Introduction

Clean water is strong demand in environment that can save the life of human beings on the earth. The clean water is used for different purposes such as drinking, sanitation and industrial application [1, 2]. Nanoscale materials have received an attractive application over different research fields particularly the clean and sustainable energy system such as fuel cells, solar cells and batteries, because of their capability to augment the yield and efficiency of the traditional devices [3,4,5,6,7,8]. A well-known example is using the nano-sized particles spread into the conventional fluid to enhance its thermal conductivity in heat transfer systems [9]. In this case, the liquid-containing suspended nanoparticles are called nanofluid, where the particles are in atomic size less than 100 nm [10,11,12,13].

Regarding that, nanotechnology-based heat transfer has changed our vision toward solar system by using nanofluids especially in solar stills (one type of desalination processes) due to the greater efficiency caused by the suspended nano-sized particles. The reason is they have a greater surface area, which decreases the thermal resistance of the brackish water and hence improves its thermal conductivity that favors water desalination process [12, 14, 15]. Therefore, nanofluids have been intensively used in forced convection of solar thermal systems as an ideal solution to improve the desalination process for potable and clean water [9, 16,17,18] due to their thermophysical properties them [8, 19].

World Health Organization reveals that about 663 million people around the world (particularly those living in rural areas) do not use improved potable water sources, due to unprotected springs and wells [16]. To obviate the needs of human, it is important to apply solar desalination system that has less pollution and make the water drinkable, even in remote areas. In that case, solar stills can provide freshwater with no greenhouse effects [20]. Since many decades ago, desalination technologies have improved enormously enabling almost all people worldwide to access the potable supply of water from brackish water (contains soluble salt) conversion [21].

Today, it is strongly important to combine desalination techniques using renewable energy resources the same as solar still, to prohibit the waste heat and to decelerate the global warming [22]. In general, it has been shown that desalination with solar still is superior over the conventional desalination processes due to the simpler installation and maintenance even in remote regions, cost-efficiency for customers and manufacturer and less air pollution due to fewer burning fuel in this process reducing the global warming effect [23, 24]. However, a comparison between conventional water desalination and the renewable one reveals that previously a quite larger area had to be allocated to such that technology in a power plants for water desalination, besides of having too many moving parts as heavy and high-pressure pump [25,26,27].

In previous years, several researches were done to study the various characteristics and elements that can improve efficiency concerning the geographical situation as well as the weather condition of a typical region [28]. However, not only from the economic point of view, but also from the environmental aspect, nanofluids are among the first options that can boost the efficiency of solar collectors [29]. In current article, a review on variants techniques to improve the rate at which the fluid starts to evaporate and condense in different solar still geometries which using nanofluid has been discussed.

General mechanisms of thermal radiation and solar stills

The stem of a solar panel is its distillation processes that can produce freshwater in the large scale in that the brackish water starts vaporizing as solar radiation influences the black surface [30] of the collector and transferring the heat to the water on its surface increasing the water temperature. Then, the vaporized water will move upward and will occupy the whole container. As they reach the glass cover, they will turn into a cooled water droplet. This cooled air can be mixed with vapor droplets and carry them (convection process) for molecular precipitation through condensation process pouring them into the trough [31,32,33,34,35].

Nanomaterial in solar stills

There are main differences among various types of solar stills concerning the geometries and building materials, however, they all have three particular components; evaporator (convert salty liquid into pure liquid droplets), condenser (convert vapor droplets into liquid) and solar thermal collector (collect electromagnetic solar radiation) [36, 37]. Many researchers have investigated different techniques to improve the productivity such as adding extra condenser, ventilation and changing the geometry of the stills as well as using nanoparticles to obtain high evaporation rate [37]. It has been seen that surface temperature of the saline H2O is greater using metallic nanoparticles in a fluid rather than non-metallic nanoparticle fluid [38] because of the thermal conductivity of metallic nanomaterial which has correlation with the size of the metallic particles [39, 40].

Different factors can affect the size of the particles including the PH, concentration, temperature and viscosity of the nanofluid [41, 42]. In fact, nano-sized particles are more stable in the suspension fluid rather than the larger-scale particles owing to higher ratio of surface area to the volume [42, 43]. Many researchers studied various aspects of thermal characteristics of the nanofluids empirically and via theoretical methods, and they found that metallic oxide nanoparticles are thermodynamically more stable and have strong ability of solar radiation absorption because of their metallic characteristic. Hence, all of these properties make them more practical for nanofluids in solar still by improving their productivity [44, 45]. The geometry of solar stills differ mostly due to the weather condition, however, active and passive solar stills are the main two groups. Passive solar stills are those that do not use any mechanical devices pumps valves and controllers for water circulation or forced convection [36].

Single slope

There are many investigations to augment the yield of the SS-solar still [46]. Among them, Rashidi et al. [47] had a numerical and mathematical model (volume of the fluid—VOF) to verify their experimental setup to evaluate the condensation and evaporation rate. Al2O3 nanomaterial with volume fraction of 5% (Eq. (1)) has been utilized for a SS-Solar still. The comparison between the conventional still without nanofluid and the still modified with nanofluid show that as nanofluid fraction rises from about 0% to around 5%, the efficiency, thermal and viscous entropy generation, all increase by 25%, 25% and 95%, respectively. They used Ansys-Fluent to compare the simulation both modified and non-modified conditions.

$$\rho_{\mathrm{eff}} = \left( {1 - \varphi } \right)\rho_{\mathrm{f}} + \varphi \rho_{\mathrm{f}}$$
(1)

Figure 1 shows that for different nanofluid concentration (\(\varphi = 0 , \varphi = 5\%\)) how the viscous and thermal entropy have changed. In addition, owing to the high-temperature gradient, the maximum viscous and thermal entropy were on those surface domains.

Fig. 1
figure 1

(Left) vapor fraction for both systems at \(\varphi = 5\%\) as time changes, (right) effect of nanofluid fraction on viscous and thermal entropy generation [47]

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Other researchers, Gnanadason et al. [48] modified the system into a vacuum SS-Solar still investigated the impact of nanofluid on evaporation and condensation rate. However, to escalate the evaporation rate, they reduced the vapor pressure through the solar still chamber using a vacuum pump (it uses the atmospheric gravity and pressure and sometime by photovoltaic system) [27, 49, 50]. Alternatively, at low temperature the latent heat of condensation was used as a heat source for evaporation. Carbon nanotube (CNT: prepared by chemical vapor deposition—CVD) was synthesized in the laboratory and combined with brine water as nanofluid and then sodium dodecyl sulfate (SDS) surfactant (which has the ability to augment of water temperature) was used to make them stable in water. Figure 2 illustrates the improved productivity of the entire unit. In addition, to increase the evaporation rate (even at low temperatures) they reduced the pressure of the container that can release the latent heat during condensation process as well. With both vacuum pump and nanofluid, the efficiency increased by more than 40%.

Fig. 2
figure 2

The modified solar still with vacuum pump [48]

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Other metallic nanomaterials were used to develop the efficiency of the solar still. Sain and Kumawat [51] used Al2O3 nanoparticles (50–100 nm) combined with black paint of basin to have a rise in overall efficiency of the system. They gained 12.18% higher thermal efficiency and 38.09% improvement in pure water production rate with using nanofluid. Table 1 demonstrates that the quality of the H2O improved as the parts per million (PPM) of the raw H2O increased due to using the nanofluid and black basin.

Table 1 The improved efficiency of the Al2O3 nanoparticles in single slope solar still at different water depth [51]
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Kabeel et al. [52] reported productivity enhancement for the smaller size particles nanofluid (average size of 10–14 nm). As exhibited in Fig. 3, they have used numerical analysis to verify their experimental results to investigate the effect of a 0.025 and 0.3% Al2O3 and Cu2O on a solar still with external condenser. In their experiments, the nanoparticles Al2O3 and Cu2O had thermal conductivity of 46 and 76.5 W m−1 K−1, respectively. They reported that evaporation and condensation ratio and the heat transfer coefficient of cuprous oxide in the new shape of solar still are higher than the non-modified one. They also verified that the daily performance of a nanofluid with Al2O3 is higher than Cu2O nanoparticles in the water in both of the conventional and modified [52,53,54].

Fig. 3
figure 3

(Left) schematic of the solar still using nanofluid and vacuum, (right) the productivity result of the system with fan and without fan [55]

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Hence in order to improve the system using Cu2O as the nanofluid, Kabeel et al. [55] applied a vacuum fan to the conventional solar still. They took the best concentration of the nanofluid by using X-ray diffraction technique. Results showed that the metallic nanoparticle of Cu2O with the presence of fan augments the productivity of the still unit thanks to the higher thermal conductivity of the cuprous oxide than aluminum oxide.

To augment the effectiveness of the nanofluid, Thakur et al. [56] used the black paint basin to boost the absorptivity ratio of the solar still. In addition, to augment the evaporation rate the nanoparticles of Al2O3 were added to the brine water. They revealed that the efficiency per 12 h (starting at 7:00 p.m.) increased to about 47.575 and 44.14% with the presence of the nanopaint and nanofluid, respectively.

Another approach to enhance the yield and productivity is using the PCM that can augment the thermal conductivity of the solar still because of its capability to store the latent heat of the material in large quantities [50, 57]. In this way, Rajasekhar and Eswaramoorthy [58] used the Al2O3 dissolved in paraffin wax (phase change material) in which it has the ability to store the latent heat of the fluid (Fig. 4). Furthermore, they compared the thermal productivity of the solar still in different conditions; using nanoparticles spread in paraffin, merely paraffin and with no paraffin. In their studies, the PCM material has the daily efficiency of about 45%, which was higher compared to 40% and 38% of solely paraffin wax and no paraffin, respectively. In Eq. (2), m is the mass of PMC (kg) in saline water per hour, L is the latent heat, S is the glass surface area (m), and I is the irradiation of solar energy (W m−2).

Fig. 4
figure 4

(Left) schematic of the experimental setup of the PCM material in basin water, (right) daily efficiency of the three different experimental setup [58]

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$$\eta_{\mathrm{daily}} = \frac{\sum mL}{S\sum I}$$
(2)

Double slope/cascade

Double slope solar still can operate at higher temperature rather than the SS-Solar stills [59, 60]. Different nanofluids are used to have an augmentation in the performance of the solar desalination unit. Sahota et al. [45, 61] revealed that the thermal energy (Eq. (3)) of 50.34% of alumina nanomaterial in the base fluid, and 46.10% of TiO2 and 43.81% of copper oxide nanomaterial in a passive double slope solar still was greater than the conventional solar still (Fig. 5). The thermal conductivity of those concentrations are 38.5, 11.8 and 17.6 W m−1 K−1 for Al2O3, TiO2 and CuO, respectively. Using a thermal modeling, they found that 35 kg Al2O3 nanofluid could make a higher temperature difference (\(\Delta T\)) between the; hence more evaporation happens (Fig. 6).

Fig. 5
figure 5

The schematic of a double slope solar still with nanoparticles [45]

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Fig. 6
figure 6

Plots for maximum temperature variation \((\Delta T)_{\hbox{max} }\) for three different nanofluids (Al2O3, TiO2 and CuO nanoparticles dissolved in water) with three different mass concentrations: a 0.2%, b 0.25% and c 0.3% [45]

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$$\begin{aligned} &\eta_{\mathrm{hourly,ex}} = \left( {\frac{1}{{0.933 \times A_{\mathrm{s}} \times I_{\mathrm{s}} \left( t \right)}}} \right)\\ & \quad \quad \left\{ {h_{\mathrm{ebf,E}} \left[ {\left( {T_{\mathrm{bf}} - T_{\mathrm{giE}} } \right) - \left( {T_{\mathrm{a}} - 273} \right)\ln \left( {\frac{{T_{\mathrm{bf}} + 273}}{{T_{\mathrm{biE}} + 273}}} \right)} \right]} \right. \\ & \quad \left. { +\, h_{\mathrm{ebf,W}} \left[ {\left( {T_{\mathrm{bf}} - T_{\mathrm{giW}} } \right) - \left( {T_{\mathrm{a}} + 273} \right)\ln \left( {\frac{{T_{\mathrm{bf}} + 273}}{{T_{E} + 273}}} \right)} \right]} \right\}A_{\mathrm{b}} \\ \end{aligned}$$
(3)

From experimental result on passive double slope solar still, Sahota and Tiwari [62] reported the improved efficiency using Al2O3 nanoparticles with 0.04%, 0.08% and 0.12% concentrations suspended into 35 and 80 kg water. The reason for choosing Al2O3 among ZnO, Fe2O and SnO, was its better productivity for passive solar stills. As a result, the daily yield improved by increasing the concentration of Al2O3 (Fig. 7) and the solar still yield enhanced for 8.4% and 12.2% for 80 and 35 kg base fluid correspondingly (Fig. 8). The point is the nanoparticle in the base fluid can directly absorb those radiation spectrums that are matched with their own optical spectrum.

Fig. 7
figure 7

The convective heat transfer coefficient for water and nanofluid for three different concentrations in 35 and 80 kg brine water [62]

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Fig. 8
figure 8

The \((\Delta T)_{\hbox{max} }\) between the nanofluid and water as based fluid [62]

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In another experimental and numerical study [63,64,65] they analyzed the finalized cost for the two types of the nanofluids (CuO and Al2O3) for the active hybrid double slope solar with and without heat exchanger using helical coil as depicted in Fig. 9. It appears that using coil increased water temperature and that resulted in a greater temperature difference. In both cases, the EPBT (Eq. (4)), the EPF (Eq. (5)) and lCCE (Eq. (6)). They revealed that these three energy parameters are lead to better annual performance and environmental cost by using nanofluid, and helical coil heat exchanger and CuO nanofluid. The maximum LCCE over 50 years of life span reached to the 14.8% and 10.8% correspondingly for both systems using CuO nanofluid while it was 2.55 for the Al2O3 nanofluid in conventional system.

Fig. 9
figure 9

The schematic of the Hybrid double slope solar still, without coil heat exchanger [64]

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$${\mathrm{EPBT}}_{\mathrm{en/ex}} = \frac{{E_{\mathrm{in}} }}{{E_{\mathrm{out,ann}} }}\,{\mathrm{years}}$$
(4)
$${\mathrm{EPF}}_{\mathrm{en/ex}} = \left[ {\frac{{ E_{\mathrm{in}} }}{{E_{\mathrm{out,ann}} }}} \right]^{ - 1}$$
(5)
$${\mathrm{LCCE}}_{\mathrm{en/ex}} = \frac{{\left( {E_{\mathrm{en /ex ,ann}} \times n} \right) - E_{\mathrm{in}} }}{{ E_{\mathrm{sol,ann}} \times n}}$$
(6)

In this regard, Rashidi et al. [66] demonstrated the impact of nanofluid on cascade solar still (Fig. 10) by decreasing the space between the glass and the surface of the collector and the time for transferring water vapor into condenser surface [67]. The CFD results show that there will be a 22% improvement in hourly productivity by augmenting the concentration of Al2O3 from 0 to 5%. The response surface methodology (RSM) proves that the effect of step’s height is more than step’s length on productivity (Table 2).

Fig. 10
figure 10

The schematic of the cascade solar still with Tg = 40 °C, Heightleft = 0.06 m, Heightright = 0.57 m, L = 0.57 m [66]

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Table 2 The comparison between the RSM and CFD results for different Al2O3 concentration [82]
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Tubular

Another type of passive solar still is tubular solar still which mostly used in humid weather condition [68, 69]. The effect of the solvent on nanofluid properties was found by Saleh et al. [70] using a different synthesis method such as FESEM, TEM, UV–vis spectroscopy and XRD, for ZnO nanomaterials. In their research, were added to the black paint basin with wide concentration range from 0 to 600 mg of each 0.001 L of the black paint. They revealed that the rod-shaped ZnO nanoparticles improved the efficiency of the solar still to 30% and 38%, respectively, compared with nanospherical-shaped particles (Figs. 11 and 12).

Fig. 11
figure 11

(Top) schematic of the tubular solar still (a) with thermocouples (b), (bottom) effect of concentration on the efficiency of the half-tubular solar still [69]

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Fig. 12
figure 12

The comparison between effect of the two morphology and concentration of the nanoparticles on the productivity and water temperature [69]

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The reason is the rod-shaped ones have more surface area and less rough edge and face, which made them to be homogeneously distributed in fluid.

However, Arunkumar et al. [71, 72] found another a way to have productivity growth of the nanofluid tubular solar still (TSS). They combined a parabolic concentrator with a tubular solar still with specific thermodynamic properties as (\(\frac{1}{{\sin \left( {\theta a} \right)}}\)), where \(\theta a\) is a half-angle of the solar incident on the receiver. Hence, this characteristic can raise the temperature besides using paraffin wax and PCM to increase the evaporation rate as they discharge the stored latent heat of the brine water [57]. Combination of PCM and the brine water can increase the evaporation rate at night (Fig. 13). They showed that heat transfer coefficient improved to about 700% and 176% for the TSS and the concentric-TSS, both with the compound parabolic concentrator, respectively, compared to the non-modified TSS which was about 50 W m−2 K−1.

Fig. 13
figure 13

The schematic of the modified system which is the combination of the concentric tubular solar still and the parabolic concentrator [81]

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Other types of solar still

There exist some solar still designs that cannot be categorized in any of the aforementioned groups. For example, Omra et al. [73, 74] conducted three case experiments for a simple corrugated wick solar still equipped with a vacuum fan and then wall mirrors [72]. They reported that the hourly distillate water was higher compared to the conventional solar stills since the enhancement in the condensation and evaporation rate was due to different factors including the nanofluids (Cu2O and Al2O3 with water), vacuum fan, V-shape wick and mirrors (Fig. 14). In which the productivity of system with vacuum and mirror was higher than with vacuum only as illustrated in Fig. 15.

Fig. 14
figure 14

The schematic of the experimental setup of the corrugated wick solar still with vacuum and mirrors [71]

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Fig. 15
figure 15

The daily distillate rate for the grooved wick solar still with vacuum and mirrors and Cu2O nanofluid [71]

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Another group also proved that using CuO in active solar stills appears to be more effective than the Al2O3 nanofluid in conventional stills. Sharshir et al. [75, 76] demonstrated that the combination of single slope solar stills can lead to a new modified design that enhances the effect of CuO (Fig. 16). They operated an experiment with various water depth and different cooling rate using different concentrations of CuO micro-flakes and graphite with thermal conductivity of 76 and 129 W m−1 K−1 and the average particle size of less than 2 μm. They found the ideal nanofluid concentration for 1% and the optimum 0.5 and 1 cm water depth for CuO micro-flakes and graphite, respectively. The maximum productivity was 53.95% with glass cooling and 43.10% without glass cooling for 1% CuO nanofluid with 0.5 depths of the brackish H2O. The daily efficiency of unit with cooling flow over the glass increased by 46% and 49% for CuO and graphite correspondingly (Fig. 17).

Fig. 16
figure 16

The experimental setup for solar still with glass shield cooling [73]

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Fig. 17
figure 17

Increase in productivity for solar still with glass cover cooling with different nanofluid concentrations and different base fluid depth for daytime and nighttime, and effect of flow rate on productivity [73]

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However, Fig. 17 shows that in all cases graphite has higher productivity during both daytime and nighttime and that endorses the superiority of graphite over CuO for the combined design. Because the graphite particles have lower density than copper (ll) oxide particles and this makes them to have homogenous distribution in the base fluid [74].

As it is mentioned, CuO nanofluid has often used in the forced convection systems. Gupta et al. [77] augmented the thermal conductivity of the passive solar still by using the CuO nanoparticles to escalate the heat transfer surface area which enhances the productivity of the modified system to 22.4% higher than the conventional one and that endorses a better yield. Their results were confirmed by Kabeel et al. [78] who used the same nanoparticles (CuO) mixed with black paint for the inside surface of the basin to increase the thermal conductivity. In their system, they used a tank to have the same initial conditions for both conventional and modified still (Fig. 18). It is proved that there is an increase in their modified system using nanofluid and the highest productivity was achieved at 40% concentration of the nanoparticles (Fig. 19).

Fig. 18
figure 18

Experimental setup for the modified solar still with the same initial condition using a same brine water tank [76]

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Fig. 19
figure 19

Improved productivity of the black-coated basin with CuO nanofluid [76]

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Among all metallic nanoparticles, silicon carbide (SiC) is a semiconductor material that can boost the conduction mode of the brine water. Chen et al. [79] synthesized the SiC nanofluid uniformly in laboratory and then showed that it has a great effect on thermal conductivity and radiation absorption of the seawater. They found that as the saltiness intensity, the stability level decreased. Besides the optical features of the nanomaterial, different salt concentration in seawater was examined to find the stable condition and the best thermal conductivity. The novelty of their experiment was using the UV–vis spectrometer and optical properties of the nanofluid to study the effect of salinity on nanofluid. They proved that metallic oxides have superior thermal conductivity and optical properties, and low luminousness improves the absorptivity for desalination. Besides the important achievements of the researchers, Table 3 demonstrates a comprehensive summary of the different types of nanofluid that researchers used for various locations and geometries of solar stills.

Table 3 Summary of the type of nanoparticles, geometry of the solar still and the location of the experiment
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Cost evaluation

Usually using renewable energy as solar desalination devices appears to be costly but in the long period. Yadav et al. [89] showed that the payback of solar stills without using the nanofluid depends on manufacturing cost (fixed cost), and operating cost (variable cost) when the lifetime of the solar stills vary between 5 and 10 years for various kinds of solar stills (Table 4). Besides the other components of the solar still such as pumps, pipes, tank and control devices, using nanofluid will adds up to the capital cost [70, 79]. Elango et al. [91] provided a table for different nanoparticles that can be combined with saline water. It appears that Al2O3 synthesizes has reasonable cost among the other types of nanopowders (Table 5). Several nanomaterials were utilized in recent years to augment thermal features [92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111].

Table 4 Cost per liter per day for different types of solar still in USA [78]
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Table 5 Thermal conductivity and cost of different quantities of nanoparticles for nanofluids [80]
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Conclusions

From the literature, the density of the nanoparticles plays a significant role in the quality of their distribution. That means the lower density leads to higher concentrations of the nanopowders within pure fluid and this boosts the thermal conductivity of the nanofluids, particularly in laminar convection. The volume fraction, size and shape of the suspended nanoparticles are the other important factors that have a correlation with thermal conductivity in either flow patterns. In all cases, the volume fraction of the nanoparticles did not go beyond a limit to avoid making it into a non-Newtonian fluid. For the modified systems that used a fan to have a forced convection, an increase in convection coefficient leads to an augment temperature gradient. In other words, higher condensation rate was achieved due to improving the thermal conductivity of the brine H2O. Furthermore, Al2O3 seemed to be the best choice in terms of the solar still with natural convection.

Suggestions for the future research

It appears that besides using nanofluids some of the researchers applied mechanical devices such as pressure and vacuum pump and fan, and using an extra condenser. However, this cycle needs some extra power and energy. It is suggested to install a photovoltaic (PV) panel on the ceiling of a building and the extra heat at the rear side of the PV panel can be used to increase the evaporation rate. Although this may not be a cost-effective approach, in long term it will have payback for users.