Energy, exergy, economic, exergoeconomic, exergoenvironment, and enviroeconomic (6E) analysis of solar stills—A critical review

Drinking water is a crucial need for human survival and solar stills (SSs) have emerged as a cost‐effective solution to meet this need from an economic point of view. In this work, the working of various SSs, their productivity factors, and various devices that can be integrated to improve their efficiency have been studied. The conversion of saline water into freshwater water using SSs has the potential to address the issue of drinking water. This study highlights the importance of SSs as a viable and sustainable solution for drinking water access. On the basis of various parameters correlating to the performance of the SSs, energy, exergy, economics, exergoeconomics, exergoenvironment, and enviroeconomics analysis has been analyzed in this study. To improve the performance and sustainability of SSs, various combinations were studied. These combinations include SSs with different nanoparticle coatings, phase‐change materials, photovoltaic modules, external condensers, wick materials, and reflectors. It is concluded that a comprehensive framework that integrates thermodynamic, economic, and environmental criteria can guide the development of more efficient and sustainable SS technologies.


| INTRODUCTION
Drinking water and continuous energy supply are two major factors that are affecting every nation's economy.Water is the most essential for all aspects of socioeconomic development and to maintain a healthy ecosystem. 1The majority of water, 97%, is in the ocean which is saline water, and the rest is in the lakes, rivers, streams, groundwater, and in the form of glaciers. 2At present, water impurity is increasing for several reasons, making it impossible for us to use it as drinking water.Due to the rapid growth in population, there is a parallel demand for luxurious comfort, air, and water.To fulfill the daily needs of human beings, massive fuel is used in various industries causing global emissions, such as water and air pollution.It is necessary to consider that human basic energy demand for daily uses is also one of the main factors for both global warming and CO 2 emissions. 3Two primary types of SS are basically used, that is, active and passive.Active solar still (SS) needed a second source, like, additional solar collectors, condensers, and fans.In passive SS, multiple basin designs and basin materials are employed to boost output.In recent years, various scholars have become interested in solar desalination to use clean and free solar energy to change saline water into drinking water.Yousef et al. 4 utilized three different systems to conduct an experiment on SSs: system 1, traditional SS; system 2, SS with cylindrical hollow fin; and system 3, SS with steel wool fiber.Research demonstrates that systems 2 and 3 are 16% and 25%, respectively, more productive than system 1.The highest possible energy ratio for systems 1, 2, and 3 are 42%, 45.5%, and 52.5%, respectively.Moreover, it was determined that systems 2 and 3 effectively reduce CO 2 by, respectively, 14.4 and 15.6 tons/ annum.In another study, Elango and Kalidasa Murugavel 5 conducted experiment using single and double basin solar still with different depths of water 1-1.5 cm under the condition, with and without insulation.After the experiment discovered that compared with a single basin, double basin solar still with and without insulation produces water 17.38% and 8.12% higher, respectively, at a depth of 1 cm.Another study was conducted on a double basin utilizing a small pile of material in the bottom of the basin.Researchers took two different pile materials: jute and black cotton.In result get that, Jute pile had a yield that was 18.02% and 21.46% greater than the black cotton pile at depths of 0.01 and 0.02 m. 6 Jani and Modi 7 experimented on a single basin double slope solar still utilizing hollow fin cross-section square and circular.Productivity obtained at different water depths.The experiment revealed, 10 mm water depth is suitable for both cross-sections to enhance the yield.
Dumka et al. 8 performed an experiment on SS augmented with 100 cotton bags filled with sand.Bags are placed in the basin to increase sensible energy storage capacity.After the experiment results revealed that the overall efficiency of modified SS improved by 28.96% and 31.31% at 30 and 40 kg of water in assessment with traditional SS.Patel et al. 9 conducted an experiment on ultra-modified double slope solar still (DSSS) to improve portable water production.Considered basin's water depth is 1 cm, and the glass tilt angle is 15°.Discovered that the water production was 9157 mL/day in summer and 6630 mL/day in winter, at a 60°reflector angle.In comparison to modified solar still (MSS), ultra-MSS had thermal efficiencies in the summer and winter of 10.4% and 10%, respectively.The calculated payback period of this setup is 111 days.Fathy et al. 10 performed an experiment with a combination of DSSS and parabolic through collector (PTC).Experimenting with three different systems, they found that SSs with PTC had greater temperatures and productivity than conventional systems, and the yield of tracked PTC is greater than fixed PTC systems.In addition, he also discovered that in winter water production is less than summer production.
There are additional ways to boost the productivity of SS done by researchers. 11Hassan et al. 12 experimented on SS combined with PTC.Following the investigation, it was discovered that employing wire mesh and sand enhanced daily freshwater production by around 13.7% and 3.1% in winter and 14.1% and 3.4%, respectively, in summer.Additionally, the utilization of sand in SS boosts efficiency by 15.3% in the summer and 13.8% in the winter.Another researcher utilized a metal matrix structure immersed in a basin as energy storage, and observed that the efficiency and productivity of modified SS increases compared with conventional Still.SS with a metal matrix structure is one of the most economical ways to generate freshwater. 13Mohamed et al. 14 utilized porous material as an absorber in the basin and fine basalt stone was used as an absorber with different particle sizes.Experiment results revealed the average Nu and Sherwood number increased in the cavity by 115% and 51.95%.Moreover, exergy efficiency calculated at different fine stone particle sizes 1, 1.5, and 2 cm was enhanced by 65%, 104.4%, and 123% as compared with SS without stone.
Recently, the utilization of various nanoparticles in the desalination system has become potentially useful due to high thermal conductivity which leads to enhancing the performance of the SS as shown in Figure 1. 15 Sahota and Tiwari 16 conducted an investigation to enhance the yield of SS using Al O 2 3   nanoparticles with liquid.After investigation, they discovered that water productivity increased by 8.4% and 12.2% for 80 and 35 kg of base fluid, respectively, when nanoparticle concentration was 0.12%.The same research group analyzed various nanofluids for DSSS and observed that nanofluids TiO 2 , Al O 2 3 , and CuO had greater thermal efficiencies than basefluids.In addition, thermal exergy was likewise high.The most effective nanofluid for generating water among these three is Al O 2 3 . 17Hedayati-Mehdiabadi et al. 18 conducted an experiment on DSSS with phase-change materials (PCMs) and photovoltaic (PV) collectors.PCMs enhance the production of pure water at night and PV collector is used to warm saline water.After experiment result showed that water yield enhanced by 10% and exergy efficiency enhanced by 27% on July 6.Sharshir et al. 19 conducted an experiment using an evacuated tube and nanofluid on pyramid SS.The experiment showed that the use of CuO and carbon black nanofluid productivity increases 27.85% and 33.59% as compared with conventional pyramid SS.In the case of compared with conventional solar still (CSS), it enhanced by 54.48% and 57.098%, respectively.Moreover, they get maximum thermal efficiency by modified pyramid SS that was 64.5%.Zanganeh et al. 20 use nanoparticles for coating the internal surface of the cover glass to increase the yield of SS.The experiment result revealed that condensate production increased by 23% after glass nanocoating and at an angle of 50°as compared to without coating SS.Moreover, utilizing Cu 2 O nanofluid and thermoelectric cooling (TEC) module in single slope solar still discovered that using TEC increased energy, exergy, and productivity by 38.9%, 31.2%, and 38.5%, respectively.In the scenario where 0.08% volume of Cu 2 O nanoparticle was added to the basin, the corresponding increases in energy, exergy, and productivity were 81.5%, 92.6%, and 82.4%. 21

| Performance enhancement of SS using various reflectors
The SS performance is typically measured as the quantity of water produced unit area of basin per day.Various types of reflectors utilized a slope basin solar still enhance productivity as shown in Figure 2. Kuhe and Edeoja 28 did a comparative study and found that using a parabolic reflector with paraffin wax PCM resulted, a maximum production of 5.243 kg/m 2 .This arrangement is considered the best for improving productivity in SSs.

| Increased output of SS by various studies using PV
Praveen Kumar et al. 34 constructed and experimented with two types of SS-CSS with a single slope and a hybrid photovoltaic/thermal (PV/T) SS. 35 Hybrid SS is equipped with NiCr heater powered by PV to produce more distillate water.The experiment conducted on April 8 showed that the conventional still produced 3.420 kg of distilled water, whereas the suggested hybrid PV/T SS generated 8.542 kg of distilled water as shown in Figure 3.

| SS performance with the use of an external condenser
Efficient condensation is crucial for optimal distillate production in SSs.The productivity of SS using a condenser is most affected by the absorptance of the evaporator. 44SS yield increases and heat loss is reduced with the attachment of an external condenser.Investigation into the effects of drawn vapor at various speeds revealed that integrating with an external condenser boosted yield by 53.2%.Additionally, the use of nanofluids further enhanced productivity by 116% when an external condenser was employed as shown in Figure 4.7][38][39][40][41][42][43] DSSS, double slope solar still; ETC, evacuated tube collector; FPC, flat plate collector; PV, photovoltaic; SS, solar still.

| Effect of wick material on SS performance
A wick is a permeable material, such as cotton or jute, that allows liquid to be transported through capillary action.As the liquid travels along the surface of the wick, it becomes heated and rapidly evaporates into vapors.Younes et al. 49 investigated SSs that use wick materials on their vertical sides, including half-barrel and corrugated absorbers.Wick material was found to minimize loss of heat and boost the evaporation rate, which resulted in higher productivity in comparison to CSS.Corrugated wick SS showed the greatest increase in yield at 43%, while the half-barrel wick solar still had a 52% increase.Moreover, adding vertical wick material to corrugated wick and half barrel wick SS further increased productivity by 112% and 123%, respectively, over the CSS (shown in Figure 5).

| Performance of SS utilizing PCM
As shown in Figure 6, Samuel Hansen et al. 54 conducted an experiment on a combination of inclined and basin SS with fins and paraffin wax use as PCM.Results revealed that inclined still with fin absorber and PCM coupled with basin still found to be 87.96% more productive.Dhivagar et al. 55 experimented on SS using slack wax PCM.Results show that still is 85.1% more productive than CSS. 56

| ENERGY AND EXERGY ANALYSIS
Rapid usage in agriculture, industry, and population growth have increased the demand for potable water, making it challenging to deliver affordable drinkable water.0][51][52][53] DSSS, double slope solar still; SS, solar still.
approach for harnessing solar energy to clean salt and brackish water is solar distillation. 60Sharshir et al. 61 presented a theoretical comparison between modified SS and CSS in terms of output and thermal characteristics.Cost estimates for desalination utilizing micro/ nanoparticles were generated for MSS that comprise saline water combined with micro/nanoparticles of graphite and copper oxide.The study calculated hourly heat transfer coefficients for convection, evaporation, and radiation with and without micro/ nanoparticles, and also examined energy loss in SS components.According to the findings, discovered that MSSs exhibited superior energy and exergy efficiencies compared with CSS.During the day, energy efficiency for graphite and CuO MSSs was found to be 41.18% and 38.61%, respectively, while that of CSS was 29.17%.
The significance of having access to potable water is discussed, as well as how direct sources of water might not always be suitable for drinking.The proposed solution is the employment of SS, which converts saline or wastewater to drinking water using solar energy.This study assesses the thermal efficiency and energy assessment of a DSSS operating in forced circulation mode.Daily SS thermal efficiency ranges 13.55%-31.07%,and exergy efficiency ranges 0.26%-1.34%. 62Asbik et al. 63 studied energy analysis of passive SS with a heat-storage system that stores and retrieves energy utilizing paraffin wax as PCM.In June 2011, the study was carried out in Errachidia, Morocco.For each component of the desalination unit and the PCM, the exergy balance equation is developed and solved in this study.The findings reveal differences in PCM temperature, and exergy destruction in both the SS and PCM.Research also discusses the impacts of exergy destruction on PCM thickness, air velocity, and basin water depth.The study concluded that latent heat storage increased water output while lowering energy efficiency.Ranjan et al. 64 presented a thorough thermodynamic model for the exergy study of passive SS in the climate of India.Estimated temperature of basin, water body, outer and inner glass cover using a computer program with energy and exergy efficiency 30.42% and 4.93%, respectively, findings suggest that SS can generate 4.17 L/m 2 of freshwater per day.The component with the highest possibility of improvement, according to the study, is the basin liner.The total exergy efficiency of SS, accounting for exergy destructions is 23.14%.
Energy efficiency can be expressed as where m w is the mass of water condensed, ℎ fg is the latent heat of vaporization, A is the solar radiation falling area, and I(t) is the solar radiation falling on the glass surface.

| ECONOMIC AND EXERGOECONOMIC ANALYSIS
The use of nanofluids in passive DSSS was examined in this study, with a focus on their impact on annual productivity, energy, and exergy.Al 2 O 3 , CuO, and TiO 2 nanofluids were tested, and all three showed significant enhancements in productivity, energy, and exergy.For each of the distinct nanofluids, energy payback period, energy output factor, lifecycle conversion efficiency, environmental cost, and exergoeconomic characteristics were evaluated, with a maximum lifespan of 50 years.
Results suggest that nanofluids have the capability to enhance the performance and economic viability of SSs for potable water production. 65Tripathi et al. 66 68 improved the effectiveness of SS utilizing cotton hung pad and nanoparticles, Fe 3 O 4 and CuO.Utilizing combinations improved evaporation rate and increased distillate productivity by 42.3%-56.6%compared with CSS.Maximum efficiency was achieved with MSS using CuO nanoparticles.When compared with CSS, exergy efficiency for MSS with CuO showed an improvement of 103%.Economic investigation revealed that MSS is less expensive in terms of per liter freshwater than CSS.Additionally, exergoeconomic and environmental studies also showed that MSS mitigates CO 2 emissions.Tiwari et al. 69 experimented on SS integrated with PVT and flat plate collector in the Delhi site.Studies have been contrasted to those of previous investigations, hourly, thermal, electrical, exergy, and thermal efficiency have all been evaluated.Additionally, demonstrated that modified active SS can satisfy both daily requirements of freshwater and DC electrical power throughout the daytime.Pal et al. 70 focused on a developed model for a modified multiwick basin-type DSSS and validated it experimentally.The model includes analytical expressions for temperatures, yield, and instantaneous efficiency.Results indicate that SS using black cotton wicks at a water depth of 1 cm has higher exergy and energy production factor and a lower CO 2 emission, and the exergoeconomic parameter is found 0.0791 kWh/Rs at an interest rate of 4% and system lifespan 50 years.Bait 71 proposed SS connected to a tubular solar collector, and compared performance with CSS of similar dimensions.Modified SS yields more distillate and has higher hourly and global exergy efficiency.However, the cost of freshwater is higher and payback period longer for improved still.The environmental cost is also higher for the modified system.Overall, enhanced SS is economical and feasible due to high yield.
where R En and R Ex were the exergoeconomic parameters based on energy and exergy and uniform annual cost.To calculate the annual production of exergy E ( ) ex. out and energy E ( ) en. out of SS have been found via the resulting equation: where the saline water is T w then ambient temperature is T a .

| EXERGOENVIRONMENT AND ENVIROECONOMIC ANALYSIS
Elbar et al. 72 studied an integrated PV module on the back of SS to heat saline water in the basin.Four SS systems were examined, including CSS and SS with PV modules and additional materials.Results showed that using PV as a reflector provided the most significant improvements in modified still energy and exergy efficiency.When compared with CSS, the use of PV in SS has been shown to be ineffective in an aspect of energy payback time.However, exergoeconomic and exergoenvironmental assessments of SS with PV were optimistic.Experiments were conducted in the summer season in Alexandria, Egypt.An experimental work is performed by Eltawil and Omara 73 using SS connected to a PV system, solar water heater, and solar air collector.In the experiment, heated air was pumped from the bottom to the top while hot water was sprayed on the surface, resulting in the water to quickly evaporate.According to the results of the investigation, this SS is 51%-148% more productive than a traditional SS and only circulated hot water enhanced the productivity by 56%.Narayanan et al. 74 reviewed SS.The essential goal of this review is to guide researchers on how to select the best approach for improving the SS.They concluded that several methods are effective: reducing the water depth increases the evaporation rate, cover glass angle also affects productivity, increasing insulation thickness to reduce heat losses, and in the basin absorber material should be utilized to improve SS productivity.Castillo-Téllez et al. 75 experimented on SS to analyze the air velocity effect on thermal efficiency.Discovered production of water and thermal efficiency increases up to air velocity 5.5 m/s and subsequently decreases at higher air velocity.The ideal air velocity was determined to be 3.5 m/s in the previous study.Sonker et al. 76 proposed for remote locations with inadequate technological infrastructure, using SSs to create freshwater might be an ideal solution for delivering freshwater during times of emergency.SS production was boosted by installing copper cylinders loaded with PCMs above the basin liner.Increasing water depth results in lower temperatures, although paraffin wax has heat-storage capacity higher than stearic or lauric acid, making it less sensitive to this change.In all three PCMs, the total distillate reduced linearly as water depth increased.Paraffin wax produced the most distillate from stearic and lauric acids.Paraffin wax has greater latent heat capacity than other PCMs.
With the development and testing of numerous SSs, multiple researchers are attempting to improve performance of SS.Even if it is challenging due to the little amount of daylight, it is not impossible to generate continuously from a standstill. 77Kabeel et al. 78 1.Overall, this table provides a brief overview of earlier studies and identifies the key elements that must be considered to improve the efficiency of SSs.

| CONCLUSION
In this work, 6E has been critically reviewed in the context of SS.It is evident from the review that SS have a significant potential for providing drinking water in areas where access to safe drinkable water is limited.The factors listed below are the main factors to operate SSs more efficiently.
(1) Performance of SS can be improved by utilizing appropriate design configurations, materials, and operating conditions.(2) Energy and exergy analyses provide valuable insights into the thermodynamic efficiency of SSs and the potential for energy recovery.(3) Incorporating nanoparticles in SSs holds promise, potentially achieving a remarkable energy efficiency of 75.12%.(4) The utilization of PCMs demonstrates significant potential, offering a productivity boost of up to 87.96% in SSs.(5) Integration of PV systems into SS setups has the capacity to yield substantial daily outputs, reaching a maximum of 12 kg.(6) Economic analysis helps in determining the feasibility and cost effectiveness of SSs, while exergoeconomic analysis provides a comprehensive framework for integrating thermodynamic and economic performance criteria.(7) Exergoenvironment and enviroeconomic analysis consider the environmental impacts and sustainability aspects of SSs.
Overall, the findings of this review paper emphasize the importance of using a comprehensive approach that integrates thermodynamic, economic, and environmental criteria to guide the development of more efficient and sustainable SS technologies.This will enable us to meet the growing demand for drinking water in a sustainable and cost-effective manner.

FUTURE SCOPE
(1) The use of novel nanoparticles can be used to enhance the performance of SSs.(2) The possibilities of new PCMs and how to best incorporate them into solar-powered systems for increased energy and efficiency can also be explored.

ORCID
Mohammad Hossein Ahmadi http://orcid.org/0000-0002-0097-2534 67alyzed thermal and exergy enhancement for four different cases (25%, 50%, 75%, and 100% PV coverage area) of partially covered photovoltaic thermal (PVT) and concentrator water collectors coupled in series, considering different weather conditions in New Delhi.A transient model is developed to optimize six collectors for the desired temperature range.Determined that Case (iv) is suitable for the requirement of electrical and overall exergy, while Case (i) first choice for thermal gain.Enviroeconomic study examines the reduction in CO 2 emission and money saved.Abdelaziz et al.67studied the effectiveness of SS utilized porous absorber consisting of tubes of activated carbon.Four different sets of absorbers are tested for energy, exergy, economic, and environmental points of view.Results indicate that SS with horizontally staggered had the best yield, energy and exergy efficiency, cost-saving capabilities.CO 2 emissions are decreased, which has favorable effects on the economic and environment.Total effectiveness of SS enhanced by employment of porous absorber.Sharshir et al.
83vestigated pyramidal SS, coated absorber plate using TiO 2 nanoparticle to analyze them under various water depths.The experiment's results demonstrate that the use of black paint, the temperature of water increased by 1.5°C and yielded 6.1% compared with CSS.Muthu Solar still energy, exergy, economic, exergoeconomic, exergoenvironment, and enviroeconomic analysis.PV, photovoltaic; SS, solar still; SSSS, single slope solar still; TSS, tubular solar still.Saravanan et al.79utilized a Kanchey marble medium for storing heat in an experiment on SS to determine the impact on overall efficiency.Discovered that SSs employing Kanchey marbles perform 16.32% better than CSS.Gad et al.80studied conical SS with a base size of 0.8 m 2 and conical angle of 31°to minimize shadow impact and maximum usage of solar radiation.The experiment's results reveal that conical SS produced 3.38 L/m 2 while CSS produced 1.93 L/m 2 in a day.The coefficients of heat transfer for conical SS and CSS were also computed, respectively, 66 and 32 W/m 2 °C.Abderachid and Abdenacer 81 simulated symmetric double-effect solar desalination to determine the influence of solar desalination orientation.He discovered that the best angle of inclination to capture maximum radiation is 10°, and the best water depth is 0.02 m.Moreover, the consistency of solar radiation observed in the north-south direction receives greater solar radiation.Pangwa and Msomi 82 studied factors affecting the performance of SS.Eliminating factors that affect the output of SS is the study's primary goal.According to the study, the main factors that influence SS performance are wind speed, temperature difference, basin depth, glass thickness, evaporation area, and solar intensity.The combination of all these elements would help the industries that depend on water for effective operation, and most essential, as water is a basic requirement, the human species would gain significantly from this.Reddy and Sharon83analyzed efficiency and enviroeconomic impact on multieffect vertical active SS.During normal and low-pressure operation mode produce the highest daily average freshwater yield of 6.78 and 21.29 kg/m 2 , respectively.Over its 20-year lifespan, evacuated unit was determined to efficiently reduce at least 81.81 tons of CO 2 emissions, with distilled water production costing Rs. 2272/m 3 .The study also revealed that the cost of producing freshwater was significantly influenced by the financing interest rate.The summary of the main key findings of SS and key factors like energy, exergy, economic, exergoeconomic, exergoenvironment, and enviroeconomic (6E) is shown in Table T A B L E 1Abbreviations: CSS, conventional solar still; DBSS, double basin solar still; DSSS, double slope solar still; PCM, phase-change material;