1. Effects of Al2O3 Nanofluid on Solar-Still Performance
Sahota and Tiwari 
investigated the effect of Al2
on the yield performance of a passive double-slope solar still (DSSS). Al2
nanoparticles were added to the base fluid (water) at two different masses of 35 and 80 kg. The passive DSSS with the nanofluid worked as follows: Solar radiation penetrates the solar still through the transparent condensing cover. The radiation is first absorbed by the nanofluid and then by the blackened surface. A plasmon-resonance absorption band of metallic nanoparticles is clearly observed near the infrared spectrum. The mixture of metallic nanoparticles in the base fluid absorbs solar radiation. A common effect of energy transfer between the nanoparticles and the basin liner is that it increases the temperature of the nanofluid in the solar still. Sahota and Tiwari 
developed and implemented a methodology to calculate the changes in hourly and daily yields resulting from the passive DSSS at 0.04%, 0.08%, and 0.12% concentrations. The findings showed an increase in the heat-transfer coefficient with an increase in the concentration of nanoparticles. One reason for this is that nanofluids directly absorb solar radiation, and the rate of absorption increases with solar intensity. Another reason is that the basin liner also transfers thermal energy to the nanofluid, resulting in an increase in temperature. Overall, the daily yield of the solar still increases with increasing concentration of Al2
nanoparticles in the base fluid.
Zhang et al. 
studied the effects of Al2
nanoparticles on reducing the energy demand within solar collectors. The investigation involved the application of two methods to facilitate the better use of solar energy. The first was a phase-change material (PCM), which was used to absorb the solar radiation, eliminating potential summer radiation problems. The second approach depended on the absorption of the solar radiation and its conversion into a source of heat to generate sanitary hot water. The findings showed that the use of nanoparticles resulted in the reduction in energy consumption within the solar collectors. In the PCM method, energy consumption was reduced by 44% on hot days, which was equal to a 267 kWh drop. On cold days, the reduction was 48%—equivalent to 2419 kWh. Year-round analysis revealed that the energy consumption was reduced by 2686 kWh with the use of the nanoparticles in solar collectors.
Farajzadeh et al. 
conducted experimental and numerical studies on the effect of Al2
on the thermal efficiency of flat-plate solar collectors. First, they determined the maximum efficiency and thermal loss of collectors with water following the American Society of Heating, Refrigerating, and Air-Conditioning Engineers methodology. The maximum efficiency of the solar still with water was 0.9, while the thermal loss was 21.2. This was followed by another experiment in which the solar still was filled with Al2
along with water. The maximum efficiency of the solar still with this mixture was 0.9, while the thermal loss was reduced to 13.74. These findings showed that Al2
did not substantially improve the maximum efficiency of a solar still but can be beneficial in reducing thermal loss. However, the authors failed to explain the reasons for the decrease in the heat loss when Al2
nanoparticles were introduced.
Faridani and Ameri 
analysed the possibility of using Al2
nanoparticles to enhance the performance of basin solar stills. They used γ-Al2
nanoparticles to effectively produce pure water in the presence of a mixer. Three analyses—Fourier-transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy—were used to verify the nanoparticles. The quantity of γ-Al2
nanoparticles, initial depth of water, intensity of solar energy, and temperatures of the glass, bottom, and brine were analysed. The results showed a distillate yield of approximately 60.03% with a 0.3 mass% of γ-Al2
. The tests with nanoparticles also showed an increase in the glass, bottom, and brine temperatures. Overall, the application of γ-Al2
nanoparticles improved the performance of the still because of the better thermal characteristics of saline water.
Negm et al. 
investigated the effect of Al2
on the efficiency of a thermosyphon flat-plate solar collector and found that the efficiency of solar collectors could be increased using nanofluids such as Al2
as the working fluid. They observed an increase in the efficiency when using nanofluids as the working fluid compared with that of designs that used distilled water alone. Al2
–water nanofluids have been found to be suitable for improving the efficiency of solar collectors.
Shoeibi et al. 
studied the effects of nano-enhanced PCM on the performance of solar stills. The Al2
nanoparticles used in the study were at concentrations of 0.3 wt%. Al2
was mixed in paraffin wax to enhance the thermal properties of PCM. The study indicated a 49.5% increase in the productivity of solar stills when Al2
nanoparticles were used at 0.3 wt% concentration. Moreover, the Al2
nanoparticles could reduce the melting point by 1.8 °C at a concentration of 0.1 wt%. The nano-coating also increased the rate of water production of the solar still by approximately 5.7%. Overall, the study showed an improved performance of the solar still with the addition of Al2
Chaichan and Kazem 
investigated the productivity improvement of a single-slope solar distillator using a PCM and Al2
nanoparticles. They added nano-Al2
to the wax of one of the distillates used in the study to enhance the thermal conductivity of paraffin wax. The addition of Al2
to paraffin wax resulted in a substantial increase in the rate of heat transfer and enhanced the distillate yield to approximately 60.53% compared with that of the simple distiller yield recorded after sunset. Muraleedharan et al. 
reported the design and construction of a modified active-solar-distillation system (MSDS) and compared its performance parameters with those of a conventional solar still. The heat-transfer fluid was an Al2
Therminol-55 nanofluid (nHTF), and different parameters—including the temperature of the saline water, hourly yield, and total yield—were compared. The experiment showed that the hourly yield of the MSDS with an nHTF of 0.1% was 45–250.27% more than that of the conventional solar still. The total yield of the MSDS with 0.1% nHTF was much higher at 12.190 L/m2
/day compared with the 3.48 L/m2
/day reported for the conventional solar still. Daily efficiency of the MSDS varied depending on the concentration of the nanofluid, but the maximum efficiency was 53.33% for an nHTF concentration of 0.1%. Thus, an MSDS system using a nanofluid showed higher productivity and better cost-effectiveness than those of a conventional solar still using only water.
Rashidi et al. 
numerically tested the significance of a nanofluid used in stepped solar stills. The glass cover and bottom surface had constant temperatures of 30 °C and 40 °C, respectively. An unstable Al2
–water nanofluid flow was used in the simulation. The nanoparticles were then added to the base fluid. Employing higher concentrations of the nanofluid led to higher rates of evaporation and condensation and quantities of water generated by the device. The results showed an increase in hourly productivity because of increasing the solid volume fraction of nanoparticles. Elevating the percentage concentration of Al2
from 0% to 5% resulted in a 22% increase in the hourly productivity.
Choudhary and Subudhi 
reported turbulent natural convection in an enclosure full of a water-based Al2
nanofluid. The nanoparticles were spherical and had a mean diameter of 40 nm, and they were mixed with distilled water to prepare the nanofluid at a concentration of 0.01 and 0.1 vol%. The resulting nanofluids remained stable for >24 h. There was an increase in Ra, suggesting an increase in heat transfer because of the use of nanomaterials in the enclosure. Bellila et al. 
used an Al2
nanofluid to enhance the productivity of a hemispherical solar still. The improvement in the yield of the solar distiller ranged from 105.8% to 121% across different volume concentrations of the nanoparticles. Tuly et al. 
evaluated the use of Al2
in a modified DSSS and found a 21.5% increase in the augmented productivity.
The effects of Al2O3 on the performance of solar-still desalination systems is summarised in Table 2.
Table 2. Summary of the effects of Al2O3 on the performance of a solar-still desalination system.
2. Effects of CuO Nanofluid on Solar-Still Performance
CuO nanoparticles have been increasingly used at different concentrations to increase the thermal conductivity of the fluid, increase the water temperature, and reduce particle precipitation at the bottom of the solar still. Dawood et al. 
investigated approaches to increasing the productivity of freshwater in a solar-still system incorporating CuO nanofluids. A wet-chemistry approach was used to synthesise CuO nanoparticles. During the preparation process, Cu (NO3
, NaOH, and acetic acid were used to obtain a fine CuO nanopowder. At 1.5% nanofluid volume concentration, the daily efficiency increased by 54%, 43%, and 36%, compared with that of the conventional solar still at water depths of 10, 20, and 30 mm, respectively.
Arunkumar et al. 
studied the effect of nano-coated CuO absorbers with polyvinyl alcohol (PVA) sponges within a single-slope solar-still (SSSS) desalination system. Four configurations were used in the experiment, two of which featured an SSSS with CuO-nano-coated absorber plates (NCAPs) and an SSSS with CuO-NCAPs with PVA sponges. The efficiency and productivity of the SSSS with CuO-NCAPs were 53% and 2995 mL/m2
/day, respectively, while those of the SSSS with CuO-NCAPs with PVA sponges were 41% and 2318 mL/m2
/day, respectively. Thus, the addition of CuO-NCAPs substantially enhanced the productivity of the solar-still system. In a different study, Arunkumar et al. 
investigated the use of sensible thermal-storage materials in solar-still systems. CuO-NCAPs were assessed within the solar still. The productivity of the SSSS with CuO-NCAPs was 2.9 L/m2
/day, while the cost of the water distilled was USD 0.0077/L. Sharma et al. 
evaluated the effect of copper fins on the freshwater production within a pyramid solar still. The experiment featured two sets of absorber plates—one with copper fins. The study showed that the incorporation of copper tubes which functioned as fins on a black absorber plate resulted in an increase in daily freshwater production, with a maximum value of 1.95 L/m2
. The distilled water output for the absorber plate with copper tubes was approximately 60% higher than that for the smooth absorber plate. In another study, Sharma et al. 
investigated the performance analysis of an active solar still in the colder Indian Himalayan region and used CuO-NCAPs in one of the experiments. The productivity of the system with pebbles was 2.80 L/m2
/day, while a CuO-NCAP-augmented solar still showed a higher rate of productivity of 2.90 L/m2
/day. The minimum productivity levels recorded from the system with the pebbles and from the CuO-NCAP-augmented solar still were 2.60 L/m2
/day and 1.90 L/m2
/day, respectively. Hence, the productivity with NCAPs was much higher, and the minimum productivity was much lower.
Rufuss et al. 
studied the effects of nano-enhanced PCMs in a solar-still system. CuO was among the nanoparticles used to enhance the PCM properties at 0.3 wt% in paraffin to form paraffin–CuO (SSNPCM-2). The findings showed an increase in the thermal conductivity and a decrease in the melting and solidification temperatures with nano-enhanced PCM similar to that of the PCM. The yield of paraffin–CuO was 5.28 L/m2
/day, corresponding to a 35% increase in productivity. An economic analysis also revealed that the cost per litre of water for paraffin–CuO was USD 0.026. With the noted advantages in productivity, the authors recommended SSNPCM-2 as the best solar still compared with those using other nanoparticles, as it provides clean water at almost half the cost of bottled water. Thakur et al. 
conducted a performance analysis of passive solar stills with and without nanoparticles. They included CuO among other nanoparticles in basin water and compared the performance of solar stills. Observations showed that the solar still with CuO nanofluids achieved 41.60% higher productivity than that of the solar still without nanofluids. CuO also showed a higher thermal conductivity than that of ZnO. The solar still with CuO also generated 2025 mL/day of distilled water, compared with the solar still without nanoparticles, which generated 1430 mL/day. Therefore, the productivity of the solar stills with CuO was found to be much higher.
Abdullah et al. 
studied the use of reflectors along with nano-enhanced PCM to improve the performance of solar stills. The authors attempted to assess the role of coating solar-still surfaces with black paint and CuO nanoparticles. The nanoparticles were hypothesised to enhance the heat-transfer characteristics between the water and the basin surface. Paraffin wax mixed with CuO nanoparticles was used as a PCM. The results showed an improvement in the freshwater yield by 108% when using the PCM with CuO nanoparticles compared with that of the reference. While conventional solar stills produced 2400 mL/m2
/day, those coated with CuO produced approximately 5000 mL/m2
/day. The thermal efficiency of the PCM-coated materials was 51.5%. Attia et al. 
experimentally assessed the methods to improve the yield of hemispherical distillers using CuO nanoparticles. The nanoparticles were used at concentrations of 0.1%, 0.2%, and 0.3%. The results showed that CuO nanoparticles at 0.3% concentration improved the freshwater productivity to 7.9 L/m2
/day compared with 3.85 L/m2
/day for the normal distiller, suggesting a 105.2% improvement. Essa et al. 
attempted to improve the thermal performance of stepped solar stills. Among the assessed methods, they applied CuO–paraffin wax on the step liner of the basin and found that the application enhanced the freshwater productivity of the distiller by 127%. The freshwater productivity of the modified stepped solar still was substantially higher at 7000 mL/m2
/day compared with that of 2600 mL/m2
/day for the conventional still.
Behura and Gupta 
assessed the effect of PCMs embedded in nanoparticles on solar-still productivity. Paraffin wax was embedded with CuO nanoparticles at 0.1%, 0.2%, and 0.3%, and the productivity was found to be 440 mL/0.25 m2
/day, 455 mL/0.25 m2
/day, and 510 mL/0.25 m2
/day, respectively. Selimefendigil et al. 
experimentally tested the use of CuO nanoparticles in an SSSS. The results showed that including CuO nanoparticles enhanced the productivity by 26.77% compared with that of conventional solar stills. The energy of the system was enhanced from 15.96% to 19.90%, and the energy efficiency increased from 1.25% to 2.01% when the MSS was used. Sharshir et al. 
analysed the energy and exergy of solar stills with nanoparticles and found that the exergy of evaporation, energy efficiency, and exergy efficiency of the MSSs were substantially higher than those of traditional solar stills. For CuO, the diurnal productivity of MSSs increased by 32.35%. Gupta et al. 
studied the performance of a passive MSS with nanoparticles of 0.12 wt% at water depths of 5 cm and 10 cm. The MSS with CuO nanoparticles produced 3445 mL/m2
/day and 3058 mL/m2
/day at water depths of 5 and 10 cm, respectively, while the conventional solar still in that study produced 2814 mL/m2
/day and 2351 mL/m2
/day for the two water depths, respectively.
Abdullah et al. 
used a corrugated absorber with wick and nano-enhanced PCM to augment the performance of a tray solar still. The results revealed that the total freshwater yield of the corrugated-tray solar still increased by 122% when a PCM with CuO nanoparticles was used compared with that of the conventional solar still. The water-production rate improved by 180%. Abdelgaied et al. 
compared the thermo-economic performance of a hemispherical MSS to that of a traditional hemispherical solar still (THSS). Paraffin wax was used as the PCM, and CuO nanoparticles were used. Based on these findings, the use of CuO–water nanofluid enhanced the productivity by 60.41% compared with that of the THSS. The daily energy efficiency of the system was 56.46% with CuO and 63.61% with paraffin wax and CuO. The use of both paraffin wax and CuO was found to be more effective, and producing fresh water was cheaper by 75% compared with traditional solar stills. Abdelgaied and Kabeel 
used a novel combination of absorber surfaces coated with CuO. This combination facilitated a higher level of performance, with the cumulative yield improving to 9885–10,015 mL/m2
/day, suggesting a 140.1–142% improvement in the performance of the traditional system. The daily thermal and exergy efficiencies also improved by values ranging between 138.1% and 140.1%, and between 243.6% and 252.9%, respectively. Sharshir et al. 
used evacuated tubes coated with nanofluids to evaluate the performance of pyramid solar stills. The augmentation with CuO improved freshwater production by approximately 27.85%. Nazari et al. 
tested an SSSS using 0.08% CuO nanofluid—which improved the productivity, energy efficiency, and exergy efficiency by 81%, 80.6%, and 112.5%, respectively. Elsheikh et al. 
used a copper corrugated absorber plate on a stepped solar still and found that the yield increased by 128% compared with that of the traditional solar stills. Elaziz et al. 
found a 100% increase in yield using Cu2
O as the nanoparticle compared with that of the conventional solar stills. The effects of the CuO nanofluid on the productivity of the solar stills is shown in Table 3
Table 3. Effects of CuO on the productivity of solar stills.
3. Effects of TiO2 on Solar-Still Productivity
Parikh et al. 
evaluated the use of TiO2
nanoparticles to enhance the productivity of a solar-still system. TiO2
was used alongside a black dye as the base paint at 20% and 40 wt%. The results showed higher water-depth productivity of the solar still. Compared with that of the conventional solar still, there was an 11–18% and a 20–23% increase in productivity based on a 20% and a 40% mixture, respectively. Ibrahim et al. 
nanoparticles to test the thermal performance of a wick-type solar still. Two types of solar stills were used: one with a pure PCM and the other with a PCM coated with TiO2
nanoparticles. The inclusion of nanoparticles enhanced the thermal conductivity of the PCM by 9.6%. Parsa et al. 
tested this effect using a 0.1 wt% TiO2
in a DSSS using a combination of thermodynamic and environmental analyses. The findings revealed a thermal efficiency of 20.7% compared with that of the conventional system. Essa et al. 
also tested this effect using a convex tubular solar still reliant on the nanocomposites. The use of jute cloth increased the daily distillate by 114% with nanocomposites. The highest productivity using jute cloth with nanocomposites was reported to be an output of 9000 mL/m2
/day against 4200 mL/m2
/day for tubular solar stills. The enhanced design enhanced the performance of solar stills.
Rufuss et al. 
conducted a numerical study on a solar desalination system using TiO2
particles were added to paraffin and tested for different thermophysical properties—including thermal conductivity. The use of nanoparticles improved the cumulative yield to 6.6 L/m2
/day. Therefore, paraffin enhanced with TiO2
yielded better productivity compared with that of the unenhanced paraffin. Sahota and Tiwari 
investigated the effect of TiO2
particles on the performance of a passive DSSS and observed a higher thermal-energy efficiency of 46.10% compared with that of 37.78% for the base fluid. The thermal exergy was also higher for nanofluids—such as TiO2
(12.38%)—than that for the base fluid (4.92%). Samneang et al. 
assessed the TiO2
concentration in a solar-still system. TiO2
was used in assorted sizes, e.g., 20 nm, 150 nm, and 400 nm. The results showed a highest temperature of 69.69 °C recorded for TiO2
of 400 nm specimens; the temperature was 15.97% higher than that for the bare plate. Zabour et al. 
attempted to improve the performance of an SSSS using various metal-oxide nanofluids—including TiO2
and water. The productivity of the solar-still nanoparticles TiO2
was 7.1 kg/m2
Kabeel et al. 
used a novel absorber plate in a solar still and coated it with TiO2
nano black paint to improve the performance of solar stills. TiO2
nanoparticles enhanced the water temperature by 1.5 °C compared with that of the absorber plate without the nanoparticle coating. The coating also enhanced the yield during sunshine hours. Overall, there was a 12% improvement in the yield at the maximum water depth when the absorber plate coated with TiO2
was used. Gandhi et al. 
developed and assessed the efficiency of a stepped-basin solar still with TiO2
as a nanomaterial. There was a 49.21% increase in efficiency with 20% and 30% coatings of the nanomaterial. The efficiency of the coated system was substantially higher than that of a solar-still system without nanoparticle coating. Shanmugan et al. 
experimented with a single-basin SSSS and found that TiO2
nanoparticles resulted in an average daily efficiency of 57.16% and 36.69% during summer and winter, respectively.
Table 4 provides a summary of the effects of TiO2 on the performance of distinct solar-still systems, as discussed above.
Table 4. Summary of the effects of TiO2 on the performance of several types of solar-still systems.
4. Effects of Other Nanomaterials on Solar-Still Productivity
ZnO is prominent among the nanomaterials proposed to increase solar-still productivity. Kumar et al. 
studied a solar-still system incorporating a nano disbanded PCM. ZnO nanoparticles were used on the crude wax. The results indicated that the addition of ZnO enhanced the output of clean water by 65.17%. Saleh et al. 
used ZnO nanoparticles to enhance the distillation capacity of a solar-still system. The use of these nanoparticles increased the productivity and efficiency of the solar still by 30% and 38%, respectively, compared with that of the system without nanomaterials. Attia et al. 
experimented with a hemispherical solar still using a series of nanomaterials including zinc trays. The modification resulted in a major increase in the rate of evaporation of the saltwater owing to better heat-transfer characteristics. The cumulative yield resulting from the use of zinc trays was 6.3 kg/m2
/day. The use of Zn also caused a 31.25% increase in productivity compared with that of the conventional solar stills. Panchal and Sadasivuni 
conducted an experiment on a modified solar still using ZnO nanoparticles and reported a 52.5% overall efficiency.
is another material that has been extensively considered by researchers. Arani et al. 
nanoparticles on an absorber plate of a tubular solar still and showed that the basin and water temperatures increased by 10.49% and 10.88%, respectively, when using SiO2
nanoparticles at 20% concentration along with black paint. Water production was enhanced by 55.18%. Sathyamurthy et al. 
used fumed-silica nanoparticles to boost the yield of stepped solar stills. The fumed silicon oxide concentration ranged from 10% to 40%. The total yield improved by 27.2%, 34.2%, 18.3%, and 18.4% at 10%, 20%, 30%, and 40% concentrations, respectively. Kumar et al. 
also used silica nanoparticles along with paraffin and noted a 67.07% increase in freshwater production. Thakur et al. 
used a nanosilicon-coated cover to augment the performance of a solar desalination unit. Augmenting the system with a silicon coating increased the water yield by 15.6% compared with that of the conventional solar stills.
Sharshir et al. 
used carbon black nanoparticles to enhance the thermal performances of a solar-still system in an economical and environmental-friendly approach. The accumulated yield in the system improved by 59.33%, while the average energy efficiency and exergy efficiency improved by 75.12% and 142.7%, respectively. In another study, Sharshir et al. 
used carbon black nanoparticles on a stepped DSSS. The addition of nanoparticles increased freshwater productivity by 80.57% and energy efficiency by 110.5% compared with those of the traditional solar stills.
Rasachak et al. 
evaluated the capability of SnO2
in enhancing solar-still productivity. SnO2
concentrations of 15 wt% resulted in the highest surface temperature of 101.61 °C—which was 53.67% higher than that of the conventional solar still. Kabeel et al. 
added graphite nanoparticles to paraffin wax in a solar-still system. The distillate production of the system ranged between 7.123 and 8.52 L/m2
/day across different concentrations. The percentage improvement in the water production ranged between 62.62% and 94.52% across different graphite-nanoparticle mass concentrations. Alqsair et al. 
used a PCM–Ag mixture to experimentally test a solar desalination system. The nanoparticles improved the production of the system by approximately 320%, with an efficiency of 72%. Gupta et al. 
tested the effectiveness of Cu2
O on an MSS and noted an efficiency of 34% compared with that of 22% recorded for the conventional still. Lawrence et al. 
used NiO on a wick-type SSSS and reported an increased yield of 5.8 L/m2
Table 5 provides a summary of the reviewed studies on different nanofluids and their effects on the performance of solar stills.
Table 5. Summary of the effects of other nanomaterials on solar-still productivity.
||Type of Study
||Type of Nanofluids
||Type of Solar Device
|Kumar et al. 
||Output of clean water enhanced by 65.17%.
|Saleh et al. 
||Productivity and efficiency increased by 30% and 38%, respectively.
|Attia et al. 
||Hemispherical solar still
||31.25% increase in productivity.
|Panchal and Sadasivuni 
||Modified solar still
||52.5% increase in overall efficiency.
|Arani et al. 
||Tubular solar still
||Basin and water temperatures increased by 10.49% and 10.88%, respectively.
Water production enhanced by 55.18%.
|Sathyamurthy et al. 
||Stepped solar still
||Total yield improved by 27.2%, 34.2%, 18.3%, and 18.4% for 10%, 20%, 30%, and 40% concentrations, respectively.
|Kumar et al. 
||67.07% increase in freshwater production.
|Thakur et al. 
||Solar desalination unit
||Water yield increased by 15.6%.
|Sharshir et al. 
||Accumulated yield in the system improved by 59.33%.
Average energy efficiency and the exergy efficiency improved by 75.12% and 142.7%, respectively.
|Sharshir et al. 
||Freshwater productivity and energy efficiency increased by 80.57% and 110.5%, respectively.
|Rasachak et al. 
||Surface temperature was 53.67% higher than that of conventional system.
|Kabeel et al. 
||The percentage improvement in water production ranged between 62.62% and 94.52%.
|Alqsair et al. 
||Solar desalination system
||Production and efficiency improved by 320% and 72%, respectively.
|Gupta et al. 
||Modified solar still
||Efficiency was higher at 34% compared with that of 22% for the conventional still.
|Lawrence et al. 
||Single-slope wick-type solar still
||Increase in the yield of 5.8 L/m2/day.