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Solar still

A solar still is a simple, passive device that harnesses solar energy to desalinate or purify water through evaporation and condensation, typically consisting of a shallow basin containing saline, brackish, or contaminated water covered by a transparent lid, such as glass or plastic, where heated water vapor rises, cools, and collects as fresh distillate.^[1]^[2] This process mimics the natural hydrological cycle, utilizing the greenhouse effect to trap solar radiation and achieve thermal efficiency ranging from 25% to 50%, with typical daily yields of 2 to 5 liters per square meter under optimal sunny conditions.^[3]^[4] Solar stills have a long history, with evidence of solar evaporation techniques used over 2,000 years ago for salt production and documented distillation methods emerging in the 16th century among Arab alchemists, evolving into large-scale installations like the 1872 greenhouse-style still in Chile that supplied fresh water to a mining community.^[4]^[2] During World War II, over 200,000 inflatable solar stills were mass-produced for U.S. Navy life rafts to provide emergency potable water.^[4] Modern designs classify solar stills into passive types, which rely solely on direct sunlight; active types, incorporating external heat sources like flat-plate collectors or solar dishes to boost evaporation; and hybrid systems, such as those integrated with photovoltaic-thermal (PV/T) panels for simultaneous water production and electricity generation.^[1]^[5] These devices are particularly valuable in addressing global water scarcity, offering a low-cost, sustainable solution for remote rural areas, coastal regions, small islands, and disaster relief scenarios where access to clean water is limited, with applications extending to agriculture, hospitals, and battery maintenance.^[1]^[4] Recent advancements, including the use of phase-change materials (PCMs) for heat storage, nanotechnology-enhanced surfaces for improved absorption, and 3D interfacial heating to minimize energy losses, have increased productivity up to 24 L per square meter per day in some optimized prototypes.^[5]^[2]^[6] Despite their simplicity and minimal maintenance needs, challenges like low output in cloudy climates and material degradation continue to drive research toward commercialization and broader adoption.^[1]

Principles of Operation

Basic Mechanism

A solar still is a simple distillation device that harnesses solar energy to evaporate saline or impure water, subsequently condensing the vapor to produce fresh water.^[7] The fundamental process begins with solar radiation penetrating a transparent cover and heating the water contained in a basin, which raises its temperature and induces evaporation. The resulting water vapor, being lighter, rises toward the cooler cover—typically made of glass or plastic—where it condenses into droplets due to the temperature differential. These droplets then trickle down the inclined surface of the cover into a collection trough or channel for harvesting as distilled water.^[8] Key components of a basic solar still include the basin, which holds and absorbs heat for the water; the transparent cover, providing the condensation surface; insulation surrounding the basin to reduce heat losses to the environment; and the collection system, which channels the condensate for storage.^[9] Heat transfer modes in the device encompass radiation, which delivers solar energy through the cover to heat the basin and water; conduction, transferring heat from the basin material to the water; and convection, facilitating the movement of humid air and vapor within the enclosed space toward the cover.^[10] The core energy balance for evaporation is expressed as

Q = m \cdot L

where Q represents the heat input to the system, m is the mass of water evaporated, and L is the latent heat of vaporization of water, approximately 2260 kJ/kg.^[11]^[12]

Thermodynamic Processes

The thermodynamic processes in a solar still involve intricate heat and mass transfer mechanisms that govern the distillation of water through solar energy. The primary energy input is solar irradiance I, which is absorbed by the basin water surface with absorptivity \alpha, yielding useful heat \alpha I. This absorbed energy is balanced against various losses: convective heat transfer to the inner cover surface with coefficient h_c, evaporative heat transfer with coefficient h_e, radiative heat transfer with coefficient h_r, and conductive heat transfer to the basin base with coefficient h_k. These terms form the core of the energy balance equation for the water body:

\alpha I = h_c (T_w - T_{gi}) + h_e (T_w - T_{gi}) + h_r (T_w - T_{gi}) + h_k (T_w - T_b)

where T_w is the basin water temperature, T_{gi} is the inner glass cover temperature, and T_b is the basin temperature. This balance ensures that the net heat drives the phase change from liquid to vapor.^[13] Mass transfer occurs primarily through the diffusion of water vapor across the air gap between the evaporating water surface and the condensing cover, approximated by Fick's law of diffusion. The vapor flux is driven by the humidity gradient, which correlates with the partial pressure difference P_w - P_{gi} (saturation pressures at T_w and T_{gi}), leading to the evaporative heat transfer coefficient h_e \approx 0.0163 h_c \frac{P_w - P_{gi}}{T_w - T_{gi}}. This process quantifies the rate at which water molecules migrate via molecular diffusion in the humid air layer, with the air gap thickness influencing the diffusion path length. The temperature difference T_w - T_{gi} is crucial, as it establishes the vapor pressure gradient essential for sustained mass flow toward the cooler cover.^[13]^[14] During evaporation at the basin surface, sensible heat from the water is converted to latent heat of vaporization L, facilitating the phase change to vapor without significant temperature rise. Conversely, at the cover's inner surface, the incoming vapor condenses, releasing latent heat that warms the cover through sensible heat transfer. This asymmetric phase change dynamic maintains the temperature gradient, with the daily distillate yield Y approximated as Y = \frac{h_e A (T_w - T_{gi})}{L} integrated over the daylight hours, where A is the basin area. Such models, rooted in foundational analyses, underscore the efficiency limits imposed by these coupled heat and mass transfers.^[13]

Historical Development

Early Inventions

Documented European interest in solar stills emerged in the 16th century, with contributions from Arab alchemists using distillation methods and Italian scholar Giovan Battista della Porta describing one of the first designs in his 1589 treatise Magia Naturalis.[^[15]] Della Porta's apparatus involved wide earthen pots exposed to sunlight to evaporate seawater or brackish water, with the vapor condensing on a sloped cover and collecting as fresh distillate; this marked a foundational step in conceptualizing solar-powered purification for practical use, such as salt production or desalination. By the 18th century, similar evaporators appeared in European patents and descriptions, including adaptations for industrial salt extraction, though large-scale implementation remained limited by material constraints.^[15] A pivotal advancement occurred in the 19th century with the patent and construction of basin-style solar stills by Swedish engineer Carlos Wilson around 1870. In 1872, Wilson oversaw the erection of the world's first large-scale solar distillation plant at Las Salinas, Chile, comprising 64 interconnected basins totaling approximately 4,700 m² to desalinate brackish water for a remote nitrate mining community. This facility, which initially produced up to 23,000 liters of fresh water daily, demonstrated the viability of solar stills for sustained community supply in isolated, drought-prone areas and influenced subsequent adoptions in colonial outposts facing water shortages, such as mining operations in arid zones of Australia and supply efforts during famines in India. The technology also gained traction for maritime survival following notable 19th-century shipwrecks, where improvised evaporation methods in lifeboats underscored the need for portable distillation amid prolonged sea ordeals.^[16]^[17] Key figures like Wilson highlighted the shift toward engineered solutions. These early inventions laid the groundwork for solar stills as a low-cost, passive technology, particularly impactful in colonial contexts where droughts exacerbated water scarcity for settlers and indigenous populations alike.^[17]

Modern Evolution

In the early 20th century, solar still technology transitioned from experimental prototypes to practical military applications, particularly during World War II. Hungarian-American inventor Mária Telkes developed a portable solar still for the U.S. Navy in the 1940s, designed as an inflatable device using clear plastic to desalinate seawater for stranded sailors and airmen in the Pacific theater.^[18] These stills were integrated into life rafts and became standard emergency equipment, producing up to 0.95 liters of fresh water per day per unit under optimal conditions.^[19] Post-World War II, a research boom propelled solar still development, driven by global water scarcity concerns and international collaboration. These efforts emphasized cost-effective scaling, though operational challenges like maintenance in dusty environments limited widespread adoption.^[20] From the late 20th to early 21st century, material innovations reduced costs and improved durability. In the 1970s, a shift to plastic sheeting and liners replaced glass and metal components in basin stills.^[21] By the 1980s, integration with greenhouses emerged as a key advancement, with designs incorporating solar still roofs to provide distilled water for hydroponic agriculture, as demonstrated in transient analysis studies showing enhanced year-round productivity in temperate climates.^[22] Key milestones included 2000s patents for multi-effect stills, such as diffusion-coupled systems that reused latent heat across stages to boost output by 2-3 times compared to single-effect models.^[23] In the 2010s to 2025, hybrid solar-thermal systems marked significant progress, combining photovoltaic panels with thermal desalination for off-grid scalability.^[24] Amid climate change pressures, focus shifted to sustainable materials, including bio-based polymers and recycled composites for absorbers, reducing environmental impact while improving heat retention in floating and wick-enhanced designs.^[25]

Types of Solar Stills

Basin and Pit Stills

The basin solar still represents one of the simplest passive designs for solar desalination, consisting of a horizontal tray painted black to maximize solar absorption, positioned beneath a sloped transparent glass cover that allows condensation to trickle down into a collection trough. Typical units feature a basin area of 1 to 2 m² and a water depth of 5 to 10 cm to optimize evaporation while minimizing heat capacity. In sunny conditions, such stills can produce 3 to 5 L of distilled water per m² per day, depending on solar insolation and ambient factors. Construction commonly employs durable materials like concrete or galvanized metal for the basin liner to withstand corrosion from saline water, with the glass cover inclined at 10 to 30 degrees for efficient condensate flow.^[9]^[26]^[9] The pit solar still, a below-ground variant suited for emergency or remote applications, involves excavating a hole approximately 0.5 to 1 m deep and 1 to 2 m wide, lining it with a waterproof plastic sheet, and covering it with a transparent plastic or glass sheet secured at the edges with soil or rocks to create a tight seal. Vegetation or moist soil placed in the pit enhances evaporation, while a central collection container captures the condensate formed on the underside of the cover; local materials such as leaves can provide additional insulation around the edges. The surrounding soil acts as natural insulation, helping to retain heat and reduce nighttime losses compared to above-ground designs. These units typically yield 1 to 3 L of water per day under optimal solar exposure, making them less productive but highly portable for individual use.^[27]^[27] Basin stills offer scalability for community-level deployment, as seen in early large-scale installations, while pit stills excel in portability for personal survival scenarios but suffer from lower yields and labor-intensive setup. Historically, basin stills gained prominence with the construction of pioneering plants in Chile during the late 19th century, such as the 1872 Las Salinas facility producing up to 22.5 m³/day across 4,757 m², and interest in such designs revived globally in the 1950s for centralized desalination efforts.^[28]^[29]

Inclined and Vertical Stills

Inclined solar stills feature a sloped transparent cover, typically at an angle of 15° to 30° relative to the horizontal, which facilitates gravity-assisted flow of condensed water droplets toward a collection trough at the lower end.^[30] The basin liner is tilted at a similar angle to ensure even distribution of saline water and minimize stagnant pooling, enhancing evaporation rates compared to flat basin designs where condensate may accumulate and reduce efficiency.^[31] This geometric adjustment can increase daily productivity by 20% to 40% over conventional horizontal basin stills by promoting faster drainage and reducing thermal losses from pooled water.^[31] Vertical solar stills employ upright sidewalls paired with vertical glazing to create a compact evaporation chamber, allowing for efficient condensation on the inner surfaces without relying on significant tilt.^[32] These designs are particularly suited for stacked multi-stage configurations, where multiple evaporation-condensation cycles are arranged vertically to amplify output in limited space, often integrated into rooftop systems for urban or remote applications.^[32] Construction of inclined stills commonly incorporates lightweight materials such as PVC sheets for the cover or fiberglass-reinforced plastic for the basin to reduce structural weight while maintaining durability against environmental exposure.^[33] Vertical stills, by contrast, favor modular aluminum frames with polycarbonate glazing for easy assembly and scalability in urban desalination setups, enabling quick deployment on building rooftops or portable units.^[32] Despite these advantages, both inclined and vertical stills face challenges from higher wind-induced material stress, particularly on upright or angled surfaces, which can lead to deformation or misalignment over time.^[34] Precise sealing of joints and glazing is essential to prevent vapor leaks and contamination, requiring robust adhesives or gaskets that withstand prolonged solar exposure and temperature fluctuations.^[34]

Wick and Transpiration Stills

Wick solar stills utilize absorbent materials to enhance evaporation by increasing the exposed surface area of water beyond the limitations of a simple basin. These designs employ porous fabrics such as cotton or jute cloth, which are draped over the basin or suspended from the cover frame, allowing water to be drawn upward through capillary action for continuous exposure to solar radiation.^[35] This configuration effectively enlarges the evaporation area by approximately 2-3 times compared to conventional basin stills, as the wick material spreads a thin film of water that promotes rapid vaporization.^[36] The increased surface area typically boosts distillate yield by 30-50%, with experimental setups demonstrating daily outputs of 5-7 L/m² under optimal conditions.^[37]^[38] In construction, the wick is typically black-dyed to absorb more solar energy and hung in a manner that keeps the lower end in contact with the basin water, ensuring sustained wetting via capillary rise while the upper portions remain exposed for evaporation.^[39] Condensation occurs on the inclined cover as in basic mechanisms, with droplets collecting in a trough. Wick stills were notably tested in 1980s experiments in arid zones, where they proved effective for low-water-volume scenarios by maximizing efficiency without requiring deep basins.^[40] Transpiration stills build on similar principles by using structures inspired by plant evaporation to facilitate water vapor generation, often incorporating biomimetic materials that mimic natural transpiration processes for enhanced efficiency in passive desalination. These designs leverage high surface area and capillary action akin to wicks but draw from biological evaporation mechanisms to improve vapor production without relying on living plants. Experimental prototypes have shown potential for improved yields through such nature-inspired enhancements.^[41]

Reverse and Multi-Effect Stills

The reverse solar still adopts an inverted configuration, where saline water trickles down the underside of a blackened transparent cover acting as the evaporator, while the underlying basin functions as the condenser. This upside-down setup minimizes the distance vapor must travel before condensing, thereby enhancing condensation rates. Developed in the mid-1990s, this design was first detailed by Badran and Hamdan, who demonstrated its operation through experimental testing under varying solar radiation and flow rates.^[42] The system employs insulated trays or channels to direct the thin water film along the cover, preventing premature dripping and ensuring efficient heat absorption from solar radiation passing through the transparent material. Compared to single-basin stills, the reverse configuration achieves productivity gains of approximately 18%, with daily yields reaching 2.8 L/day under optimal tilt angles of 47°.^[43] Multi-effect solar stills utilize a cascaded arrangement of multiple evaporation-condensation stages, where the latent heat released from condensation in one basin preheats the saline water in the subsequent stage, improving overall energy utilization. Common designs feature 2 to 5 stacked basins, with vapor from the upper stage transferring heat passively to the lower one via conduction or natural convection, often incorporating inter-stage partitions for separation. This passive approach, avoiding external energy inputs, typically yields 8-12 L/m²/day, as evidenced by analyses under arid conditions showing up to 10.7 L/m²/day for two-effect systems.^[44] Design elements include insulated stacking to minimize lateral heat loss and, in some variants, embedded heat pipes to facilitate inter-stage thermal transfer without mechanical components. Early concepts trace to the 1960s, but modern implementations in the 2020s, such as thermally localized multistage devices, have scaled to 10 stages with production rates exceeding 5 L/m²/hour under one-sun illumination, demonstrating potential for higher throughput in passive setups.^[45] Despite these advances, multi-effect stills face challenges in maintaining airtight seals between stages, which can result in 10-20% vapor leakage and reduced efficiency. Such issues necessitate precise fabrication, often using silicone gaskets or welded joints, to preserve pressure differentials essential for continuous operation. In regions like the Mediterranean, experimental plants incorporating multi-effect designs have achieved daily outputs up to several thousand liters through modular scaling, though commercial adoption remains limited by construction complexity.^[46]

Enhancements and Modifications

Additives and Materials

Additives such as dyes are commonly introduced to the basin water of solar stills to enhance solar radiation absorption, thereby increasing the absorptivity of the water surface. Black ink, for instance, serves as an effective dye that darkens the water, significantly increasing solar radiation absorption and improving photothermal conversion, with reported productivity increases of up to 45%.^[9] Advanced materials, including nanoparticles, further augment the thermal performance of solar stills by improving heat transfer within the basin. Aluminum oxide (Al₂O₃) and titanium dioxide (TiO₂) nanoparticles, when applied as coatings or dispersed in the water, boost thermal conductivity by up to 25% compared to base fluids, leading to higher basin temperatures and greater distillate yields.^[47] Phase change materials (PCMs), such as paraffin wax, are integrated to store excess solar heat during peak daylight hours and release it gradually, extending the evaporation process into the evening when solar input diminishes.^[48] These materials are typically applied through simple mixing or embedding techniques to ensure compatibility with passive solar still designs. For nanoparticles, concentrations of 0.1-1% by volume are mixed directly into the basin water, promoting uniform dispersion and avoiding agglomeration that could reduce efficacy.^[49] PCMs like paraffin are embedded in the basin walls or as thin layers beneath the absorber plate, leveraging their typical latent heat capacity of 150-250 kJ/kg to maintain elevated temperatures without external power.^[48] In the 2020s, innovations have focused on nanomaterial-enhanced surfaces, such as graphene-modified absorbers and covers that improve wettability and condensation dynamics, potentially increasing efficiency by up to 49%.^[50] Recent anti-fouling coatings using nanoparticles have emerged to reduce biofouling in saline environments, maintaining performance for up to 3 months.^[51] The integration of PCMs has demonstrated particularly notable impacts, boosting daily distillate output by 25-40% in low-sunlight regions by prolonging effective operating hours beyond direct solar exposure.^[48]

Hybrid and Active Systems

Hybrid systems combine traditional passive solar stills with photovoltaic (PV) panels to power auxiliary mechanisms such as electric stirring devices, fans, or pumps, thereby enhancing evaporation and circulation beyond reliance on natural convection alone. For instance, integrating a solar-electric pump to circulate water in the basin can increase distillate yield by 50-100% compared to passive configurations, as demonstrated in experimental setups where PV-driven circulation raised productivity from baseline levels of 3-5 L/m²/day to 6-10 L/m²/day under similar conditions.^[52] Active solar stills incorporate optical or mechanical elements to intensify solar input or airflow, including mirrors and parabolic concentrators that focus sunlight onto the basin for a concentration gain factor of 2-5 times the direct irradiance. These systems often employ internal fans powered by small PV modules to induce forced convection, accelerating vapor removal and condensation rates. Additionally, reflector arrangements in active designs can elevate effective irradiance on the water surface to approximately 1000 W/m², even under variable weather, leading to improved thermal gradients and higher output.^[53] In PV-hybrid designs, photovoltaic panels rated at 10-20 W per m² of still area are commonly integrated to drive low-power components like 5-10 W fans or pumps without exceeding the system's energy balance. Recent prototypes, such as the 2024 enhanced hybrid solar desalination system utilizing PV for augmented evaporation, have achieved daily yields of up to 15 L/m² under optimized conditions, showcasing potential for scalable deployment in remote areas.^[54] As of 2025, further enhancements include thermal membrane distillation integrated with stepped solar stills, improving yields by up to 50% in experimental setups.^[55] As of 2024, initial setup costs for hybrid and active systems are typically 20-50% higher than passive stills, with production costs around $0.009-0.09 per liter depending on configuration, enabling short payback periods through enhanced productivity.^[54]^[56]

Performance and Efficiency

Influencing Factors

The performance of solar stills is fundamentally driven by thermodynamic processes involving solar heating, evaporation, and condensation, with various environmental and operational variables significantly impacting the yield of distilled water. Among environmental factors, solar irradiance serves as the primary energy input, typically reaching peak values of 800-1000 W/m² under clear skies, directly correlating with higher evaporation rates and overall productivity.^[57] Ambient temperature also plays a crucial role, as rising from 20°C to 40°C can approximately double the distillate yield by enhancing the temperature differential between the basin water and the cover. Wind speed contributes by promoting convective heat loss from the cover, potentially increasing yield by 10-20% per m/s through improved cooling and condensation efficiency.^[57] Conversely, high humidity diminishes the vapor pressure gradient essential for condensation, reducing yield by up to 40%.^[57] Operational parameters further modulate efficiency; for instance, maintaining water depth at an optimal 1-5 cm balances heat absorption and evaporation without excessive sensible heat storage that delays vaporization, while depths exceeding this can reduce yield by about 30% due to prolonged heating times.^[57] The cover tilt angle, when adjusted to match the site's latitude, maximizes solar exposure year-round, optimizing the angle of incidence for incident rays.^[57] Material properties influence heat transfer dynamics, with basin surfaces exhibiting high absorptivity greater than 0.9—achieved through black coatings—maximizing solar energy capture and minimizing reflection losses.^[57] Similarly, an air gap spacing of 5-10 cm between the water surface and the condensing cover minimizes conductive and convective heat losses while facilitating vapor diffusion.^[58] Geographic location affects baseline performance, with equatorial regions benefiting from consistent high irradiance to achieve daily yields of 6-8 L/m², compared to 2-3 L/m² in temperate zones where seasonal variations limit solar input.^[57] These factors often interact; for example, elevated humidity in humid equatorial areas can counteract the advantages of high irradiance, underscoring the need for site-specific considerations in deployment.

Measurement and Optimization

The performance of solar stills is quantified primarily through daily yield, expressed as the volume of distilled water produced per unit basin area (L/m²/day), which typically ranges from 2 to 5 L/m²/day for passive single-slope designs under standard conditions.^[9] Thermal efficiency (η) serves as a key metric, calculated as the ratio of the energy used for evaporation to the total incident solar energy, given by the formula:

\eta = \frac{Y \cdot L \times 1000}{I \times 86400}

where Y is the daily yield in kg/m², L is the latent heat of vaporization (approximately 2257 kJ/kg), I is the average daily solar irradiance in W/m², and the factors 1000 and 86400 account for unit conversions from kJ to J and seconds in a day, respectively.^[59] Measurements rely on instruments such as pyranometers to record solar irradiance with high accuracy (up to 1280 W/m² range) and thermocouples to monitor temperatures of water, vapor, and surfaces, enabling precise data logging via multi-channel acquisition devices.^[60]^[61] Optimization of solar stills involves computational techniques like computational fluid dynamics (CFD) modeling, which has become standard in the 2020s for simulating heat and mass transfer flows, often yielding 10-20% improvements in productivity through virtual design iterations without physical prototypes.^[62] Machine learning algorithms further enable site-specific predictions by training on local climate data (e.g., irradiance and humidity) to forecast yields and recommend tailored parameters like basin depth or cover tilt, enhancing adaptability in diverse environments such as humid subtropics.^[63]^[64] Experimental setups for evaluation typically involve controlled outdoor or indoor tests, systematically varying factors like water depth (e.g., 4-10 cm) and cover inclination (20-60°) to assess impacts on yield, with results validated using thermal modeling software such as TRNSYS for transient simulations of multi-effect systems.^[65]^[66] Studies from 2024-2025 have employed AI optimization for pyramid solar stills with thermal storage, achieving efficiency improvements of up to 50%, while other designs integrated with textiles have attained thermal efficiencies around 62%.^[67]^[68] Benchmarks indicate that passive basin solar stills generally attain thermal efficiencies of 30-40%, while active hybrid systems, incorporating external energy inputs like flat-plate collectors, reach 50-80% under optimized conditions.^[9]^[69]

Applications

Desalination in Arid Regions

Solar stills have been deployed in arid regions of the Middle East as modular basin and multi-effect systems to desalinate seawater and brackish water, providing a sustainable alternative to energy-intensive conventional methods. For instance, experimental multi-effect solar desalination systems with capacities up to 10 m³/day (10,000 L/day) have been commissioned and tested, demonstrating feasibility for scaling in water-scarce areas like those in the Arab region.^[70] These modular designs allow for easy expansion, with units integrated into larger plants to address chronic water shortages in countries such as Saudi Arabia and Oman, where solar irradiation exceeds 2,000 kWh/m² annually. In agricultural applications, greenhouse-integrated solar stills combine water production with crop cultivation in arid environments, utilizing the still's structure to capture solar heat for both evaporation and controlled humidity. Such systems reduce cooling loads by absorbing excess irradiance, enabling year-round farming in regions like Egypt's coastal deserts, where they produce irrigation water while minimizing evaporation losses from open fields.^[71]^[72] Case studies in remote Egyptian communities show these hybrids supporting sustainable plantations by desalinating brackish groundwater on-site, with outputs directed toward hydroponic setups.^[73] Process adaptations for arid desalination include pre-filtration stages to mitigate salt buildup on evaporation surfaces, ensuring consistent operation with seawater at 35 ppt salinity. Sedimentation and coarse filters remove particulates before feeding into the still, preventing scaling that could reduce efficiency by up to 30%. Yields typically range from 4 to 10 L/m²/day under high solar conditions, with advanced designs achieving 7.4 L/m²/day through enhanced heat transfer.^[74]^[75]^[76] In India, particularly Gujarat's coastal villages, solar still projects from 2015 onward have provided community-scale desalination for smaller groups by integrating stills into rural water supply networks. The Awania plant in Gujarat, operational since 1979, exemplifies long-term use, producing potable water for non-electrified areas facing brackish groundwater issues. A 2018 pilot initiative in Olpad, Gujarat, leverages solar thermal desalination to supply clean water to a village serving around 300 residents, with economic viability estimated at $0.01–0.02/L due to low capital and operational costs compared to grid-powered alternatives.^[77]^[78]^[79] The sustainability of these deployments stems from zero ongoing energy input, relying solely on passive solar heating, and low maintenance requirements, with systems lasting 5–10 years before major refurbishments like liner replacements. In arid settings, this eliminates fuel dependencies, reducing carbon emissions while providing reliable output with minimal intervention beyond periodic cleaning.^[9]^[26] Scale-up efforts involve transitioning from single units to large arrays covering up to km², as seen in conceptual modular expansions for MENA regions, incorporating automated cleaning mechanisms like brush systems or coatings to combat dust accumulation without halting production. These arrays enhance overall yield proportionally, with prototypes showing 20–50% efficiency gains through interconnected multi-effect configurations.^[80]^[81]

Survival and Emergency Use

Solar stills play a vital role in survival and emergency situations, providing a simple, low-tech method for individuals or small groups to produce potable water from limited or contaminated sources. Portable designs, such as pit and wick stills, emphasize ease of construction using readily available materials like tarps, plastic bottles, or ponchos, requiring no specialized tools or electricity. These devices leverage solar energy to evaporate and condense water, yielding typically 0.5 to 2 liters per day per unit under optimal conditions, sufficient for basic hydration needs in crises.^[82]^[83] In desert survival scenarios, such as those faced by trekkers in arid environments like the Sahara, pit stills constructed by digging a shallow hole (approximately 1 meter wide and 0.5 meters deep), lining it with moisture-rich vegetation, and covering it with a clear plastic sheet prove essential for extracting water from soil or plants. A military poncho can serve as an improvised cover, secured with rocks and weighted in the center to create a drip point into a collection container, potentially producing 1-2 liters daily for one person when sunlight is abundant. Wick stills, using absorbent materials like cloth or sponges soaked in brackish water and draped over a frame under plastic sheeting, offer a compact alternative for mobile users, enhancing evaporation through capillary action.^[82]^[84] During disaster relief efforts, including the widespread flooding in Asia during the 2020s that contaminated freshwater supplies, portable solar stills enable on-site purification for displaced populations. For instance, bottle-based wick designs—where a halved plastic bottle holds feedstock and a wick draws liquid to the sun-exposed surface—allow quick setup in refugee camps or flooded areas, producing small volumes of clean water without fuel. Techniques involving urine or plant sap as feedstock are particularly useful; solar evaporation distills these into safe drinking water by leaving behind salts and toxins, while transpiration methods, such as enclosing non-toxic green vegetation in a sealed plastic bag or pit, harness natural plant moisture for detoxification and yield up to 1 liter per day from sap-rich sources.^[82]^[85]^[86] Historically, solar stills have saved lives in maritime emergencies; during World War II, inflatable lifeboat stills developed by engineer Maria Telkes provided U.S. Navy crews adrift in the Pacific with distilled seawater, yielding about 0.5 liters per unit daily and contributing to the survival of numerous stranded personnel. In modern contexts, compact units like the Aquamate solar still are included in emergency kits distributed by organizations akin to FEMA, as a long-established inflatable design that produces 0.25 to 1.8 liters per day, ideal for individual backpacks or small-group aid packages.^[87]^[83] Despite their utility, solar stills have limitations in emergency use, with maximum outputs of 0.5-3 liters per day constrained by sunlight exposure—typically requiring 4-6 hours of direct sun for effective operation—and environmental factors like humidity or cloud cover, making them unsuitable as a sole long-term solution.^[82]^[83]

Wastewater Treatment

Solar stills have been adapted for wastewater treatment through designs such as multi-effect systems and wick-type configurations, often incorporating pre-sedimentation to remove suspended solids before evaporation.^[88]^[89] In these adaptations, contaminated water from domestic or industrial sources is first allowed to settle in a pre-treatment basin to minimize clogging and enhance evaporation efficiency, followed by solar heating in the still.^[90] The evaporation process effectively removes organics and heavy metals, achieving up to 99% purity in the distillate by leaving non-volatile contaminants concentrated in the basin for subsequent disposal.^[88]^[91] During operation, volatile compounds may evaporate alongside water vapor but can be separated through selective condensation in multi-effect stages, where cooler surfaces promote pure water droplet formation while volatiles remain in vapor form or are minimized by thermal decomposition.^[92] Non-volatile residues, including salts, metals, and organics, accumulate in the basin, allowing for their concentrated removal and proper disposal, thus preventing environmental release.^[93] This phase-change mechanism ensures high contaminant rejection, with total dissolved solids (TDS) reduced from levels exceeding 5000 ppm to potable or reusable standards below 100 ppm.^[91] In rural India during the 2010s to 2025, solar stills have been deployed for greywater treatment, yielding 3-5 L/m² of clean water daily from inputs with up to 5000 ppm TDS, supporting household and agricultural reuse in water-scarce regions like Rajasthan.^[94] Similarly, pilot projects in Bangladesh's textile industry have utilized solar evaporative stills to treat garment wastewater, reducing chemical oxygen demand (COD) by over 86% and enabling effluent reuse in non-critical processes.^[95]^[96] Solar exposure in these systems provides ultraviolet (UV) radiation that complements thermal disinfection, further inactivating pathogens during evaporation.^[97] The resulting distillate is typically suitable for non-potable applications, such as irrigation or industrial cooling, meeting standards for limited human contact.^[90] Treatment efficiency for pathogens reaches 90-99% removal of coliforms and indicators, with studies confirming near-complete elimination of viruses and bacteria through combined heat and UV effects.^[98] For small-scale plants, operational costs are approximately $0.0165-0.05 per liter, making it viable for decentralized deployment in developing regions.^[99]^[90]

Remote and Off-Grid Deployment

Solar stills are particularly valuable in remote and off-grid environments where access to conventional water treatment infrastructure is limited, such as isolated research bases, expeditionary operations, and nomadic lifestyles. These devices operate passively or with minimal active enhancements, relying solely on solar energy without the need for fuel or electricity grids, making them ideal for locations with unreliable power supplies. In such settings, hybrid active solar stills integrate auxiliary components like flat-plate collectors or photovoltaic panels to boost evaporation rates, often paired with battery storage to power fans or pumps during low-sunlight periods. For instance, in polar regions like Antarctic research stations, renewable energy systems incorporating solar thermal technologies support off-grid energy needs, with water production via other desalination methods supplementary to broader strategies.^[9]^[100] Portable designs, including inclined and wick-type models, enhance deployability in mobile scenarios. Inclined solar stills, which tilt the basin to optimize solar incidence, achieve yields of approximately 4.1 L/m²/day in experimental remote setups, while wick variants using absorbent materials like jute or cotton facilitate higher evaporation through capillary action, increasing productivity by up to 25% compared to basin types. These wick stills are lightweight and collapsible, suitable for military patrols or nomadic groups, where no-fuel operation reduces logistical burdens in harsh terrains. Vertical stills, another compact option, yield around 2.6 L/m²/day when augmented with reflectors, offering space-efficient solutions for temporary shelters. Overall, in variable climates, standard single-slope stills produce 2–6 L/m²/day, providing sufficient potable water for small groups without extensive maintenance.^[9]^[101] Integration with complementary off-grid technologies further extends utility. Solar stills can couple with solar cookers to preheat feedwater, elevating basin temperatures and boosting distillate output by expanding the absorption area, as demonstrated in experimental hybrids that enhance exergy efficiency. In expeditionary contexts, these systems mount within shelters or alongside photovoltaic arrays for dual water and energy production. Logistics emphasize modularity: kits like inflatable wick stills weigh under 1.5 kg and pack into compact volumes (e.g., 36 × 22 × 6 cm), enabling easy transport by hand or vehicle. Maintenance involves basic local training on cleaning wicks and basins, using readily available materials to ensure longevity in isolated deployments, with daily yields scaling to 1–1.8 L per small unit for emergency use.^[102]^[83]

Challenges and Limitations

Technical and Environmental Issues

One major technical challenge in solar stills is salt accumulation, which leads to fouling and scaling on the evaporator surface. This buildup reduces the heat transfer area and evaporation rate, causing a significant drop in overall productivity; for instance, in wick-based systems, solar-to-vapor conversion efficiency can fall below 60% due to crystallization.^[103] In prolonged operation, such fouling can cause a significant decrease in yields over time without intervention, as salt crystals block vapor pathways and lower thermal conductivity.^[104] Maintenance requirements further complicate deployment, particularly in remote areas. Regular cleaning is essential to remove accumulated salt and dust from the basin and cover; dust deposition on the transparent cover can reduce solar transmittance, while salt requires manual scraping or flushing depending on salinity levels.^[105] Material degradation exacerbates these issues, as plastic covers commonly used in low-cost designs suffer UV-induced breakdown, leading to embrittlement and reduced lifespan of 2-5 years in intense solar exposure.^[106] Environmentally, solar stills have a modest land footprint, typically requiring about 1 m² to produce 5 L of fresh water per day under optimal conditions, though large-scale installations can compete with agricultural land use. Some active systems may require additional water for cooling in arid regions. Climate variability poses additional hurdles, with cloudy conditions reducing solar radiation and thus output compared to clear days, as productivity is directly tied to insolation levels. Recent reviews as of 2025 emphasize low productivity rates (typically 2-5 L/m²/day) and increasing insolation variability due to climate change as barriers to scalability.^[107]^[25] Mitigation strategies focus on design innovations to address these challenges. Anti-scale approaches promote better brine drainage and reduce fouling by facilitating natural convection. Incorporating recyclable materials, like waste polypropylene trays or polyethylene sheets, enhances sustainability while minimizing environmental degradation from disposal.^[108] These measures, including wick-free structures with confined water layers, enable salt rejection up to 20 wt% brine without performance loss.^[103]

Contamination Mechanisms

Although solar stills produce distilled water through evaporation and condensation, trace ions can contaminate the output primarily via aerosol entrainment, where non-volatile salts from the feed water or ambient air are carried into the vapor phase as fine droplets or particles. These aerosols enter through microscopic leaks in the still's seals or joints and serve as nucleation sites for water vapor condensation, resulting in residual ion concentrations of 0.02–0.54 mg/L for fluoride and 0.04–0.25 mg/L for sodium in typical setups.^[109] Studies confirm that ions such as Na⁺, Cl⁻, F⁻, NO₃⁻, and SO₄²⁻ appear in distillates due to this process, with imaging revealing aerosol-induced droplets in the vapor space above the hot basin water.^[109] Mechanical carryover from splashing or bubbling in the basin can exacerbate this, entraining saline droplets into the rising vapor stream.^[110] Back-diffusion of ions across thin boundary layers at the water-vapor interface or through imperfect seals represents another pathway, driven by concentration gradients in the humid boundary layer near the evaporating surface. High operating temperatures in the basin enhance this process. Cover material leaching contributes marginally.^[110] Mitigation strategies focus on minimizing entry points and post-processing to achieve potable quality. Sealing leaks with materials like parafilm or silicone reduces aerosol ingress by up to 90%, while double distillation—cascading output from one still into another—further purifies by an order of magnitude.^[109] Ion-exchange resins applied post-distillation can remove residual traces. With proper seals and maintenance, solar stills can achieve potable water quality.^[93]

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