Impact of various design parameters on solar still systems performance: a review

Several researchers have experimented with various design parameters on solar stills to optimize the distillate. It has been observed that various design/operating parameters affect productivity and efficiency such as angle of solar radiation, angle of inclination, cover plate transmissivity, depth of water, wind velocity, basin surface absorptivity, flow rate, convective heat transfer coefficient, insulation of wall of basin, and so on. These parameters help to modify and design solar desalination to be more productive. The present study refers to the evaluation of various design parameters for simple solar stills impacting yielding rate and heat transmission characteristics. The observations for the utility indicators and performance comparisons of several solar stills have been mentioned along with the significant interpretations, and finally, recommendations for future improvement of the novel concepts were also made in the proposed work.

Murugavel et al. (2010) designed a double-slope solar still with an outer surface of mild steel and observed the performance of the system with a level of water and various heat-sensible storage materials, including quartzite rock, concrete cement pieces, red brick-washed stones, and iron scraps. They concluded that the basin material was quartzite rock that was still only an inch thick. Additionally, the temperature of the glass and water as well as the production rate was negatively impacted by the still's capacity and materials. In Ghaziabad, India (2840° N, 7725° E), Ashmad & Hameed (1997) performed thermal modeling for a double-slope solar still with 30 cm of water depth operating in the natural circulation mode. They concluded that, despite the double-slope active solar still providing a yield that was 51% larger than that of the double-slope passive solar, the passive system had a higher efficiency than the active system did. Furthermore, the double-slope active solar system still had greater energy efficiency than the passive system.

Rajaseenivasan & Murugavel (2013) used the Microsoft Excel program to research the effectiveness, productivity, and dependent variables of a double-slope solar still and a double-basin solar still under the meteorological conditions of Tamil Nadu, India. In contrast to the single basin system, they concluded that adding an extra basin increased productivity by 85%, reaching 2.99 l/m²/day. Although the lower basin had higher productivity, the rate of yield was reduced when the water mass in both the upper and lower basins increased. To determine the best parameters of design for both types of systems under the meteorological conditions of Constantine, Algeria (latitude 36° 22′ N, longitude 6°37′ E), Abderachid & Abdenacer (2013) investigated the effect of both orientations (east–west and south–north) on the performance of productivity of a symmetrical double-slope solar still compared with an asymmetric solar still with a double effect. According to their research, both stills should be inclined at an angle of 10° for maximum solar light absorption and higher productivity. Higher daily production was shown to be associated with a 20 cm depth of water and a south–north (S-N) orientation, particularly in asymmetric solar stills with a double effect (22.57%) as opposed to symmetric solar stills with a double slope (16.23%).

The performance and output of a double-slope solar still with a capillary film-type condenser were examined by Belhadj et al. (2015) using FORTRAN 90 under the climate conditions of Adrar, Algeria (longitude of 0.17°W, an altitude of 264 m, and a latitude of 27.53°). Their findings indicated that an increase in the flow of saline water feeding the condensing chamber and the space between its two plates has an adverse effect on productivity. The double-slope solar still with capillary film type condenser, however, produced 7.15 kg/m²/day while the typical sun still produced just 4.52 kg/m²/day. Results present the algorithm they employed to compute the temperatures and distill freshwater.

The research was done by Singh (2023) on the exergo-economic and environmental economics of a partially covered PVT-FPC active solar distillation system. They discovered that the use of DSSS was incredibly cost-effective for the production of drinkable water. Additionally, the commercialization of PVT active solar was still viable and affordable in outlying places.

The theory of radiation heat transfer justifies that black surfaces have maximum absorptivity for any incoming irradiation. Sathyamurthy et al. (2020) used this concept with the inner side wall of the chamber so that the maximum part of incident solar radiation is absorbed by black, and hence the temperature of water and cabin both increases. Hence, the rate of evaporation of water increases, and the performance of solar still enhanced. With a 20% concentration of nanoparticles in black paint, the average temperature of the water, basin, and glass was raised by 10.4, 12.2, and 12.3%, respectively. Beyond 20% of nanoparticle concentration, there is no discernible improvement in water, glass, or basin temperature. When utilizing 10 and 20% nanoparticles in black paint, the yield from solar stills is increased by 27.2 and 34.3%, respectively, and is higher than the output from solar stills using regular black paint.

Dhiman (1988) discussed a spherical solar still system and studied it using mathematical modeling. Spherical solar still has a combination of spherical top cover and liner basin. He also compared it to SSSS, and the offered system reported 30% more yield than SSSS because it does not require any solar tracking equipment but instead absorbs solar radiation. Basel later conducted an experimental investigation for the mutable hemispherical solar still (HSSS) and discovered up to 5.7 l/m²/day with 33% efficiency. With the exception of the second system's portability advantage, both systems are free from solar tracking.

Kabeel et al. (2019a) exposed, till the research in solar still from the economic angle, nanoparticles of TiO₂ are an economical way to mix with paint and paint on the inner surfaces of the basin. The availability of nanoparticles enhances the heat flow rate because of the enhanced thermal conductivity of the mixture as compared to without it. Such painted surface enhanced their absorbing capacity of solar and thermal energy. Such a concept enhances the evaporation rate of water and yields solar still. Nanoparticles can indeed enhance the heat flow rate and improve thermal conductivity when incorporated into a mixture or material. This phenomenon is commonly observed in nanofluids, which are liquids containing suspended nanoparticles. By adding nanoparticles to a mixture, such as a paint or coating, the thermal conductivity of the material can be significantly increased. This enhanced thermal conductivity allows for better heat transfer within the material, resulting in a higher heat flow rate. As a result, the material can absorb and distribute thermal energy more efficiently. When applied to the inner surfaces of solar still, these nanoparticle-enhanced coatings can enhance their absorbing capacity for both solar and thermal energy. The nanoparticles can help capture a greater amount of incoming solar radiation, converting it into thermal energy that can be utilized or stored. This makes the painted surface more effective at harnessing both types of energy.

Enhanced productivity of fresh water is a need of society and hence day by day research is happening. Condensation at the cover surface has an important role in this phenomenon. Condensation is based on the combination of convection and conduction heat transfer. Dimri et al. (2008) observed that the productivity of solar was further enhanced with a higher value of thermal conductivity of the cover surface. When the cover surface of a solar panel or solar thermal collector has higher thermal conductivity, it facilitates better heat transfer from the surface to the underlying components, such as the absorber or heat transfer fluid. To achieve higher thermal conductivity in the cover surface, various techniques can be employed but it's also important to consider other factors as well, such as cost, durability, and compatibility with the specific application, when selecting or designing the cover surface material.

The thermal study is performed at two distinct water levels of 3 and 12 cm at an inclination of 23°, according to Prakash et al.’s (2021) analysis. When the water depth increases for the active mode of operation, the daily yield production rises. The daily output yield falls with increasing water depth and increases with increasing h_ew and h_cw. When the evaporative surface temperature is higher and the condensing surface temperature is lower, the output of the solar still distillate increases. The optimal yield output is 0.23885 kg/h at 12 cm of water depth, while it is 0.13 kg/h at 3 cm.

Potable water is collected after condensing generated steam on the cover surface through both convective and conductive heat transfer. Solar stills with fins for condensation are a type of solar desalination system that utilize fins to enhance the condensation process. Solar stills are devices that use solar energy to evaporate water and then collect and condense the vapor to produce fresh water. Velmurugan & Srithar (2011) observed fins increase the surface area available for condensation, allowing for more efficient heat transfer, water vapor condensation in a single solar still, and efficiency enhancement by 45.5%. The extended surface area helps in maximizing the contact between the warm vapor and the cooler condensing surface, facilitating faster condensation and higher water production rates. The fins used in solar stills can be made from materials with good thermal conductivity to aid in the transfer of heat from the vapor to the condensing surface.

By introducing a pin fin into a paraffin wax bed of a single slope solar still it performed more productively. Such experiments are observed with several depths of water from 2 to 4 cm in a modified solar still where productivity was higher at 2 cm and also achieved yield 3,750 ml/m² for modified solar still while for the same depth, the yield for conventional solar still was 3,017 ml/m² (Suraparaju & Natarajan 2021a).

Suraparaju et al. researched better productivity of still with the use of ridge gourd natural fiber in the single slope solar still. Two identical SSSS are used to compare the impact of modification on productivity and the result found that such kind of modification has no impact on the productivity of solar stills (Suraparaju & Natarajan 2020).

Singh (2021) observed that in comparison to SSSS, Tiwari & Selim (1984) showed that the tubular solar still (TSS) performs better over a longer period of time when there are no nighttime conditions because it allows for a bigger basin area and better evaporation. A TSS consists of a rectangular tray as a still basin in a tube that is covered. A model of TSSS with several wicks in place of a rectangular basin was provided by Kumar & Anand (1992). This desalting technology thus provides more space for condensation and superior results to SSSS. Then, Amimul et al. (2012) enhanced and provided TSSS in new design parameters with a basin of metal material covered by a thin film of vinyl chloride. The straightforward or traditional TSSS was performed after desalination. Due to the greater temperature difference between the basin water and the thin film tubular covering in this setup, the yield was improved. Then, Arunkumar et al. (2016) tested with TSSS undercooled or uncooled settings, together with PSS and CPC, and the successful findings were 7.77 l/m²/day yield. They also concluded that the system's heavy initial setup cost is offset by its overall enhanced efficiency and the distillate that results from it. Later, Arunkumar & Kabeel (2017) introduced PCM with TSS-CPC and analyzed TSS-CPC with PCM, and compared for without PCM; the results showed that TSS-CPC with PCM performed better, with a daily yield of 5.78 l/m² compared to 5.33 l/m² for TSS-CPC alone. The use of PCM, which provides continuous heat as supplemental thermal energy by altering its phase, allowed the system to operate more efficiently. As a result, latent heat from PCM was transferred to the basin liner between periods of cooling. In a recent study, Jing et al. examined concentric TSS, which consists of two layers, the outer of which has a wet wick and the inner of which is normal. As distillate output, the outcome was superior to the earlier TSS system. In comparison to other systems, TSS has superior solar radiation usability due to a smaller layer of basin water mass, and the usage of PCM expands its capability. Thus, superior output outcomes are obtained with appropriate PCM for TSS-CPC systems.

Cooper et al.'s (2018) novel idea for superheated steam is provided. In this experiment, the solar absorber does not come into direct contact with the water; instead, it is filled with solar radiation energy, which is then used to superheat the steam with 38% efficiency (under laboratory conditions) and distill water up to 2.5 l/m² each day.

As opposed to this, Kashyap et al. (2019) demonstrated the circulating CO₂ capture into a grapheme aerogel material for corresponding conversion into water and CaCO₃ with the aid of solar localized heating for 1,000 W/m² (one sun) solar intensity in a laboratory setting. Diffusion solar stills and weir-type solar stills function well in the various solar desalting systems represented by the body of work. It has a greater effective condensation and evaporation surface area with the lowest water mass at the basin liner or at the diffusion surface which directly leads to the corresponding result. Stills with CPC, FPCs, PCM, or additional condenser, etc. lead directly to the freshwater production under the same surface area of the basin (Singh & Gautam 2022).

Several solar still designs that used a storage tank for continuous water circulation in stepped solar stills (STSS) to extend production many times were analyzed (El-Agouz 2014; Kabeel et al. 2019b). The STSS absorber plate has a surface area of 1 m², which is separated into 10 equal steps. The feed water (i) seawater and (ii) salt water, and the basin conditions (i) black absorber basin and (ii) cotton absorber basin were changed during the experiments. Black absorber STSS is experimentally explored in the first case of basin circumstances, and the outcomes were compared with simple solar still (SSS). It has been determined that the STSS and SSS black absorbers' daily efficacy for desalinating seawater is 61 and 42%, respectively, and for desalinating saltwater is 55 and 37%. In the second instance, a cotton absorber is used in place of the black absorber. According to the experimental findings, the STSS and SSS cotton absorbers' daily efficacy for desalinating seawater is 61 and 40%, respectively, and for desalinating saltwater, it is 70 and 48%. For seawater desalination and saltwater desalination, respectively, the black absorber STSS enhances the freshwater production rate by up to 43 and 48% more than that of SSS. For seawater and saltwater desalination, respectively, cotton absorber STSS enhances the freshwater production rate by up to 53 and 47% more than that of SSS. STSS using seawater as the intake yielded 6.1 and 6.3 kg/m²/day in yield.

Various types of single slope simple solar stills (SSSS) have been discussed along with SSSS and the overall performances of SSSS. The variations between productivity and efficiency show the effective performance of solar water still, that's why it is recognized easily, the worth of full solar desalination among all the respective parameters under conditions for simple and integrated solar still. The summarized observed data show that cascade-type weir solar desalination systems with and without PCM, multi-step solar still with multiple absorbers are better while weir solar still system without PCM performs more effectively with 3.05% yield and 5.96% efficiency due to the effective utilization of collected solar energy to evaporate saline water at the same time. SSSS performs effectively with 24% more productivity per unit area of basin per day while most of the part is at a mid-time. PSS performs efficiently because PSS does not require any solar tracking machine for solar energy because of its design. This has a per day average efficiency of 45% with distillate as 4.01 l/m²/day at $ 0.065/l rate. Variations in the performance of various NSDSs based on its novelty, working principle, different physical parameters, utilized material (smart materials, nanoparticles, etc.), designing concepts, and intensity of solar radiation per confined laboratory conditions and open atmospheric conditions. Singh (2021) analyzed new conceptual designs with enhanced techniques with creative novelty representing better performance, as nanoporous aluminum oxide membrane with Au nanoparticles solar distillations system gives better performance by 11.4 l/m²/day for one solar intensity due to the wide band of absorption of solar radiation delivering huge heat to evaporated and regularly more distillation. On the same side, a self-assembled plasmatic absorber (restricted porous substance filled Au/NPs) solar still system and affordable narrow gap solar evaporation and still system perform efficiently with more efficiency by 90% because of its restricted specialty, i.e. initial system has tightly packed NPs in the membrane which design a porous structure suitable for taking all spectral solar intensities and for another case, still was concentrated via parabolic concentrator and this concentrated solar radiation heat is received by smart material which is further consumed to evaporate water via a narrow void under the affected of localized heat that helps the performance of the improved system. It is observed that improved heat transfer through various parameters helps in several ways as also mentioned in Tables 1 and 2.

Enhanced absorption: A covered surface with higher thermal conductivity can more effectively transfer absorbed solar energy to the underlying absorber material. This allows for better utilization of the incident sunlight, resulting in increased energy absorption and overall productivity of the solar system.

Reduced temperature gradients: Efficient heat transfer helps in reducing temperature gradients across the cover surface. This is particularly important for solar panels or collectors because high-temperature gradients can lead to thermal stress, which can degrade the performance and lifespan of the system. By minimizing temperature differentials, a cover surface with higher thermal conductivity promotes more uniform heating and reduces the risk of localized hotspots.

Faster heat dissipation: In solar thermal systems, faster heat dissipation from the cover surface improves thermal management and prevents overheating. When the cover surface can efficiently transfer heat to the surroundings, it helps maintain optimal operating temperatures, which leads to improved overall system performance.

Increased surface area: By spreading the water across a larger surface area through the wicks, the solar still increases the exposure to the surrounding air. This enables a greater surface area for evaporation, leading to enhanced water vapor production.

Improved heat transfer: The wicking materials can help in efficient heat transfer within the system. As the water evaporates from the wicks, it absorbs heat from the surrounding environment, including solar energy. The wicks facilitate the movement of water and heat, ensuring a continuous supply of water for evaporation.

Facilitated condensation: Once the water vapor is generated, it needs to be condensed to produce fresh water. The wicking materials can also play a role in the condensation process. They can act as condensation surfaces or provide a structure on which the condensed water droplets can accumulate and flow down into a collection area.

Based on the above study, a frame of conclusion has been made accordingly:

• i.

  Conventional single-slope stills are effective in summer weather conditions and vice versa for double-slope solar stills.

• ii.

  The modified solar stills perform better according to the augmentation of active components and effective design considerations under suitable meteorological conditions.

• iii.

  A solar still with a wet cloth on the side walls provides high porosity with thin film evaporation thus improving distillation. A solar still with wicks is a type of solar desalination system that utilizes wicking materials to enhance the evaporation and condensation processes.

• iv.

  Solar stills with wicks are particularly useful in situations where there is limited access to freshwater sources or in areas with high salinity levels.

The selection of appropriate wicking materials is important to optimize the performance of the solar still. Materials with good capillary action and high evaporation rates are typically chosen. Factors such as porosity, surface area, and durability should be considered when selecting the wicking materials for a specific application. It's worth noting that the design and configuration of the solar still with wicks should be carefully considered to achieve optimal performance. Factors such as wick spacing, thickness, and arrangement can affect water distribution, evaporation rates, and condensation efficiency. Additionally, environmental conditions such as solar radiation, temperature, and humidity levels also impact the overall effectiveness of the system.

The first author sincerely acknowledges Dr Ashok Kumar Singh, Associate Professor, Galgotias College of Engineering and Technology, Greater Noida for his appreciable help while completing this article and is also grateful to Centre for Energy and Environment, Delhi Technological University, Delhi (India) for providing basic facility in compiling this manuscript.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.