Performance Improvement of Solar Desalination System Based on CeO₂-MWCNT Hybrid Nanofluid

Abstract

There is a scarcity of freshwater resources and their quality is deteriorating. As a result, meeting human needs is getting more and more challenging. Additionally, significant health problems are brought on by a shortage of freshwater. Therefore, finding a sustainable alternative technique for producing clean water is necessary. Solar distillation is one of the methods that can be implemented to enhance the overall production of pure water. In this work, a hybrid nanofluid was prepared using a two-step method with cerium oxide (CeO₂) nanoparticles and multi-walled carbon nanotubes (MWCNTs) in a ratio of 80:20. The concentrations of hybrid nanofluids investigated were 0.02%, 0.04%, and 0.06%. The surfactant cetyltrimethylammonium bromide (CTAB) was used to keep the hybrid nanofluid stable. The studies were carried out over three days in both conventional and modified stills at a constant depth of 1 cm of hybrid nanofluid. The modified still (MS) achieved a maximum production of 1430 mL compared to the conventional still’s (CS) maximum output of 920 mL. The CPL (Cost per liter) for MS was USD 0.039, and for CS, it was USD 0.045. The levels of TDS in the MS and CS were 96.38% and 92.55% lower than those in saline water. The fluoride ion level of saline water was 0.635 mg/L, whereas the distilled water of MS and CS are 0.339 mg/L and 0.414 mg/L, respectively.

1. Introduction

One of the major issues confronting the world today is providing people with safe water to drink. Groundwater levels are gradually declining due to the continuous exploitation of water resources from the soil for commercial and residential use. Surface water is easily accessible, but much is brackish and unsafe for humans to drink [1]. It is of utmost importance to develop effective strategies for the removal of a wide range of environmental pollutants from surface water by utilising solar energy [2]. The textile, cosmetic, plastic, and food sectors are the leading producers of the wastewater [3]. Even at minute quantities, a family of contaminants poses a substantial risk to humans, plants, and other animals [4]. Therefore, a comprehensive process is required to remove salts and other impurities from water and get the pH level up to where it needs to be for human consumption. Conventional desalination techniques use a lot of energy and are quite expensive [5]. Therefore, it is vital to design a water desalination technique that is cost-effective, sustainable, and renewable. Solar energy appears to be the most efficient and feasible option.

Researchers have come up with a wide range of ways to improve the production of distilled water. Hitesh et al. [6] employed the magnesia waste brick material as energy storage to investigate the solar still performance. Kaviti and co-workers [7] enhanced the desalination performance with the help of using aluminum fins as solar absorbing material. Mevada et al. [8] utilized black granite and marble stones to increase the performance, obtaining a daily efficiency of 72.6% more than the traditional still. Kaviti et al. [9] improved distillate output by 36.35% using camphor-soothed banana stems. Kabeel et al. [10] enhanced distillate productivity by 45% compared to traditional still using red bricks coated with cement. Chen et al. [11] discussed the environmental applications of derived carbon materials from plastic wastes for sustainable green energy. Kumar and co-workers [12] studied different hydrogels to treat brine water desalination. Abdelaziz et al. [13] developed a porous absorber with activated carbon tubes which has a favorable effect on energy, efficiency, economy, and environmental performance. Kumar and co-researchers [14] enhanced the desalination percentage by 104.54% by augmenting the magnets and charcoal in the stepped solar still. Kaviti et al. [15] improved solar desalination by fabricating hierarchical structures with an efficiency of 60%. Further, they studied the morphology and optical properties of the nanostructure at room temperature [16]. Wei et al. [17] regulated the salt concentration in the desalination mechanism for a freshwater generation. Attia et al. [18] acquired 5.27 kg of productivity by utilizing the material phosphate bag as energy storage. Kaviti et al. [19] evaluated the solar still performance by incorporating the parabolic and truncated conic fins. They also investigated the fin geometry effect on the desalination yield output [20]. Chen et al. [21] significantly elaborated on the different strategies for developing functional materials from wastes for the remediation of wastewater.

Singh et al. [22] deployed a wick and nanofluid to improve distilled water production, obtaining an efficiency of 89.9%. Hossain and Sahin [23] utilized the hybrid nanofluid with a combination of Al₂O₃-water-SiO₂, resulting in thermal efficiency of 37.76% with an output of 4.99 kg/m² per day. Iqbal et al. [24] evaluated a decade of the progress of nanofluid-assisted solar still desalination. Gamal et al. [25] enhanced solar still performance with a combination of the wick, carbon black (CB) nanofluid, and aluminum corrugated with a V-shape. They reported that an ideal scenario includes using a wick and a v-corrugated basin with 1.5 weight percent CB nanofluid and 3 weight percent CB nanoparticles. Kandeal et al. [26] optimized the thermo-economic efficiency of solar desalination by using nanofluid, copper chips, and nano-based phase transition material. The cost of producing freshwater was also reduced by 35.3%. Balachandran et al. [27] utilized nano-ferric oxide, which is a thermally conductive and enhanced performance of a solar still by 68%. Panchal et al. [28] used the TiO₂ and MgO nanofluids to evaluate the annual performance of solar still. They reported that using TiO₂ and MgO nanofluids at 0.1% and 0.2% concentrations increases the distillate production of the solar still by 4.1%, 20.4%, 33.33% and 45.8%, respectively. The above discussed literature’s desalination percentage and type of material used for the performance enhancement were mentioned in

Table 1. Desalination percentage and type of material used for performance enhancement.

S No	Author	Material Type	Desalination Percentage
1.	Hitesh et al. [6]	Magnesia waste brick	NA
2.	Mevada et al. [8]	Black granite and marbles stones	72.6%
3.	Kaviti et al. [9]	Camphor soothed stems	36.35%
4.	Kabeel et al. [10]	Red bricks coated with cement	45%
5.	Kumar et al. [14]	Combination of magnets and charcoal	104.54%
6.	Kaviti et al. [15]	Hierarchical structures	60%
7.	Singh et al. [22]	Combination of wick and nanofluid	89.9%
8.	Hossain and Sahin [23]	Hybrid nanofluid (Al2O3-water-SiO2)	37.76%
9.	Balachandran et al. [27]	Nano-ferric oxide	68%
10.	Panchal et al. [28]	TiO2 and MgO nanofluids	20.4% and 45.8%

.

The most popular nanoparticles so far are Al₂O₃, MgO, carbon black, CuO₂, ZnO, SiO₂, and TiO₂. The concentrations ranged from 0.02% to 0.3% for Al₂O₃, 0.02% to 0.2% for CuO₂, and up to 0.1% for SnO₂ and ZnO. The authors sought to employ cerium oxide (CeO₂) as nanoparticles in solar distillation applications due to its potential improvement of thermo-optical characteristics. For example, the thermo-optical property of the tube solar collector is improved by up to 34% at lower concentrations of CeO₂ and it was reported by Sharafeldin and Gyula [29]. Further, Kumar and co-researchers [30] improved the performance of the desalination system by 27.40% with the use of cerium oxide (CeO₂) nanoparticles.

The novelty of the current work compared to the previous reports on cerium oxide-based frameworks and logic behind the use of CeO₂ nanoparticles and MWCNTs in a hybrid nanofluid is that they can improve the thermal conductivity and heat transfer properties of the fluid. CeO₂ nanoparticles have been shown to have high thermal stability and good thermal conductivity, while MWCNTs have high aspect ratios and excellent thermal conductivity. By combining these two types of nanoparticles in a hybrid nanofluid, the resulting fluid can have significantly improved thermal properties compared to traditional nanofluids used in solar stills. The enhanced thermal properties of the hybrid nanofluid can result in higher evaporation rates and improved efficiency in solar stills. This means that less energy is required to produce freshwater, which can be a significant advantage in areas with limited access to energy resources. Additionally, the use of CeO₂ and MWCNTs is considered to be environmentally friendly, as they are both relatively low toxicity materials.

This work aims to manufacture a hybrid nanofluid with the appropriate concentrations to determine the performance of a hybrid nanofluid at 1 cm water depth and to optimize the concentrations for increased distillate in single slope glass solar still. Using a two-step approach, a hybrid nanofluid was developed by combining cerium oxide (CeO₂) nanoparticles with multi-walled carbon nanotubes (MWCNTs) at a ratio of 80:20. The experiments were conducted with hybrid nanofluid concentrations of 0.02%, 0.04%, and 0.06% for the three consecutive days. Further water quality analysis was carried out because of the harmful nature of nanofluids to humans, and the outcomes were within the WHO acceptable limits. Economic analysis was also reported to know whether it was economically viable or not.

2. Preparation of Hybrid Nanofluid, Experimental Setup, and Procedure

2.1. Hybrid Nanofluid Preparation

The selected nanoparticles for hybrid nanofluid were cerium oxide (CeO₂) and multi-walled carbon nanotubes (MWCNTs). To stabilize the hybrid nanofluid, we decided to add the surfactant. Since it is a hybrid nanofluid, we have chosen a cationic surfactant, i.e., CTAB (Cetyl Trimethyl Ammonium Bromide), to stabilize it. Then, we weighed the two nanoparticles CeO₂-MWCNTs and CTAB, using the digital weighing machine for different concentrations such as 0.02%, 0.04%, and 0.06%. Next, we prepared the hybrid nanofluid of 5 L/day for 1 cm depth of water by using a two-step method, which is discussed in detail below and illustrated in Figure 1. Then, CeO₂-MWCNTs are put together in a beaker in the ratio of 80:20 with base fluid, i.e., water. Then, the mixture was stirred with the glass rod for 5 min to prevent lump formation, followed by placing the mixture in ultra sonicator for 15 min to get a homogeneous mixture, then added the surfactant as per the desired nanoparticle to surfactant mixing ratio, i.e., 3:2, to enhance the stability of hybrid nanofluid. The ultrasonication was carried out for up to 90 min at 25 °C temperature to avoid the evaporation of the hybrid nanofluid. After sonication, we performed the magnetic stirring for 30 min for 5 L at 800 rpm (approximately). Finally, we got CeO₂-MWCNTs/water-based hybrid nanofluid. Similarly, we prepared the hybrid nanofluid for the other two concentrations as per prescribed standards and a ratio of varying concentrations.

Cerium oxide (CeO₂) and multi-walled carbon nanotubes (MWCNTs) are two materials that have been studied for their potential use in desalination. In a hybrid desalination system, these two materials can work together to improve the efficiency and effectiveness of the desalination process. Cerium oxide is a versatile material with several properties that make it useful in desalination. It is a photocatalyst that can help to break down contaminants in the water, and it also has good thermal stability, making it useful in high-temperature applications. Cerium oxide can also absorb and store solar energy, which can be used to drive the desalination process. It has been shown to improve the efficiency of desalination systems, particularly when used in conjunction with other materials.

MWCNTs are a type of carbon nanotube that have multiple layers. They have a high surface area, which makes them useful for adsorption and separation processes. MWCNTs can be used as a filter to remove salt and other impurities from the water, and they can also be used to trap and concentrate solar energy. MWCNTs can also be used as a support material for other functional materials, such as cerium oxide. When combined, cerium oxide and MWCNTs can form a hybrid material that takes advantage of their individual properties. The MWCNTs can act as a scaffold to support the cerium oxide particles. Overall, this hybrid nanofluid can be an effective way to desalinate water using solar energy.

2.2. Thermal Conductivity

The thermal conductivity of the hybrid nanofluid (CeO₂ + MWCNTs) at different concentrations was measured and summarized in

Table 2. Thermal conductivity of hybrid nanofluid measured by utilizing the TPS method.

S No	Hybrid Nanofluid Concentration (CeO2 + MWCNTs)	Thermal Conductivity (W/m-k)
1	0.02%	0.78936
2	0.04%	0.79534
3	0.06%	0.80132

. The TPS (Transient Plane Source) technique was used to determine the thermal conductivity of a hybrid nanofluid. The TPS approach combines a plane sensor and a unique mathematical model explaining heat conductivity with electronics, allowing the method to determine thermal properties. It may measure various materials, including solids, liquids, pastes, thin films, etc. This technique may also be used to evaluate both isotropic and anisotropic materials, according to the TPS standard, and the process is described below.

The TPS approach generally involves two sample halves, with the sensor sandwiched between them, as described in Figure 2. Ideally, samples should be homogenous; however, it is feasible to utilize TPS testing on heterogeneous materials if the sensor size is chosen to enhance sample penetration. This approach can also be employed in a single-sided arrangement with the addition of a recognized insulating material as sensor support. The flat sensor is made up of an uninterrupted double helix of nickel (Ni) metal, which conducts electricity and is carved out of a thin foil. A thin polyimide sheet called kapton is sandwiched between two layers of the nickel spiral. These thin kapton sheets provide the sensor’s mechanical stability and electrical insulation. The sensor is positioned between two measuring sample halves. A steady electrical action throughout the conducting spiral during the measurement raises the sensor’s temperature. On each side of the sensor, the heat produced is allowed to escape into the sample at a rate determined by the material’s thermal transport capabilities. By recording the sensor’s response to changes in temperature over time, it is possible to determine the material’s thermal conductivity.

Further, the prepared hybrid nanofluids were compared to existing models to evaluate their thermal conductivity. Based on theoretical models for hybrid nanofluids produced by Jake et al. [31], The thermal conductivity of the hybrid nanofluid is evaluated using the Chamkha et al. [32] empirical correlation and is given as:

$$k_{hnf}=k_{bf}\left[\frac{\frac{{\varnothing }_1{k}_{p,1}+{\varnothing }_2{k}_{p,2}}{{\varnothing }_{hnf}}+2k_{bf}+2\left({\varnothing }_1{k}_{p,1}+{\varnothing }_2{k}_{p,2}\right)-2k_{bf}{\varnothing }_{hnf}}{\frac{{\varnothing }_1{k}_{p,1}+{\varnothing }_2{k}_{p,2}}{{\varnothing }_{hnf}}+2k_{bf}+\left({\varnothing }_1{k}_{p,1}+{\varnothing }_2{k}_{p,2}\right)-k_{bf}{\varnothing }_{hnf}}\right]$$

(1)

where $k_{p,1}, {\varnothing }_1 \mathrm{and} k_{p,2}, {\varnothing }_2$ are the thermal conductivity and individual concentrations of nanoparticles and ${\varnothing }_{hnf}={\varnothing }_1+{\varnothing }_2$ is the hybrid nanofluid concentration.

The model proposed by Devi and Devi [33] for determining the thermal conductivity of hybrid nanofluid is as follows:

$$k_{hnf}=k_{bf}\left[\frac{k_{p,1}+\left(n_1-1\right)k_{bf}-\left(n_1-1\right){\varnothing }_1\left(k_{bf}-k_{p,1}\right)}{k_{p,1}+\left(n_1-1\right)k_{bf}+{\varnothing }_1\left(k_{bf}-k_{p,1}\right)}\right]\left[\frac{k_{p,2}+\left(n_2-1\right)k_{bf}-\left(n_2-1\right){\varnothing }_2\left(k_{bf}-k_{p,2}\right)}{k_{p,2}+\left(n_2-1\right)k_{bf}+{\varnothing }_2\left(k_{bf}-k_{p,2}\right)}\right]$$

(2)

where $n_1 \mathrm{and} n_2$ are the nanoparticle’s shape factors.

Similarly, the Chougule and Sahu [34] model for determining the thermal conductivity of hybrid nanofluids is mathematically represented as:

$$k_{hnf}=k_{bf}\left[1+\frac{k_{p,1}{\varnothing }_1{r}_{bf}}{k_{bf}\left(1-{\varnothing }_{hnf}\right)r_{p,1}}+\frac{k_{p,2}{\varnothing }_2{r}_{bf}}{k_{bf}\left(1-{\varnothing }_{hnf}\right)r_{p,2}}\right]$$

(3)

where $r_{p,1} and r_{p,2}$ are the radii of the nanoparticles.

From the studies, it was observed that Chougule and Sahu [34] model thermal conductivity values were closer to the experimental values when compared to the other two models and were mentioned in

Table 3. Comparison of the hybrid nanofluid thermal conductivity with the existing models.

S No	Hybrid Nanofluid Concentration (CeO2 + MWCNTs)	Thermal Conductivity (W/m-k)
		Theoretical			Experimental
		Chamkha et al. [32]	Devi and Devi [33]	Chougule and Sahu [34]	Transient Plane Source Method
1	0.02%	0.60967	0.61273	0.67989	0.78936
2	0.04%	0.62092	0.62319	0.76180	0.79534
3	0.06%	0.63173	0.63638	0.85412	0.80132

. It was also noticed that the difference between theoretical and experimental thermal conductivity values was more anticipated [31].

2.3. Experimental Setup and Procedure

Two identical solar stills, conventional still (CS) and modified still (MS), were manufactured as shown in Figure 3. The galvanized iron sheet with the dimension of 50 cm × 50 cm × 0.1 cm was used to fabricate the basin of the solar still. The basin of solar still was powder coated in black color to absorb solar intensity. To prevent heat losses, all sides of the solar still were insulated with a 0.8 cm thickness of plywood. The solar still front wall was 20 cm in height, while the back wall was 36 cm. The solar still was covered with a transparent glass cover of 0.4 cm thickness, making an angle of 17° horizontal. The back wall of the solar still consists of a 1.27 cm hole used for saline water supply and drainage. The distillate output of solar still was collected through a channel provided on the front wall inner side.

The investigations were carried out in Hyderabad’s climate conditions (78.49° E longitude, 17.39° N latitude). The experiments were conducted over three days while increasing the concentrations of hybrid nanofluid from 0.02% to 0.04% to 0.06%. The particle size of CeO₂ and MWCNTs are as follows. CeO₂: 50 nanometers diameter on average. And MWCNTs: 10–20 nm diameter on an average and 10–30 μm length on an average. The nanoparticles (CeO₂), cerium oxide, also known as ceric oxide, are acquired from Sigma-Aldrich. It is pale yellow in color. The surfactant (CTAB) and MWCNTs were purchased from Merck.

Multiple measuring instruments were employed to evaluate different aspects of the performance of the solar still, as shown in

Table 4. Standard uncertainty, percentage error, and accuracy of the measuring instruments.

Measuring Device	Standard Uncertainty	% Error	Accuracy
Hukseflux pyranometer (SR05-D1A3)	5.77 W/m2	10	±10 W/m2
RTD sensors	0.5 °C	0.25	±0.8 °C
Data logger	0.06 °C	1.3	±0.1 °C
Measuring jars	3 mL	5	±5 mL
Anemometer	0.06 m/s	10	±0.1 m/s

. A 16-channel data logger was used to record temperatures at three different locations using RTD (Resistance Temperature Detector) sensors, which have an accuracy of ±0.8 °C. The temperatures of the inner glass (T_g) cover, the water (T_w), and the basin plate (T_b) were the important areas to measure. The solar intensity and ambient temperature were monitored using a hukseflux pyranometer (accuracy ± 10 W/m²). An anemometer with a ± 0.1 m/s precision was utilized to measure wind speed. A one-liter measuring jar calibrated with a ±5 mL accuracy was used to estimate the water production.

Here, the solar stills are oriented southward to capture the maximum solar radiation when the sun moves from east to west. The prepared hybrid nanofluid is poured into the basin of the solar still via the intake channel provided at the back wall. Compared to the conventional still, which merely uses saline water, the modified still uses a combination of cerium oxide (CeO₂) nanoparticles and MWCNTs. The transparent glass cover allows maximum solar radiation into the solar still. As a consequence, the water temperature in the solar still increases and initiates evaporation. The evaporated saline water condenses on the inner surface of the glass cover because the temperature at this surface is low. The distillate output of solar still was collected through a channel provided on the front wall inner side in the form of droplets. Further, these water droplets were collected in a one-liter measuring jar provided at the CS and MS outlet.

2.4. Uncertainty Analysis

Uncertainty analysis is simply the difference between the true value and the computed value, which is also known as an error. There are two different categories of ambiguity errors: type A and type B. Errors of type A are random and quantifiable by mathematical and repetitive investigation. Errors of type B are systematic and may be computed using the instrument’s calibration report or data book. The normal uncertainty is deduced from the following mathematical theorem as follows.

$$\mathrm{u}=\frac{\mathrm{a}}{\sqrt{3}}$$

(4)

where

$$\mathrm{u}=\mathrm{normal} \mathrm{uncertainty}, \mathrm{a}=\mathrm{measuring} \mathrm{instrument} \mathrm{precision}$$

3. Results and Discussions

3.1. Technical Analysis

Solar irradiance and ambient temperature significantly influence a desalination system’s performance. The values of ambient temperatures and irradiance on three consecutive days, using hybrid nanofluid with various concentrations, are graphically represented in Figure 4. On day 1 (23 May 2022 with 0.02% concentration of hybrid nanofluid), the solar irradiance with a peak value of 987 W/m² at 12 h was recorded. On day 2 and day 3 (24 and 25 May 2022, with 0.04% and 0.06% concentrations of hybrid nanofluid), it reached 976 and 939 W/m², respectively. Similarly, on all three days, the outside temperature ranged from 31 to 33 °C at 9 h to 39 to 42 °C at mid-day. Solar irradiation exhibits a small amount of variation on each day of experimentation. Since the irradiance was high initially, the ambient temperature continued to be high for a long time.

The temperatures at three locations of still, i.e., (i) in water (T_w), (ii) on the basin (T_b) (iii), and on inner glass (T_g), have been measured using three different thermocouples and graphically represented in Figure 5 on three different days, respectively. The highest water temperature in an MS was measured at 70 °C, 69 °C, and 68 °C for hybrid nanofluid with 0.02%, 0.04%, and 0.06% concentrations, respectively. The highest water temperature for CS was 58 °C, 56 °C, and 59 °C during the three-day testing period. The presence of MWCNTs and CeO₂ nanoparticles in hybrid nanofluid is responsible for the variation in water temperature readings between CS and MS. A considerable percentage of incoming solar energy flux is transferred to the surrounding water as the hybrid nanofluid in the basin absorbs additional solar flux and develops heat localization due to the resonance effect between the MWCNTs and CeO₂ nanoparticles. The maximum basin temperatures for MS were 75 °C, 73 °C, and 70 °C at 0.02%, 0.04%, and 0.06% concentrations of hybrid nanofluid, respectively, whereas for CS, they were 59 °C, 58 °C, and 60 °C. Energy transfers between the solar still basin and the hybrid nanofluid are responsible for this. The maximum distillate production was achieved because of these water and basin temperatures, which helped to enhance the evaporation rates. For MS, the highest glass interior temperatures were 68 °C for 0.02%, 66 °C for 0.04%, and 67 °C for 0.06% concentrations of hybrid nanofluid. CS’s maximum glass interior temperature was 55 °C, 53 °C, and 56 °C on three different days, respectively. The most significant temperature values were recorded at noon on each of the three consecutive days, and they exhibit a pattern consistent with the solar irradiation and ambient temperature.

The hourly yields and cumulative yields of both the stills, i.e., CS and MS, with three different nanofluid concentrations, have been represented graphically in Figure 6 and Figure 7, respectively. It is observed that the maximum yields (in one-hour duration) are produced at noon session, between 13 h and 15 h. After completion of experimentation, 1430 mL, 1310 mL, and 1170 mL of distillate were produced at concentrations of 0.02%, 0.04%, and 0.06%, respectively, when hybrid nanofluid is used in the MS. It can be inferred that the distillates are maximum when the concentrations are minimum, irrespective of the type of nanofluid used in the solar stills. In the case of CS, less distillate is produced in contrast to MS, but in the case of hybrid nanofluid, at higher concentrations, the viscosity might be the key reason behind the poor distillation results. Of course, the precipitation can be prevented by adding a surfactant, but it must be selected wisely because the surfactant plays a role with respect to the type of nanofluid, i.e., different surfactants are used to stabilize mono and hybrid nanofluid separately. In other words, the surfactant used for mono nanofluid might not stabilize the hybrid nanofluid. Here, in the latter case, we used CTAB surfactant to stabilize the hybrid nanofluid, which cannot be used to stabilize the mono nanofluid.

Based on irradiance, most distillate must be produced post-noon, i.e., after 12 h. As irradiance increases, the basin absorbs heat and transfers heat to in-contact water, which takes some time. Thus, we can observe that the distillate produced is maximum, between 14 h and 15 h. One can see that the highest distillate produced is using hybrid nanofluid on day 1, which is of 0.02% concentration among all other concentrations because lower concentrations gave more stability in contrast to higher concentrations, so we considered 0.02%, 0.04%, and 0.06% vol fractions for our study. Additionally, extremely low concentrations (such as ≤0.01%) did not significantly increase the thermal efficiency, as reported by Sharafeldin and Gyula [29]. The MS with a 0.02% concentration of hybrid nanoparticles produces 55.43% more distillate when compared with CS. Since there is a drastic reduction in distillate using a 0.06% concentration of hybrid nanofluid, an optimum percentage concentration lies between 0.02% and 0.04% for the generation of enhanced distillate, by limiting the usage of hybrid nanoparticles, leading to reduce the cost of water produced per liter. Further, the present study compared with the previous literature is described in

Table 5. Comparison of the present study with the previous literature.

S No	Author	Nanofluid	Distillate Output (L/m2)	Efficiency (%)
1.	Kaviti et al. (present study)	Hybrid nanofluid (CeO2 + MWCNTs)	2.86	55.43
2.	Elango et al. [35]	Aluminum oxide (Al2O3)	1.870	29.95
3.	Elango et al. [35]	Zinc oxide (ZnO)	1.50	12.67
4.	Elango et al. [35]	Tin oxide (SnO2)	1.610	18.63
5.	Masoud et al. [36]	Silver (Ag)	1.915	25.42
6.	Kabeel et al. [37]	Aluminum oxide (Al2O3)	2.095	NA
7.	Kabeel et al. [37]	Cuprous oxide (Cu2O)	2.240	NA
8.	Sahota and Tiwari [38]	Aluminum oxide (Al2O3)	1.725	5.9
9.	Hafs et al. [39]	Fins + cuprous oxide (Cu2O)	2.61	20
10.	Kandeal et al. [26]	Cu chips + copper oxide (CuO)	2.36	56
11.	Singh et al. [22]	Al2O3-water-SiO2	2.49	44.95
12.	Panchal et al. [28]	TiO2 and MgO	3.5 and 2.7	33.33 and 45.80

.

3.2. Assessment of Water Quality

To ensure that the distillate water is within the BIS (Bureau of Indian Standards) and WHO (World Health Organization) guidelines, the water purity of the CS and MS has been assessed both before and after desalination.

Table 6. Water quality assessment.

Water Quality Metrics	Prior to Desalination	After Desalination (CS)	After Desalination (MS)	Maximum Allowable Levels in Drinking Water (WHO and BIS Standards) [40]
pH	8.24	7.38	7.12	8.5
Fluoride (mg/L)	0.635	0.414	0.339	1.5
Hardness (mg/L)	350	170	130	200
TDS (ppm)	470	35	17	500
Chloride (mg/L)	62.4	12.64	8.43	250

provides an overview of the outcomes of the water quality tests, which were done at the VNRVJIET Environmental Engineering Laboratory in Hyderabad.

The pH of the brine water was lowered by the CS and MS from 8.24 to 7.38 and 7.12, respectively. Total dissolved solids (TDS) were at 470 ppm prior to desalination; however, post-desalination, TDS levels were drastically lowered to 17 ppm for MS and 35 ppm for CS, respectively. The MS TDS levels were 96.38% lower than those of saline water. For brine water, the hardness values were 350 mg/L, 170 mg/L for CS, and 130 mg/L for MS. The fluoride ion content of brine water was 0.635 mg/L, compared to 0.339 mg/L and 0.414 mg/L for the MS and CS, respectively. The BIS and WHO, India, permitted standards were met for all the MS and CS water quality measures [40].

4. Monetary Analysis

Equations (5)–(13) are utilized to evaluate the monetary analytical modeling. These mathematical formulae are provided by [41,42].

$$\mathrm{CRF} \left(\mathrm{Capital} \mathrm{recovery} \cos \mathrm{t}\right)=\frac{\mathrm{i}{(1+\mathrm{i})}^{\mathrm{y}}}{[\left(1+\mathrm{i}{)}^{\mathrm{y}}-1\right]}$$

(5)

Following are the assumptions:

Life of solar still (y) = 10 Years

Interest rate (i) = 12%

Number of sunny days (n) = 250

$$\mathit{FAC} \left(\mathrm{Fixed} \mathrm{annual} \cos \mathrm{t}\right)=\mathrm{P}\left(\mathrm{Capital} \cos \mathrm{t}\right)\times \mathrm{CRF}$$

(6)

$$S \left(\mathrm{Salvage} \mathrm{value}\right)=0.2\times \mathrm{P}$$

(7)

$$\mathit{SFF} \left(\mathrm{Sin}\mathrm{king} \mathrm{fund} \mathrm{factor}\right)=\frac{\mathrm{i}}{[\left(1+\mathrm{i}{)}^{\mathrm{y}}-1\right]}$$

(8)

$$ASV \left(\mathrm{Annual} \mathrm{salvage} \mathrm{value}\right)=SFF\times S$$

(9)

$$AMC \left(\mathrm{Annual} \mathrm{maintenance} \mathrm{operational} \cos \mathrm{t}\right)=0.15\times FAC$$

(10)

$$AC \left(\mathrm{Annual} \cos \mathrm{t}\right)=FAC+AMC-ASV$$

(11)

$$M \left(\mathrm{Average} \mathrm{annual} \mathrm{productivity} \mathrm{in} \mathrm{liters}\right)=c\times n$$

(12)

where

n = Sunny days per year                                                                                

c = Yield/day                                                                                                   

$$CPL \left(\mathrm{Distilled} \mathrm{water} \cos \mathrm{t} \mathrm{per} \mathrm{liter}\right)=\frac{AC}{M}$$

(13)

It costs USD 54 and USD 73 to fabricate CS and MS, which covers all of the components required for manufacture and are stated in

Table 7. Fabrication cost of solar stills.

S. No	Material/Service	Area/Quantity Per Still	CS (USD)	MS (USD)
1.	Glasses	2.5 m2	15	15
2.	PVC collecting channel	1	3	3
3.	Black powder coating	1 m2	2	2
4.	Glass cover, 0.4 cm	0.5 m2	1	1
5.	Double-sided foam tape	1.5 m	1	1
6.	Silicon glue	1	2	2
7.	CeO2 nanoparticles	25 gm	-	5
8.	MWCNTs	10 gm	-	10
9.	CTAB (surfactant)	10 gm	-	4
10.	Fabrication charges	-	30	30
	Total cost	-	USD 54	USD 73

.

Table 8. Economic analysis of CS and MS.

Parameters in USD	Conventional Still (CS)	Modified Still (MS)
P	54	73
CRF	0.177	0.177
FAC	9.5	12.9
S	10.8	14.6
SFF	0.05698	0.05698
ASV	0.6153	0.8319
AMC	1.42	1.94
AC	10.30	14.00
M	225	357
CPL	0.045	0.039

provides a list of the financial analysis inputs used in mathematical modeling. The MS significantly enhanced its yearly productivity by 58.67%, proving its supremacy in terms of output due to its greater daily distillate output (1430 mL/day) than CS. The corresponding CPL of CS and MS were USD 0.045 and 0.039. The CPL of MS was 15.38% lower than that of its equivalent CS. As a result, MS has become more inexpensive in terms of the CPL.

5. Conclusions

As the concentration of hybrid nanofluid (Cerium oxide and MWCNTs) increases, the production rate of distilled water decreases. Compared to CS, a maximum yield of 1430 mL was produced by MS with a 55.43% performance enhancement. The MS with 0.02% enhanced its performance by 9.16% and 22.22% compared to 0.04% and 0.06% concentrations of hybrid nanofluid. The CPLs were USD 0.039 for MS and USD 0.045 for CS. In comparison to CS, the CPL of MS was 15.38% lower. As a result, MS became more affordable.

The CS and MS successfully brought the brine water’s pH down from 8.24 to 7.38 and 7.12, respectively. Total dissolved solids (TDS) levels were 470 ppm before desalination but dramatically lowered to 35 ppm for CS and 17 ppm for MS after desalination. The TDS levels in the MS were 96.38% lower than those in the saline water. The hardness values for brine water were 350 mg/L, 130 mg/L for MS, and 170 mg/L for CS.