Performance analysis of snail shell biomaterials in solar still for clean water production: nature-inspired innovation for sustainability

The burgeoning global concern regarding the reduction of CO₂ emissions and the finite nature of fossil fuel resources has significantly propelled the industrial sector toward the adoption of solar energy. Besides energy, the widening gap between the limited supply of freshwater resources and the rising demand for water makes water scarcity a serious threat to sustainable development. The growing world population, fast urbanization, and industry have driven up the amount of water that is needed. Water scarcity is a global phenomenon that arises from the depletion of freshwater reservoirs due to various contributing factors, including population growth, poor water management practices, and climate change (Taner 2019; Reddy et al. 2022). Desalination has the promise of becoming the solution to this urgent problem. To make brackish or saltwater suitable for agriculture and human consumption, contaminants and salt must be removed (Ebhota & Tabakov 2023). Desalination becomes a critical tactic as traditional freshwater sources are overused and eventually run out. Furqan et al. (2023) stated that thermal desalination is a crucial technique among the numerous desalination technologies since it uses heat to effectively separate salt and contaminants from water. Some advantages of desalination like increased freshwater supply, diversification of water sources, independent of climate conditions, technology advancements, addressing water scarcity, support for agriculture and industry and emergency water supply.

Despite these advantages, it's important to note that desalination also has some challenges and considerations, including high-energy requirements, environmental impacts, and the potential for brine disposal issues (Suraparaju et al. 2021). Balancing the benefits and drawbacks is essential in implementing sustainable desalination projects. Additionally, ongoing research and development focus on addressing these challenges and making desalination more efficient and environmentally friendly. In response to these challenges, there is a burgeoning interest in integrating renewable energy sources, particularly solar energy, into the desalination process to foster sustainability. Systems for desalinating water driven by renewable energy significantly lower greenhouse gas emissions, supporting international efforts to tackle climate change (sustainable development goal, SDG). Furthermore, by promoting ecosystem resilience and biodiversity preservation, this integration lessens the total environmental impact of desalination. Additionally, the cooperative relationship between desalination and renewable energy helps to build a more sustainable infrastructure for the provision of water and sanitation, thereby achieving the overarching goal of sustainable management for all (Selfa et al. 2024).

The solar still basin uses materials that store sensible heat to achieve higher productivity. As a result, the dry cow dung was used in solar still basin as biomaterial heat storage and increased the productivity by 35% than conventional solar still (CSS) (Panchal 2015). Samuel et al. (2016) reported that solar thermodynamic fluid in a plastic container yielded almost 66% more water than CSS. Bhargava & Yadav (2019) observed that jute fabric, bamboo cotton, and dry cotton enhanced productivity significantly. In this, bamboo cotton resulted in around 52% higher productivity than other materials. Similarly, the jute fabric and sand were utilized in the solar still basin and enhanced the productivity by 15% (Kabeel et al. 2018). In the same way, the sand-filled coal powder and cotton cylinders were incorporated in the solar still basin and achieved 31% higher productivity than CSS (Dumka et al. 2019). As a result, the overall system performance was greatly enhanced in cotton cylinders. In another work, the pumice stones were introduced in a solar still basin and increased the productivity by 28% than CSS (Bilal et al. 2019). As an external energy storage medium, the gravel coarse aggregate assisted in the solar still improved the cumulative productivity to 4.21 kg/m² (Dhivagar et al. 2021a).

Several researchers have found ways to enhance the energy efficiency of solar stills using various materials that store sensible heat. El-Sebaii et al. (2009) reported that the salts in solar still basin increased energy efficiency by 37.8%. Similarly, the estimated energy efficiency in black granite gravel-assisted solar still was around 52% greater than CSS (Sakthivel & Shanmugasundaram 2008). Results from this experiment show that granite stone considerably increased the system efficiency. The evaporation rate in V-type solar stills was significantly increased by floating charcoal in the basin (Kumar et al. 2008). The energy efficiency achieved in this trial was 30% higher than CSS. In addition, the heat produced at the charcoal surface accelerated the condensation. Similarly, the aluminium-coated sheets improved the energy efficiency of multi-effect solar still basin by 62% than CSS (Rahim 2003). In this experiment, the stored energy in the daytime was significantly utilized during nighttime to improve efficiency. Bilal et al. (2019) employed 10 kg of pumice stones in a solar still basin increasing overall energy efficiency to 28.8%.

Many researchers estimated the exergy efficiency to assess the energy losses in the solar still system. Vaithilingam et al. (2022) investigated the exergy efficiency of solar stills with copper fins and found a higher efficiency than CSS. Dinesh et al. (2022) used the solar still basin made of black granite, colored glass balls, and white ball marbles to determine the highest exergy efficiency in CSS. Kateshia & Lakhera (2022) used lauric acid and pin fins in a solar still basin and estimated the exergy efficiency to be 2.23%. Similarly, Selimefendigil et al. (2022) used the CuO nanoparticles as an absorber coating and discovered a maximum exergy efficiency of about 2%. Dhivagar & Mohanraj (2021) estimated the exergy efficiency of 4.4% in the solar still basin using graphite plate fins in conjunction with magnets. Balachandran et al. (2021) used eggshells in a solar still basin which had the maximum exergy efficiency of about 2.36%.

Many studies have analyzed the cost-effectiveness of solar still employing sensible heat storage materials. Sellami et al. (2016) used Portland cement in the solar still basin and estimated the CPL of about 0.00103 USD. Similarly, Kabeel et al. (2019) found that the usage of composite materials in the solar still basin had a CPL of about 0.0013 USD. In addition, the overall estimated CPL of CSS without energy storage material was about 0.027 USD. Samuel et al. (2016) applied encapsulated salt to the CSS basin and attained a CPL of 0.0094 USD. Rufuss et al. (2018) discovered that the CPL of solar stills that utilized phase change materials and graphite oxide was about 0.12 USD. Dhivagar et al. (2021b) employed magnets as heat storage devices inside the solar still basin and estimated the CPL of around 0.213–0.216 USD.

Many researchers use enviro-economic analysis to demonstrate solar stills' environmental performance. Joshi & Tiwari (2018) estimated the environmental effect of an active solar still and found that it releases about 7.14 tons of CO₂ in a year. In another study, Piyush et al. (2018) found that the jute and cotton fabric solar stills released 7.82 and 8.69 tons of energy-based CO₂ emissions during the experimentation. Khanmohammadi & Khanmohammadi (2019) found that paraffin wax can release exergy-based CO₂ emissions by 0.139 tons/year. Bait (2019) analyzed that the CO₂ emission in active solar stills was about 4.42 tons. Similarly, Hassan et al. (2020) evaluated the exergy-based CO₂ emissions as 5.9 tons/year.

Some of the recent studies in the enhancement of solar still performance are done using natural source materials. Dhivagar et al. (2023) used the conch shell biomaterial as an energy storage medium and porous media in a solar still conch shell solar still (CSSS) and investigated its impact on water productivity and efficiency. Experiments demonstrated a notable enhancement in performance compared to CSS. The CSSS showcased a 10.8% increase in cumulative water productivity, along with superior energy and exergy efficiency by 10.3 and 9%, respectively. Furthermore, carbon footprint reduction and CO₂ emissions mitigation were significantly improved, highlighting the potential of this innovative approach for sustainable desalination. Similarly, Ramzy et al. (2023) aimed to optimize solar still performance by testing different absorbing materials in arid conditions. Two solar stills were constructed and compared using materials like black luffa and steel wool pads. Results from testing in Egypt showed steel wool pads yielded the highest productivity at 4.3 kg/m² and the highest thermal efficiency at 32.74%. Moreover, steel wool pads had the lowest CPL at 0.0034 USD/kg. This study suggests steel wool pads as a promising modification for improving solar still performance in addressing water scarcity. A study by Abdullah et al. (2023b) explored enhanced designs for spinning wick solar stills to address freshwater scarcity. Two designs were tested: one with a wick belt in an ‘LC’ shape path and the other in an ‘L’ shape path. Wick materials, rotation direction, and timing were varied. Jute wicks outperformed cotton. Counterclockwise rotation yielded higher productivity. The best output was with 30 min off and 5 min on, achieving 17% more yield than the other design. With additional enhancements like reflectors, fans, and nanofluids, productivity increased by 28%, yielding freshwater at CPL of 0.016 USD.

Elsawy et al. (2023) conducted an experimental study aimed at enhancing freshwater production efficiency and reducing costs using minimal energy. They evaluated the performance of a hemispherical solar still under various configurations, employing charcoal from guava tree wood (CHL) and carbonized corncobs (CCC) as floating agricultural waste materials. Both materials underwent physical and chemical activation to improve their photothermal properties. Tests were carried out using seawater and lake water in Kafrelsheikh, Egypt. Results showed significant improvements in thermal and exergy efficiencies, daily output, and cost-effectiveness compared to conventional distillation methods. Abdullah et al. (2023c) address the escalating challenge of freshwater scarcity, proposing solar distillers as viable solutions for remote locales and small households. However, conventional models suffer from low efficiency. They examined two modified designs of rotating wick distillers: one with an ‘L’-shaped path (l-RWSS) and another with an ‘LC’ shape (LC-RWSS). The study evaluated different wick materials, rotation directions, and intervals. Results indicate that jute wicks outperformed cotton. LC-RWSS, especially with counterclockwise rotation, exhibited superior productivity, enhanced further with graphene quantum dots nanofluid, offering a promising solution to freshwater scarcity at reduced costs. In recent research by Khalili et al. (2023), stepped solar stills' absorber surfaces were examined with a focus on enhancing their performance. Metal fins were introduced to augment radiant energy absorption. The study varied input factors such as inlet flow rate, number, angle, height, and material of fins. Results indicated a productivity increase with higher fin numbers and a decrease with rising inlet flow rates. Notably, copper fins outperformed others, with +30° angled fins proving the most effective. The optimal configuration yielded around 171% efficiency boost compared to fins-less designs.

In harnessing solar energy for sustainable water production, researchers have explored various sensible heat-storing materials in solar still basins. Among these, using snail shells as a biomaterial is a novel and eco-friendly approach which contributes to the broader objectives of SDG. Solar stills with snail shells in the absorber basin are a creative adaptation of traditional solar still technology. In this setup, snail shells are utilized as a means to enhance the efficiency of the still in collecting and purifying water. Here's a more precise breakdown:

• 1. Design: The solar still consists of a basin-like structure that collects water. Inside this basin, snail shells are placed as absorbers. These shells absorb solar radiation during the day, heating up the water inside the basin.

• 2. Evaporation: As the water in the basin heats up, it begins to evaporate. The evaporated water vapor rises and condenses on a transparent cover placed over the basin. This cover allows the condensed water to trickle down to a collection point, where it is gathered as potable water.

• 3. Role of snail shells: The snail shells act as absorbers of solar radiation, increasing the temperature of the water in the basin. This acceleration of heating helps in enhancing the evaporation process, thereby improving the overall efficiency of the solar still.

• 4. Sustainability: Snail shells are abundant and renewable, making them an eco-friendly alternative to synthetic materials.

• 5. Cost-effectiveness: Snail shells are inexpensive or even free, reducing the overall cost of the solar still.

Challenges associated with this setup:

• 1. Shell availability: One challenge is ensuring an adequate supply of snail shells. Depending on the location and scale of the project, sourcing a sufficient quantity of shells may be difficult.

• 2. Shell preparation: Preparing snail shells for use in the absorber basin can be labor-intensive and time-consuming, especially if large quantities are needed.

• 3. Shell durability: Snail shells may degrade over time due to exposure to water and sunlight, potentially reducing their effectiveness as condensation surfaces.

• 4. Maintenance: Like any component of solar still, the snail shells require regular maintenance. They may need cleaning to prevent the buildup of dirt or algae, which could reduce their effectiveness as absorbers. Keeping the absorber basin clean and free from biofouling or mineral buildup can be more challenging with snail shells present, requiring regular maintenance.

• 5. Temperature regulation: While the shells can help in heating the water, there might be challenges in regulating the temperature within the basin. If the water gets too hot, it could lead to excessive evaporation or even damage to the still components.

• 6. Design optimization: Designing the solar still with snail shells requires careful consideration of factors such as shell placement, spacing, and orientation to maximize their effectiveness. Optimization may involve experimentation and iterative improvements. Ensuring a consistent distribution of snail shells in the absorber basin can be challenging, impacting the overall performance of the solar still.

• 7. Scalability: Scaling up the technology to meet larger water demands may pose challenges in terms of cost-effectiveness and practicality. It's essential to assess scalability issues before implementing the technology on a larger scale.

Despite these challenges, solar stills with snail shells offer a promising approach to water purification, especially in regions with abundant sunlight and limited access to clean water sources. Therefore, the study seeks to enhance the productivity of solar stills by incorporating snail shells as a sensible heat storage material. By evaluating the increase in productivity under consistent climatic conditions, a comparative analysis is conducted between SSSS and CSS. The investigation explores multiple dimensions: energy, exergy, economics, and environment. This comprehensive analysis provides a holistic understanding of the implications of utilizing snail shells in solar stills compared to CSS. The evaluation not only scrutinizes the performance of snail shells but extends its purview to assess the economic viability and environmental impact and align the research with key pillars of SDG. Furthermore, snail shells are naturally recyclable which makes them a sustainable resource and a green way to produce freshwater. This feature is in perfect harmony with the SDG, especially the ones that deal with affordable, clean energy, clean water and sanitation, and responsible consumption and production. The incorporation of snail shells as a biomaterial in solar stills improves efficiency and is in line with international efforts for a more sustainable and just future. All things considered, this research is a testimony to innovation in sustainable technology. The study conclusions add to the current discussion on substitute materials for applications in renewable energy, highlighting the significance of comprehensive strategies that take environmental responsibility and technical advancement into account.

In material analysis, the researchers employed the Scanning Electron Method (SEM), a cutting-edge technique involving a concentrated stream of high-energy electrons. This technique has revealed the crystalline structure and surface roughness of the snail shells, as seen in Figure 3 at different magnification levels. The results from SEM reveal a porous surface on the snail shell which was the crucial finding that holds significant promise for SDG. A key revelation from the SEM analysis is that the porous nature of the snail shell's surface enhances its capacity to absorb solar radiation. This novel insight contributes to developing eco-friendly technologies, aligning with SDG by harnessing solar energy efficiently.

The experiment included performing a porosity test on ten different weights of snail shells, each of which had a different size. To determine porosity, the dry weight of each sample was meticulously assessed using a cutting-edge digital scale. The method was used which required soaking the samples in hot water for an hour to saturation. The detailed list of properties of the snail shells used in this innovative investigation is summarized in Table 1. In basin, the saline water warms with the heat stored in snail shells and evaporates. The dissipated vapor condenses on the inside surface of the glass cover finally.

The study examined the energy and exergy efficiency of SSSS and CSS systems, evaluating their individual contributions to the system's overall thermodynamic performance. This comprehensive study attempted to bring out the complex details of each system's performance, taking into account both energy and exergy factors.

The relationship between the evaporation rate and the amount of net solar energy was examined in order to evaluate the efficiency of the solar still.

By applying the second law of thermodynamics to energy analysis, the exergy analysis was estimated as mentioned below. The rate of evaporation is expressed as a percentage of total solar energy gained.

Solar stills efficiently produce distillate and have a quicker payback time. The relations listed below are used to examine it (Parsa et al. 2020):

The annual productivity can be considered (picked 270 days) on clear sunny days (Dhivagar et al. 2022).

Here, POW stands for the price of water in units of 0.1 USD/kg, while is the annual productivity in kg. Therefore, the PCR needs to be higher than one for the investment to be effective.

The environmental and economic analysis shows how much carbon dioxide (CO₂), a solar still emits over its lifetime and how much carbon credits it generates. Conventional power stations that burn fossil fuels release an estimated 1.58 kg/kWh of carbon dioxide, as taken in the present study (Dwivedi & Tiwari 2012):

This section explores the findings of the SSSS and CSS experiments under similar weather conditions which were conducted in Ongole City, India.

The integration of snail shells into the basin of solar still markedly increased the production of clean water in the SSSS compared to the CSS. The primary factor contributing to this enhancement is the increased basin water temperatures and the expanded wet surface area, attributable to the inherent porosity of the snail shells. These shells, when included in the absorber basin, significantly improved the absorption of solar radiation, thereby elevating the overall system temperatures. The porous nature of the snail shells and their capacity for water absorption increased the wet surface area within the basin, which in turn enhanced the evaporation rate. Additionally, the heat-retaining properties of the snail shells contributed to higher water and absorber temperatures in the SSSS. The empirical data clearly indicate that the temperatures of both the water and the absorber in the SSSS were significantly higher than those in the CSS. This increase in basin water temperature in the SSSS facilitated a greater evaporation rate within the absorber basin, thereby resulting in higher water productivity. Consequently, the improved system temperatures and the increased wet surface area in the absorber basin of the SSSS led to a substantial improvement in productivity when compared to the CSS. In conclusion, the strategic inclusion of snail shells in the absorber basin not only augmented the absorption of solar radiation but also enhanced the heat storage and water evaporation processes. These modifications significantly boosted the performance and productivity of the SSSS relative to the CSS, underscoring the effectiveness of this innovative approach.

It is observed that the outcomes from the snail shell based solar still are significant and are in line with the previous studies such as solar still with sand troughs (Nagaraju et al. 2022), PCMs (Sahu et al. 2023; Suraparaju & Natarajan 2023; Suraparaju et al. 2024a), photothermal absorbers similar to snail shells and natural fibers (Suraparaju et al. 2024b). Therefore, the current small scale prototype SSSS can be scaled up for industrial use and sustainable drinking water uses in rural and coastal areas.

In this comprehensive study, the performance of SSSS and CSS were analyzed under the same climatic conditions. The SSSS proved to be a more effective system which slightly increased the productivity due to the use of snail shells inside the basin to absorb and store sensible heat.

• The SSSS hourly productivity increased by 6.8% when compared to CSS. Although both systems produced around the same cumulative productivity averaging 2.28 kg/m² for SSSS and 2.18 kg/m² for CSS, the hourly increases highlighted the better performance in SSSS.

• Energy efficiency is an important metric and is preferred the SSSS with a rate of 24.1% which was slightly higher than the CSS with a rate of 23%. Additionally, the system's capacity to generate work was measured as exergy efficiency and showed a little but significant lead in SSSS at 2.8% and in CSS at 2.7%.

• The CPL for the SSSS and CSS were estimated to be around 0.028 USD and 0.029 USD, respectively. Moreover, the PBP of the CSS and SSSS were calculated to be around 147 days and 141 days, respectively, which demonstrates the system's economic feasibility and the SSSS had a slightly faster return on investment.

• The environmental impact and emissions of both systems were evaluated. The estimated CO₂ emissions from the SSSS and CSS during a 10-year period were 7.5 and 6.4 tons, respectively. Simultaneously, the CCE of both SSSS and CSS have resulted in around 180 USD and 153.6 USD, respectively, which demonstrates the financial impact of carbon mitigation initiatives.

The research results indicate that the SSSS is a feasible and environmentally conscious replacement for CSS, which is particularly relevant to SDG. To sum up, the SSSS demonstrates its effectiveness in raising output and reducing energy consumption. It is a concrete step in the direction of sustainable development, combining social effect, environmental responsibility, and economic viability in a harmonious way.

There is no funding support for this research work.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.