermal efficiency of the basin solar still is the ratio of instantaneous evaporative heat transfer to the instantaneous solar radiation intensity incident over its aperture and is expressed mathematically in Equation (23) [54,65].

$${\eta }_{ith}={\displaystyle \frac{Q_{ew}}{I_t\times A_a}}\times 100$$

(23)

The instantaneous loss efficiency of the basin solar still is the ratio of heat energy needed to increase basin water temperature at any instant to the instantaneous solar radiation intensity incident over its aperture and is expressed mathematically in Equation (24) [54,65].

$${\eta }_{iL}={\displaystyle \frac{m_{bw}{c}_{bw}\left(T_{bw(t+1)}-T_{bw(t)}\right)}{I_t\times A_a\times \Delta t}}\times 100$$

(24)

In this work, the data obtained from 9:15 a.m. to 13:15 p.m. on effective sunny hours of a clear day (Day 4) have been used to plot and generate the characteristic equation for instantaneous thermal and loss efficiency. The unrealistic data like negative instantaneous loss efficiency values and instantaneous thermal efficiency values greater than 100%, which occur due to a drop in basin water temperature and low solar radiation values in early morning and late evening hours, respectively, must be eliminated [66]. The linear characteristic equations of the basin solar still’s instantaneous thermal and loss efficiency appertained to its water temperature, ambient temperature and solar radiation intensity [66] and are represented by Equations (25) and (26), respectively. The unknown parameters in the above Equations (25) and (26) can be obtained from the regression equation generated by plotting the realistic data in Microsoft Excel.

$${\eta }_{ith}=F^{\prime }{\left(\alpha \tau \right)}_{eff}+F^{\prime }{U}_{eff}\left[{\displaystyle \frac{\left(T_{bw}-T_a\right)}{I_t}}\right]$$

(25)

$${\eta }_{iL}=F^{″}{\alpha }_e-F^{″}{U}_L\left[{\displaystyle \frac{\left(T_{bw}-T_a\right)}{I_t}}\right]$$

(26)

The variation in instantaneous thermal and loss efficiency with ${\displaystyle \frac{\left(T_{bw}-T_a\right)}{I_t}}$, along with the generated linear characteristic equations, is shown in Figure 9. The instantaneous thermal efficiency plot showed a positive slope indicating higher evaporation rates with increased ${\displaystyle \frac{\left(T_{bw}-T_a\right)}{I_t}}$. The negative intercept seen in the characteristic equation of instantaneous thermal efficiency indicated the existence of a solar radiation intensity threshold. The threshold value can be estimated by equating the generated linear characteristic equation of instantaneous thermal efficiency to zero. Beyond this threshold value, the basin solar still can perform effectively [66]. The estimated threshold solar radiation intensity of the investigated basin solar still was about 517.0 W/m², which occurred at 9:00 a.m. on Day 4 (clear day). The summation of instantaneous thermal and loss efficiencies must not be greater than 60.0% [64]. The instantaneous loss efficiency plot has a negative slope in Figure 10, thus indicating that maximum thermal efficiency is possible only when the loss efficiency is minimum. The thermal efficiency can be improved or the loss efficiency can be minimized by providing proper insulation, installing the still in shadow-free zones and making the still airtight.

2.9. Water Quality Analysis

The comparison of sea water and desalinated water quality is shown in

Table 4. Water quality analysis results of feed sea water and obtained desalinated water.

S. No.	Parameter	Sea Water	Desalinated Water	Removal Efficiency	Indian Drinking Water Standards [68,69]
		Initial Feed	(Average of First, Second and Last Days)	(%)
1.	pH	7.80	5.14 ± 0.13	-	6.5 to 8.5
2.	Electrical Conductivity (µS/cm)	52,810.00	4.67 ± 0.94	99.99	Not Available
3.	Salinity (ppm)	34,200.00	3.33 ± 1.89	99.99	Not Available
4.	Total Dissolved Solids (ppm)	34,300.00	3.33 ± 1.89	99.99	500 ppm

. When the desalinated water was compared to the feed sea water, a huge decrease in the quantity of ions was observed. This resulted in the lowering of electrical conductivity from 52,810.0 µS/cm in feed sea water to about 4.67 ± 0.94 µS/cm in the desalinated water. The low pH value of the desalinated water (5.14 ± 0.13) compared to the feed sea water pH value 7.80 could be attributed to the removal of OH⁻ ions [67]. The salinity, i.e., the salt content in the desalinated water, was about 3.33 ± 1.89 ppm, which was several times lower than the salinity of the feed sea water (34,200.0 ppm). The total dissolved solids (TDS) present in the feed sea water was also similarly reduced from 34,300.0 ppm to 3.33 ± 1.89 ppm in the desalinated water. All of the measured desalinated water qualities were found to be well below the Central Pollution Control Board (CPCB) of India limits and this desalinated water can be made fit for human consumption by adding essential minerals to it. The electrical conductivity, salinity and TDS removal efficiencies of the basin solar still were found to be about 99.99%. The huge improvement in the water quality justified the solar still’s ability to purify non-potable water to clean water at low expenditure without using any expensive and non-renewable resources.

3. Sites Investigated for Potential Solar Still Application

Climate change has affected the rainfall pattern in Latin American and Caribbean countries which in turn has led to prolonged periods of dryness, leading to increased water stress even in the urban regions. Financial damage worth USD 13 billion has also been reported in Latin America over the past 40 years due to droughts [1]. Moreover, intermittent piped water supply in rural regions has forced women to spend many hours in collecting water. Women have also been subjected to violence and mistreatment at water collection points [70]. Though the water resources are plenty in these countries, they still suffer from the lack of solutions to tackle rising fresh water demands. Countries such as Cuba [71], Honduras [72], Haiti [73], Jamaica [74] and Nicaragua [75] face severe issues of terrible plumbing in their water supply systems, leading to fresh water losses and water contamination due to contacts with sewage. Belize [76], Guatemala [77] and El Salvador [78] face large-scale water contaminations, mainly due to improper industrial and agricultural waste disposals and fewer wastewater treatment plants. Poor government policies have caused improper water sanitation in Dominican Republic [79].

Water demand is projected to increase by 43.0% in Latin American and Caribbean countries [80]. Around 25.0% of the population lacks access to clean drinking water and one in six children live in the highly water scarce regions of these countries [80,81]. The development of proper water supply infrastructures, the erection of desalination plants and encouraging wastewater treatment and reuse have been identified as suitable options to address the rising water stress [82]. Hence, investments are needed in rural and urban regions, especially for maintaining watersheds, building water storage facilities, reducing energy requirements for water treatment, developing policies for water governance systems and predicting future water risks [1]. Investment worth 1.3% of the region’s Gross Domestic Product (GDP) is needed over the next ten years to provide clean water and proper sanitation facilities to achieve UN’s Sustainable Development Goal 6 in those regions [81]. In this study, nine lower- and upper middle-income countries of the Central American (part of Latin American counties) and Caribbean countries have been considered for assessing the viability of F-SSS desalination plants. The map indicating the location of the sites is shown in Figure 10. The geographical position, sea water salinity, piped water supply price and treated bottled water price for the considered sites are tabulated in

Table 5. Geographical position, feed water salinity and water price details of the sites considered for the study.

S. No	Country	Classification [84]	Site	Geographical Position	Salinity of Sea Water [85]	Piped Water Supply Price [86]	Treated Bottled Water Price [87]
					(ppm)	(USD/m3)	(USD/m3)
							Minimum	Maximum
1.	Guatemala	Upper middle income	Guatemala City	14.63° N, 90.50° W	34,000	3.36	520.0	830.0
2.	Belize	Upper middle income	Belize City	17.50° N, 88.19° W	36,000	5.08	460.0	3710.0
3.	El Salvador	Upper middle income	San Salvador	13.69° N, 89.21° W	34,000	0.10 *	240.0	510.0
4.	Honduras	Lower middle income	Tegucigalpa	14.06° N, 87.17° W	34,000	10.55	520.0	1380.0
5.	Nicaragua	Lower middle income	Managua	12.11° N, 86.23° W	34,000	0.40	470.0	3720.0
6.	Cuba	Upper middle income	Havana	23.11° N, 82.37° W	36,000	NA	320.0	720.0
7.	Haiti	Lower middle income	Port-au-Prince	18.59° N, 72.30° W	36,000	6.30	410.0	1700.0
8.	Jamaica	Upper middle income	Kingston	18.01° N, 76.80° W	36,000	19.26	340.0	1710.0
9.	Dominican Republic	Upper middle income	Santo Domingo	18.46° N, 69.93° W	36,000	8.83	1330.0	3000.0
10.	India	Lower middle income	Visakhapatnam	17.70° N, 83.20° E	34,200	0.06 *	90.0	500.0

NA—Not Available; *—indicates only connection charges, and consumption charges are not provided.

. The sea water salinity in the considered Central American sites was about 34,000 ppm, and 36,000 ppm in the Caribbean sites. The treated bottled water price is higher in all of the sites compared to the conventional piped water supply price. The daily hourly climatic conditions of the sites for an entire year were assessed from the PVGIS online database [83].

3.1. Year-Round Performance of the Solar Still in Central American and Caribbean Sites

The desalinated water productivities, thermal efficiencies and exergy efficiencies of the solar still were estimated under the climatic conditions of Central American and Caribbean countries and an Indian experimental site, Visakhapatnam, by simulation. In the simulated solar still, EPE foam insulation was replaced by a 0.04 m thick rockwool insulation (0.0372 W/m-K thermal conductivity) to facilitate the effective operation of the still over a long duration. However, other parameters of the still, including optical properties and dimensions, were kept similar to the actual experimental system. Feed water salinity was considered similar to the sea water salinity of the site as listed in Table 5. The yearly average solar radiation energy potential of the considered sites in Central America and the Caribbean ranges between 18.7 and 21.2 MJ/m²d and the yearly average solar radiation potential of Visakhapatnam is about 18.1 MJ/m²d. The solar radiation energy incident over the still and the desalinated water productivity of the still in different sites are shown in Figure 11 and Figure 12, respectively. The yearly desalinated water productivity of the still was observed to be directly proportional to the incident solar radiation energy. The yearly desalinated water productivity of the still in Belize city, Guatemala City, San Salvador, Tegucigalpa, Managua, Havana, Kingston, Port-au-Prince, Santo Domingo and Visakhapatnam was about 836.0, 844.0, 904.0, 845.0, 891.0, 856.0, 917.0, 979.0, 895.0 and 824.0 L/year, respectively. The daily average distillate yield of the still was observed to be superior in Port-au-Prince (2.68 L/d) followed by Kingston (2.51 L/d) and San Salvador (2.48 L/d) amongst all of the considered sites. The lowest daily average desalinated water productivity of the still was observed to be about 2.26 to 2.31 L/d in Belize city, Guatemala City, Tegucigalpa, Havana and Visakhapatnam. The month-wise thermal and exergy efficiency of the solar still in the investigated sites is shown in Tables S2 and S3, respectively, of the Supplementary Materials. The yearly average thermal and exergy efficiency of the still ranged between 36.5 and 39.8% and 2.9 and 4.2%, respectively. The lowest thermal efficiency was observed mostly during the months of November and December (31.3 and 31.4%) in the considered Central American and Caribbean sites.

3.2. Sizing of Family-Scale Solar Still Desalination Plant

The average number of individuals per family in Central America, the Caribbean and India ranges between three and four and four and five, respectively [88]. The amount of fresh water essential per individual for quenching thirst is about 3.0 L/d, whereas the fresh water requirement for survival, including drinking, hygiene practices and cooking, ranges between 7.5 and 15.0 L/d per person [89]. In this work, the average number of persons per family has been taken as five. The quantity of fresh water needed per family of five persons for quenching thirst is about 15.0 L/d (Case 1). Similarly, the lowest and highest quantity of fresh water needed per family of five members for addressing drinking, hygiene practices and cooking are about 37.5 L/d (Case 2) and 75.0 L/d (Case 3), respectively. In this work, solar still desalination plants were sized based on the estimated fresh water demand per family.

The number of solar stills required to address the fresh water demand of a family for the above three cases was estimated by Equation (27). The latent heat of water evaporation at 50 °C is about 2381.6 kJ/kg and the same has been used for the calculation in Equation (27). The number of solar stills estimated for the three cases in the considered sites, along with their arrangement plan and required plumbing components is tabulated in

Table 6. Description of detailed items in family-scale solar still desalination plant proposed for Case 1, Case 2 and Case 3.

Cases	Cities	No. of Solar Stills	Solar Still Arrangement			Plumbing Components
			Columns	Rows	Number of Stills to Be Removed/ Included from the Last Row	Sea Water Pipe Length	Condensate Pipe Length	No. of Valves	No. of Elbows	No. of Tee Joints	Feed Water Tank Capacity	No. of 100 L Capacity Condensate Tanks
						(Feet)	(Feet)				(L)
Case 1	Belize City	11	3	4	−1	47	69	8	7	14	300 L	1
	Guatemala City	10	3	4	−2	45	66	8	7	14	200 L	1
	San Salvador	9	3	3	0	36	53	6	6	10	200 L	1
	Tegucigalpa	11	3	4	−1	47	69	8	7	14	300 L	1
	Managua	10	3	4	−2	45	66	8	7	14	200 L	1
	Havana	11	3	4	−1	47	69	8	7	14	300 L	1
	Kingston	10	3	4	−2	45	66	8	7	14	200 L	1
	Port-au-Prince	9	3	3	0	36	53	6	6	10	200 L	1
	Santo Domingo	11	3	4	−1	47	69	8	7	14	300 L	1
	Visakhapatnam	10	3	4	−2	45	66	8	7	14	300 L	1
Case 2	Belize City	28	4	7	0	102	166	14	10	33	700 L	2
	Guatemala City	25	4	6	1	89	144	12	9	28	500 L	2
	San Salvador	22	4	6	−2	83	136	12	9	28	500 L	2
	Tegucigalpa	29	4	7	1	104	168	14	10	33	700 L	2
	Managua	25	4	6	1	89	144	12	9	28	500 L	2
	Havana	27	4	7	−1	100	163	14	10	33	700 L	2
	Kingston	25	4	6	1	89	144	12	9	28	500 L	2
	Port-au-Prince	23	4	6	−1	85	139	12	9	28	500 L	2
	Santo Domingo	28	4	7	0	102	166	14	10	33	700 L	2
	Visakhapatnam	24	4	6	0	87	141	12	9	28	500 L	2
Case 3	Belize City	56	7	8	0	165	322	16	11	62	1500 L	3
	Guatemala City	50	7	7	1	146	284	14	10	54	1000 L	3
	San Salvador	45	7	7	−4	136	271	14	10	54	1000 L	3
	Tegucigalpa	57	7	8	1	167	325	16	11	62	1500 L	3
	Managua	50	7	7	1	146	284	14	10	54	1000 L	3
	Havana	53	7	8	−3	159	315	16	11	62	1500 L	3
	Kingston	49	7	7	0	144	281	14	10	54	1000 L	3
	Port-au-Prince	46	7	7	−3	138	274	14	10	54	1000 L	3
	Santo Domingo	56	7	8	0	165	322	16	11	62	1500 L	3
	Visakhapatnam	49	7	7	0	144	281	14	10	54	1000 L	3

. The number of solar stills needed for addressing only the drinking water demands of a family range between nine and eleven in the considered sites. The number of stills needed for meeting the lowest and highest water demands associated with drinking, hygiene practices and cooking for a family range between 22 and 29 and 45 and 57, respectively. The proposed layout of the F-SSS desalination plant is shown in Figure 13.

$$\mathrm{Number} \mathrm{of} \mathrm{Solar} \mathrm{Stills}={\displaystyle \frac{\mathrm{Daily} \mathrm{Water} \mathrm{Requirement} (\mathrm{kg}/\mathrm{d}) \times \mathrm{Latent} \mathrm{Heat} \mathrm{of} \mathrm{Vaporization} (\mathrm{kJ}/\mathrm{kg})}{\left(\begin{array}{c}\mathrm{Lowest} \mathrm{Efficiency} \\ \mathrm{of} \mathrm{the} \mathrm{Still} \mathrm{in} \mathrm{the} \mathrm{site}\end{array}\right) \times \left(\begin{array}{c}\mathrm{Lowest} \mathrm{Solar} \mathrm{Radiation} \\ \mathrm{Energy} \mathrm{Potential} \\ {\mathrm{of} \mathrm{the} \mathrm{Site} (\mathrm{kJ}/\mathrm{m}}^2\mathrm{d})\end{array}\right){ \times \mathrm{Still} \mathrm{Aperture} \mathrm{Area} (\mathrm{m}}^2)}}$$

(27)

The gap between each solar still was taken as 2 feet. The layout consists of a number of solar still strings which are arranged parallel to each other such that each row is supplied with feed water from the tank separately. This arrangement facilitates the easy maintenance of stills without halting the operation of the entire plant. Distillate collection pipes from each still are collected together and are drained to the common distillate collection tanks that are connected in series. Feed water is to be supplied to the stills only once in a week. The feed water tank capacity was estimated based on the feed water requirement of all of the solar stills (20 L per still) and rounded off to the nearest capacity available in the local market. The number of condensate collection tanks of 100 L capacity each, needed for Case 1, Case 2 and Case 3, have been fixed as one, two and three, respectively. These numbers are set after considering the factor of safety to avoid overflow as the stills are oversized to provide sufficient water even in the worst operating conditions.

The total length of the feed water distribution pipe was estimated by Equation (28),

$$\begin{array}{cc}\hfill \mathrm{Feed} \mathrm{Water} \mathrm{Pipe} \mathrm{Length}& =\left(\mathrm{No}. \mathrm{of} \mathrm{Stills} \mathrm{in} \mathrm{the} \mathrm{Plant} \times \mathrm{Gap}\right)\hfill \\ & +\left(\left(\mathrm{No}. \mathrm{of} \mathrm{Rows} - 1\right) \times \mathrm{Still} \mathrm{Width}\right)\\ & +\left(2\times \left(\mathrm{No}. \mathrm{of} \mathrm{Rows} \times \mathrm{Gap}\right)\right)\end{array}$$

(28)

The total length of the distillate collection pipe was estimated by Equation (29),

$$\begin{array}{cc}\hfill \mathrm{Distillate} \mathrm{Collection} \mathrm{Pipe} \mathrm{Length}& =\left(\mathrm{No} \mathrm{of} \mathrm{Stills} \times \mathrm{Distillate} \mathrm{Pipe} \mathrm{Outlet} \mathrm{Length} \mathrm{from} \mathrm{each} \mathrm{Still}\right)\hfill \\ & +\left(\mathrm{No} \mathrm{of} \mathrm{Stills} \times \mathrm{Gap}\right)+\left(\begin{array}{l}\left(\mathrm{No} \mathrm{of} \mathrm{Columns} - 1\right) \times \\ \mathrm{No} \mathrm{of} \mathrm{Rows} \times \mathrm{Breadth} \mathrm{of} \mathrm{each} \mathrm{Still}\end{array}\right)\\ & +\left(\mathrm{No} \mathrm{of} \mathrm{Rows} \times \mathrm{Gap}\right)+\left(\left(\mathrm{No} \mathrm{of} \mathrm{Rows} - 1\right) \times \mathrm{Breadth} \mathrm{of} \mathrm{each} \mathrm{Still}\right)\end{array}$$

(29)

The total number of tee joints, elbows and valves required were estimated by Equations (30)–(32), respectively.

$$\mathrm{No} \mathrm{of} \mathrm{Tee} \mathrm{joints}=\left(2 \times \left(\mathrm{No} \mathrm{of} \mathrm{Rows} - 1\right)\right)+\left(\left(\mathrm{No} \mathrm{of} \mathrm{Columns} - 1\right) \times \mathrm{No} \mathrm{of} \mathrm{Rows}\right)$$

(30)

$$\mathrm{No} \mathrm{of} \mathrm{Elbows}=\mathrm{No} \mathrm{of} \mathrm{Rows}+3$$

(31)

$$\mathrm{No} \mathrm{of} \mathrm{Valves}=\mathrm{No} \mathrm{of} \mathrm{Rows} \times 2$$

(32)

3.3. Family-Scale Solar Still Desalination Plant Economics

The prices of components associated with the F-SSS desalination plant are tabulated in

Table 7. Price of components in the family-scale solar still desalination plant.

S. No.	Component	Size	Price
			INR
1.	Basin solar still with stand	0.88 × 0.87 m2 aperture area	16,750.00 per still
2.	CPVC condensate pipe [92]	1-inch nominal diameter	45.00 per feet
3.	CPVC sea water pipe [92]	1-inch nominal diameter	45.00 per feet
4.	CPVC tee joint [93]	1.25-inch nominal diameter	95.00 per piece
5.	CPVC elbow [94]	1.25-inch nominal diameter	74.00 per piece
6.	CPVC valve [95]	1.25-inch nominal diameter	474.00 per piece
7.	Sea water storage tank (Make: Sintex Titus) [96]	200 L capacity	1800.00 per piece
		300 L capacity	2600.00 per piece
		500 L capacity	4000.00 per piece
		700 L capacity	4700.00 per piece
		1000 L capacity	7900.00 per piece
		1500 L capacity	11,800.00 per piece
8.	Condensate storage tank [97]	100 L capacity	3382.00 per piece
9.	Labor cost [45]	-	USD 0.50 per solar still
USD 1 = INR 84.0

. The price of the experimental solar still per piece was around INR 16,750 (USD 199.40) and the same has been considered for the economic analysis. The total capital price of the F-SSS desalination plant is the summation of direct price, indirect price, labor price and the price associated with component replacement (once every 5 years). The direct price is the price associated with the solar still and plumbing items. The indirect price occupies around 30.0% of the direct price and includes design, planning, insurance, taxes and transport prices [45]. The annual operation and maintenance price and annual distillate post treatment price are about 1.50% and 10.0% of the fixed annualized price, respectively [45]. Salvage value and replacement price are about 15.0% of the plant’s useful materials price and 5.0% of the direct price, respectively [45,46]. The price per unit volume of desalinated water is the ratio of total annualized price (in USD) to the annual desalinated water productivity of the F-SSS desalination plant (in m³) [90]. The interest rates in Belize city, Guatemala City, San Salvador, Tegucigalpa, Managua, Havana, Kingston, Port-au-Prince, Santo Domingo and Visakhapatnam are 2.25%, 5.0%, 5.53%, 4.0%, 7.0%, 2.25%, 6.75%, 17.0%, 6.5% and 6.5%, respectively [91]. The direct price, indirect price and labor price of F-SSS desalination plants in various sites under the three cases considered are shown in Table S4.

Fixed annualized price was estimated by Equation (33) [46],

$$FAP=\left(DP+IDP+LP+\left(DP\times f_{rp}\times N_t\right)\right)\times {\displaystyle \frac{i{\left(1+i\right)}^{LT}}{\left({\left(1+i\right)}^{LT}-1\right)}}$$

(33)

Total annualized price was estimated by Equation (34) [50],

$$TAP=FAP+\left(FAP\times f_{O\&M}\right)+\left(FAP\times f_{WPT}\right)-\left(P_{UM}\times {\displaystyle \frac{i}{\left({\left(1+i\right)}^{LT}-1\right)}}\times f_{SV}\right)$$

(34)

The price per unit volume of potable water generated was estimated by Equation (35), including the brine disposal price of 0.66 USD/m³ of desalinated water [45],

$$PPV={\displaystyle \frac{TAP}{\left(\frac{M_Y\times N_{ss}}{1000}\right)}}+P_{BD}$$

(35)

The time taken to obtain a return on the investment made on the F-SSS desalination plant was estimated by Equation (36),

$$FPB={\displaystyle \frac{\left(DP+IDP+LP+\left(DP\times f_{rp}\times N_t\right)\right)}{\left(\frac{M_Y\times N_{ss}}{1000}\right)\times P_{\mathrm{Bottled} \mathrm{Water}}}}$$

(36)

The potable water production prices of an F-SSS desalination plant for the three considered cases under different lifetimes in the considered sites are shown in Figure 14. It can be seen clearly that the lifetime of a solar still desalination plant has a significant impact on its overall potable water production price. In spite of the high annual potable water generation of this plant in Port-au-Prince, high potable water production prices were also observed here, due to the high interest rate in this city (17.0%). Belize City and Havana sites recorded the lowest potable water production prices due to their low interest rates (2.25%). Visakhapatnam recorded the lowest annual potable water productivity among all of the considered sites, and thereby it has the highest potable water price next to Port-au-Prince due to its relatively low interest (6.5%), comparatively.

Potable water prices in the considered sites were the least in Case 3, followed by Case 2 and Case 1. However, the difference in the potable water price for the three cases in each of the considered sites is nearly negligible. Potable water prices of the solar still desalination plants in all of the considered sites were found to become nearly stable after the 20-year lifetime. A solar still desalination plant can operate effectively for 25 years [45]. The maximum potable water prices per m³ produced by the F-SSS desalination plant of a 25-year lifetime were about USD 20.69 in Belize City, USD 28.35 in Guatemala City, USD 27.89 in San Salvador, USD 25.30 in Tegucigalpa, USD 32.80 in Managua, USD 20.22 in Havana, USD 31.15 in Kingston, USD 60.62 in Port-au-Prince, USD 31.06 in Santo Domingo and USD 33.92 in Visakhapatnam. It is interesting to observe that the potable water generated by the solar still desalination plant even for the lowest lifetime of 5 years in all sites is much lower than the treated bottled water price tabulated in Table 5. This clearly indicates the effectiveness of solar still desalination plant in meeting the fresh water demands of the population in an economical way even without any subsidy.

The impact of a subsidy on an F-SSS desalination plant’s potable water production price in the considered sites is shown in Figure 15. The potable water production price dropped by about 12.0 USD/m³ for every 20.0% increase in subsidy in Port-au-Prince, which has a high interest rate. In sites with the lowest interest rate of 2.25%, namely Belize City and Havana, a drop of around 4.0 USD/m³ was observed for every 20.0% increase in subsidy. For the other sites with interest rates between 5.0 and 7.0%, the potable water production price drop was about 5.4 to 6.6 USD/m³ for every 20.0% increase in subsidy. The provision of an 80.0% subsidy brought down the potable water production price/m³ much below conventional piped water supply tariff/m³ in Belize City, Tegucigalpa, Kingston and Santo Domingo. The potable water production price/m³ upon using F-SSS desalination plants with an 80.0% subsidy was about USD 2.69, USD 6.53, and USD 6.15 higher than the conventional piped water supply tariff/m³ in Guatemala City, Managua and Port-au-Prince, respectively. Potable water price per m³ was observed to be USD 6.02, 6.92 and 7.12 with an 80.0% subsidy in San Salvador, Managua and Visakhapatnam, respectively. It could be interesting to observe that the higher the subsidy, the higher the price drop in sites possessing good annual desalinated water production capacities and high interest rates. At an 80.0% subsidy, the potable water production price of an F-SSS desalination plant ranged between 4.50 and 6.92 USD/m³ in the Central American and Caribbean sites considered in this study, except for Port-au-Prince (12.45 USD/m³). These observations clearly indicate the need for concerned governments to subsidize F-SSS desalination plants to make affordable clean water accessible to all of their households in a sustainable way with reduced economic burden on the citizens. Moreover, this would also be a benefit to both citizens and the governments by cutting off the medical expenses related to largely waterborne diseases, thereby strengthening the economy [98].

The influence of the F-SSS desalination plant’s lifetime on the finance payback time of the project in the considered sites for various cases is tabulated in Table S5 through Table S7 of the Supplementary Materials. The finance payback time was evaluated based on the minimum and maximum price of the treated bottle water in the corresponding locations. The finance payback time was found to drop and increase slightly, with an increase in the plant’s capacity and lifetime, respectively. The maximum and minimum finance payback time of the plant with a 25-year lifetime ranges between 3.2 (Santo Domingo) and 17.6 (San Salvador) months, and 1.1 (Managua) and 8.3 (San Salvador) months, respectively, among the Central American and Caribbean sites considered in this study. The lowest and the highest finance payback times of the plant in Visakhapatnam were around 9.3 and 51.4 months, respectively. The observed highest payback time here is mainly due to a low annual desalinated water production capacity and a low treated bottled water price. The finance payback time of the F-SSS desalination plant in Central American and Caribbean sites (less than 2 years) and Visakhapatnam (less than 5 years) is much lower compared to its lifetime (25 years).

3.4. Environmental Aspects of Family-Scale Solar Still Desalination Plant

Solar energy-driven systems are expected to have positive impacts on the environment and mitigate climate change. The F-SSS desalination plant proposed in this study operates by utilizing solar thermal energy. Hence, it becomes essential to quantitatively assess the environmental aspects of this plant to justify its environmental friendliness. Environmental aspects are quantified by evaluating its embodied energy payback time, specific CO₂ emission, decarbonization potential and sustainability index. Embodied energy payback time is the time duration taken to regain the energy spent on the plant and is estimated by Equation (37). It is the ratio of plant’s embodied energy to the annual energy output of the plant [99]. The energy output of the desalination plant is measured in terms of the energy needed to cause water evaporation [99].

$$\mathrm{Energy} \mathrm{Payback} \mathrm{Time} \left(\mathrm{Yr}\right)={\displaystyle \frac{\left(\mathrm{Embodied} \mathrm{Energy} \mathrm{of} \mathrm{the} \mathrm{Plant}\right)}{\left(\frac{M_Y\times h_{fg}\times N_{ss}}{3600}\right)}}$$

(37)

The specific CO₂ emission of the plant is the ratio of its embodied CO₂ emission to the quantity of desalinated water expected to be produced by the plant over its lifetime and is estimated by Equation (38). Embodied CO₂ emission was evaluated by multiplying the embodied energy of the plant and CO₂ emission intensity. The CO₂ emission intensity refers to the emission from the power generation system in the particular country/site per kWh of electrical energy generated. The CO₂ emission intensity evaluated in various sites by considering transmission losses is tabulated in Table 8

$$\left.\begin{array}{c}{\mathrm{Specific} \mathrm{CO}}_2 \mathrm{emission} \\ {(\mathrm{g} \mathrm{CO}}_2 \mathrm{per} \mathrm{L} \mathrm{of} \mathrm{Desalinated} \mathrm{Water})\end{array}\right\}={\displaystyle \frac{\left({\mathrm{Embodied} \mathrm{Energy} \mathrm{of} \mathrm{the} \mathrm{plant} \times \mathrm{CO}}_2 \mathrm{Emission} \mathrm{Intensity}\right)}{\left(M_Y\times N_{ss}\times LT\right)}}$$

(38)

The utilization of the F-SSS desalination plant eliminates the energy utilized from fossil fuel-related systems to generate desalinated water and thereby eliminates CO₂ emission. The decarbonization potential of the F-SSS desalination plant was evaluated by Equation (39) [100],

$$\left.\begin{array}{c}\mathrm{Decarbonization}\\ \mathrm{Potential} \\ \left(\mathrm{tons}\right)\end{array}\right\}=\left[\left({\displaystyle \frac{M_Y\times h_{fg}\times N_{ss}\times LT}{3600}}\right)-\left(\begin{array}{c}\mathrm{Embodied} \mathrm{Energy} \\ \mathrm{of} \mathrm{the} \mathrm{Plant}\end{array}\right)\right]\times {\mathrm{CO}}_2 \mathrm{Emission} \mathrm{Intensity}$$

(39)

The sustainability index of the desalination plant was evaluated based on its exergy efficiency using Equation (40). The higher the exergy efficiency, the higher the sustainability index [101].

$$\mathrm{Sustainability} \mathrm{Index}={\displaystyle \frac{1}{1-\left(\frac{{\eta }_{ex}}{100}\right)}}$$

(40)

The mass of a CPVC 1-inch nominal diameter pipe, 1.25 nominal diameter CPVC valve, elbow and tee-joint are about 0.20 kg/ft [102], 0.68 kg per piece [103], 0.15 kg per piece [104] and 0.20 kg per piece [105], respectively. The mass of a 100 L, 200 L, 300 L, 500 L, 700 L, 1000 L and 1500 L high-density polyethylene (HDPE) water tank is estimated to be around 1.5, 3.0, 4.5, 7.5, 10.5, 15.0 and 22.5 kg, respectively [106]. The embodied energies of CPVC components and HDPE tanks are about 55.0 and 103.0 MJ/kg, respectively [106,107]. The embodied energy of the F-SSS desalination plant for the three cases in the considered sites was evaluated and tabulated in Table 8. Embodied energy per solar still was estimated to be 333.44 kWh. The embodied energy of the F-SSS desalination plant ranged between 3507 and 4335 kWh, 9232 and 11,157 kWh, and 17,141 and 21,662 kWh among the sites for Case 1, Case 2 and Case 3, respectively. The higher the desalination plant capacity, the higher its embodied energy, which was noticed from its values for Case 3. Similarly, the higher the number of stills required in a site to meet a similar water demand, the higher the embodied energy in that site for the same case. The lowest embodied energy was observed in San Salvador and Port-au-Price for Case 1 while the highest was noticed in Tegucigalpa for all three cases. The annual energy output from the desalination plant ranged between 5382 kWh (San Salvador) and 6513 kWh (Santo Domingo) in Case 1, 13,083 kWh (Visakhapatnam) and 16,579 kWh (Santo Domingo) in Case 2 and 26,711 kWh (Visakhapatnam) and 30,971 kWh (Belize City) in Case 3.

Table 8. Emission intensity of power plants, embodied energy and annual energy output of the family-scale solar desalination plant in various sites.

Site	Emission Intensity [108]	Electricity Transmission and Distribution Losses [109]	Emission Intensity Including Losses	Embodied Energy of the F-SSS Desalination Plant			Annual Energy Output from the F-SSS Desalination Plant
	(gCO2e per kWh)	(%)	(gCO2e per kWh)	Case 1	Case 2	Case 3	Case 1	Case 2	Case 3
				(kWh)	(kWh)	(kWh)	(kWh/Y)	(kWh/Y)	(kWh/Y)
Belize City	225.81	12.0	252.91	4335	10,810	21,315	6084	15,486	30,971
Guatemala City	328.27	14.0	374.23	3945	9579	18,877	5584	13,959	27918
San Salvador	224.76	12.0	251.73	3507	8538	17,141	5382	13,157	26,912
Tegucigalpa	282.27	25.0	352.84	4335	11,157	21,662	6149	16,211	31,864
Managua	265.12	25.0	331.40	3945	9579	18,877	5894	14,736	29,472
Havana	637.61	15.0	733.25	4335	10,462	20,273	6229	15,290	30,013
Kingston	555.56	24.0	688.89	3945	9579	18,530	6066	15,166	29,726
Port-au-Prince	567.31	56.0	881.64	3507	8885	17,489	5829	14,896	29,792
Santo Domingo	580.78	34.0	778.25	4335	10,810	21,315	6513	16,579	33,157
Visakhapatnam	713.01	19.0	848.48	3988	9232	18,530	5451	13,083	26,711

Table 8. Emission intensity of power plants, embodied energy and annual energy output of the family-scale solar desalination plant in various sites.

Site	Emission Intensity [108]	Electricity Transmission and Distribution Losses [109]	Emission Intensity Including Losses	Embodied Energy of the F-SSS Desalination Plant			Annual Energy Output from the F-SSS Desalination Plant
	(gCO2e per kWh)	(%)	(gCO2e per kWh)	Case 1	Case 2	Case 3	Case 1	Case 2	Case 3
				(kWh)	(kWh)	(kWh)	(kWh/Y)	(kWh/Y)	(kWh/Y)
Belize City	225.81	12.0	252.91	4335	10,810	21,315	6084	15,486	30,971
Guatemala City	328.27	14.0	374.23	3945	9579	18,877	5584	13,959	27918
San Salvador	224.76	12.0	251.73	3507	8538	17,141	5382	13,157	26,912
Tegucigalpa	282.27	25.0	352.84	4335	11,157	21,662	6149	16,211	31,864
Managua	265.12	25.0	331.40	3945	9579	18,877	5894	14,736	29,472
Havana	637.61	15.0	733.25	4335	10,462	20,273	6229	15,290	30,013
Kingston	555.56	24.0	688.89	3945	9579	18,530	6066	15,166	29,726
Port-au-Prince	567.31	56.0	881.64	3507	8885	17,489	5829	14,896	29,792
Santo Domingo	580.78	34.0	778.25	4335	10,810	21,315	6513	16,579	33,157
Visakhapatnam	713.01	19.0	848.48	3988	9232	18,530	5451	13,083	26,711

The energy payback time, specific CO₂ emission, decarbonization potential and sustainability index of the F-SSS desalination plant for the three cases is tabulated in

Table 9. Energy payback time, specific CO₂ emission, decarbonization potential and sustainability index of F-SSS desalination plant in various sites.

Site	Energy Payback Time (Year)			Specific CO2 Emission (gCO2/L of Desalinated Water)			Decarbonization Potential in 25 Year Lifetime (Tons)			Sustainability Index
	Case 1	Case 2	Case 3	Case 1	Case 2	Case 3	Case 1	Case 2	Case 3
										(-)
Belize City	0.71	0.70	0.69	4.77	4.67	4.61	37	95	190	1.03
Guatemala City	0.71	0.69	0.68	7.00	6.80	6.70	51	127	254	1.04
San Salvador	0.65	0.65	0.64	4.34	4.32	4.24	33	81	165	1.04
Tegucigalpa	0.71	0.69	0.68	6.58	6.43	6.35	53	139	273	1.03
Managua	0.67	0.65	0.64	5.87	5.70	5.62	48	119	238	1.03
Havana	0.70	0.68	0.68	13.50	13.28	13.11	111	273	535	1.03
Kingston	0.65	0.63	0.62	11.85	11.51	11.36	102	255	499	1.03
Port-au-Prince	0.60	0.60	0.59	14.04	13.92	13.70	125	320	641	1.04
Santo Domingo	0.67	0.65	0.64	13.71	13.43	13.24	123	314	629	1.03
Visakhapatnam	0.73	0.71	0.69	16.43	15.84	15.58	112	270	551	1.03

. The energy payback period was observed to be less than a year in all of the sites and it typically ranged between 0.60 and 0.73 years, 0.60 and 0.71 years and 0.59 and 0.69 years for Case 1, Case 2 and Case 3, respectively. The specific CO₂ emission of the plant per liter of desalinated water produced ranged between 4.34 (San Salvador) and 16.43 g (Visakhapatnam) in Case 1, 4.32 (San Salvador) and 15.84 g (Visakhapatnam) in Case 2 and 4.24 (San Salvador) and 15.58 g (Visakhapatnam) in Case 3, respectively. The specific CO₂ emission was observed to decrease slightly with an increase in plant size for all of the sites. The F-SSS desalination plant can cut off CO₂ emission in the range of 33 to 125 tons, 81 to 320 tons, and 165 to 641 tons during its operating lifetime of 25 years in Case 1, Case 2 and Case 3 scenarios, respectively, in the considered sites. The sustainability index of the plant ranged between 1.03 and 1.04 in the considered sites. In spite of the benefits associated with desalination, public acceptance of desalinated water is possible only through effective government policies and campaigns that project the reliability, safety and necessity of desalinated water [110].

4. Discussion

The basin solar still experiment in Visakhapatnam, India, under non-continuous mode operation showed a thermal and exergy efficiency of about 23.6 to 36.4% and 0.93 to 2.71%, respectively. Moreover, the still was found to generate high-quality desalinated water from highly saline water and showed 99.99% salt removal efficiency. The thermodynamic model developed to replicate solar still performance was more capable in predicting daily desalinated water productivity closer to the actual values. The simulated solar still’s performance was found to be superior in the Caribbean and Central American sites than in the experimental site (Visakhapatnam) due to the high solar radiation potential (>than 18.0 MJ/m²d) of those sites. The sizing of F-SSS desalination plants of varying capacities (capable of supplying at least 15.0, 37.5 and 75.0 L/d per day) for each of the sites was carried out and its layout with required components and economics was also presented in detail. Size and environmental aspects of the plant were dependent on the performance, local solar radiation potential and local interest rates. The price per m³ of potable water produced by the F-SSS desalination plant was significantly influenced by the plant’s lifetime, site climatic conditions, the interest rate of financial institutions at the site and subsidies. The estimated potable water production price of the plant was lower than the price of bottled clean water available locally. Moreover, the finance payback time was much less. The minimum and maximum specific CO₂ emission per L of potable water produced by the plant was around 4.24 and 14.04 g, respectively. The F-SSS desalination plant was estimated to have a maximum decarbonization potential of about 165 to 641 tons during its lifetime of 25 years.

The comparison of the main contributions in the present work with other relatively similar works is tabulated in Table 10. It could be noticed that solar still desalination plants have been of great interest for addressing water demands in most of the continents. Moreover, they were found to viable in all of the investigated sites. Earlier studies were focused on centralized large-scale plants. On the contrary, this study focused on an F-SSS desalination system applicable for a single household under three different cases. Apart from performance aspects, detailed economics and environmental benefits have also been assessed and reported in this study. The relatively high potable water price observed in this study was due to the use of a stainless-steel basin still, the consideration of local actual interest rates and the incorporation of the replacement and brine disposal prices. The problems faced by other solar still desalination plants have been eliminated in this proposed plant and thereby a sustainable clean water supply for a longer operation time is ensured.

The long-term expected benefits of the F-SSS desalination plant are listed below:

• The elimination/mitigation of waterborne diseases thereby enhancing human resource utilization for nation building;
• The availability of clean water at the point of need thereby enhances the safety of children and women and improves their contribution in other essential activities;
• The creation of additional financial savings due to the elimination of water-related medical issues and dependence on expensive bottled drinking water;
• The creation of self-sustained communities leading to peaceful co-existence with adjacent communities by avoiding water-related conflicts;
• The mitigation of CO₂ emission thereby contributing to nations’ CO₂ emission cut-off targets for the United Nations.

Experience from earlier real-time solar still desalination plants in Chile [111], Australia [112] and Mexico [42] confirmed the durability of this technology for clean water supply. However, the potential challenge will be its maintenance. The common problems that may occur are vapor leakage, glass cracks, bird droppings, dust accumulation, algae growth, the peeling of black paint, salt deposition and issues with brine disposal. In this study, the price associated with pretreatment, post treatment, brine disposal and maintenance have been considered during economic analysis. As the plant is for household purposes, the glass cover can be cleaned daily in the early morning by a family member. Each row of the F-SSS desalination plant can be cleaned once in a month to re-paint and remove the deposited silt and salt if any.

The different techniques available for brine disposal are surface water discharge, sewer discharge, deep-well injection and land application. These techniques have negative impacts on the environment and seem expensive. Brine has potential application as a coolant in power plants, as a source for the recovery of salts and in the production of chemicals [113]. However, the better strategy would be for the concerned district authorities to collect the reject concentrated brine via a proper schedule and transport the same to a centralized zero liquid discharge plant to extract salts/minerals without any further waste generation.

Table 10. Comparison of the present study with other peer-reviewed literature on solar still desalination plants.

Site	Solar Still Type	Absorber Type	Size of Solar Still Desalination Plant	Investigation				Observations
				Feed Water	(Performance— Theoretical or Experimental or Both)	Economics Aspects	Environmental Aspects
Quillagua, Chile [111]	Double-slope basin solar still	Cement trays	107 m2	10,000 ppm saline river water	Experiment	1.68 USD/m3	NA	Cracking
Valparaiso, Chile [111]	Double-slope basin solar still	Asbestos–cement trays	103 m2		Experiment	2.25 USD/m3	NA	Accurate fabrication seems difficult
Coober Pedy, Australia [112]	Double-slope basin solar still	Galvanized iron troughs	3530 m2 (19.0 m3/d)	24,000 ppm saline underground water	Experiment	0.73 USD/m3	NA	Algae growth in feed water, soil movement, animal interference
Awania, India [43]	Double-slope basin solar still	Brick with cement plaster coated black	1867 m2 (5.0 m3/d)	3000 to 5000 ppm well water	Experiment	NA	NA	Cracks, algae growth, glass breakage by slipping
Delhi, India [114]	Inclined wick type solar still	Galvanized iron sheet with jute wicks	28 m2 (70 L/d)	1095 ppm tap water	Experiment	NA	NA	Bleaching of wick, corrosion of still, need for black dyes
NEOM City, Saudi Arabia [115]	Single-slope basin solar still	Metal basin	NA (2.8 to 6.5 L/m2d)	Saline water (salinity NA)	Simulation	NA	NA	Productivity depends on climate and this technology is suitable for NEOM city
Baja California Sur, Mexico [42]	Double basin solar still	Ferrocement basin	384 m2 (1.0 m3/d)	Sea water (Salinity NA)	Experiment	NA	NA	Capable of addressing fresh water demands of 100 local inhabitants
Rural regions of California, USA [45]	Single basin solar still	Concrete	10,000 m2 (30.0 m3/d)	0 to 35,000 ppm water	Simulation	At least 6.89 USD/m3	NA	More economical than solar photovoltaic-powered reverse osmosis system
Coastal cities of Somalia [46]	Single basin solar still	Fiber reinforced plastic	555 to 1079 m2 (At least 3.0 m3/d)	35,000 ppm sea water	Simulation	8.66 to 9.48 USD/m3; 4.0 to 13.0 years finance payback time	2.5 to 13.6 kilotons of CO2 emission mitigation	Lifetime and interest rates affect economics of the plant significantly
Central American and Caribbean sites [Present study]	Single basin solar still	Stainless steel basin	9 to 57 m2 (At least 15.0 to 75.0 L/d)	34,000 to 36,000 ppm sea water	Experimentation in Visakhapatnam and simulation	20.22 to 60.62 USD/m3; less than 2.0 years finance payback time	33.0 to 641.0 tons of CO2 emission mitigation	Good quality desalinated water; more economical than bottled water price; subsidy seems essential

NA—Not Available.

Table 10. Comparison of the present study with other peer-reviewed literature on solar still desalination plants.

Site	Solar Still Type	Absorber Type	Size of Solar Still Desalination Plant	Investigation				Observations
				Feed Water	(Performance— Theoretical or Experimental or Both)	Economics Aspects	Environmental Aspects
Quillagua, Chile [111]	Double-slope basin solar still	Cement trays	107 m2	10,000 ppm saline river water	Experiment	1.68 USD/m3	NA	Cracking
Valparaiso, Chile [111]	Double-slope basin solar still	Asbestos–cement trays	103 m2		Experiment	2.25 USD/m3	NA	Accurate fabrication seems difficult
Coober Pedy, Australia [112]	Double-slope basin solar still	Galvanized iron troughs	3530 m2 (19.0 m3/d)	24,000 ppm saline underground water	Experiment	0.73 USD/m3	NA	Algae growth in feed water, soil movement, animal interference
Awania, India [43]	Double-slope basin solar still	Brick with cement plaster coated black	1867 m2 (5.0 m3/d)	3000 to 5000 ppm well water	Experiment	NA	NA	Cracks, algae growth, glass breakage by slipping
Delhi, India [114]	Inclined wick type solar still	Galvanized iron sheet with jute wicks	28 m2 (70 L/d)	1095 ppm tap water	Experiment	NA	NA	Bleaching of wick, corrosion of still, need for black dyes
NEOM City, Saudi Arabia [115]	Single-slope basin solar still	Metal basin	NA (2.8 to 6.5 L/m2d)	Saline water (salinity NA)	Simulation	NA	NA	Productivity depends on climate and this technology is suitable for NEOM city
Baja California Sur, Mexico [42]	Double basin solar still	Ferrocement basin	384 m2 (1.0 m3/d)	Sea water (Salinity NA)	Experiment	NA	NA	Capable of addressing fresh water demands of 100 local inhabitants
Rural regions of California, USA [45]	Single basin solar still	Concrete	10,000 m2 (30.0 m3/d)	0 to 35,000 ppm water	Simulation	At least 6.89 USD/m3	NA	More economical than solar photovoltaic-powered reverse osmosis system
Coastal cities of Somalia [46]	Single basin solar still	Fiber reinforced plastic	555 to 1079 m2 (At least 3.0 m3/d)	35,000 ppm sea water	Simulation	8.66 to 9.48 USD/m3; 4.0 to 13.0 years finance payback time	2.5 to 13.6 kilotons of CO2 emission mitigation	Lifetime and interest rates affect economics of the plant significantly
Central American and Caribbean sites [Present study]	Single basin solar still	Stainless steel basin	9 to 57 m2 (At least 15.0 to 75.0 L/d)	34,000 to 36,000 ppm sea water	Experimentation in Visakhapatnam and simulation	20.22 to 60.62 USD/m3; less than 2.0 years finance payback time	33.0 to 641.0 tons of CO2 emission mitigation	Good quality desalinated water; more economical than bottled water price; subsidy seems essential

NA—Not Available.

5. Conclusions

The possibility of an F-SSS desalination plant in addressing the clean water demands of nine Central American and Caribbean sites in a sustainable way under three cases (Case 1—at least 15.0 L/d per family; Case 2—at least 37.5 L/d per family and Case 3—at least 75.0 L/d per family) has been performed and reported. The major conclusions hence drawn are listed below:

• The developed solar still is effective in desalting feed water of high salinity (43,100 ppm) to generate improved quality water.
• The number of solar stills required per family ranges between 9 and 11, 23 and 29 and 46 and 57 in the considered sites under Case 1, Case 2 and Case 3, respectively.
• The potable water price per m³ and finance payback time of the F-SSS desalination plant remain nearly the same in all three cases for a site, but are highly dependent on lifetime, interest rate and the still’s performance at the concerned site.
• The potable water price per m³ ranges between USD 19.75 (Havana) and USD 60.62 (Port-au-Prince) which seems to be several times lower than the locally available treated bottled water price.
• The finance payback time of the F-SSS desalination plant is less than 2 years.
• Subsidies seem to play a significant role in increasing acceptance among the public and this is reflected through a reduced potable water price with an increased subsidy.
• The energy payback time, specific CO₂ emission and decarbonization potential of the F-SSS desalination plant range between 0.59 and 0.69 year, 4.24 and 13.70 gCO₂/L of desalinated water and 33 and 641 tons of CO₂, respectively.
• Maintenance and brine disposal would be a serious issue to address in reality. However, dedicated maintenance from family members and strategies for centralized concentrated brine management via a zero liquid discharge plant run by district/local authorities can ease this issue and make the proposed plant truly sustainable.

Effective saline water desalination ability and low potable water generation price, along with a significant decarbonization potential justifies the suitability of the F-SSS desalination plant in addressing the clean water demands at a household level in low- and middle-income Central American and Caribbean countries. However, this transition is highly possible only through subsidies, which reduce the burden on the general public significantly. Policy and guideline development to facilitate the local government in the easy implementation of this task will be a focus in future works to achieve a sustainable future for all.