Water is one of the most essential resources for sustaining life, supporting agriculture, enabling industrial processes, and ensuring overall human well-being. With only about 2.5% of the Earth’s water being fresh, and much of it locked in glaciers or inaccessible aquifers, the availability of potable water is severely limited. Rapid population growth, urbanization, industrial expansion, and climate change are intensifying global water scarcity, making access to safe drinking water a critical challenge particularly in arid, remote, and coastal regions. According to the United Nations, by 2050 nearly 5 billion people could experience water stress, highlighting the urgent need for sustainable freshwater generation technologies.

Solar desalination offers a promising, eco-friendly, and cost-effective solution to address this challenge, particularly in areas with abundant solar irradiation but limited freshwater resources. Among various solar desalination techniques, the solar still stands out as a simple, low-maintenance device that uses solar energy to evaporate saline or brackish water and condense the vapor into fresh water. Despite its advantages, conventional solar stills are often constrained by low productivity due to limited heat storage capacity, heat loss, and inefficient solar energy utilization. These limitations have motivated researchers to explore advanced materials, innovative designs, and integrated thermal energy storage solutions to improve solar still performance.

A single-basin solar still operates on passive solar distillation, where a shallow, black-coated basin absorbs solar irradiance to heat saline or contaminated water. Evaporation occurs, leaving impurities behind, while vapor condenses on the inclined transparent cover’s cooler surface, typically glass. Gravity directs the condensate into collection channels. This design provides an energy-autonomous, low-maintenance, and cost-effective purification system, particularly advantageous for decentralized freshwater production in rural or resource-constrained environments. Contemporary solar stills incorporate diverse configurations, including single-effect, double-effect, and tubular systems, frequently augmented with evacuated tubes, extended fins, selective absorber coatings, and thermal insulation to enhance operational efficiency. However, a significant research gap persists concerning the mechanistic justification for integrating nanoparticle–phase change material (Nano-PCM) composites rather than conventional PCMs. The present work addresses this limitation by investigating Nano-PCM interactions and their impact on stepped single-basin solar still (SBS) distillation performance. Solar desalination, which relies on solar-induced evaporation and condensation of saline water, is increasingly recognized as a sustainable strategy for freshwater generation in remote regions. With escalating global freshwater scarcity, such innovations are imperative to ensure accessible and safe potable water supplies.

The development of natural and sustainable desalination systems is essential to meet this challenge. Since the 1960s, the number of desalination plants has steadily increased, reaching 16,758 worldwide by 2023 (Jones et al., 2019). Freshwater is vital for all life on Earth, and its demand continues to rise. Shanmugan et al. (2013) designed a double-slope wick-type solar still and, through energy balance modeling, established that 0.05 m insulation beneath tilted-wick regions optimized distillation efficiency, achieving 41.25% freshwater conversion performance. Shanmugan and Krishnamoorthi (2013) demonstrated that incorporating a transparent honeycomb structure into a double-exposure solar still enhanced radiation absorption, resulting in a 30% higher daily distillate yield compared to conventional designs. Panchal et al. (2020) compared finned and conventional solar stills using Suryjot pond water, reporting average yields of 2.375 L (inclined fins), 2.322 L (vertical fins), and 1.873 L (CSS), with negligible differences between fin orientations. This study provides a detailed analysis of how solar desalination, enhanced with PCMs and nanoparticles, can significantly improve natural drinking water production, as illustrated in Figure 1a.

Over the last two decades, innovations in solar still design have enabled advanced material integration, particularly the utilization of nanoparticles for scientific, technological, and industrial applications. Carbon-based nanomaterials are increasingly investigated due to their exceptional attributes, including high specific surface area, low toxicity, superior thermal and electrical conductivity, mechanical robustness, and tunable structures (Deng et al., 2016). Such properties make them highly suitable for applications in solar energy, electronics, information technology, and biomedical systems. Nanoscale engineering (Khan et al., 2019) has facilitated optimization of material performance through controlled structural design. Carbon nanostructures, such as fullerenes, carbon nanofibers, diamonds, graphite, and carbon nanotubes, offer significant potential across multiple domains (Abdullah et al., 2025; Omara et al., 2024; Gaur et al., 2021). As illustrated in Figure 1b and Table 1, these nanomaterials have been systematically evaluated in solar desalination systems, with studies from 2015 to February 2023 demonstrating their role in enhancing energy efficiency and overall system performance.

Fossil fuel utilization has become economically unsustainable compared to renewable alternatives, particularly when evaluating the contribution of SBS in mitigating anthropogenic climate change. Escalating freshwater scarcity, driven by rapid population expansion, has intensified the urgency for renewable-based desalination technologies. Solar energy, abundant and sustainable, plays a pivotal role in decentralized water production, as emphasized by Amy et al. (2017). Among global concerns, the depletion of potable water resources remains paramount, necessitating innovative freshwater generation strategies. Kabeel et al. (2016) demonstrated that single-effect solar stills provide an effective approach to alleviate water shortages (Figure 1d), offering a low-cost, eco-friendly solution. Furthermore, Vaithilingam and Esakkimuthu (2015) reported a conventional CSS-based SBS configuration, achieving daily freshwater productivity of 1.485 L/m² with an efficiency of 30.960%. Collectively, these studies underscore the technical viability of solar stills as sustainable, scalable, and resource-efficient systems for addressing the accelerating global water crisis.

Phase Change Materials (PCMs) undergo a solid-to-liquid thermophysical transition when ambient temperatures surpass their fusion point. During this process, they exhibit latent heat storage, absorbing significant thermal energy without a substantial change in temperature. This isothermal energy absorption maintains thermal equilibrium. Upon solidification, the stored energy is discharged, facilitating temperature regulation. The application of PCMs spans diverse thermal management systems, including thermal energy storage, heat recovery, and solar thermal applications, addressing the imperative for sustainable energy solutions amid escalating greenhouse gas emissions and energy scarcity (Wu et al., 2020). Solar distillation systems, incorporating PCM-based heat transfer methods (Figure 1e), offer a promising solution for producing freshwater. A thorough evaluation of vertical solar still modifications by Ahmed et al. (2025) revealed that wicks, coils, PV modules, and reflectors improve thermal performance, resulting in 9–107% productivity gains while reducing resistance and footprint and directing future compact designs. Almeshaal and Shanmugan (2024) demonstrated green-sourced PTP nanocomposites in SWS, with 30% loading optimizing thermal dynamics, delivering 8.70 L/m²/day distillate, including 1.95 L/m² nocturnal yield, achieving 42.74% efficiency. Palaniappan et al. (2024) developed a stepped M-shaped solar distiller with PRSNME, yielding 8.106/6.357 mL·m⁻²·day⁻¹ (summer/winter), achieving 54.68%/44.46% efficiency, 26% productivity enhancement, 34.8 t CO₂ mitigation, and reduced water cost (0.014 $/L). A simulative inquiry into three distinct solar still architectures, Abu-Arabia and Zurigat (2005), leveraging empirical data from Marmul, Oman, investigated the augmentative potential of double-glass cooling technology on regenerative thermal effects. The analysis encompassed the impact of basin water mass, collection efficiency, and dual-glazing configurations on systemic heat dissipation. The study further characterized the structural parameters and performance of PCMs, including paraffin waxes, hydrated salts, and metallics. The efficacy of medium-grade paraffin wax in electronic thermal management was specifically noted due to its congruent melting point, as shown in the energy and material cost analysis in Figure 1f.

Enhancement of SBS performance increasingly relies on advanced materials such as PCMs and engineered nanoparticles. Contemporary research demonstrates that integrating PCMs and nanofluids substantially augments thermal regulation, heat storage, and evaporation dynamics, thereby improving freshwater yield and overall efficiency. This review critically examines state-of-the-art techniques employing PCMs and nanofluids, elucidating their synergistic role in elevating SBS functionality. Experimental investigations confirm notable productivity improvements, with empirical outcomes (Figure 1g) validating the effectiveness of these modifications, as highlighted by Vaithilingam and Esakkimuthu (2015). Collectively, these findings establish that material-driven innovations provide transformative potential for scaling SBS systems toward sustainable and efficient desalination. This study aims to map and explore the research landscape of energy storage in solar desalination systems using PCMs enhanced with bionanoparticles. The research specifically seeks to evaluate the integration of bio-sourced nanoparticles with PCMs, such as paraffin wax infused with copper nanoparticles and agro-based additives, to improve the thermal efficiency of SBS. Different SBS configurations, which combine PCMs, nanofluids, and concentrators, will be experimentally evaluated under controlled solar exposure to determine their effects on heat transfer, evaporation-condensation dynamics, and freshwater yield. Furthermore, the paper outlines future research directions for compact, high-performance, and sustainable solar desalination systems, including optimal material combinations and design characteristics.

The experimental setup (Figure 2a) by Vaithilingam and Esakkimuthu (2015) and schematic diagram (Figure 2b) was designed to evaluate the performance of a SBS for freshwater generation. The still was fabricated with a basin area of 1.0 m² and a water depth of 0.1 m, dimensions chosen to ensure adequate exposure to solar radiation while maintaining efficient evaporation (Mohandass Gandhi et al., 2022; Elsheikh et al., 2022). The basin surface was coated with matte black paint to maximize solar absorptivity and minimize reflection losses. To enhance condensation, a transparent glass cover of 4 mm thickness was installed at an inclination of 30°, allowing maximum incident solar radiation while enabling the condensed vapor to flow easily into the distillate collection channel (Alsaiari et al., 2022; Essa et al., 2022). The stepped design was implemented by dividing the basin into multiple horizontal levels, each facilitating thin-film water spreading, thereby increasing the evaporation surface area. Paraffin wax was embedded in aluminum containers placed beneath the basin surface, serving as a PCM for thermal energy storage. This enabled heat retention during peak solar hours and the release of stored energy during off-sunshine periods, extending freshwater production into late evening hours. For enhanced thermal conductivity, Cu NPs at a concentration of 0.1 wt% were dispersed within the PCM. Additionally, agro-based fibers were laid across the steps to promote uniform water distribution and improve capillary action, reducing dry patches within the basin. A PCM concentrator ratios and SBS were integrated into selected experiments to increase the incident radiation on the basin (Elsheikh et al., 2022). Environmental parameters, including solar intensity, basin water temperature, PCM temperature, glass cover temperature, and ambient conditions, were continuously monitored using calibrated K-type thermocouples and a pyranometer. Distillate output was measured at hourly intervals using a graduated cylinder. The setup was tested under controlled outdoor conditions, ensuring consistent comparisons across different configurations.

A promising development in solar desalination technology is the integration of PCMs and nanofluids with SBS. The use of PCMs provides superior heat storage capacity, enabling the system to sustain evaporation even under low solar intensity by retaining thermal energy for extended periods. Simultaneously, nanofluids significantly enhance thermal conductivity, thereby accelerating the evaporation–condensation cycle and improving overall heat transfer rates. Compared to conventional systems, SBS performance can be markedly improved to achieve higher freshwater yields through the utilization of diverse material combinations. Moreover, this approach is inherently scalable, eco-efficient, and sustainable, offering a viable pathway for next-generation desalination technologies to address global water scarcity while reducing energy consumption and environmental impacts.

Kabeel et al. (2017) reported SBS yield performance (Figure 3a) using nanofluids such as Al₂O₃ (73.80%) and CuO (84.20%). Sadasivuni et al. (2022) and Elango et al. (2015) studied SBS enhanced with nanofluids including SnO₂, Fe₂O₃, ZnO, and Al₂O₃, with Al₂O₃ achieving the highest production efficiency at 29.95%. Asha et al. (2022), Shanmugan (2013), Panchal et al. (2021), Gandhi et al. (2021), and Abdullah et al. (2021) also investigated similar modifications. Dsilva Winfred Rufussa et al. (2018) and Panchal et al. (2021) demonstrated that nano-PCM composites (e.g., paraffin with TiO₂) improved SBS solar thermal efficiency to ~88.64%, Essa et al. (2021) with comparative effectiveness ratios of 100, 100, 88.47, and 76.93%. Chaichan and Kazem (2018) achieved 60% productivity using Al₂O₃ nanofluid with paraffin wax, resulting in a total daily distillate yield of 12,190 mL/m². Kumar et al. (2021), Devi and Shanmugan (2021), Panchal et al. (2021), and Gupta et al. (2016) utilized CuO nanofluids in SBS, reporting a harvest rate of 3.51 L/m²/day at water depths of 50 and 100 mm, further confirmed by Panchal et al. (2021). Nazari et al. (2019) incorporated a thermoelectric channel with Cu₂O nanofluid, achieving power and energy efficiencies of 81, 80.6, and 112.5%, respectively. Lawrence et al. (2022) developed wick-type solar distillers with NiO nanoparticles, reporting efficiencies of 52.5, 5.83, and 36.36% under different conditions. Essa et al. (2020) and Kandeal et al. (2021) incorporated PCMs, nanofluids, and copper chips into distillers, with MDSSS performance improvements of 72, 110.3, 97, 138, 112.5, and 190% (Figure 3b). Shanmugan et al. (2020) and Essa et al. (2020) further validated these findings.

At a nanoparticle mass fraction of 0.3%, the incorporation of CuO and Al₂O₃ nano-enhanced phase change materials (NPCMs) within solar stills augmented thermal efficiency by 55.8 and 49.5%, respectively, while concurrently attenuating peak basin temperatures by 2.1 °C and 1.8 °C. Shanmugan et al. (2018) investigated the synergistic integration of PCMs with nanostructured additives in SBS, wherein copper plates were surface-treated with black paint and Al₂O₃ nanoparticles, with C₁₈H₃₆O₂ employed as the latent heat storage medium. Their study reported performance enhancements of 59.14% under winter operating conditions and 27.13% in summer when coupled with feed-water cooling. Similarly, Manoj Kumar et al. (2022) analyzed three SS configurations: a baseline system utilizing crude wax PCM (PCM-SS), and two nanoparticle-dispersed configurations (NDPCM-SS) containing ZnO-reinforced wax (Sangeetha et al., 2022). The findings indicated freshwater yield increments of 50.24% for PCM-SS and 65.17% for NDPCM-SS. In parallel, Alwan et al. (2021) introduced an absorber design with finned aluminum bars that markedly elevated distillate flux, while Saleh et al. (2024) demonstrated further improvements through geometric optimization. Tuly et al. (2022) reported high distillate production rates, 140 mL/m²/h using aluminum absorber plates and 100 mL/m²/h with glass covers. Comparative trials by Sangeetha et al. (2024) revealed that case 3, with an efficiency of 42.85%, surpassed cases 1 and 2, exhibiting productivity rates of 46.2, 25.9, and 38.1%, respectively, with an energy efficiency improvement of 111% relative to the initial cases. A comprehensive synthesis of SBS configurations incorporating PCMs with diverse thermal storage and nanoparticle additives is compiled in Table 2 (Kabeel et al., 2017; Selimefendigil et al., 2022; Kumar et al., 2023; Chaichan and Kazem, 2018; Shanmugan et al., 2023; Shalaby et al., 2016; Arshad et al., 2011; Gupta et al., 2016; Sharshir et al., 2018; Balan et al., 2011; Mahmoud et al., 2019; Sharshir et al., 2019; Venugopal et al., 2012; Elango et al., 2015; Shanmugan et al., 2012; Modi and Shukla, 2018; Tuly et al., 2022; Manikandan et al., 2013; Grewal and Kumar, 2022; Gangavathi et al., 2022).

Recent innovations in solar desalination leverage advanced material strategies across multiple domains: photothermal absorbers such as carbon blacks, graphite/graphene, CNTs, plasmonic metals (Au, Al), MXenes, black-TiO₂, and selective solar coatings enhance light capture and conversion; thermal management approaches including PCMs, finned absorbers, multilayer insulation, heat pipes, and nano-enhanced PCMs (metal oxides, carbon-based) stabilize temperature fluctuations and extend nocturnal productivity; interfacial evaporators built from foams, aerogels, wood-derived and cellulose-based structures, as well as biopolymeric hydrogels like chitosan/alginate with Janus designs, enable efficient evaporation with directional water transport; fluids and additives such as nanofluids (Al₂O₃, CuO, SiO₂, ZnO, graphene), surfactants, and anti-foaming agents improve heat transfer and surface stability; antifouling and antiscaling measures, including superhydrophilic/superhydrophobic coatings, TiO₂ photocatalytic layers, zwitterionic polymers, and electrostatic salt-rejecting structures, mitigate performance degradation; while condensation efficiency is enhanced by patterned wettability, drop wise-promoting surfaces, thermally conductive foils, and cooled condensers. Finally, diverse wick and substrate choices, jute, cotton, glass fiber, carbon cloth/felt, and melamine sponge, facilitate capillary-driven water transport, enabling robust and scalable solar distillation architectures.

In SBS, cotton sacks filled with sand have been employed as a low-cost thermal energy storage (TES) medium, yielding mean efficiencies of 28.56% with a basin charge of 30 kg and 31.31% with 40 kg, as reported by Dumka et al. (2019). Deshmukh and Thombre (2017) investigated SBS configurations incorporating solvothermal intermediate oils beneath the basin liner, identifying an optimal storage capacity in the range of 8–15% kJ·m⁻²·K⁻¹. The role of pumice stone as a porous TES medium was further examined at incremental loadings of 0, 5, and 10 kg, where efficiency declined from 35.1% (no pumice) to 29.3% (5 kg) and 28.8% (10 kg), respectively (Figures 4, 5). Temporal performance analysis indicated peak solar energy harvesting near solar noon. Bilal et al. (2019), corroborated by Dumka et al. (2019), demonstrated up to 49.17% enhancement in internal efficiency when the feedwater was magnetically conditioned using permanent magnets (Figure 6). Furthermore, Mohamed et al. (2019) evaluated black basalt stones of varying granulometries (1, 1.5, and 2 cm) as alternative TES media in SBS units (Figure 7).

The utilization of diverse energy storage media (ESM) in SBS and CSS has been extensively explored to augment thermal retention and freshwater productivity. Basalt stones of increasing granulometry (1–2 cm) demonstrated progressive gains in solar-to-thermal conversion, with reported efficiencies of 19.8, 27.86, and 33.37%, corresponding to thermal efficiencies of 19.09, 26.84, and 32.07%, respectively. Quartzite was tested by Nagaraj et al. (2020), yielding a modest productivity of 0.05 L·m⁻²·day⁻¹, whereas Subbarama Kousik et al. (2021) observed average efficiencies of 34.9% with marble spheres, producing 22.8% on day one and 21.3% over 2 days. Attia et al. (2021) employed cotton sacks packed with phosphate, achieving freshwater outputs of 4.52 kg (1 cm depth) and 3.99 kg plus 0.88 kg (2 cm depth), translating to output improvements of 28 and 22.5%, respectively (Figure 8). In parallel, Kabeel et al. (2019) demonstrated that red-brick–cement composites could elevate potable water production by up to 150% compared to CSS at optimal water mass loading (>20 kg), coupled with a 25% enhancement in evaporative heat transfer relative to Dunkle’s model. Graphite has also emerged as a highly conductive ESM, with Kabeel et al. (2018) reporting daily water productivity of 7.731 L·m⁻² and thermal efficiencies approaching 60%. Dhivagar et al. (2022) emphasized the techno-economic and enviro-economic feasibility of SBS employing magnetic media, where block magnet solar stills (BMSS) achieved 3.15 kg·m⁻² of output, outperforming disc magnet solar stills (DMSS, 2.82 kg·m⁻²) and CSS (2.15 kg·m⁻²), with a 20.7% gain in saline water temperature and 5.8% cumulative productivity advantage. Sakthivel and Arjunan (2018) reported that cotton cloth (6 mm) functioned effectively as a passive heat sink, elevating SBS efficiency to 23.8%. Mohammed et al. (2022) demonstrated the utility of indigenous clay as an ESM, achieving 5.39 L·m⁻² of output with a 38.7% increase in yield and 47% daily efficiency. Similarly, Dhivagar et al. (2020) optimized gravel-based TES at 1 cm depth, reaching 32% efficiency, while Attia et al. (2020) incorporated aluminum spheres in photovoltaic-integrated SBS systems, achieving 5.09 kg of freshwater daily with efficiency gains from 31.6 to 40.1% (Figure 9). Harris Samuel et al. (2016) investigated salt-filled spherical capsules coupled with sponge matrices, producing 2.7 kg·m⁻² in SBS compared to 2.2 kg·m⁻² in CSS (Figure 10). Finally, Essa et al. (2020) introduced a coffee-derived colloidal medium, which enhanced productivity by 35.14% over CSS, with SBS exhibiting a 26.96% higher heat transfer coefficient and 35.34% superior energy efficiency (Figure 11).

Table 3 (Tiwari and Tiwari, 2006; Badran and Al-Tahaineh, 2005; Phadatare and Verma, 2007; Tiwari et al., 2009; Kumar and Tiwari, 2008; Tiwari, 1987; Nafey et al., 2000; Feilizadeh et al., 2016; Modi and Nayi, 2020; Kabeel et al., 2019) displays the cooling and water depth analysis for SBS. Energy Storage of Bio-Nanoparticle-Enhanced PCM for Solar Desalination does not directly analyze the engineering scalability of SBS systems. However, the bibliometric insights reveal emerging research on modular thermal storage, hybrid configurations, and techno-economic optimization, which collectively indicate potential pathways for future large-scale and community-based implementations. In this study, the focus is on mapping the scientometric landscape rather than conducting direct experimental synthesis. However, the reviewed literature highlights the incorporation of bio-nanoparticles such as ZnO, TiO₂, Al₂O₃, SiO₂, and carbon-based nanostructures within organic and inorganic PCMs to improve thermal conductivity, latent heat storage, and stability. These nanoparticles are often surface-functionalized or dispersed using biopolymer stabilizers (e.g., chitosan, cellulose) to enhance compatibility and reduce agglomeration. The scientometric analysis reveals that ZnO- and Al₂O₃-based bio-nanoparticle composites are most frequently reported for solar desalination, owing to their superior thermal conductivity, photothermal conversion efficiency, and cost-effectiveness. Thus, our framework contextualizes how these nanoparticle-enhanced PCMs are advancing energy storage for solar desalination, even though experimental validation is beyond the scope of this work.

Research on solar desalination has evolved from simple conventional solar stills (CSS) toward advanced passive, active, and hybrid configurations aimed at overcoming low productivity and efficiency challenges. Passive designs include single- and double-slope basin stills, stepped and wick-type systems, while active and hybrid systems integrate flat plate collectors, evacuated tube collectors, PV/T units, or waste-heat recovery to enhance thermal energy input. Recent studies emphasize nanomaterials, photothermal coatings, and PCMs to improve heat absorption, thermal storage, and nocturnal productivity. Interfacial evaporators such as carbon-based foams, aerogels, and hydrogels, along with nano-enhanced PCMs (Al₂O₃, CuO, ZnO, graphene), are widely explored for boosting evaporation and maintaining stability under varying solar intensity. Parallel developments target condensation enhancement using hydrophobic/hydrophilic surface patterning, dropwise promoters, and subcooling techniques to increase distillate recovery. Advanced systems such as humidification–dehumidification (HDH), membrane distillation (MD), and solar-driven RO hybrids extend the technology landscape toward higher output and scalability. Performance is typically reported in terms of daily freshwater yield (L·m⁻²·day⁻¹), thermal/exergy efficiency, water quality compliance, and levelized cost of water (LCW). However, the field still faces gaps in standardization, long-term outdoor stability, fouling and scaling resistance, and economic viability. Emerging trends highlight salt-rejecting evaporator structures, dropwise condensation surfaces, nano-enhanced PCMs, and AI-driven optimization as promising solutions for reliable 24-h productivity. Despite progress, further research is required in scaling control, brine management, and durable low-cost materials, especially for community-scale deployment in water-stressed regions.

Hydrogel/aerogel evaporators with broadband absorption and self-salt-excretion; advanced condensation strategies like patterned wettability and heat-pipe-assisted cooling; salt-rejecting architectures that enable continuous purging; nano-enhanced PCMs for nocturnal yield and thermal stability; and hybrid solar desalination systems that combine energy and water production are all examples of how state-of-the-art solar desalination is progressing. Bottlenecks continue to exist despite encouraging laboratory results because of a lack of standardised test procedures, a dearth of long-term field testing, difficulties managing brine and scaling, the stability of condensation in humid environments, and limitations related to cost and manufacturing. Uniform feed water and weather conditions, multi-day outdoor testing with cloudy periods, and open reporting of yields, temperatures, salt balances, uncertainties, and manufacturing details are all important components of comparative reporting. Future prospects include low-cost condenser upgrades, community-scale hybrid solar still systems with storage, PCM-used basin area retrofits for historic stills, long-lasting salt-rejecting IPE systems verified over >90 days.